Yoshinari Miyamoto, Osaka Univ
Barry Rabin, Lockheed Idaho Technologies Co
Ivar Reimanis, Colorado School of Mines
Juergen Roedel, FB 21
Sanjay Sampath, SUNY-Stony Brook
Philip Morris Research
- U.S. Department of Energy, Office of Basic Energy Sciences
* Invited paper
SESSION GG1: MODELING
Chairs: Yoshinari Miyamoto and Barry Rabin
Tuesday Morning, December 2, 1997
Daniel Webster/Courier (W)
8:45 AM *GG1.1
SURFACE CRACKING AND DEBONDING OF FGM COATINGS. F. Erdogan, Lehigh University, Mechanical Engineering & Mechanics, Bethlehem, PA.
Surface cracking and debonding are among the most important modes of failure in FGM coatings. In the first part of this lecture the analytical benchmark problem for a FGM coating bonded to a homogeneous substrate will be considered. It will be assumed that the mechanical properties of the bond coat are approximately same as that of the substrate and the coating contains a surface crack perpendicular to the boundary. The composite medium may be under mechanical or thermal loading. The stress intensity factors for three typical crack geometries, namely ah will be presented, where a and h are, respectively, the crack length and coating thickness. Also presented will be the results of the nonlinear crack/contact problem in which, despite the positive stress intensity factor at the crack tip, due to the compressive stress near the boundary, the crack may actually be closed near and at the coating surface. The primary objective of the study is to investigate the influence of the crack-component geometry, the nature of the external loads and the material inhomogeneity parameter on the stress intensity factor. The second part of the lecture will deal with the debonding of FGM coatings. It will be assumed that the debonding crack initiates at a stress-free boundary and propagates along the nominal interface. Due to the formation of a thin oxide layer, the interface will be assumed to be the weak fracture plane. The composite medium having finite dimensions will be under steady-state heat conduction with convective heating at the FGM surface, forced cooling at the substrate surface and very weak convective cooling at the ends. An additional important factor considered in this problem will be the influence of partial insulation across the crack surfaces on the temperature distribution, total heat flow and the strain energy release rate. In heat transfer problems involving cracks it is generally assumed that the crack surfaces are either fully insulated or fully conducting.
9:30 AM GG1.2
FRACTURE MECHANICAL MODELING OF FUNCTIONALLY GRADED THERMAL BARRIER COATINGS. Ines Pflugbeil, Hans-Achim Bahr, Herbert Balke, Dresden University of Technology, Institute of Solid Mechanics, Dresden, GERMANY.
The fracture and damage behaviour of high temperature materials can be essentially modified by realising well-defined property gradients. Crack propagation perpendicular and parallel to the heated surface are the crucial phenomena. Crack propagation parallel to the surface leads to delamination and damage. A gradient effect based on fracture mechanics is demonstrated with the modelling of thermal barrier coatings (TBCs), taking into account the effective material properties derived in a self-consistent way. It is shown that the energy release rate G for delamination in TBCs under stationary heat flow can be reduced by grading towards decreasing thermal misfit between TBC and the substrate, even though the coating must be made thicker for compensation of the higher thermal conductivity. According to the fracture criterion, G has to be compared with the critical energy release rate Gc. Therefore, information on Gc has to be gathered. Damage is avoided by keeping G below Gc. The 4-point bending test after Charalambides has been modified such that Gc of the delamination crack of thin layers can be measured. The G required for crack propagation is obtained by attaching a stiffening layer to the TBC. As another advantage of this modification, segmentation of the layer and plastic deformation of the substrate are avoided. The experiment is analysed by FEM. In order to measure Gc, one needs the critical load and the crack length. The latter is derived by FEM from the measured compliance of the sample. With the providing of fracture mechanical material parameters and the optimization of a material gradient by minimization of G/Gc, this example is meant to contribute to improving the performance of layered systems and thermal barrier coatings in particular.
9:45 AM GG1.3
STRESS DISPLACEMENT FUNCTIONS IN EMBEDDED DUCTILE LAYERS: THE INFLUENCE OF CONSTITUTIVE LAWS, DAMAGE, DEBONDING, FRACTURE MODE AND DEFORMATION GEOMETRY. John Heathcote, Dept of Materials, G. Robert Odette and Glenn E. Lucas, Dept of Mechanical and Environmental Engineering, University of California, Santa Barbara, Santa Barbara, CA.
A basic set of design criteria was determined for elastic/elastic-plastic microlaminates, composed of alternating micron-scale metal and intermetallic layers. An analysis combining the finite element method of constrained metal deformation and a large scale bridging was used to calculate the effect of various microscale fracture processes and composite parameters on the stress-displacement function, s(u), of the ductile layer and both the corresponding resistance curve toughness and fracture strength of composite specimens with specified flaw sizes. Microscale processes included residual stresses, tunnel crack growth, fracture path selection (e.g., slanted and offset cracks), internal inclusion debonding/microvoid damage and effective layer debonding by brittle matrix splitting. The constituent parameters included ductile layer strength, strain hardening exponents and intrinsic brittle matrix toughness. The effect of loading rate, fracture mode and statistical distributions in key parameters were also modeled. Fracture strength is enhanced by a high metal layer strength and strain hardening, and, for small initial flaw sizes, by tensile residual stress and a high degree of deformation constraint. Voids in the ductile layers reduce fracture strength at large flaw sizes; for large voids, the fracture strength is reduced at all flaw sizes. Interfacial debonding or splitting cracks in the elastic layers reduce constraint, which, along with layer thickness, decreases the fracture strength for small flaws and increases it for larger cracks.
10:00 AM GG1.4
MICROMECHANICAL MODELING OF RESIDUAL STRESSES AND LOCAL RESIDUAL STRESS CONCENTRATION IN FUNCTIONALLY GRADED MATERIALS. Ming Dao, Pei Gu, Robert J. Asaro, Division of Structural Engineering, University of California-San Diego, La Jolla, CA.
A physically based computational micromechanics model is developed to study the random and discrete microstructures in functionally graded materials (FGMs). The influences of discrete microstructure on residual stress distributions at grain size level are examined with respect to material gradient and FGM volume percentage (within a ceramic/FGM/metal three layer structure). Both thermoelastic and thermoplastic deformation are considered, and the plastic behavior of metal grains is modeled at the single crystal level using crystal plasticity theory. The results are compared with those obtained using a continuous model which does not consider the microstructural randomness and discreteness. In an averaged sense both the micromechanics model and the continuous model give practically the same macroscopic stresses; whereas the discrete micromechanics model predicts fairly high residual stress concentrations at grain size level, i.e. around 6 FGM can have an averaged stress p () beyond 700MPa with only 300oC temperature drop in a Ni-Al2O3 FGM system. Statistical analyses show that local residual stress concentrations are insensitive to material gradient and FGM volume percentage in this case. The need to consider microstructural details in FGM microstructures is evident. The results obtained provide some insights for improving the reliability of FGMs against fracture and delamination.
10:45 AM GG1.5
SIMULATION OF GRADED MATERIALS. Masao Doyama, Yoshiaki Kogure, Keiko Aoki and Takayuki Omae, Teikyo University of Science and Technology, Uenohara, Yamanashi, JAPAN.
A computer program for the materials design minimizing the thermal stress and strength using finite element method has been developed. In the case of metals and alloys, the thermal conductivity can be estimated by Wiedemann-Franz rule. Applying the rule, the relation between the thermal conductivity and the concentration of alloys are obtained. When the temperature gradient exists along the thickness, the constant thermal gradient can be obtained by adjusting the concentrations of the alloy, so that the thermal stress can be minimized.
11:00 AM GG1.6
CRACK TIP STRESS FIELDS AND CREEP CRACK GROWTH OF INTERFACE CRACKS IN FUNCTIONALLY GRADED/LAYERED MATERIALS. S.B. Biner, Ames Laboratory, Iowa State University, Ames, IA.
In this study, the growth behavior of interface cracks in bimaterials and in graded/layered materials resulting from the creep cavitation was studied. The growth model includes the effects of material deposition resulting from the growth of creep cavities on the crack tip stress fields. The results indicate that in graded/layered materials under identical applied loading, the location of the interface crack strongly influence the amplitude of the stress field (C*) at steady-state. Due to large variation in the distribution of the stresses ahead of the interface cracks at creep regime, depending upon the crack location, the creep crack growth rates will be significantly different from each other under identical loading for a given graded/layered material.
11:15 AM GG1.7
MESO-SCALE SIMULATIONS OF GRADED MICROSTRUCTURE EVOLUTION. C. D. Van Siclen and B. H. Rabin, Idaho National Engineering Laboratory, Idaho Falls, ID.
A new modeling approach is being developed to investigate the morphological changes occurring on the mesoscopic scale in two-phase graded microstructures. The method involves use of kinetic Monte Carlo simulations involving a new agent-site exchange mechanism for transforming the microstructure. In this method, the sites hypothetically represent large atomic groups or clusters of atomic groups, and the agents are analogous mobile vacancies in a crystal lattice. Agent-site exchange rules determine the microstructure that is evolved; an agent-site exchange is accepted with Boltzmann probability determined by the energy of the site alone. Energy barriers are empirically determined, but can be related to phase diagram features. The simulation time scale is related to real time by a factor that can be determined by comparing simulation results with a sequence of experimental microstructures. In this paper, details of this new modeling approach are discussed, and preliminary results for hypothetical graded structures are presented.
11:30 AM GG1.8
METHOD OF MOMENTS ANALYSIS OF CURRENT MODULATION IN PERCOLATING-LIKE MATERIALS. Paul J. Kemper, Jr., Lisa M. Lust, Signature Technology Laboratory, Georgia Tech Research Institute, Atlanta, GA.
The electromagnetic characteristics of conducting geometric structures or patterns are theoretically produced by resonant modes across them. Percolating-like materials have long-range order (connectivity) between conducting elements which can produce strong electromagnetic characteristics. Artificial interruption of this long range order leads to the creation of materials with a modulatable current distribution. This paper presents analysis of the current distribution in periodically replicated unit cells of percolating-like materials, an approach which allows optimally modulatable materials to be identified. Visualization of the current also assists in identifying the optimal switching points at locations that greatly modify resonances in the structure. By placing active circuit elements at these points, modulatable materials may be realized. This paper presents calculations of current distributions on these percolating-like patterns using a Method of Moments computer code and shows the effects of the modifications of the patterns upon it.
SESSION GG2: PROCESSING I
Chairs: S. C. Deevi and John J. Petrovic
Tuesday Afternoon, December 2, 1997
Essex East (W)
1:30 PM GG2.1
FUNCTIONALLY GRADED MATERIALS BY INFILTRATION OF POROUS PREFORMS. A. Neubrand, R. Jedamzik, J. Rödel, Institute of Materials Science, Technical University Darmstadt, GERMANY.
A novel method to produce gradient materials is presented. It is based on the infiltration of refractory porous preforms with a metal, glass or polymer melt. The porosity gradient is introduced in the preform by an electrochemical process. For this purpose, a gradient of the electrochemical potential is set up inside a porous material, which serves as an active electrode in an electrolytic cell. This potential gradient leads to a position dependent rate of electrochemical dissolution or deposition of the preform material and thus to a graded porosity. A macrohomogeneous model of the electrode kinetics was developed to predict the influence of experimental parameters like current density, electrode and electrolyte resistivity and geometrical factors on the gradation profiles obtained. The porosity gradients predicted with this model agree well with experimental observations. The potential of the method will be demonstrated on the example of W/Cu-FGM's which are prepared by producing a graded tungsten preform by anodic dissolution and subsequent infiltration with copper. The compositional and thermomechanical property profiles of the tungsten/copper FGM's produced have been determined. The residual stresses calculated in the FGM's with these data are compared with experimental results from neutron diffraction measurements. The process can be modified for the production of electrochemically inactive ceramic preforms. In this case, porous carbon is used as an electrode material. After electrochemical dissolution, a carbon preform with graded porosity is retained. This graded preform is infiltrated with a ceramic slip and the carbon is burnt out during the subsequent sintering step. The resulting porosity graded ceramic preform is then infiltrated with metal or polymer. Preliminary results on this indirect route which allows the production many metal-ceramic FGM's will be presented.
1:45 PM GG2.2
A NOVEL METHOD OF PRODUCING GRADIENT MATERIAL BY CONTINUOUS CASTING USING MANY-STREAM-POURING TECHNIQUE. Ge Yu and Weiwen Zhang, City University of Hong Kong, Dept Physics and Materials Science, Kowloon, HONG KONG.
It is proposed to produce the gradient material to meet the demands for different mechanical properties at the different position of the cross-section of materials for both structural and functional applications. A method to achieve the gradient distribution of the alloy composition by continuous casting is introduced. The technique of many-stream-pouring is used. In the stationary state, the flow and the solidification of the outside stream and the inside stream can be controlled by changing the parameters, such as the casting temperature and the volume ratio of the two streams. Different composition profile in the as-cast structure can be yielded. First samples from Al-Si system, Al12wt%Si for the outside metal and pure Al for the inside metal, shows that the silicon composition, the mechanical property and the microstructure change continuously from the surface to the centre of the samples.
2:00 PM GG2.3
PROCESSING AND CHARACTERIZATION OF FUNCTIONALLY GRADED MOSi2-Si3N4 COMPOSITES. S.C. Deevi, Research Center, Philip Morris, Richmond, VA.
Among the high temperature materials, both MoSi2 and Si3N4 are unique due to their high melting points, excellent oxidation resistance, and good chemical compatibility. Their chemical compatibility allows us to design high temperature composite materials with the desired electrical and mechanical properties. A functionally graded structure based on Si3N4 and MoSi2 would exhibit properties varying across their dimensions based on the contents of MoSi2 and Si3N4. Functionally graded composites based on MoSi2- Si3N4 were processed with different volume fractions of MoSi2 and Si3N4, while keeping the MoSi2/ Si3N4 ratio constant in each layer. Two to five layered composites were processed by varying the gradient across each layer. Thermal expansion differences between MoSi2 and Si3N4 led to separation of the layers when the gradient exceeded by 30% volume fraction across each layer. In this paper, processing methodology, microstructure, microhardnesses, electrical properties, and oxidation resistance of functionally graded structures will be presented.
2:15 PM GG2.4
GRADIENT COMPOSITES PRODUCED BY CENTRIFUGAL CONSOLIDATION OF Al2O3 AND Y-ZrO2. Robert J. Moon, Erik N. Drewry, Keith J. Bowman, and Kevin P. Trumble, School of Materials Engineering, Purdue University, West Lafayette, IN.
Layered Al2O3 and Y-ZrO2 composites were produced by centrifugal consolidation of Al2O3 and Y-ZrO2 suspensions. Symmetric stepwise gradient composites (8 total layers) were produced by incrementally increasing (or decreasing) the Al2O3/Y-ZrO2 ratio within suspensions. The influence of layer composition and layer stacking sequence (increasing or decreasing Al2O3/Y-ZrO2 ratio from composite top surface to composite center) on Hertzian cone cracking behavior will be discussed. Difficulties in producing gradient composites by centrifugal consolidation will also be addressed. This research is supported by the National Science Foundation through grant DMR 95-28928 and the Army Research Office MURI grant DAAH04-96-1-0331.
2:30 PM GG2.5
CONTINUOUSLY GRADED INTERMETALLIC/CERAMIC MATERIALS FABRICATED BY WET-MOLDING PART (III) - MoSi2/ZrO2(2Y) AND WSi2/ZrO2(2Y) SYSTEM. Kenji Arata, Masaru Yoshinaka, Ken Hirota, Osamu Yamaguchi, Doshisha Univ., Faculty of Engineering, Kyo-Tanabe, Kyoto, JAPAN.
Bulk graded materials with laminated-structures have interfaces between layers with different compositions. Concentrated stress induced at these interfaces often results in origins of cracking and chipping in the materials. For this reason, much effort has been devoted for the fabrication of graded materials with a smooth transition. In the present study, continuously graded intermatallic/ceramic (MoSi2/ZrO2(2Y)) materials with high density (97.5% of theoretical) have been fabricated by uniaxial wet-molding, followed by hot pressing (1000C/1h/30MPa) and hot isostatic pressing (1400C/2h/196MPa); the uniaxial wet-molding was adopted to control the sedimentation velocities of particles in a slurry by verifying the viscosities of dispersion media used in the molding process. Their composition profiles are greatly influenced by the viscosity of the mixed solution of glycerin (934 mPA's) and ethanol (1.067 mPa's) used as dispersion media; a linear compositional gradient from MoSi2/ZrO2(2Y)70/30 to 20/80 mol% is obtained from the solution (50/50 vol%) with the viscosity of 20 mPa's. The mechanical properties are evaluated, in comparison with those of laminated materials with three-layers of MoSi2/ZrO2(2Y) (70/30)/(50/50)/(30/70) mol%. Vickers hardness Hv increased linearly from 9.7 to 12.4 GPa with increasing distance from the bottom, reflecting a linear composition profile. On the other hand, Hv in laminated materials changes in stepwise. Fracture toughness Kc in continuously graded materials increases gradually from 5.1 to 6.1 mPam1/2 in the 80 mol% ZrO2(2Y) region, while the Kc of laminated materials shows a stepwise change in the same behavior as with Hv. These differences between both materials are discussed, in relation to their microstructures and composition profiles. Moreover, the fabrication and evaluation of WSi2/ZrO2(2Y) materials will be presented.
2:45 PM GG2.6
ELECTRON BEAM PROCESSING OF FGM BOND/ENVIRONMENTAL COATINGS. M.R. Jackson, GECRD, Schenectady, NY; and Jogender Singh, The Pennsylvania State University, State College, PA.
Adherence of ceramic thermal barrier coatings to Ni-base superalloys is enhanced by the use of bond coatings. These bond coatings must also be extremely oxidation resistant, since oxygen can reach the bond coat/TBC interface to grow an intervening oxide layer. Minimizing the growth rate of the oxide can extend the time the TBC remains adherent. Once the TBC is spalled from the surface, the bond coating is all that remains to prevent attack of the superalloy. Many FGM bond coatings have been evaluated previously as a means to grade the expansion behavior from the relatively high expansion of the superalloy to the lower expansion of the TBC. Expansion grading must be achieved in coatings of less than 100 microns (for jet engine applications, somewhat thicker for gas turbine applications). In the current study, electron beam physical vapor deposition has been used to produce graded structures. Small volumes of Al-base alloys have been evaporated to near-exhaustion to produce graded structures on superalloy substrates. Deposition has been on substrates at low temperature (500C) so that minimal reaction with the substrate occurred in situ. Chemical gradients could then be studied for both as-deposited structures and for NiAl-base structures formed by reaction with the substrate at 1100C. Alloys of Al, AlCr, AlSi, and AlCrSi base have been deposited. The gradients in chemistry produced in the structures can be interpreted in terms of the expected vapor pressures of Al, Cr and Si above superheated pools. The use of small pool volumes evaporated to near-exhaustion further enhances chemical gradient formation.
3:15 PM GG2.7
FUNCTIONALLY GRADED BORON CARBIDE MATERIALS. John J. Petrovic, Kenneth J. McClellan, Charles D. Kise, Richard C. Hoover, W. Kent Scarborough, Materials Science & Technology Division, Los Alamos National Laboratory, Los Alamos, NM.
Functionally graded boron carbide-to-metal and boron carbide-to-polymer materials have potential important uses in lightweight body armor as well as wear-related applications. Boron carbide ceramics with gradients in porosity are being produced using grading of densification aids, and different powder sizes and size distributions. Furfuryl alcohol is being employed as a precursor for the carbon densification aid. Boron carbide powders with average sizes of 0.5, 5, and 25 microns are being employed. Techniques for producing graded green ceramic bodies of boron carbide have been developed. These graded green bodies are then hot pressed at temperatures in the range of 1800-2000 C to produce gradients in porosity from 0-50%. The graded porosity boron carbide forms are subsequently vacuum liquid infiltrated with molten aluminum or polymers to produce graded ceramic-metal and ceramic-polymer materials. Details of the FGM processing of these materials will be described and initial mechanical property characteristics discussed.
3:30 PM GG2.8
INTEGRAL SKIN FOAMS WITH A SIGNIFICANT SPATIAL DENSITY STRUCTURE ANALYSIS AND SIMULATION OF THE DENSITY STRUCTURE FORMATION. Prof. Dr.-Ing. Walter Michaeli, Dipl.-Ing. Hubert Ehbing, Institute of Plastics Processing (IKV), Technical University of Aachen, Aachen, GERMANY.
Polyurethane Integral-Skin-Foams have a significant density structure over the cross-section of the molded part. For part thickness of 5 to 10 mm, the density values are usually about 800 kg/m3 in the skin and 300 kg/m3 in the core region. This structure relies on the local different release of blowing agent during the foam formation. Because of this sandwich structure those parts generally have good specific mechanical properties in relation to their weight. Applications are housings and automotive components. The complex behavior during the foam formation could not be entirely described yet. So far the technical design of the foaming process is made by means of empirical methods (``trial and error''). Therefore a simple mold and a standard processing equipment is used to analyze the mechanisms of the graded density formation. The influence of process parameters on the density structure is investigated. E.g. it is shown that by increasing the amount of foaming agent and by decreasing the mold temperature the sandwich profile becomes more distinct. Hence the foamed parts have a higher stiffness in bending. Because the common methods for analyzing density distributions are time-consuming, a new measurement process has been introduced based on image.
3:45 PM GG2.9
GRADIENT CERAMIC COMPOSITES VIA DIFFERENTIAL SPONGE COMPRESSION. Frank R. Cichocki, Jr., Kevin P. Trumble, Purdue University, Materials Engineering, W. Lafayette, IN; Juergen Roedel, J. R. Ceramics Group, TH Darmstadt, GERMANY.
A technique for producing gradient ceramic composites via compression and subsequent infiltration of polymer sponges with ceramic slurry has been developed. Polyurethane sponges have been compression molded to produce sponge networks with a controlled gradient in porosity. Pressure infiltration followed by pyrolysis of the sponge and firing of the ceramic produces a ceramic preform that is essentially the negative of the compressed sponge; gradient composites are then produced by infiltrating the preform with metal. Connectivity, morphology, and volume fraction of the porosity in the sintered preforms have been quantitatively assessed. A relationship between compressive strain in the sponge and porosity in the sintered ceramic has been established. The applicability of this technique to the manufacture of complex composite shapes will be demonstrated.
4:00 PM GG2.10
FABRICATION OF METAL/INTERMETALLIC COMPOUND FGMS BY EUTECTIC BONDING OR EUTECTIC COATING METHOD. Tokuzou Tsujimoto, Soshu Kirihara, Seiji Kobayashi, Yo Tomota, Ibaraki Univ., Dept. of Materials Science, Hitachi, JAPAN.
Metal/Intermetallic compound FGMs have been studied as structural material in the present study. Intermetallic compounds are the materials which have properties between metals and ceramics. This concept also holds for ductility and thermal expansion coefficients, which makes alloy design of FGMs easy. In this study, a metal and an intermetallic compound, which have a eutectic reaction between them in the phase diagram, are joined using eutectic bonding method. This process is simply realized by heating a pair of the metal block and the intermetallic compound block in close contact above the eutectic temperature. Mixing of the metal and the intermetallic compound is achieved on solidification of the generated eutectic melt. Composition of the melt changes gradually from that at the metal/melt interface to that at the intermetallic compound/melt interface, and dendrites of the constituents develop from both the interfaces on cooling of the melt. That is, composition gradient is formed through segregation on solidification of the inhomogeneous melt. When powder of the intermetallic compound and a shaped metal part are used in the above procedure, the metal product is covered by functionally graded coating. In this process which is named eutectic coating, the eutectic melt is yielded at the interface between the metal part and the powder, and the melt is sucked up to the vacant spaces of the powder by the capillary effect. In this method mixing the components, fixing the powder, and forming composition gradient is achieved through penetration of the melt into the powder layer. Experiments on the eutectic bonding and the eutectic coating were carried out for Ti/Ti3Sn, Ti/Ti5Si3 and Ni/TiC system. Composition gradient was formed in all the cases, and generation of any cracks was not found.
4:15 PM GG2.11
JOINING OF DISSIMILAR CERAMICS USING FUNCTIONALLY GRADED INTERLAYERS. Mani Gopal, Lutgard C. DeJonghe & Garth Thomas, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA.
When dissimilar ceramics are joined, residual stresses are generated due to differences in the coefficient of thermal expansion. These stresses can be minimized by using a FGM interlayer as the strain is better accommodated by the gradient. This is a report on the joining of Si3N4 to Al2P3 using an FGM interlayer. Chemical compatibility is ensured because the FGM is Si3N4 rich in one end and Al2O3 rich at the other end. The FGM is synthesized using a novel differential sedimentation technique. The advantage of this process is that the gradient is formed in a single step. Further, the process can be used for any system - metal or ceramic, conductor or insulator, etc. Control over the gradient is achieved by varying the particle sizes and its distribution. A model has been developed to predict the composition profile of the FGM as a function of the starting particle distributions. The model shows excellent agreement with experimental observations and is used to control the composition profile which in turn controls the stresses in the joint. Characterization of the FGM and the joint has been performed using scanning and transmission electron microscopy. Preliminary mechanical characterization has been performed by indentation techniques.
4:30 PM GG2.12
COPPER GRADIENT FORMATION IN A LAYERED TIC FGM VIA FIELD-ASSISTED SHS. Ellen M. Carrillo-Heian, Zuhair A. Munir, Univ of California Davis, Dept of Chemical Engineering and Materials Science, Davis, CA.
Titanium carbide diluted with copper was synthesized via field-assisted SHS from elemental powders in two-layered samples with differing concentrations of copper. The copper concentration profiles at the interfaces between the layers were measured via EMA. The results showed a dependence of the concentration profile on the magnitude of the applied field. The results demonstrate the use of an electric field to effect a desired transition at the interfaces of layered FGMs to reduce associated stress concentrations. Ideally, this can lead to a situation where the properties of the easily-synthesized layered FGM approach the more difficult continuous FGM. In addition, the present work demonstrates the influence of the electric field in SHS on improving the properties of a synthesized material over the same material synthesized without field.
4:45 PM GG2.13
STRUCTURAL FEATURES OF FUNCTIONALLY GRADED MATERIALS AT THE BASIS OF SUPER ALLOYS OBTAINED BY SHS TECHNOLOGY. Valery V. Kovylyayev, Victor P. Solntsev, Alexader P. Lyapunov, Myroslav O. Korbutyak, Valery V. Kartuzov, Institute for Problems in Materials Science, NASU, Kyiv, UKRAINE.
High speed of reactionary processes occurring under SHS and their large remoteness from equlibrium state stipulates the possibility to obtain new structural states ensuring the improvement of all complex of properties of functionally graded materials. Electron microscope investigation on character of samples fracture, on chemical composition of phases and their structure allows to determine that under SHS initially discrete distribution of layers is changed due to reactionary diffusion into smooth changes of composition and structure. Main feature of character of destroing graded super alloys obtained by SHS technology is viscous-plastic fracture of intermetallic constituents. Active and passive components of super alloys are determined which enables purposeful formation of materials structure particularly in case of need to suppress the process of accelerated transport, for example nickel, aluminium and niobium under reactionary diffusion.
5:00 PM GG2.14
DEVELOPMENT OF CONTROLLED TECHNOLOGY FOR SINTERING FUNCTIONALLY GRADED MATERIALS USING SHS. Victor P. Solntsev, Myroslav O. Korbutyak, Valery V. Kartuzov, Valery V. Kovylyayev, Tatyana A. Solntseva, Institute for Problems in Materials Science, NASU, Kyiv, UKRAINE.
Existing phenomenological representations for mechanism of combustion wave propagation during solid phase exothermal reactions do not consider the dynamics of non-linear interaction of all accompanying processes. Theoretical and experimental investigation on interaction processes allowed to determine that the temperature of combustion front is not constant. It is changed under front motion and is a function of enthalpies of chemical reaction, phase transitions of the first kind (melting, cristallization, phase transformation), of thermal conductivity and is also determined by dissipative properties of powder and porous medium (sintering processes). At the same time these quantities depend on temperature. Thus one can create physicochemical model which reflects all mechanisms of non-linear interaction at the basis of synergetic approach. It enables to calculate a trajectory of temperature changes of reaction front and to specify the sintering mode for every subsystem by a program way.
SESSION GG3: PROCESSING II
Chair: F. Erdogan
Wednesday Morning, December 3, 1997
Essex East (W)
8:30 AM GG3.1
ON SELECTIVE LASER SINTERING OF GRADED OXIDE CERAMICS. A.V. Ragulya, I.N. Frantsevich Institute for Problems of Material Science, Kiev, UKRAINE.
Selective laser sintering is considered as a prospective method to manufacture the multilayer graded materials. Zirconia-based ceramics doped by titania, alumina and yttria was chosen to be the object of the present study. Nd-YAG permanent laser with wave length of 1.06 m was applied to sinter the multilayer ceramics of variable porosity composition and, therefore, properties. It has been shown that necessary gradients of properties can be controlled by means of laser parameter. The variations of liquid phase quantity, time of melt existence and cooling rates allow the control over microstructure development during selective laser sintering. Several experimental results will be compared to ones of computer simulation.
8:45 AM GG3.2
GENERATION OF GRADED METAL-CARBIDE MULTILAYER STRUCTURES BY THE POWDER-FED LASER CLADDING PROCESS. Thomas Seefeld, Emil Schubert, Gerd Sepold, BIAS Bremen Institute of Applied Beam Technology, Bremen, GERMANY.
The application of functionally gradient materials and graded surface coatings allows for extended performance and lifetime of a component by matching the material's property profile with the demands of the component's service requirements. A compositional gradient may be obtained within a material or coating by stepwise application of subsequent layers of material of varying composition. In the present work, various nickel based alloy powders are mixed in-situ with chromium carbide powders and deposited on steel substrates using a direct deposition laser cladding process. For the cladding of multiple layers with increasing carbide content, process parameters have to be adapted appropriately. The paper highlights the effect of process parameters on the material's behaviour during processing.
9:00 AM GG3.3
CHLOROSILANE EFFECTS ON THE PREPARATION OF FUNCTIONALLY GRADED SiC/C MATERIALS THROUGH CHEMICAL VAPOR DEPOSITION FROM CHLOROSILANE-HYDROCARBON-HYDROGEN MIXTURES. Igor M. Kostjuhin and Stratis V. Sotirchos, Dept of Chemical Engineering, University of Rochester, Rochester, NY.
A comparative study of the preparation of functionally graded SiC/C materials through chemical vapor deposition (CVD) from methyltrichlorosilane-ethylene-hydrogen and tetrachlorosilane-ethylene-hydrogen mixtures is presented in this study. CVD experiments were carried out in a hot-wall CVD reactor, coupled to a microbalance and thus allowing for continuous monitoring of the deposition rate. X-ray diffraction, electron microscopy, and energy dispersive X-ray analysis were employed for the characterization of the deposits. The results showed that SiC/C functionally graded films and coatings can be prepared through SiC and C codeposition from either of the two mixtures. Several qualitative similarities were found to exist in the two systems in the variation of the deposit stoichiometry and of the deposition rate with the feed composition and the location in the reactor. Both deposition systems exhibited multiple steady states, a situation that manifested itself as abrupt changes in the deposition rate and the deposit stoichiometry as some operating parameters were varied. In the absence of multiple steady states, the composition of the deposit changed between C and SiC over a broader range of feed composition in the case of methyltrichlorosilane. It is thus believed that it will be much easier to control the composition profile in the deposit by manipulating the composition of the feed mixture when this gas is used as silicon source.
9:15 AM GG3.4
YTTRIUM-DOPED ALPHA ALUMINA: RE-VISITED. Mehmet A. Gulgun, Valeri Poutlaiev, and Manfred Ruehle, Max-Planck-Institut fuer Metallforschung, Stuttgart, GERMANY.
Using multiple techniques of electron microscopy (EM), the issue of yttrium segregation in polycrystalline alpha alumina was re-addressed. Hot-pressed and crept samples were both doped with 1000 ppm of yttrium (weight ratio of Y/Al). At this doping level and an average grain size of 2 to 4 microns, both types of samples precipitated a yttrium-rich second phase, yttrium aluminate garnet (YAG). Scanning and transmission electron microscopy on a multiplex of specimens from both types of samples showed that grain size and shape strongly depended on the precipitation behavior. Both samples exhibited a bimodal alumina grain size distribution: small equiaxed grains and larger tabular grains. Grain boundaries (GB) of grains from either category contained varying levels of yttrium and silicon, a common impurity in alumina. The maximum amount of yttrium at GBs was 1/3 of a monolayer coverage as measured with EDS in a dedicated scanning transmission electron microscope (STEM). Electron energy loss spectroscopy (EELS) was used to gain insight into the chemical and structural make of general and some special grain boundaries.
9:30 AM *GG3.5
PROCESSING - MICROSTRUCTURE RELATIONSHIP DURING PLASMA SPRAYING OF FGMS. H.Herman, W.C.Smith, T.J.Jewett and S.Sampath, Center for Thermal Spray Research, Department of Material Science and Engineering, SUNY-Stony Brook, Stony Brook, NY.
Functionally graded materials (FGMs) offer solutions to numerous engineering problems, especially those involving multi-layer systems with large differences in CTE, i.e. thermal barrier coatings. By allowing continuous change in the properties over definable distance FGMs thus minimizing sharp interface effects. By its nature, plasma spraying is well-suited to the fabrication of FGMs. However, due to large gradients in the plasma spray flame and the inherently large differences in physical properties between the FGM species, complexities arise in processing. It is therefore important to develop inter-relationships between particle size distributions, injection orientations , feed rate and process parameters. This is the objective of the present investigation. A series of experiments consisting of particle diagnostics, deposition efficiency measurements, and microstructural analysis were carried for Ni-alumina and NiCrAlY zirconia FGMs. The results of these experiments and linkages to feedsto ck characteristics and processing parameters will be presented.
10:00 AM GG3.6
SINGLE AND MULTIPLE INJECTOR SPRAY PATTERN AND PARTICLE TRAJECTORY EFFECTS IN THE THERMAL SPRAY PROCESSING OF FGMS. J. R. Fincke, W. D. Swank, D. C. Haggard, Optical and Plasma Physics, Idaho National Engineering Laboratory, Idaho Falls, ID
In the thermal spray processing of FGMs multiple particle types with significantly different size distributions, densities, and melting points are co-deposited on a substrate. These materials may be either singly injected through multiple injectors or co-injected in a single injector. successful processing requires that the spray patterns of different particle types coincide at impact and that each particle type arrive with the appropriate temperature and degree of melting. Particle trajectory is a major controlling factor. Using in-flight diagnostics of particle size, velocity, and temperature, and spray pattern shape and trajectory, particle spray field characteristics are detailed and the controlling parameters examined for both singly and co-injected ceramic and metallic particles.
SESSION GG4: FRACTURE AND DEFORMATION I
Chair: Sanjay Sampath
Wednesday Morning, December 3, 1997
Essex East (W)
10:45 AM *GG4.1
THE ENGINEERING OF SURFACES THROUGH GRADED MICROSTRUCTURES. Subra Suresh, Massachusetts Institute of Technology, Dept of Materials Science and Engineering, Cambridge, MA.
This presentation will provide an overview of recent experimental and theoretical advances in the understanding of thermomechanical behavior of graded materials. A variety of techniques will be discussed by recourse to which the onset of cracking can be suppressed in graded materials subjected to thermomechanical loading, indentation and impact. Specific guidelines will also be presented for the control of processing-induced and thermally-induced internal stresses. Applications of the fundamental studies and theory to practical situations will also be examined. The presentation will conclude with discussions of new methods for the measurement of local properties of graded materials through micro- and macro-indentation, where recent advances in the theory for indentation of graded materials will be highlighted.
11:30 AM GG4.2
DELAMINATION AND INTERFACE FAILURE MECHANISMS OF THERMAL BARRIER AND OTHER HARD COATINGS ON SUPERALLOY AND CERAMIC SUBSTRATES. R. Suryanarayanan, Joanna Mroz and Reiner H. Dauskardt, Stanford University, Materials Science and Engineering, Stanford, CA.
Thermal barrier and other hard ceramic coatings are used extensively to prolong component life in high temperature environments and in cutting tool applications exposed to high wear conditions. Spalling of the coatings, associated with progressive debonding driven by thermal strains from thermal expansion mismatch across the interface and alternating thermomechanical loading during service, is a critical concern for long-term reliability. Pull-off and scratch techniques have been used to characterize interface adhesion but reproducible and quantitative data remains elusive. We describe a fracture mechanics approach to obtain critical adhesion values and to characterize progressive (time or loading cycle dependent) debonding of the coating-substrate interface. This is achieved by employing a novel sample geometry consisting of a ceramic coating sandwiched between two rigid substrates, coupled with a high resolution piezoelectric loading system. Our research focuses on zirconia/NiAlPt thermal barrier multi-layers on single crystal superalloy substrates and TiN, TiAlN, and TiCN hard coatings and multi-layers on cemented carbide and cermet substrates. Mechanisms of debond fracture in these highly anisotropic systems together with interface engineering strategies to promote adhesion are discussed.
11:45 AM GG4.3
HERTZIAN-CRACK SUPRESSION IN CERAMICS WITH ELASTIC-MODULUS-GRADED SURFACES. Juthamas Jitcharoen, and Nitin P. Padture, University of Connecticut, Department of Metallurgy and Materials Engineering, Storrs, CT; Antonios E. Giannakopoulos and Subra Suresh, MIT, Department of Materials Science and Engineering, Cambridge, MA.
Hertzian (spherical) indentation experiments were carried out in a graded alumina-glass composite whose Young's modulus (E) increased with depth (z) beneath the indented surface. A novel, in-situ processing method inlvolving impregnation of a dense, fine-grained alumina by an aluminosilicate glass was employed to fabricate such a composite, free of macroscopic, long-range residual stresses. The unidirectional variation in E under the indenter is shown to fully suppress the formation of Hertzian cone cracks. Without these elastic-modulus gradients, cone-crack formation was observed in bulk glass and alumina. It is reasoned that the present innovations, involving functionally-graded surfaces and their in-situ processing, provide new possibilities for enhancing certain contact-damage resistance characteristics in various ceramic materials for a broad range of engineering applications. Furthermore, since this contact-damage-resistance phenomenon in functionally-graded ceramics is elastic in nature, these ceramics are likely to be immune to mechanical fatigue.
SESSION GG5: FRACTURE AND DEFORMATION II
Chairs: Fumio Nogata and Ivar E. Reimanis
Wednesday Afternoon, December 3, 1997
Essex East (W)
1:30 PM *GG5.1
GRADED COATINGS FOR GAS TURBINE APPLICATION AT ELEVATED TEMPERATURE. W.A. Kaysser, M. Peters, U. Schulz, U. Leushake, Institute of Material Research, DLR - German Aerospace Research Estb., Cologne, GERMANY.
Improved thermal barrier coatings can lead to increased engine operating efficiency of aeroengine and stationary turbines. A significant problem is that these multilayer systems are not thermally stable at elevated temperatures. Phenomena such as interdiffusion, sintering and oxidation take place limiting coating system lifetime during service. In order to extend overall lifetimes and/or increase application temperatures, several modifications based on functionally graded material concept can be made. Possible application targets will be described in detailed according to their mechanism and their potential to extend the lifetime. Chemical graded bond coats can be applied to reduce interdiffusion based coating system damage. To improve thermal shock resistance microstructural graded zirconia layers were found to be successful. The dominating failure mechanism is the bond coat oxidation and the formation of a thermally grown oxide (TGO) at the interface BC/TBC during service at elevated temperatures. To overcome this failure mode a concept of a graded transition layer from alumina to zirconia for improved lifetime TBCs will be presented, which allows interfacial control and reduces the TGO growth rate. The fabrication of these graded alumina/zirconia transitions by co-evaporation from two sources using a jumping electron beam coater will be briefly discussed.
2:15 PM *GG5.2
THERMAL FRACTURE MECHANISMS IN FUNCTIONALLY GRADED THERMAL BARRIER COATINGS. Klod Kokini, Purdue Univ, School of Mechanical Engineering, West Lafayette, IN.
Thermal barrier coatings are becoming increasingly important for the development of advanced high temperature machines such as diesel engines, gas turbines and aircraft engines. The design of such systems, however, is complex because the high heat fluxes, and the transient temperatures that the materials are exposed to affect the properties of these materials with time, ultimately resulting in cracking, delamination and spalling of the coating. Functionally graded coatings provide a more gradual transition in properties and change the effective properties of the system possibly making the overall structure more durable under the applied thermal loads. However, the presence of many material combinations also generates the possibility of different fracture modes. Such fracture modes include surface fracture, multiple surface fracture, interface fracture and edge fractures which are dependent on the behavior and properties of the materials, the applied thermal loads and the architecture of the materials. This paper will review the research activities in these areas performed by the present investigator and his collaborators. In particular, the results of experiments performed using a 1.5 kW CO2 laser and a high intensity focused infra-red lamps on the behavior of coatings will be presented. The analytical models which simulate and explain the thermal fracture behavior under different heating and cooling conditions will be discussed.
3:00 PM GG5.3
EXPERIMENTAL INVESTIGATION ON FRACTURE BEHAVIOR OF FUNCTIONALLY GRADED METAL/CERAMIC MATERIALS. Lianchao Sun, Leon L. Shaw, University of Connecticut, Dept. of Metallurgy and Materials Engineering, Storrs, CT.
Few experimental studies have been done on the fracture behavior of functionally graded materials. In this paper, three functionally stepwise graded materials (FGMs), Ti-6Al-4V/SiC, Ni/Y2O3 and Ti-6Al-4V/Y2O3, were prepared through the powder metallurgy approach. Four-point bending fracture test were carried out with chevron-notched specimens. Crack propagation behavior during bending tests were directly observed and recorded with a Questar long distance microscopic vision system. The microhardness distribution in these FGMs was measured. Microstructure and fracture surface were also examined with optical and scanning electric microscopes. The crack extension processes observed through the Questar system were correlated to the recorded load-displacement data. The results showed that three FGMs exhibited different crack propagation behaviors. It was found that crack growth resistance continued to increase when cracks propagated from the high ceramic content side to the high metal content side. In addition, fracture behavior was affected by two types of interfaces present in the FGMs: one is the interface between two adjacent layers and the other is the interface between metal and ceramic constituents within each layer. Depending on the interface strength, cracks can be deflected at both interfaces with large increase in fracture energy when cracks are deflected at the interfaces between the adjacent layers. Fracture behavior of FGMs was also found to be dictated by the combination of metal and ceramic constituents. Among three systems, Ti-6Al-4V/Y2O3 exhibited the highest fracture energy. The results suggested that interface properties and chemical compatibility between metal and ceramic constituents are two major parameters in controlling the fracture energy of FGMs.
3:15 PM GG5.4
DEFORMATION AND FRACTURE OF NICKEL/ALUMINA COMPOSITES WITH AND WITHOUT GRADED MICROSTRUCTURES. Andrew Winter, Ivar Reimanis, Colorado School of Mines, Dept of Metallurgical and Materials Engineering, Golden, CO; Barry Rabin, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID.
Compositionally graded and non-graded composites consisting of alumina and nickel, ranging from 0 to 90 volume percent nickel have been synthesized by hot isostatic pressing. A systematic study on the role of powder additives in achieving desirable green densities was conducted so the amount of sintering shrinkage for each composition could be controlled; in this way, the differential shrinkage for each composition could be minimized. The microstructures have been controlled using several different starting particle sizes of alumina and nickel. The deformation and fracture of graded and non-graded composites was examined by conducting crack propagation experiments with a compact tension specimen geometry. In-situ viewing utilizing a long distance optical microscope has facilitated understanding how the microstructure controls fracture behavior. It has been observed that the interface between nickel and alumina is important in governing the fracture behavior. Specifically, weak interfacial bonding was observed in many of the composites, resulting in little plastic deformation in the nickel phase during fracture, and correspondingly low fracture energies. The relative particle sizes are also important in determining the fracture behavior. The relationships between processing, deformation, and fracture of these composites are discussed.
3:30 PM GG5.5
CHARACTERIZATION OF LINEAR AND NON-LINEAR NiAl-Al2O3 FUNCTIONALLY GRADIENT COMPOSITES. Hexiang Zhu, Durbha Padmavardhani, Reza Abbaschian, Dept of Materials Science and Engineering, University of Florida, Gainesville, FL.
Functionally gradient materials (FGMs) are of increasing interest in a wide range of engineering applications since the composition gradient in FGMs can be tailored to meet their specific requirements. In this presentation, the effect of composition gradient on the microstructures and mechanical properties of NiAl-Al2O3 functionally gradient composites (FGCs) will be discussed. The linear and non-linear FGCs were fabricated by reactive hot compaction of Ni and Al powders in which one or both of them were preoxdized. The gradient in the composition was obtained by stacking different powder mixtures of desired compositions. The in-situ alumina formed a continuous network around the NiAl during the compaction process. The microhardness near the interfaces and fracture toughness of FGCs were characterized.
4:00 PM GG5.6
THERMAL FRACTURE BEHAVIOR OF METAL/CERAMIC FUNCTIONALLY GRADED THERMAL BARRIER COATINGS STUDIED BY BURNER HEATING TEST. Akira Kawasaki, Masashi Yamaguchi and Ryuzo Watanabe, Department of Materials Processing, Graduate School of Enginering, Tokohu University, JAPAN
The cyclic thermal fracture mechanism of metal/ceramic functionally graded thermal barrier coatings was studied by burner heating test. The main problem in thermal barrier coatings applied to gas turbine components is the spallation of ceramic coating under thermal cycling environments. To clarify the spalling behavior of plasma-sprayed FGM coatings, the burner heating test was conducted where the actual thermal environment is simulated. In the test the top surface is heated by combustion flame and the bottom is cooled by water flow. The monitoring of acoustic emission (AE) allows us to in-situ detection of cracking in the ceramic coat during testing. The TCB's consist of NiCrAlY bond coat, ZrO2-8wt.%Y2O3 top coat and FGM inter layer where coating layers were deposited by air plasma spraying on stainless substrates. Three types of FGM coatings, having the same thermal resistance with different compositional profiles, were fabricated. The spallation mechanism has been discussed on the basis of the crack observation, frequency analysis of AE events and the variation of effective thermal conductivity. The sequence of spalling behavior is found to be; vertical crack formation on the top surface during cooling, parallel crack formation at the top coat/bond coat interface during heating, and the growth of parallel cracks and their coalescence which leads eventually the ceramic coat to spall. Spallation life has also been discussed.
4:15 PM GG5.7
EFFECTS OF INTERNAL STRESS ON FRACTURE BEHAVIOR OF SYMMETRICALLY GRADED Al2O3/TiC/Ni. Junshan Lin, Yoshinari Miyamoto, Joining & Welding Research Inst, Osaka Univ, Ibaraki, Osaka, JAPAN.
The symmetrically graded material in the Al2O3/TiC/Ni system was prepared by using SHS/HIP process. Internal stress distribution was calculated by using the finite element method and determined by X-ray diffraction technique. The fracture toughness in the graded structure was measured as a function of the initial crack length. The effect of stress relaxation, which was induced by the initial crack, on the fracture toughness was analyzed as well. The R-curve behavior of the Al2O3/TiC/Ni material determined by indentation-strength method showed that the surface compressive stress can significantly enhance the R-curve behavior, resulting in the improvement of the crack growth resistance and damage tolerance.
4:30 PM GG5.8
THE EFFECT OF MICROSTRUCTURE AND THERMAL RESIDUAL STRESSES ON THE MECHANICAL BEHAVIOR OF NiA1-TiB2 GRADED COMPOSITES. R.D. Torres, L.E. Reimanis, J.J. Moore, G.G.W. Mustoe, Colorado Center for Advanced Ceramics, Colorado School of Mines, Golden, CO.
NiAI - TiB2 graded composites are potential materials for high temperature application due to high thermal stability, high thermal conductivity and good oxidation resistance. In this study a correlation between mechanical properties such as elastic modulus, modulus of rupture and fracture toughness with microstructure has been made. The microstructure features that affect the mechanical behavior of the composites are TiB2 particle spacing, the nature of the interface formed between NiAI and TiB2, NiAI grain size and the amount of porosity. The finite element computer code Abaqus has been used in the determination of thermal residual stresses developed within the graded region. The constitutive law, which describes the properties of a specific composite in the graded region, was determined by measuring the Young's modulus and the coefficient of thermal expansion (CTE) as a function of composition. The variation of the Young's modulus and CTE with composition will be compared with rule-of-mixture models.
4:45 PM GG5.9
CREEP BEHAVIOR OF MoSi2-Si3N4 FUNCTIONALLY GRADED LAYERED COMPOSITES. C.R. Feng, K. Sadananda, Naval Research Laboratory, Washington, DC; S.C. Deevi, Research Center, Phillips Morris USA, Richmond, VA.
Both molybdenum disilicide and silicon nitride possess high melting point and excellent oxidation resistance and are thermodynamically stable with each other. In this study, two to five layered composites are made with each layer containing different volume fractions of MoSi2 and Si3N4 The ratio of the two constituent materials in each layer is kept constant. For example in the five layer functionally graded material, the layers consist of l00%MoSi2, 80%MoSi2/20% Si3N4, 60%MoSi2/40% Si3N4, 40%MoSi2/60%Si3N4, 20%MoSi2/80%Si3N4. In each layer, the two constituents are uniformly distributed. Constant load compression creep tests were conducted in air at 1200C. The load was increased in steps after the minimum creep rate was observed at each load. The stress range in these incremental load tests was 38-400 MPa. Tests were conducted on two different orientations with the layers perpendicular and parallel to the loading direction. In addition, the creep behavior of each single layer was also studied for base line data. There was a gradient in the creep deformation with increasing volume fraction of the Si3N4. The deformation was ductile at the high MoSi2 end while it was brittle with extensive cracking at the Si3N4 end. Assuming a free sliding interface, the creep rates of the composite are computed using the creep rates of the individual layers using series and parallel interfaces. Predictions agree reasonably with the data except when cracking occurred.
5:00 PM GG5.10
RESIDUAL STRESSES IN PLASMA SPRAYED NI-AL2O3 AND NICRALY-YSZ FGM'S. Jiri Matejicek, Sanjay Sampath, Center for Thermal Spray Research, State University of New York, Stony Brook, NY; Paul C. Brand, Henry J. Prask, National Institute of Standards and Technology, Gaithersburg, MD; Olivera Kesler, Subra Suresh, Massachusetts Institute of Technology, Cambridge, MA.
Functionally graded plasma sprayed deposits are now gaining more and more attention thanks to their use as thermal barrier coatings and other high-performance applications. One of the factors that need consideration is residual stress, originating from the plasma spraying process and thermal expansion mismatch between dissimilar materials. The advantage of graded coatings lies in alleviation of the thermal expansion discontinuities at sharp interfaces. Plasma spraying, however, produces deposits which do not have completely continuous composition gradients - rather two-phase mixtures with different ceramic/metal ratio are produced. The knowledge of residual stresses in each phase at different composition can elucidate the factors influencing the stress distribution and magnitude in a whole graded coating. Together with known thermomechanical properties of the constituents, service conditions and failure analysis, optimum compositional profile and processing conditions can be designed for a specific application. Neutron diffraction as a phase distinctive method was used to measure residual stress in plasma sprayed Ni-Al2O3 and NiCrAlY-YSZ cermets of five different compositions each. The stress was determined for each constituent phase separately, i.e. the coating's metallic phase, coating's ceramic phase and the substrate. The diffraction experiments were complemented by calculations based on experimentally determined thermal and mechanical properties. The significance of these properties in determining the sign and magnitude of residual stresses are discussed.
SESSION GG6: DESIGN FOR PERFORMANCE
Chairs: Philippe Colomban and W. A. Kaysser
Thursday Morning, December 4, 1997
Essex East (W)
8:30 AM *GG6.1
FUNCTIONALLY GRADED THERMAL BARRIER COATINGS FOR DIESEL ENGINES. M.B. Beardsley, Catepillar Inc., Peoria, IL.
Caterpillar has been engaged in the development of thermal barrier coatings for advanced, low heat rejection diesel engines since the mid-1980's. This effort is in partnership with DOE Office of Transportation Technologies with past efforts being supported by both the Heavy Duty Transportation Technology Program administered by NASA Lewis Research Center and the Ceramic Technology Program administered by ORNL.
Caterpillar has demonstrated a decrease in specific fuel consumption of up to 5% by insulating the combustion chamber with functionally graded thermal barrier coatings. This performance gain can be increased to an overall 54% thermal efficiency for advanced diesel engine concepts that use turbocompounding technologies. Caterpillar's engine testing has used thick (>2 mm) thermal barrier coatings (TTBCs) applied to diesel engine combustion chambers. Advanced modeling techniques have been used to predict engine conditions and combine this information with fundamental property evaluation of TTBC systems to predict engine performance and TTBC stress states. Engine testing supported by the Heavy Duty Transportation Technology Program has been used to verify the predicted performance of the TTBC systems and provide information on failure mechanisms. A review of past engine performance testing of TTBC insulated engines will be given.
9:00 AM *GG6.2
CONCEPT AND DEVELOPMENT OF THE GERMAN RESEARCH PROGRAM ON FUNCTIONALLY GRADED MATERIALS. A. Neubrand, Institute of Materials Science, Technical University, Darmstadt, GERMANY.
In November 1995, the German Research Society started a research program on materials in which a built-in property gradient is essential for optimum performance. This program on functionally graded aterials (FGM's) will have a duration of six years and intends to improve basic knowledge of the processing, properties and theory of FGM's. Unlike its two predecessors in Japan, the program has an pen structure which allows incorporation of new research projects each year, which seems appropriate in a field where new ideas are still emerging at a high rate. The German program on FGM's with it bottom-up structure sets no limitations concerning methods or applications, in contrast to other programs that concentrate research efforts concerning a particular application or material. It is be oming evident now that this structure was able to stimulate FGM research in fields such as graded polymers or FGM's for acoustic applications, which had previously been only partially involved.
To date, the program has gathered more than 40 research projects on the processing, characterization and theory of FGM's. Examples of some typical projects will be given, e.g. new gradation methods or FGM's, and new methods for the position-dependent characterization of FGM's. Theoretical groups in the program have two tasks : Firstly, they support the processing groups in improving their proc ssing steps, e.g. drying or sintering, and second, they predict the FGM behavior under load and calculate optimized property profiles for the FGM's produced by the program participants. Additionally collaboration exists between participants with related topics.
The second phase of the program will be devoted to the stimulation of industrial interest in FGM's by removing the main obstacles towards their introduction into components. For the participants, the following challenges result from this goal:
1. Elaborate processes suitable for mass production
2. Ensure that the cost of the FGM does not interfere with the application in mind
3. Prefer methods capable of near-net shape parts with three-dimensional gradients
4. Develop methods applicable to different material combinations without major changes
5. Promote simple rules where a gradient will be advantageous
6. Determine a number of optimized gradation profiles for some important applications
7. Develop simple methods for the quality control of gradient materials
9:30 AM *GG6.3
FUNCTIONALLY GRADED MICROWAVE ABSORBING CERAMIC MATRIX COMPOSITES. Philippe Colomban, ONERA, Dept of Materials, Chatillon and CNRS, Lasir, Thiais, FRANCE.
Stealthiness is now a required specification for modern weapons, and parts made of specific materials may be used to minimize the electromagnetic wave reflected in the direction of the radar observer. A significant decrease in microwave cross section, or radar equivalent surface (RES), may be obtained by shape optimization of mobiles. The shaping possibilities are limited for high velocity missiles due to the stringent thermomechanical and aerodynamic requirements. Consequently, microwave absorbing ceramic matrix composites are desired. The control of the electromagnetic absorption is possible by the combination of dielectric and conducting materials and/or by the dispersion of ferro/ferrimagnetic nanoparticles in a dielectric matrix. The main problem arising from the preparation of such microwave absorbing materials consists in the combining/alternating various refractory fibre reinforced materials in the same body: low permittivity material is desirable at the surface to favour good penetration of the wave, and a conducting material will dissipate within the body. Fibre/matrix interfacial reaction needs to be controlled to prevent the formation of high conducting interphase. The (model) examples reported here show how sol-gel route allows tailoring of the macro, micro and nanostructure of multiphase and composite materials. The preparation route allows one to manucfacture composite parts combining several matrices (mullite, Nasicon, celsian, zirconia, etc), several fibres (C ,SiC, mullite, alumina) and fabrics, and to embed these fabrics in various interphases. Typical sets of conductivity/permittivity values measured for FG CMC's in microwave range are discussed.
10:15 AM *GG6.4
INNOVATIVE DESIGN ARCHITECTURE OF FGMs THROUGH GUIDANCE OF ADAPTIVE GROWTH>MECHANISM IN NATURAL MATERIALS. Fumio Nogata, Himeji Institute of Technology, Department of Mechanical Engineering, Himeji, JAPAN.
Future technological advances are becoming increasingly dependent on our ability to design and develop new processing of materials with specific physical and chemical properties. The goal of this work is to understand the principle of design and processes found in natural materials, and to apply to the development of new and superior structural concepts for materials, e.g., creating composites of multiphased and functionally graded materials. As a typical example, the ingenious construction of bamboo is introduced herein to help understanding of the principle and design process found in natural tissue which is FGMs. It is concluded that the bamboo structure is designed to have uniform strength at all positions in both the radial direction on the transverse section and the lengthwise direction, which is managed by the adaptive self-optimizing graded structure constructed by a cell-based sensing system for external mechanical stimuli. Furthermore, we found that the biological electric signal from the cells controls the growth activity of load carriers. It is also demonstrated that the specified electric signals may control the shape of plant branch. From the findings in the ingenious construction of some biomaterials, it is considered that this process is the best choice to survive in hard competition for energy and external mechanical condition with minimum volume of materials. Thus rather than developing new materials with high stiffness it may be more advantageous to create structures using the optimal placement of fibers as seen in bamboo, various microstructures in mollusk shell, or porous or cellular structures. In addition the self-optimizing, cell-based sensing system for analyzing external physical stimuli adapted by plants could serve as a useful model for the construction of new intelligent multiphase materials.
11:00 AM GG6.5
MOLECULAR COUNTERPARTS OF MECHANICAL GRADIENTS IN NATURAL FIBERS: MUSSEL BYSSAL THREADS. J.H. Waite, Dept Chemistry/Biochemistry, University of Delaware, Newark, DE.
Mussel byssal threads are composite materials consisting of anisotropically packed discontinuous fibers in an amorphous matrix all coated by a protective cuticle. The threads are imparted with a longitudinal gradient of mechanical properties ranging from stiff and tough distally to extensible and rubbery proximally. Collagen is the principal structural polymer as determined by fiber x-ray diffraction, however, the mechanical properties of typical tendon are poorly matched with those of byssus. In order to reconcile these differences, a fuller examination of byssal collagen has been undertaken to explain this discrepancy. Distal byssal collagen (precol-D; 97 kD) consists of a central collagenous domain (45 kD) flanked by two fibroin-like domains with polyalanine runs and GGX repeats. At both terming there are H-rich clusters. Proximal collagen (precol-I3; 95 kD) has a similar domain structure except that the flanking domains resemble elastin sequence motifs. Immunohistochemical studies of byssus suggest that precol-D and precol-P are distributed as complementary gradients along each thread. The former predominates distally and the latter proximally. Extensibility and stiffness may thus be imparted by the narrow silk-like and elastic domains in the collagens. The structural contribution of collagen per se remains a vexed question.
11:15 AM GG6.6
FUNCTIONALLY GRADED WC-Co-DIAMOND COMPOSITES. R.K. Sadangi, O.A. Voronov, G.S. Tompa, Diamond Materials Inc., Piscataway, NJ; B.H. Kear, S.-C. Liao, Dept. of Ceramics and Materials Engg., Rutgers University, Piscataway, NJ; C.C. Wilson, Dept. of Industrial & Manufacturing Systems Engg., Kansas State University, Manhattan, KS; J. Yorston, Tempo Technologies Corporation, Somerset, NJ.
Preliminary results of a functionally graded WC-Co-Diamond triphasic composite, wherein the volume fraction of diamond phase progressively decreases from surface to interior of the material, is described. Starting with available WC-Co powder, the new process involves high pressure/high temperature (HPHT) consolidation of WC-Co-diamond composite. Initial granite-log test results have demonstrated that our new triphasic composites, which have 40 vol.% diamond phase in a WC-15 Co matrix, have a wear ratio that is about half that of conventional polycrystalline diamond compacts. This result compares favorably with conventional WC-Co composites. The graded microstructure ensures a gradual transition in elastic modulus and thermal expansion coefficient in the composite. When used as an interlayer, these new triphasic materials can minimize residual stress problems in consolidation of polycrystalline diamond compacts and hence, their perfomance.
11:30 AM GG6.7
AN OXIDATION RESISTANT METAL-CERAMIC FUNCTIONAL GRADIENT MATERIAL. M. Willert-Porada, R. Borchert, University of Dortmund, Deptm. Chem. Engineering, Div. Mater. Sci., Dortmund, GERMANY.
The paper describes a new concept for achieving oxidation resistance in a metal-ceramic FGM for thermal barrier applications. Opposite to classical zirconia coatings wžth a metallic interlayer protecting the substrate from oxidation, FGM's require a ``volumetric'' oxidation resistance, because no sharp interface exists between the ceramic and the metallic zone of the material. Furthermore, the high thermomechanical stability in a metalceramic FGM is achieved on the expense of an increased themal conductivity, due to the higher density of the FGM-ceramic zone as compared to coatings obtained by thermal spraying. Therefore, it is necessary to simultaneously decrease the thermal conductivity in the dense ceramic region and to increase the oxidation resistance within the metal-ceramic transition zone of the FGM. ZrSiO4, introduced as third component into the FGM, was found to lower the thermal conductivity and to increase the oxidation resistance within the gradient material. By adjusting the microstructure of the composite, a 8Y-ZrO2NiCr8020-FGM is achievable, which is stable to oxidation at 1200C (ceramic side) for > 1000 h. Details on preparation, phase composition, microstructure and oxidation as well as thermomechanical data will be presented. The function of ZrSiO4 is explained in terms of thermal expansion mismatch, oxygen diffusion inhibition, solubility for oxides and low thermal conductivity of this silicate.
11:45 AM GG6.8
ADJUSTMENT OF CONCENTRATION GRADIENTS IN ALUMINUM ALLOYS BY DIRECTIONAL SOLIDIFICATION. M. Rettenmayr, B. Siber, H.E. Exner, Technical University Darmstadt, Materials Science Dept., Darmstadt, GERMANY.
Bridgeman furnaces for directional solidification are generally optimized for producing homogeneous concentration distributions. However, under special processing conditions, notably strong convection in the melt, concentration gradients can be generated. The possibilities of adjusting concentration gradients during directional solidification is demonstrated for samples with diameters between 8 and 55 mm. The influence of some experimental parameters (solidification front velocity, temperature gradient, direction of heat flow, crucible material, crucible diameter) is investigated. Natural thermosolutal convection, stimulated by an appropriate choice of the temperature gradient and the direction of the solidification front, leads to gradients of concentration and microstructure. Another method for introducing convection is using induction heating. In this case the stirring of the melt is more effective. Plane front solidifcation throughout the experiments and concentration profiles close to Scheil profiles (representing the maximum achievable gradient) are obtained. The samples are used for determining tie-lines in multicomponent phase diagrams. The gradient in the hardness due to the graded concentration distribution can be enhanced by a heat treatment after solidification leading to precipitation hardening in the solute rich parts of the sample. The hardened samples are used to investigate the influence of the gradient in the mechanical properties on crack formation in a rotating beam testing machine.
SESSION GG7: THERMAL AND ELECTRICAL PROPERTIES
Chairs: Akira Kawasaki and Andrew J. Slifka
Thursday Afternoon, December 4, 1997
Essex East (W)
1:30 PM *GG7.1
THE FGM RESEARCH PROGRAM FOR DEVELOPMENT OF HIGHLY EFFICIENT THERMOELECTRIC CONVERTERS IN JAPAN. Yoshinari Miyamoto, Osaka University, Joining and Welding Research Institute of Technology, Ibaraki, Osaka, JAPAN; Junji Yoshino, Tokyo Institute of Technology, Tokyo, JAPAN; and Isao A. Nishida, National Research Institute of Metals, Tsukuba, JAPAN.
A five years research program ``Development of Energy Conversion Materials by Tailoring Graded Structures'' is promoted under the auspices of science and technology agency in Japan since 1993. About 30 research groups from national institutes, universities, and companies join and promote the research aiming to develop highly efficient thermoelectric and thermionic converters which are expected to contribute to energy problems. Thermoelectric and thermionic converters can produce electric power directly from heat energy by the electron flow generated in space, or in solid along high temperature gradient, respectively. Design and optimization of the gradient fields in electronic potential and in elastic potential would lead to higher conversion efficiency with relaxation of thermal stress. This paper reports the coordinated studies of modeling, processing, and evaluation for the development of graded thermoelectric converter units of Bi2Te3, PbTe, SiGe, and other related compounds with the graded concentration of dopants, electrode joints, and their cascade combination for a wide temperature range of 50C^to achieve the overall conversion efficiency over 15%.
2:00 PM GG7.2
THERMAL STABILITY OF GRADED (Si-MoSi2)/SiGe THERMOELECTRIC CONVERSION UNIT. Junshan Lin, Yoshinari Miyamoto, Joining & Welding Research Inst, Osaka Univ, Ibaraki, Osaka, JAPAN; Hiroyasu Kido, Osaka Municipal Technical Research Inst, Morinomiya, Jhoto-Ku, Osaka, JAPAN; Kenrou Shibata, Ken Hirota, Osamu Yamaguchi, Faculty of Engineering, Doshisha Univ, Kyotanabe, Kyoto, JAPAN.
The thermoelectric conversion units of SiGe with graded (Si-MoSi2) electrode were fabricated in one step by HIP sintering. The electrical resistivity, thermoelectric power, thermal conductivity, and Hall mobility were measured for p- and n-type SiGe units. The thermal stability of the thermoelectric properties was evaluated through annealing in a temperature range from 500C-1200C for different time. The results showed that the (Si-MoSi2)/SiGe units fabricated in one step can give a well bonded interface of the electrode/SiGe and stable thermoelectric properties. The electrical resistivity of the graded electrode was 1.010-4cm. The annealing experiments suggested that the thermoelectric conversion unit can satisfy the requirement of service life in a temperature range of 500C-900C.
2:15 PM GG7.3
FUNCTIONALLY GRADED MATERIALS FOR THERMOELECTRIC CONVERTERS UNDER LARGE TEMPERATURE GRADIENTS. Lev Bulat, Sergey Blekher, St. Petersburg State Academy of Refrigeration, Dept of Electrical Engineering, St. Petersburg, RUSSIA.
A problem of alternative methods of refrigeration from the point of view of a danger for ozone layer and the effect on global warming of the climate can be solved on principle with using of the thermoelectric method. Unfortunately, applied thermoelectric refrigerators are competitive nowadays only for cooling power below 100-150 W. The main part of thermoelectric refrigerator price is a price of using semiconductor materials. A natural way to reduce this price is a decreasing of material mass per cooling power (i.e. micro-miniaturization of converters). So, we must use large temperature gradient and large electric current density in thermoelectric converter to leave its energetic parameters. It is quite evident that under these conditions a traditional theory of transport in solids is unacceptable. The theory of transport under large temperature gradients must be non-linear and non-local. The traditional definition of the thermoelectric figure of merit becomes erroneous in such conditions. So, we have used hear a direct way to determine the efficiency of a thermoelectric converter. We introduced a function of spatial variations in thermoelectric material doping and took into consideration a temperature and coordinate dependence of all kinetic coefficients. We have employed the solution of the system of equations of the energy balance for electrons and phonons to solving the problem. Here we used three approaches to the solution of this system: 1) a perturbation theory; 2) an approximating solution of differential equations; 3) a numerical experiment. As a result, we have determined the special kinds of the functions of spatial variations in thermoelectric material doping, leaded to essential increasing of the efficiency of the thermoelectric converter in the conditions of large temperature gradients.
2:30 PM GG7.4
THERMOELECTRIC PROPERTIES OF SEGMENTED Pb-Sn-Te SYSTEM WITH GRADED CARRIER CONCENTRATIONS. Y.Shinohara, Y. Imai, Y. Isoda, I.A. Nishida, National Research Institute for Metals, STA Tsukuba, JAPAN; H.T. Kaibe, Tokyo Metropolitan Univ., Tokyo, JAPAN; I. Shiota, Kogakuin Univ., Tokyo, JAPAN.
Thermoelectric properties are investigated on the segmented p-type Pb-Sn-Te. The homogeneous solidified ingots of Pb1-xSnxTe were prepared by rocking furnace. x was changed in the range of 0.1 0.5. The carrier concentration of ingot at room temperature was 5.6x1024 m-3 in the case of x=0.1 and was increased with x. The electrical resistivity was lower with larger x in the temperature range of 300650K. The temperature corresponding to the maximum thermoelectric power was shifted to higher temperature with increasing x. The temperature dependence of figure of merit Z(=/) was evaluated from the above data and the calculated thermal conductivity . The calculation of was based on that lattice thermal conductivity was inversely proportional to temperature T. Z showed the maximum at 400, 480, 540 and 680K for x=0.1, 0.15, 0.25 and 0.5, respectively. When the temperature difference 500K was given at the cold side temperature of 300K, the segmented material of ingots with x=0.1, 0.25 and 0.5 had more than 37% higher average Z than the homogeneous ingots. This result shows the segmentation with graded carrier concentration is effective to improve the thermoelectric performance of p-type Pb-Sn-Te. The segmented materials are fabricated by diffusion bonding technique and their thermoelectric properties including maximum output power are reported.
3:15 PM GG7.5
OXIDATION BEHAVIOR OF FGM Ni-Cr-Al COATINGS. M.R. Jackson, J. Dobbs and R. Corderman, GECRD, Schenectady, NY.
In FGM bond coatings used in ceramic thermal barrier systems protecting Ni-base superalloys, the FGM must function to provide adequate oxidation resistance to prevent attack of the substrate, while at the same time achieving a graded thermal expansion behavior across its own thickness to reduce stresses at the metal/ceramic interface. The use of an FGM of NiAl and bcc-Cr can further produce a bond coating with reasonable ductility above the ductile-to-brittle-transition-temperatures of the component phases (600C for NiAl). The expansion behavior of NiAl is nearly equivalent to advanced superalloys, while the expansion behavior of bcc-Cr is somewhat lower than yttria-stabilized zirconia. With bond coats of NiAl/0.4-0.5 bcc-Cr (volume fraction), the metallic composite is essentially equivalent to YSZ in thermal expansion over the temperature range envisioned for the TBC/bond coat interface (<1200C). However, at this level of Cr, monolithic NiAl/bcc-Cr composites have inadequate ability to form alumina scales and maintain them for extended periods of thermal exposure. Using electron beam evaporation from two pools, of NiAl and of Cr, and controlling the power input to the Cr pool while holding constant the power input to the NiAl pool, coatings can be produced with nominally the same total Cr content, but with the Cr either dispersed nearly uniformly, or concentrated close to the surface serving as the interface with the TBC. Structures with nominally equal overall Cr content were produced with Cr surface levels varying from nominally 10 to 50 a/o. The Al source strength for alumina formation is essentially the same for these structures, but the Al available at the oxidizing interface is dependent on diffusion to that surface. Measured oxidation behavior (1200C) as a function of FGM microstructure can be optimized with estimated thermal expansion behavior of the FGM at the bond coat/TBC interface.
3:30 PM GG7.6
THERMAL CONDUCTIVITY OF FUNCTIONALLY GRADED THERMAL BARRIER COATINGS. A.J. Slifka, B.J. Filla, J.M. Phelps, NIST, Materials Reliability Division, Boulder, CO; M.B. Beardsley, Caterpillar Inc., Peoria, IL; A. Kumakawa, National Aerospace Laboratories, Kakuda Research Center, Kakuda, JAPAN.
Thermal conductivity measurement of two functionally graded thermal barrier coatings from 400 K to 1200 K using an absolute, steady-state guarded-hot-plate technique will be described. One coating is a 1.10 mm thick linearly graded coating of Ni20Cr/8% yttria-stabilized zirconia. The second coating is a 1.80 mm thick linearly graded NiCoCrAlY/25% ceria-2.5% yttria-stabilized zirconia. We conducted repeat tests on the first coating and included a moderate thermal shock between tests to evaluate the effect of microcracking. The thermal conductivity of this coating ranged from 0.8 to 1.8 W/m-K; the second coating ranged from 0.95 to 1.45 W/m-K. We discuss the behavior of the thermal conductivity in relation to the coating microstructures.
3:45 PM GG7.7
SI-GE GRADED CRYSTALS FOR X-RAY SYNCHROTRON OPTICS. Veniamin Shekhtman, Eugene Shulakov, Salavat Khasanov, Irina Smirnova, Inst. Solid State Physics RAS, Chernogolovka, RUSSIA; Alexei Erko, Wolfgang Gudat, Markus Veldkamp, BESSY GmbH, Berlin, GERMANY; Nikolai Abrosimov, Sergei Rossolenko, Volker Alex, Inst. Crystal Growth, Berlin, GERMANY.
The experimental and theoretical data on laterally and depth graded aperiodic structures as a novel type of optical elements for synchrotron radiation applications in the X-ray range are presented. Crystals are grown by the Czochralski method on the basis of Si1-xGe_x alloys. For such a graded single crystal (containing up to some atomic % of germanium in solid solution) the lattice parameter changes nearly linearly along growth axis. Therefore, by appropriated cutting of the crystals, we obtained a plate with constant gradient of the lattice parameter. Laterally graded crystal - monochromators with a variable d-spacing offer the possibility, as well as curved ones, to increase the diffracted flux for the required wavelength considerably. Alternatively, a change of the d-spacing of the crystal in depth is also possible. Depth graded crystals can be used to increase the integral reflectance. Thus the variation of the Bragg angles for divergent incident beam light or temperature gradient on the crystal surface can be compensated for. This opens up the possibility to develop a crystal monochromator system for SR with suitable resolution parameters in the whole energy range above 2 keV. Samples of graded single crystals up to 60 mm x 50 mm in size have been tested by x-ray scattering technique with standard diffractometers and soft X-ray SR station at BESSY. The results of X-ray diffraction topography were used to analyse the concentration and real structure parameters of laterally-graded crystals. The reflecting properties of gradient crystals were studied by simulation of diffracted waves based on the Takagi equations. Furthermore, the essential features of graded crystal SR-optics are discussed. This work was supported by the Deutsche Forschung Gemeinschaft in frame of the DFG-Project AL 456/1-1 and the Russian Foundation of Basic Research, grant 96-02-18116a.
4:00 PM GG7.8
INTERFACE PHYSICS FOR MULTILAYER THIN FILM COATINGS ON MICROSPHERES IN A PACK BED ELECTROLYTIC CELL. George H. Miley ang Gokul Narne, Dept. of Nuclear Engineering, University of Illinois, Urbana, IL.
Reaction rates in a hydrogen electrolytic cell are enhanced by the use of multiple layers of thin films (200-1000) of hydrogen absorbing metals with different Fermi levels. In this case, the electron density at the interfaces between the layers is greatly enhanced, accelerating the local reaction rate (1) The theory behind this, termed swimming electron (SEL) theory is described in Ref. 1 and references therein. To examine this theory over a dozen experiments with various thin film coatings have been carried out using a 1-molar Li2SO4 light water electrolyte.. About 1000 microspheres (0.5 cm3 volume) were used in the packed-bed cell. Titanium electrodes were employed in most runs. The preheater allows control of the entering temperature of the electrolyte (1 molarLi2SO4/H2O), with flow rates 11 ml./min. Voltages across the bed were held at 2-3 V, with several mA of current, giving an electrical input power of approximately 0.06 W. Inlet-outlet thermocouples provided a measure of the temperature increase of the flowing electrolyte. Loading of hydrogen into the thin-film is done at low (25C) temperatures, requiring several hours, as observed by an initial increase in the voltage across the bed, followed by an eventual equilibrium voltage level of +2-3 V. Then the cell inlet temperature is slowly raised (over 4-8 hours) to the maximum allowed with the present plastic cell construction, near 60-70 C. Run times of several weeks were typical. Further details about the construction and operation of this type of cell are given in Ref.2 and Ref. 3. Results from these runs generally confirm the benefit of creating SEL conditions using multiple layers. Thus, a heat output of 2 0.5 W were obtained with a five layer coating consisting of alternate layers of nickel and palladium. In contrast, a single layer of nickel or a single layer of palladium produced 0.5 0.4 W. An examination of the composition of the metal interface region using a Sims revealed significant quantities of Fe Si, Mg, Cu, Cr, Zn, and Ag. Additional data from an Auger depth scan and from Neutron Activation Analyses and ICP Mass Spec Analyses will be presented.
4:15 PM GG7.9
INFLUENCE OF PHOTON RECYCLING EFFECT ON STATIC AND DYNAMIC PERFORMANCES OF THE GRADED SEMICONDUCTOR DIODES WITH HIGH INTERNAL QUANTUM EFFICIENCY. V.Rossokhaty, N. Rossokhata,* Kiev University, Dept. of Semiconductor Electronics (*Cybernetics Dept.), Kiev, UKRAINE.
Graded A3B5-semiconductors are perspective materials for devices with a wide range of applications owing to their peculiarities regarding many physical effects. One of them, connected with the effect of photon recycling in semiconductors with high internal quantum efficiency may essentially influence on the parameters of devices operating at high levels of excitation (power diodes and transistors, LED, lasers). Because of complexity of the expression describing the effect of reemission, many authors used approximations and simplifications such as linear recombination rate, low level of excitation etc., which restrict the practical use of such models (for example, [1-3]). Besides, traditional description of the electron-hole plasma in semiconductor devices in terms of the Poisson equation for charge, equations of the drift-diffusion transfer of the electrons and holes and the continuity equations  makes the models rather cumbersome, complex in solving and demanding a lot of computed expenditures. In this work, we present transient one-dimensional mathematical model of the diode structure based on graded-gap semiconductors with high efficiency of spontaneous radiative recombination, which does not contain mentioned restrictions, much more economic from calculation viewpoint and hence, is more appropriate for practical use. Numerical scheme for it computer realisation, results of calculations and experiment are also under discussion. We consider p+-n-diode structure which base region is homogeneously doped by shallow donors of concentration Nd and has a high efficiency of band-to-band radiative recombination. The energy gap Eg(x) in the base linearly changes along the direction of current (perpendicular to the p+ -n-junction flat), that provides a constant built-in quasielectric field . The ohmic contact is located on the outer surface of the epitaxial layer (x=1).In one-dimensional approach, we use the standard form for electron and hole current densities and the continuity equations. We suppose the base region of structure to be neutral, what corresponds to operation of many kinds of injective semiconductor devices. This gives opportunity to reduced the input system of equations to one second-order parabolic equation for the electron or hole concentration. The photon recycling is described in general form in terms of binary radiative recombination. Finally, the model was reduced to the initial-boundary value problem of nonlinear integro-differential equation. The peculiarity of the problem is the nonlocal integral term with singular kernel. In order to investigate influence of the photon recycling effect on the most important static and dynamic performances of the diode structure such as the value of forward voltage in steady state Vst, the relaxation time under forward bias application (switch-on time ton) and the time of high reverse conductivity (switch-off time toff) when the device is switched from the direct current mode to the reverse one, the mentioned stiff problem was numerically calculated and transient position distribution of nonequilibrium charge carrier density p(x,t) was obtained. The method of lines was applied and special algorithm to calculate nonlinear nonlocal integral term was developed . The numerical experiment has shown that photon recycling under appropriate conditions can significantly transform the transient position carrier distribution and thus, result in even qualitative changes in device performances. So, for instance, the graded structures with Eg narrowing towards the ohmic contact exhibit the S-shaped I-V-characteristics, with their parameters depending strongly on the Eg gradient and optical properties of the boundaries. It was also obtained that the photon recycling in graded semiconductors can both improve and deteriorate dynamic device performances. In particular, it can reduce the switch-off time, what is significant, for example, for power electronics. Such influence of the photon transport on device performances was experimentally proved using graded diode structures. In particular, the S-shaped I-V-characteristics were observed and studied in dependency of such factors as Eg gradient, temperature, optical properties of surfaces. It was experimentally studied the influence of the photon recycling on transient processes in the structures.