Micro-abrasion wear testing of triode plasma diffusion and duplex treated Ti–6Al–4V alloy

In this paper micro-abrasion wear testing is used to evaluate the wear resistance of triode plasma diffusion-treated, single-layered TiN-, CrAlN-, and WC/C-coated and duplex-diffusion and coated Ti–6Al–4V under uniform three-body rolling abrasion. Nanoindentation, Knoop microhardness, mechanical surface profilometry, optical microscopy, scanning electron microscopy and atomic force microscopy, were used to characterise the surfaces under investigation. Optimum testing conditions for rolling abrasion were established by varying the test parameters and resultant severity of contact. Very low normal loads and high volume fractions of particles in the abrasive slurry are necessary to obtain predictable and reproducible results. Relatively coarse SiC abrasive particles, having a mean diameter of around 3 μm, appear more suitable for micro-abrasion testing of the samples investigated, compared to finer Al2O3 particles. Problems associated with the measurement of the scar volume and subsequent calculation of the wear rate for hard coatings deposited on relatively soft metals like titanium are identified, and suitable testing and measurement techniques are suggested. Three-dimensional wear scar maps generated by mechanical stylus profilometry were used to measure the wear volumes. Under the test conditions used, wear coefficients can be determined from perforating and non-perforating tests, although perforating tests provide more consistent results. Triode plasma diffusion treatments, plasma-assisted (PA) PVD TiN and PAPVD CrAlN can reduce the specific wear rate of Ti–6Al–4V, while PACVD-based WC/C coatings do not provide suitable protection against abrasive wear. The combination of triode plasma oxynitriding diffusion treatments and PVD coatings to create duplex treatments can also lead to further reductions in the coating wear coefficient when compared to non-duplex coatings deposited on non-pretreated substrates.