Mechanical and microstructural characterization of thermally grown alumina on Kanthal APMT

Abstract

Thermally grown α-alumina layers on Kanthal APMT were characterized and mechanically tested. Different growth durations and post growth heat treatments were evaluated. Gaugeless ring pull tests were conducted with digital image correlation to measure the spallation strain of the oxide layer, providing tens of separate measurements per test. The strain at oxide spallation failure had significant spread with average failure occurring at 0.5 - 2 % strain in both tension and compression. It was also found that the failure strain was dependent on oxide thickness with the thicker 2 and 3 µm oxides tending to fail at lower strains. The thicker oxide layers also experienced more catastrophic failure, with larger spallation flakes leaving more of the base metal exposed. Transmission Kikuchi diffraction was used to characterize the microstructure of the oxide layer. All oxide layers showed columnar grains with grain boundaries extending across the entire thickness of the oxide.