Change in Mechanical Strength and Bone Contact Ratio of Beta-Type TNTZ Subjected to Mechanical Surface Modification

Abstract

Ti-29Nb-13Ta-4.6Zr (TNTZ), which is one of metastable beta-type Ti alloys, has developed as one of representative biomedical and dental Ti alloys in Japan. TNTZ subjected to solution treatment shows Young’s modulus of 60 GPa, which is close to that of cortical bone. In addition, TNTZ has very low cytotoxicity and good bone biocompatibility as well. Heat treatment like solution treatment and aging (STA) is mainly used for improving the mechanical properties of metastable beta-type Ti alloys because of alpha precipitates, while Young’s modulus also rises drastically. This study was investigated the effects of mechanical surface modifications such as fine particle bombarding (FPB) with steel and hydroxyapatite particles or friction stir processing (FSP) on the mechanical strength of TNTZ in order to maintain low Young’s modulus. The relative bone contact ratios between the cancellous bones of Japanese white rabbits and column-shaped TNTZ subjected to FPB of steel particles were also evaluated. Vickers hardness (HV) of TNTZ subjected to FPB with fine particles of steel and hydroxyapatite particles increased by HV30 to 200 at the edge of the specimen surface to around 100 to 300 mm in depth as compared with that of TNTZ subjected to solution treatment. The hydroxyapatite layer was formed on the specimen surface by FPB with fine particles of hydroxyapatite particles, although the trend was not significant by FPB with steel particles. Furthermore, the fatigue strength in high cycle fatigue region of TNTZ subjected to FPB with steel particles was improved and the fatigue limit showed around 400 MPa, although that of TNTZ subjected to FPB with fine particles of hydroxyapatite particles were around 60 MPa higher than that to TNTZ subjected to solution treatment (230 MPa). TNTZ with a rough surface texture (Ra: 0.65 μm) showed a relative bone contact ratio of more than 80% after undergoing FPB with fine particles of steel particles; this value was significantly higher than that of TNTZ with a surface texture (Ra: 0.07 μm). Lastly, the microstructure of TNTZ subjected to FSP showed the recrystallization area by the frictional heating with very fine equiaxed beta phase with an average grain diameter of 3.0 μm. The change in Vickers hardness of TNTZ subjected to FSP was almost identical to that of Young’s modulus and showed the almost same trend of FPB.