The surface hardness and wear resistance of titanium alloys can be improved by micro-arc oxidation (MAO). Coating microstructure and performance can be further improved by suitable pretreatment. Four constant-voltage pretreatments (135 or 180 V for 3 or 5 min) were applied to TC4 titanium alloy. Each was followed by constant-current MAO at 0.7 A for 7 min. A group without pretreatment was used as the control. Voltage responses were recorded during MAO. Surface and cross-sectional morphologies were examined by scanning electron microscopy (SEM). Surface porosity was measured by image analysis. Coating–substrate interfacial performance was assessed by progressive-load scratch testing. The results showed that after 180 V pretreatment, a slower voltage rise was observed during subsequent MAO. Surface porosities of 6.964–7.786% were obtained for pretreated groups, compared with 13.755% for the control. The lowest porosity (6.964%) and greatest mean coating thickness (12.22 ± 1.61 μm) were obtained after 135 V pretreatment for 5 min. The highest mean scratch critical load, L_c2, was obtained after 135 V pretreatment for 3 min. A value of 6.508 ± 0.404 N was recorded, 23.7% above the control. Lower mean critical loads were measured for both 180 V groups than for the control.
 

Micro-arc oxidation,TC4 titanium alloy,Constant-voltage pretreatment,Microstructure,Coating adhesion