Different oxidation protection mechanisms of HAPC silicide coating on niobium alloy over a large temperature range

Oxidation behaviors of halide activated pack cementation (HAPC) silicide coatings on niobium alloy were elaborately studied by experimental and theoretical analysis. The HAPC process was performed with a commonly applied powder pack of Si, NH₄F, and Al₂O₃ filler. Adsorptive Al₂O₃ particles from the pack from HAPC process played a positive role against oxidation at 700 °C (the so-called pest temperature). Above this temperature, the passion behavior rather than pulverization gradually appeared by the occurrence of seal-healing SiO₂. However, the seal-healing effect was meanwhile weakened by the generation of the volatilized Si-O (g) species when the temperature rose up to the eutectic point of Nb₂O₅-SiO₂ system. Continuously temperature-elevated (1500–1700 °C) oxidation overwhelmingly destroyed the surface protective scale by the depletion of melting Nb₂O₅ and the volatilization of SiO₂ (g), leading to a catastrophic failure of structural integrity for the coated samples.