Threshold strength and residual stress analysis of zirconia–alumina laminates

We dedicate this work to Wolfgang Pompe, a very good friend and a good friend of the Materials Faculty at UCSB where he and his wife, Gisela, shared their lives and fellowship. Three different zirconia–alumina laminates, AZ50, AZ80, and AZ95, were fabricated via tape-casting thick Zr(3Y)O2 layers, dip-coating the zirconia tapes in a slurry containing a mixture of zirconia and alumina, stacking and bonding the coated zirconia tapes, followed by densification. Each composite had a different compressive stress by using different mixtures of alumina and zirconia (the number associated with each composite system refers to the volume fraction of alumina in the thin, compressive layers). After densification, the Zr(3Y)O2 layers were *425 lm thick, and the thin alumina/zirconia layers were 60 * 65 lm thick. The threshold strengths, below which the probability of failure is zero, were determined to be 255 ± 8 MPa, 311 ± 7 MPa, and 421 ± 12 MPa for AZ50, AZ80, and AZ95 laminates, respectively. These values are about 60 * 70% of those calculated from a previously reported function. The surface and interior stresses were determined using a finite element analysis. The compressive stresses of the outer alumina/zirconia layers were determined using a piezospectroscopy method. The surface stresses for the thick zirconia layers were estimated using an indentation– crack length relation. The piezospectroscopy, analytical and finite element analysis results for the outer compressive layer were in good agreement. But, the finite element analysis showed that the compressive stress component on the surface of the compressive layers was approximately half the value of the biaxial compressive stresses deep within the thin, compressive layers. It appears that the much smaller compressive stresses on the surface of the compressive layers is one important factor that resulted in the lower threshold strength than predicted by the previously developed function used to estimate the threshold strength.