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not show any significant difference. For example, for a testing temperature of 500°C a value of 2.5% was obtained for the virgin specimen and 2.6% for the aged specimen.Changes in specimen dimensions, weight, and density as a function of aging treatment were investigated but no significant changes were observed. This result is not wholly unexpected if it is considered that the total oxygen loss in firing is approximately 1% in the entire specimen, that only about 10% of the specimen is involved in the reoxidation, and that since there is no discernible change in color, it is clear that only a minor fraction of the possible reoxidation occurs.Microhardness tests, however, indicate that the aging has affected the properties of the surface of the specimen. The results of more than one thousand measurements on the sur-face and the interior of ten specimens may be summarized as follows: (a) Titanium-modified zirconia fired at 20OO0C in vacuo has a hardness of approximately 1032 f 50 DPH. (b) Titanium-modified zirconia later aged 20 hours a t 6OO0C a t 1 X 10-3 torr has a hardness of approximately 960 f 50 DPH.(c) Alpha titanium containing zirconium and oxygen found as a second phase in this study has a hardness of approximately 940 f 50 DPH (aged and unaged).

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