Effect of Gradient Transition Layer on the Cracking Behavior of Ni60B (NiCrBSi) Coatings by Laser Cladding

Laser cladding technology is an effective method for producing wear-resistant coatings on damaged substrates, improving both wear and corrosion resistance, which extends the service life of components. However, the fabrication of hard and brittle materials is highly susceptible to the problem of cracking. Using gradient transition layers is an effective strategy to mitigate the challenge of achieving crack-free laser-melted wear-resistant coatings. This study presents the cracking issue of laser cladding Ni60B (NiCrBSi) coatings on 38CrMoAl (18CrNiMo7-6) steel by designing a gradient transition layer infused with varying amounts of Ni powder. We examine how different levels of Ni doping in the transition layer influence the fabrication of the Ni60B coating. The results indicate that the cracking mechanism of Ni60B is primarily due to the brittleness and hardness of the fusion cladding layer, which can result in cold cracks under residual tensile stress. Increasing the nickel content in the transition layer reduces the difference in thermal expansion coefficients between the cladding layer and the substrate. Additionally, the nickel in the transition layer permeates the cladding layer due to the laser remelting effect. The physical phase within the cladding layer transitions from the initial CrB, M₇C₃, and γ-Ni solid solution to γ-Ni solid solution and Ni-B-Si eutectic, with a small amount of boride and carbide hard phases. As the nickel doping in the transition layer increases, the proportion of the toughness phase dominated by Ni elements significantly rises, leading to a decrease in the hardness of the fused cladding layer. However, the average hardness of the fusion cladding layer in crack-free samples was measured at 397.5 ± 5.7 HV_0.2, which is 91% higher than that of the substrate.