钎焊温度对柔性镍基碳化钨金属布真空钎焊涂层组织及性能的影响

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

摘要:

将由WC粉末、NiCrBSi粉末和粘结剂构成的混合金属粉末制备成柔性镍基碳化钨金属布，借助真空钎焊在Q235钢表面制备镍基碳化钨涂层。这种柔性金属布可以任意裁剪、任意折叠，实现了一些曲面异形结构件表面厚度可控涂层的制备。本工作探讨了钎焊温度对镍基碳化钨涂层组织、界面结合强度、显微硬度和耐磨性的影响。结果表明：钎焊温度的升高不改变涂层微观组织构成，镍基碳化钨涂层/钢基体界面处主要由Fe的固溶体、Ni₃Si和Ni₃B构成，钎缝区主要是Ni的固溶体、WC、W₂C、CrB和Cr₇C₃等化合物构成。随着钎焊温度的升高，镍基碳化钨涂层的孔隙率逐渐降低，孔隙平均面积先减小后略有增大，涂层的致密度逐渐提高，从而涂层的硬度和耐磨性逐渐增加，这主要是因为柔性碳化钨金属布熔化后，随着钎焊温度的增加，熔融NiCrBSi金属对WC颗粒的包裹性和填充WC颗粒之间间隙的能力增强，减少了涂层中孔隙，提高了涂层的致密度，涂层具有较高的硬度和耐磨性。而镍基碳化钨涂层/钢基体结合强度随着钎焊温度的升高先增大后减小，当钎焊温度为1080 ℃时，涂层抗剪切强度最大，为336.5 MPa。这是因为随着钎焊温度升高至1080 ℃，涂层/基体界面化合物层厚度从4.7 μm增加至9.2 μm，厚度适宜的化合物增加了涂层与基体之间的结合强度。但钎焊温度进一步升高至1140 ℃，界面化合物层厚度增加至13.9 μm，过厚的化合物层导致涂层抗剪切强度降低。

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.