Oxidation Behavior of Sputtered DD98M Nanocrystalline Coating at 1000 A°C

The isothermal and cyclic oxidation behavior of single-crystal superalloy DD98M and its sputtered nanocrystalline coating at 1000 A degrees C in air were investigated. The as-sputtered DD98M coating possessed a columnar structure in the deposition direction and the width of the columnar grains ranged from 30 to 100 nm. The coating was composed solely of gamma phase. Complex oxides formed on the surface of DD98M alloy when oxidized at 1000 A degrees C, which stratified, cracked, and spalled. The faceted-like AlN and the particle-like and strip-like TiN formed in the alloy. The oxides that formed on the sputtered nanocrystalline coating mainly consisted of alpha-Al2O3, which was dense, adherent, and continuous. After 500-h oxidation, the mass gain of the DD98M nanocrystalline coating was only one half that of DD98M. Nanocrystalline coating with the same composition as DD98M alloy promoted the selective oxidation of aluminum at the initial stage of the oxidation and the maintenance of an exclusive alumina external scale after prolonged oxidation, and enhanced the adhesion of the oxide scale, thus improving the oxidation resistance of the DD98M alloy. Neither an apparent interdiffusion zone nor an SRZ was observed in the DD98M superalloy after oxidation. STEM image and the corresponding elemental X-ray mappings of DD98M-sputtered nanocrystalline coating after 1-h oxidation at 1000 A degrees C. From the STEM and elemental X-ray mapping results, the oxide scale formed on the sputtered coating after 1-h oxidation was merely Al2O3, which indicates that nanocrystallization can promote selective oxidation and rapid formation of the Al2O3 at initial stage. It was worth to notice that oxide pegs formed due to the oxidation along the columnar grain boundaries, which anchor the scale to the substrate, enhancing the bonding strength of the scale.