ENHANCED ADHERENCE OF AL2O3 COATING LAYER BY CONCURRENT S SUPPRESSION AND TIC SURFACE PRECIPITATION

Our previous works have demonstrated that the adherence of Al2O3 coating layer to alloys is decreased by interfacial segregation of S but increased by interfacial precipitation of TiC. In this work we tried to improve the adherence of the Al2O3 coating layer by simultaneous suppression of S segregation and promotion of TiC precipitation. For this purpose surface analysis by AES was made at 1 100 K on Ti containing alloys doped with Ce or Y2O3 for suppressing S segregation, and other reference alloys. The adherence was evaluated through cyclic oxidation test at 1 100 K in pure O2. In accord with the expectation, the Al2O3 coating layer was the most protective on MA956-0.52Y2O3 alloy which generates TiC surface precipitation and suppresses S surface segregation very well. In order to elucidate the Al2O3/alloy interfacial phenomena, some alloys partially coated with an extremely thin Al2O3 layer were heated in an apparatus of XPS. Chemical shift of the peaks of O 1s and Al 2p by heating was observed on an alloy on which the surface segregation of S takes place extensively, but not on alloys on which S segregation is slight. This result, along with the oxidation result, affords an important evidence of our previous conclusion that Al2O3/alloy bonding is largely weakened by S segregated at the interface.