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

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

Wear ] (]]]]) ]]]–]]] Contents lists available at SciVerse ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Sliding wear and frict...

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Wear ] (]]]]) ]]]–]]]

Contents lists available at SciVerse ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

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 Universidade Federal do Rio Grande do Sul, RS, Brazil c ´lica do Rio de Janeiro, RJ, Brazil Pontifı´cia Universidade Cato b

a r t i c l e i n f o

abstract

Article history: Received 20 September 2012 Received in revised form 31 January 2013 Accepted 31 January 2013

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 1C, 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 Xray 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 (GDOES), 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, plasmanitriding 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.

Keywords: Friction and wear CrN-coating Ethanol Oil

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

n

Corresponding author. Tel.: þ55 54 3218 2764. E-mail addresses: [email protected], [email protected] (M.C.M. Farias).

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 tribocorrosion 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 engineering 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) technologies [4]. The type of thin film that is deposited by PVD determines

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Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

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different friction coefficients and, in the case of ethanol interacting with CrN, the formation of chromium ethoxide was detected as boundary film [5]. This incipient knowledge cannot provide a deep insight about the tribological behavior of coated steels in contact with liquid solutions of bioethanol and oil under wear conditions. Consequently, tribological problems are not well understood, and there is a need to know what types of products are formed and developed new models to analyze, in particular, tribological performance of the top piston rings/ethanol/lubricant oil system. The objective of this work is to investigate the friction and wear behavior of the AISI 4140 steel after both plasma-nitriding and a combined treatment of plasma-nitriding and physical vapor deposition of CrN-coating. In order to analyze the effect of the fuel dilution in the lube oil, sliding wear tests were conducted in ethanol fuel, oil and ethanol–oil mixture.

2. Experimental procedure 2.1. Material, plasma nitriding and coating procedures The friction and wear characteristics of a CrN-coating were investigated and compared with uncoated and plasma-nitrided AISI 4140 steel (C 0.43, Si 0.35, Mn 1.00, P 0.03, S 0.04, Cr 1.10, Mo 0.25, and balance Fe in wt%). Discs were cut from previously quenched (870 1C, 30 min, oil) and tempered (550 1C, 60 min) AISI 4140 steel bars to a final dimension of 40 mm in diameter and 5 mm in thickness. The discs were ground with SiC grinding paper up to 1200 grit. After grinding, discs were polished with a 3 mm diamond paste. Samples were then washed with soap, degreased with acetone in an ultrasonic bath for 30 min and dried with hot air. The plasma nitriding surface treatment was conducted in a laboratory-scale chamber at 500 1C for 5 h, in a 10% N2 þ90% H2 gas mixture under 200 Pa. After the nitriding stage, the samples were cooled down under vacuum inside the chamber. After plasma nitriding process, some test samples were submitted to plasma vapor deposition by DC-magnetron sputtering process (MS-PVD) to grow the CrN-coating with a base pressure better than 4  10  5 Pa. A 99.95% purity Cr target in a reactive nitrogen atmosphere with a mixture of 45% of argon and 55% of nitrogen gases at a working pressure of 1 Pa were used. During the deposition, the power was kept at 50 W and the temperature of samples was held at 300oC approximately. The deposition rate of CrN is 15 nm min  1. 2.2. Sliding wear tests The tribological study of both plasma nitriding and PVD CrNcoated AISI 4140 steel samples (PN–CrN) was performed in a conventional ball-on-disc test machine (Plint & Partners TE-79 multiaxial tribometer). A silicon nitride ball with a diameter of 6 mm was loaded with 10 N against the discs, which corresponds to initial maximum contact stress of 0.40 GPa. The tangential velocity of the discs was 0.01 m/s. The unidirectional sliding wear tests were performed at room temperature (23 1C), at controlled humidity (50%) and were interrupted after 2 h. The total sliding distance was 188.4 m. All the tests of the PN–CrN samples were run in (i) dry (reference tests), (ii) ethanol fuel (hydrated ethanol fuel, 94.5% ethanol and 4.5% water, pH6), (iii) 5W40 synthetic engine oil and (iv) ethanol fuel and engine oil blend (5% ethanol fuel and 95% engine oil). Additional reference wear tests were conducted in unlubricated conditions and ethanol fuel for the quenched and tempered steel (QT) and the plasma nitriding steel (PN). In all cases, we have performed three wear tests for each

condition and the friction and wear coefficients are an average of these three independent values. 2.3. Characterization of material, coating and worn surfaces Before the sliding wear tests, microstructure analyses of substrate and nitrided layer were conducted by scanning electron microscopy (SEM) using a Shimadzu SSX-500) microscope. The coating thickness was also estimated by SEM. The elementary composition of the CrN film was determined by Rutherford backscattering spectrometry (RBS) that was also used to estimate coating thickness. The structural details of the steel substrate were determined by X-ray diffraction (XRD) with a conventional y/2y Bragg–Brentano symmetric geometry. The crystalline structure of plasma-nitrided layer and CrN-coating were determined by glazing angle X-ray diffraction analysis (GAXRD), at an incident angle of 21. Both analyses were conducted using a Shimadzu XRD-6000 diffractometer with Cu Ka radiation (40 kV and 30 mA), in a range of 30– 901 and 2y scan step of 0.051. All the samples were rotated during the XRD experiments for the reduction of texture effects. In order to characterize all present phases in the diffractograms, the powder diffraction file number 11-0065 for CrN from CrN JCPDSInternational Centre for Diffraction Data and a previous work were used, where iron nitrides used in AISI 4140 were detected [6]. The hardness of the substrate was determined by conventional Rockwell C measurements with a load of 60 kgf. Coating hardness was accessed by nanoindentation measurements using a NanoTest-600 machine manufactured by Micro Materials Ltd., equipped with a three-side pyramid diamond tip (Berkovich indenter). The nanoindentation tests were conducted at a loading rate of 0.1 mN/s, with a creep time of 10 s and maximum indentation depth of 80 nm, which ensured the minimum influence of the substrate. A top nanohardness profile of the nitride layer was obtained by the same technique at the indentation depth range of 100 –1400 nm. The average hardness was calculated using the Oliver and Pharr method [7] from a minimum of 10 to 30 points for each published measurement, in order to decrease the standard deviation, using the unloading portion of load–depth curves. After the sliding wear tests, the morphology of the wear tracks was analyzed by SEM. The element distribution of the wear tracks were analyzed with an energy dispersive energy spectroscopy (EDS) unit coupled to the scanning electron microscope. Raman spectroscopy was used to identify the tribofilm formed after the sliding tests. Raman spectra were obtained using a NT-MDT integraSpectra spectrometer equipped with a microscope used to locate the micro-spots on the sample. The excitation wavelength used was 514.5 nm provided by an Ar þ laser. The spectra were collected from 2000 cm  1 to 200 cm  1 with an acquisition time of 30 s. The surface chemical composition was evaluated indepth by glow discharge optical emission spectroscopy (GD-OES). A GD-Profiler-2 from Horiba—Jovyn Ivons was used. The technique was employed in the qualitative mode. To quantify the wear, the wear scar depth was measured using a stylus surface profilometer Ambios XP2.

3. Results and discussion 3.1. Physicochemical and structural properties of the materials The X-ray diffractograms for the different treatments of the steel substrate are shown in Fig. 1. After plasma nitriding irontype nitride g’-Fe4N was detected. The hardness of the quenched and tempered AISI 4140 steel was 3.9 GPa, which was lower than

Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

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the plasma-nitrided (6.4 GPa) and plasma-nitriding and CrNcoated (21 GPa) samples. The RBS analysis of the CrN film deposited on a silicon substrate indicated the formation of a stoichiometric film (Cr/N¼1.01) with 1 mm of thickness, approximately. SEM analysis of the coated steel also showed similar value of coating thickness. 3.2. Friction behavior Fig. 2 shows the evolution of friction coefficient with sliding distance for the substrate, plasma nitride and coated substrate in unlubricated and ethanol and oil lubricated conditions. The unlubricated reference tests indicated the critical sliding contacts where scuffing damage occurs. Under these conditions friction coefficients increased rapidly after about 10 min and stabilized after about 80 min of sliding. The average coefficients of friction corresponding to the steady-state stage are shown in Fig. 3. The combined treatment of plasma nitriding and CrN-coating 30

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The comparison of the wear rates shown in Fig. 4 indicates that there was a significant difference between the wear rates of the treated materials run in ethanol fuel. The uncoated steel





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(PN–CrN) showed the lowest coefficient of friction in unlubricated conditions compared with the QT and PN treatments Fig. 3(a), which indicate the lubrication properties of the CrN-coating. The tests run with ethanol fuel produced a lower and steadier coefficient of friction during the sliding contact. The average coefficient of friction was significantly reduced with ethanol fuel, as can also be seen in Fig. 3(b). As in the case of dry tests, the coated steel substrate had the lowest friction for the lubrication in ethanol fuel. In order to simulate the effect of engine cold-starting on friction and wear, the CrN-coating running in ethanol–oil blend was studied. From Figs. 2(b) and 3, it is possible to observe that the engine oil provided the lowest friction of the coated system, but the average coefficient of friction did not change when the tests were run in the blend of ethanol fuel and engine oil. Compared with the two oil-containing lubrication conditions (engine oil and ethanol–oil mixture), the ethanol fuel had less lubricity when alone, observing its higher values of average coefficient of friction. Further evidence of wear mechanisms and tribofilm formation will be given below, which will help to elucidate the friction behavior.

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Fig. 2. Variation of friction coefficient of the treated materials (QT, PN and PN–CrN) in (a) unlubricated and (b) lubricated conditions.

Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

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experienced the highest wear, followed in descending order by the plasma-nitrided and the coated steel. This indicated that, in addition to its lubricating property, the plasma nitriding and CrNcoating treatment had anti-wear capability. Furthermore, the wear of the PN–CrN steel samples were very similar when run in engine oil alone or in the blend of ethanol fuel and engine oil. Even if the addition of ethanol fuel had decreased the lubricant viscosity, this did not likely change the nature of the tribofilm.

Wear Rate (x10-12 mm3/N.m)

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PN

PN-CrN

Fig. 4. Wear rates of uncoated, plasma-nitrided and coated steel in lubrication conditions in ethanol fuel and ethanol–oil blend.

3.3.1. QT and PN steel substrates in dry and lubricated conditions The morphologies of the worn surfaces after dry sliding tests are shown in Fig. 5(a) and (b) for the uncoated and the plasmanitrided steel, respectively. The formation of a film on the worn surface of the uncoated steel (QT) can be observed in Fig. 5(a). The film cracked and partially detached from the contact spots, due to rubbing action. The elemental distribution maps of Fe and O shown in Fig. 6(a) and (b), respectively, for the same region of Fig. 5(a), indicate the presence of the elements Fe and O in the film micro-area, which probably means that an iron oxide was formed. In the micro-area where film detachment occurred, the EDS maps show the presence of Fe only. Thus, in this region, the oxide was completely removed, exposing the bare steel substrate to the test environment. After the subsequent stage, this clean micro-area may have reacted with the air oxygen and reoxidized. The wear sequence already described is characteristic of oxidative wear, specifically the oxidation–scrape–reoxidation mechanism [8]. Moreover, the formed metallic oxide particles, which will be demonstrated below, can also promote abrasive wear due to the typical morphology of thin grooves in the wear track. For the nitrided steel (PN) run in dry sliding conditions, SEM (Fig. 5(b)) and EDS analyses suggested the formation of more protective iron oxide films, since no film removal was observed on the worn surface. This kind of wear resulted in a lower coefficient of friction than in the oxidation–scrape–reoxidation mechanism generated during the dry sliding of the QT sample, as shown in Fig. 3. The oxidized regions appeared plastically deformed, which is further evidence of the occurrence of total oxide mechanism [4]. Moreover, the formed metallic oxide

FexOy

FexOy

Substrate

FexOy

10 μm

2 μm

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FexOy

FexOy

FexOy Substrate

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Fig. 5. SEM morphologies of the worn disc surfaces after unlubricated (a,b) and with ethanol fuel sliding tests (c,d). (a) QT in dry test; (b) PN in dry test; (c) QT in ethanol fuel test and (d) PN with ethanol fue.

Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

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Fig. 6. Elemental distribution maps obtained by EDS analysis of the worn disc surfaces of uncoated steel (QT) run in dry sliding (a,b) and plasma-nitrided steel (PN) run with ethanol fuel (c,d).

particles can promote abrasive wear due to the typical morphology of thin grooves in the wear track The worn surface of the QT steel run in ethanol fuel (Fig. 5(c)) was covered by a thin film, containing Fe and O, as well, that is also partially removed during sliding rubbing contact. For this condition, abrasion grooves were observed in the wear track, which may be produced by the retained oxide particles. This kind of wear is also typical of the oxidation–scrape–reoxidation mechanism. One must remark that the formed grooves in ethanol fuel condition are thicker than those created in dry condition. A more resistant iron oxide film partially built up on the worn surface of the PN steel run in ethanol fuel (Figs. 5(d) and 6(c,d)), indicating the occurrence of total oxide mechanism. In this case, the interaction of the plasma-nitrided surface with the ethanol fuel produced a lower friction and wear rate than the QT steel (Fig. 4). A freshly exposed iron nitride-containing (g’-Fe4N) sliding surface is rapidly oxidized by the oxygen in the hydrated ethanol fuel.

3.3.2. PN–CrN steel substrates in dry and lubricated conditions Fig. 7 shows the SEM morphologies of the PN–CrN steel substrate after sliding tests in dry and lubricated conditions. The formation of microfilm partially covering the worn surfaces after dry sliding can be seen (Fig. 7(a)). According to the elemental distribution maps of the elements Cr, Fe and O in worn surface after dry sliding, there was the formation of a microfilm composed of iron and chromium oxides (Fig. 8(a–c)). Moreover, it is important to stress that the morphology of both metallic oxides (FexOy and CrxOy) are quite different when compared to the original CrN (please, see the regions where each compound is

apparent in Fig. 7a). Likely, the CrN-coating partially transformed into oxide chromium that was subsequently removed to some extent, exposing the steel substrate. The regions containing Fe and O elements indicate that new microfilm areas probably were formed by the oxidation of the exposed steel substrate. The CrNcoated samples from dry sliding tests had the lowest average coefficient of friction, compared with the QT and PN samples, which can be associated with the formation of a more lubricous chromium oxide. For the PN–CrN samples run in ethanol lubricated tests, the worn surface were covered by a film (Fig. 7(b)) probably composed of chromium and iron oxides, as indicated by the chemical mapping of Cr, Fe and O (Fig. 8(d–f)). In this condition, the tribochemical mechanism occurred by the reaction of the oxygen in the hydrated ethanol fuel with the CrN-coating, forming a boundary layer of chromium oxide. Additionally, when the protective layer was removed, the exposed steel substrate was also oxidized, forming a micro-area composed of iron oxide. However, the oxide chromium reactive layer probably had good lubricity proprieties and load-carrying capacity, since the average coefficient of friction and wear rate changed to a lower value, as compared with QT and PN samples run in ethanol fuel (see Figs. 3 and 4). For the two oil-containing lubrication conditions, the SEM analyses also showed the formation of a protective tribofilm (Fig. 7(c,d)). The chemical mapping in the covered micro-area indicated the formation of a tribofilm likely composed of chromium oxide. The regions free of tribofilm were composed of iron oxide. The different chemical structures of the tribofilms developed in the PN–CrN worn surfaces can explain its greater efficiency in

Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

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CrxOy FexOy

CrN CrxOy

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FexOy

5 μm

5 μm

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FexOy

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CrxOy

5 μm

5 μm

Fig. 7. SEM images of the worn surfaces of the plasma-nitrided and CrN-coated steel substrate (PN–CrN) after sliding tests in (a) dry, (b) ethanol fuel, (c) engine oil and (d) ethanol–oil mixture.

the reduction of friction and wear. The nature of the tribofilms comes from the interaction between the ethanol fuel, its constituents/contaminants, and the engine oil with the contact surfaces and can be described in terms of its chemistry and nanomechanical properties. The stability of the tribofilm, that is, formation, removal and replenishment, also affects friction and wear properties. 3.4. Raman and GD-OES analysis of the worn surfaces In order to confirm the evidence highlighted in EDS analyses, the types of oxides of Fe and Cr formed in the wear tracks after unidirectional sliding tests were investigated by Raman spectroscopy (Fig. 9). The worn surfaces of the QT and PN samples were similar in their Raman spectrum, in which oxides of Fe were formed, namely magnetite (Fe3O4) and hematite (Fe2O3). The characteristic bands of magnetite were observed in 544, 677 and 707 cm  1, being consistent with the literature [9–11]. For the hematite, bands in 344, 1401 and 1624 cm  1 were observed [9,12]. In the Raman spectrum of the PN–CrN worn surface, a CrO2 band at 680 cm  1 was observed [11,13]. The broader bands in the spectra indicate a higher structural disorder probably due to plastic deformation under the action of the rubbing contact pressures. As a complementary tool of surface characterization, GD-OES experiments were performed in order to study the qualitative chemical structure of the outermost and underneath layers after wear tests. Fig. 10a and b shows the GD-OES spectra of uncoated (QT) and nitrided plus CrN deposition on steel substrates after

wear tests in ethanol fuel, respectively. One can see that in both cases, there is a outermost oxide layer after wear tests. For the QT sample, an iron oxide layer is formed, whereas an chromium oxide layer is developed for the nitrided and CrN coated sample. These outermost layers are in the order of 50 nm, according to the average sputtering rate of the experiment due to the power used (30 W). In despite of qualitative data only were obtained to identify the chemical structure of the outermost layer (tribofilm), we have determined unequivocally that after wear tests, the system without CrN coating renders iron oxides while the system with CrN coating renders chromium oxides at the top. These experimental evidences will be used below due to the chromium oxides are more lubricious than the iorn oxides. 3.5. Nano-mechanical properties of the worn surfaces The nano-mechanical properties of the worn surfaces were accessed by nanoindentation tests conducted in a range maximum indentation depth of 200 –1400 nm. Fig. 11 shows that compared with the unworn samples, both the worn surfaces and the regions nearly below the wear surface of the QT and PN samples suffered hardening. The higher hardness values of the worn surfaces at 200 nm depth of the QT and PN samples (10.2 GPa and 12.1 GPa, respectively) can be attributed to the presence of the iron oxide tribofilms, while the hardening of the sub-surface regions can be explained by the strain field developed by the action of normal and shear pressures in the rubbing contacts. For the PN–CrN samples, it can be seen in Fig. 11 that

Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

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Fig. 8. Elemental distribution maps obtained by EDS analysis of the worn disc surfaces of CrN-coated (PN–CrN) steel in dry sliding (a–c) and ethanol fuel (d–f).

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7 Fig. 9. Raman spectra of the wear tracks for the uncoated (QT), plasma-nitrided and CrN-coated steel after wear tests in ethanol fuel and for the coated steel in the mixture of ethanol fuel and engine oil.

the chromium oxide tribolfim formed in ethanol fuel and ethanol– oil mixture were softer (13.8 GPa and 14.6 GPa, respectively) compared with the unworn CrN-coating (21 GPa), which likely offers good lubricating property. In addition, the strain hardened PN–CrN steel sub-surface provides sufficient support to the lubricious tribofilm and wear resistance. In order to have further explanation for the wear rate behavior, the H/En ratios of the tribofilms (‘elastic strain to failure’) were also analyzed. Fig. 12 shows the hardness ratio (relationship between the hardness of worn surface or tribofilm, Htf, and the hardness of the unworn surface, Huw), the elastic strain to failure (relationship between

0

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Fig. 10. GD-OES spectra of the wear tracks for the uncoated (a) and nitrided and CrN-coated steel (b) after wear tests in ethanol fuel, respectively.

the hardness and reduced modulus of the tribofilm, Htf/Entf) and the wear rate for the untreated, plasma-nitrided and coated steel substrate after wear tests in ethanol and ethanol–oil mixture. The H/En ratio of the tribofilm can be used as indicator of the deformation capability of the tribofilm and the wear resistance of the surface. Fig. 12 shows that as the H/En ratio of the tribofilm increased, the wear rate decreased.

Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

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QT in Ethanol PN in Ethanol PN-CrN in Ethanol PN-CrN in Ethanol-oil

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Fig. 11. Nano-hardness measurement as a function of depth on the wear tracks for the heat treated (QT), plasma-nitrided only (PN) and the duplex system of PN plus CrN-coated steels after wear tests in ethanol fuel and the duplex system of PN plus the coated steel in the mixture of ethanol fuel and engine oil.

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Wear rate

Fig. 12. Wear rate, hardness ratio and plasticity index associated to the worn surfaces for the heat treated (QT), plasma-nitrided only (PN) and the duplex system of PN plus CrN-coated steels after wear tests in ethanol fuel and the duplex system of PN plus the coated steel in the mixture of ethanol fuel and engine oil.

compound with low chemical interaction with organic molecules that constitute the oil formulation. In our system, the effect of the tribofilms on friction behavior can also be explained based on the ‘‘crystal chemical’’ approach [15], which states that lubricious oxides are those with higher ionic potential. Conversely, low ionic potential oxides will lead to high friction. The ionic potential (j) is expressed in terms of the cationic charge (Z) and the radius of the cation (r) as j ¼Z/r. In general, those oxides with high ionic potential had their cations completely screened by the oxygen anions and had less ability to interact chemically with the surrounding cations. These oxides are generally soft and, hence, had low shearing strength, providing low friction. On the other hand, in low potential oxides, the cations can interact with other, forming strong covalent or ionic bonds. As a results, they are generally hard and offer a high resistance to shearing, having higher friction. As shown above, the chromium oxides found in PN–CrN worn surfaces are softer than the unworn surfaces, composed of the CrN hard film. Based on the ‘‘crystal chemical’’ approach, it was found that the CrO2 is a high ionic potential oxide (7.3). Consequently, it deforms and shears easily during the sliding contact, which explains the lower values of average coefficient of friction and wear rate obtained for PN–CrN samples. On the other hand, the iron oxides formed in the worn surfaces of QT and PN samples have low ionic potential (4.3 for Fe2O3) and are harder than the unworn surface and, consequently, do not have a low shear resistance. These facts could also explain the higher values of the coefficient of friction and wear rate of the QT and PN samples. In Fig. 13a complete picture of friction and wear behavior of the materials is presented, considering the wear mechanisms, wear rate, coefficient of friction, chemistry and mechanical of the tribofilm (type, ionic potential, hardness, and reduced modulus). The relationships can be briefly explained as follow: the higher the ionic potential of the oxide, the lower the friction coefficient; the higher the ratio Htf/Huw is, the higher the wear rate is; the increase in Htf/Entf ratio increases the wear resistance. Two limiting cases can be identified: (1) oxidational wear mechanism with the formation of a relatively hard oxide with low ionic potential and (2) tribochemical wear mechanism in which a softer and high ionic potential oxide is formed. In that sense, the uncoated (QT) and plasma-nitrided (PN) surfaces underwent an oxidative wear mechanism. For these two material treatments, the contact with the hydrated ethanol fuel produced the formation of a low ionic potential iron oxide (Fe3O4 and Fe2O3) with

4. Discussion of the tribological behavior CrN and DLC coatings are emerged materials to functionalize auto parts such as piston rings, valves and fuel injectors in order to decrease wear and friction. Although DLC coatings show lower friction coefficient than 0.08 in no lubricant condition using polished flats, the friction coefficients are slightly greater in lubricant condition than those observed with uncoated surfaces [14]. In our case, CrN coatings show lower friction coefficients in dry condition than those observed with uncoated and nitridedonly surfaces. Moreover, ethanol and mixture ethanol–oil conditions did not show substantial changes in the friction coefficients (approximately 0.1) when CrN coatings were used. Indeed, DLC coatings are carbon-based materials as organic oils. We suggest that depending on the deposition process, the DLC may contain hydrogen in its structure allowing a chemical interaction with the aliphatic molecules of oil and, consequently, increasing the viscosity in the boundary layer. Otherwise, CrN is an inorganic

Fig. 13. Relationships of the oxide ionic potential, Htf/Huw and lubrication condition with friction and wear mechanism of the treated steel.

Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i

A.L. Bandeira et al. / Wear ] (]]]]) ]]]–]]]

high friction properties. Additionally, the high rations of Htf/Huw, provide low carrying-load capability of subsurface regions, leading to the removal of the oxide layer and, consequently, a high wear rate. On the other hand, the coated steel experienced a tribochemical wear mechanism, through the formation of a chromium oxide tribofilm. The chromium oxide (CrO2) tribofilm that resulted from the reaction between the CrN-coating with the oxygen in the ethanol fuel has a high ionic potential and good lubricity, reducing the coefficient of friction. Since the ration Htf/Huw is lower, the tribofilm can be supported by the subsurface regions, offering a higher wear resistance. The contact with engine oil pure or blended with ethanol fuel leads to the formation of a chromium oxide (CrO2) tribofilm with a higher ionic potential and provides a lower coefficient of friction. The lower ratio Htf/Huw provides a lower wear rate.

5. Conclusion In this work, the friction and wear behavior of the plasmanitrided and CrN-coated AISI 4140 steel was investigated in ethanol fuel and ethanol–oil mixture. The friction and wear of the CrN-coated AISI 4140 steel were lower than in the uncoated steel. Moreover, the CrN-coated steel run in ethanol fuel had a lower coefficient of friction and wear rate than in the dry-sliding condition. In engine oil lubricating sliding tests, friction and wear decreased, compared with the tests run in ethanol fuel. However, compared with ethanol fuel, no significant differences appeared in the friction and wear behaviors of the sliding tests run in ethanol–oil blend, which is related to the similar chemical structure of the tribofilms that were formed. In the presence of ethanol, oil and ethanol–oil mixture, the wear mechanism of the CrN-coated steel was a tribochemical reaction, in which a lubricious and relatively soft tribofilm containing chromium oxide was formed. The chromium oxide was a product of the CrN reacting with the oxygen in the ethanol fuel. Such chromium oxide forms an outermost layer, as detected by GD-OES, in the wear track. According to the crystal chemical approach, the ionic potential of CrO2 is higher than Fe3O4 and offers a low-friction property, producing a better lubricious tribolfilm than those constituted by iron oxides.

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Acknowledgments This work is partially supported by CNPq: Projects INES-INCT and ETANOL.

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Please cite this article as: A.L. Bandeira, et al., Sliding wear and friction behavior of CrN-coating in ethanol and oil–ethanol mixture, Wear (2013), http://dx.doi.org/10.1016/j.wear.2013.01.111i