Influence of the surface chemistry-structure relationship on the nanoscale friction of nitrided and post-oxidized iron

The relationship between the friction behavior and surface properties such as surface chemistry, morphology and phase distribution is not fully understood. In the case of surface treatments involving steels, the alloying elements introduce non-controllable variables in tribological experiments. We further advance this discussion by designing experiments where the friction behavior of a conical diamond tip sliding on different plasma nitrided and post-oxidized pure iron substrates was determined. Also, a detailed chemical and microstructural surface characterization using discharge optical emission spectroscopy, X-ray diffraction, and atom probe tomography was performed. Implementing composition profiling and localized elemental distribution analysis throughout the outermost layer, the formation of iron oxide with non-homogenous morphology was detected. Although the coefficient of friction decreases as a function of the post-oxidation time, the friction force is under a relative great dispersion at intermediate post-oxidation times. The dispersion of the friction force obtained from the sliding analyses can be understood as a combination of surface chemistry and different phases (structure) at the surface.