INFLUENCE OF THE PHASE COMPOSITIONS ON THE TRANSIENT-STAGE HIGH-TEMPERATURE OXIDATION BEHAVIOR OF AN NICOCRALY COATING MATERIAL

The oxidation behaviour of a commercial NiCoCrAlY coating material (H. C. Starck Inc.; Amperit 410) was investigated in the temperature range 850-1200-degrees-C. The measurements were made in air under isothermal and cyclic conditions. The oxidation process was quantified by thermogravimetry. The oxide compositions were characterized by energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectroscopy (SIMS). The corresponding oxide morphologies were studied by scanning electron microscopy (SEM). This study was focused on the oxide growth of a NiCoCrAlY alloy at temperatures between 850 and 1200-degrees-C up to oxidation periods of 25 h. Complementary information obtained from various surface-sensitive techniques are summarized in a simplified scheme of the oxide growth. The oxidation behaviour in the stated temperature range is explained by the different chromium and aluminium contents of the phases. Yttrium becomes effective at temperatures above 1000-degrees-C. The good protective properties of the NiCoCrAlY at temperatures above 1000-degrees-C and the fairly poor protection below 950-degrees-C are interpreted in terms of alumina formation and chromia formation as rate-determining factors.