Influence of Si₃N₄ interface chemistry on both grain morphology and fracture resistance

Abstract
Quantitative microstructural analysis was performed on Si₃N₄ materials doped with 5 wt% Y₂O₃ and 5 wt% Sc₂O₃ as sintering aids. Two different processing routes were utilized to achieve complete densification : (i) gas-pressure sintering of Si₃N₄-starting powders (SSN) and (ii) post-sintering of reaction-bonded Si₃N₄ (SRBSN). Apart from quantitative evaluation of grain diameter and aspect ratio of the matrix grains, the fracture toughness was determined. While the two Y₂O₃-doped materials (SSN, SRBSN) showed identical K_IC-values, a large discrepancy in fracture toughness was found for the Sc₂O₃-doped Si₃N₄ (ΔK_IC = 5 MPa√m). SEM crack propagation studies and additional TEM interface characterization showed no signifïcant difference between these materials. Quantitative microstructure analysis, however, revealed a pronounced variation in grain diameter and aspect ratio after thermal treatment. A correlation between interface chemistry and both resulting microstructure and fracture resistance is discussed.