Takefumi Nakanishi

Mechanical and Phase Stability of Zirconia Toughened Alumina

The JMM-ZTA, a kind of zirconia-toughened alumina, is a bearing ceramic newly developed for artificial joints. Although it has already been reported that mechanical strength and fracture toughness are higher in the JMM-ZTA than in alumina, the stability of the JMM-ZTA has not been studied in detail yet. In the present study, the stability of the JMM-ZTA with respect to mechanical strength and crystalline phases was examined under hydrothermal environment, and the results were compared with those of alumina. Both the 4-point bending strength and the fraction of monoclinic zirconia unchanged even after the aging test at 121°C for 300 h. These results indicate that the JMM-ZTA possesses quite excellent stability to be used as bearing material of artificial joints.

The Characteristics of the New Ceramic Material for Artificial Joints

A new zirconia-toughened alumina ceramic (JMM-ZTA) for a ceramic-on-ceramic hip replacement has been developed. The JMM-ZTA has a fracture toughness superior to zirconia, and the JMM-ZTA/JMM-ZTA combination has superior wear-resistant performance to alumina/alumina. Moreover, the JMM-ZTA shows an excellent crystalline phase stability. Therefore, the JMM-ZTA is believed to be useful for artificial hip prostheses.

Phase Stability of Zirconia Toughened Alumina Composite for Artificial Joints

The phase stability under aqueous condition and changes in the wear region after the Pin-on-Flat wear testing were evaluated using Raman and fluorescence spectroscopy techniques and X-ray diffractometry for a newly developed zirconia toughened alumina (JMM-ZTA). This study suggests that JMM-ZTA is a highly stable material under aqueous environment in the human body and has a high wear resistance in Pin-on-Flat wear tests. This results from a transformation toughening mechanism operative in JMM-ZTA. Therefore, JMM-ZTA appears to have a possibility to improve the performance of monolithic alumina as a material for hip and knee joint prostheses.

Characteristics of Low Temperature Degradation Free ZTA for Artificial Joint

Low temperature degradation free Zirconia toughened alumina (ZTA) has been developed. It is reported that ZTA has higher mechanical strength compared to alumina due to the stress induced transformation and spontaneously transformation of zirconia phase on some ZTA have been occurred. For achieving the higher reliability of artificial joint prosthesis alternative to alumina and other ceramic materials, it is necessary to improve and validate the both mechanical characteristics and phase stability at the same time. We evaluated that microstructure, mechanical characteristics and phase stability of newly developed ZTA (BIOCERAM® AZUL). It was confirmed that four-point bending strength and weibull modulus were extreamly high, and ZTA has higher reliability. There were no significant changes and deterioration in four-point bending strength, crystal structure and wear property with and without accelerated aging test. Newly developed ZTA not only with high mechanical characteristics but also with phase stability could be quite useful as bearing materials in artificial joints for longer clinical use.

Phase Stability and Fracture Toughness of Zirconia Toughened Alumina for Joint Replacement

As for zirconia toughened alumina (ZTA) with various ratios of alumina/zirconia, crystal and micro structures, fracture toughness and phase stability were evaluated by X-ray diffraction, Raman spectroscopy, and aging test in hydrothermal environment. The grain size and monoclinic fraction of zirconia phase and residual stress in alumina matrix changed as a function of zirconia content. The ZTA showed higher fracture toughness than conventional alumina. The fracture toughness of ZTA was highest at which the content of tetragonal zirconia was maximum. The monoclinic fraction of ZTA did not increase even after aging test at 121°C for 150 hr. This study indicates that the optimization of tetragonal zirconia content is essential for achieving higher fracture toughness of ZTA. The ZTA with phase stability as well as with fracture toughness is expected as bearing materials which could extend lifetime of artificial joints in clinical use.