High-strength alumina/alumina:calcium-hexaluminate layer composites

The strength degradation of alumina/alumina:calcium-hexaluminate/alumina trilayers, after damage from Hertzian contacts, is evaluated, Relative to the monolithic alumina and alumina:calcium-hexaluminate constituent layer materials, the trilayers show markedly improved strength retention in the damaged state at high contact loads, The outer, fine-grained alumina layers are classically brittle, characterized by cone cracking, whereas the inner alumina:calcium hexaluminate layer is essentially quasiplastic, with a well-defined "yieId" zone that consists of distributed microdamage, The improved strength behavior of the trilayer composite is rationalized in terms of a synergistic interaction between the contact-induced deformation modes in the two layers, with each mode partially negating the effectiveness of the other as a source of failure. This result offers the prospect of hybrid structures with hard outer layers, to provide wear resistance, and soft, tough underlayers, to inhibit brittle fracture.