Effect of heat treatment after accelerated aging on phase transformation in 3Y-TZP.

Our purpose was to investigate the effect of heat treatment on the reversibility of the tetragonal to monoclinic transformation in 3Y-TZP, and associated surface roughness. The goals were to determine the onset temperature of the reverse transformation, and characterize surface roughness after accelerated aging, and after aging followed by heat treatment. 3Y-TZP disc-shaped specimens were sintered at temperatures from 1300 to 1550 degrees C. The reversibility of the transformation was investigated by X-ray diffraction (XRD) after accelerated aging, followed by heat treatment at temperatures from 350 degrees C up to 850 degrees C. The surface roughness (R(rms)) was measured by atomic force microscopy after polishing, after accelerated aging for 1 or 10 h, and after aging followed by heat treatment. XRD showed that the fraction of m-phase increased linearly with grain size after aging for 10 h (1.0-29.8%). The transformation was reversed for all groups after heat treatment at 850 degrees C/min., with only trace amounts of m-phase remaining for the group sintered at 1550 degrees C. A significant increase in mean surface roughness was observed after accelerated aging (1.59-7.45 nm), compared to polished groups (0.83-1.0 nm). However, the mean surface roughness after accelerated aging for either 1 or 10 h, followed by heat treatment at 850 degrees C/min. (1.18-2.1 nm), was not significantly different from that of the polished groups. This was attributed to the reverse transformation. XRD revealed that the monoclinic to tetragonal transformation, was complete after heat treatment at 500 degrees C for 1 min, for specimens sintered at 1550 degrees C and aged 10 h.