Oxidation Properties of NIAI Intermetallic Coatings Prepared by High Velocity Oxy-Fuel Thermal Spraying

A reaction-formed NiAl intermetallic compound (IMC) powder has been deposited as a coating onto low carbon steel test coupons by the High Velocity Oxy-Fuel (HVOF) process using both gaseous and liquid fuels. The microstructure of this coating has been examined using scanning electron microscopy and x-ray diffraction and was found to depend on spraying conditions. Oxidation tests on the coating in air, between the temperatures of 800°C-1200°C, revealed that an α-alumina (Al₂O₃) scale formed on the coating's surface. At 1200°C, a nickel spinel (NiO/NiAl₂O₄) and haematite (Fe₂O₃) phases were observed. Diffusion studies were performed to calculate an activation energy for iron ion diffusion in NiAl.