Preparation and Tribological Properties of Ni₃Al Matrix Self-Lubricating Composite Coating

In this study, a feed of Ni₃Al matrix self-lubricating composite powder has been fabricated by high-energy ball milling combined with spray granulation methods. Then it was deposited on the steel surface by plasma spraying to form a self-lubricating coating. The friction and wear properties of the Ni₃Al matrix self-lubricating composite coating at elevated temperatures in atmosphere were tested on a HT-1000 pin-on-disk type high-temperature tribometer. In addition, the microstructure, phase composition and wear mechanism of the coating were characterized and analyzed by scanning electron microscopy, energy dispersive spectrometer and Raman. The results show that the coating had a good self-lubricating property from 25 ℃ to 800 ℃, characterized by friction coefficients of 0.14~0.42 and wear rates of 2.41×10^–4 ~ 5.76×10^–4 mm³/(Nm). The wear mechanism can be explained as follows: (1) within temperature range 25 ~ 400 ℃, an effective transfer lubricating film formed on the worn surface due to the increase of the toughness of Ni₃Al intermetallic and the improved plastic-deformation ability of soft metallic silver at higher temperature, thereby improving the friction and wear properties of the coating, the wear mechanisms were brittle fracture, abrasive wear and adhesive wear. (2) At 600 ℃, the wear surface was partially covered by a complete lubrication film due to the excessive softening of Ag and incomplete brittle-plastic transformation of BaF₂/CaF₂ eutectic, thus deteriorating the tribological properties, and then the wear mechanism was peeling and abrasive wear. (3) At 800 ℃, due to the formation of a continuously smooth enamel layer rich in high-temperature lubricant (NiO, Ag₂MoO₄ and NiCr₂O₄) on the worn surface, the friction coefficient and the wear rate were greatly reduced, and the wear mechanism was mainly oxidation wear and adhesive wear.