Transformation and oxidation of a sputtered low-expansion Ni-Cr-Al-Ti-Si bond coating for thermal barrier systems

A model low-expansion Ni-Cr-Al-Ti-Si bond coating was applied to nickel-base IN 100 and CMSX-4 substrates by magnetron sputtering. The single-phase coating consisted of Ni(Cr, Al, Ti, Si) phase with cubic B2 structure. Transformation of the as-sputtered coating at increasing temperatures was investigated by XRD and DSC. Essentially two-step transformation occurs during hearing. In the first step gamma-Ni forms due to diffusional loss of Ti and Si from the B2 phase which subsequently forms cubic Ni16Ti6Si7. After long-term exposure at high temperatures alpha-Cr precipitates within the coating. Oxidation of the Ni-Cr-Al-Ti-Si coating was investigated by isothermal exposure to air between 900 and 1100 degrees C up to 4000 h. Marked interdiffusion with the IN 100 substrate leads to higher oxidation rates of the coating compared with the CMSX-4 substrate. Oxide scale morphology and composition strongly depends on the temperature as shown for Ni-Cr-Al-Ti-Si coated IN 100. Mixed oxide scales were found containing alpha-Al2O3 + Theta-Al2O3 at 900 degrees C and CY-Al2O3 + TiO2 at 1000 and 1100 degrees C. However, on Ni-Cr-Al-Ti-Si coated CMSX-4 alpha-Al2O3 is formed exclusively even after 4000 h of exposure at 1000 degrees C, which imparts considerably reduced mass gain. Oxide scales remain adherent to the bond coating even during cyclic thermal loading. (C) 1997 Elsevier Science S.A.