Nano-indentation to the Passive Metal Surfaces in Solution
Masahiro Seo, Makoto Chiba, Yusuke Kurata
This article reviewed the recent results of nano-indentation tests in solution for in-situ evaluation of the mechanical properties of passive iron and titanium surfaces electrochemically controlled in pH 8.4 borate solution. The hardness of passive metal surfaces in the solution could be determined from the load-depth curves measured with nano-indentation tests. In the absence of chromate treatment, the hardness of the passive iron (110) surface was larger by 10% than that of the passive iron (100) surface which was ascribed to the difference in surface atomic density of the substrate. The chromate treatment increased the hardness of the passive iron (100) surface. The effect of chromate treatment on hardness was explained in terms of high repassivation rate at the ruptured sites of passive film during nano-indentation in solution.
The hardness of the titanium surface obtained with nano-indentation under the electrochemical control at 5V (RHE) after anodic oxidation at 5V for 1h in the solution was 3-4 times as large as that of the titanium surface obtained with nano-indentation in air after anodic oxidation at the same condition. This large difference in hardness was attributed to high repassivation rate at the ruptured sites of anodic oxide film under the electrochemical control at 5V in the solution as well as the effect of the chromate treatment on hardness for the passive iron surface.