Dry sliding wear behavior of electroless NIP and NIP-Al2O3 composite coatings

Abstract

Nickel phosphorous (Ni-P) coating and co-deposited nickel phosphorous- Al2O3 (Ni-P- Al2O3) composite coatings reinforced with nanoparticles were prepared using electroless deposition process on a mild steel substrate. The coatings were heated treated at 400oC for 1 h. Reasonable increase in micro-hardness is obtained due to the incorporation of Al2O3 nanoparticles in Ni-P. Heat treatment of both coatings increased the hardness by 70.5% and 82.5% compared to as deposited conditions. Improvement in hardness is due to crystallization of amorphous nickel and the formation of nickel phosphide (Ni3P) during heat treatment. The wear behaviour of as deposited and heat treated Ni-P and Ni-P- Al2O3 coatings were investigated in dry sliding conditions. Under these conditions wear involves oxidation and adhesion in Ni-P coating and a combination of oxidation, adhesion and abrasion in Ni-P- Al2O3 coatings. The formation of an oxide film at the surface during the wear test increases wear resistance of Ni-P coating with increase in sliding distance. The incorporation of Al2O3 particles in the Ni-P matrix results in increased wear resistance compared to the Ni-P coating. On heat treated coatings the wear tests induced micro cracks and their propagation is observed as the test proceeds. Delamination and peeling of the coating is observed in Ni-P coating heat treated at 400 °C with a relatively low wear resistance. The incorporation of Al2O3 nano particles delays the propagation of micro cracks during the wear test. Thus, the maximum hardness and maximum wear resistance were obtained for Ni-P- Al2O3 heat treated at 400 °C.