Characterization and nano-mechanical properties of tribofilms using Cu nanoparticles as additives

Cu nanoparticles were dispersed into SN 650 oil to improve the lubricating properties of the oil and in-situ generate a thin copper film on the rough steel surface in the process of sliding friction. SEM, XPS and nano-indentation tester were utilized to investigate the morphology, chemical state and nano-mechanical properties of the film, respectively. Results show that the friction-reducing and anti-wear properties of SN 650 oil have been improved by adding Cu nanoparticles. A metallic copper film, with a low hardness and a thickness of approximately 5 μm, has been formed on the worn steel surface under the lubrication of SN 650 oil containing 1 wt.% Cu nanoparticles. The film, which bonds well with the substrate steel by a metallurgical bonding, separates the two contact surfaces and provides a fast relaxation of the contact stress. An ideal tribological structure with a soft surface film and a hard sub-surface has been formed on the worn surface, which was considered as the main reason why Cu nanoparticles possessed the excellent tribological properties.