In Situ Observation of Sliding Deformation in Commercially Pure Titanium Sheet with TiO2 Film

To investigate the factors that cause variations in the friction coefficients of commercially pure titanium sheets with titanium oxide films, the sliding deformation during ball-on-block sliding tests was observed in situ and compared with electron backscatter diffraction analyses of the same regions. Under a vertical load of 0.1 N, the friction coefficient was stable at a low level of approximately 0.12. By contrast, at 0.5 N, the friction coefficient fluctuated widely between 0.20 and 0.80. At 2.0 and 4.0 N, the friction coefficient was stable again at a high level of approximately 0.30 and 0.40, respectively. The fluctuation in friction coefficient at a vertical load of 0.5 N was investigated further based on the Taylor factor for the uniaxial compression of the titanium grains directly beneath the contact point. Notably, the friction coefficient was negatively correlated with the Taylor factor of the underlying grains. Thus, it can be presumed that the plowing term of the friction coefficient increases as the compressive strain on titanium increases. At vertical loads of 2.0 and 4.0 N, the contact area is larger, so the ball is always in contact with multiple grains. Thus, the influence of the Taylor factor of individual grains can be assumed to be averaged, thereby reducing the variation in friction coefficient.