Impact of Si concentration on microstructure and high temperature oxidation behavior of IN718 coating by plasma cladding: An experimental and first principle study

Abstract

To improve the properties of IN718 coatings, Si-modified IN718 coatings with an equiaxed structure and enhanced high-temperature oxidation resistance were fabricated by plasma cladding in this paper. The addition of Si aggravated the precipitation of the Laves phase and Cr/Nb segregation. First-principles calculations revealed that the increase in repulsive forces between NbFe atoms and CrNi atoms is the essential reason for the promotion of Cr/Nb segregation. The hardness of IN718 increases with Si addition, mainly attributed to grain refinement and dislocation strengthening. High-temperature oxidation experiments revealed that the 3Si-IN718 coating generated a double oxide layer at 1000 °C, with Cr2O3 in the external layer and SiO2 in the internal layer; therefore, the oxidation resistance of the coating was greatly enhanced. Si addition significantly reduced the internal oxidation of Al/Nb and cracking of the oxide layer. However, Si has a modest contribution to oxidation resistance, and the oxide layers were all Cr2O3 at 900 °C. According to the thermodynamic calculations, the elemental activity of Si only increased significantly above 900 °C, which may be the reason for the different results at different experimental temperatures. Calculations of surface adsorption illustrate that Si doping enhances the ability of the Ni-Fe-Cr surfaces to bind O atoms and raises the potential barrier for O atoms to diffuse into the sublayer, thereby enhancing the resistance of the IN718 coating to high-temperature oxidation.