High-Temperature Molten Vermiculite Degradation Mechanisms of APS AlCoCrFeNiTi High-Entropy Alloy Coatings at 1200 °C

Abstract

This study investigates the high-temperature corrosion mechanisms of a Ti-doped AlCoCrFeNi high-entropy alloy (HEA) coating system under severe molten vermiculite (VM) exposure. A CoNiCrAlY bond coat was deposited on Inconel 718 via HVOF, followed by an APS-sprayed Ti-doped HEA top layer. The APS CoNiCrAlY/AlCoCrFeNiTi coating system, consisting of an approximately 100 & micro;m CoNiCrAlY bond coat and an APS-sprayed HEA top coat, was exposed to molten vermiculite at 1200 degrees C for 1, 3, 5, 10, and 24 h, and the resulting microstructural evolution was investigated using SEM, XRD, and EDS. The coating exhibited strong resistance to molten silicate infiltration, with penetration largely confined to near-surface regions. This behavior may be associated with the formation of dense Al2O3 and Cr2O3 scales. In addition, diffusion-limiting mechanisms frequently reported in HEA systems may also contribute to the observed behavior. However, prolonged exposure led to selective dissolution of oxide layers and spinel phases due to chemical interaction with the silicate melt. In addition, thermal expansion mismatch and stress accumulation during cooling induced localized spallation. Overall, the Ti-doped HEA acts as an effective diffusion barrier, significantly delaying corrosion progression despite gradual thickness loss under severe conditions. These findings provide novel insight into the transition from protective oxidation to chemically driven degradation in HEA coatings under severe molten silicate attack, and highlight the unique potential of HEA coatings as dynamically self-renewing barriers against molten VM-related damage in gas turbine applications.