Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture

In this work, the friction and wear behavior of CrN-coating deposited on steel substrate was investigated under dry and lubricated sliding conditions. The surface of quenched and tempered AISI 4140 steel was coated by a combined treatment of plasma-nitriding and physical vapor deposited CrN-coating and submitted to unidirectional sliding wear tests using a commercial tribometer with ball-on-disc contact geometry. CrN-coated discs of ∅40 mm×5 mm were run against Si3N4 ∅6.35 mm ball counterbodies. All the tests were conducted in the same sliding conditions with a normal load of 10 N, tangential velocity of 0.01 m/s, in dry, ethanol fuel and ethanol–oil mixture, at room temperature of 25 °C, in air with 50% relative humidity. Reference sliding tests were also conducted with both uncoated AISI 4140 steel and plasma-nitrided steel. The elementary composition of CrN-coating deposited by DC magnetron sputtering was determined by Rutherford backscattering spectrometry (RBS) that was also used to estimate coating thickness. The crystalline structure of nitride-layer and CrN-coating were determined by glazing angle X-ray diffraction analysis (GAXRD). The hardness of the nitriding layer and CrN-coating were accessed by nanoindentation measurements. The worn surfaces were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), glow discharge optical emission spectroscopy (GD-OES), and Raman spectroscopy, which allowed elucidating the wear mechanisms and the chemical structure of tribofilms formed during the sliding contact. Compared with the dry sliding conditions, there was a significant decrease in the levels of the wear rate and friction coefficient of the uncoated, plasma-nitriding and CrN-coated samples run in lubricated conditions, which was attributed to the physical and chemical reaction of ethanol and oil lubricants with the sliding surfaces, forming protective tribofilms with lubricity and anti-wear properties. In these conditions, the CrN-coated samples showed the best tribological behavior.