Damage accumulation and cyclic fatigue in Mg-PSZ at Hertzian contacts
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David B. Marshall Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, California 91360 (Received 3 February 1995; accepted 8 June 1995)
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: microfracturedominated 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.
I. INTRODUCTION Zirconia-based ceramics can be tailored with uncommonly high long-crack toughnesses and strong ^-curves. 1 " 3 Most widely studied is magnesia-partiallystabilized zirconia (Mg-PSZ), where toughening is due primarily to tetragonal-monoclinic transformation of precipitates within a constraining cubic-phase matrix. 12 These zirconia ceramics are subject to fatigue in tests with traditional long-crack (compact tension) specimens.4'5 Crack extensions in cyclic loading are typically several orders of magnitude higher than in corresponding static loading, indicating a true mechanical fatigue effect, although rate effects from environmental moisture are also in evidence. The growth rates depend on the aging time, declining from the initial (as-fired) state up to a critical aging time (peak-aged), and thereafter increasing (over-aged). This trend correlates with the long-crack toughness values, governed in turn by the extent of tetragonalmonoclinic transformation within the crack-tip shielding a)
Guest Scientist, from Departamento de Fisica, Universidad de Extremadura, 06071-Badajoz, Spain. Guest Scientist, from Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201.
b)
J. Mater. Res., Vol. 10, No. 10, Oct 1995 http://journals.cambridge.org
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zone.2'3 However, while the role of transformation in the toughening process appears well understood, the underlying mechanisms of fatigue are only recently being elucidated.4'6'7 A shortco
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