R-CURVE BEHAVIOR AND FLAW INSENSITIVITY OF CE-TZP/AL2O3 COMPOSITE

A ceria-partially-stabilized zirconia-alumina (Ce-TZP/Al2O3) composite optimized for transformation toughening was used to demonstrate its flaw insensitivity due to R-curve behavior. Four-point bend specimens fabricated with a controlled distribution of spherical pores showed nearly the same characteristic strength and strength variability (Weibull modulus) as specimens fabricated without the artificial pores. In situ observations confirmed stable growth of cracks initiated at pores and the crack lengths at fracture instability were much greater than the pore sizes, thus resulting in fracture strengths insensitive to the pores. The small variability in the fracture strength was found to be associated with variability in the R-curve and the instability crack lengths. An analysis based on the fracture instability criterion for rising crack growth resistance accounted for the strength variability due to variability in the R-curve. Comparable four-point bend experiments were also conducted on a sintered yttria-partially-stabilized zirconia (2Y-TZP) ceramic. This ceramic showed significant degradation of strength due to the presence of the pores. This flaw sensitivity is attributed to its steep rising R-curve over short crack lengths.