Damage accumulation and cyclic fatigue in Mg-PSZ at Hertzian contacts

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Abstract
Hertzian contact damage in as-fired, peak-aged, and over-aged Mg-PSZ is studied, in single-cycle and multiple-cycle loading. Indentation stress-strain curves reveal a monotonically increasing quasi-plasticity component in the contact deformation with increasing aging time. A bonded-interface technique is used to obtain surface and subsurface views of the damage zones beneath the spherical indenter. Analytical techniques, including optical and scanning electron microscopy, acoustic emission, Raman spectroscopy, and thermal wave imaging, are used to characterize the damage. The damage patterns are fundamentally different in the three aging states: microfracture dominated in as-fired; tetragonal-monoclinic phase-transformation-dominated in peak-aged; monoclinic-phase twinning-dominated in over-aged. The damage accumulates with increasing 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 predominantly 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 in concentrated stress configurations, with special relevance to the evolution of flaws at the microstructural level, are considered.