At present, a well-designed hip joint has a lifetime of 10-15 years. Since this is rather short compared with an increasing lifetime of human, there is great need to improve the durability and clinical lifetime of artificial joints. Reduction of wear debris through the coating of the material surface is one of methods to improve the durability. Here we grow carbon nanotube (CNT) array and composite film (CNT terminated with graphite layers) on the artificial bone surface (ASTM F75 (Co: 62 wt%, Cr: 30 wt%, Mo: 5 wt %)), and examin the tribological property by scratch measurement.

 Alcohol chemical vapor deposition (ACVD) was employed to grow CNT arrays and composite films. Growth temperature, pressure and time were 800 C, 160 torr and 5 min. Ball-on-plate type tribo-test machine (Tribostation Type 32, Shinto Sci. Co., Ltd.) was employed. Normal loads were set at 50 g or 1 g. According to the recipe [1], CNT arrays standing perpendicular to the substrate and composite films were successfully grown on the ASTM surface. Tribological measurement with 50 g load reveals that both carbon film gives lower friction coefficient than a bare ASTM surface. In addition, CNT arrays showed a smaller friction than the composite films. SEM images of both surfaces after wear experiment under 1 g load show that CNT arrays fall flat to the substrate, and that cracks form in the graphite layers of the composite films. This study demonstrates possible use of CNTs as a solid lubrication for artificial hip joint metal materials.

[1] Y. Matsuoka et al., J. Vac. Sci. Technol. B29, 061801 (2011).