XPS study of carboxylic acid layers on oxidized metals with reference to particulate materials

Low-molecular-weight organic additives such as stearic acid are commonly used as surface additives in powder injection moulding (PIM). It is therefore important to know how the additives interact with the surface of the powder used. In this study, such interactions are studied by means of controlled adsorption of carboxylic acids on the oxides of interest. The oxides are prepared by oxidation of flat samples of Fe, Cr, Mn and Si. Surface chemical characterization is done by means of XPS, the main approach on flat samples being a comparison of angle-resolved analysis and the use of the Tougaard nanostructure analysis technique. Taking advantage of this comparison, the Tougaard method is then applied in the evaluation of XPS analyses of stainless-steel powder with adsorbed stearic acid. In addition, time-of-flight SIMS analysis is used to verify the adsorption of stearic acid on the powder surface. It is shown that Tougaard nanostructure analysis can be used for determining the thickness of an organic layer on particulate material. The layer thickness of adsorbed stearic acid was estimated to be similar to20 Angstrom, corresponding to monolayer adsorption. Time-of-flight SIMS analysis verified the adsorption of stearic acid on the powder surface. From the XPS analysis of flat samples it was determined that the use of the metal/oxide universal cross-section in Tougaard nanostructure analysis best described the increased background due to adsorption of carboxylic acids, and that information about molecular orientation could be gained. Copyright (C) 2003 John Wiley Sons, Ltd.