Effect of the long-term heat treatment on the cyclic oxidation behavior of Fe-based amorphous/nanocrystalline coatings prepared by high-velocity arc spray process

- Fe-based nanocrystalline coatings were obtained by crystallization of amorphous coatings.
- The grain size of the annealed Fe-based coatings was significantly increased as the pre-annealing time prolonged.
- The cyclic oxidation resistance of the annealed coatings was unexpectedly improved as the increase in grain size.
- The parabolic rate constant kp value of the coating pre-annealed at 700 °C for 100 h was as low as 1.7 × 10− 11 g2 cm− 4 s− 1.
- The enhanced oxidation resistance of these coatings was attributed to the elimination of cracks and splat boundaries.

In this study, Fe-based nanocrystalline coatings were obtained by crystallization of amorphous coatings through heat treatment with different annealing time of 0.5 h, 5 h, 10 h and 100 h, respectively. Cyclic oxidation behaviours of the Fe-based amorphous coating as well as these annealed ones at 750 °C in still air were investigated and compared. The results showed that the grain size of the annealed Fe-based coatings was significantly increased as the pre-annealing time prolonged. However, the cyclic oxidation kinetic curves indicated that the cyclic oxidation resistance of the annealed Fe-based coatings was unexpectedly improved as the increase in grain size. The coating, which was annealed at 700 °C for 100 h, exhibited the lowest accumulated weight gain of 2.8 mg/cm2 and parabolic rate constant kp values of 1.7 × 10− 11 g2 cm− 4 s− 1, indicating the highest oxidation resistance. The enhanced oxidation resistance of the annealed coatings was mainly attributed to the elimination of cracks, pores and splat boundaries caused by the intersplat sintering during heat treatment. Cracks and splat boundaries were the primary diffusion paths for the oxygen, resulted in the different diffusion models of oxygen or metal atoms in Fe-based nanocrystalline coatings.