Effect of Nb particles on the microstructure and tribological properties of laser cladding Ni60/Ag(Cu)/nanoNi/h-BN composite coatings

Nickel-based self-lubricating composite coatings (Ni60/Ag(Cu)/nanoNi/h-BN) with different amounts of Nb were manufactured by laser cladding on the surface of 42CrMo steel. The effects of carbide forming element Nb on the microstructure evolution and tribological properties of the composite coatings were investigated. The results show that Nb is easily precipitated at grain boundaries and reacts with carbon to form the in-situ synthesized phase NbC. The grains were refined and rich substructures appeared inside, which improved the strength and hardness of the coatings. The average microhardness of the coatings with 5.5 wt% Nb addition reaches 731.5H3/0.2, which is 192.1H3/0.2 higher than that of NCA0(0 wt%), and 3.17 times that of the substrate. Compared to NCA0, the average friction coefficient and wear rate of NCA3 decreased by 38.1% and 30.6%, respectively. The solid lubricant formed a lubricant film during wear, and the fine grains facilitated oxidation wear, resulting in the formation of oxidation film. The lubricant film, oxidation film and the increase in hardness together improved the tribological properties of the coating.