Performance Enhancement in Borocarburized Low-Carbon Steel by Double Glow Plasma Surface Alloying

coatings

Article Performance Enhancement in Borocarburized Low-Carbon Steel by Double Glow Plasma Surface Alloying

Zheng Ding, Qiang Miao *, Wenping Liang, Zhengang Yang and Shiwei Zuo

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210000, China; [email protected] (Z.D.); [email protected] (W.L.); [email protected] (Z.Y.); [email protected] (S.Z.) * Correspondence: [email protected]

Received: 12 October 2020; Accepted: 2 December 2020; Published: 10 December 2020

Abstract: In this paper, the performance of low-carbon steel is enhanced after introducing a·borocarburized diffusion layer via double glow plasma surface alloying technology. Due to the boron-carbon gradient structure of low-carbon steel, the protective coating exhibits an excellent wear and corrosion resistance. Interestingly, the borocarburized layer consists of a 64 µm carburized layer and a 27 µm boride layer, which plays an effective role in enhancing the microhardness of borocarburized low-carbon steel, exhibiting a 1440 Vickers hardness increase in the surface microhardness of low-carbon steel. The potentiodynamic polarization measurement and impedance measurement results indicate that the boride protective film can effectively prevent aggressive chloride ions from invading the substrate, which indicates an excellent property of corrosion resistance. This systematic study paves a promising way for the future application of hard coatings in severe environments.

Keywords: borocarburizing; double glow; gradient structure; Fe2B; corrosion resistance

1. Introduction Due to the desirable weldability, plasticity and toughness, low-carbon steel has gained worldwide attention for its versatile applications in construction and mechanical parts. The surface hardened low-carbon steel can be widely used, for example, in wear-resistant parts of vehicles and ships [1–4]. The fast development of relative industries has boosted the demand of low-carbon steel, but features more and more rigorous market criteria on friction damage since the caused indirect damage could be much more expensive than the value of the material itself. Moreover, the inferior wear resistance, low hardness and poor corrosion resistance of low-carbon steel largely limits its application in these areas. In recent years, most surface modify techniques such as nitriding [5], carburization and nitrocarburizing [6–8] are based on studying the tribological behavior of low-carbon steel. Among all the available techniques, boriding has been proven to be an effective surface modification method that can diffuse boron atoms into substrate to form a boride layer of FeB phase and Fe2B phase [9], thereby enhancing wear resistance and hardness. However, due to the brittleness of the single boride layer, especially FeB phase, inevitable inner stress during the cooling process generates, leading severe crack propagation in the boride layer [10,11]. Based on these issues, a series of research studies have been carried out. S.A. Kusmanov et al. [12] pointed out that the brittle failure phenomenon is more likely to occur with a boride layer with FeB phase and Fe2B phase than with a boride layer with single Fe2B phase. The thermal expansion coefficient difference of FeB and Fe2B will induce a non-uniform distribution of thermal stress, thereby leading to a spalling of the boride layer during cooling process.

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In this case, the large stress difference between the boride layer and substrate will also result in a peeling phenomenon on the boride layer during its working time [13,14]. Hence, an effective method to acquire a work piece with desirable hardness and wear resistance is quite urgent both for researchers and markets. Up untilCoatings now, 2020 lots, 10, x ofFOR techniques PEER REVIEW and methods have been carried out to obtain a hardness2 of 13 layer on multiple metals,during cooling such as process. nitriding, In this electroplating, case, the large stre ionss difference implantation, betweenspark the boride plasma layer and sintering, substrate chromium plating andwill laser also result modification in a peeling withphenomenon Mo or on Ni the [boride15–22 layer]. The during technique its working of time double [13,14]. glowHence, has been successfullyan employedeffective method to fabricate to acquire various a work kindspiece with of coatings. desirable hardness This technique and wear employsresistance ais plasmaquite region, which is producedurgent both by for double researchers glow and discharges, markets. to fabricate a coating on substrate surface. The target Up until+ now, lots of techniques and methods have been carried out to obtain a hardness layer bombardedon by multiple Ar and metals, the elementssuch as nitriding, from the electroplating, target can beion depositedimplantation, on spark the substrateplasma sintering, [23–25 ]. Due to the metallurgicalchromium bonding plating and eff laserects, modification the double with glow Mo techniqueor Ni [15–22]. can The providetechnique aofgradient double glow structure has with desirable bondingbeen successfully strength employed between to fabricate the substrate various ki andnds coating.of coatings. Additionally, This technique employs the preparation a plasma of large area coatingsregion, can which also beis produced realized by by double the double glow discharges glow technique, to fabricate [26 a coating–28]. In on thissubstrate paper, surface. Q235 The low-carbon target bombarded by Ar+ and the elements from the target can be deposited on the substrate [23–25]. steel was borocarburizedDue to the metallurgical by the bonding double effects, glow the technique. double glow The technique morphological, can provide structural,a gradient structure wear resistance and corrosionwith resistance desirable bonding of samples strength under between diff theerent substr speedate and conditions coating. Additionally, with and without the preparation a borocarburized of layer werelarge systematically area coatings studied.can also be The realized friction by the performance double glow technique of the coating [26–28]. In under this paper, different Q235 loads has been shownlow-carbon in our previous steel was borocarburized study [29]. by the double glow technique. The morphological, structural, wear resistance and corrosion resistance of samples under different speed conditions with and without a borocarburized layer were systematically studied. The friction performance of the coating 2. Materials and Methods under different loads has been shown in our previous study [29]. As observed in Figure1, the workpiece and target are both cathodes, the device chamber is 2. Materials and Methods an anode. Argon is continuously introduced during the double glow process, which can be excited to As observed in Figure 1, the workpiece and target are both cathodes, the device chamber is an active plasma ions (Ar+). Due to the hollow cathode effect, the sharply increased plasma in the furnace anode. Argon is continuously introduced during the double glow process, which can be excited to cavity canactive bring plasma an increased ions (Ar+). temperatureDue to the hollow to cathode target effect, and workpiece.the sharply increased The sputtered plasma in the atoms furnace and ions of the target willcavity increase can bring subsequently. an increased temperature The strike to target of these and workpiece. particles willThe sputtered also increase atoms and the ions temperature of of the substrate.the target By adjusting will increase the subsequently. work voltage The strike and of current, these particles the working will also increase temperature the temperature is maintained in of the substrate. By adjusting the work voltage and current, the working temperature is maintained the ideal range. in the ideal range.

Figure 1.FigureSchematic 1. Schematic of the of the double double glow glow plasmaplasma surface surface metallurgy metallurgy technique. technique.

Q235 low-carbon steel with a size of 10 mm × 10 mm × 8 mm was selected as substrate. The Q235 low-carbon steel with a size of 10 mm 10 mm 8 mm was selected as substrate. The chemical chemical composition (wt.%) of Q235 steel ×was Mn 0.35%,× Si 0.3%, C 0.18%, S 0.04%, P 0.04%, Fe compositionbalance. (wt.%) Firstly, of Q235 abrasive steel paper was of Mn1200 0.35%,meshes was Si 0.3%, used to C polish 0.18%, theS samples; 0.04%, after P 0.04%, that, the Fe polished balance. Firstly, abrasive papersubstrates of 1200 were meshes ultrasonically was usedcleaned to polishin acetone the to samples; remove surface after that,contamination. the polished The average substrates were ultrasonicallyroughness cleaned of the in substrate acetone was to 0.26 remove μm. The surface whole fabrication contamination. process contained The average two steps. roughness Firstly, of the pure carbon target was used for depositing the carbon layer that was deposited on the substrate; after substrate was 0.26 µm. The whole fabrication process contained two steps. Firstly, pure carbon target that, B4C target was used for depositing the boron layer. After a series of orthogonal experiments, the was used foroptimum depositing technological the carbon parameters layer of that the waslayer deposited preparation on were the obtained. substrate; The aftervoltage that, of source B4C target was used for depositingelectrode, thevoltage boron of the layer. cathode, After working a series pres of orthogonalsure, distance experiments, between the source the optimum electrode technologicaland parameterscathode, of the treatment layer preparation time and argon were flow obtained.rate of C deposition The voltage was 900 V, of 450 source V, 35 Pa, electrode, 15 mm, 3 h voltage and of the 70 sccm, respectively. The voltage of source electrode, voltage of the cathode, working pressure, cathode, workingdistance between pressure, the distancesource electrode between and the cathode, source treatment electrode time and and cathode,argon flow treatment rate of B time and argon flow rate of C deposition was 900 V, 450 V, 35 Pa, 15 mm, 3 h and 70 sccm, respectively. The voltage of source electrode, voltage of the cathode, working pressure, distance between the source electrode and cathode, treatment time and argon flow rate of B deposition was 900 V, 550 V, 38 Pa, 10 mm, 4 h and Coatings 2020, 10, 1205 3 of 13

50 sccm, respectively. The temperature measurement of layer preparation was conducted by infrared thermometer (Marathon MR1S, Raytek, Santa Cruz, CA, USA). The morphological characteristics of samples were examined by scan electron microscopy (JSM-6360LV, JEOL, Tokyo, Japan) equipped with EDS (XMS60S, EDAX Inc., Mahwah, NJ, USA). The average testing time of EDS is 5 min for each sample. The structural information of the sample was analyzed by X-ray diﬀraction (Smartlab, Rigaku, Tokyo, Japan) with step of 0.02◦ and counting time of 6 min. The micro-hardness of borocarburized layer was detected under the load of 100 g by Vickers indenter (DHV-1000/Z, SCTMC, Shanghai, China). An electrochemical workstation (CHI-660d, ChenHua, Shanghai, China) with a three-electrode cell system was used to calculate the corrosion resistance of the borocarburized layer in 3.5% sodium chloride solution. All the samples were sealed by epoxy resin, which exposed a surface area of 0.5 cm2. The platinum electrode with 4.0 cm2 exposed surface was used as auxiliary electrode, and saturated calomel electrode (SCE) was used as reference electrode. The electrochemical impedance spectroscopy (EIS) spectra were detected according to open-circuit potential with scanning range from 10,000 kHz to 100 kHz. A potentiodynamic polarization test was carried out in a range of 0.5 V of open circuit potential. The wear test was conducted using a ball-on-disk wear tester (HT-500, KaiHua, Lanzhou, China) without lubrication in air with relative humidity of (45 5)% and morphology of wear surface was measured by depth-of-ﬁeld system ± (VHX-1000, Keyence, Osaka, Japan). The load and turning radius of the wear test were set as 730 g and 2 mm, respectively. The sliding speeds were 0.083 m s 1, 0.167 m s 1 and 0.333 m s 1. During the wear · − · − · − test, the friction pair of all samples was a 5 mm-diameter ZrO2 ball.

3. Results and Discussion The microtopography of sample is shown in Figure2. As observed in Figure2a, the carburized sample exhibits a homogeneous and needle-like microstructure without cracks. In Figure2b, the boronized layer after carburizing consists of submicron-sized particles and the average roughness of the layer was 1.189 µm. The cross-section microstructure of the borocarburized layer is uniform and even, which is different from the single boronized layer [30]. The boride layer and the carburized layer can be obviously observed along with the depth of the borocarburized layer. Moreover, the penetration of boron and carbon atoms can also reduce the brittleness of the boride layer [3]. As shown in Figure2d, the EDS result of the cross-section from surface of the coating to along the depth direction of the substrate indicates that the thickness of the boride layer and carburized layer is 27 µm and 64 µm, respectively. It can be found that boron and carbon show a gradient change in the coating, showing complete metallurgical diffusion behavior, which helps to improve the adhesion between the coating and the substrate. In the boronized layer, the concentration of carbon atoms is very low, which is in line with the viewpoint that the solubility of carbon atoms in the boronized layer is very low [31]. Coatings 2020, 10, x FOR PEER REVIEW 4 of 13

Figure 2. SEM images of the borocarburized layer (a) C deposition, (b) B deposition, (c) cross-section Figure 2. SEM imagesimage (d) EDS of theresult. borocarburized layer (a) C deposition, (b) B deposition, (c) cross-section image (d) EDS result. As we can see from the XRD results in Figure 3a, apart from the detected orthorhombic Fe7C3 phase (PDF75-1499) and tetragonal C0.09Fe1.91 phase (PDF44-1292), the carburized sample exhibits a main phase of monoclinic Fe5C2 phase (PDF89-7272). Furthermore, an individual phase of tetragonal Fe2B (PDF89-1993) phase with desirable purity is observed in Figure 3b after introducing the borocarburized layer. Interestingly, an orthorhombic FeB (PDF76-0092) phase for the traditional boronizing layer [32] is not detected in the XRD results. In the process of boronizing, due to the continuous bombardment of various particles, the substrate surface becomes clean and a large numbers of defects are formed on it, which can accelerate the diffusion rate of boron atom and hinder the formation of boron-rich FeB. At the same time, at the temperature of layer preparation (working temperature 1050~1100 °C), the FeB phase can be transformed into Fe2B phase [13]. The disappearance of the FeB phase can effectively reduce the residual stress caused by the large difference in thermal expansion coefficient between the Fe2B phase and FeB phase, thereby reducing the spalling possibility of the boride layer during the cooling process. This is also the difference between the double glow plasma device and some other traditional pieces of equipment. The single Fe2B phase layer can be obtained via the double glow plasma device without any other heat treatment process [9,13,14].

Figure 3. XRD patterns of the borocarburized layer (a) after the carburizing process, (b) after the boriding process.

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Figure 2. SEM images of the borocarburized layer (a) C deposition, (b) B deposition, (c) cross-section Coatingsimage2020 ,(d10), EDS 1205 result. 4 of 13

As we can see from the XRD results in Figure 3a, apart from the detected orthorhombic Fe7C3 As we can see from the XRD results in Figure3a, apart from the detected orthorhombic Fe 7C3 phase phase (PDF75-1499) and tetragonal C0.09Fe1.91 phase (PDF44-1292), the carburized sample exhibits a (PDF75-1499) and tetragonal C0.09Fe1.91 phase (PDF44-1292), the carburized sample exhibits a main main phase of monoclinic Fe5C2 phase (PDF89-7272). Furthermore, an individual phase of tetragonal phase of monoclinic Fe5C2 phase (PDF89-7272). Furthermore, an individual phase of tetragonal Fe2B Fe2B (PDF89-1993) phase with desirable purity is observed in Figure 3b after introducing the (PDF89-1993) phase with desirable purity is observed in Figure3b after introducing the borocarburized borocarburized layer. Interestingly, an orthorhombic FeB (PDF76-0092) phase for the traditional layer. Interestingly, an orthorhombic FeB (PDF76-0092) phase for the traditional boronizing layer [32] boronizing layer [32] is not detected in the XRD results. In the process of boronizing, due to the is not detected in the XRD results. In the process of boronizing, due to the continuous bombardment of continuous bombardment of various particles, the substrate surface becomes clean and a large various particles, the substrate surface becomes clean and a large numbers of defects are formed on numbers of defects are formed on it, which can accelerate the diffusion rate of boron atom and hinder it, which can accelerate the diffusion rate of boron atom and hinder the formation of boron-rich FeB. the formation of boron-rich FeB. At the same time, at the temperature of layer preparation (working At the same time, at the temperature of layer preparation (working temperature 1050~1100 ◦C), the FeB temperature 1050~1100 °C), the FeB phase can be transformed into Fe2B phase [13]. The disappearance phase can be transformed into Fe B phase [13]. The disappearance of the FeB phase can effectively of the FeB phase can effectively reduce2 the residual stress caused by the large difference in thermal reduce the residual stress caused by the large difference in thermal expansion coefficient between the expansion coefficient between the Fe2B phase and FeB phase, thereby reducing the spalling possibility Fe B phase and FeB phase, thereby reducing the spalling possibility of the boride layer during the of 2the boride layer during the cooling process. This is also the difference between the double glow cooling process. This is also the difference between the double glow plasma device and some other plasma device and some other traditional pieces of equipment. The single Fe2B phase layer can be traditional pieces of equipment. The single Fe B phase layer can be obtained via the double glow obtained via the double glow plasma device without2 any other heat treatment process [9,13,14]. plasma device without any other heat treatment process [9,13,14].

Figure 3. XRDXRD patterns patterns of of the the borocarburized borocarburized layer layer ( (aa)) after after the the carburizing carburizing process, process, ( (bb)) after the boriding process.

Figure4 depicts the microhardness distribution of the borocarburized layer. As observed in this ﬁgure, the maximum hardness of 1650 Vickers hardness (HV) is obtained for the top boride layer. However, the hardness of the original steel substrate is only 210 HV, indicating an obvious enhancement in hardness after introducing the borocarburized layer. Additionally, the hardness of the transition carburized layer is around 600 HV. Therefore, the introduction of the carburized layer reduces the hardness gradient of the single boride layer and improves the comprehensive mechanical properties of the coating [33]. Meanwhile, the surface hardness of the borocarburized layer is much higher than that of the carburized layer, which is beneﬁcial to wear resistance. In fact, the surface microhardness and brittleness of the Fe2B phase layer are lower than those of FeB phase. Due to the extremely high internal stress during cooling, there are many risks of cracks and spalling in the surface area of FeB/Fe2B layers [3,9]. Coatings 2020, 10, x FOR PEER REVIEW 5 of 13

Figure 4 depicts the microhardness distribution of the borocarburized layer. As observed in this figure, the maximum hardness of 1650 Vickers hardness (HV) is obtained for the top boride layer. However, the hardness of the original steel substrate is only 210 HV, indicating an obvious enhancement in hardness after introducing the borocarburized layer. Additionally, the hardness of the transition carburized layer is around 600 HV. Therefore, the introduction of the carburized layer reduces the hardness gradient of the single boride layer and improves the comprehensive mechanical properties of the coating [33]. Meanwhile, the surface hardness of the borocarburized layer is much higher than that of the carburized layer, which is beneficial to wear resistance. In fact, the surface microhardness and brittleness of the Fe2B phase layer are lower than those of FeB phase. Due to the extremely high internal stress during cooling, there are many risks of cracks and spalling in the Coatings 2020, 10, 1205 5 of 13 surface area of FeB/Fe2B layers [3,9].

FigureFigure 4. 4.Microhardness Microhardness distribution distribution of of the the borocarburized borocarburized layer. layer.

TheThe friction friction coefficients coefficients of samples of samples with with and withoutand without layers layers under under dry-sliding dry-sliding against against ZrO2 ball ZrO with2 ball 1 1 1 0.083 m s , 0.167−1 m s and 0.333−1 m s speed−1 at room temperature are shown in Figure5. As for the Q235 with 0.083· − m·s , 0.167· − m·s and ·0.333− m·s speed at room temperature are shown in Figure 5. As for substrate,the Q235 a rapidsubstrate, increase a rapid in friction increase coefficient in friction is observed coefficient at the is initialobserved stage. at Withthe initial the gradual stage. increaseWith the ingradual wear time, increase the friction in wear coefficient time, the tendsfriction to becoefficien stable att tends the range to be of stable 0.7 to at 1. the The range large of fluctuations 0.7 to 1. The inlarge the friction fluctuations coefficient in the of friction the substrate coefficient are dueof the to substrate the greater are plasticity, due to the which greater is prone plasticity, to adhesive which is wear.prone Additionally, to adhesive at wear. the initial Additionally, friction stage, at the the initial wear debrisfriction embedded stage, the in wear the substrate debris embedded is also prone in tothe generatesubstrate relatively is also largeprone friction. to generate For further relatively increased large wearfriction. time, For the further wear debris increased is easily wear broken, time, andthe wear the frictiondebris force is easily is reduced. broken, It and can bethe seen friction from force the figure is reduce thatd. the It frictioncan be seen coefficient from ofthe the figure substrate that the increases friction withcoefficient the increase of the in thesubstrate rotation increases speed. This with is because the incr theease increased in the rotationrotation speedspeed. can This bring is anbecause increase the inincreased the temperature rotation of thespeed contact can surface bring andan increase accelerates in the the oxidation, temperature while of the the iron contact oxide issurface not dense and andaccelerates is unable tothe protect oxidation, the substrate. while the Instead, iron oxide it increases is not the dense hardness and is of unable the wear to debris protect and the the substrate. friction coefficient.Instead, it As increases for the borocarburized the hardness layer, of the the wear friction debris coefficient and isthe much friction lower coefficient. than that of As the for Q235 the substrate,borocarburized which is layer, basically the stable friction between coefficient 0.15 and is much 0.28. Thelower above than difference that of the in Q235 relatively substrate, stable frictionwhich is coefficientsbasically canstable be attributedbetween 0.15 to the and difference 0.28. The in specimenabove diffe hardnessrence in (see relatively Figure4), stable which friction has been coefficients verified bycan previous be attributed researchers to the [34 difference]. The friction in specimen coefficient hardness of the borocarburized (see Figure 4), layer which also has increases been verified with the by increaseprevious in rotatingresearchers speed. [34]. However, The friction for conventionalcoefficient of FeBthe orborocarburized FeB/Fe2B layers, layer there also is increases a risk of peeling with the offincrease the surface, in rotating which speed. may lead However, to friction for fluctuationsconventional of FeB the or friction FeB/Fe coefficient2B layers, there [3,9]. is This a risk shows of peeling that a single-phaseoff the surface, Fe2 whichB may may provide lead better to friction anti-friction fluctuations performance. of the friction coefficient [3,9]. This shows that a single-phase Fe2B may provide better anti-friction performance.

Figure 5. Friction coeﬃcients under dry-sliding against ZrO ball with 0.083 m s 1, 0.167 m s 1 and 2 · − · − 0.333 m s 1 speed (a) substrate; (b) borocarburized layer. · −

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The wear morphology of Q235 low-carbon steel and the borocarburized layer at different speeds Coatings 2020, 10, x FOR PEER REVIEW 7 of 13 is shown in Figure6. As observed in Figure6a, obvious tears and scratches are distributed in the wear area of the substrate when the speed is 0.083 m s 1. It can be found from the amplified Figure6b micro cracks, indicating an excellent wear resistance· − of the borocarburized layer under high speed thatconditions. there are many furrows on the wear track surface, containing partly tearing and sticking areas. As shownFigure in 7 Figure is the6c, three-dimensional the wear track of substratemorphology after introducingof Q235 low-carbon a borocarburized steel substrate layer is slightlyand the scratchedborocarburized and presents layer at a narrowerdifferent sp scareeds. width, It can which be seen shows from a desirable the figure wear that resistance. the wear track Uniform width wear of morphologythe substrate ofincreases the borocarburized significantly with layer the can increa be obviouslyse in speed. observed Additionally, from the the amplified accumulated Figure height6d, showingof the wear a smoothtrack edge characteristic also increases of theas well, entire which wear can track be explained area. The by wear the trackincreased morphology degree of of wear the substratewith the increased at low speed speed, indicates leading that to thesevere wear friction mechanism and plastic is mainly deformation abrasive on wear, the accompanied sample surface. by slightAs for adhesive the borocarburized wear, while layer, the slight the increased wear track speed morphology will also is bring observed an increase at low speed in width on theand surface depth ofof thewear borocarburized track; however, layer, the showingdegree of the increase characteristics is much ofsmaller abrasive than wear. that The of the above substrate. phenomenon The above can beresults well correspond explained by well the with high previous hardness wear of the morphology. boronized layer.

1 FigureFigure 6.6.Wearmorphology Wear morphology of wear of trackwear of track (a,b) substrateof (a,b) undersubstrate 0.083 munders− speed, 0.083 (cm·s,d) borocarburized−1 speed, (c,d) 1 1 · layerborocarburized under 0.083 layer m s− underspeed, 0.083 (e,f) substratem·s−1 speed, under (e, 0.167f) substrate m s− speed, under ( g0.167,h) borocarburized m·s−1 speed, layer(g,h) 1 · 1 · underborocarburized 0.167 m s −layerspeed, under (i,j) substrate0.167 m·s under−1 speed, 0.333 (i m,j) s−substratespeed, (underk,l) borocarburized 0.333 m·s−1 layerspeed, under (k,l) 1 · · 0.333 m s speed. −1 borocarburized· − layer under 0.333 m·s speed. With the increased speed of 0.167 m s 1, the wear track morphology of Q235 low-carbon steel · − substrate and borocarburized layer can be seen in Figure6e–h. As shown in Figure6e, the wear track width signiﬁcantly increases with the increased speed. The enlarged morphology in Figure6f reveals an obvious tearing and warping on the surface of the wear track, and a large area of sticking phenomenon occurs. After introducing a borocarburized layer, only a little plastic deformation in the wear track area is observed in Figure6g, and the wear track width slightly increases compared to that at low speed. In summary, when the speed is increased to 0.167 m s 1, the substrate is dominated by · − adhesive wear, accompanied by fatigue wear and abrasive wear. However, the coating undergoes a slight plastic deformation, and the wear mechanism shows a characteristic of fatigue wear, which can be explained as follows. During the wear process, friction wear of the ZrO2 ball and sample surface will increase with the increase in speed, bringing increased heat generated by friction and sharply rising temperature in the local area of the wear track surface. However, the borocarburized layer contains a boronized layer with high hardness, which can eﬀectively reduce the actual contact area of the friction pair and the sample surface, thereby improving the wear resistance.

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With the further increased speed of 0.333 m s 1, as shown in Figure6i, the wear track width of the · − substrate is further increased and a large area of wear scar is warped and peeled, leaving many furrows with different depths. As shown in the enlarged morphology in Figure6j, an amount of abrasive debris is randomly scattered on the wear track surface. Additionally, the spalling phenomenon occurs in some areas. Figure6k shows the wear track morphology after introducing the borocarburized layer at 0.333 m s 1 speed. It can be found that the wear track width is slightly wider than that at low speed · − and the wear track surface is relatively complete. The enlarged morphology depicted in Figure6l shows that only plastic deformation occurs on the wear track surface, where a few micro cracks remain, and no damage occurs. In summary, for the speed of 0.333 m s 1, the wear mechanism of the substrate · − becomes a coexisting wear mechanism of cohesive wear, abrasive wear, and oxidative wear. As for the borocarburized layer, the wear track morphology exhibits negligible micro cracks, indicating an excellent wear resistance of the borocarburized layer under high speed conditions. Figure7 is the three-dimensional morphology of Q235 low-carbon steel substrate and the borocarburized layer at different speeds. It can be seen from the figure that the wear track width of the substrate increases significantly with the increase in speed. Additionally, the accumulated height of the wear track edge also increases as well, which can be explained by the increased degree of wear with the increased speed, leading to severe friction and plastic deformation on the sample surface. As for the borocarburized layer, the increased speed will also bring an increase in width and depth of wear track; however, the degree of increase is much smaller than that of the substrate. The above Coatings 2020, 10, x FOR PEER REVIEW 8 of 13 results correspond well with previous wear morphology.

Figure 7. Three-dimensional morphology of substrate and borocarburized layer at diﬀerent speeds Figure 7. Three-dimensional morphology1 of substrate and borocarburized 1layer at different speeds (a) substrate under 50.083 m s− speed, (b) borocarburized layer under 0.083 m s− speed, (c) substrate 1 −1 · 1 · −1 (a) substrateunder under 0.167 m50.083s− speed, m·s (speed,d) borocarburized (b) borocarburized layer under la 0.167yer under m s− speed, 0.083 (m·se) substrate speed, under (c) substrate 1 · 1 · under 0.1670.333 m·s m s−1 speed,speed, ( f()d borocarburized) borocarburized layer underlayer 0.333under m s0.167− speed. m·s−1 speed, (e) substrate under 0.333 · · m·s−1 speed, (f) borocarburized layer under 0.333 m·s−1 speed. Sliding wear results of the substrate and borocarburized layer at diﬀerent speeds are shown in Table1. In order to measure the friction and wear properties, the wear volume and wear rate can be Slidingcalculated wear as results follows of [35 the,36]: substrate and borocarburized layer at different speeds are shown in Table 1. In order to measure the friction and wear properties, the wear volume and wear rate can be calculated as follows [35,36]:

V = (2πhr (3h2 + 4b2))/6b (1)

K = V/PS (2) with V being the wear volume (mm3); r the rotating radius (mm); b the width of wear track (mm); h the depth of wear track (mm); K the specific wear rate (10−7·mm3·N−1·m−1); P the load (N); S the sliding distance (m). As high speed can bring an increase in frictional heat and the degree of wear of the sample, consequently, the wear volume increases significantly at high speed. The specific wear rate of the substrate at the speed of 0.083 m·s−1, 0.167 m·s−1 and 0.333 m·s−1 is 11.47 × 10−7 mm3·N−1·m−1, 33.71 × 10−7 mm3·N−1·m−1 and 83.18 × 10−7 mm3·N−1·m−1. For the borocarburized layer, the specific wear rate is reduced by 92.24%, 93.53% and 96.51%, respectively. The improved wear resistance can be attributed to the introduced borocarburized layer, which fundamentally enhanced the hardness of the coating.

Table 1. Sliding wear results of substrate and borocarburized layer at different speeds.

Depth Volume Loss Wear Rate Specific Wear Rate Sample Width b/mm h/μm V/10−4·mm3 v/10−6·mm3·m−1 K/10−7mm3·N−1·m−1 Substrate—0.083 m·s−1 0.21 0.7 12.31 8.21 11.47 Substrate—0.167 m·s−1 0.36 1.2 36.17 24.11 33.71 Substrate—0.333 m·s−1 0.41 2.6 89.26 59.51 83.18 Borocarburized layer—0.083 m·s−1 0.06 0.19 0.95 0.64 0.89 Borocarburized layer—0.083 m·s−1 0.1 0.28 2.34 1.56 2.18 Borocarburized layer—0.333 m·s−1 0.12 0.31 3.11 2.08 2.90

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V = (2πhr (3h2 + 4b2))/6b (1)

K = V/PS (2) with V being the wear volume (mm3); r the rotating radius (mm); b the width of wear track (mm); h the depth of wear track (mm); K the specific wear rate (10 7 mm3 N 1 m 1); P the load (N); S the − · · − · − sliding distance (m). As high speed can bring an increase in frictional heat and the degree of wear of the sample, consequently, the wear volume increases significantly at high speed. The specific wear rate of the substrate at the speed of 0.083 m s 1, 0.167 m s 1 and 0.333 m s 1 is 11.47 10 7 mm3 N 1 m 1, · − · − · − × − · − · − 33.71 10 7 mm3 N 1 m 1 and 83.18 10 7 mm3 N 1 m 1. For the borocarburized layer, the specific × − · − · − × − · − · − wear rate is reduced by 92.24%, 93.53% and 96.51%, respectively. The improved wear resistance can be attributed to the introduced borocarburized layer, which fundamentally enhanced the hardness of the coating.

Table 1. Sliding wear results of substrate and borocarburized layer at diﬀerent speeds.

Depth Volume Loss Wear Rate Speciﬁc Wear Rate Sample Width b/mm h/µm V/10 4 mm3 v/10 6 mm3 m 1 K/10 7mm3 N 1 m 1 − · − · · − − · − · − Substrate—0.083 m s 1 0.21 0.7 12.31 8.21 11.47 · − Substrate—0.167 m s 1 0.36 1.2 36.17 24.11 33.71 · − Substrate—0.333 m s 1 0.41 2.6 89.26 59.51 83.18 · − Borocarburized 0.06 0.19 0.95 0.64 0.89 layer—0.083 m s 1 · − Borocarburized 0.1 0.28 2.34 1.56 2.18 layer—0.083 m s 1 · − Borocarburized 0.12 0.31 3.11 2.08 2.90 layer—0.333 m s 1 · −

Electrochemical characteristics of the substrate and borocarburized layer were carried out via potentiodynamic polarization measurement to determine the corrosion mechanism. The potentiodynamic polarization curve is depicted in Figure8. The cathodic reaction in the polarization curve corresponds to hydrogen evolution, while the anodic reaction curve represents the corrosion reaction of the boronized layer. The anodic polarization slope is higher than that of the cathodic polarization, which indicates that the whole corrosion reaction is mainly controlled by the anodic reaction. In addition, electrochemical parameters evaluated from potentiodynamic polarization curves 2 such as corrosion current density Icorr (A/cm ), corrosion potential Ecorr (V vs. SCE) and polarization resistance Rp are listed in the inset table in Figure8. As observed in Figure8, corrosion potential E corr moves towards the positive direction after introducing the borocarburized layer. The corrosion current 6 2 6 2 density Icorr decreases from 2.025 10− A/cm (substrate) to 1.480 10− A/cm (borocarburized layer). × × 2 2 Additionally, polarization resistance Rp increases from 10,894 Ω cm of substrate to 18,150 Ω cm of · − · − borocarburized layer. The above results indicate that the borocarburized layer can improve corrosion resistance of low-carbon steel to some extent. Coatings 2020, 10, x FOR PEER REVIEW 9 of 13 curve corresponds to hydrogen evolution, while the anodic reaction curve represents the corrosion reaction of the boronized layer. The anodic polarization slope is higher than that of the cathodic polarization, which indicates that the whole corrosion reaction is mainly controlled by the anodic reaction. In addition, electrochemical parameters evaluated from potentiodynamic polarization curves such as corrosion current density Icorr (A/cm2), corrosion potential Ecorr (V vs. SCE) and polarization resistance Rp are listed in the inset table in Figure 8. As observed in Figure 8, corrosion potential Ecorr moves towards the positive direction after introducing the borocarburized layer. The corrosion current density Icorr decreases from 2.025 × 10−6 A/cm2 (substrate) to 1.480 × 10−6 A/cm2 (borocarburized layer). Additionally, polarization resistance Rp increases from 10,894 Ω·cm−2 of substrate to 18,150 Ω·cm−2 of borocarburized layer. The above results indicate that the borocarburized layer can improve corrosion resistance of low-carbon steel to some extent. Coatings 2020, 10, 1205 9 of 13

Figure 8. Potentiodynamic polarization curve of the borocarburized layer and substrate. Figure 8. Potentiodynamic polarization curve of the borocarburized layer and substrate. In order to determine the corrosion behavior of the interface between solution and metal, Inan order impedance to determine measurement the wascorrosion carried out.behavior Nyquist of plots the and interface Bode plots between of borocarburized solution layerand metal, an impedanceand substrate measurement are presented was carried in Figure out.9. The Nyquist observed plots typical and capacitive Bode plots loop of of borocarburized the Nyquist plot layer and substrateindicates are presented that the reaction in Figure is controlled 9. The by observed charge transfer typical [37]. capacitive Therefore, theloop diameter of the of Nyquist the capacitive plot indicates loops in the Nyquist plots and the impedance value of the Bode plots both have a close connection that thewith reaction corrosion is controlled resistance. Asby obviously charge transfer shown in [3 Figure7]. Therefore,9a, the diameter the diameter of the capacitive of the loopcapacitive of loops in the Nyquistthe borocarburized plots and layer the is impedance larger than that value of the of substrate, the Bode indicating plots both an enhancement have a close in corrosion connection with corrosionresistance resistance. after introducing As obviously the borocarburized shown in Figure layer. Previous 9a, the studies diameter [38,39 of] have the provencapacitive that the loop of the borocarburizedformation oflayer boride is waslarger advantageous than that toof improve the substrate, the corrosion indicating resistance an of enhancement steels, which is in in corrosion resistanceaccordance after introducing with our results. the Asborocarburized observed in Figure layer.9b, the Previous impedance studies value of[38,39] the borocarburized have proven that the layer is larger than that of substrate, which also indicates a better corrosion resistance. It can be seen formationfrom of Figure boride9b that was there advantageous is only one time to constant improve in the th corrosione corrosion process resistance of both the substrateof steels, and which is in accordanceCoatingsthe layer. with 2020, This 10 our, x FOR phenomenon results. PEER REVIEW As may observed be due toin theFigu smallre 9b, impedance the impedance value of the value corrosion of the products, borocarburized10 of 13 layer iswhich larger cannot than be that reﬂected of substrate, in the Bode which diagram. also indicates a better corrosion resistance. It can be seen from Figure 9b that there is only one time constant in the corrosion process of both the substrate and the layer. This phenomenon may be due to the small impedance value of the corrosion products, which cannot be reflected in the Bode diagram.

Figure 9. ElectrochemicalElectrochemical impedance impedance spectroscopy spectroscopy plots plots of ofborocarburized borocarburized layer layer and and substrate substrate (a) Nyquist(a) Nyquist plots, plots, (b) (Bodeb) Bode plots. plots.

The equivalent circuit is proposed in Figure 10 to further analyz analyzee the electrochemical process of the electrode/electrolyte surface, where R is the solution resistance from reference electrode to sample, the electrode/electrolyte surface, wheres Rs is the solution resistance from reference electrode to C is the double layer capacitors in the reaction interface, R is the charge transfer resistance of the sample,dl Cdl is the double layer capacitors in the reaction interface,f Rf is the charge transfer resistance reaction interface. As for the layer, it possesses a double-layer structure, Q represents the constant of the reaction interface. As for the layer, it possesses a double-layer structure,d1 Qd1 represents the constant phase angle element associated with the boronized layer, and Rpore represents the porous resistance of the boronized layer.

Figure 10. The equivalent circuit model used for fitting the EIS plots (a) substrate, (b) the borocarburized layer.

During the corrosion test, Rf is proportional to corrosion resistance. According to Table 2, the Rf of the borocarburized layer is larger than that of the substrate. The above results indicate that the corrosion resistance of low-carbon steel is improved after introducing the borocarburized layer. Moreover, the protective film of boride can resist the transport of aggressive Cl−, thereby protecting inner metal from the aggressive environment. Returning to the electrochemical measurements (Figure 8), the increased Rp and the decreased Icorr revealed that the corrosion rate of the coating declines after the formatting of the borocarburized layer on Q235 low-carbon steel, which can also explain the improved corrosion resistance to some extent. The corrosion resistance of the boronized layer is related to micro cracks, so the corrosion resistance performance of the borocarburized layer is largely due to the uniform Fe2B layer without cracks. Meanwhile, for the conventional FeB/Fe2B layers, stress concentration tends to occur at the FeB/Fe2B phase boundary due to the different

Coatings 2020, 10, x FOR PEER REVIEW 10 of 13

Figure 9. Electrochemical impedance spectroscopy plots of borocarburized layer and substrate (a) Nyquist plots, (b) Bode plots.

The equivalent circuit is proposed in Figure 10 to further analyze the electrochemical process of the electrode/electrolyte surface, where Rs is the solution resistance from reference electrode to sample, Cdl is the double layer capacitors in the reaction interface, Rf is the charge transfer resistance Coatings 2020, 10, 1205 10 of 13 of the reaction interface. As for the layer, it possesses a double-layer structure, Qd1 represents the constant phase angle element associated with the boronized layer, and Rpore represents the porous resistance ofphase the angleboronized element associatedlayer. with the boronized layer, and Rpore represents the porous resistance of the boronized layer.

Figure 10. The equivalent circuit model used for fitting the EIS plots (a) substrate, (b) the Figure 10. borocarburizedThe equivalent layer. circuit model used for fitting the EIS plots (a) substrate, (b) the borocarburized layer. During the corrosion test, Rf is proportional to corrosion resistance. According to Table2, the R f of the borocarburized layer is larger than that of the substrate. The above results indicate that the corrosion Duringresistance the corrosion of low-carbon test, R steelf is isproportional improved after to introducing corrosion the resistance. borocarburized According layer. Moreover, to Table 2, the Rf of the borocarburizedthe protective layer film of is boride larger can than resist thethat transport of the ofsubstrate. aggressive The Cl−, therebyabove protectingresults indicate inner that the corrosion resistancemetal from theof aggressivelow-carbon environment. steel is Returning improved to the after electrochemical introducing measurements the borocarburized (Figure8), layer. the increased Rp and the decreased Icorr revealed that the corrosion rate of the coating declines− after the Moreover, theformatting protective of the borocarburized film of boride layer ca onn Q235 resist low-carbon the transport steel, which of canaggressive also explain Cl the, improvedthereby protecting inner metalcorrosion from resistancethe aggressive to some extent. environment. The corrosion resistanceReturning of the to boronized the electrochemical layer is related to micro measurements (Figure 8), cracks,the increased so the corrosion Rp resistanceand the performance decreased of theIcorr borocarburized revealed that layer the is largely corrosion due to the rate uniform of the coating declines afterFe2 Bthe layer formatting without cracks. of the Meanwhile, borocarburized for the conventional layer on FeB Q235/Fe2B layers,low-carbon stress concentration steel, which can also tends to occur at the FeB/Fe2B phase boundary due to the different expansion coefficients between explain the theimproved two phases. corrosion Thus, micro resistance cracks in these to FeBsome/Fe2 extent.B layers areThe feasible corrosion during theresistance preparation of and the boronized layer is relatedservice to [ 39micro,40]. cracks, so the corrosion resistance performance of the borocarburized layer is largely due to the uniformTable Fe 2. EIS2B fittinglayer results without of the substrate cracks. and Meanwhile, borocarburized layer.for the conventional FeB/Fe2B layers, stress concentration tends to occur at the FeB/Fe2B phase boundary due to the different Rs Cdl Rpore Qdl Rf (Ω cm 2) (F cm 2) (Ω cm 2) (Ω 1 cm 2 s n) (Ω cm 2) · − − · − − · − · − · − Borocarburized 1.47 10 6 1.692 10 8 31.02 5.236 10 4 8750 layer × − × − × − Q235 1.82 10 7 3.341 10 8 — — 1858 × − × −

4. Conclusions In our research, the wear and corrosion behaviors of the borocarburized low-carbon steel were investigated. The following conclusions can be drawn from the whole article: A crack-free and uniform borocarburized diﬀusion layer was successfully fabricated by double glow plasma surface alloying technology. Only a Fe2B phase was detected on the surface of the borocarburized layer. Coatings 2020, 10, 1205 11 of 13

The frictional and wear properties of the substrate and borocarburized layer at different speeds were systematically investigated. Due to the enhanced hardness of the coating from 200 HV to 1700 HV after introducing the borocarburized layer, the coating exhibited an excellent wear resistance. When the rotation speed was 0.083 m s 1, 0.167 m s 1 and 0.333 m s 1, the specific wear rate was reduced by · − · − · − 96.52%, 96.50% and 96.51%, respectively. The borocarburized layer effectively prevented the formation of the phase of FeB/Fe2B. The crack-free Fe2B layer resisted the transport of aggressive Cl-, and thereby improved the corrosion resistance.

Author Contributions: Conceptualization, Z.D. and Q.M.; Formal analysis, Z.D., W.L. and Z.Y.; Investigation, Z.D.; Project administration, Q.M. and W.L.; Validation, Z.D., Q.M., Z.Y. and S.Z.; Writing—original draft, Z.D.; Writing—review and editing, Q.M. and W.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51874185. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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