Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture A.L. Bandeira a , R. Trentin a , C. Aguzzoli a , M.E.H. Maia da Costa c , A.F. Michels b , I.J.R. Baumvol a,b , M.C.M. Farias a,n , C.A. Figueroa a a Universidade de Caxias do Sul, RS, Brazil b Universidade Federal do Rio Grande do Sul, RS, Brazil c Pontifı ´cia Universidade Cato ´lica do Rio de Janeiro, RJ, Brazil article info Article history: Received 20 September 2012 Received in revised form 31 January 2013 Accepted 31 January 2013 Available online 8 February 2013 Keywords: Friction and wear CrN-coating Ethanol Oil abstract 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 Si 3 N 4 |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 1 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. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Bioethanol has been considered as an alternative fuel in automotive vehicles, because it is a renewable, biodegradable and environmentally-friendly fuel [1]. After the introduction of the so-called flex-fuel engines, the consumption of bioethanol has increased markedly. Therefore, the bioethanol is in contact with different metallic parts of the engine. However, few works were devoted to study the bioethanol effect on wear of materials, in particular those that make part of flex-fuel engines. Recently, more severe wear problems caused by fuel dilution, due to bioethanol addition, in the lube oil in cold-starts have been reported [2]. Indeed, the ethanol fuel shows limited lubricity and its acidity produces more severe wear and induces more scuffing earlier than low acidity fuels [3]. Therefore, the tribo- corrosion damage produced by the reaction of the hydrated ethanol fuel with the steel parts is another challenge to be overcome. Piston rings are one of the flex-fuel engine parts that have been affected by the higher severity of the loading contact and by scuffing wear. Plasma-assisted diffusion technologies are able to modify metallic surfaces by incorporation of light elements such as carbon, nitrogen, boron, and oxygen. Indeed, such surface engi- neering techniques are widely used in the automobile industry due to the diversified properties given on the surfaces of auto parts. In particular , the piston ring surfaces can be modified by plasma nitriding and physical vapor deposition (PVD) technolo- gies [4]. The type of thin film that is deposited by PVD determines Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear 0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.111 n Corresponding author. Tel.: þ55 54 3218 2764. E-mail addresses: mcmfarias@ucs.br, mariacmf@yahoo.com.br (M.C.M. Farias). Wear 301 (2013) 786–794