Numerical simulation study on the formation mechanism of hydroxyapatite-silver gradient bioactive ceramic coatings under wide-band laser cladding

•The simulation model of coating by wide-band laser cladding is established.•The formation and evolution process of the coating molten pool is studied.•The heat transfer mechanism and the melt flow behaviors are researched.•The microstructure evolution mechanism of the coating is identified.•The formation process and mechanisms of the coating are revealed. In this paper, a numerical simulation model is established for the three-layer structure HA-Ag gradient ceramic coatings under wide-band laser cladding to reveal the coating formation process and correlated physical mechanisms. Based on the proposed model, the formation and evolution process of the molten pool is studied, as well as the solidification front characteristics of the molten pool. The dominant heat transfer mode of the molten pool is heat conduction when laser cladding the barrier layer and the transition layer, while the main heat transfer mode of the molten pool is heat convection when laser cladding the bioactive layer. The driving force affecting the melt flow in the molten pool of each layer is dominant the surface tension of the melt. The temperature gradient (G) and solidification rate (R) at the solidification front, and the variations of the solidification behavior mainly depend on the change of solidification rate. The simulation results can be used to guide the process parameters optimization of laser cladding three-layer structure gradient HA-Ag bioactive ceramic coating.