Sintering by hot-pressing of the system, Si
3N
4-Al
2O
3, was investigated.
The purities of the raw materials were about 98.5% for Si
3N
4 and 99.99% for Al
2O
3. 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/cm
2, respectively.
The following results were obtained. The quantity of Al
2O
3 necessary for full densification of the mixture Si
3N
4-Al
2O
3 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 Al
2O
3-concentration exceeds 40mol%. The activation energies for sintering under the pressure of 250kg/cm
2 were determined to be 100kcal/mol for 80mol% Si
3N
4-20mol%Al
2O
3 and 67kcal/mol for 60mol% Si
3N
4-40mol% Al
2O
3. Oxidation resistance of sintered Si
3N
4-Al
2O
3, evaluated by the weight gain, increased with Al
2O
3-concentration, reaching the maximum at 65mol% of Al
2O
3, followed by decreasing when Al
2O
3-concentration exceeds 65mol%. At the Al
2O
3-concentration of 65mol%, the oxidation resistance was more than 10 times as high as that of the system, Si
3N
4-MgO. Abrasive resistance of the sintered body, evaluated by the weight-loss during a abrasive test with abrasive powder, SiC #800, increased with Al
2O
3-concentration, reaching the maximum at the Al
2O
3-concentration of 50mol%, followed by decreasing when Al
2O
3 concentration exceeds 50mol%. The maximum abrasive resistance was about 4 times as high as that of sintered 85mol% Si
3N
4-15mol% MgO, which is in practical use.
By hot-pressing, the solid solution of Si
3N
4 and Al
2O
3, Si
3N
4ss, was found to be formed, which was already pointed out by the present authors. The solubility limit of Al
2O
3 in Si
3N
4s.s. was estimated to be about 67mol% at 1750°C and 78mol% at 1850°C. From the observation of specific gravity of Si
3N
4ss, the solid solution is thought to be of a substitutional type. A new compound was also found, whose composition was estimated to be 2Si
3N
4⋅3Al
2O
3. 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 Si
3N
4-Al
2O
3.
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