Tribological effect of iron oxide residual on the DLC ﬁlm surface under seawater and saline solutions R.P.C. Costa ⁎, F.R. Marciano, D.A. Lima-Oliveira, E.J. Corat, V.J. Trava-Airoldi Instituto Nacional de Pesquisas Espaciais-INPE, Avenida dos Astronautas 1758, C.P. 515, São José dos Campos, SP, Brazil abstract article info Article history: Received 7 May 2010 Accepted 12 January 2011 Available online 20 January 2011 Keywords: DLC Tribocorrosion Salinity effects Surface energy This paper discusses the seawater and saline solutions effects on the tribological behavior of diamond-like carbon (DLC) ﬁlms. The adsorption of Fe on DLC surface is one of the mechanisms that is believed to be the cause of the decrease in dispersive component of the surface energy and increase of the I D /I G ratio leading to low friction coefﬁcient and wear rate under corrosive environments. Tribological behaviors DLC ﬁlms were experimentally evaluated under corrosive environments by using steel ball and DLC coated steel ﬂat under rotational sliding conditions. The DLC ﬁlms were prepared on 440 stainless steel disks by DC-pulsed PECVD using methane as a precursor gas. Two different set of tribological system was assembled, one when the liquids and the pairs were put inside of a stainless steel vessel and others inside of a PTFE. Every tribological test was performed under 10 N normal load120 mms -1 of sliding speed. The friction coefﬁcients were evaluated during 1000 cycles. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction The recent interest of the petroleum companies on improving the efﬁciency and viability of oil ducts ﬂow have been increased in the several last years [1,2]. However the commercial extraction viability has some obstacles, such as drilling of the salt layer that can block and hold the drill pipes. Drill and ﬂow pipes are made with seamless steel coated with resin to decrease the wear and corrosion. However, this resin wears very quickly due to contact sliding between steel cables and inner wall of the drill pipes. In addition, corrosion of the steel components due to great pressure and the sulfur dioxide presence decreases the efﬁciency and viability. The water from the oil extraction generally contains high salinity, oil particles in suspension and chemicals added as demulsifying and defoaming [3]. The corrosive environment can result deposition/precipitation of corro- sion products in the metal surface and increase wear inside of the drill and ﬂow pipes. DLC coatings are well known for their low friction, high wear resistance, and the combination of inert character of DLC ﬁlms to most chemicals and their amorphous structure which makes them almost defect free makes these coatings good candidates for oil ducts. Technologies based on the ﬂow of oil through ducts coated with DLC ﬁlms represent an alternative way [1]. DLC coatings can potentially increase the efﬁciency by reducing the friction, wear and corrosion on the drill pipes. Such an improvement will reduce the operating costs by reducing the equipment downtime for maintenance and extend the life of the existing machinery. The most common way to control corrosion in oil ducts is by chemical inhibition, but it is not fully understood. The mechanism can be dominated by inhibitor adsorption on the surface of ﬁlm and/or metal; the inhibitor provokes a small extent of corrosion on the ﬁlm surface and/or metal and is also adsorbed forming a compact protective thin layer. The inhibitor forms a precipitate on the surface metal in such a way to protect it. The protective combination of corrosion product/inhibitor layer is dynamic [4]. Inhibitors have also been shown to be effective in reducing erosion [5]. The tribocorrosion area involves rheological properties and interaction among mechanical friction, wear processes and several chemical corrosion processes [6]. Tribocorrosion includes the solid particle erosion, sliding wear and oxidation as surface degradation process. The presence of suspended solids, the erosion–corrosion rate can increase by 25–100 times greater than the uniform corrosion rate [7]. In severe operating conditions, such as erosion–corrosion conditions, corrosion resistant alloys such as 13Cr martensitic stainless steel (CRAs) are widely used in oil industry [8]. The martensitic grades are mainly used where hardness, strength, wear and corrosion resistance are required, and chromium improves oxidation resistance of the steel in sulfuric atmospheres. The objective of the present work is to study the behavior of friction coefﬁcient and wear rate when iron oxide residual is deposited on DLC ﬁlm surface under seawater and saline solutions of 3 and 5 mass% NaCl in distilled water. This experimental study presents the correlation of iron oxide residual with decreasing of dispersive component of surface energy and increase of DLC ﬁlm ID/IG ratio, in order to understand the chemical mechanisms evolved under aggressive environments. Surface Science 605 (2011) 783–787 ⁎ Corresponding author. Tel.: +55 1232086578; fax: +55 1232086717. E-mail address: rominapccosta@gmail.com (R.P.C. Costa). 0039-6028/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.01.018 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc