Lubrication and anti-wear behavior of duplex annealed nanodiamonds/PEO coating on Ti6Al4V: Functional mechanism of structural transformation

Titanium alloys have been widely used due to its low density, high strength and excellent corrosion resistance, but the poor tribological properties restrict its engineering application. Nanodiamond particles provide excellent lubrication and high wear resistance due to the unique physical-chemical properties, but the mismatched mechanical properties severely restrict the solid lubrication application of nanodiamond for titanium alloys. In this study, plasma electrolytic oxidation coatings were firstly prepared on Ti6Al4V substrates, and core-shell annealed nanodiamond particles obtained by high-temperature annealing at 1300 degrees C with nanodiamond core and graphitic outer shell were deposited on the PEO coatings. The plasma electrolytic oxidation-annealed nanodiamond coatings led to a 46.67 % and 96.13 % reduction of coefficient of friction and wear volume comparing to plasma electrolytic oxidation coatings. Annealed nanodiamond with unique core-shell structure can reduce the abrasion of friction pairs; and the friction-induced structural transformation of annealed nanodiamond particles in the top-layer tribofilm further enhanced the lubrication and wear-resistance. The proposed lubrication mechanism has implications for the development of novel nanomaterials with core-shell structure and duplex coating systems for engineering applications.