Tribological characteristics and tribo-chemical mechanisms of Al–Mg–Ti–B coatings under water–glycol lubrication

Abstract

In this work, the tribological characteristics and tribo-chemical mechanisms of the water–glycol lubricated Al–Mg–Ti–B coatings under various normal loads were studied by pin-on-disk unidirectional sliding test, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). The water–glycol lubricated Al–Mg–Ti–B coatings showed a steady-state friction coefficient as low as 0.02 and this value was not influenced by the applied normal loads. As the normal loads increased, the friction coefficients of the Al–Mg–Ti–B coatings decreased more rapidly. Examination of the wear tracks of water–glycol lubricated Al–Mg–Ti–B coatings by SEM and EDX shows that the sliding wear process was dominated by a polishing effect. XPS results indicate that significant surface boron oxidation of Al–Mg–Ti–B coatings occurred during sliding in a water–glycol lubricated environment; such effect was more pronounced at higher normal loads. However, there is no clear evidence of the presence of boric acid in XPS spectra at all applied normal loads. This is thought to be due to the unstable nature of boric acid and in situ surface characterization of the water–glycol lubricated Al–Mg–Ti–B coatings needs to be done to clarify this issue.