Effect of laser-induced nanocrystallisation on the properties of electroless Ni-P/Ni-W-P duplex coatings



This paper presents a comparative study of nanocrystallisation and the wear resistance of electroless plated Ni-P/Ni-W-P duplex coatings with a single Ni-W-P coating before and after high- power diode laser treatment. Effects of the laser operating parameters on microstructures, in terms of crystallisation, porosity formation, phase transformation and grain growth, were investigated using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and quantitatively X-ray Diffraction (XRD). Microhardness and wear behaviour of the coatings before and after laser treatment were evaluated by measurement of coating surface and cross-section hardness as well as un-lubricated friction and wear tests. The results revealed that in the case of laser treatment, the Ni-P/Ni-W-P duplex coatings offered better wear resistance than the single Ni-W-P coating, while the as-plated, single Ni-W-P coating showed better wear resistance than the Ni-P/Ni-W-P duplex coatings. Adhesive wear mechanism prevails in the laser-treated coatings when subjected to wear test against hardened steel material. The effects of microstructural characteristics in the coatings, in particularly the grain size of Ni3P phase and the degree of crystallisation, on the adhesive wear behaviour have been investigated and found to be dominant besides the effect of hardness.