Contact damage resistance and strength degradation of glass-infiltrated alumina and spinel ceramics.

All-ceramic crowns are coming into widespread use because of their superior esthetics and chemical inertness. This study examines the hypothesis that glass-infiltrated alumina and spinel core ceramics are resistant to damage accumulation and strength degradation under representative oral contact conditions. Accordingly, Hertzian indentation testing with hard spheres is used to evaluate damage accumulation in alumina and spinel ceramics with different pre-form grain morphologies and porosities. Indentation stress-strain curves measured on fully infiltrated materials reveal a marked insensitivity to the starting pre-form state. The glass phase is shown to play a vital role in providing mechanical rigidity and strength to the ceramic structures. All the infiltrated ceramics show subsurface cone fracture and quasi-plastic deformation above critical loads P(C) (cracking) and P(Y) (yield), depending on sphere radius, with P(Y) < P(C). Strength degradation from accumulation of damage in Hertzian contacts above these critical loads is conspicuously small, suggesting that the infiltrated materials should be highly damage-tolerant to the "blunt" contacts encountered during mastication. Failure in the strength tests originates from either cone cracks ("brittle mode") or yield zones ("quasi-plastic mode"), with the brittle mode more dominant in the spinels and the quasi-plastic mode more dominant in the aluminas. Multi-cycle contacts at lower loads, but still above loads typical of oral function, are found to be innocuous up to 10(5) cycles in air and water, although contacts at 10(6) cycles in water do cause significant strength degradation. By contrast, contacts with Vickers indenters produce substantial strength losses at low loads, suggesting that the mechanical integrity of these materials may be compromised by inadvertent "sharp" contacts.