Effect of Fe- and Si-induced flaws on fracture of Si{sub 3}N{sub 4}

Fracture studies were performed to detect and assess the effect of flaws on the fracture behavior of hot-pressed Si{sub 3}N{sub 4} with Fe or Si inclusions. The addition of 5 and 0.5 wt.% Fe inclusions of 88--250 {micro}m size reduced the strength of Si{sub 3}N{sub 4} specimens by {approx} 40 and 15%, respectively. Similarly, addition of 1 and 0.5 wt.% Si inclusions of < 149 {micro}m size reduced the strength of Si{sub 3}N{sub 4} specimens by {approx} 50 and 39%, respectively. Fractography indicated that failure occurred primarily from internal flaws which included Fe- and Si-rich inclusions and/or regions of Si{sub 3}N{sub 4} matrix that were degraded as a result of reaction between Si{sub 3}N{sub 4} and molten Fe or Si. For inclusion-induced internal flaws, the critical flaw sizes calculated by fracture mechanics were always larger than the fractographically measured flaw sizes. This observation suggested local degradation in fracture toughness of the Si{sub 3}N{sub 4} matrix. A ratio, K, of {approx} 3.5--4.2 appeared to exist between the calculated and measured values of the critical internal flaw sizes of specimens that contained Fe inclusions. A similar ratio of 1.7--3.1 was observed for specimens that contained Si inclusions. The ratio K has important implications for strength predictions that are based on observed flaw size