Effect of Brazing Temperature on Ni-Based Tungsten-Carbide Coatings Prepared by Vacuum Brazing Using Flexible Metal Cloth as Brazing Filler Metal

Abstract:

A flexible Ni-based tungsten-carbide metal cloth, which was prepared by mixing WC powder, NiCrBSi powder and the binder, was coated on the surface of Q235 steel by vacuum brazing. This flexible metal cloth can be arbitrarily cut and folded, enabling the preparation of coatings with controllable thickness on the surfaces of curved and irregularly shaped structural components. The effects of brazing temperature on the microstructure, shear strength, microhardness and wear resistance of the Ni-based WC coatings were explored. The results show that increasing brazing temperature does not change the microstructure composition of the coating. The interface between Ni-based tungsten-carbide coating and steel substrate is mainly composed of Fe solid solution, Ni₃B and Ni₃Si compounds, while the brazing seam zone is mainly composed of Ni solid solution, WC, W₂C, CrB and Cr₇C₃ compounds. With the increase in brazing temperature, the porosity and average pore area of the coatings gradually decrease, while the density gradually increases. Consequently, the microhardness and wear resistance of the coating are enhanced. This is mainly because after the flexible Ni-based tungsten-carbide metal cloth melts, as the brazing temperature increases, the ability of the molten NiCrBSi alloy to encapsulate WC particles and fill the gaps between them is enhanced. This reduces the porosity in the coatings, improves the density, thus endowing the coatings with higher hardness and wear resistance. The bonding strength between Ni-based tungsten-carbide coating and steel substrate increases first and then decreases with the increase in brazing temperature. When the brazing temperature is 1080 ℃, the shear strength of the coating reaches the maximum value of 336.5 MPa. This is because the thickness of the compound layer at coating/substrate interface increases from 4.7 μm to 9.2 μm when the brazing temperature increases to 1080 ℃, and the compound layer with appropriate thickness increases the bonding strength between the coating and the substrate. However, the thickness of the interface compound layer increases to 13.9 μm when the brazing temperature increases to 1140 ℃, and the compound layer with the excessive thickness leads to a decrease in bonding strength of the coating.