| Oil-based lubricants are widely used in almost all mechanical equipments. Oil-based lubricants not only waste energy, but also bring pollution to the environment. Water-glycol lubricant has the advantages of pollution-free, easy accessibility and energy-saving, which is an ideal environment-friendly lubricating medium. Development of a novel frictional material or surface coating, which is compatible with the water-glycol lubricant, has become a hot research area in tribology research. The tribological studies performed on the Al-Mg-Ti-B nanocomposite coatings showed low frictional behavior associated with these coatings under certain lubrication conditions. Previous studies suggested that the low-friction behavior of these coatings is due to he formation of boric acid, B(OH)3, which is a reaction product of boron oxide (B2O3) with moisture. Weak van der Waals-bonded atomic layers in B(OH)3 tend to slide against each other with little resistance to applied shear stresses. However, with the water-glycol as a lubricating medium, the origin of B(OH)3, the low-friction mechanism and the wear behavior of Boron-rich borides, are still unclear.AlMgB14 coating, TiB2 coating and Al-Mg-Ti-B coating were prepared by radio frequency (RF) or direct-current (DC) magnetron sputtering respectively. These coatings were then annealed at 700℃. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectrascopy were employed to characterize these coatings. Results show that boron at annealed AlMgB14 coating surface can be oxidized; surprisingly, such oxidation does not lead to the formation of boric acid in ambient air. Instead, boric acid can be produced at the surface of annealed TiB2 coating and Al-Mg-Ti-B coating. It is shown, via the water contact angle measurements, that these boride coatings exhibit distinct surface wettability characteristics, which are believed to result in the observed surface hydration processes. Furthermore, anatase TiO2 formation plays a major role in the surface wetting behaviors for these boride coatings.In this work, the tribological characteristics and tribo-chemical mechanisms of the water-glycol lubricated Al-Mg-Ti-B nanocomposite coatings under different loads and sliding speeds were studied by pin-on-disk unidirectional sliding test using UMT- 3 tribometer. The water-glycol lubricated Al-Mg-Ti-B coating showed a steady-state friction coefficient as low as 0.02 and this value was not influenced by the applied normal loads and sliding speeds. As the normal loads and sliding speeds increased, the friction coefficients of the Al-Mg-Ti-B coating decreased more rapidly. Examination of the wear tracks of water-glycol lubricated Al-Mg-Ti-B coating by SEM and EDX shows that the sliding wear process was dominated by a polishing effect. XPS results indicate that significant oxidation of Al-Mg-Ti-B coating 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 and sliding speeds. 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 coating needs to be done to clarify this issue.In this work, we also studied the effect of ultraviolet (UV) irradiation on the surface chemical states and tribological properties of the water-glycol lubricated Al-Mg-Ti-B coating. XPS results indicate that boric acid is present at both the wear track and unworn surface of the UV-irradiated Al-Mg-Ti-B coating, as opposed to the case of Al-Mg-Ti-B coating untreated by UV irradiation, in which there is no clear evidence of the boric acid formation. We believe this is due to the UV-induced strong hydrophilicity in the presence of anatase TiO2. In addition, the observed distinct surface chemistries result in the different tribological behaviors for these coatings. This study, thereby, provides further insights into the role of anatase TiO2 in the tribological characteristics of Al-Mg-Ti-B coating. |
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