Evaluation of the thickness and roughness of homogeneous surface layers on spherical and irregular powder particles

The present study describes the development of a theoretical model for estimating the thickness of homogeneous surface layers on spherical and irregular shaped powder particles using the XPS depth profiling. As opposed to flat surfaces, such an approach for substrates of specific geometry is not straightforward. One needs to consider those geometrical factors associated with the experimental setup and sample’s roughness, which impose an angle dependence on the photoelectrons peak intensity, X-ray flux and ion etch rate over the surface in question in order to evaluate it. The novelty of the current model lies in the introduction of geometrical freedom in connection to the experimental arrangement, which can be tailored to match the needs of contemporary instruments. The model was evaluated experimentally by analyzing the surface oxide layers on metal powder grades of different morphology. Complementary analytical techniques such as high resolution (HR) SEM and focused ion beam (FIB) were used in combination to further characterize the surface layers prior to the XPS investigations. The results reveal that the estimation of the oxide/metal interface from the measured relative metal intensity signal as function of etch depth using the model is in extremely good agreement with the corresponding values from direct measurement of HR SEM on FIB cross-sectioned powder samples. The model deviates slightly only for the irregular shaped powder, which can be regarded as means for quantification of surface roughness of the material. The model is used as a basis for a computer software that estimates the thickness of surface layers for powdered materials.