Development of β-Si3N4 for Self-Reinforced Composites
β-Si3N4 was obtained by the doping of a high-purity, commercial Si3N4 powder with 2 wt.% Y2O3. With additional processing the B-Si3N4 was used as seed material for the development of self-reinforcing microstructures in a Si3N4 composite, containing Al2O3 and Y2O3 sintering aids.
A Si3N4 composition (A2Y6), containing a 5 wt.% addition of β seed, was processed by turbomilling and pressure casting. Discs of the baseline Si3N4 (composition without seeds) and the seeded Si3N4 were sintered to 99–100% of theoretical density at 1880°C for 4 h at 43.5 KPa (300 psig) N2. Based on limited data, 4-pt flexural strength was found to be 925–1025 MPa for the baseline Si3N4 and 750–950 MPa for the seeded Si3N4. Fracture toughness, using the Cook and Lawn modified indentation method, was found to be 8–10 MPa-m1/2 for the baseline Si3N4 and 10–13 MPa-m1/2 for the seeded Si3N4. SEM results show that the seeded Si3N4 developed a more mature β-Si3N4 grain structure than the baseline composition.
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