Micropitting Resistance of Oil-Soluble Nano-LaF₃ in Gear Oil

Micropitting is a kind of hard gears, bearings and other slip-roll contact parts occurring in the border or mixed lubrication state due to poor lubrication caused by surface fatigue damage phenomenon on a microscopic scale. The appearance of micropitting will not only affect the accuracy of the transmission system, but also cause noise, vibration problems, while the further evolution of micropitting will also cause pitting, abrasion and other serious forms of failure, and even lead to broken teeth. Oil-soluble nano LaF₃ as a gear oil additive has excellent friction reduction and anti-wear properties as well as extreme pressure carrying capacity, and has a broad application prospect in gear oils, but there are fewer studies on applying nano-particles to anti-pitting, so this paper investigated the potential of LaF₃ nano-particles as an anti-micro-pitting additive to improve the micro-pitting and wear behavior of bearing steel under boundary lubrication conditions. Bis(2-ethylhexyl) phosphoric acid (D2EHPA)-modified LaF₃ nanoparticles were added into gear oil at the optimal additive amount, and simulated contact fatigue experiments were carried out with a friction and wear testing machine (SRV5), and the three-dimensional structure of the wear traces and the wear volume were measured with a three-dimensional optical profiler (Contour GT-K), and the wear surface of the steel disc was analyzed with a scanning electron microscope and an energy spectrometer. The morphology and elemental distribution of the steel disk wear surface were analyzed by scanning electron microscopy and energy spectrometry, which further revealed the micropitting resistance mechanism of nano LaF₃. The simulated contact fatigue experiments and three-dimensional characterization results showed that the bis(2-ethylhexyl)phosphoric acid (D2EHPA)-modified LaF₃ nanoparticles had excellent resistance to micropitting. The surface analysis results showed that with the increase of load and the extension of experimental time, the micropitting damage on the surface of the gear oil lubricated steel disk became more and more serious, while the addition of LaF₃ nanoparticles made the wear surface smoother and flatter, and the roughness was reduced significantly, effectively inhibiting the emergence and development of micropitting, but the deposition of La and F elements was not detected on the wear surface. Under the action of contact pressure in the friction process, LaF₃ nanoparticles entered the contact surface, and with the action of shear force, the large particle size nano LaF₃ dynamically polished the friction surface, eliminates the roughness peaks on the friction surface, made the surface smooth, weakened the stress concentration, and reduced the shear force in the friction process, thus inhibiting the emergence of micropitting on the friction surface.