Stress-State Effects on Strength and Fracture of Partially-Stabilized Zirconia

Stress-state effects on the transformation yield stresses of commercial-grade ceria-partially-stabilized zirconiaalumina Ce-TZPAl2O3 composite and MgO-partially-stabilized zirconia MG-PSZ were studied in various biaxial tests and in hydraulic compression tests. The yield envelopes in tension-tension and tension-compression stress quadrants and the pressure sensitivity of the yield stress in uniaxial compression were rationalized by a hypothesis where stress-induced transformation is triggered by a critical value of the interaction energy calculated according to the method of Eshelby. Crack-tip transformation zone shapes and sizes and rising crack-growth-resistance behaviors were studied in Ce-TZPAl2O3. The shape of the crack-tip transformation zones was not consistent with the derived yield criterion however, the crack shielding developed by the elongated zones could be explained by zone shielding theories that took into account the observed zone shape and the variations in the monoclinic content within the zones. The variation of the R-curves with the sintering temperature was consistent with the effects of transformation zone shape and size on zone shielding. The Ce-TZPAl2O3 composites were also shown to exhibit a genuine fatigue crack-growth phenomenon. Characteristics of their fatigue behavior included higher crack-growth rates in tension-tension cyclic loading as compared to crack-growth rates in peak sustained loads, lower threshold stress intensity for initiation of crack growth in fatigue and a systematic decrease of the thresholds with increasing transformation yield stress.