Industrial Alloys Studied by Surface Sensitive Techniques

Abstract: This thesis reports on surface studies of industrial materials whose importance for society manifests in the vast range of applications. In industrial materials alloying is performed in order to improve the parent material's physical and mechanical properties such as strength, corrosion and wear resistance, as well as high temperature performance in comparison to the pure metal.Even though metals like copper, tin, and zinc have been alloyed since approximately 2500 BCE, current research projects still try to unravel the complex interactions between the single alloying elements on an atomic scale. This thesis focuses on two groups of metallic alloys: aluminum alloys and steels. For both alloy groups, the properties of their protective surface oxides are of major importance as they determine the material's performance with respect to corrosion, erosion, wear, joining, and coating. Therefore, a surface science approach was employed to study the chemical composition, thickness and distribution of phases and particles at the oxide surfaces. Contemporary research in this area is characterized by attempts to bridge the gap between classical model systems in highly controlled environments and industrial complex alloys in experimental conditions mimicking their working environment.Here, the material gap is bridged by transitioning from single crystals to industrial alloy standards, see Papers I and IV. Custom made composite aluminum alloys made for brazing applications, Papers II and III, and multiphase steel, Paper V, are investigated in this thesis. Simultaneously to the material gap, the pressure gap is addressed by exposing the materials to more realistic conditions using ambient pressure X-ray photoelectron spectroscopy, see Papers III and IV. This technique allows for measurements while the sample is exposed to different gases up to the lower mbar regime. By comparing X-ray photoelectron spectroscopy with standard UHV measurements major differences in the resulting surface oxide composition are observed. The thickness of the native oxide films of the different samples is determined by X-ray reflectivity measurements performed at different experimental conditions ranging from UHV to air and water environments, Papers I and IV. X-ray photoelectron emission microscopy and low-energy electron microscopy was used to follow the surface development during heating of aluminum and steel samples on a sub-micron length scale, Papers II, IV, and V. The microscopic imaging allows for the identification of the chemical state of the alloying elements and their lateral distribution in the surface layer.