Investigation on Microstucutres and properties of ion nitriding layer of Al0.5CoCrFeNiTi0.25 HEAs cladding coating

To investigate the effect of ion nitriding on the microstructures and properties of high entropy alloy coatings, the Al_0.5CoCrFeNiTi_0.25 high entropy alloy coating was fabricated via laser cladding and subsequently subjected to ion nitriding treatment. A comparative analysis were conducted to investigate the phase compositions, microstructures and properties of cladding coatings and ion nitriding layer. Furthermore the formation mechanism of the nitriding layer was studied. XRD results revealed that Al_0.5CoCrFeNiTi_0.25 HEA coatings consisted of duplex FCC and BCC phases, whereas the nitriding layer was composed of high-nitrogen FCC solid solutions and nitrides such as AlN, CrN, Fe₄N and TiN. Following ion nitriding, the surface morphology of nitriding layer presented “cauliflower-like” nitride particles, which became denser with the extended nitriding time. Notably, the microhardness of nitriding layer was approximately four times that of HEA coating, and the wear resistance and the corrosion resistance of cladding coating were improved significantly after ion nitriding. During ion nitriding process, N atoms reacted with the alloying elements to form nitrides, which preferentially deposited at grain boundaries and BCC phases and grew horizontally. As the deposition layer spread across the entire surface, the nitriding mechanism shifted from reaction -controlled to diffusion-controlled, with N atoms diffusing inward to form a diffusion layer with high nitrogen solid solutions. With the prolong of nitriding time from 1h to 12h, the thickness of the diffusion layer increased from 11.77 μm to 96.07 μm.