Voltage-Dependent Evolution of Microstructure and Tribo-Mechanical Behaviour of Coatings Formed on CP-Ti from a Wollastonite–Hydroxyapatite Electrolyte by Micro-Arc Oxidation

Commercially pure titanium is widely used for load-bearing implants but its low surface hardness and high friction coefficient limit durability. In this work, calcium- and silicon-bearing coatings were deposited on CP-Ti (VT1-0/ASTM Grade 2) by micro-arc oxidation (MAO) in an aqueous electrolyte of 30 wt.% H3PO4, 60 g/L hydroxyapatite and 75 g/L wollastonite, at a fixed filling-parameter setting of 8% and two voltages, 200 and 300 V. The coatings were characterised by SEM, EDS, FIB cross-sectioning, progressive-load scratch testing, instrumented indentation (ISO 14577-1) and single-pass ball-on-flat sliding with 3D profilometry. Raising the voltage from 200 to 300 V thickened the layer from about 28 to 37 µm but rendered the cross-section coarsely porous and the coating–substrate interface markedly rougher. The first cohesive critical load fell from 6.15 N at 200 V to 3.73 N at 300 V, and the kinetic friction coefficient rose from 0.41 to 0.75. Instrumented indentation gave comparable properties for both coatings (Martens hardness 2254 versus 2169 N/mm2; modulus 103 versus 102 GPa), indicating that the higher-voltage penalty lies in coating architecture and interface quality rather than in intrinsic hardness. The lower voltage is therefore preferable for tribologically loaded coatings of this type.