Understanding of failure mechanisms of the oxide scales formed on nanocrystalline coatings with different Al content during cyclic oxidation

Cyclic oxidation behaviors of three nanocrystalline coatings with phase constitutions of γ/γʹ, single γʹ, and γʹ/β, respectively, tailored by adjusting the Al content, are investigated. The oxide scale on γ/γʹ coating exhibits serious surface rumpling accompanied by frequently cracking at the crests of undulations after 500 cycles at 1100°C. The premature degradation of the coating in microstructure (i.e. γ′ phase dissolution and concurrent grain coarsening), arising from high oxidation rate and low original Al content, not only increases its CTE to introduce larger thermal stress, but also decreases its hardness and elastic modulus, making it susceptible to rumpling and further inducing cracking of oxide scale. Comparatively, benefited from lower oxidation rate and higher Al content, the degradation of γʹ and γʹ/β coatings is less significant. They are subjected to lower thermal stress but possess higher deformation resistance during cycling. However, local spallation of the oxide scale occurs on γʹ/β coating since it is insusceptible to rumpling and thus the thermal stress is difficult to release in time by plastic deformation. The γʹ coating outperforms its counterparts of γ/γʹ and γʹ/β coatings, presenting low oxidation rate, high resistance to scale cracking and spallation, meanwhile, limited elements-interdiffusion with the underlying substrate.