CONSTRAINED DENSIFICATION OF ALUMINA ZIRCONIA HYBRID LAMINATES .2. VISCOELASTIC STRESS COMPUTATION/

To analyze the inhibited densification during sintering and differenti al shrinkage during cooling of Al2O3/ZrO2 symmetric and asymmetric lam inates, viscoelastic formulations, in which the viscosity and elastic modulus vary with time, have been developed. The viscoelastic mismatch stresses have been numerically computed over the entire processing cy cle, including the heating period, the isothermal period, and the cool ing period. The viscosity and free sintering rates that are needed for stress computation have been obtained by modifying the parameters tha t are measured for a normal isotropic densifying compact using cyclic loading dilatometry. The modification is based on the available sinter ing models to account for the effect of strain history on compact visc osity and sintering rates. The stress calculation shows that, with the exception of the initial heating period, the viscoelastic stress is i dentical to the viscous stress that is calculated solely from the stra in rate mismatch, Sintering damage in the laminates is shown to occur during densification under conditions where the differential sintering stress is smaller than the intrinsic sintering pressure. The magnitud e of residual stress in hybrid laminates on cooling is dependent on th e cooling rate, and slower cooling rates are capable of almost complet ely relaxing the expansion mismatch stress at temperatures of >1200 de grees C.