Tribological Properties of Anti-Wear Additive ZDDP in Poly α-Olefin Base Oil

The anti-wear mechanism and film-forming ability of ZDDP in lubricants were studied. The functional groups of the PAO10 base oil and ZDDP were characterized by Fourier transform infrared spectroscopy (FTIR), respectively. The influence of ZDDP on the friction and wear properties of base oil was studied by four-ball friction testing machine. The diameter, morphology and surface chemical elements of the wear scars were analyzed by electron microscopy (SEM) and energy dispersive spectrometer (EDS). The 3D morphology of the wear scars were scanned by confocal laser microscope and the roughness was measured at the same time. The 3D morphology and roughness of ZDDP thermal films were measured by Atomic force microscopy (AFM), and the surface functional groups of the thermal films were analyzed by Raman spectroscopy. The influence of ZDDP on the film forming ability was studied by using the point contact ball-disc optical interference oil film thickness test device. The results of friction test showed that ZDDP exhibits high friction and low wear characteristics. At 23 ℃, the friction coefficient of ZDDP with 2% mass fraction was increased by 0.05 compared with base oil, and the diameter of wear scar was reduced by 110 μm. At 40 ℃, the friction coefficients of ZDDP with different mass fractions changed little, but also higher than base oil. At 100 ℃, the friction coefficient increased further with the increase of temperature. In the friction and wear test, the high friction and low wear characteristics of ZDDP are caused by the formation of friction reaction film in the contact zone of ZDDP, which inhibited the entrainment of lubricant in the inlet zone into the contact zone. The results showed that the test temperature and the concentration of additives were important factors to improve the performance of ZDDP. The tribochemical mechanism was explained according to the relationship between the thickness of thermal reaction film and the surface roughness under steady state. Atomic force microscopy (AFM) results showed that ZDDP thermal film was a dynamic growth process. ZDDP thermal decomposition occurred at 150 ℃, chemical bonds were broken and a layer of thermal reaction film was formed on the steel surface through chemical reaction. The thickness of the thermal reaction film was increased by about 150 nm compared to test blocks without ZDDP added. Under the fully-flooded conditionIn, the additive had little effect on film thickness. In the limited oil-supply condition, ZDDP additive increased the film thickness of the lubricants and effectively alleviated the oil starvation in the contact area.