Effect of Vacuum Heat Treatment on the Structural-Phase State of Detonation-Sprayed NiCrAl Coatings

The effect of vacuum heat treatment on the structural-phase state of detonation-sprayed NiCrAl coatings was investigated. The samples were annealed in vacuum at temperatures of 800, 900, 1000, and 1100 °C with a holding time of 4 h. It was established that the as-sprayed coating is characterized predominantly by the presence of β-NiAl and a non-equilibrium lamellar structure formed under conditions of high-velocity detonation deposition and rapid particle cooling. At 800 °C, diffusion processes are activated, and a more complex multiphase state involving Ni–Al- and Cr-containing intermetallic phases is formed. Increasing the temperature to 900 °C promotes further structural and chemical homogenization of the coating while retaining β-NiAl as one of the main phases. At 1000 °C, along with continued homogenization, Cr2O3 and NiO appear, while interdiffusion between the coating and the Fe-containing substrate becomes more pronounced near the interface. After treatment at 1100 °C, the most profound phase transformation is observed, with formation of Al2O3, Cr2O3, NiO, and NiAl2O4, significant redistribution of the Ni–Al intermetallic constituent, and formation of a chemically heterogeneous transition zone with local microvoids. Increasing temperature is also accompanied by changes in surface morphology and a moderate decrease in roughness. Surface roughness showed a non-monotonic temperature dependence, with Ra values of 6, 5.3, 5.3, and 5.9 μm after heat treatment at 800, 900, 1000, and 1100 °C, respectively; the minimum roughness was obtained at 900 °C.