Enhanced tribological performance of TiO₂-hBN/CNT double-layer coating by CNT-assisted plasma electrolytic oxidation with nanoparticles addition

Hexagonal boron nitride (h-BN) is considered a very promising anti-friction material, however, how to assemble them densely and uniformly on metal surfaces remains a challenge. In this study, CNT-assisted plasma electrolytic oxidation (PEO) with the addition of h-BN nanoparticles was employed to create a double-layer coating on titanium alloy, demonstrating exceptional tribological characteristics. The CNTs not only acted as lubricating phases to improve the anti-friction performance but also as additives to facilitate the transition from the nanocomposite coating to the double-layer coating. The later stage of the PEO process with CNT doped in the electrolyte produces an interesting sudden rise in current density, which generates strong and dense discharge that facilitates the deposition and sintering of h-BN. As a result, the double-layer coating has a higher adhesion and hardness as well as more h-BN nanoparticles incorporated into the coating compared to coatings without CNTs-assisted deposition. Further, the double-layer coating exhibits a lower friction coefficient (∼ 0.11), attributed to the high release rate of h-BN under shear to form a lubrication film in conjunction with CNTs on the contact surface. The simple technique offers a novel approach to fabricating self-lubricating ceramic coatings on light alloys, which has huge potential for application in the anti-friction materials field.