Hot-Pressing of Si<sub>3</sub>N<sub>4</sub>-Al<sub>2</sub>O<sub>3</sub>

Abstract

Sintering by hot-pressing of the system, Si3N4-Al2O3, was investigated.The purities of the raw materials were about 98.5% for Si3N4 and 99.99% for Al2O3. Their grain sizes were, both, about 1μ in diameter. Temperature, time and pressure in the hot-pressing were 1650°-1850°C, 6-60min and 250kg/cm2, respectively.The following results were obtained. The quantity of Al2O3 necessary for full densification of the mixture Si3N4-Al2O3 was more than 20mol%, which agreed with that given by Deeley et al. Grain-growth in the sintered body was observed to be remarkable when Al2O3-concentration exceeds 40mol%. The activation energies for sintering under the pressure of 250kg/cm2 were determined to be 100kcal/mol for 80mol% Si3N4-20mol%Al2O3 and 67kcal/mol for 60mol% Si3N4-40mol% Al2O3. Oxidation resistance of sintered Si3N4-Al2O3, evaluated by the weight gain, increased with Al2O3-concentration, reaching the maximum at 65mol% of Al2O3, followed by decreasing when Al2O3-concentration exceeds 65mol%. At the Al2O3-concentration of 65mol%, the oxidation resistance was more than 10 times as high as that of the system, Si3N4-MgO. Abrasive resistance of the sintered body, evaluated by the weight-loss during a abrasive test with abrasive powder, SiC #800, increased with Al2O3-concentration, reaching the maximum at the Al2O3-concentration of 50mol%, followed by decreasing when Al2O3 concentration exceeds 50mol%. The maximum abrasive resistance was about 4 times as high as that of sintered 85mol% Si3N4-15mol% MgO, which is in practical use.By hot-pressing, the solid solution of Si3N4 and Al2O3, Si3N4ss, was found to be formed, which was already pointed out by the present authors. The solubility limit of Al2O3 in Si3N4s.s. was estimated to be about 67mol% at 1750°C and 78mol% at 1850°C. From the observation of specific gravity of Si3N4ss, the solid solution is thought to be of a substitutional type. A new compound was also found, whose composition was estimated to be 2Si3N4⋅3Al2O3. The new compound was stable under slightly oxidizing atmosphere and unstable under neutral or slightly reducing atmosphere. The eutectic point in the system was estimated to be 1870°±30°C. An approximate phase diagram was given for Si3N4-Al2O3.