Aus Wissenschaft und Forschung 5 Tribologie + Schmierungstechnik · 65. Jahrgang · 5/ 2018 1 Introduction The promising direction for enhancing wear resistance is the development of environmentally friendly lubricants. This is due to the fact that “traditional lubricants”, getting into the surrounding environment, are only partially removed or rendered harmless as a result of natural oxidation processes, photochemical reactions, and biodegradation. The main part is a source of sustainable pollution of the soil, reservoirs and the atmosphere - this is especially dangerous in permafrost zones, where the natural balance is very fragile. As the environmental regulations become more stringent, the costs for the utilization of lubricants will increase, which will make the development of production of ecologically safe products more profitable. Furthermore, the working surfaces of the tribo-pair should possess appropriate properties providing high hardness, strength, wear resistance and low friction coefficient. To obtain such properties of surfaces, the method of micro-arc oxidation is considered most expedient. The micro-arc oxidation (MAO) technology is a relatively new and promising method for surface treatment of materials [1-4]. With its help, it is possible to form coatings with a wide range of properties, such as high wear resistance, heat resistance, corrosion and erosion resistance [5-7]. The essence of this technology lies in the formation of a structured surface predominantly consisting of aluminium oxides formed from the aluminium substrate itself. Thereby, the part is immersed in an electrolyte, and its surface is exposed to micro-arc discharges. These multiple micro-arc discharges interact with the surface resulting in the transformation of the surface into a high-strength ceramic-like coating which consists of aluminium oxides and other compounds of the electrolyte components and includes, in particular, the high-strength phase of alumina - α-Al 2 O 3 (corundum). MAO-coatings are characterized by high performance and include following advantages: - Possibility of forming stable and ultra-strong coatings; - High adhesion of the coating to the surface of the substrate; - Large range of coating thicknesses (from several micrometers to 400 - 500 μm); - Well developed technology for valve metals and alloys (titanium, magnesium, tantalum, zirconium and especially aluminium), can be used for ferrous metals too. Tribological performance of micro-arc oxidation coatings with base oils V.N. М alyshev, N. Dörr, O.Yu. Elagina, M. Rodríguez Ripoll, N.S. Poches* This paper describes the tribological performance of coatings formed by micro-arc oxidation (MAO) on aluminium alloy lubricated with base oils. The used base oils were perfluorinated polyester (PFPE), polyethylene glycol (PEG), diisotridecyl adipate ether (DI- TA) and silicone oil. Polyalphaolefin (PAO) base oil was used as reference. The experiments were performed under different loads and sliding conditions. The selected conditions encompassed milder contact conditions up to extreme contact pressures. The results obtained showed that the morphology of the MAOcoatings had a determining effect on the interaction with base oils. In general, it was observed that MAOcoatings with a more porous structure showed a higher wear resistance with all lubricants, except for PFPE. Furthermore, it was observed that MAO-coatings have an excellent performance with DITA and PEG base oils that resulted in a superb wear resistance even under extreme pressure conditions independently of their morphology. Finally, selected experiments were performed using base oils with zinc dialkyldithiophosphate (ZDDP). A positive effect of ZDDP on friction and wear could be observed particularly under oil starvation conditions. These results highlight the potential of MAO-coatings for operation in lubricated contacts. DITA and PEG being superior to other synthetic base oils basically open the door for prospective environmentally acceptable formulations using green base oils and additives. Keywords Micro-arc oxidation, tribotechnical characteristics, lubricants Abstract * Prof. Vladimir Malyshev 1 Dr.techn. Nicole Dörr 2 Prof. Oxana Yu. Elagina 1 Dr.-Ing. Manel Rodríguez Ripoll 2 Nikita S. Poches 1 1 Gubkin Russian State University of Oil and Gas (National Research University), Moscow, 119991, Russia 2 AC2T research GmbH, 2700 Wiener Neustadt, Österreich T+S_5_18.qxp_T+S_2018 28.08.18 13: 50 Seite 5 software, which allowed determining not only the average pore size but also the pore size distribution within the investigated sections of the coating surface [17]. Base oils comprising polyalphaolefin (PAO), silicone oil (Silicon oil), perfluorinated polyester (PFPE), polyethylene glycol (PEG) and diisotridecyl adipate ether (DITA) were used as lubricants. Lubricants were selected in such a way that their kinematic viscosities at 40 °C were similar, with a value of about 25 mm 2 / s. The friction tests were carried out under two contact conditions: reciprocating sliding using a “ball-on-disk” friction scheme (Schwing-Reib-Verschleiß Tribometer SRV3) and unidirectional sliding on a “pin-on-disk” friction scheme (tribometer of own design at Gubkin University) according to ASTM G133 [18] and ASTM G99 [19], respectively. During the experiments, the coefficient of friction was continuously monitored and recorded. Volume and mass loss on the MAO-coated aluminium samples were determined using 3D microscopy images of the wear scars. 2.1 Reciprocating ball-on-disk sliding test rig The reciprocating sliding experiments were performed using balls with a diameter of 10 mm (3/ 8”) made from steel 100Cr6 (analogue of Russian steel ШХ15) and disks made from aluminium alloy 2024 (D16) Ø 24 x 8 mm with MAO-coatings. Table 1 shows the average values of thickness and porosity of the MAOcoatings determined in 5 - 6 points on the coatings, depending on the electrolyte used. Aus Wissenschaft und Forschung 6 Tribologie + Schmierungstechnik · 65. Jahrgang · 5/ 2018 However, some disadvantages of the MAO-method are: - Relatively high energy consumption; - Some difficulties of forming a uniform coating of the required thickness on parts of complex shape. MAO-coatings have a wide range of applications in the industry. In the oil and gas industry, this method is used to process the plungers of pumps, gate valves, and friction pairs of mechanical seals. In mechanical engineering, fast-rotating parts of submerged water and oil-free vacuum pumps and compressors can be added as applications [8-12]. Under extreme operating conditions, the requirements for materials become tougher and more complex. Therefore, the development of technologies for modifying the surface properties of parts and products is an urgent and demanded task. In this sense, the modification by microarc oxidation method is one of the most promising technologies in the field of wear-resistant coatings [13]. The lubricants applied have a tremendous influence on the tribotechnical characteristics. Optimal matches between materials and lubricants allow reducing the amount of wear in tens and hundreds times, in comparison with the same processes in the absence of lubrication [14-16]. The goal of the presented paper was the investigation of tribotechnical characteristics of MAO-coatings in contact with steel in various lubricants. 2 Materials and methods of investigation Micro-arc oxidation coatings were formed on samples with the dimensions ∅ 24 x 8 mm and ∅ 10 x 6 mm from aluminium alloy 2024 of the Series 2 Al-Cu-Mg (duralumin D16 in Russia). For the formation of coatings on these samples, two types of weakly alkaline electrolytes were used - electrolyte №1 and electrolyte №2. Before the coatings were formed, the samples were honed on the slab using alumina powder and kerosene, then washed, degreased and dried in air. The formation of coatings was carried out in the anodic-cathode mode under the following conditions: - Current density, A/ dm 2 6 - 7; - Voltage at the anode, V 550 - 600; - Voltage at the cathode, V 150 - 260; - Time, min 55 - 60 Afterwards, the samples were honed on the slab with a diamond paste, polished, cleaned, washed, and dried, followed by thickness control and porosity measurements using a NIKO optical microscope. The porosity of coatings was also evaluated on micro-sections using the method of texture image analysis with the help of “Image J” Table 1: Average values of thickness and porosity of the MAO-coatings on disks Electrolyte Average thickness Average porosity µm % №1 84±5 27±7 №2 77±5 36±7 The study was carried out on a SRV3 tribometer in two modes: with increasing load and at constant loads. For the tests with increasing load, the following parameters were used: - Frequency of oscillation, Hz 50; - Reciprocating movement, mm 1.0; - Ambient temperature, °C 40; - Running-in at 50 N, min 15; - Increasing of load from in steps of 25 N 400 N to 600 N every 7 min; - Test duration, min 80 T+S_5_18.qxp_T+S_2018 28.08.18 13: 50 Seite 6 At least two repetitions per lubricant and test configuration were carried out. Based on the results with increasing load, the best-performing oils were selected for subsequent experiments at constant loads. For these tests in the second stage, the load was kept at 400 or 600 N for a duration of 60 minutes while applying the same values for the other parameters as described above. 2.2 Unidirectional pin-on-disk sliding test rig In this case, the samples were made from the alloy 2024 (D16) in the form of pins with dimensions of Ø10x6 mm and coated by MAO. A disk with a diameter of 110 mm made from 100Cr6 was used as a counter body. Table 2 shows the average values of the thickness and porosity of MAO-coatings in the second stage, also determined in 5-6 points of the coatings. The experiments were performed at least twice with the following parameters: - Load, N 200; - Rotational speed of the disk, rpm 38; - Sliding speed, m/ s (calculated) 0.16; - Duration, min 60 Tests under the friction scheme “pin-on-disk” were carried out in two modes: in lubricated mode (via drop lubrication) and emergency mode, i.e., under conditions of oil starvation or in other words only with the oil that remained in the pores of the coating. Based on the results of the first stage, lubricants PEG, PAO and DITA were selected as the lubricants for further testing in the third stage. The composition of the lubricants was expanded by the addition of additives. Each oil was blended with anti-wear additive zinc dialkyldithiophosphate (ZDDP) in an amount of 1600 ppm (0.16 %), and 5 % DITA was dissolved in PAO. According to [16], the introduction of zinc dialkyldithiophosphate in the oil protects against pitting, scoring, and increasing of wear. Additionally, ZDDP can contribute to antioxidant and corrosion protection. 3 Test results 3.1 Reciprocating sliding experiments with increasing load The friction curves obtained for samples formed in the electrolyte №1 by lubrication with DITA and PAO performed using the SRV tribometer are shown in Figure 2. As it can be seen, friction with PAO base oil is not stable, Aus Wissenschaft und Forschung 7 Tribologie + Schmierungstechnik · 65. Jahrgang · 5/ 2018 Table 2: Average values of thickness and porosity of the MAO-coatings on pins Electrolyte Average thickness Average porosity µm % №1 100±5 38±7 For these friction tests, an own-developed experimental setup was used realizing the friction scheme “pin-ondisk”. The working part of the installation (see Figure 1) was equipped with a force sensor the signals of which were then transformed into friction coefficient values. The steel disk (3) in the unit was fixed using a clamping washer (4). The loading device (2) was made in the form of a rod moving in guides equipped with rolling bearings. On the lower part of the rod, a pin holder was fixed through the screw, and sinkers were installed on its upper part. Figure 1: Pin-on-disk test rig: 1 - sample with MAOcoating; 2 - loading device; 3 - steel disk; 4 - clamping washer Figure 2: Exemplary friction curves obtained with DITA and PAO for MAO coatings produced using electrolyte №1 under reciprocating sliding with increasing load (first 900 s are running-in without friction measurement) T+S_5_18.qxp_T+S_2018 28.08.18 13: 50 Seite 7 In Figure 3a, PEG, DITA, and PFPE showed the lowest values of friction coefficients. However, the samples after the tests with PFPE were characterized by higher wear volumes for the coatings formed in the electrolyte №2, while PEG and DITA provided low wear values (Figure 3b). It is also noted that DITA showed the best Aus Wissenschaft und Forschung 8 Tribologie + Schmierungstechnik · 65. Jahrgang · 5/ 2018 and an early break down of the MAO-coating was observed on the two performed experiments, after about 8.3 and 41.6 min, respectively. For DITA, stable and constant friction coefficient occurred throughout the test despite increasing the load up to extreme pressure values. Figure 3a: Average friction coefficients of MAOcoated aluminium alloy against steel by using the reciprocating ball-on-disk sliding scheme with increasing load Figure 3b: Average wear volumes of MAO-coated aluminium alloy obtained after tests by using the reciprocating ball-on-disk sliding scheme with increasing load Figure 4: Photos of wear scars on MAO-coatings formed in (a) electrolyte №1 and (b) electrolyte №2 lubricated with PEG, DITA and PFPE Figure 5: Topographies of wear scars on MAO-coatings formed in (a) electrolyte №1 and (b) electrolyte №2 lubricated with PEG, DITA and PFPE. Note different height scale in image DITA b) T+S_5_18.qxp_T+S_2018 28.08.18 13: 50 Seite 8 tribotechnical characteristics in tests with increasing load. After tests with increasing load, wear scars were examined. Figure 4 shows the surfaces after testing with PEG, DITA and PFPE. Worn surfaces were similar for electrolyte №1 (a) and electrolyte №2 (b), except for PFPE where MAO-coating formed in electrolyte №2 showed severe failure. Figure 5 shows the topographies of the worn surfaces corresponding to the previous figure. As MAO-coatings did not fail when they were lubricated with PEG and DITA, PEG and DITA were selected for constant load tests. 3.2 Reciprocating sliding experiments at constant loads Tests at the constant loads of 400 and 600 N with PEG and DITA revealed that the best tribotechnical characteristics were found for MAO-coatings lubricated with DITA base oil even at a load of 600 N (see Figure 6). It is noted that at a load of 400 N, the friction coefficients for MAO-coatings formed in both electrolytes are approximately equal with DITA. Applying a constant load of 600 N, results suggest a more stable friction with MAO-coatings formed in electrolyte №2. 3.3 Unidirectional sliding experiments using various lubricants As shown in Figure 7a, the lowest values of friction coefficients were obtained for PAO and PEG at the load of 200 N. The wear value with PEG base oil is lower in the emergency mode than with PAO (see Figure 7b). In general, it can be said that in lubricated mode the MAOcoatings showed comparable wear values with any lubricant. The addition of ZDDP to PEG, PAO and DITA resulted in comparable friction and wear performance in the lu- Aus Wissenschaft und Forschung 9 Tribologie + Schmierungstechnik · 65. Jahrgang · 5/ 2018 Figure 7: Comparison of (a) friction coefficients and (b) wear of MAO-coated aluminium alloy obtained in lubricated mode and emergency mode by using the unidirectional pin-on-disk sliding scheme Figure 6: Average friction coefficients at a constant load of (a) 400 N and (b) 600 N by using the reciprocating ball-on-disk sliding scheme (a) (b) (a) (b) T+S_5_18.qxp_T+S_2018 28.08.18 13: 50 Seite 9 5 Conclusions As main findings of the work made, following conclusions can be drawn: - MAO-coatings showed good wear resistance with DITA and PEG base oil at different loads and contact types; - MAO-coatings with a more porous structure during testing have a lower wear volume in all lubricants, except for PFPE; - A positive effect of ZDDP on friction and wear was observed in the mode of oil starvation. As a recommendation, oils based on DITA and PEG can be suggested, because in tests with high contact loads (ball-on-disk tests), unlike other oils, they showed low wear and lower friction coefficient. At low loads, PAO based oils can also be recommended besides DITA and PEG. 6 Acknowledgements This work was funded by the “Austrian COMET-Program” in the frame of K2 XTribology (project no. 849109) and parts of the work were carried out within the “Excellence Centre of Tribology” (AC2T research GmbH). 7 References [1] Microarc Oxidation. Science and Humanity, Znanie, Moscow, Russia, 1981. [2] Markov G. A., Belevantsev V. I., Terleeva O. P., Shulepko E. K., and Slonova A. I. Microarc oxidation/ / Vestnik Mashinostroeniya, vol. 1, series 6, p. 34, 1992. [3] Yerokhin A. L., Nie H., Leyland A., Matthews A. and Dowey S. J. Plasma electrolysis for surface engineering./ / Surface and Coatings Technology, vol. 122, no. 2-3, pp. 73-93, 1999. [4] Yerokhin A. L., Snizhko L. O., Gurevina N. L., Leyland A., Pilkington A. and Matthews A. Discharge characterization in the plasma electrolytic oxidation of aluminium./ / Journal of Physics D, vol. 36, no. 17, pp. 2110-2120, 2003. [5] Petrosyants A. A., Malyshev V. N., Fedorov V. A. and Markov G. A. Wear kinetics of coatings deposited by microarc oxidation./ / Trenie i Iznos, vol. 5, no. 2, pp. 127- 130, 1984 (in Russian). [6] Malyshev V. N. Coating formation by anodic-cathodic microarc oxidation./ / Zashchita Metallov, vol. 32, no. 6, pp. 662-667, 1996 (in Russian). Aus Wissenschaft und Forschung 10 Tribologie + Schmierungstechnik · 65. Jahrgang · 5/ 2018 bricated mode except PEG+ZDDP with increased friction. In emergency mode, i.e., in the mode of oil starvation, the friction coefficient by using DITA and wear values by PAO and DITA could be reduced by using ZDDP. The addition of 5 % DITA to PAO base oil revealed some improvement in comparison with PAO without additive. 4 Discussion Analyzing the results obtained, it can be concluded that the nature of the lubricants plays an important role regarding the tribological characteristics of the MAOcoating-steel pairing. With increasing load in the ballon-disk contact, the lowest values of the friction coefficient were found with PEG, DITA and PFPE (Figure 3a) whereas high friction was observed for PAO and silicone oil. Similar results were obtained for wear: PEG, DITA, and PFPE have provided low wear during the test (Figure 3b). The study of wear scars (Figure 4 and Figure 5) revealed that the MAO-coatings formed in both electrolytes did not break down when they were lubricated with PEG and DITA. When friction tests were conducted with PFPE, destruction was observed for the MAO-coatings formed in electrolyte №2. In all other cases, rubbing destroyed the MAO-coatings. Concluding the first stage of research, PEG and DITA showed the best results in the reciprocating ball-on-disk sliding scheme with increasing load. Accordingly, PAO, PFPE and silicone oil were eliminated from further testing whereas PEG and DITA were subjected to tests applying constant loads. In the second stage, reciprocating ball-on-disk sliding tests were performed at a constant load of 400 and 600 N. The best results on friction coefficient were obtained with DITA at both loads (Figure 6). By trend, MAOcoatings formed in electrolyte №2 provided a more stable friction at 600 N. In the final stage of the studies, tests in lubricated and emergency mode were carried out using the unidirectional pin-on-disk sliding scheme. In general, the results obtained were similar to those elaborated in the first stage, except PAO which also showed relatively good tribotechnical properties. The addition of ZDDP to the base oils contributed to the improvement of the tribotechnical characteristics of oils and hence can be considered for the formulation of new lubricants (Figure 7). The beneficial role of such additive is more visible in the oil starvation (emergency) mode when the amount of oil is limited by the volume left in pores of the MAOcoating. T+S_5_18.qxp_T+S_2018 28.08.18 13: 50 Seite 10 [7] Malyschev V. N. Mikrolichtbogen-Oxidation-ein neuartiges Verfahren zur Verfestigung von Aluminiumoberflaechen./ / Metalloberflaeche, no. 8, pp. 606-608, 1995. [8] Alekhin V. P., Fedorov V. A., Bulychev S. I., Tyurpenko O. A. Features of microstructure of hardened surface layers obtained by microarc oxidation. / / Physics and Chemistry of Materials Processing, no. 5, pp. 121-126, 1991. [9] Malyshev V. N. Modification of the surface layer of parts by the method of microarc oxidation: state, possibilities, prospects / / Quality management in the oil and gas complex, no. 1, pp. 15-20, 2014. [10] Malyshev V. N. Wear resistance evaluation of ceramic coatings formed by microarc oxidation method. / / Sb. Works of international scientific and practical. Conference “Fundamental problems and modern technologies in mechanical engineering”. Moscow. Mechanical Engineering, p. 571, 2010. [11] Malyshev V. N., Bulychev S. I., Markov G. A. Physicomechanical characteristics and wear resistance of coatings deposited by the method Microarc oxidation. / / Physical chemistry and treatment, №1, pp. 82-87, 1985 (in Russian). [12] Malyshev V. N., Markov G. A., Fedorov V. F., Petrosyants A. A., Terleeva O. P. Features of the structure and properties of coatings applied by the method of microarc oxidation / / Chemical and petroleum engineering. №1, pp. 26- 28, 1984. [13] Crete B., Ludin V., Suminov I. et al. MAO-synthesis of nanoceramic layers on details of metals and alloys. / / Nanoindustry, №2, pp.12-13, 2010. [14] Pichugin V. F., Gantimirov B. M., Nesterov N. B. Tribotechnics of lubricants. Moscow: GANG, 1996. [15] Mang T., Drezel U. Lubricants. Production, application, properties. Reference book / trans. With the English. 2 nd ed.; Ed. V.M. Shkolnikova, St. Petersburg: OCP “Profession”, Ill, 944 p., 2015. [16] Fitch J., Troyer D. Analysis of oils. Fundamentals and applications / Trans. With the English. 2 nd ed.; Ed. E.A. Novikova, M.V. Kiriukhina, St. Petersburg: OCP “Profession”, 176 p., 2015 (in Russian). [17] Malyshev V. N., Volkhin A. M. Technique of coatings porosity determining. / Proceeding of XI International technical and scientific conference “Progressive Technology in Modern Mechanical Engineering”, Penza, pp. 26-30, 2016 (in Russian). [18] ASTM Standard G133-02. Standard Test Method for Lineary Reciprocating Ball-on-Flat Sliding Wear. ASTM Standards. Philadelphia, PA, USA. 2002 [19] ASTM Standard G99-04. Standard Test Method for Wear Testing with Pin-on-Disk Apparatus. ASTM Standards Philadelphia, PA, USA. 2004 Aus Wissenschaft und Forschung 11 Tribologie + Schmierungstechnik · 65. 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