Four types of zirconia ceramic, Ce-TZP, 3Y-TZP, 8Y-FSZ and Mg-PSZ, were heat treated in hydrogen gas. Only Ce-TZP was very effectively reduced after the heat treatment. The drastic weight loss and the increase of internal friction, which were observed in the reduced Ce-TZP, were due to the high number of oxygen vacancies introduced into the matrix. The various mechanical properties of Ce-TZP before and after the heat treatment were investigated. Sufficient reduction effectively increased the hardness but unfortunately also embrittled the sample. The serious increase in embrittlement could not be attributed to the hydrogen introduced into the matrix, but was due principally to the low transformability of the reduced Ce-TZP. On the other hand, the anelastic property of the reduced Ce-TZP was considerably improved by the increase in oxygen vacancies and, furthermore, the tensile fracture strength was influenced due to the fact that anelasticity occurs instead of phase transformation.
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