Influence of the chemical surface structure on the nanoscale friction in plasma nitrided and post-oxidized ferrous alloy

Full text: In the friction phenomenon at the nanoscale we encounter a correlation between theory and practice not established yet [1] Therefore, to generate knowledge in nanotribology area can assist in define parameters helping the selection of materials and/or surface treatment processes, aiming a increase on energy efficiency related to reduction of friction. To that end, by a chemical approach, some authors have established a qualitative correlation between the friction coefficient of several oxides and the ionic surface potential [2,3]. Furthermore, the influence of phonons has been incorporated in the effort to understand the friction phenomenon at nanoscale level [4]. The aim of this paper is to investigate the nanoscale friction behavior due to a fast plasma post-oxidation process on a previously plasma nitrided steel. The chemical structure of the outermost layers was determined by GD-OES. The crystalline phases and microstructure were analyzed by GA-XRD and SEM, respectively. Sliding friction tests were performed at different normal loads in order to probe the friction behavior between a diamond tip and the nitrided/oxidized steel surfaces, using a Micro Materials Nanotest-600 nanoindenter. Due to constant results the evolution of CoF as a function of the surface chemistry can be assessed without any influence from the change of the mechanical properties. These experimental results can be explained using a phononic model relying on the characteristic vibrational frequency of the most external layers of the material. The model shows a reasonable agreement with the experimental results, with a difference of 3% between theoretical and practical results. References [1] Bhushan, B. Nanotribology and nanomechanics. Berlin: Springer (2011). [2] Erdemir, A. Surf. and Coat. Technol., v. 200, n. 5, p. 1792-1796 (2005). [3] Erdemir, A.; Li, S.; Jin, Y. Intern. Jour. of Mol. Scien., v. 6, n. 6, p. 203-218 (2005). [4] Cannara, R. J. et al. Science 318, 780–783 (2007). (author)