Constrained Densification of Alumina/Zirconia Hybrid Laminates, I: Experimental Observations of Processing Defects

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Abstract
Various forms of damage were observed in pressure‐less‐sintered Al 2 O 3 /ZrO 2 symmetric laminates and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel cracks in the ZrO 2 layers, Al 2 O 3 edge‐effect cracks parallel to the layers, delamination in the Al 2 O 3 layers, and debonding between the Al 2 O 3 and ZrO 2 layers. Based on detailed microscopic observations, the defects were attributed to sintering rate and thermal expansion mismatch between the layers. Cracks or cracklike defects were formed in the early stages of densification, and these cracks either opened during sintering or acted as preexisting flaws for thermal expansion mismatch cracks. Consequently, the extent of cracking could be reduced or even eliminated by decreasing mismatch stresses during the sintering and cooling stages. This can be accomplished by reducing the heating and/or cooling rates or by adding Al 2 O 3 in the ZrO 2 layers. The sintering mismatch stresses were estimated from the degree of curling in asymmetric laminates and from layer viscosities that were obtained by cyclic loading dilatometry. The measured curvature was an indication of the mismatch in sintering strain between Al 2 O 3 and ZrO 2 and were consistent with the dilatometric data that were obtained for the component layers.

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