Vacuum Thermal Treatment for Achieving Macroscale Superlubricity by Nanodiamond and Hexagonal Boron Nitride on H-DLC Film Surfaces in Dry Nitrogen

Hydrogenated diamond-like carbon (H-DLC) films are limited by their poor thermal stability, which significantly affects the tribological applications and needs improvement. Accordingly, nanodiamond (ND) and hexagonal boron nitride (h-BN) are used to address this issue. When the H-DLC surface is deposited using ND+h-BN mixture with mass ratio of 1:1 and a concentration of 0.1 mg mL −1 and vacuum-heated at 200 °C and 1 × 10 −5 Pa, a superlow friction coefficient of 0.0015 can be obtained, with a reduction of 98.33% as compared to pure H-DLC. Correspondingly, the wear rates of wear scar and wear track decreased by 81.95% and 24.83%, respectively. High vacuum thermal treatment can purify the adsorbed species on the surfaces of ND and h-BN, and produce newly-exposed dangling bonds. Simultaneously, new bonds of C-N are formed between ND and h-BN, and the nanoparticles adhere together to form a polymer-like structure under friction. Furthermore, the ND can support h-BN and reduce its agglomeration. Under the action of tribochemical reaction, the layer spacing of hexagonal boron nitride is increased to obtain a better shear slip. The combination of these factors resulted in ultra-low friction. This study paved the way for developing functional anti-friction additives for durable and high-performance lubrication.