Tribological Evaluation of Plasma-Sprayed Hydroxyapatite Coating on Wire Arc Additively Manufactured Ti6Al4V

Wear-induced deterioration in additively manufactured Ti6Al4V components restricts their long-term performance in biomedical and structural applications due to severe frictional contact and abrasive conditions. Therefore, this study aimed to enhance the surface durability and tribological reliability of wire arc additively manufactured (WAAM) Ti6Al4V substrates by depositing hydroxyapatite (HAP) coatings through plasma spraying. The optimized HAP coatings exhibited a uniform lamellar microstructure with thicknesses ranging from 134 to 169 µm and strong interfacial bonding. Tribological performance was evaluated through slurry abrasion and pin-on-disk tests against an EN31 hardened steel counterface, supported by response surface methodology (RSM)-based optimization. Slurry abrasion testing demonstrated that the HAP-coated specimens achieved an average reduction in mass loss of approximately 35-45% relative to uncoated Ti6Al4V across the investigated parameter space. Similarly, dry sliding pin-on-disk tests revealed a mass loss reduction of approximately 40-50% for the HAP-coated surfaces. Post-wear SEM analysis indicated that uncoated Ti6Al4V predominantly underwent severe plastic deformation, deep plowing, and adhesive wear. In contrast, the HAP-coated surfaces exhibited controlled microabrasion, inter-splat crack deflection, and localized brittle flaking governed by the lamellar ceramic microstructure and coating phase stability. These mechanisms restricted subsurface deformation and limited direct damage to the substrate. Statistical analysis using response surface methodology confirmed strong model predictability.