EFFECT OF CRYSTALLIZATION ON FATIGUE-CRACK GROWTH IN A SIC-REINFORCEDSILICON-NITRIDE COMPOSITE AT 1200-DEGREES-C

Subcritical crack growth in a high strength, SiC-reinforced Si3N4, und er sustained and cyclic loads at high temperature has been investigate d to determine the influence of crystallization on crack growth veloci ty. Crystallization of the grain boundary glass phase did not produce an observable increase in fracture toughness but did produce increased fatigue crack growth resistance at high temperature. TEM study indica ted that this improvement resulted from an observed reduction in glass flow and crack tip cavitation due to the significantly reduced glass phase content after crystallization. It is shown that high temperature fatigue crack growth is predominantly loading time dependent with a w eak effect of stress cycles. An influence of loading frequency on crac k growth rates occurred only at high loading rates in uncrystallised ( high glass phase content) specimens where the crack lip stress intensi ty was reduced due to viscous bonding of the glass phase at the crack surfaces. (C) 1998 Acta Metallurgica Inc.