Creep mechanism and microstructure evolution in silicon nitride ceramics

Tensile creep processes in a commercial grade of silicon nitride, SN 88, are reviewed with the focus on cavitation and microstructure changes. Experimental measurements showed that cavitation at multigrain junctions is the rate controlling creep mechanism. The cavitation creep model of Luecke and Wiederhorn was extended to explain non-power law behaviour and strong creep asymmetry. Secondary phase composition changes after prolonged creep tests resulted in the formation of Yb-disilicate phase at the expense of Yb-oxynitride due to oxidation. Such secondary phase changes affect residual glass composition, which has crucial effect on the processes involved in cavitation and resulting creep behaviour.