metals

Article Effect of Hydrogen on the Wear Resistance of Steels upon Contact with Plasma Electrolytic Oxidation Layers Synthesized on Aluminum Alloys

Volodymyr Hutsaylyuk 1,* , Mykhailo Student 2, Volodymyr Dovhunyk 2, Volodymyr Posuvailo 2, Oleksandra Student 2, Pavlo Maruschak 3 and Ihor Koval’chuck 2

1 Department of Machine Design, Military University of Technology, Gen. S. Kaliskiego str. 2, 00-908 Warsaw, Poland 2 Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, 79060 Lviv, Ukraine; [email protected] (M.S.); [email protected] (V.D.); [email protected] (V.P.); [email protected] (O.S.); [email protected] (I.K.) 3 Department of Industrial Automation, Ternopil National Ivan Pul’uj Technical University, 46001 Ternopil, Ukraine; [email protected] * Correspondence: [email protected]; Tel.: +48-22-261-839-245

 Received: 16 January 2019; Accepted: 22 February 2019; Published: 1 March 2019 

Abstract: The different nature of the effect of hydrogen on the tribological behavior of two carbon steels (st. 45 and st. U8) upon their contact with super solid plasma electrolytic oxidation (PEO) layers synthesized on two light alloys (AMg-6 and D16T alloys as the analogists of the A 95556 UNS USA and AA2024 ANSI USA alloys correspondently) was investigated in the medium of mineral oil of the I-20 type. To compare the effect of hydrogenation on the tribological properties of the analyzed contact pairs, similar tests were also performed on the same mineral oil with clear water or an aqueous solution of glycerine added to its content. A spinel-type film (hercynite) was formed upon friction of two contacting surfaces—the -carbon steels and PEO layers synthesized on the AMg-6 alloy. This film was a reliable protection against wear of the surface subjected to the effect of hydrogen. When steels came into contact with the PEO layers synthesized on the D16T alloy, surface protection against wear was ensured by another mechanism. The phenomenon of selective metal transfer in the friction zone (from one to another friction surfaces) was revealed.

Keywords: plasma electrolytic oxidation (PEO); hydrogen effect; additives to lubrication; water and glycerine effect

1. Introduction Due to their high specific strength, aluminum structural alloys are widely used in the aviation industry, automotive industry, power production, and the construction industry [1,2]. However, the parts from aluminum alloys are characterized by a short service life due to their low abrasive wear resistance and a tendency to point capture in the process of friction interaction with other materials. To increase their serviceability, the hard chrome plating is galvanically applied to the working surfaces of the parts [3–7]. However, the process of galvanic chrome plating is environmentally dangerous and, therefore, is gradually withdrawn from technological processes. The synthesis of oxide layers on aluminum alloys by the method of plasma electrolytic oxidation (PEO) in weakly alkaline electrolytes can be an alternative to galvanic chrome plating. Oxide layers synthesized on aluminum alloys have a high hardness (1200 HV 0.2–2000 HV 0.2) and wear and resistance [7–16]. In addition, PEO layers synthesized on lightweight alloys have significant advantages over other coatings used in

Metals 2019, 9, 280; doi:10.3390/met9030280 www.mdpi.com/journal/metals Metals 2019, 9, 280 2 of 14 friction units. However, the insufficient knowledge about the behavior of oxide layers in the conditions of tribological tests hinders the introduction of this technology into production. To use light alloys in friction units, their surface layers are modified. The most promising is PEO synthesis technology. This technology provides for the transformation of light alloy surfaces into composite oxide-ceramic layers with high wear resistance. Such PEO layers are an ideal material for use in friction units of internal combustion engines, turbine components, slide bearings, and the like. Aluminum alloys are often exploited under conditions of boundary lubrication in friction pairs “PEO layer–steel”. In the process of frictional interaction of these friction pairs, hydrogen is formed due to the destruction of technological media (oils or aqueous solutions), causing the degradation of the tribological characteristics of these friction pairs. Permanent cyclic deformation and destruction of the protrusions formed on surfaces of contacting friction pairs occur during their tribological interaction. As a result, juvenile (freshly formed) surfaces are formed during friction process, and conditions are created for synthesizing the structurally new (secondary) compounds with high wear resistance that provide for the low wear of metal on these local sites. The intensity of these processes depends on the properties of the lubricating medium used, the load applied to a friction pair, the composition and structural condition of materials of the contacting surfaces [16]. The degradation of the lubricating medium in the friction zone occurs more easily due to the high local temperatures in the points of maximum contact pressure of the friction surfaces. Atomic hydrogen formed in these local contact zones acts as a catalyst of the tribochemical reactions [17]. There are several features that describe the interaction between hydrogen and metals (in particular, steels) caused by the existence of diffusive and residual hydrogen [18–20]. In the atomic form, hydrogen can be “diffusion active” and move freely in the crystal lattice of the metal. Alternatively, it can be captured by low-energy crystal traps (vacancies, dislocations, grain boundaries, and sub-boundaries). The “residual” hydrogen can also be present in the metal. In the atomic form, hydrogen will be bound in high-energy traps of the crystal lattice. In molecular form, it will be located in bulk defects (pores and microcracks) [21]. Such hydrogen is difficult to remove from the crystal lattice of the metal. However, during frictional interaction, when the temperature of the surface layers between the friction pairs exceeds 450 ◦C, hydrogen in atomic form can be released from the crystal lattice near the friction surface. The purpose of this research is to determine the effect of hydrogen on the tribological properties of friction pairs consisting of conductive steel elements and dielectric PEO layers synthesized on aluminum alloys.

2. Materials and Methods The behavior of friction pairs of “steel–PEO layers synthesized on aluminum alloys of two widely used groups” was investigated. The first group is Al-Mg alloys (Mg 2 ... 6 wt.%), which cannot be hardened by heat treatment. The second group is Al-Cu alloys (Cu 3 ... 6 wt. %), which can be strengthened by heat treatment. PEO layers with dielectric properties were synthesized on the light alloys AMg-6 and D16T. Steel 45 and steel U8 with pronounced conductor properties were used as tribological pairs during wear tests. The chemical compositions of the materials used are presented in Table1. The disc-shaped specimens with a thickness of 10.0 mm and a diameter of 42.0 mm were made of alloys AMg-6 and D16T. The high hardness layers were synthesized by PEO methods on the cylindrical surfaces of these specimens in an electrolyte with 3.0 g/l of KOH + 2 g/l of Na2SiO3 using the cathode-anode mode of the pulsating current with a frequency of 50 Hz. The ratio of the cathode 2 and anode currents was Ic/Ia = 1.0 at a current of 20.0 A/dm . The thickness of the synthesized layer was 250–300 µm. The microhardness of the PEO layer synthesized on the AMg-6 alloy was 1200–1400 HV 0.2, and that of the PEO layer synthesized on the D16T alloy was 1400–1005 HV 0.2. Before the tribological tests, all specimens with PEO layers synthesized on their cylindrical surfaces Metals 2019, 9, 280 3 of 14 were polished by diamond grinding wheel to obtain the outer diameter d = 42.0 ± 0.02 mm at a surface roughness Ra = 0.6 µm.

Table 1. The chemical compositions of the investigated materials, wt. %.

Elements AMG-6 D16T Steel 45 Steel U8 C - - 0.43 0.8 Al 91.5 92.3 - - Mg 6.2 1.5 - - Cu 0.1 4.1 0.06 0.05 Mn 0.6 0.6 0.62 0.19 Fe 0.2 0.2 - - Si 0.3 0.4 0.23 0.21 Zn 0.1 0.03 - - Cr - 0.07 0.08 0.09 Ti - 0.012 - - Be - - - - Ni - - 0.1 0.12 S - - 0.04 0.028 P - - 0.035 0.03

Steel specimens were previously annealed in vacuum at 800 ◦C for 1.0 h. The hardness value (HV0,2) of steel 45 steel was about 180 HB, and that of steel U8 was about 190 HB. After annealing, the specimens were electrolytically hydrogenated in the 1N H2SO4 solution with the addition of 10 mg/L of As2O3 as a stimulator of the hydrogenation process. The steel specimens were hydrogenated for 1.0 h at a density of the cathode current of 1.0, 2.0, and 3.0 A/dm2, or at a density of 2.0 A/dm2 for 1.0, 2.0 and 3.0 h. This electrolyte solution was used at room temperature together with the platinum grid as an anode. The release of hydrogen from the acid solution can be represented by a total reaction: − 2H3O + 2e → H2 + 2H2O. The effect of diffusion hydrogen on the tribological properties of both steels was estimated after holding hydrogenated specimens in air for 15 min, whereas the effect of the residual hydrogen on these properties was determined after holding the hydrogenated specimens for 24.0 h. The tribological properties of both steels in contact with the obtained PEO layers were estimated on the testing machine SMC-2 (Tochmashpribor, Ivanovo, Russia). The schematic diagram of these friction tests is shown in Figure1. It was believed that this test pattern was most relevant for simulating the frictional behavior of crankshafts under boundary lubrication conditions. The “disk-block” friction scheme that describes the contact between disc-shaped specimens from light alloys and cylindrical surfaces of the steel specimens in the form of disc segments was used. Their diameters in the contact zone corresponded to one another (42.0 ± 0.02 mm). The ratio between the friction surfaces of both elements was 0.125. During the tribological tests, the fixed element of the friction pair in the form of disk-segment (namely, the block) was clamped by means of a self-adjusting device that guaranteed the stability of pressing of the surfaces in the zone of contact, and the stable mutual position of the contacting surfaces during the long-term wear resistance tests. Metals 2018, 8, x FOR PEER REVIEW 3 of 14

Zn 0.1 0.03 - - Cr - 0.07 0.08 0.09 Ti - 0.012 - - Ве - - - - Ni - - 0.1 0.12 S - - 0.04 0.028 P - - 0.035 0.03

The disc-shaped specimens with a thickness of 10.0 mm and a diameter of 42.0 mm were made of alloys AMg-6 and D16T. The high hardness layers were synthesized by PEO methods on the cylindrical surfaces of these specimens in an electrolyte with 3.0 g/l of KOH + 2 g/l of Na2SiO3 using the cathode-anode mode of the pulsating current with a frequency of 50 Hz. The ratio of the cathode and anode currents was Ic/Ia = 1.0 at a current density of 20.0 A/dm2. The thickness of the synthesized layer was 250–300 µm. The microhardness of the PEO layer synthesized on the AMg-6 alloy was 1200–1400 HV 0.2, and that of the PEO layer synthesized on the D16T alloy was 1400–1005 HV 0.2. Before the tribological tests, all specimens with PEO layers synthesized on their cylindrical surfaces were polished by diamond grinding wheel to obtain the outer diameter d = 42.0 ± 0.02 mm at a surface roughness Ra = 0.6 µm. Steel specimens were previously annealed in vacuum at 800 °C for 1.0 h. The hardness value (HV0,2) of steel 45 steel was about 180 НВ, and that of steel U8 was about 190 НВ. After annealing, the specimens were electrolytically hydrogenated in the 1N H2SO4 solution with the addition of 10 mg/l of As2O3 as a stimulator of the hydrogenation process. The steel specimens were hydrogenated for 1.0 hour at a density of the cathode current of 1.0, 2.0, and 3.0 A/dm2, or at a density of 2.0 A/dm2 for 1.0, 2.0 and 3.0 hours. This electrolyte solution was used at room temperature together with the platinum grid as an anode. The release of hydrogen from the acid solution can be represented by a total reaction: 2Н3О + 2е−→ Н2 + 2Н2O. The effect of diffusion hydrogen on the tribological properties of both steels was estimated after holding hydrogenated specimens in air for 15 min, whereas the effect of the residual hydrogen on these properties was determined after holding the hydrogenated specimens for 24.0 hours. The tribological properties of both steels in contact with the obtained PEO layers were estimated on the testing machine SMC-2 (Tochmashpribor, Ivanovo, Russia). The schematic diagram of these friction tests is shown in Figure 1. It was believed that this test pattern was most relevant for simulating the frictional behavior of crankshafts under boundary lubrication conditions. The “disk-block” friction scheme that describes the contact between disc-shaped specimens from light alloys and cylindrical surfaces of the steel specimens in the form of disc segments was used. Their diameters in the contact zone corresponded to one another (42.0 ± 0.02 mm). The ratio between the friction surfaces of both elements was 0.125. During the tribological tests, the fixed element of the friction pair in the form of disk-segment (namely, the block) was clamped by means of a selfMetals-adjusting2019, 9, 280 device that guaranteed the stability of pressing of the surfaces in the zone of contact,4 of 14 and the stable mutual position of the contacting surfaces during the long-term wear resistance tests.

FigureFigure 1. 1. SchematicSchematic diagram diagram of of the friction tests: 11 —rotating— rotating specimen specimen in in the the form form of of a a disk disk from from the the D16TD16T or AMg16 AMg16 light light alloys alloys with with plasma plasma electrolytic electrolytic oxidation oxidation (PEO) (PEO) layers layers synthesized synthesized on their on outer their cylindrical surface; 2—blocking segment-like specimen from the 45 or U8 steels; 3—reservoir with

lubricant, 4—lubricating medium; P—pressing force.

This made it possible to correctly assess the changes in the tribological characteristics of the materials studied. During all test, the changes of two parameters—the frictional moment and heating temperature—were recorded simultaneously in the friction zone. The frictional moment was estimated by an inductive sensor located on the rotating shaft of the testing machine with the investigated disk fixed on it. The heating temperature in the contact zone of the friction pair was determined using the chromel- thermocouple fixed in the holes made on steel specimens at a distance of 0.5 mm from the surface of their contact. The sliding rate was 0.67 m/s, the specific load in the friction zone between both tribo-elements was 4.0 MPa, the test duration was 4.0 h. These specific load levels in friction testing (electrical signals in millivolts from the corresponding strain gauges) were controlled and recorded in the computer memory with increments of 0.25 sec, and were converted into friction coefficient values. Tribological characteristics (mass loss and coefficient of friction) were determined as the arithmetic mean value of 5 measurements. The maximum deviation of the current measurements of the weight loss by each specimen investigated did not exceed 5.0% of the mean arithmetic value for 5 measurements. Since the variance of this parameter was very small, its in-depth statistical analysis was not performed. The environmentally friendly additives, in particular, aqueous solutions of glycerine, which are well combined with traditional mineral oils, have been used in recent years to increase the performance of friction pairs. Such mixtures have better lubricating properties than pure mineral oil. If they are used, the coefficient of friction and wear of metal pairs contacting at conditions of the boundary lubrication can be reduced by an order of magnitude [22,23]. Mineral oil of the I-20 type (GOST 20799-88, Moscow, SU) was used as a base lubricant in the friction test. To reveal the effect of glycerine on the tribo-characteristics of steel specimens, a 1.0% aqueous solution of glycerine was added to this oil. As a result, the contents of glycerine and water in one liter of the final composition of the lubricating medium were 0.25 mL and 10 mL, respectively. To reveal the effect of water on the friction properties of both steels, 0.5 vol. % of distilled water was also added to the base oil composition. Wear resistance of the steel specimens was estimated using the data on their weight loss by the gravimetric method using the Radwag WAA 160 analytical balance (Radwag Balances and Scales, Radom, Poland) with a measurement error ±104 g. The phase analysis of the PEO layers was carried out on a diffractometer DRON-3M (Bourevestnik JSC, Saint-Petersburg, Russia) using the Cu-Kα radiation. The secondary structures formed on the surfaces of the steel specimens due to its friction with PEO layers was analyzed using the Co-Kα radiation. The features of the surfaces formed after the friction tests were studied using the Zeiss Stemi-2000c microscope (Carl Zeiss Mikroskopie, Jena, Germany). The microstructure and elemental analysis were Metals 2019, 9, 280 5 of 14 obtained using the EVO-40XVP (Carl Zeiss Mikroskopie, Jena, Germany) scanning electron microscope with the INCA-Energy system for the X-ray microanalysis.

3. Results

3.1. Phase Composition of PEO Layers Synthesized on Light Alloys It is known that the phase composition of the PEO layers formed on light alloys is determined by the chemical compositions of the substrate materials and the electrolytes used. PEO layers synthesized on the AMg-6 and D16T light alloys were examined by the phase analysis using the X-ray diffraction method (Figure2, Table2). In the PEO layer synthesized on the AMg-6 alloy, in addition to the well-known phases such as α-Al2O3 and γ-Al2O3 [24–29], we found the phase Metalsof Mg 2018 and, 8, anx FOR insignificant PEER REVIEW amount of sillimanite MgAl2Si3O10. The nanoscale particles of Cu5 of and 14 Cu2O were additionally found in the PEO layer synthesized on the D16T alloy (Tables2 and3)[ 13–16]. The revealed revealed differences differences in in the the phase phase composition composition of the of synthesized the synthesized PEO PEOlayers layers may affect may their affect theirtribological tribological properties properties significantly significantly upon contact upon with contact the with carbon the steels carbon in different steels in media, different Figure media,3. Figure 3.

(a)

(b)

Figure 2. 2. X-X-rayray diffraction diffraction patterns patterns of the of PEO the PEOlayers layers synthesized synthesized on AMg on-6 AMg-6(a) and (Da )16T and (b D) alloys. 16T

(Theb) alloys. designations The designations of revealed of revealed phases: phases: (a) 1 (—a)α 1—-Alα2O-Al3, 2O 2—3,Mg. 2—Mg. 3— 3—-Alγ-Al2O32, O 43—, 4—MgAlMgAl2Si23OSi103O; 10 (b; ) (1b—)α 1—-Alα2-AlO3,2 2O—3,Cu, 2—Cu, 3— 3—-Alγ2O-Al3, 42O—3Cu, 4—Cu2O, 5—2O,SiO 5—SiO2. 2.

Table 2. Phase composition of the PEO layers synthesized on AMg-6 and D16T alloys.

PEO layer formed on the AMg-6 alloy PEO layer formed on the D16T alloy Components SG* mass. % Components SG* mass. %

α-Al2O3 R -3 C 54.72 α-Al2O3 R -3 C 56.26 Mg P63/mmc 2.69 Cu F M 3 M 2.45

-Al2O3 F D -3 M 34.56 -Al2O3 F D -3 M 37.08

MgAl2Si3O10** P 6 2 2 8.03 Cu2O P N 3 M 2.93

- - - SiO2 P 3 2 1.28

* SG—spatial group; ** MgAl2Si3O10—spinel composition (MgO·Al2O3·3SiO2).

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Table 2. Phase composition of the PEO layers synthesized on AMg-6 and D16T alloys.

PEO Layer Formed on the AMg-6 Alloy PEO Layer Formed on the D16T Alloy Components SG * mass. % Components SG * mass. %

α-Al2O3 R -3 C 54.72 α-Al2O3 R -3 C 56.26 Mg P63/mmc 2.69 Cu F M 3 M 2.45 γ-Al2O3 F D -3 M 34.56 γ-Al2O3 F D -3 M 37.08 Metals 2018, 8, x FOR PEER REVIEW 6 of 14 MgAl2Si3O10 ** P 6 2 2 8.03 Cu2O P N 3 M 2.93 --- SiO2 P 3 2 1.28 Table 3.* Element SG—spatial composition group; ** MgAl of the2Si 3PEOO10—spinel layers compositionsynthesized (MgO on АМg·Al2O-63 ·and3SiO D16T2). alloys.

Table 3. Element composition of the PEOPEO layers layer synthesized on AMg-6 and D16T alloys. AMg-6 D16T Element mass.%PEO Layerat.% mass.% at.% Na AMg-60.1 0.1 0.2 D16T0.3 Element Mgmass.% 1.8 at.%0.8 1.1 mass.% 0.6 at.% NaAl 0.149.4 0.137.4 42.6 0.2 33.0 0.3 MgSi 1.81.2 0.80.7 4.2 1.1 3.2 0.6 AlK 49.40.5 37.40.4 2.1 42.6 1.0 33.0 SiCa 1.2- 0.7- 0.3 4.2 0.2 3.2 K 0.5 0.4 2.1 1.0 CaCu -- -- 3.1 0.3 1.1 0.2 CuO -50.0 -59.8 46.4 3.1 60.4 1.1 O 50.0 59.8 46.4 60.4

(a) (b)

FigureFigure 3.3. MicrostructureMicrostructure ofof PEOPEO layerslayers synthesizedsynthesized onon thethe alloys alloys AMg-6 AMg-6 ( a(a),), D D 16T 16T ( b(b).).

3.2. Results of Tribological Tests During friction friction tests tests of of all all types, types, weight weight loss loss of PEO of layersPEO layers was not was recorded not recorded in either in the either AMG-6 the alloyAMG- or6 alloyD16T or alloy. D16T Therefore, alloy. Therefore, only the massonly theloss mass of segmented loss of segmented steel specimens steel wasspecimens analyzed. was 2 Itanalyzed. was found It was that found an increase that an in increase current in density current from density 1.0 tofrom 3.0 1 A/dm.0 to 3.0during A/dm2short-term during short (1.0-term h) hydrogenation(1.0 h) hydrogenation of specimens of specimens from steel from 45 steel had practically45 had pra noctically effect no on effect their on wear their resistance wear resistance due to theirdue to tribocontact their tribocontact with disc with specimens disc specimens with a PEO with layer a PEO synthesized layer synthesized on the AMg-6 on the alloy AMg to-6 provide alloy to diffusiveprovide anddiffusive residual and hydrogen residual (Figure hydrogen4a). With (Figure an increase 4a). With in the anduration increase of hydrogenation, in the duration wear of resistancehydrogenation, of steel wear specimens resist decreasesance of steel significantly, specimens in the decreases case of both significantly, diffusive and in residual the case hydrogen of both (Figurediffusive4b). and The residual effect of hydrogen diffusive (Figure and residual 4b). The hydrogen effect of on diffusive wear was and evaluated residual after hydrogen the specimens on wear 2 2 2 werewas evaluated hydrogenated after at the a currentspecimens density were of hydrogenated 1.0 A/dm (columns at a current 2), 2.0 density A/dm of(columnrs 1.0 A/dm 3),2 (c 3.0olumns A/dm 2), 2 (columnrs2.0 A/dm2 4)(columnrs for 1.0 h, 3), Figure 3.0 A/dm4a, and2 (columnrs at the current 4) for density 1.0 h, Figure of 2.0 A/dm4a, and atfor the 1.0 current h (columns density 2), 2.0of 2.0 h (columnsA/dm2 for 3), 1.0 and h (columns 3.0 h (columns 2), 2.0 h 4), (columns Figure4b. 3), and 3.0 h (columns 4), Figure 4b.

Metals 2019, 9, 280 7 of 14 Metals 2018, 8, x FOR PEER REVIEW 7 of 14

(a) (b)

FigureFigure 4. Weight 4. Weight losses losses of specimensof specimens from from steel steel 45, 45, takingtaking into account account preliminary preliminary hydrogenation hydrogenation at at 2 2 a currenta current density density of 1.0 of A/dm1.0 A/dm2 (a )(a and) and 2.0 2.0 A/dm A/dm2 (b)) during their their tribocontact tribocontact with with the the PEO PEO layer layer synthesized on the AMg-6 alloy, determined by testing in mineral oil I-20 by contact load 4.0 MPa; synthesized on the AMg-6 alloy, determined by testing in mineral oil I-20 by contact load 4.0 MPa; 1—without preliminary hydrogenation; 2–4 — taking into account hydrogenation (hydrogenаtion 1—without preliminary hydrogenation; 2–4—taking into account hydrogenation (hydrogenation parameters are shown on columns). parameters are shown on columns). Although the hydrogen content in the steel specimens increases with an increase in current Althoughdensity from the 1.0 hydrogen to 3.0 A/dm content2 during in their the electrolytic steel specimens hydrogenation increases for with1.0 h, anthis increase concentration in current is 2 densitynot from sufficient 1.0 to for 3.0 the A/dm formationduring of microdamage. their electrolytic Therefore, hydrogenation such a mode for of 1.0 hydrogenation h, this concentration has no is noteffect sufficient on wear for resistance the formation of the steel of microdamage. specimens. If the Therefore, hydrogenation such atime mode increases of hydrogenation from 1.0 to 3.0 has h, no effectwear on wear resistance resistance of steel of specimens the steel specimens.decreased nearly If the 5- hydrogenationfold under the influence time increases of residual from hydrogen, 1.0 to 3.0 h, wearand resistance up to 12 of-fold steel under specimens the influence decreased of diffusion nearly hydrogen. 5-fold under It is possible the influence to explain of residualthis effect hydrogen, by an and upincrease to 12-fold in the under hydrogen the influencecontent in ofthe diffusion near-surface hydrogen. layers of Itsteel is possible specimens to causing explain microdamage this effect by an increasein metal. in the As hydrogen a result, the content steel wear in the resistance near-surface is decreased layers significantly. of steel specimens causing microdamage in The effect of hydrogenation on the tribological properties of st. 45 and st. U8 upon their friction metal. As a result, the steel wear resistance is decreased significantly. with the PEO layers synthesized on the AMg-6 and D16T alloys was also compared (Figure 5). Steel The effect of hydrogenation on the tribological properties of st. 45 and st. U8 upon their friction specimens were friction tested in the pure mineral oil of I-20 type or mineral oil with the distilled withwater the PEO or anlayers aqueous synthesized solution on with the glycerine AMg-6 andadded D16T to it. alloys Non-hydrogenated was also compared steel specimens (Figure5 ). and Steel specimensspecimens were after friction preliminary tested electrolytic in the pure hydrogenation mineral oil ofwere I-20 compared. type ormineral Water in oil different with the states distilled is waterquite or an often aqueous found in solution working with oils (as glycerine process addedenvironments), to it. Non-hydrogenated so, oil with water added steel to specimens it was also and specimensused in after laboratory preliminary tests. First electrolytic of all, water hydrogenation may be dissolved were or compared.chemically bonded Water with in different oil. It should states is quitebe often noted found that if in water working is not oilsremoved (as process by conventional environments), methods, so, it is oil pr withactically water harmless added at to a itcontent was also usedup in laboratoryto 0.15%. tests. First of all, water may be dissolved or chemically bonded with oil. It should be noted thatWater if may water also is notbe present removed in the by form conventional of emulsion methods, characterized it is practically by bonds of harmless different atstrength a content with molecules of organic oil. In this case, the water content in the oil can reach up to 2.0–3.0%. Due up to 0.15%. to the formation of such suspensions, oil can freely leak from the crankcase of internal combustion Water may also be present in the form of emulsion characterized by bonds of different strength engines and oil tanks, so there is no need to use controlling devices [22,23]. with moleculesWear resistance of organic of oil. the In previously this case, hydrogenatedthe water content specimens in the fromoil can st. reach 45 and up st. to U8 2.0–3.0%. upon the Dueir to the formationfrictional interaction of such suspensions, with PEO layers oil synthesized can freely on leak AMg from-6 and the D16T crankcase alloys ofin mineral internal oil combustion of type enginesI-20, and with oil the tanks, addition so there of water is no need (Figure to 5, use columns controlling 2) or devices an aqueous [22,23 solution]. of glycerine, was Wearincreased resistance (Figure of5, columns the previously 5). Regardless hydrogenated of the test specimensconditions, fromthe wear st. resistance 45 and st. of U8both upon steels their frictionalupon interaction friction contact with with PEO PEO layers layer synthesized synthesized on on AMg-6 the surface and D16T of the alloys AMg in-6 mineralalloy is nearly oil of type5–10 I-20, withtimes the addition higher than of waterupon contact (Figure with5, columns the PEO 2)layer or synthesized an aqueous on solution the surface of glycerine, of the D16T was alloy. increased (Figure5, columns 5). Regardless of the test conditions, the wear resistance of both steels upon friction contact with PEO layer synthesized on the surface of the AMg-6 alloy is nearly 5–10 times higher than upon contact with the PEO layer synthesized on the surface of the D16T alloy.

Metals 2019, 9, 280 8 of 14

Metals 2018, 8, x FOR PEER REVIEW 8 of 14

(a) (b)

FigureFigure 5. The mass loss of the 4545 andand U8U8 steelssteels afterafter theirtheir tribocontacttribocontact withwith PEOPEO layerslayers synthesizedsynthesized onon the the AMg AMg-6-6 (a (a) )and and D16T D16T ( (bb)) alloys alloys estimated estimated by by the the weight weight losses Δ∆W of specimens tested at unitunit loadload level level of of 4 4 MPa MPa within within 4 4 h: h: in in the the pure pure mineral mineral oil oil of the I-20I-20 type ((1);); afterafter theirtheir hydrogenationhydrogenation withinwithin 1 1h hat at current current density density 2.0 2.0 A/dm A/dm2 with2 with following following friction friction in the in mineral the mineral oil of oil the of I- the20 type I-20 typewith thewith addition the addition of 0.5 of vol. 0.5 vol. % distilled % distilled water water (2); ( 2 aft); afterer hydrogenation hydrogenation of of specimens specimens followed followed by their their holdingholding in in air air within within 15 15 min min ( (33)) and and 24 24.0.0 h h ( (44)) to to provide the diffusive or residualresidual hydrogenhydrogen inin thethe metal,metal, correspondently; correspondently; in in the the mineral mineral oil oil of of the the I I-20-20 type type with the addition of 1.0 vol. % 2.5% aqueous sosolutionlution of of glycerin glycerin (5 (5).).

Hence,Hence, it it was was concluded that that if if the the hydrogen hydrogen content content in the in the surface surface layers layers of steel of steelspecimens specimens does doesnot create not create favorable favorable conditions conditions for microdamage for microdamage formation, formation, the wear the resistance wear resistance of steels increases of steels increasesregardless regardless of the hydrogenation of the hydrogenation conditions. co Suchnditions. conditions Such are conditions realized in are all realized of the analyzed in all of cases the analyzedwhen both cases steels when are inboth contact steels with are thein contact PEO layer with synthesized the PEO layer on the synthesized AMg-6 alloy on afterthe AMg a short-term-6 alloy after(1.0 h)a short preliminary-term (1.0 hydrogenation hour) preliminary of steel hydrogenation specimens (Figure of steel5a). specimens In addition, (Figure the current5a). In addition, density theduring current short-term density hydrogenationduring short-term (in particular, hydrogenation of steel (in 45) particular, does not significantlyof steel 45) does affect not its wearsignificantly during affectfriction its testswear (Figureduring4 frictiona). tests (Figure 4a). TheThe friction friction coefficient coefficient of of both both investigated investigated steels steels at at their tribotribo-contact-contact with the PEO layer layer synthesizedsynthesized on on the the AMg AMg-6-6 alloy alloy in in pure pure mineral mineral oil oil is is decreased significantly significantly under the influenceinfluence ofof bothboth types types of of hydrogen hydrogen (diffusive (diffusive and and residual) residual) in in the the surface surface layers layers (Figure (Figure 66a,a, columnscolumns 33 andand 4).4). TheThe exception exception is is the the effect effect of of residual residual hydrogen hydrogen on on this this coefficie coefficientnt for the U8 steel (Figure6 6a,a, columncolumn 3).3). It It is is also also difficult difficult to to explain explain the the highest highest values values of of the friction coefficientcoefficient for both steelssteels withoutwithout preliminarypreliminary hydrogenation hydrogenation upon upon contact contact with with the the PEO PEO layer layer synthesized on the AMg-6AMg-6 alloy within ll hourh in in the the course course of of their their tribo-testing tribo-testing in oilin oil with with the the addition addition of 0.5%of 0.5% water water (Figure (Figure6a, column6a, column 2). 2). UnderUnder the the influence influence of of hydrogen, hydrogen, the the friction friction coefficients coefficients of of both steels with the PEO layer layer synthesizedsynthesized on on the the D16T D16T alloy alloy changed changed approximately approximately in in the the same same way way as as in contact with thethe PEOPEO layerlayer synthesized synthesized on on the the AMg AMg-6-6 alloy, exceptexcept for for the the case case when when glycerine glycerine was was added added to theto the lubricant. lubricant. In Inthis this case, case, the the coefficient coefficient of frictionof friction decreased decreased by approximatelyby approximately 20.0 20.0 times times (Figure (Figure6b, columns 6b, columns 5). This 5). Thisresult result is in agreementis in agreemen witht the with heating the heating temperature temperature due to tribocontact due to tribocontact in the friction in the zones friction (Figure zones6d, (Figurecolumns 6d, 5). columns The temperatures 5). The temperatures in friction zones in friction between zones both between steels and both the PEO steels layer and synthesized the PEO layer on synthesizedthe D16T alloy on inthe oil D16T with alloy the aqueous in oil with solution the aqueous of glycerine solution added of are glycerine up to 40–80 added degrees are up lower to 40 than–80 degreesin other lower conditions than in of other friction conditions tests (Figure of friction6d, columns tests (Figure 5). 6d, columns 5).

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(a) (b)

(c) (d)

FigureFigure 6. Averaged 6. Averaged values values of the of friction the friction coefficients coefficients (a,b ) (a,b and) andthe heating the heating temperatures temperatures in the in vicinitythe of thevicinity tribocontact of the tribocontact zones (c,d )zones between (c,d) the between PEO layersthe PEO synthesized layers synthesized on the AMg-6 on the (AMga,c)- and6 (a,c D16T) and (b,d) alloysD16T and (b,d specimens) alloys and from specimens the 45 and from U8 the steels 45 and without U8 steels hydrogenation without hydrogenation (1), after (their1), after preliminary their hydrogenationpreliminary withhydrogenation the diffusive with (the3) or diffusive residual (3 hydrogen) or residual (4 hydrogen) under tribo-testing (4) under tribo in- thetesting pure in mineralthe pure mineral oil of the I-20 type, or without hydrogenation of steel specimens and with their oil of the I-20 type, or without hydrogenation of steel specimens and with their tribo-testing in mineral tribo-testing in mineral oil of the I-20 type with the addition of the 0.5 vol. % distilled water (2), or of oil of the I-20 type with the addition of the 0.5 vol. % distilled water (2), or of the 1.0 vol. % aqueous the 1.0 vol. % aqueous solution of glycerin (5). Current density during electrolytic hydrogenation of 5 solutionsteel ofspecimens glycerin was ( ). 2.0 Current A/dm2 densityand the timeduring of hydrogenation electrolytic hydrogenation was 1.0 h. The of specific steel specimens load in the was 2 2.0 A/dmfriction zoneand thewas time 4.0 MPa. of hydrogenation Testing time was was 4.0 1.0 h. h. The specific load in the friction zone was 4.0 MPa. Testing time was 4.0 h. The analysis of the results obtained has shown that despite the high friction coefficients and triboThe-heati analysisng temperatures of the results upon obtained friction contact has shown between that both despite steels theand highthe PEO friction layer coefficientssynthesized and tribo-heatingon the AMg temperatures-6 alloy, the weight upon frictionlosses by contact steel specimens between during both steels the wear and process the PEO are layer minimal synthesized and on theare AMg-6 further alloy,decreased the weight in the presence losses by of steel hydrogen specimens in different during states the wear in the process surface are layer. minimal The high and are furtherwear decreased resistance inof theboth presence steels under of hydrogen the action inof differenthydrogen statesis explained in the by surface the formation layer. The of a high black wear resistanceglassy film of both with steels high wear under resistance the action on their of hydrogen surfaces. is explained by the formation of a black glassy film with high wear resistance on their surfaces. 4. Phase Analysis of the Layer Formed on the Friction Surface of the Steel 4. PhaseThe Analysis phase of analysis the Layer of tribo Formed-contact on thelayers Friction has shown Surface that of this the film Steel was represented by a complex iron oxide of spinel type (FeAl2O4). Components such as iron, oxygen and a small amount of The phase analysis of tribo-contact layers has shown that this film was represented by a complex aluminum, , and dioxide were found in the composition of this film (Figure 7). ironThe oxide thickness of spinel of type this (FeAl film2 O can4). beComponents increased bysuch preliminary as iron, oxygen electrolytic and a small hydrogenation amountof of aluminum, steel magnesium,specimens and or by silicon using dioxide mineral wereoil with found water in or the an composition aqueous solution of this of filmglycerine (Figure added7). Theduring thickness wear of thistests. film can be increased by preliminary electrolytic hydrogenation of steel specimens or by using mineral oil with water or an aqueous solution of glycerine added during wear tests.

MetalsMetals 20182019, ,89, ,x 280 FOR PEER REVIEW 1010 of of 14 14

I a.u 500

1 - FeAl O 4 450 2 4 4 2 - SiO 4 2 400 3 - Fe C 3 350 4 - -Fe

300 3 3 3 3 3 3 3 1 3 1 1 1 2 250 1 2 2 2 1 2 1 200

150 30 40 50 60 70 80 90 100 110 2

Figure 7. XRD pattern of the layer formed on the friction surface of the 45 steel after the friction testing Figurein pair with7. XRD the pattern PEO layer of the synthesized layer formed on the on AMg-6 the friction alloy surface in mineral of the oil of45 the steel I-20 after type the with friction the testingaddition in ofpair 1.0 with vol. the % aqueous PEO layer solution synthesized of glycerin. on the The AMg specific-6 alloy load in inmineral the friction oil of zonethe I was-20 type 4.0 MPa. with theThe addition designations of 1.0 of vol. revealed % aqueous phases: solution1—FeAl of2 Oglycerin.4; 2—SiO The2; 3 s—Fepecific3C; load4—α -Fe.in the friction zone was 4.0 MPa. The designations of revealed phases: 1—FeAl2O4; 2—SiO2; 3—Fe3C; 4—-Fe. Consequently, the spinel-like oxide layer (hercynite (Fe0,816·Al0,184)·(Al1,816·Fe0,184·O4) type) was synthesizedConsequently, on the specimen the spinel surfaces-like oxide of steel layer 45 during (hercynite the friction (Fe0,816 process.·Al0,184)·(Al As1,816 it is·Fe known,0,184·O4) such type) oxide was synthesizedprotects the onsteel the surfaces specimen against surfaces damage of steel under 45 the during hydrogen the friction effect. In process. the absence As it of is a known, mechanical such oxideeffect, protects hercynite the is synthesizedsteel surfaces under against various damage conditions. under the It is hydrogen usually synthesized effect. In the at temperatures absence of a ◦ mechanicalexceeding 1500 effect,C inhercynite the electric is synthesized arc furnaces underas a result various of the conditions. direct reaction It is between usually FeO synthesized and Al2O at3. temperaturesIt is often represented exceeding by 1500 the formula°C in the Fe(Fe electric0.5·Al arc1.5 )furnaces·O4. Hercynite as a result can be of synthesized the direct reaction at temperatures between ◦ FeOof 490–600 and Al2OC3. under It is often the combinedrepresented action by the of nitrogenformula Fe(Fe and hydrogen0.5·Al1.5)·O4 [.24 Hercynite,25]. In this can research, be synthesized it was atfound temperatures that under of the490– influence600 °C under of the the mechanical combined factor action upon of nitrogen tribocontact and hydrogen between the[24,25]. PEO In layer this research,synthesized it was on thefound AMg-6 that alloyunder and the carboninfluence steels, of the hercynite mechanical can be factor formed upon on tribocontact the friction surfacesbetween ◦ theat a PEO temperature layer synthesized of about 120 on theC in AMg the contact-6 alloy zone. and A carbon hercynite steels, layer hercynite synthesized can be on formed the surface on the of frictionhydrogenated surfaces steel at a specimens temperature before of about friction 120 tests °C in prevents the contact the wear zone. of A the hercynite steel substantially. layer synthesized on theTribo-layers surface of hydrogenatedformed on the steel friction specimens surfaces before of steel friction specimens tests were prevents also the examined wear of using the steel the substantially.optical and electron microscopes. Studies using an optical microscope revealed that, at first glance, the morphologyTribo-layers of formed the friction on the surfaces friction does surfaces not change of steel after specimens the wear were tests. also However, examined the elemental using the opticalcomposition and electron of the formedmicroscopes. films Studies has shown using some an optical differences microscope between revealed them. Moreover,that, at first a clusterglance, thestructure morphology is characteristic of the friction of the surfaces formed does tribo-layers. not change The after local the areaswear withtests. practicallyHowever, the white, elemental light-, compositiondark-grey and of practically the formed black films color has were shown identified some in differences the structure between of this layer. them. White Moreover, areas contained a cluster structuremore iron, is less characteristic aluminum, of and the oxygen formed (Figure tribo-layers.8a, spectrum The local 4). areas On the with contrary, practically a dark white, gray light area-, darkcontained-grey moreand practic aluminum,ally black magnesium, color were and oxygen identified (Figure in the8a, spectrum structure 2 of and this 3). layer.Mainly White iron and areas a containedsmall amount more of iron, oxygen less were aluminum, revealed and in light oxygen grey (Figure areas (Figure 8a, spectrum8a spectrum 4). 1).On Apparently, the contrary, not a onlydark grayoxides area but contained also compounds more aluminum, doped with magnesium, oxygen were and formed oxygen locally. (Figure Black 8a, areasspectrum contained 2 and impurities3). Mainly ironof aluminum, and a small magnesium amount and of oxygen a significant were amount revealed of oxygen in light (Figure grey 8 areasa spectrum (Figure 5). 8a spectrum 1). Apparently,In two cases, not only a predominantly oxides but also light compounds grey film was doped formed with on the oxygen friction were surfaces formed of the locally. specimens Black areasfrom contained steel U8 upon impurities tribo-contact of aluminum, with the PEOmagnesium layer synthesized and a sign onificant the AMg-6amount alloy of oxygen during (Figure testing in8a spectrumthe I-20 oil 5). (Figure 8c–f). In the first case, it is formed after the electrolytical hydrogenation of steel specimensIn two before cases, friction a predominantly test (Figure8 c). light And grey in the film second was case, formed it is formedon the after friction the additionsurfaces of of the the specimenswater-glycerine from solution steel U8 to upon friction tribo lubricant-contact (Figure with the8e). PEO It means layer that synthesized in both cases, on iron the AMg oxides-6 with alloy duringan insignificant testing in admixture the I-20 of oil aluminum (Figure 8 oxides c–f). In predominate the first case, on the it isfriction formed surfaces. after the An insignificantelectrolytical hydrogenationamount of black of areassteel wasspecimens revealed before on tribo friction surfaces, test (Figure too. 8c). And in the second case, it is formed after the addition of the water-glycerine solution to friction lubricant (Figure 8e). It means that in both cases, iron oxides with an insignificant admixture of aluminum oxides predominate on the friction surfaces. An insignificant amount of black areas was revealed on tribo surfaces, too.

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Spect Rum C N O Al Mg Si S Fe Number 1 – – 43 2.2 0.2 0.8 1.0 52.8 2 10.6 4.3 51.0 1.0 0.1 0.5 0.5 31.7 3 11.8 2.8 55.4 0.8 0.1 0.7 0.7 27.7 4 8.3 1.2 29.1 1.3 0.1 0.6 0.4 59.1 5 – – 52.1 6.1 0.5 2.0 2.3 37.0

(a) (b)

Spect Rum O Al Mg Si S Fe Number 1 13.4 0.1 0.1 0.2 0.1 62.3 2 34.9 5.5 0.4 0.5 0.2 58.3 3 37.7 13.3 0.7 0.6 0.1 47.5 4 26.6 5.0 0.2 0.5 0.1 67.5 5 8.2 0.1 - 0.7 - 91.0 6 24.4 0.2 0.2 0.5 0.1 74.6 7 27.9 0.6 0.2 0.5 0.1 70.5

(c) (d)

Spectrum C O Al Si Fe Number 1 20.2 - - 0.6 79.3 2 29.5 18.7 - 0.5 51.13 3 47.2 - - 45.2 7.6 4 12.4 40.1 0.7 0.4 45.8 5 14.2 36.0 0.9 0.4 48.3 6 37.9 - - - 62.1 7 17.8 20.2 1.8 0.9 59.2 8 19.3 19.0 0.6 0.5 60.7

(e) (f)

Figure 8. The structure of the friction surfaces (a,c,e) and the results of their microspectral analysis Figure 8. a,c,e (b,d,f)The with structure elemental of composition the friction (at.surfaces %) of ( the various) and the regions results of of tribo their-layers microspectral formed on analysis the (b,d,fsurfaces) with elementalof specimens composition from the 45 (at. (a,b %)) and of the U8 various(c–f) steels regions after oftheir tribo-layers electrolytic formed pre-hydrogenation on the surfaces of specimens(hydrogen from of diffusion the 45 ( a,b type)) and and U8 after (c–f ) their steels subsequent after their wear electrolytic test in pre-hydrogenation tribo-pair with PEO (hydrogen layer of diffusionsynthesized type) on and the afterAMg- their6 in the subsequent pure mineral wear oil test of the in tribo-pairI-20 type (a,b with) and PEO with layer thesynthesized addition of 0.5 on the AMg-6vol. in % the water pure (c,d mineral) or of 1 oil.0 vol. of the % of I-20 an type aqueous (a,b )solution and with of theglycerine addition (e,f) of at 0.5 the vol. specific % water load (inc,d the) or of 1.0 vol.friction % of zones an aqueous of 4.0 MPa. solution of glycerine (e,f) at the specific load in the friction zones of 4.0 MPa.

TheThe effect effect of of the the diffusive diffusive hydrogen hydrogen (coming (coming into into the the zones zones of of tribocontact tribocontact from from a a pre-hydrogenatedpre-hydrogenated steel steel specimen specimen used used as as counter counter bodi bodies)es) and and atomic atomic one one (formed (formed as a as result a result of of lubricantlubricant destruction destruction in in the the tribocontact tribocontact zones)zones) on the the friction friction process process was was explained explained as follows. as follows. The following mechanism is proposed for the formation of spinel on the surfaces of steel The following mechanism is proposed for the formation of spinel on the surfaces of steel specimens specimens as a result of friction. Atomic hydrogen released on the surface of hydrogenated steel as a resultspecimens of friction. or formed Atomic as a result hydrogen of the released destruction on ofthe water surface and of oil hydrogenated in the contact zonesteel ofspecimens friction or formed as a result of the destruction of water and oil in the contact zone of friction surfaces cannot contribute to the formation of an oxide film on the steel surface. However, it can be assumed that atomic hydrogen interacts with the molecular oxygen dissolved in the oil with the formation of water. Metals 2019, 9, 280 12 of 14

During friction, juvenile surfaces are formed due to local deformation and damage with flashes of temperature. This temperature can be higher than 400 ◦C[26]. It is known that 1–3 µm thick oxide films are formed on the steel surface in the water vapor environment at these temperatures. This film is similar to the one formed on the steel surfaces during their bluing. An oxide film layer (FeAl2O4) is not formed on the steel surface when the steel wears in a tribo-pair with a PEO layer synthesized on the surface of the alloy-D16T. It was explained by the appearance of nanoscale inclusions in the structure of the PEO layer. These copper particles were actually found on the surface of steel specimens upon completion of the wear test. In this case, the phenomenon of selective transfer is realized [17]. The copper ions from the surface layer with nanoscale copper particles in the PEO structure are selectively transferred to the surface of the steel specimen (as a friction pair) under the influence of hydrogen ions H+. This phenomenon is similar to the destruction of steel under the influence of hydrogen. An abnormally low coefficient of friction (0.006) (Figure6b, columns 5) is due to the formation of a glycerate film on the steel surface. Thus, in the process of tribological destruction of glycerol added to the lubricant, the oxides of + copper and iron are reduced under the action of hydronium ion H3O , and friction polymers are created in the form of copper and iron glycerates [17]. These tribosynthetic compounds protect steel surfaces from the destructive action of hydrogen. However, friction polymers arose only in the case of triboelectron emission. Oxide-coated surfaces (oxide ceramics or PEO layers synthesized on aluminum alloys) are the strongest sources of triboelectrons emission [17]. High local (flash) temperatures on contacted micro-areas caused by their interaction at friction can activate the emission of triboelectrons, which leads to tribochemical reactions in accordance with the concepts of NIRAM (the negative-ion-radical-action mechanism), due to which the reactivity of surfaces increases [27–31].

5. Conclusions On the surfaces of light alloys with high-strength PEO layers, tribo-destruction of glycerol added to the lubricant takes place, oxides of copper and iron are reduced under the action of hydronium + ion H3O , and copper and iron glycerates are formed. If the hydrogen content in the steel specimens (used as the counter body in the wear test) is not sufficient to form microdamage in the surface layer at the boundary lubrication conditions, then hydrogen has no negative effect on their wear resistance, regardless of its nature (diffusive or residual). The minimum loss of weight by the specimens from steel 45 was revealed regardless of the hydrogen nature formed in the surface layer upon friction in the tribopair with PEO layers synthesized on the surface of the AMg-6 alloy. A wear-resistant spinel film is formed on the surface of steel specimens during the friction process as a result of tribosynthesis. This film is a complex oxide, which contains iron and small amounts of aluminum, magnesium, and silica. The film also prevents the intensified wear of the steel specimen under the effect of hydrogen. It has been revealed that during friction interaction in a friction pair “PEO layer synthesized on D16T alloy (with dispersed copper particles) - steel under conditions of extreme lubrication in the medium of motor oils with glycerine”, the effect of selective mass transfer occurs due to the formation of glycerate iron films on the friction surface of steel, which increases the tribological characteristics of this friction pair significantly. In this case, the coefficient of friction and wear of a friction pair decrease by an order of magnitude, even at high specific loads.

Author Contributions: Conceptualization, M.S.; formal analysis, P.M. and V.H.; investigation, M.S., V.H., V.D., P.M., V.P., O.S., and I.K.; methodology, M.S. and P.M.; project administration, V.H.; validation, M.S., V.H., V.D., P.M., V.P., O.S., and I.K.; writing—original draft, M.S., O.S., P.M.; writing—review and editing, V.H. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. Metals 2019, 9, 280 13 of 14

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