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Possibilities of increasing wear resistance of steel surface by plasma electrolytic treatment
Wear 386-387 (2017) 239–246
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Wear journal homepage: www.elsevier.com/locate/wear
Possibilities of increasing wear resistance of steel surface by plasma electrolytic treatment S.A. Kusmanov n, S.A. Silkin, A.A. Smirnov, P.N. Belkin Kostroma State University, Dzerzhinskogo, 17, Kostroma 156005, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 8 September 2016 Received in revised form 6 November 2016 Accepted 15 December 2016
The paper considers the effect of the anode plasma electrolytic treatment (PET) on tribological properties of steels after their boriding (PEB), nitriding (PEN), and nitrocarburising (PEN/C). Electrolyte compositions and processing temperatures improved wear resistance of samples under dry friction condition using a ball-on-disk test with an Al2O3 ball as counter-body are determined. The anode PEB of medium carbon steel in solution of boric acid and ammonium chloride (850 °C, 5 min) followed by quenching in electrolyte enables to decrease friction coefficient, surface roughness 3 by times, and volume loss by 3.7 times. This treatment results in the change of wear mechanism from abrasive-adhesive wear to polishing mode. The PEN of medium carbon steel in a solution containing ammonia and ammonium chloride (650 °C, 5 min) followed by quenching in electrolyte leads to decrease in friction coefficient from 0.65 to 0.45, surface roughness by a factor of 1.74, and volume loss by 4.6 times. PEN/C of low-carbon steel in electrolyte containing glycerol, ammonium chloride and ammonium nitride at 950 °C also promotes surface roughness and wear rate reduction of by 9.5 times. & 2017 Elsevier B.V. All rights reserved.
Keywords: Plasma electrolysis Boriding Nitriding NitrocarburisingFriction coefficient Wear resistance
1. Introduction Case hardening of steel parts occupies an important place among the methods of increasing their wear resistance. Technologies of diffusion saturation of steel include PET which enables to decrease the processing time to several minutes and does not require expensive equipment or toxic components. Effective application of plasma electrolytic saturation of steels with nitrogen, carbon or boron for enhancement of their wear resistance is presented in many publications. Cathode PEN in a solution of carbamide at 400–600 °С for 3–10 minutes diminishes wear rate of cast iron G3500 by 2.5 times and steel S0050A by 3times under dry friction with WC balls as counter-bodies (5 N normal load, 0.1 m/s sliding speed, and 200 m sliding distance) [1]. However, electrical discharges inherent to cathode treatment lead to friction coefficient increase from 0.37 to 0.4 for steel S0050A and from 0.14 to 0.4 for cast iron G3500. Similar results were obtained for PEN of high speed steel R6M5 [2] or structural steel 34CrNi1Mo [3] where abrasive wear resistance decreased by a factor of 1.5. The cathodic PEC of pure iron in glycerol-based electrolyte decreases wear rate by one order but the friction coefficient increases [4]. Pulse PEC of high-carbon steel T8 in glycerol-based n
Corresponding author. E-mail addresses: [email protected] (S.A. Kusmanov), [email protected] (S.A. Silkin), [email protected] (A.A. Smirnov), [email protected] (P.N. Belkin). http://dx.doi.org/10.1016/j.wear.2016.12.053 0043-1648/& 2017 Elsevier B.V. All rights reserved.
electrolyte also results in the formation of diamond-like carbon layer which provides an increase in the wear rate by 5 times and reduces friction coefficient by 2 times approximately [5]. Maximal values of microhardness and wear resistance of low-allow steel H13 after its PEC in glycerol electrolyte were obtained at frequency of 10 kHz [6]. The wear resistance of steels can be enhanced by means of PEN/C, as well. In particular, the steel treatment in carbamide-based electrolytes reduces the weight wear of low carbon steel 1020 by an order of magnitude approximately [7], Q235 [8] or stainless steel 316 L [9]. Moreover, PEN/C of steel 316 L enhances its fatigue strength [10]. Wear resistance of cast iron can also be increased using PEN/C in electrolyte containing acetamide and glycerol [11]. Additional increase in wear resistance of AISI 304 stainless steel can be obtained using a diamond-like carbon coating on the PEN/C pre-treated substrate [12]. This treatment results in a simultaneous reduction of the friction coefficient and wear rate due to changes in the wear mechanism from adhesion/ abrasion to asperity deformation and polishing. It is established that this coating does not get damaged under 10–25 N loads against different counter-bodies [13]. Positive results were obtained for PEB of steels. Weight wear rate of steel H13 after its treatment in borax-based electrolyte (969 °С, 10 min) decreased by 13 times in comparison with untreated samples [14]. Anode PEB of medium carbon steel in electrolyte containing borax also results in diminishment of friction coefficient from 0.26 to 0.16 and weight wear rate by a factor of 7 during dry wear testing against hardened steel disk (HRC 45–50)
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[15]. Some better results are obtained with anodic PEB of medium carbon steel in a solution of boric acid and ammonium chloride (900 °С, 5 min) where surface roughness decreased by 3 times, friction coefficient from 0.85 to 0.15 and wear rate was reduced by a factor of 15 [16]. Simultaneous saturation of steels with boron and other elements is investigated in some studies. Borocarburising of lowcarbon steel Q235 in a solution of borax and glycerol provides weight wear decrease by 12 times in comparison with raw sample after dry wear testing against ZrO2 ball [17]. In this case, friction coefficient reduces from 0.6 to 0.17 in spite of a rise of surface roughness by an order. Replacing glycerol with other organic substance can further improve wear resistance of steel Q235 by a factor of 19 under the same test conditions [18]. Simultaneous saturation of H13 steel with boron and nitrogen in electrolyte containing borax and sodium nitrite provides growth of wear resistance by more than 17 times in comparison with untreated steel [19]. A number of authors note that cathode treatment increases surface roughness of steels [7–9 and etc.]. The anode processes are characterized by a decrease in surface roughness of steel due to anode dissolution of samples which enables to reduce friction coefficient and wear rate. The anode PEN of steel 40Cr in a solution of ammonia and ammonium chloride (750 °С, 5 min) leads to diminishment of friction coefficient from 0.42 to 0.34 and wear rate by 15 times for dry wear testing with pin of sintered TiC as a counter-body [20]. Anode PEN/C in carbamide-based electrolyte provides decrease in surface roughness of low-carbon steel by 9 times and wear rate by 6.7 times during lubricant wear testing against hardened steel disk (HRC 50) as a counter-body with normal load of 315 N, sliding speed of 0.47 m/s, and 1000 m sliding distance [21].Finally, we present an example of anode saturation of mild steel with boron, nitrogen and carbon in electrolyte containing boric acid, carbamide and ammonium chloride [22]. In this case, surface roughness diminishes from 1.0 μm to 0.8 μm, dry friction coefficient from 0.16 to 0.11, and weight loss during testing from 6.8 mg to 3.8 mg at sliding distance of 1000 m. Analysis of publications shows the prospects for plasma electrolytic processes to improve wear resistance of steel parts. However, these positive results were obtained under various and fixed conditions of wear testing. Morphology of friction tracks and the mechanism of wear have not been the focus of most studies. The aim of this work is to study the effect of temperature saturation of steels with interstitial elements on the coefficient of friction and wear rate of the steel samples treated in different ways. Results of the anode processes of PEB, PEN, and PEN/C of low-carbon and medium carbon steels including the structure of the modified layers, their phase composition, microhardness, surface roughness and morphology of the worn surface will be considered.
2. Materials and methods 2.1. Materials and characteristics of treatment Cylindrical samples (∅ 10 15 mm) of low-carbon 20 and medium carbon 45 steels (Table 1) were ground with SiC abrasive Table 1 Chemical composition of the samples (wt. %). Grade
C
Mn
Si
P
S
Cr
Ni
Cu
As
20 45
0.20 0.44
0.38 0.59
0.21 0.23
0.014 0.017
0.013 0.017
0.17 0.16
0.09 0.09
0.17 0.2
0.01 0.01
paper grit size P320 to Ra ∼1.0 μm and ultrasonically cleaned with aсetone. Plasma electrolytic saturation was carried out in a cylindrical electrolyzer (volume of 1.5 l) with an axially symmetric electrolyte flow supplied through a nozzle located at the bottom of the electrolyzer [23]. In the upper part of the electrolyzer, the electrolyte was overflowing into the sump and was further pumped through a heat exchanger at a rate of 2.6 l/min which was measured with a RMF-0.16 GUZ flow meter (accuracy of 72.5%). This scheme provides stabilization of processing conditions. The solution temperature was measured using thermocouple placed at the bottom of the chamber. The electrolyte temperature was maintained at 307 2 °C. The samples were connected as positive output and the electrolyzer was connected as negative output of the 15 kW DC power supply. Voltage and current were measured using DP6-DV voltmeter and DP6-DA ampermeter (70.5%). After switching the voltage of 200 V, the samples were immersed in the electrolyte at speed of 1–2 mm/s. At slow immersion vapour-gaseous envelope is easily formed in the samples initial area adjoining electrolyte, and further extends across the sample as it submerges. As far as a sample is immersed at a depth equal to its height, voltage is corrected for reaching prescribed sample's temperature (850–950 °C). The sample temperature was measured with MY K-type thermocouple and multimeter APPA109N (accuracy to 3% over a temperature range of 400–1000 °C). Thermocouple fixed in a hole made in the samples at a distance of 2 mm from the sample bottom. The treatment time was 5 min. The treatment temperature varied from 650 to 950 °C. After PET samples were quenched in the electrolyte (hardening). Aqueous solutions of boric acid H3BO3 (3 wt.%) with ammonium chloride NH4Cl (10 wt.%), ammonia NH3 (5 wt.%) with ammonium chloride NH4Cl (10 wt.%), and glycerol (8 wt.%) with ammonium chloride NH4Cl (15 wt.%) and ammonium nitrate NH4NO3 (5 wt.%) were used as the working electrolytes. This is a cost-effective, ecologically friendly and non-hazardous components compared to the organic, highly flammable and toxic electrolytes. 2.2. Surface characterization Microstructural studies of the surface layers were performed using conventional techniques. Quanta 3D 200i scanning electron microscopy (SEM) (FEI Company) with backscattering electron detector (working distance was 15.2 mm and acceleration voltage was 10 kV) was used to observe the structure of the surface layer of the samples after polishing and etching with the use of a 4 wt. % nitric acid solution in ethanol for 5–10 s. The phase composition of the surface layers after PET was investigated with the use of an ARL X’tra x-ray diffractometer (Thermo Fisher Scientific) with Cu Kα radiation at a simple scanning in the theta-2theta-mode and scanning rate of 2°/min. Microhardness of the samples’ surface layer after PET was measured on a PMT-3M apparatus at a loading of 50 g (4 measurements). Surface roughness before and after PET was investigated using a roughness tester TR200 (5 measurements). A ball-on-disk tribometer was applied to evaluate wear resistance of the untreated and treated samples under dry testing conditions with 10 N normal load, 0.2 m/s sliding speed, and 240 m sliding distance (diameter of track is 9 mm) with an Al2O3 ball (6.35 mm in diameter) as counter-body. The typical morphologies of wear tracks were analyzed with an optical microscope. The profiles of wear tracks were investigated using a roughness tester TR200. The volume loss was tested using 5 measurements. To enhance reproducibility of experimental data (before wear tests) the friable part of the oxide layer was removed
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mechanically using abrasive paper with grains size of 3 μm. All wear tests were performed under ambient atmospheric conditions and repeated 3 times to confirm the validity of the results.
3. Results and discussion 3.1. Plasma electrolytic boriding Tribological properties of materials surface are affected by its structure. Anode PEB of medium carbon steel results in the formation of oxide and modified layers (Fig. 1). According to the data of X-ray analysis oxide layer contains FeO, α-Fe2O3, and Fe3O4, and modified layer contains FeB and Fe2B, martensite, and solid solution of boron in iron (Fig. 2). Oxide layer formation occurs due to the samples’ oxidation, borides are the results of boron diffusion into material structure, martensite, and retained austenite are formed owing to hardening in electrolyte from temperature of saturation with boron. Fig. 3 shows microhardness profile of modified layer. The rise of PEB temperature results in diminishment of maximal microhardness of modified layer and its thickness. We suppose that the temperature growth increases the oxide layer thickness which inhibits the boron diffusion and decreases the modified layer thickness. Similar results were obtained for anode PEC [23], PEN [24], PEN/C [21] and PEB [16] of steels. Moreover oxidation and diffusive saturation, the anode process is characterized by anode dissolution which leads to a decrease in the surface roughness. The temperature rise is established to intensity both anode dissolution of steels and their oxidation [21]. In this case, simultaneous effect of their processes causes an increase in surface roughness due to thickening of the porous oxide layer which can delaminate irregular flaking during rapid cooling in the electrolyte. Therefore, the minimum roughness is reached after treatment at 850 °C (Table 2). Wear test shows that wear resistance of borided samples is always higher than that of untreated ones (Table 2). The minimal volume loss during wear test is observed after PEB at 850 °С with the slight growth when processing temperature rises. The PEB at this temperature leads to the lowest friction coefficient (Fig. 4). However, this coefficient is more than that of untreated sample at higher processing temperatures. Moreover, the lower the treatment temperatures is, the shorter is the lengths of initial part of wear track where running-up and stabilization of
Fig. 2. X-ray diffraction patterns of surface layers of medium carbon steel after anode PEB at 850 °С.
Fig. 3. Microhardness distribution in the modified layer after anode PEB at different treatment temperatures. Table 2 Surface characteristics and properties of the untreated and treated samples in the solution of boric acid and ammonium chloride. Treatment temperature [°C]
Oxide layer thickness [μm]
Modified lay- Surface roughness er thickness [μm] [μm]
850 900 950 Untreated
20 75 40 75 80 75 –
1107 5 1107 5 30 75 –
0.30 7 0.02 1.007 0.05 1.30 7 0.06 1.007 0.05
Volume loss [mm3]
0.117 0.02 0.16 7 0.03 0.18 70.04 0.40 70.06
friction coefficient occur. Similar part of wear track is not found for the raw sample. The PEB temperature rise leads to an increase in the oxide layer thickness, surface roughness and reduction of microhardness which result in growth of friction coefficient. Therefore, the observed enhance of tribological behavior is associated with the increase in surface microhardness and roughness decrease. Observations of worn surfaces of the raw sample show a combination of abrasive microcutting and adhesive wear which leads to the greatest destruction of steel surface by the counterbody. Conversely, anode PEB polishes of the treated surface and reduces material removal (Fig. 5). 3.2. Plasma electrolytic nitriding Fig. 1. SEM image of cross-section of medium carbon steel surface after anode PEB at 850 °С: 1 – oxide layer, 2 – modified layer, 3 – initial structure.
The structure of medium carbon steel after its anode PEN
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Fig. 4. Friction coefficient behaviour of untreated and treated samples after anode PEB for different treatment temperature.
includes oxide layer containing FeO and Fe3O4 and modified layer with nitride zone (Fe4N and Fe2-3N) and diffusive one the latter being solid solution of nitrogen in iron (Fig. 6). Nitrides and solid solution are formed owing to the nitrogen diffusion. Quenching of samples in electrolyte results in appearance of martensite providing high hardness of diffusion zone (Fig. 7), and retained austenite. The distribution of microhardness obtained at different temperatures is determined by intensity of nitrogen diffusion. The lower microhardness at a depth of 15 m can be associated with the higher concentration of softer iron nitrides. The maximal
Fig. 6. SEM image of cross-section of medium carbon steel surface after anode PEN at 750 °С: 1 – oxide layer, 2 – nitride zone, 3 – diffusion zone, 4 – initial structure.
microhardness is reached in martensite zone. The thicknesses of nitride and diffusion zones rise as temperature of PEN increases to 750 °С that probably is associated with acceleration of nitrogen diffusion (Table 3). The PEN temperature growth up to 800 °С reduces the thicknesses of nitride and diffusion zones along with a decrease in microhardness. This result may be associated with inhibition of nitrogen diffusion by growing oxide layer and deterioration of ammonia adsorption on the sample surface [25]. The decrease in the surface roughness can be explained by action of anode dissolution. The increase in the ammonium chloride concentration evidently results in the rise of the anode dissolution
Fig. 5. Micrograph and profile of worn surface of untreated sample (a) and treated samples after anode PEB at 850 °С (b), 900 °С (c), 950 °С (d).
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Fig. 7. Microhardness distribution in the modified layer after anode PEN at different treatment temperatures. Table 3 The surface characteristics and properties of the untreated and treated samples in the solution of ammonia and ammonium chloride. Treatment temperature [°C]
Oxide layer thickness [μm]
Nitride zone thickness [μm]
Diffusion zone thickness [μm]
Surface roughness [μm]
Volume loss [mm3]
650 750 800 untreated
47 2 127 2 16 72 –
47 2 14 72 87 2 –
307 5 657 5 607 5 –
0.60 7 0.03 1.30 70.06 1.75 70.07 1.007 0.05
0.09 70.02 0.217 0.05 0.107 0.02 0.40 70.06
243
rate and reduction of surface roughness [21]. As the temperature rises the roughness increase associated with the growth of oxide layer highlights the dominant role of steel oxidation. Wear test shows enhancement of wear resistance of steel after PEN at all processing temperatures (Table 3). The greatest reduction of wear rate (by a factor of 4.6) occurs after PEN at 650 °С and about the same at 850 °C. Unlike boriding, the correlation between wear rate of nitride steel and its microhardness is not observed. Moreover, according to measurements, the depth of wear track after removing the oxide layer is greater than the thickness of a relatively soft nitrided zone for samples treated at 650 °С (Fig. 8). Consequently, the counter-body slides on harder martensite layer after deletion of nitrided zone. In this case, wear rate is minimal. Contrary, wear of sample nitride at 750 °С takes place only within the soft nitrided zone with the maximal wear rate. It can be assumed for the sample nitrided at 800 °С that the counter-body slides near the boundary between nitrided and martensite zones where the concentration of nitrides is lower and martensite concentration is higher. An intermediate value of the wear rate is observed in this case. No correlation of wear resistance with microhardness occurs because the martensitic layer is not always involved in the process of wear. Maximal decrease in the friction coefficient is also observed after PEN at 650 °С (Fig. 9). Here the wear mechanism changes from abrasive/adhesion wear for untreated samples (Fig. 8a) to polishing (Fig. 8b) which results in the minimal wear rate of the samples after PEN. Friction coefficient is also reduced after PEN at 750 °С but in a lesser degree than after PEN at 650 °С. The traces of abrasive wear are observed on the worn surface (Fig. 8c) with the increase in the wear rate. When the treatment temperature rises to 800 °С, friction coefficient moves higher than that of the untreated samples where adhesive wear mechanism is observed (Fig. 8d).
Fig. 8. Micrograph and profile of worn surface of untreated sample (a) and treated samples after anode PEN at 650 °С (b), 750 °С (c), 800 °С (d).
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Table 4 Surface characteristics and properties of the untreated and treated samples in the solution of glycerol, ammonium nitrite and ammonium chloride. Treatment temperature [°C]
Oxide layer thickness [μm]
Modified lay- Surface roughness er thickness [μm] [μm]
750 850 950 untreated
457 5 70 75 957 5 –
1007 5 130 75 145 75 –
0.50 70.03 0.707 0.04 0.90 70.05 1.00 70.05
Volume loss [mm3]
0.06 70.01 0.05 70.01 0.09 70.02 0.49 70.11
Fig. 9. Friction coefficient behaviour of untreated and treated samples after anode PEN for different treatment temperature.
3.3. Plasma electrolytic nitrocarburising Fig. 10 shows oxide layer (FeO and Fe3O4) and modified one (solid solution of nitrogen and carbon in iron) formed on the surface of low-carbon steel after its anode PEN/C. Quenching of samples in the electrolyte after their saturation with N and C results in the formation of martensite in diffusive zone. Thicknesses of all layers grow as the processing temperatures rises (Table 4). Accordingly, the depth of the hardened zones and the maximum value of microhardness increase (Fig. 11). The surface roughness after PEN/C decreases at all treatment temperatures (Table 4). When the PEN/C temperature rises, the surface roughness increases in parallel with thickening of oxide layer. Tribological tests reveal that wear rates of treated samples at all processing temperatures are lower than that of untreated samples (Table 4). Wear rate decreases in the greatest degree after PEN/C at 850 °С. In this case, the leading factor is surface roughness which compensates for lower values of microhardness and higher friction coefficient (Fig. 12). In result, the PEN/C of steel provides the transition of abrasive/adhesion mechanism, specify to raw samples, to polishing (Fig. 13).
Fig. 11. Microhardness distribution in the modified layer after anode PEN/C at different treatment temperatures.
Fig. 12. Friction coefficient behaviour of untreated and treated samples after anode PEN/C for different treatment temperature.
4. Conclusions
Fig. 10. SEM image of cross-section of low-carbon steel surface after anode PEN/C at 850 °С: 1 – oxide layer, 2 – modified layer, 3 – initial structure.
1. Anode boriding of medium carbon steel (0.45 wt. % С) in electrolyte containing 3 wt.% of boric acid and 10 wt.% of ammonium chloride results in the formation of the structure containing the external oxide layer and modified layer – a solid solution of boron in iron enriched by its borides and martensite. The anode PEB at 850 °С during 5 min enables to increase microhardness of the layer to 1800 HV, the reduction of surface roughness from 1.01 μm to 0.31 μm, of dry friction coefficient from 0.65 to 0.55, and volume loss from 0.40 mm3 to 0.11 mm3 at sliding distance of 240 m in comparison with untreated
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Fig. 13. Micrograph and profile of worn surface of untreated sample (a) and treated samples after anode PEN/C at 750 °С (b), 850 °С (c), and 950 °С (d).
samples. Wear test with Al2O3 ball as counter-body at normal load of 10 N and sliding speed of 0.2 m/s reveals that the PEB provides transfer from abrasive/adhesion wear for raw samples to polishing for treated ones. The minimal wear rate is reached when microhardness of surface layer is maximal. 2. Anode nitriding of medium carbon steel (0.45 wt. % С) in electrolyte with 5 wt.% of ammonia and 10 wt.% of ammonium chloride enables to form external oxide layer, nitride zone and nitrogenous ferrite. The layer microhardness increases to 540 HV, the surface roughness decreases from 1.01 μm to 0.58 μm, friction coefficient reduces from 0.65 to 0.45, and volume loss diminishes from 0.40 mm3 to 0.09 mm3 at sliding distance of 240 m in comparison with untreated samples. The lowest wear rate is reached providing that nitride zone thickness is minimal. 3. Oxide layer and solid solution of nitrogen and carbon in iron are formed after anode nitrocarburising of low-carbon steel (0.2 wt. % C) using electrolyte containing 5 wt.% of ammonium nitrate, 8 wt.% glycerol and 15 wt.% of ammonium chloride. The highest microhardness of 880 HV is reached at PEN/C temperature of 950 °С, assumedly, owing to intensive carbon diffusion. The maximal wear resistance is observed after treatment at 850 °С during 5 min. In this case, the layer microhardness increases up to 620 HV, surface roughness decreases from 1.01 μm to 0.7 μm, and volume loss reduces from 0.49 mm3 to 0.06 mm3 at sliding distance of 240 m in comparison with untreated samples. The lowest wear rate approximately corresponds to the minimum surface roughness.
Acknowledgments This work was financially supported by the Russian Science Foundation (Contract no. 15-13-10018) to the Kostroma State University.
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Wear journal homepage: www.elsevier.com/locate/wear
Possibilities of increasing wear resistance of steel surface by plasma electrolytic treatment S.A. Kusmanov n, S.A. Silkin, A.A. Smirnov, P.N. Belkin Kostroma State University, Dzerzhinskogo, 17, Kostroma 156005, Russia
art ic l e i nf o
a b s t r a c t
Article history: Received 8 September 2016 Received in revised form 6 November 2016 Accepted 15 December 2016
The paper considers the effect of the anode plasma electrolytic treatment (PET) on tribological properties of steels after their boriding (PEB), nitriding (PEN), and nitrocarburising (PEN/C). Electrolyte compositions and processing temperatures improved wear resistance of samples under dry friction condition using a ball-on-disk test with an Al2O3 ball as counter-body are determined. The anode PEB of medium carbon steel in solution of boric acid and ammonium chloride (850 °C, 5 min) followed by quenching in electrolyte enables to decrease friction coefficient, surface roughness 3 by times, and volume loss by 3.7 times. This treatment results in the change of wear mechanism from abrasive-adhesive wear to polishing mode. The PEN of medium carbon steel in a solution containing ammonia and ammonium chloride (650 °C, 5 min) followed by quenching in electrolyte leads to decrease in friction coefficient from 0.65 to 0.45, surface roughness by a factor of 1.74, and volume loss by 4.6 times. PEN/C of low-carbon steel in electrolyte containing glycerol, ammonium chloride and ammonium nitride at 950 °C also promotes surface roughness and wear rate reduction of by 9.5 times. & 2017 Elsevier B.V. All rights reserved.
Keywords: Plasma electrolysis Boriding Nitriding NitrocarburisingFriction coefficient Wear resistance
1. Introduction Case hardening of steel parts occupies an important place among the methods of increasing their wear resistance. Technologies of diffusion saturation of steel include PET which enables to decrease the processing time to several minutes and does not require expensive equipment or toxic components. Effective application of plasma electrolytic saturation of steels with nitrogen, carbon or boron for enhancement of their wear resistance is presented in many publications. Cathode PEN in a solution of carbamide at 400–600 °С for 3–10 minutes diminishes wear rate of cast iron G3500 by 2.5 times and steel S0050A by 3times under dry friction with WC balls as counter-bodies (5 N normal load, 0.1 m/s sliding speed, and 200 m sliding distance) [1]. However, electrical discharges inherent to cathode treatment lead to friction coefficient increase from 0.37 to 0.4 for steel S0050A and from 0.14 to 0.4 for cast iron G3500. Similar results were obtained for PEN of high speed steel R6M5 [2] or structural steel 34CrNi1Mo [3] where abrasive wear resistance decreased by a factor of 1.5. The cathodic PEC of pure iron in glycerol-based electrolyte decreases wear rate by one order but the friction coefficient increases [4]. Pulse PEC of high-carbon steel T8 in glycerol-based n
Corresponding author. E-mail addresses: [email protected] (S.A. Kusmanov), [email protected] (S.A. Silkin), [email protected] (A.A. Smirnov), [email protected] (P.N. Belkin). http://dx.doi.org/10.1016/j.wear.2016.12.053 0043-1648/& 2017 Elsevier B.V. All rights reserved.
electrolyte also results in the formation of diamond-like carbon layer which provides an increase in the wear rate by 5 times and reduces friction coefficient by 2 times approximately [5]. Maximal values of microhardness and wear resistance of low-allow steel H13 after its PEC in glycerol electrolyte were obtained at frequency of 10 kHz [6]. The wear resistance of steels can be enhanced by means of PEN/C, as well. In particular, the steel treatment in carbamide-based electrolytes reduces the weight wear of low carbon steel 1020 by an order of magnitude approximately [7], Q235 [8] or stainless steel 316 L [9]. Moreover, PEN/C of steel 316 L enhances its fatigue strength [10]. Wear resistance of cast iron can also be increased using PEN/C in electrolyte containing acetamide and glycerol [11]. Additional increase in wear resistance of AISI 304 stainless steel can be obtained using a diamond-like carbon coating on the PEN/C pre-treated substrate [12]. This treatment results in a simultaneous reduction of the friction coefficient and wear rate due to changes in the wear mechanism from adhesion/ abrasion to asperity deformation and polishing. It is established that this coating does not get damaged under 10–25 N loads against different counter-bodies [13]. Positive results were obtained for PEB of steels. Weight wear rate of steel H13 after its treatment in borax-based electrolyte (969 °С, 10 min) decreased by 13 times in comparison with untreated samples [14]. Anode PEB of medium carbon steel in electrolyte containing borax also results in diminishment of friction coefficient from 0.26 to 0.16 and weight wear rate by a factor of 7 during dry wear testing against hardened steel disk (HRC 45–50)
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[15]. Some better results are obtained with anodic PEB of medium carbon steel in a solution of boric acid and ammonium chloride (900 °С, 5 min) where surface roughness decreased by 3 times, friction coefficient from 0.85 to 0.15 and wear rate was reduced by a factor of 15 [16]. Simultaneous saturation of steels with boron and other elements is investigated in some studies. Borocarburising of lowcarbon steel Q235 in a solution of borax and glycerol provides weight wear decrease by 12 times in comparison with raw sample after dry wear testing against ZrO2 ball [17]. In this case, friction coefficient reduces from 0.6 to 0.17 in spite of a rise of surface roughness by an order. Replacing glycerol with other organic substance can further improve wear resistance of steel Q235 by a factor of 19 under the same test conditions [18]. Simultaneous saturation of H13 steel with boron and nitrogen in electrolyte containing borax and sodium nitrite provides growth of wear resistance by more than 17 times in comparison with untreated steel [19]. A number of authors note that cathode treatment increases surface roughness of steels [7–9 and etc.]. The anode processes are characterized by a decrease in surface roughness of steel due to anode dissolution of samples which enables to reduce friction coefficient and wear rate. The anode PEN of steel 40Cr in a solution of ammonia and ammonium chloride (750 °С, 5 min) leads to diminishment of friction coefficient from 0.42 to 0.34 and wear rate by 15 times for dry wear testing with pin of sintered TiC as a counter-body [20]. Anode PEN/C in carbamide-based electrolyte provides decrease in surface roughness of low-carbon steel by 9 times and wear rate by 6.7 times during lubricant wear testing against hardened steel disk (HRC 50) as a counter-body with normal load of 315 N, sliding speed of 0.47 m/s, and 1000 m sliding distance [21].Finally, we present an example of anode saturation of mild steel with boron, nitrogen and carbon in electrolyte containing boric acid, carbamide and ammonium chloride [22]. In this case, surface roughness diminishes from 1.0 μm to 0.8 μm, dry friction coefficient from 0.16 to 0.11, and weight loss during testing from 6.8 mg to 3.8 mg at sliding distance of 1000 m. Analysis of publications shows the prospects for plasma electrolytic processes to improve wear resistance of steel parts. However, these positive results were obtained under various and fixed conditions of wear testing. Morphology of friction tracks and the mechanism of wear have not been the focus of most studies. The aim of this work is to study the effect of temperature saturation of steels with interstitial elements on the coefficient of friction and wear rate of the steel samples treated in different ways. Results of the anode processes of PEB, PEN, and PEN/C of low-carbon and medium carbon steels including the structure of the modified layers, their phase composition, microhardness, surface roughness and morphology of the worn surface will be considered.
2. Materials and methods 2.1. Materials and characteristics of treatment Cylindrical samples (∅ 10 15 mm) of low-carbon 20 and medium carbon 45 steels (Table 1) were ground with SiC abrasive Table 1 Chemical composition of the samples (wt. %). Grade
C
Mn
Si
P
S
Cr
Ni
Cu
As
20 45
0.20 0.44
0.38 0.59
0.21 0.23
0.014 0.017
0.013 0.017
0.17 0.16
0.09 0.09
0.17 0.2
0.01 0.01
paper grit size P320 to Ra ∼1.0 μm and ultrasonically cleaned with aсetone. Plasma electrolytic saturation was carried out in a cylindrical electrolyzer (volume of 1.5 l) with an axially symmetric electrolyte flow supplied through a nozzle located at the bottom of the electrolyzer [23]. In the upper part of the electrolyzer, the electrolyte was overflowing into the sump and was further pumped through a heat exchanger at a rate of 2.6 l/min which was measured with a RMF-0.16 GUZ flow meter (accuracy of 72.5%). This scheme provides stabilization of processing conditions. The solution temperature was measured using thermocouple placed at the bottom of the chamber. The electrolyte temperature was maintained at 307 2 °C. The samples were connected as positive output and the electrolyzer was connected as negative output of the 15 kW DC power supply. Voltage and current were measured using DP6-DV voltmeter and DP6-DA ampermeter (70.5%). After switching the voltage of 200 V, the samples were immersed in the electrolyte at speed of 1–2 mm/s. At slow immersion vapour-gaseous envelope is easily formed in the samples initial area adjoining electrolyte, and further extends across the sample as it submerges. As far as a sample is immersed at a depth equal to its height, voltage is corrected for reaching prescribed sample's temperature (850–950 °C). The sample temperature was measured with MY K-type thermocouple and multimeter APPA109N (accuracy to 3% over a temperature range of 400–1000 °C). Thermocouple fixed in a hole made in the samples at a distance of 2 mm from the sample bottom. The treatment time was 5 min. The treatment temperature varied from 650 to 950 °C. After PET samples were quenched in the electrolyte (hardening). Aqueous solutions of boric acid H3BO3 (3 wt.%) with ammonium chloride NH4Cl (10 wt.%), ammonia NH3 (5 wt.%) with ammonium chloride NH4Cl (10 wt.%), and glycerol (8 wt.%) with ammonium chloride NH4Cl (15 wt.%) and ammonium nitrate NH4NO3 (5 wt.%) were used as the working electrolytes. This is a cost-effective, ecologically friendly and non-hazardous components compared to the organic, highly flammable and toxic electrolytes. 2.2. Surface characterization Microstructural studies of the surface layers were performed using conventional techniques. Quanta 3D 200i scanning electron microscopy (SEM) (FEI Company) with backscattering electron detector (working distance was 15.2 mm and acceleration voltage was 10 kV) was used to observe the structure of the surface layer of the samples after polishing and etching with the use of a 4 wt. % nitric acid solution in ethanol for 5–10 s. The phase composition of the surface layers after PET was investigated with the use of an ARL X’tra x-ray diffractometer (Thermo Fisher Scientific) with Cu Kα radiation at a simple scanning in the theta-2theta-mode and scanning rate of 2°/min. Microhardness of the samples’ surface layer after PET was measured on a PMT-3M apparatus at a loading of 50 g (4 measurements). Surface roughness before and after PET was investigated using a roughness tester TR200 (5 measurements). A ball-on-disk tribometer was applied to evaluate wear resistance of the untreated and treated samples under dry testing conditions with 10 N normal load, 0.2 m/s sliding speed, and 240 m sliding distance (diameter of track is 9 mm) with an Al2O3 ball (6.35 mm in diameter) as counter-body. The typical morphologies of wear tracks were analyzed with an optical microscope. The profiles of wear tracks were investigated using a roughness tester TR200. The volume loss was tested using 5 measurements. To enhance reproducibility of experimental data (before wear tests) the friable part of the oxide layer was removed
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mechanically using abrasive paper with grains size of 3 μm. All wear tests were performed under ambient atmospheric conditions and repeated 3 times to confirm the validity of the results.
3. Results and discussion 3.1. Plasma electrolytic boriding Tribological properties of materials surface are affected by its structure. Anode PEB of medium carbon steel results in the formation of oxide and modified layers (Fig. 1). According to the data of X-ray analysis oxide layer contains FeO, α-Fe2O3, and Fe3O4, and modified layer contains FeB and Fe2B, martensite, and solid solution of boron in iron (Fig. 2). Oxide layer formation occurs due to the samples’ oxidation, borides are the results of boron diffusion into material structure, martensite, and retained austenite are formed owing to hardening in electrolyte from temperature of saturation with boron. Fig. 3 shows microhardness profile of modified layer. The rise of PEB temperature results in diminishment of maximal microhardness of modified layer and its thickness. We suppose that the temperature growth increases the oxide layer thickness which inhibits the boron diffusion and decreases the modified layer thickness. Similar results were obtained for anode PEC [23], PEN [24], PEN/C [21] and PEB [16] of steels. Moreover oxidation and diffusive saturation, the anode process is characterized by anode dissolution which leads to a decrease in the surface roughness. The temperature rise is established to intensity both anode dissolution of steels and their oxidation [21]. In this case, simultaneous effect of their processes causes an increase in surface roughness due to thickening of the porous oxide layer which can delaminate irregular flaking during rapid cooling in the electrolyte. Therefore, the minimum roughness is reached after treatment at 850 °C (Table 2). Wear test shows that wear resistance of borided samples is always higher than that of untreated ones (Table 2). The minimal volume loss during wear test is observed after PEB at 850 °С with the slight growth when processing temperature rises. The PEB at this temperature leads to the lowest friction coefficient (Fig. 4). However, this coefficient is more than that of untreated sample at higher processing temperatures. Moreover, the lower the treatment temperatures is, the shorter is the lengths of initial part of wear track where running-up and stabilization of
Fig. 2. X-ray diffraction patterns of surface layers of medium carbon steel after anode PEB at 850 °С.
Fig. 3. Microhardness distribution in the modified layer after anode PEB at different treatment temperatures. Table 2 Surface characteristics and properties of the untreated and treated samples in the solution of boric acid and ammonium chloride. Treatment temperature [°C]
Oxide layer thickness [μm]
Modified lay- Surface roughness er thickness [μm] [μm]
850 900 950 Untreated
20 75 40 75 80 75 –
1107 5 1107 5 30 75 –
0.30 7 0.02 1.007 0.05 1.30 7 0.06 1.007 0.05
Volume loss [mm3]
0.117 0.02 0.16 7 0.03 0.18 70.04 0.40 70.06
friction coefficient occur. Similar part of wear track is not found for the raw sample. The PEB temperature rise leads to an increase in the oxide layer thickness, surface roughness and reduction of microhardness which result in growth of friction coefficient. Therefore, the observed enhance of tribological behavior is associated with the increase in surface microhardness and roughness decrease. Observations of worn surfaces of the raw sample show a combination of abrasive microcutting and adhesive wear which leads to the greatest destruction of steel surface by the counterbody. Conversely, anode PEB polishes of the treated surface and reduces material removal (Fig. 5). 3.2. Plasma electrolytic nitriding Fig. 1. SEM image of cross-section of medium carbon steel surface after anode PEB at 850 °С: 1 – oxide layer, 2 – modified layer, 3 – initial structure.
The structure of medium carbon steel after its anode PEN
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Fig. 4. Friction coefficient behaviour of untreated and treated samples after anode PEB for different treatment temperature.
includes oxide layer containing FeO and Fe3O4 and modified layer with nitride zone (Fe4N and Fe2-3N) and diffusive one the latter being solid solution of nitrogen in iron (Fig. 6). Nitrides and solid solution are formed owing to the nitrogen diffusion. Quenching of samples in electrolyte results in appearance of martensite providing high hardness of diffusion zone (Fig. 7), and retained austenite. The distribution of microhardness obtained at different temperatures is determined by intensity of nitrogen diffusion. The lower microhardness at a depth of 15 m can be associated with the higher concentration of softer iron nitrides. The maximal
Fig. 6. SEM image of cross-section of medium carbon steel surface after anode PEN at 750 °С: 1 – oxide layer, 2 – nitride zone, 3 – diffusion zone, 4 – initial structure.
microhardness is reached in martensite zone. The thicknesses of nitride and diffusion zones rise as temperature of PEN increases to 750 °С that probably is associated with acceleration of nitrogen diffusion (Table 3). The PEN temperature growth up to 800 °С reduces the thicknesses of nitride and diffusion zones along with a decrease in microhardness. This result may be associated with inhibition of nitrogen diffusion by growing oxide layer and deterioration of ammonia adsorption on the sample surface [25]. The decrease in the surface roughness can be explained by action of anode dissolution. The increase in the ammonium chloride concentration evidently results in the rise of the anode dissolution
Fig. 5. Micrograph and profile of worn surface of untreated sample (a) and treated samples after anode PEB at 850 °С (b), 900 °С (c), 950 °С (d).
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Fig. 7. Microhardness distribution in the modified layer after anode PEN at different treatment temperatures. Table 3 The surface characteristics and properties of the untreated and treated samples in the solution of ammonia and ammonium chloride. Treatment temperature [°C]
Oxide layer thickness [μm]
Nitride zone thickness [μm]
Diffusion zone thickness [μm]
Surface roughness [μm]
Volume loss [mm3]
650 750 800 untreated
47 2 127 2 16 72 –
47 2 14 72 87 2 –
307 5 657 5 607 5 –
0.60 7 0.03 1.30 70.06 1.75 70.07 1.007 0.05
0.09 70.02 0.217 0.05 0.107 0.02 0.40 70.06
243
rate and reduction of surface roughness [21]. As the temperature rises the roughness increase associated with the growth of oxide layer highlights the dominant role of steel oxidation. Wear test shows enhancement of wear resistance of steel after PEN at all processing temperatures (Table 3). The greatest reduction of wear rate (by a factor of 4.6) occurs after PEN at 650 °С and about the same at 850 °C. Unlike boriding, the correlation between wear rate of nitride steel and its microhardness is not observed. Moreover, according to measurements, the depth of wear track after removing the oxide layer is greater than the thickness of a relatively soft nitrided zone for samples treated at 650 °С (Fig. 8). Consequently, the counter-body slides on harder martensite layer after deletion of nitrided zone. In this case, wear rate is minimal. Contrary, wear of sample nitride at 750 °С takes place only within the soft nitrided zone with the maximal wear rate. It can be assumed for the sample nitrided at 800 °С that the counter-body slides near the boundary between nitrided and martensite zones where the concentration of nitrides is lower and martensite concentration is higher. An intermediate value of the wear rate is observed in this case. No correlation of wear resistance with microhardness occurs because the martensitic layer is not always involved in the process of wear. Maximal decrease in the friction coefficient is also observed after PEN at 650 °С (Fig. 9). Here the wear mechanism changes from abrasive/adhesion wear for untreated samples (Fig. 8a) to polishing (Fig. 8b) which results in the minimal wear rate of the samples after PEN. Friction coefficient is also reduced after PEN at 750 °С but in a lesser degree than after PEN at 650 °С. The traces of abrasive wear are observed on the worn surface (Fig. 8c) with the increase in the wear rate. When the treatment temperature rises to 800 °С, friction coefficient moves higher than that of the untreated samples where adhesive wear mechanism is observed (Fig. 8d).
Fig. 8. Micrograph and profile of worn surface of untreated sample (a) and treated samples after anode PEN at 650 °С (b), 750 °С (c), 800 °С (d).
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Table 4 Surface characteristics and properties of the untreated and treated samples in the solution of glycerol, ammonium nitrite and ammonium chloride. Treatment temperature [°C]
Oxide layer thickness [μm]
Modified lay- Surface roughness er thickness [μm] [μm]
750 850 950 untreated
457 5 70 75 957 5 –
1007 5 130 75 145 75 –
0.50 70.03 0.707 0.04 0.90 70.05 1.00 70.05
Volume loss [mm3]
0.06 70.01 0.05 70.01 0.09 70.02 0.49 70.11
Fig. 9. Friction coefficient behaviour of untreated and treated samples after anode PEN for different treatment temperature.
3.3. Plasma electrolytic nitrocarburising Fig. 10 shows oxide layer (FeO and Fe3O4) and modified one (solid solution of nitrogen and carbon in iron) formed on the surface of low-carbon steel after its anode PEN/C. Quenching of samples in the electrolyte after their saturation with N and C results in the formation of martensite in diffusive zone. Thicknesses of all layers grow as the processing temperatures rises (Table 4). Accordingly, the depth of the hardened zones and the maximum value of microhardness increase (Fig. 11). The surface roughness after PEN/C decreases at all treatment temperatures (Table 4). When the PEN/C temperature rises, the surface roughness increases in parallel with thickening of oxide layer. Tribological tests reveal that wear rates of treated samples at all processing temperatures are lower than that of untreated samples (Table 4). Wear rate decreases in the greatest degree after PEN/C at 850 °С. In this case, the leading factor is surface roughness which compensates for lower values of microhardness and higher friction coefficient (Fig. 12). In result, the PEN/C of steel provides the transition of abrasive/adhesion mechanism, specify to raw samples, to polishing (Fig. 13).
Fig. 11. Microhardness distribution in the modified layer after anode PEN/C at different treatment temperatures.
Fig. 12. Friction coefficient behaviour of untreated and treated samples after anode PEN/C for different treatment temperature.
4. Conclusions
Fig. 10. SEM image of cross-section of low-carbon steel surface after anode PEN/C at 850 °С: 1 – oxide layer, 2 – modified layer, 3 – initial structure.
1. Anode boriding of medium carbon steel (0.45 wt. % С) in electrolyte containing 3 wt.% of boric acid and 10 wt.% of ammonium chloride results in the formation of the structure containing the external oxide layer and modified layer – a solid solution of boron in iron enriched by its borides and martensite. The anode PEB at 850 °С during 5 min enables to increase microhardness of the layer to 1800 HV, the reduction of surface roughness from 1.01 μm to 0.31 μm, of dry friction coefficient from 0.65 to 0.55, and volume loss from 0.40 mm3 to 0.11 mm3 at sliding distance of 240 m in comparison with untreated
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Fig. 13. Micrograph and profile of worn surface of untreated sample (a) and treated samples after anode PEN/C at 750 °С (b), 850 °С (c), and 950 °С (d).
samples. Wear test with Al2O3 ball as counter-body at normal load of 10 N and sliding speed of 0.2 m/s reveals that the PEB provides transfer from abrasive/adhesion wear for raw samples to polishing for treated ones. The minimal wear rate is reached when microhardness of surface layer is maximal. 2. Anode nitriding of medium carbon steel (0.45 wt. % С) in electrolyte with 5 wt.% of ammonia and 10 wt.% of ammonium chloride enables to form external oxide layer, nitride zone and nitrogenous ferrite. The layer microhardness increases to 540 HV, the surface roughness decreases from 1.01 μm to 0.58 μm, friction coefficient reduces from 0.65 to 0.45, and volume loss diminishes from 0.40 mm3 to 0.09 mm3 at sliding distance of 240 m in comparison with untreated samples. The lowest wear rate is reached providing that nitride zone thickness is minimal. 3. Oxide layer and solid solution of nitrogen and carbon in iron are formed after anode nitrocarburising of low-carbon steel (0.2 wt. % C) using electrolyte containing 5 wt.% of ammonium nitrate, 8 wt.% glycerol and 15 wt.% of ammonium chloride. The highest microhardness of 880 HV is reached at PEN/C temperature of 950 °С, assumedly, owing to intensive carbon diffusion. The maximal wear resistance is observed after treatment at 850 °С during 5 min. In this case, the layer microhardness increases up to 620 HV, surface roughness decreases from 1.01 μm to 0.7 μm, and volume loss reduces from 0.49 mm3 to 0.06 mm3 at sliding distance of 240 m in comparison with untreated samples. The lowest wear rate approximately corresponds to the minimum surface roughness.
Acknowledgments This work was financially supported by the Russian Science Foundation (Contract no. 15-13-10018) to the Kostroma State University.
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