Durability of underaluminized thermal barrier coatings during exposure at high temperature

Abstract

In the present work the durability of the APS-TBC systems with bi-layer bond coat consisting of HTLA-CVD aluminide as bottom layer and APS-flashcoat of MCrAlY as upper layer was investigated. The additional factors potentially influencing TBC lifetime, namely substrate surface roughness prior to aluminizing and bond coat heat-treatment, were taken into account. The obtained results showed that the substrate surface roughness influences the IDZ morphology, however, IDZ disappears partly after heat-treatment and completely after high temperature exposure until the end of TBC life. The results of the lifetime measurement showed that the heat-treated TBC systems revealed up to three times longer lifetime (250 and 350 hot hours for systems with ground and grit-blasted substrate surface respectively) in comparison with non heat-treated ones (100 and 110 hot hours for systems with ground and grit-blasted substrate surface respectively). Moreover, TBC system with grit-blasted substrate revealed longer lifetime (350 hot hours) than system with ground substrate surface (250 hot hours). It was found that in both cases, the lifetime was limited by the Al-reservoir in the CVD layer and APS-flashcoat. The TBC failure was caused by the breakaway oxidation of depleted MCrAlY flashcoat particles that have led to the formation and propagation of cracks during cyclic exposure. Based on the results obtained in the present work, further elongation of the TBC lifetime is proposed by increasing of thickness of CVD aluminide layer.