Formation of silicon nitride based materials by nitridation and sintering
Abstract: Todays demands on silicon nitride based materials for structural applications focus on microstructures rendering high strengths and reliability at reasonable costs. This work was mainly aimed at production of sialons for these types of applications by nitridation with subsequent sintering. For this purpose both the influence of different factors on sialon formation and the effect of additives in nitriding was studied, as well as different sintering methods. Y-alfa-sialons with x=0.1 to 0.9 in the formula Yx(Sil2-4.5x,Al4.5x)(01.5x,N 16-1.5x) prepared from silicon nitride powders were sintered in different ways. The crystalline phase composition varied from alfa/beta sialon at x=0.2 to alfa sialon at x=0.4 and alfa sialon + polytypes at x=0.8. The highest alfa-sialon content and density was obtained for x=0.4 with an excess of yttria after sintering 1h at 1750°C and post HIPing 1h at 1750°C and 200 MPa. No glass encapsualtion was needed as closed porosity was obtained in the sintering step. Less residual glass was also present after this processing than when just HIPing. Sintering without pressure, however was not enough to densify the material. Sintering experiments by HT-XRD and in a conventional furnace of an x=0.4 alfa-sialon composition without excess yttria, showed that the amount of alfa-sialon formed was relatively insensitive to small changes in composition. Assuming that the formation mechanisms during the early stages of sintering (first 90-120 min) did not change with time and temperature, therefore made it possible to determine the kinetics of alfa-sialon formation. The activation energy was estimated as 330 kJ/mol. The effect of additives on nitridation was studied by adding silica or additives for sialon formation (AlN, Al2O3, Y2O3, CaO) to silicon. Formation of silicon oxynitride in the case of silica additions and sialon in the case of sialon additions was then observed after nitriding. The amounts of sialon formed depended on the liquid phase properties of the different compositions and the nitriding conditions. Fast nitridation resulted in more sialon formation. By nitriding with different schedules it was shown that this formation could be controlled and also that the nitridation could be speeded up when additives were present. Large amounts of additives made the pore size distribution insensitive to nitriding gas composition. The presence of hydrogen in the gas, however, did increase the amount of reaction at low temperatures and thereby influenced the phase composition. Densification of the materials nitrided with silica present was not possible by pressureless sintering and standard glass encapsulated HIPing. Most of the nitrided sialon compositions, on the other hand, sintered well by most sintering methods used at temperatures of 1850°C, and resulted in homogeneous microstructures. Sinter HIPing rendered unusually elongated grains in the (alfa-sialon (x=0.4), while the beta-sialon (z=2) had high grain aspect ratios for all sintering methods. HIPing at 1750°C gave the highest densities in most cases but resulted in inhomogeneities in the alfa-sialon. These looked very similar to inhomogeneities obtained when AlN additive powder with larger grain sizes was used. This work shows that nitridation with subsequent sintering of sialon compositions is a very promising way of manufacturing high performance structural ceramics based on silicon nitride.
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