Concentration depth-profile reconstruction from angle-resolved XPS data using the maximum entropy method: characterization of surface film formed on Ni-18P alloy

A knowledge of the depth concentration profile of materials is very important for research and technological development. Methods reported in the literature are often destructive and/or based on severe approximations; their application is limited to relatively simple profiles (number of components ≤ 3). A reconstructed depth-profile should be consistent with the ARXPS data. However, transformation of XPS intensities vs. emission angle into concentrations vs. depth data is an ill posed mathematical problem. The main goal of this work was thus to develop a new, iterative algorithm based on the maximum entropy method (MEM) that allows to solve this problem. The new algorithm involves an iterative procedure for calculating the IMFP values taking into account the actual depth-profile of the sample surface under investigation. In a first phase, numerical experiments were performed on a large series of computer generated, ideal and error containing, ARXPS data from model depth-profiles with up to four layers and up to eight components. In a second phase, the tested algorithm was implemented using real ARXPS data obtained from technologically important, highly corrosion and wear resistant Ni-P alloys. In conclusion, the new algorithm proved to be at least as accurate as depth-profiling methods reported in the literature but more powerful than any of the existing algorithms as depth profiles with up to eight components can be reconstructed from ARXPS data. Combining information on the chemical state of the different phosphorus compounds in the layered interface with the reconstructed in-depth profile it can be concluded that the high corrosion and wear resistance of Ni-P alloys is due to a thin, self-repairing nickel-(poly)phosphate film formed on a strongly phosphorus enriched surface.