A single crystal-tantalum (100) surface was anodically oxidized at 5.0 V (SHE) for 1 h in pH 8.4 borate buffer solution. Nano-scratching in solution at a constant normal force of 300 µN was performed to the tantalum (100) surface kept at 5.0 V (SHE) after anodic oxidation. Nano-scratching in air was also performed to the tantalum (100) surface after anodic oxidation. A stick-slip was always observed in lateral force vs. lateral displacement curves during nano-scratching in solution. The degree

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... f stick-slip, however, decreased with increasing scratching rate. The average friction coefficient obtained with nano-scratching in solution was significantly larger than that obtained with nano-scratching in air and increased linearly with logarithm of time required for scratching. In contrast, the friction coefficient obtained with nano-scratching in air was independent of time required for scratching. The protrusion at the scratching end of the groove produced by nano-scratching in solution was observed from the AFM image, while no protrusion was observed for nano-scratching in air. It has been proposed that the protrusion consists of anodic oxide accumulated on the moving front of the indenter tip during nano-scratching in solution and contributes to the increase in lateral force, i.e., friction coefficient.