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) oriented beta-NiAl single crystals, with and without Pt doping (50 nm), were followed at 973 K using LEED and AES. Upon oxidation, Al2O3 is selectively formed. On NiAl (110) a complex surface structure is observed which according to Jager et al. [13] is a duplex oxide structure similar to alpha- or gamma-Al2O3. On the (100) face, submicroscopic facetting 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, the phenomena observed for the Pt-free sample are not significantly changed. However the following differences exist: 1. the formation of the Al2O3 oxide is more difficult for the (110) and (111) samples but Pt doping facilitates facetting and oxide formation on the (100) oriented sample; 2. sputtering of the oxide layers on Pt-doped samples is retarded in all cases, a very stable and dense oxide seems to be formed which adheres well; 3. Pt seems to hinder the Al diffusion to the unoxidized surface of the samples.