Differences in kinetics of phase transformation of 3Y-TZP ceramics between aging test under hydrothermal environment and hip simulator wear test

The phase stability of yttria (3 mol%) stabilized tetragonal polycrystalline zirconia (3Y-TZP) has been evaluated by X-ray diffraction and laser microscope. The differences in kinetics of phase transformation in the aging test under hydrothermal environment and during the hip simulator wear test were compared. It was found that the phase transformation during aging test experiences a Mehl-Avrami-Johnson (MAJ) equation and the phase stability of tetragonal phase can be improved by controlling the sintering condition. Hip simulator wear test using zirconia femoral head with 4 different monoclinic fractions was performed against conventional UHMWPE (GUR 1050) acetabular socket up to 6 million cycles. During hip simulator testing, slight increase in monoclinic fraction of some zirconia femoral head was observed. Increasing rate of monoclinic phase depended on initial monoclinic fraction. But, from the result of measurement of UHMWPE wear rate, mutual relationship between the monoclinic fraction on zirconia femoral head and the wear rate of UHMWPE socket was not observed. From the results of laser microscope observation, in the case of phase transformation during hip simulator test, large number of small monoclinic spots were observed in contrast to the phase transformation during aging test. Moreover an increase in monoclinic fraction on the surface alone was observed significantly compared to the progress into the bulk in aging test. These facts indicate that the nucleation of monoclinic phase on the surface was enhanced by the sliding action against UHMWPE cup. Thus, it was suggested that the phase transformation during hip simulator test was affected on tribochemical reaction during sliding condition.