The impact of Nb on the microstructure and mechanical properties of Stellite 21 alloy coatings

With the increasing speed of high-speed train, the performance requirements of brake disc friction surface become more and more strict, so it is necessary to study the performance of brake disc friction surface of high-speed train. This study successfully prepared Stellite 21 alloy coatings by laser cladding with the addition of Nb elements. EDS, XRD, SEM, EBSD and mechanical properties test methods were used to investigate the effect of Nb on the microstructure, microhardness, micro-shear properties and wear resistance of Stellite 21 alloy coating. The results indicate that the addition of Nb prevents the continuous growth of columnar grains in the Stellite 21 alloy coating, resulting in more short and small columnar grains, dendritic grains, and cellular grains. The Stellite 21 alloy coating is mainly composed of γ-Co, with a small amount of M₂₃C₆ carbides precipitated. The addition of Nb induces martensitic phase transformation (γ-Co → ε-Co) and the precipitation of Laves strengthening phase in the coating, which increases with increasing Nb content. Particularly, when the Nb content reaches 5 %, the ε-Co content reaches approximately 5.9 %. The hardness and shear strength of the coating increase with the increasing of Nb addition, which is mainly attributed to the solid solution strengthening and precipitation strengthening of the Stellite 21 alloy coating caused by Nb addition, which increases the hardness of the coating by about 21.4 %, thereby improving the wear resistance of the coating. However, it also leads to a decrease in the plasticity of the coating. Nevertheless, compared to the alloy coating without Nb, the shear strength and wear resistance of the coating are increased by approximately 31.5 % and 49 %, respectively. The study demonstrates that the addition of Nb plays an important role in improving the shear performance, hardness, and wear resistance of Stellite21 alloy coatings.