Influence of Laser Energy Density on Interfacial Diffusion Bonding and Surface Density of Chromium Coating by Multi-Arc Ion Plating on Zirconium Alloy

Abstract

The chromium coating prepared by multi-arc ion plating on the surface of zirconium alloy is easy to fall off under extremely harsh conditions due to the defects of larger particles, pores and weak adhesion between the coating and the zirconium alloy substrate. Here we apply a new composite process for the fabrication of Cr coating by laser melting multi-arc ion plating technology. The numerical model of temperature field laser melting treatment was established, and based on the analysis of the above temperature field of laser melting treatment, the laser with an energy density of 9 × 104 J/cm2–18 × 104 J/cm2 was selected as the heat source. Laser energy density has a great influence on the diffusion behavior of Cr at the coating-substrate interface. When the energy density of laser is 9 × 104 J/cm2, the laser heat source cannot provide enough energy to melt the surface of the coating, however, the Cr element diffuses slightly at the membrane base interface, forming a Cr diffusion zone of about 4 μm. When the energy density of the laser increases to 13.5 × 104 J/cm2, the Zr element of the substrate diffused to the whole Cr coating, and the original Cr coating disappeared, forming a Zr-Cr diffusion and fusion zone of 30 μm. As the laser preparation process of the coating is a mixed sintering process of liquid phase sintering and solid phase sintering, the coating surface forms a dense zone and a loose zone respectively, resulting in the increase of porosity and particle rate. With the laser energy density increasing gradually, the Zr-Cr fusion zone is formed, and the coating surface particles were gradually refined and the pores were significantly reduced. With the further increase of laser energy density of 18 × 104 J/cm2, the Zr-Cr fusion zone was extended to 60 μm, and the porosity and particle rate were reduced to 0.3% and 0.4% respectively. In addition, the original stratification cracking of the film base at the fracture of the tensile specimen disappears, and the protrusion of the interface is formed. Coatings with dimple fractures were found, which indicates that the coating exhibited ductility and interfacial metallurgical bonding.