INFLUENCE OF SI3N4 INTERFACE CHEMISTRY ON BOTH GRAIN MORPHOLOGY AND FRACTURE-RESISTANCE

Quantitative microstructural analysis was performed on Si3N4 materials doped with 5 wt% Y2O3 and 5 wt% Sc2O3 as sintering aids. Two differen t processing routes were utilized to achieve complete densification: ( i) gas-pressure sintering of Si3N4-starting powders (SSN) and (ii) pos t-sintering of reaction-bonded Si3N4 (SRBSN). Apart from quantitative evaluation of grain diameter and aspect ratio of the matrix grains, th e fracture toughness was determined. While the two Y2O3-doped material s (SSN, SRBSN) showed identical K-IC-values, a large discrepancy in fr acture toughness was found for the Sc2O3-doped Si3N4 (Delta K-IC = 5 M Pa root m). SEM crack propagation studies and additional TEM interface characterization showed no significant difference between these mater ials. Quantitative microstructure analysis, however, revealed a pronou nced variation in grain diameter and aspect ratio after thermal treatm ent. A correlation between interface chemistry and both resulting micr ostructure and fracture resistance is discussed.