DAMAGE ACCUMULATION AND CYCLIC FATIGUE IN MG-PSZ AT HERTZIAN CONTACTS

Hertzian contact damage in as-fired, peak-aged, and over-aged Mg-PSZ i s studied, in single-cycle and multiple-cycle loading, Indentation str ess-strain curves reveal a monotonically increasing quasi-plasticity c omponent in the contact deformation with increasing aging time. A bond ed-interface technique is used to obtain surface and subsurface views of the damage zones beneath the spherical indenter. Analytical techniq ues, including optical and scanning electron microscopy, acoustic emis sion, Raman spectroscopy, and thermal wave imaging, are used to charac terize the damage. The damage patterns are fundamentally different in the three aging states: microfracture-dominated in as-fired; tetragona l-monoclinic phase-transformation-dominated in peak-aged; monoclinic-p hase twinning-dominated in over-aged. The damage accumulates with incr easing number of cycles, most strongly in the as-fired state. It also increases with increasing test duration in the as-fired and over-aged states, but not perceptibly in the peak-aged. The results imply predom inantly mechanical fatigue effects, augmented by a chemical component in the as-fired and over-aged states. Broader implications in relation to the susceptibilities of zirconia ceramics to fatigue degradation i n concentrated stress configurations, with special relevance to the ev olution of flaws at the microstructural level, are considered.