Effect of La₂O₃ addition on microstructure and hardness of induction heating iron-based composite coatings

Using 45# steels as the base materials, the Fe–Ni–Cr–La₂O₃ composite coatings were deposited on the 45# steel surface by medium and high-frequency induction heating. The effects of La₂O₃ addition content (mass fraction) on the microstructure, element distribution, surface hardness, and cross-sectional microhardness of the composite coatings were investigated by optical microscopes, scanning electron microscopes, energy dispersive spectrometers, microhardness testers, and Rockwell hardness testers. The optimal La₂O₃ addition content for preparing the composite coatings was determined. The results show that the addition of La₂O₃ is beneficial to improve the element diffusion between the internal components of the composite coatings and between the coatings and the substrate, and enhance the surface hardness and microhardness of the coatings. When the mass fraction of La₂O₃ is 2%, the internal defects of the Fe–Ni–Cr–La₂O₃ composite coatings are the fewest. The element inter-diffusion between the coatings and the substrate is obvious, the dendritic structure at the interface is refined, and the component segregation within the coating grains and at the grain boundaries is the least. The surface hardness of the coatings is HRC 60.2, which is 7.3% higher than that of the coatings without adding La₂O₃, and the microhardness of the coatings in longitudinal section is the highest.