High Temperature Resistant Silicon-Nitride Ceramics by Optimized Two-Step Gas Pressure Sintering

Gas Pressure Sintering of Silicon Nitride Powder Coated with Al2O3 and TiO2

“…An important observation 15,16 obtained in the case of two-stage sintering of silicon nitride compact containing 5 wt% yttria and 2-3 wt% silica reveals that pressurization at the stage when the relative density is as low as 88% could lead to a higher final relative density equivalent to 99% TD. A similar observation has also been found 17 in which an increase in densification rate, as well as the maximum final density, occurs when the pressurization in the second step is applied exactly after the completion of the pore closure, which has been observed to take place at nearly 90% of the relative density. The efficiency of the pressurization, i.e., the achievement of higher densification rates and final density, is reduced when it is accomplished after substantial grain growth to form a skeleton.…”

“…Similar variations of population of intergranular crystalline melilite phases have been noticed 38 in samples sintered with Y 2 O 3 and SiO 2 . Lowering of density due to volatilization of materials from the sintering compact has been reported by several workers [15][16][17][18][19] during gas pressure sintering of silicon nitride. Weight loss data (Table I) show that the weight losses are higher than usually observed.…”

“…S INTERING of silicon nitride, a highly covalent material 1 (70%), is accomplished with high nitrogen pressures to overcome the problem of dissociation at high temperatures and minimize the additive content. Gas pressure sintering of silicon nitride has been studied [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] using different sintering additives and a range of gas pressures. Pores developed by additive loss (bloating) during prolonged annealing 18 at high temperatures and pressures cause problems in densification.…”

Silicon Nitride and Related Materials

“…For good mechanical properties (e.g., toughness and strength), a high-aspect-ratio (although fine) grain microstructure was required. 136 A critical point in the densification process was a relative density of ϳ0.91 of theoretical, when pore closure occurred. Above this density, high gas pressures could be applied to accelerate sintering (gas-pressure sintering).…”