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 1 mu in diameter. Temperature, time and pressure in the hot-pressing were 1650 degrees similar to 1850 similar to C, 6 similar to 60 min and 250 kg/cm(2), respectively. The following results were obtained. The quantity of Al2O3 necessary for full densification of the mixture Si3N4-Al2O3 was more than 20 mol%, which agreed with that given by Deeley et ai. Grain-growth in the sintered body was observed to be remarkable when Al2O3-concentration exceeds 40 mol%. The activation energies for sintering under the pressure of 250 kg/cm(2) were determined to be 100 kcal/mol for 80 mol% Si3N4-20 mol% Al2O3, and 67 kcal/mol for 60 mol% Si2N4-40 mol% Al2O3. Oxidation resistance of sintered Si3N4-Al2O3, evaluated by the weight gain, increased with Al2O3-concentration, reaching the maximum at 65 mol% of Al2O3 followed by decreasing when Al2O3-concentration exceeds 65 mol%. At the Al2O3-concentration of 65 mol%, 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 50 mol%, followed by decreasing when Al2O3 concentration exceeds 50 mol%. The maximum abrasive resistance was about 4 times as high as that of sintered 85 mol% Si3N4-15 mol% MgO, which is in practical use. By hot-pressing, the solid solution of Si3N4 and Al2O3, Si(3)N(4)ss, was found to be formed, which was already pointed out by the present authors. The solubility limit of Al2O3 in Si(3)N(4)s.s. was estimated to be about 67 mol% at 1750 degrees C and 78 mol% at 1850 degrees C. From the observation of specific gravity of Si(3)N(4)ss, the solid solution is thought to be of a substitutional type. A new compound was also found, whose composition was estimated to be 2Si(3)N(4). 3 Al2O3. 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 degrees+/-30 degrees C. An approximate phase diagram was given for Si3N4-Al2O3.
