Load-Dependent Tribological Transition of PVD Coatings on Ti-6Al-4V in Wet and Dry Environments

Abstract

The tribological behavior of nitride coatings is highly influenced by the applied load, which dictates contact stress, micro-crack formation, and plastic deformation. This study examines the load-dependent performance of TiN- and TiCN-coated Ti-6Al-4V in both dry and wet sliding conditions. We did ball-on-disk tests at 1, 2, and 5 N for 3000 cycles with 6 mm alumina counterfaces. The friction coefficient (µ), wear morphology, and counterface scars were characterized systematically. At 1 N, both TiN and TiCN showed an unstable COF because of the formation of adhesive junctions. At 2 N, TiCN partially stabilized at ~0.45 through hardness and strain hardening, but TiN stayed at ~0.5. At 5 N, TiCN had a stable COF of about 0.4, but it also had brittle micro-cracks. TiN, on the other hand, had plowing grooves with a COF of about 0.45. Wet sliding lowered the COF in all cases, but brittle cracking and plowing stayed the same. Wear tracks showed that TiCN had mechanisms that controlled fractures, and TiN had wear that was mostly caused by abrasion. Counterface analysis corroborated abrasive debris transfer instead of self-lubrication. These findings indicate that load dictates the shift among adhesion, strain hardening, and brittle/plowing mechanisms in nitride coatings, with TiCN providing superior hardness-controlled protection yet diminished fracture resistance.