Effect of heat treatment on microstructure and high-temperature oxidation behavior of flame-sprayed Ni-5 wt.% Al coating on low carbon steel

Different thermal spray methods and size controls of splats have been tried to improve Ni–Al thermal spray coatings, which are usually expensive. In contrast, post-spray heat treatment is a simple and cost-effective alternative. In this study, heat treatment was applied to a flame-sprayed Ni-5 wt.% Al coating deposited on ST37 substrate for improving oxidation resistance. After spraying, the coatings were heat-treated under vacuum for 1 h at 950, 1050, and 1150 °C. Scanning electron microscopy was used to investigate the microstructure, while elemental and phase analysis were conducted by energy dispersive spectroscopy and X-ray diffraction. The results revealed that the coatings contained a Ni matrix with nonstoichiometric aluminum oxide at spalt boundaries and regions of Al/Ni mixed oxide. Heat-treated coatings demonstrated reduced porosity compared to as-sprayed one and established a stronger metallurgical bond with the substrate. High-temperature oxidation tests in air at 800 °C showed that mixed oxides of Al and Ni formed as scales on all coatings. The as-sprayed substrate experienced substantial oxidation, while heat-treated coatings provided improved protection despite internal NiO formation. The coating heat-treated at 1050 °C exhibited the lowest oxidation rate constant (3.58 × 10 −3 mg 2 /cm 4 h), minimal depth of internal NiO (∼220 μm), and reduced substrate oxidation, attributed to its lowest porosity (10.3 %) and enhanced adhesion from improved metallurgical bonding. • Coating was Ni–Al solid solution, Al/Ni mixed oxide and Al 2-x O 3 at splat boundaries. • Flame-sprayed coating remained the steel substrate unprotected during oxidation. • Heat treatment lowered porosity and improved adhesion by metallurgical bonding. • The heat-treated coating at 1050 °C showed the lowest SOR and internal NiO region. • Depletion of Al at splat boundaries led to a less protective scale in oxidation.