Abstract
Alumina and zirconia have been extensively used for orthopedic implants, such as hip and knee joint replacements. In 1982, Dr Hironobu Oonishi and Kyocera Corp. put the world’s first ceramic knee component, KOM, to practical use. This ceramic knee component shows excellent clinical results for long-time use. Now, the ceramic material of the knee component is changed from the alumina to the zirconia, and over 5000 ceramic components have been used in Japan so far.
The 3 mol% yttria stabilized polycrystalline zirconia (3Y-TZP) has been used as surgical grade zirconia. The 3Y-TZP possesses higher fracture strength and toughness as compared to monolithic alumina. However, it is generally known that spontaneous transformation may also occur at relatively low temperatures in hydrothermal environment as in the case of human body for the 3Y-TZP. Therefore, there is a concern of the degradation of mechanical and wear properties as a consequence of the transformation (low temperature degradation).
Our previous studies confirmed that low temperature degradation can be prevented by optimizing sintering temperature and adopting a Hot Isostatic Press process. In this study, we evaluated the effects of surface nature of ceramic material and heat treatment. Since grinding and polishing of the ceramic implants (e.g. femoral heads) might induce phase transformation, residual stress and microcracks, it is needed to examine the validity of the manufacturing process.
At first, the 3Y-TZP samples with grinded (#400) and mirror polished surfaces were prepared and exposed in saturated vapor in an autoclave at 121°C for 150 h (aging test). Some of the samples were subjected to a heat treatment and then to aging test. Before and after aging test, change in crystal structure was evaluated by X-ray diffraction. Then, for evaluation of the aging effects for microscopic area of the ceramic, a Vickers indentation was introduced on the surface before the aging test. Changes in crystal structure and residual stress were evaluated by a Raman spectroscopic technique.
In the results of the aging test, it was found that the degree of the increase in monoclinic fraction of both grinded and polished surfaces was lower in the samples with heat treatment than in those without heat treatment. These results indicate that the phase stability of the 3Y-TZP was improved by the heat treatment after machining.
Around the indentation, circular-like deformation zone and cracks extending from the four corners of the indentation were observed. After the aging test, the transformed area gradually spread towards neighborhood regions depending on the aging time. Besides, a distinct progress of phase transformation at around the crack was also observed. On the other hand, in the samples subjected to the heat treatment, no transformed area was observed around the indentation. These results suggest that low temperature degradation could be prevented by the heat treatment after the machining.
The residual stress fields induced in phase-transformed areas increased during aging test, and heat treatment after the machining was also able to prevent phase transformation even if surface damages, such as indentation or machining, were introduced on the samples.
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