Synthesis and tribological behaviour of electroless Ni-P-WS2 composite coatings

WS2 particles, as solid lubricant, were incorporated in electroless nickel-phosphorous to produce a new composite coating with enhanced tribological properties. Coating deposition was carried out in a laboratory electroless bath followed by heat treatment at 400 °C. The process included pretreatment of steel substrate by grinding and polishing to reach an appropriate roughness, surface activation by degreasing and acid cleaning and preparation of WS2 powder before addition to the electroless bath. Evaluation of friction coefficient and wear resistance was performed by a pin-on-disc tribotester at room temperature. Characterization of the surface and interface was achieved by scanning electron microscopy, microhardness measurements, X-ray diffraction and energy dispersive microanalysis to identify the chemical composition and phases in the composite coatings. Experimental results indicated that use of WS2 particles in the coating decreased the hardness and modified the wear behaviour of conventional Ni-P coating; the coefficient of friction was reduced from above 0.6 to about 0.1. The percentage of solid lubricant particles added and their distribution in the coating structure were found to be the key factors in improving the frictional behaviour of this composite coating.

► Incorporation of WS2 solid lubricant particles in Ni-P coating reduced the hardness. ► Use of WS2 particles in the coating modified the wear behaviour of Ni-P coating. ► WS2 particles as solid lubricant reduced the friction coefficient significantly. ► The dominant wear mechanisms for the coatings are adhesive and abrasive.