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Abstract: We have investigated the fully-oxidized surface that forms on (Pt,Ni)3Al(111) at temperatures ranging from 300 to 1000K and at oxygen pressures of ca. 10−6 to 10−7 Torr, using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Based on X-ray photoelectron spectroscopy data that were published previously, oxidation temperatures below 700K at these pressures produces a mixture of surface Al2O3 and NiO, with NiO being the predominant oxide. At 800–1000K, pure Al2O3 exists. In this work, STM data from a sample containing 10at% Pt show that oxidation causes an increase in roughness, relative to the clean surface. The apparent roughness correlates directly with NiO content, both of which reach a maximum at an oxidation temperature of 500K. The oxide surface is smoothest when it consists of pure Al2O3, i.e. after oxidation at 800–1000K. In terms of crystallinity, LEED data show that the Al2O3 which forms at 1000K is ordered, but its structure on the Pt-containing samples is different than reported previously for the Pt-free surface. This is true despite the structure of the clean surface being unperturbed by Pt, based on STM and LEED. The different structure of the oxide probably relates to the fact that the oxide is also thinner in the presence of Pt. The change in oxide structure undoubtedly correlates with a change in stability and adhesion of the oxide, both of which are of paramount importance in industrial applications. [Copyright &y& Elsevier]

Abstract: We have studied the early-stage of oxide formation on (111)-oriented (Pt,Ni)3Al single crystals in air. From X-ray photoelectron spectroscopy (XPS), the predominant surface oxide changed from NiO to Al2O3, as oxidation temperature increased from 900 to 1300K. Some NiAl2O4 (spinel) also formed at the higher temperatures. Under conditions where NiO and/or NiAl2O4 were present, it resided atop a layer of aluminum oxide, mixed in some cases with metallic Ni. By comparing samples that contained 0, 10 and 20at% Pt in the bulk, we found that the effects of Pt were to (1) promote the preferential formation of aluminum oxide over nickel oxide(s) in the top layer at all temperatures studied, (2) suppress the amount of metallic Ni mixed with Al2O3 in the underlying oxide region, (3) reduce the total oxide layer thickness, and (4) sharpen the interface between the oxide and the metallic alloy. [Copyright &y& Elsevier]

Abstract: Experimental evidence for surface segregation of Pt at (111) surfaces of ternary (Pt, Ni)3Al alloys is presented, based upon Auger electron spectroscopy, low energy ion scattering, and angle-resolved X-ray photoelectron spectroscopy. Density functional calculations in the dilute limit confirm that Pt segregation is energetically favored. [Copyright &y& Elsevier]

Abstract: The fully-oxidized surface that forms on (111) oriented Ni3Al single crystals, with and without Pt addition, at 300–900K under oxygen pressures of ca. 10−7 Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al x O y . At high-temperature (i.e., 700–800K), NiO is replaced by a thick layer of Al x O y . By comparing samples that contain 0, 10 and 20at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al x O y ; and (2) shift the establishment of the thick Al x O y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface. [Copyright &y& Elsevier]