G. Petzow

Tailoring of mechanical properties of Si[3]N[4] ceramics

Preface. I: Processing and Densification F.F. Lange, E.P. Luther, J.G. Heinrich, H. Kruner, R. Oberacker, G. Grathwohl. II: Microstructure -- Development, Characterization and Influence on Mechanical Properties M.J. Hoffmann, F. Mucklich, J. Ohser, S. Hartmann, M.J. Hoffmann, G. Petzow, P.F. Becher, S.L. Hwang, H.T. Lin, T.N. Tiegs, Y. Tajima, K. Urashima. III: Phase Diagrams and SiA1ON Composites M. Hillert, D.P. Thompson, T.S. Yen, Th. Ekstrom. IV: Grain Boundary Phases. IV.1. Properties of Oxynitride Glasses R.E. Loehman, T. Rouxel, H. Lemercier, J.-L. Besson, St. Hampshire. IV.2. Devitrification in Silicon Nitride Ceramics R. Raj, M.H. Lewis, M.J. Hoffmann, H. Schubert, E. Gehrke. IV.3. Analysis and Characterization of Intergranular Grain Boundary Films H.-J. Kleebe, M.K. Cinibulk, I. Tanaka, J. Bruley, J.S. Vetrano, M. Ruhle, H. Gu, M.J. Hoffmann, R.M. Cannon, D.R. Clarke. V: High Temperature Properties S.M. Wiederhorn, W.E. Luecke, B.J. Hockey, G.G. Long, D.S. Wilkinson, T. Ohji, W. Pompe, H. Kessler, R.H. Dauskardt, G.A. Schneider, I. Tanaka. VI: Mechanical Performance and Applications M. Matsui, M. Masuda, C. Saraiva Martins, M. Steen, J. Bressers, L. Guerra Rosa, K.D. Morgenthaler, H. Buhl, Y, Tajima, K. Mizuno, K. Matsubara, M. Watanabe. Author Index. Subject Index.

Analysis of particle growth by coalescence during liquid phase sintering

Abstract A statistical approach has been applied to particle coarsening during liquid phase sintering assuming direct particle coalescence as basic growth mechanism instead of Ostwald ripening. The coalescence process controlled by diffusion through the melt results in an increase of the average particle size proportional to the cube root of sintering time. After a short initial sintering interval the particle size distribution approaches a unique normalized form which is considerably broader than forms predicted by Ostwald ripening theories. The effect of preferred coalescence possibilities for definite particle size ranges and the effect of concurrent coalescence and Ostwald ripening are treated and discussed.

On the calculation and representation of multicomponent systems

Abstract A data set was established to calculate the phase equilibria in the systems Si-Al-Zr-O-N and Si-Al-B-O-N. Complete phase diagrams of the quasibinary and quasiternary boundary systems, calculated by this data set, agree well with literature phase diagrams. To calculate equilibria of the multicomponent systems, Erikssons program was modified to accept ionic species. The equilibria of some concentrations are plotted versus the temperature in phase amount diagrams.

Strengthening and toughening of boride and carbide hard material composites

Abstract Many of the well-established strengthening and toughening mechanisms for oxide ceramics are also applicable to hard materials based on borides and carbides. Chemical complications, however, limit a similar successful combination of phases, yielding metal matrix cemented borides and transformation toughening by ZrO 2 additives. Hence the most encouraging methods for improvement of the mechanical properties are grain size refinement by growth-retarding additives and crack-propagation-influencing mechanisms such as crack deflection, microcrack interaction and crack impediment. Ternary cemented borides, liquid-phase-sintered TiB 2 , B 4 C- and SiC-based composites as well as ZrO 2 -toughened TiB 2 are discussed as successful examples.

Formation of silicon oxinitride from Si3N4 and SiO2 in the presence of Al2O3

Abstract The formation of Si 2 N 2 O from SiO 2 and α-Si 3 N 4 powder mixtures with addition of 3 mol % Al 2 O 3 was investigated. An eutectic at 98 mole % SiO 2 and 2 mole % Si 3 N 4 was formed at 1863 K which is decreased by the Al 2 O 3 , thus enabling the formation of Si 2 N 2 O via the liquid phase. Microstructural analysis by TEM showed a strong orientation between Si 2 N 2 O and α-Si 2 N 4 crystals. Oxidation resistance and bending strength were also measured.

Model experiments concerning abnormal grain growth in silicon nitride

Abstract Model experiments were designed to study abnormal grain growth in Si 3 N 4 -based ceramics. Experiments relating inhomogeneous crystalline secondary-phase distribution to exaggerated grain growth conclusively showed that abnormal grain growth is not governed by secondary-phase distribution, because a rapid homogenization of locally formed liquid occurs via capillary forces. Further investigations were focussed on intrinsic properties of the α-Si 3 N 4 starting powders. The influence of: (i) β-Si 3 N 4 -grain morphology; (ii) β-Si 3 N 4 -nuclei density, and (iii) β-Si 3 N 4 . grain-size distribution of the powder blends on microstructural development were analyzed. The results revealed that a large basal plane of β-Si 3 N 4 seeds energetically and kinetically favours grain growth. However, this effect is only partly responsible for abnormal grain growth. The formation of elongated Si 3 N 4 grains, such as in in situ reinforced Si 3 N 4 materials, strongly depends on the amount and grain-size distribution of β-Si 3 N 4 nuclei present in the α-Si 3 N 4 -starting powder.

Grain growth kinetics of Si3N4 during α/ß-transformation

Abstract The growth of s-Si3N4 particles, highly diluted in an oxynitride glass, was analysed by SEM and the growth features of individual crystals were discussed. The observed linear time law for the length growth during α/s-transformation was correlated with results of a theoreticl analysis taking into account the diffusion in the liquid phase as well as the interface reaction at α- and s-crystals. The number of initial nuclei was found to be important for a linear time law of growth. Finally, the considerations of the dilute particle system were applied to a Si3N4 ceramic. The opposing influence of growth anisotropy of sSi3N4 in the presence of a melt and the packing requirements demanding a reduction of the specific surface area is discussed.

Sintering and HIPping of silicon nitride-silicon carbide composite materials☆

Abstract Si 3 N 4 composite materials containing up to 60 vol.% of dispersed β-SiC particles were sintered with Y 2 O 3 and Al 2 O 3 at 1850°C and 0·1 MPa N 2 . Fractional density decreased from 0·97 to 0·91 when the SiC content increased from 0 to 60 vol.%. Simultaneously a retardation of grain growth and reduced pore size was found. Subsequent HIPping at 2000°C with N 2 -pressure of 100 MPa resulted in almost complete elimination of presintered closed pores and a final density of 0·99 up to 20 vol.% of SiC. During HIPping Si 3 N 4 is formed by reaction of SiC or C with the SiO 2 intergranular glass in the presence of high N 2 -pressure. While fracture toughness shows no significant influence of SiC content, a reduction of critical defect size with increasing SiC content results in a distinct increase of fracture strength particularly after HIPping.

Particle growth by coalescence during liquid phase sintering of Fe-Cu

Abstract Particle growth during liquid phase sintering of Fe-30 wt% Cu and Fe-60 wt% Cu was measured and compared with a recently developed coalescence theory. Measured normalized particle size distribution, time-dependence of average particle size and dependence of growth rate on solid phase volume fraction agree well with calculations based on the assumption of a diffusion-controlled coalescence process where all component particles are assumed to have equal coalescence frequency. Slight discrepancies can be explained as the results of nonspherical particle shape and of favored coalescence opportunity for larger particles.

Relations between crystal structure and growth morphology of β-Si3N4

Abstract A PBC analysis of β-Si3N4 has been carried out by computer calculations taking into account the atom positions for F-face identification and bond lengths as a measure of bonding energy for face prominence. A rod-like habit with the prism {100} and the pyramids {101} and {10 1 } has been theoretically deduced, which can be observed on crystals grown under a reducing atmosphere. In the presence of oxides, preferably silica, (001) and (00 1 ), which are of K-type, dominate over {101} and {10 1 }. This is attributed to face specific adsorption. The growth rate anisotropy between {100} and (001) and (00 1 ) faces can be explained by a model derived from the crystal structure according to the growth theory of Kossel and Stranski. Conclusions are drawn concerning interface roughness and aspect ratio development.