Role of microstructure on contact damage and strength degradation of micaceous glass-ceramics.

OBJECTIVES: This study examines the hypothesis that microstructure plays a critical role in the accumulation of strength-degrading damage in dental ceramics. A series of micaceous glass-ceramics crystallized from a common glass composition, using heat treatments to increase the diameter and aspect ratio of mica platelets, is used as a model ceramic system.
METHODS: Damage modes are investigated by Hertzian contact testing. Four-point bend tests on indented specimens quantify the influence of single-cycle and multi-cycle damage on strength.
RESULTS: Two competing damage modes are observed: fracture, by tensile-induced cone cracking at the macroscopic level; and quasi-plastic deformation, by shear-induced yield at the microscopic level. The quasi-plastic mode becomes more dominant as the microstructures become coarser and more elongate. Bend tests show severe strength losses in the finer grain structures where cone cracking dominates, but relatively small losses in the coarser grain structures where quasi-plasticity dominates. Whereas natural strengths decline with increasing crystallization temperature, the strengths after indentation damage attain a maximum at intermediate crystallization temperatures. Multiple-cycle contact loading reduces strengths even further, and at relatively low indentation loads, indicating susceptibility to fatigue. Finite element modelling is carried out to evaluate the stress components that drive the damage modes. SIGNIFICANCE: Microstructure is confirmed to be a controlling factor in determining the nature and degree of strength-impairing damage accumulation in dental ceramics. The Hertzian test provides a means of characterizing such damage in the context of clinical function.