The Cl+ ion implantation to TiN film resulted in low friction coefficient of TiN film against a stainless steel ball. The mechanism of reduction of the friction coefficient by Cl+ implantation was studied with XPS from a viewpoint of surface reaction. The surface oxygen concentration was high for the specimen with low friction. The formation of titanium dioxide with oxygen defects was concluded to be important. Titanium dioxide with oxygen defects works as a lubricant. The titanium dioxide was worn away during friction. The re-production of the titanium dioxide is important for continuous low friction. The re-production of the titanium dioxide was controlled by the supply of oxygen from water vapor in the atmosphere and friction heat. This is the reason why the friction behavior largely depended on the friction speed.
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