AES AND LEED STUDIES OF THE INITIAL-STAGES OF OXIDATION OF BETA-NIAL WITH AND WITHOUT PLATINUM DOPING

The initial stages of oxidation of clean (110), (100), and (111) orien ted beta-NiAl single crystals, with and without Pt doping (50 nm), wer e followed at 973 K using LEED and AES. Upon oxidation, Al2O3 is selec tively formed. On NiAl (110) a complex surface structure is observed w hich according to Jager et al. [13] is a duplex oxide structure simila r to alpha- or gamma-Al2O3. On the (100) face, submicroscopic facettin g occurs during oxidation. For the (111) oriented sample the hexagonal structure remains and only an increased background intensity for the LEED pattern is observed for the oxidized surface. After Pt doping, th e phenomena observed for the Pt-free sample are not significantly chan ged. However the following differences exist: 1. the formation of the Al2O3 oxide is more difficult for the (110) and (111) samples but Pt d oping facilitates facetting and oxide formation on the (100) oriented sample; 2. sputtering of the oxide layers on Pt-doped samples is retar ded in all cases, a very stable and dense oxide seems to be formed whi ch adheres well; 3. Pt seems to hinder the Al diffusion to the unoxidi zed surface of the samples.