CONSTRAINED DENSIFICATION OF ALUMINA ZIRCONIA HYBRID LAMINATES .1. EXPERIMENTAL-OBSERVATIONS OF PROCESSING DEFECTS/

Various forms of damage were observed in pressureless-sintered Al2O3/Z rO2 symmetric laminates and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel cracks in the ZrO2 layers, AL(2)O(3) edge-effect cracks parallel to the laye rs, delamination in the Al2O3 layers, and debonding between the Al2O3 and ZrO2 layers. Based on detailed microscopic observations, the defec ts were attributed to sintering rate and thermal expansion mismatch be tween the layers. Cracks or cracklike defects were formed in the early stages of densification, and these cracks either opened during sinter ing or acted as preexisting flaws for thermal expansion mismatch crack s. Consequently, the extent of cracking could be reduced or even elimi nated by decreasing mismatch stresses during the sintering and cooling stages. This can be accomplished by reducing the heating and/or cooli ng rates or by adding Al2O3 in the ZrO2 layers. The sintering mismatch stresses were estimated from the degree of curling in asymmetric lami nates and from layer viscosities that were obtained by cyclic loading dilatometry. The measured curvature was an indication of the mismatch in sintering strain between Al2O3 and ZrO2 and were consistent with th e dilatometric data that were obtained for the component layers.