The isothermal tetragonal‐to‐monoclinic transformation of a 3Y‐TZP ceramic is investigated from 70° to 130°C in water and in steam by X‐ray diffraction and optical interferometer techniques. Aging kinetics followed by X‐ray diffraction are fitted by the Mehl‐Avrami‐Johnson law, suggesting nucleation and growth to be the key mechanisms for transformation. Optical interferometer observations of highly polished samples effectively reveal a nucleation and growth micromechanism for tetragonal‐to‐monoclinic transformation. A model based on surface change analysis is developed that fits closely to the X‐ray diffraction results.

A detailed study of a commercial zirconia (Y-TZP) aged at 37 degrees C held in Ringer's solution or transplanted in animals or humans is reported. No degradation in strength was found for MOR bars or for hip joint heads subjected to stresses caused by pressing titanium alloy trunnions into the head tapers with loads up to 30 kN. Polished specimens also showed no increase in the monoclinic phase percentage upon aging for 1 year. Small specimens transplanted in animals experience no change in the micro-indentation KIC toughness value, suggesting no degradation. Zirconia heads recovered after 2 years of human implantation showed no loss of strength. The difference between the excellent performances of this ceramic presented here and other studies can be accounted by the microstructural design and quality of the zirconia.

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Quantitative data are presented on the infrared (IR) absorption of interstitial oxygen in oxygen-rich silicon using Fourier transform spectroscopy. Besides the well-known 515 and 1106 cm-' room temperature IR bands, due to the symmetric and antisymmetric vibrations of the Si20 entity, respectively, three other bands at 1227, 1720, and 1013 cm-' are reported, whose intensities are scaled with those of the 515 and 1106 cm-' bands. The band at 1227 cm -1 has often been confused with an oxygen precipitate band observed at 1225 cm-' in annealed silicon. Evidence is given that the 1227 cm -~ band is related to interstitial oxygen. It is also shown that another band at 1720 cm -1 is a combination of the antisymmetric mode of Si20 with a phonon combination of the silicon lattice. A weak band at 1013 cm-' is reported for the first time, and it is attributed to an overtone of the 515 cm-' mode.