Hydrophobic mechanochemical treatment of metallic surfaces: Wettability measurements as a means of assessing homogeneity

Hydrophobicity, lubrication and anticorrosion properties of steel substrates have been obtained by a deposition of thin film (i.e. by mechanochemical treatment) at room conditions. Stearic acid and paraffin were chosen as reactive molecules. Different abrasive powders were selected to generate active sites on the treated surfaces for adsorption of the reactive molecules and then, the results were compared. The surfaces were analyzed by reflection–absorption infrared spectroscopy (RAIRS). The results emphasize that, a thick layer of mixed stearic acid/paraffin was deposited onto the metallic surface after the treatment. After hexane rinsing we could only detect a very thin layer of oriented stearic acid molecules chemically adsorbed onto the metallic surface and which engages strong interactions with it. Whereas, RAIRS only provides molecular analysis, the XPS technique was complementary for discriminating the different surfaces. It was possible to show differences in thickness as well as in coverage according to the size and shape of abrasive particles. Furthermore, we could conclude that deposit layer is not uniform. Defects were always present and were dependent on abrasive powders used. Then wettability was assessed as a way to test the homogeneity of thin films generated by the mechanochemical treatment. In agreement with theoretical data, receding contact angle was very dependent on the defects in the deposited film. If holes or aggregates were increased in the deposit layer, the receding contact angle was decreased while advancing contact angles and equilibrium contact angles remained constant. A very important point for technological applications was that the homogeneity of the deposited film was governed by abrasive powder involved in mechanochemical treatment and contact angle values were a direct measurement of the homogeneity of surfaces generated by mechanochemical treatment.