Effect of tensile load on high temperature oxidation of CoNiCrAlY coating

Oxide growth plays a critical role in the failure mechanisms of thermal barrier coating (TBC) systems. This study is aimed to investigate the chemo-mechanical coupling oxidation behavior of CoNiCrAlY coating which is widely used as bond coat in TBC systems. Oxidation experiments at 1000 degrees C were conducted on both load-free and tension-loaded specimens. Microscopic examination was also performed using scanning electron microscopy and Raman spectroscopy. The experimental results show that the oxidation of the CoNiCrAlY coating was accelerated by the applied tensile load. In the oxide scale of the load-free specimen, both alpha-alumina and theta-alumina were present when the oxidation duration was short (<= 15 h), but the theta-alumina transformed to alpha-alumina after 20-h oxidation. By contrast, in the tension-loaded specimen, mixed alpha-alumina and theta-alumina were found even after oxidation for 20-60 h. This phenomenon suggests that the tensile load can cause delay in the phase transformation from fast-growing theta-alumina to slow-growing alpha-alumina. A new mechanism based on the theoretically derived effect of mechanical work on the free energy for alumina phase transformation has been proposed to explain the experimental observations.