Wear resistance of smooth TiO2 coating obtained by anodic oxidation and sol-gel dip-coating combination

Abstract

This study investigates the wear behavior of crystalline TiO₂ coatings on Ti-6Al-4V substrates: a sol-gel only film (TSS; thickness 90 nm) and a combined anodic-oxidation and sol-gel system (TASS; thickness 600 nm). Wear tests employed reciprocating ball-on-flat apparatus with a conical diamond counterface (hemispherical tip radius 10 µm) to replicate prosthetic-valve leaflet motion under dry, unlubricated conditions at service-like or higher sliding speeds and contact pressures (∼10 000 MPa). Tests were run at loads of 39, 78, 117 and 156 mN for 1–4 min. Wear scars and debris were examined qualitatively, while wear volume, wear rate and wear factor were quantified with statistical analysis and uncertainty propagation. At the lowest loads (39 and 78 mN), both TSS and TASS outperformed the bare Ti-6Al-4V, exhibiting significantly lower wear volumes. At 117 mN, TSS exhibited the highest wear volumes—attributed to abrasive TiO₂ debris—whereas TASS remained comparable to the bare alloy; at 156 mN, TASS maintained lower wear than TSS and matched the substrate. Coatings that did not fail—i.e., did not expose the Ti-6Al-4V surface—showed wear factors of 2x10^-4 to 3x10^-4 mm3 N⁻¹ m⁻¹, consistent with literature. Wear-rate maps, created separately for bare Ti-6Al-4V, TSS and TASS by fitting wear rate versus load and time, illustrate each system’s load-time response and reveal that TASS extends the low-wear regime relative to TSS. These maps offer a predictive framework for optimizing coating design in durable biomedical devices.