Microstructure and phase transformation of WC/Ni60B laser cladding coatings during dry sliding wear

WC/Ni60B coatings were deposited by laser cladding with conventional WC size (WCm) or nanoscale WC (WCn). The components of the coatings are respectively 30%WCn/Ni60B, 15%WCm–15%WCn/Ni60B and 30%WCm/Ni60B so as to compare the effect of WCn content. Dry sliding wear tests of the coatings were conducted with a block on ring apparatus. The microstructure of the coatings and the phase transformation during wear were observed and analyzed with XRD, scanning electron microscopy (SEM) and TEM. The microhardness of the coatings before and after the wear was measured. The microstructure of the original coatings consists of γ-Ni based solid solution, borides, dendrite and agglomerated WC particles. During the sliding wear, only element diffusion in the worn zone occurs under lower load, however a new phase precipitation was observed under higher load due to both the actions of frictional force and heating. The new fine precipitated phase WC or WC1−x, with the size range of tens to hundreds nanometer, makes the hardness increase of the coatings and the amount of the precipitated phase increases with the WCn content increasing in the coatings.