Friction and wear mechanisms in AlMgB14-TiB2 nanocoatings

Recent tribological studies of AlMgB14-TiB2 nanocomposite coatings, sliding unidirectionally against AISI 52100 steel counterfaces under boundary-lubricated conditions, have shown sustained friction coefficient values as low as 0.02. While many industrial applications for wear-resistant coatings employ mineral oils as the lubricant, there is growing interest in the use of water-based, green alternatives. Additional performance evaluations of nanocomposite coatings in component-scale dynamometer tests, and using water-based lubricants, reveal a significantly lower surface roughness and sliding friction than comparable tests using oils. Evidence suggests that the primary contribution to the observed low-friction behaviour of these films is boric acid, B(OH)3, which is a reaction product of boron oxide, B2O3, with moisture. Weak, van der Waals-bonded atomic layers of B(OH)3 tend to slide against each other with little resistance to applied shear stresses. X-ray photoelectron spectroscopy (XPS) was employed to identify which of the two constituent phases (AlMgB14 and TiB2) was the primary contributor to the low friction behaviour associated with this nanocomposite coating. Results have shown that the boric acid is associated almost exclusively with the crystalline TiB2 component, rather than the AlMgB14 amorphous phase. This finding suggests that the compositions of the coatings could be custom-tailored to address the wear and friction requirements of specific applications.