| Al-Mg-Ti-B coatings with a typical nanocomposite microstructure, where TiB2 nanocrystallites are dispersed within the amorphous Al-Mg-B matrix, can substantially reduce the friction coefficients of mechanical components and improve the operation efficiency of mechanical systems under the water-glycol boundary lubricated condition. Water-glycol lubricant,as a green, pollution-free lubricating medium, conforms with the current energy situation and research direction, with the characteristics of a wide range of sources, high safety, and good fluidity. Therefore, the water-glycol lubricated Al-Mg-Ti-B coatings have a broad application area and are worth researching. A thorough analysis of the friction and wear mechanisms suggested that boric acid (H3BO3) is the primary contributor to the low friction behavior observed for these coatings. However, the tribological performances of the lubricated Al-Mg-Ti-B coatings in response to various tribological conditions have not been fully exploited. Thus there is a need for greater understanding of the fundamental friction and wear mechanisms of Al-Mg-Ti-B coatings under various tribological conditions; particularly, the effects of various tribological conditions on the tribo-chemical reactions for these coatings in a water-based lubricant need to be thoroughly investigated.In this work, Al-Mg-Ti-B coatings and TiB2 coatings were deposited on bearing steel and silicon substrates by direct-current (DC) magnetron sputtering. The tribological characteristics of the water-glycol lubricated Al-Mg-Ti-B coatings under various normal loads were studied by ball-on-disc unidirectional sliding test, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). As the normal loads increased, the friction coefficients of the Al-Mg-Ti-B coatings decreased more rapidly and sharply. And the curve of the friction coefficients becomes smooth because the adhesive wear between friction pair is suppressed. However, too high normal load would make it difficult for tribofilm to be stable on the surface of coating, and the lubricant would lose faster, eventually leading to lubrication failure.The differences in the tribological characteristics of the water-glycol lubricated Al-Mg-Ti-B coatings and TiB2 coatings were further studied by pin-on-disk unidirectional sliding test. It is shown that the friction coefficients of both the two coatings present a slowly-descending tendency, and the value of steady-state friction coefficient of Al-Mg-Ti-B coatings is lower. Examinations of the adjacent unworn region and the wear tracks of water-glycol lubricated Al-Mg-Ti-B coatings and TiB2 coatings by SEM and EDX show 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. The tribofilm formed in the tribo-chemical reaction provided a lower friction coefficient. And the adhesion of Fe was observed on the wear track of water-glycol lubricated TiB2 coatings after sliding against bearing steel ball.In this work, we also systematically studied the tribological characteristics of the water-glycol lubricated Al-Mg-Ti-B coatings by ball-on-disc reciprocating sliding test. Significant coating spallings were observed on the wear track of Al-Mg-Ti-B coatings under no lubrication condition. The water-glycol lubricated Al-Mg-Ti-B coatings showed a substantial adhesion of Fe. The traces of friction coefficients of water-glycol lubricated Al-Mg-Ti-B coatings untreated and treated by ultraviolet (UV) irradiation showed the same tendency. Whereas, the steady-state value of the friction coefficient of Al-Mg-Ti-B coatings treated by UV irradiation was lower. Examinations of the wear track of water-glycol lubricated Al-Mg-Ti-B coatings treated by UV irradiation by SEM and EDX showed that the width and depth of the wear track were smaller. It indicates that the wear of Al-Mg-Ti-B coatings treated by UV irradiation is lower. XPS results indicate that UV irradiation enhances the reaction between boron oxide (B2O3) and water (H2O) to form boric acid (H3BO3), thus reducing friction and wear of Al-Mg-Ti-B coatings. As the reciprocating frequency increased, the value of friction coefficients of the Al-Mg-Ti-B coatings became lower, and the depth of the wear track became shallower. It is shown that higher reciprocating frequency not only contributes to the formation of tribofilm to reduce friction, but also helps to push away the surfaces of friction pair to reduce wear. |
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