Laser cladding Ni-based alloy/nano-Ni encapsulated h-BN self-lubricating composite coatings

Nano-Ni encapsulated h-BN/Ni-based alloy (Ni60) self-lubricating composite coatings on a medium carbon steel were fabricated by laser cladding using two types of lasers: a 5 kW continuous wave (CW) CO2 laser and a 400 W pulsed Nd:YAG laser, respectively. A high-energy ball milling method was adopted to clad nano-Ni onto nano-h-BN with an aim to enhance the compatibility between the h-BN and the metal matrix during laser cladding processing. The microstructure, phase structure and wear properties of the self-lubricating composite coatings were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD), as well as dry sliding wear testing. The research indicated that laser cladding of the self-lubricating composite coatings demonstrates sound cladding layers free of cracks and porosities. It was found that a reaction between h-BN and Ni-based alloy occurred, which generated hard phase CrB and Ni3B leading to the increasing of the microhardness of the coatings by CO2 laser cladding, while laser molten pool suppressed h-BN floating up to upper regions of coating for lower temperature and rapid solidification by YAG laser cladding. The high energy ball milling of nano-Ni onto nano-h-BN significantly improved the interfacial compatibility between h-BN and Ni60 matrix. The friction coefficient of the laser-clad Ni60/nano-Ni encapsulated h-BN coating was reduced obviously.