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Modification of steel surface by plasma electrolytic saturation with nitrogen and carbon
Materials Chemistry and Physics 175 (2016) 164e171
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Modification of steel surface by plasma electrolytic saturation with nitrogen and carbon S.A. Kusmanov*, Yu.V. Kusmanova, A.A. Smirnov, P.N. Belkin Nekrasov Kostroma State University, 1 May, 14, Kostroma 156961, Russia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Aqueous solution (12.5% NH4Cl, 5% ammonia, 5% acetone) is proposed for PEN/C steels. Microhardness of steel (0.2% C) is 930 HV due to PEN/C for 5e10 min at 800 C. Anode PEN/C of low carbon steel decreases its roughness (Ra) from 1.013 to 0.054 mm. Anode PEN/C decreases friction coefficient of low carbon steel from 0.191 to 0.169 Anode PEN/C decreases wear loss of low carbon steel from 13.5 mg to 1.0 mg.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2015 Received in revised form 15 February 2016 Accepted 13 March 2016 Available online 24 March 2016
The effect of the electrolyte composition with ammonia, acetone, and ammonium chloride on the structure and properties of low carbon steel was studied in anode plasma electrolytic nitrocarburising. An X-ray diffractometer, a scanning electron microscopy (SEM) and an optical microscope were used to characterize the phase composition of the modified layer and its surface morphology. Surface roughness was studied with a profilometereprofilograph. The hardness of the treated and untreated samples was measured using a microhardness tester. The sources of nitrogen and carbon are shown to be the products of evaporation and thermal decomposition of the electrolyte components. It is established that the influence of concentration of ammonia, acetone, and ammonium chloride on the size of the structural components of the hardened layer is explained by the competition of the anode dissolution, hightemperature oxidation and diffusion of the saturating component. The electrolyte composition (10 e12.5% ammonium chloride, 5% acetone, 5% ammonia) and processing mode (800 C, 5e10 min) of low carbon steels allowing to obtain the hardened surface layer up to 0.2 mm with microhardness 930 HV and with decrease in the roughness (Ra) from 1.013 to 0.054 mm are proposed. The anode plasma electrolytic nitricarburising is able to decrease friction coefficient of the treated low carbon steel from 0.191 to 0.169 and wear rate from 13.5 mg to 1.0 mg. © 2016 Elsevier B.V. All rights reserved.
Keywords: Coatings Heat treatment Diffusion Hardness Microstructure Wear
1. Introduction * Corresponding author. E-mail addresses: [email protected] (S.A. Kusmanov), yulia.kusmanova@ yandex.ru (Yu.V. Kusmanova), [email protected] (A.A. Smirnov), belkinp@ yandex.ru (P.N. Belkin). http://dx.doi.org/10.1016/j.matchemphys.2016.03.011 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Plasma electrolytic nitrocarburising (PEN/C) of steels is carried out in the solutions containing components generating diffusing
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carbon and nitrogen. Therefore, other proprietary ingredients may be added in the electrolyte primarily for adjustment of its electrical conductivity. There are also components that are capable of donating nitrogen and carbon simultaneously. These substances include carbamide [1], monoethanolamine [2], and acetonitrile [3]. The carbamide-base electrolytes can be served as only nitrogen source at low temperature. Nitriding of high-speed steel at 550 C during 7 min enables to obtain a layer with maximum microhardness of 1250 HV [4]. Increase in the processing time leads to an unjustified increase in roughness [5]. The cathode nitriding of structural steel 34CrNi1Mo in the carbamide electrolyte results in the surface hardness of 600 HV [6]. Electrolyte compositions used for the steels nitrocarburising process and its results are shown in Table 1. The PEN/C of low carbon steel promotes the increase in their microhardness to 930 HV. The medium carbon or austenitic stainless steels after their PEN/C are characterized by higher values of microhardness due to the formation of compounds of nickel and chromium. The nitrocarburising of AISI 304 stainless steel with a diamond-like carbon coating is of special concern [19]. The duplex treatment combining the PEN/C with a plasma-immersion ion-assisted deposition applied to AISI 316 stainless steel using a modified aqueous solution of urea as the treatment electrolyte can provide both a very low friction coefficient and excellent wear resistance [1]. Despite the sharp increase in roughness from 0.002 to 0.15 mm the average friction coefficients of the treated samples were slightly lower than that of the substrate. The sample treated at 250 V during 1 min demonstrated a much higher wear resistance than the untreated one. Ploughing occurred on the untreated substrate and on other samples treated under different conditions. Additional increase in wear resistance of AISI 304 stainless steel can be obtained using diamond-like carbon (DLC) coating on the PEN/C pre-treated substrate [19]. This treatment results in a simultaneous reduction of the friction coefficient and of the wear rate due to changes in the wear mechanism from adhesion/abrasion to asperity deformation and polishing. The primary observed wear mechanism observed is one of mild asperity deformation and polishing and, in extreme
165
cases, localised delamination of the DLC coating. The volumetric wear coefficient was almost constant for loads up to 15 N, particularly when sliding against WCeCo, where material loss from the ball counterface is less severe than with bearing steel 52100, despite the higher contact pressure at an equivalent load. No failure of the DLC coating was observed for 10 and 15 N loads against WCeCo and at up to 25 N against bearing steel 52100 steel [20]. It is shown that the PEN/C of the samples made of steel 316 L in the solution based on urea containing NH4Cl, Na2CO3, and other additives results in higher resistance with superimposed cyclic stress and contact pressure [21]. The weight loss of samples decreases from 58 mg for untreated steel to 35 mg for nitrocarburised one. The corrosion resistance of the nitrocarburized samples was increased in relation to the untreated sample. The corrosion current densities and corrosion potential voltages of untreated and treated samples are determined on the base the classic Tafel analysis by extrapolating the linear portions of the potential vs. log current plot back to their intersection [10]. AISI 1020 steel disk specimens were treated in the aqueous solution of urea and sodium carbonate. The corrosion potential of sample nitrocarburized at 240 V (715 C, 5 min) increased from 795 mV (for the untreated substrate) to 545 mV. Moreover, the corrosion current densities for treated samples were lower than those for the untreated ones. Similar results are obtained for the anode PEN/C of medium carbon steel [22]. In conclusion, we present the results of rapid anodic hardening of austenite stainless steel 12Cr18Ni10Ti through saturation with nitrogen and carbon in chlorideecarbamide electrolyte with subsequent hardening [23]. Treatment at 850 С for 10 min makes it possible to obtain a hardened surface area up to 80 mm thick with microhardness of 450 HV. Intercrystalline corrosion test of nitrocarburized steel does not reveal characteristic failures along the grain boundaries. Corrosive failures with depths approximately equal to their widths are detected on specimens, i.e., corrosion spots. The depth of the corrosion spots on the specimens treated for 5 min at all temperatures did not exceed 30 mm. The average velocity of corrosion probably did not exceed the corresponding value
Table 1 Examples of the PEN/C. Nomenclature: U e voltage, T e sample temperature, d e layer thickness, t e treatment time, dc e duty cycle. Electrolyte composition
U (V)/T ( C)
Material and sizes sample (mm)
d (mm)
65% carbamide, 27% water и 8% sodium carbonate
200/700
0.02
Ethanolamine in water
150, 50 Hz
AISI 316L ∅ 25 3 Q235 8 6 1.5
Formamide-ethanolamine aqueous solution Carbamide and sodium carbonate
150 240/715
Acetamide, glycerol and sodium chloride
350; dc 50%; 100 Hz
Saturated solution of carbamide
220
10% ammonium chloride, 3% sodium carbonate, carbamide Monoethanolamine and potassium chloride
150/400 220
Carbamide 1 kg/l, potassium hydroxide 10 g/l
300
5e10% nitric acid, 5e15% ammonium chloride, 10e15% glycerol 20% ammonium chloride, 17% ammonium borftoride, 15% glycerol 10% carbamide и 10% ammonium chloride
/850 /860
10% acetonitrile, 10% ammonium chloride
/850
/850
Q235 AISI 1020 ∅ 25 5 Cast iron 15 10 1 AISI 1045 ∅ 20 2 316L 20 13 1 Q235 50 10 2 316 L 10 10 1,5 Steel (0.1% C) Steel ∅ 10 Steel ∅ 10 Steel ∅ 10
(0.4% C) 18 (0.2% C) 15 (0.2% C) 15
t (min)
HV
Research group P. Taheri et al. [7]
0.05 0.07 0.09 0.05 0.23
0.75 1.25 2.00 0.5 5
750
D.J. Shen et al. [8]
730 720 930
J. Li et al. [9] M.K. Zarchi et al. [10]
0.02
1.0
550
H. Pang et al. [11]
0.1
9
1280
10
783
N. Afsar Kazerooniy et al. [13]
3
779
Y.F. Jiang et al. [14]
30
550
L.C. Kumruoglu et al. [15]
0.19
4
700
B.R. Lazarenko et al. [16]
0.46
5
950
V.N. Duradzhy et al. [17]
0.18
10
740
Kusmanov et al. [18]
0.18
10
740
Kusmanov et al. [3]
0.065 0.7
A.R. Rastkar et al. [12]
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Fig. 1. X-ray diffraction patterns of surface layers before (a) and after anode PEN/C in a solution of ammonia (5%), acetone (5%) and ammonium chloride (10%) for 5 min at 800 С (b). In the diffraction pattern are shown interplanar distances (10e10 m).
Fig. 2. SEM image of cross-section of steel surface after anode PEN/C in a solution of ammonia (5%), acetone (5%) and ammonium chloride (10%) for 5 min at 800 С (1 e oxide layer, 2 e nitrocarburised layer, 3 e martensite layer, 4 e diffusion layer, 5 e initial pearlite-ferrite structure).
for reference specimens. The most common component for saturation of steel with carbon is glycerol. The cathode carburizing in this electrolyte at 850e900 С enables to increase the hardness of the steel 30CrMnSiA to 750 HV [24] and steel 12Cr18Ni10Ti to 485 HV [25]. It is established that the highest nitrogen concentration (7 wt.%) is attainable on the steel surface when using the electrolyte with ammonia under plasma electrolytic nitriding [26]. The highest carbon concentration (1.27 wt.%) is reached on the steel surface using the electrolyte with acetone [27]. It might be expected that the high volatility of these compounds provides the maximum concentration of the nitrogen and carbon in the vapour-gas envelope (VGE). This paper considers anode PEN/C of low-carbon steel in aqueous solution containing ammonia and acetone. 2. Material and methods 2.1. PEN/C treatment Cylindrical samples (∅ 10 15 mm) of low carbon steel (0.2% C) were nitrocarburised in a cylindrical working chamber with an
Fig. 3. EDX nitrogen and carbon distributions in the surface layers of samples treated by anode PEN/C in a solution of ammonia (5%), acetone (5%) and ammonium chloride (10%) for 5 min at 800 С (1 e oxide layer, 2 enitrocarburised layer, 3 e martensite layer, 4 e diffusion layer, 5 e initial pearlite-ferrite structure).
axially symmetric electrolyte flow supplied through a nozzle located at the bottom of the chamber [27]. In the upper part of the chamber, the electrolyte was overflowing into the sump and was further pumped through a heat exchanger at a rate of 3 l/min. The volume flow rate was measured with a RMF-0.16 GUZ flowmeter (accuracy of ±2.5%). The solution temperature was measured using thermocouple placed at the bottom of the chamber. The electrolyte temperature was maintained at 20 ± 1 C. The samples were connected as the positive output of the power supply. The chamber was connected as the negative output of the power supply. After switching the voltage, the samples were immersed in the electrolyte at a depth equal to their height. The voltage was measured using an LM-1 voltmeter (accuracy ±0.5%). The current was probed with an MS8221 multimeter. The sample temperature was measured with another MS8221 multimeter and M89-K1 thermocouple accuracy to 2% over a temperature range of 400e1000 C. Temperature measurements were performed using thermocouple fixed in a hole made in the samples at a distance of 2 mm from the heated surface. Rise of temperature is carried out by increase in voltage. The treatment time varied from 2 to 10 min. The treatment temperature varied from 650 to 800 C with an increase
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length of 50 m, an inner diameter of 0.32 mm, and a liquid stationary phase layer 0.5 mm thick containing polyethylene glycolmodified nitrotereftalat. The initial temperature of the column was 60 C after an isothermal exposure for 6 min. The column temperature was then increased to 132 C at a rate of 12 C/min and further to 180 C at a rate of 25 C/min. The temperature of the capillary evaporator was maintained at 160 C, and the detector temperature was maintained at 200 C. Nitrogen was used as the carrier gas with the flow rate of 30 ml/min and the pressure of 30 kPa. The flow rates of the hydrogen and the air were 20 and 200 ml/min, respectively. The volume of gas sample used for analysis was 1.0 ml. Fig. 4. Chromatogram of the VGE during anode PEN/C of steel in an electrolyte with ammonia (5%), acetone (5%) and ammonium chloride (10%) at 800 C (1 e acetone, 2 e acetonitrile).
in voltage of DC power supply from 140 to 220 V. After PEN/C all samples were quenched in the electrolyte (hardening). Aqueous solution of ammonia NH3, acetone (CH3)2CO and ammonium chloride NH4Cl was used as the working electrolyte. The concentration of the electrolyte components ranged from 2 to 15 wt.%. The solution concentration was regulated by measuring its density with a densimeter (accuracy ±0.001 g/cm3). 2.2. Surface characterization The phase composition of the surface layers after PEN/C was investigated with the use of an ARL X'tra x-ray diffractometer (Thermo Fisher Scientific) with Cu Ka radiation at a simple scanning in the theta-2theta-mode and scanning rate of 2 /min. Quanta 3D 200i scanning electron microscopy (FEI Company) (SEM) with an energy-dispersive spectroscope and a silicon drift detector (Apollo X, Amptek Inc.) was used to observe the structure of the surface layer of the samples and for the subsequent elemental microanalysis after PEN/C. In addition, the thickness of the diffusion layers on cross section of samples was measured under an EU METAM CM 21 optical microscope after polishing and etching with the use of a 4% nitric acid solution in ethanol for 5e10 s. The microhardness of the sample surface layer after PEN/C was measured on a PMT-3M apparatus at a loading of 50 g. The surface roughness before and after PEN/C was investigated using a 130-model profilometereprofilograph (Proton-MIET). A pin-on-disc tribometer was applied to evaluate friction coefficient of the untreated and treated samples at lubricated conditions (engine oil “LITOL” contained petroleum oil with viscosity of 60e75 mm2/s at 50 C, lithium soap of 12-hydroxy acid and antioxidant additive) with 315 N normal load, 0.47 m/s sliding speed, and 1000 m sliding distance with hardened steel (50 HRC) disk. The weight loss is measured by an electronic balance with accuracy ±0.0001 g. 2.3. Chromatographic analysis of VGE The VGE composition was determined with the use of the chromatographic method. A sample of the gas-vapour mixture from the working chamber used to perform the PEN/C process was collected in a gas collector and passed through double-distilled water. This aqueous solution was subjected to chromatographic analysis with the use of a Crystal 2000 M gas chromatograph (Chromatec) with a flame ionisation detector (frequency 25 Hz) and a Zebron ZB-FFAP (Phenomenex) silica capillary column with a
3. Results and discussion 3.1. X-ray diffraction Fig. 1 presents the X-ray diffraction (XRD) patterns of the untreated steel and treated steel samples at 800 C temperatures for 5 min. The X-ray study shows that the Fe3O4 oxide, Fe4N and Fe2-3N nitrides, ferrite, martensite, and austenite phases are detected after PEN/C. It suggests the diffusion of carbon and nitrogen into the steel surface and the high-temperature oxidation of surface. 3.2. Microstructure and EDX-analysis of the surface layer According to metallographic photos, the structure of the treated steel surface is composed of the following layers (Fig. 2): e e e e
oxide layer including the Fe3O4 oxide phase, nitrocarburised layer including the Fe2-3N nitride phase, martensite layer (nitrides, martensite and remained austenite), diffusion layer (solid solution of nitrogen and/or carbon depending on the treatment temperature and cooling conditions), and initial pearlite-ferrite structure.
According to the results of EDX-analysis the nitrogen concentration reaches its maximum value in the nitrocarburised layer and further decreases into the sample surface (Fig. 3). The carbon concentration is found to reduce near the edge of the surface layer. This is probably due to the displacement of carbon atoms by diffusing nitrogen. 3.3. Formation of the diffusion layers The resulting structure is conditioned by the simultaneous occurrence of several boundary processes at the sample e VGE interface: the electrochemical dissolution of the anode material, the oxidation of the surface steel by water vapour and oxygen, and also the adsorption of compounds of nitrogen and carbon. The oxide layer formation is associated with the occurrence of hightemperature oxidation on the surface of the sample and the formation of iron oxide [28]. Sources of the diffusing atoms are the electrolyte components and their fragments formed after the substance transfer from electrolyte to the VGE and further to the sample [3]. Acetone and acetonitrile are found in the VGE by means of chromatographic method during PEN/C at 800 С (Fig. 4). The obtained data suggest the following description of the transport mechanism of saturating components. During PEN/C acetone is evaporated in the VGE where it is subjected to thermal decomposition to form CO and radical CH3: (CH3)2CO / 2CH3 þ CO
(1)
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Fig. 5. Interfaces of oxide layer (1), nitrocarburised layer (2), martensite layer (3) and diffusion layer (4) vs treatment temperature (a) and time (b), concentration of ammonia (c), acetone (d) and ammonium chloride (e) at: a) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 5 min; b) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 800 C; c) 5% (CH3)2CO, 10% NH4Cl, 5 min, 800 C; d) 5% NH3, 10% NH4Cl, 5 min, 800 C; e) 5% NH3, 5% (CH3)2CO, 5 min, 800 C.
Carbon monoxide and radical CH3 are adsorbed on the sample as a source of carbon. The synthesis of acetonitrile can be explained by reacting acetone with ammonia: (CH3)2CO þ NH3 / CH3CN þ CH4 þ H2O
NH3 þH2 O/NH4 OHðNH3 $H2 OÞ/NH4 þ þOH
(3)
Ammonia is also adsorbed on the sample and decomposed to atomic nitrogen.
(2)
Thermodynamic possibility of this reaction is confirmed by a decrease in the Gibbs energy (484.34 kJ/mol). Acetonitrile is adsorbed and subjected to thermal decomposition to form the atomic nitrogen and carbon on the sample surface [3]. Methane is adsorbed on the sample as a source of carbon. Ammonia was found after dissolving the gas mixture (from VGE) in water with the use of the potentiometric method for the presence of ammonium ions NHþ 4:
3.4. Thickness of the diffusion layers Fig. 5 shows the effect of the processing condition on the thicknesses of all layers of modified samples. The oxide layer is formed due to the oxidation of the surface steel by water vapour and oxygen therefore the thickness of this layer increases with the rise of the temperature, treatment time and concentration of ammonia promoting iron oxidation. In the latter case the oxide layer growth due to the decrease in current density and anode dissolution rate from 2.09 to 1.27 А/cm2 and 4.4 to 3.8 mg/
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Fig. 6. Microhardness distribution in the modified layer after anode PEN/C for different treatment temperature (a) and time (b), concentration of ammonia (c), acetone (d) and ammonium chloride (e) at: a) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 5 min; b) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 800 C; c) 5% (CH3)2CO, 10% NH4Cl, 5 min, 800 C; d) 5% NH3, 10% NH4Cl, 5 min, 800 C; e) 5% NH3, 5% (CH3)2CO, 5 min, 800 C.
(cm2 min) respectively. The increase in the ammonium chloride concentration facilitates the anode dissolution that affects the oxide layer thickness. The intensification of the anode dissolution results in a faster transport of iron atoms from the sample into the VGE and inhibition of the oxide layer formation. When the concentration of the ammonium chloride in the solution increases from 7.5% to 12.5%, the growth of the thicknesses of the nitrocarburised, martensite and diffusion layers thickness is observed due to the decrease in the thickness of the oxide layer (Fig. 5e). The oxide layer thickness is decreased due to the increase in current density and anode dissolution rate from 1.18 to 2.00 А/cm2 and 2.3e10.0 mg/(cm2 min) respectively. Acetone does not affect the oxide layer thickness (Fig. 5d) unlike glycerol promoting to iron oxidation [27].
The nitrocarburised layer is detected only at the treatment temperature not lower than 700 C and the ammonia concentration over 2 wt.%. Hence, this layer formation is affected primarily by the nitrogen diffusion therefore its thickness increases with the rise in temperature, processing time, and ammonia concentration. The concentration of acetone in solution lower 5 wt.% does not affect the external layer thickness but this layer formation is inhibited when the acetone concentration is higher than 5 wt.% (Fig. 5d). This may be associated with the decrease of the nitrogen sources on the surface with a concomitant increase in the carbon sources. This also explains the growth of the thickness of the martensite layer with increasing concentration of acetone in the solution. The thickness of the martensite layer also rises with the temperature and treatment time increase suggesting the diffusion
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nature of its formation. This layer growth is promoted by the increase in the acetone concentration to a greater extent and to the concentration of ammonia in a lesser degree (Fig. 5c and d). Consequently, in this case the carbon diffusion is manifested more prominently than nitrogen diffusion. The diffusion layer is known to grow with the rise of the temperature and time treatment. In this case, the effect of the electrolyte composition on this layer thickness is complex. The oxide layer is found to slow down the nitrogen [29] and carbon [18] diffusion therefore the decrease in the oxide layer thickness leads to the rising of the other layers thickness when the ammonium chloride concentration increases to 10 wt.% (Fig. 5e). The diffusion layer thickness grows thinner when the ammonium chloride concentration is more than 10% and the ammonia or acetone concentration is more than 5%. This fact appears to reflect a complex competition between adsorption and diffusion. Fig. 7. Wear behaviour of untreated and treated samples in a solution of ammonia (5%), acetone (5%) and ammonium chloride (12.5%) for 5 min at 800 С.
3.5. Microhardness of the diffusion layers Fig. 6 shows the distribution of microhardness in the modified layer. This result is associated with the phase composition of the modified layers. The nitrocarburised layer comprising nitrides and retained austenite has a lower hardness and apparently a higher ductility. It can be expected that such a layer will provide an increase in the wear resistance of the material in the presence of a martensite sublayer. The maximum value of microhardness is reached in the zone of the highest concentration of martensite. The martensite layer microhardness rises with the increase in treatment temperature from 650 to 800 C (Fig. 6a). Variation of the treatment time does not affect the maximum values of microhardness but influences the width of the hardened zone (Fig. 6b). The highest value of microhardness (930 HV) is reached at all concentrations of electrolyte components except low content of ammonia (Fig. 6c) and acetone (Fig. 6d) and high content of ammonium chloride (Fig. 6e). The hardness obtained is greater than that observed after nitrocarburised in urea electrolyte [30]. The hardened layer thickness associated with the depth of carbon and nitrogen penetrating is affected by the treatment temperature, treatment time and electrolyte component concentrations. 3.6. Surface roughness and wear behaviour The data in Table 2 demonstrate the results of surface roughness measurements of the untreated and treated samples with different treatment temperature, processing time and electrolyte component concentrations. It is shown that after anode PEN/C surface roughness in most cases is reduced by one order of magnitude
Table 2 The surface roughness and anode dissolution rate of the untreated and treated samples. Nomenclature: C1 is the concentration of the ammonia, C2 is the concentration of the acetone, C3 is the concentration of the ammonium chloride, T is the treatment temperature, t is the treatment time, m is the weight loss of the sample, and Ra is the arithmetic average surface roughness. C1 (wt.%)
C2 (wt.%)
1 5 5 2 5 5 3 5 5 4 5 5 5 5 5 6 2 5 7 6.5 5 8 5 2 9 5 5 Before treatment
C3 (wt.%)
T ( C)
t (min)
m (g)
Ra (mm)
10 10 10 10 10 10 10 10 15
700 750 800 800 800 800 800 800 800
5 5 5 2 10 5 5 5 5
0.1100 0.0934 0.1144 0.0572 0.1653 0.1217 0.1058 0.1233 0.2756
0.164 0.054 0.117 0.056 0.075 0.085 0.100 0.131 0.054 1.013
which is associated with the electrochemical dissolution of the sample material. The wear behaviour of untreated and treated samples is shown in Fig. 7. It can be seen from the figure that the wear rate of the nitrocarburised samples is one order of magnitude lower than that of the untreated steel. The nitrocarburised samples exhibited a lower friction coefficient compared to untreated ones: 0.169 and 0.191 respectively. Hence, the obtained structure improves the tribological behaviour.
4. Conclusions (1) The structure of the modified layer of low carbon steel treated by anode PEN/C in an aqueous solution of ammonia, acetone and ammonium chloride is detected. External part of layer comprises iron oxides formed due to high-temperature oxidizing with water vapour. The increase in concentration of nitrogen and carbon is assumed to lead to the formation of dispersed iron nitrides and carbon, surrounded by a solid solution of nitrogen and carbon. The nitrocarburised layer has a relatively low hardness, but usually plays a positive role in friction pairs, providing good wear resistance which is confirmed by wear tests. Under this part of layer martensitic sublayer is located. This phase and remained austenite are formed owing to diffusion of nitrogen and carbon followed by rapid cooling in the electrolyte. Further, a solid solution of carbon and nitrogen in iron take place. (2) The mechanism of nitrogen and carbon liberation from electrolyte under study is proposed. Acetone and ammonia are supposed to evaporate in the VGE with the following thermal decomposition on the sample surface. Acetone also reacts with ammonia to form acetonitrile found in the VGE through the chromatographic method and methane. Acetonitrile and methane decompose in VGE as well. (3) According to study of influence of the processing regimes and electrolyte composition on the structural characteristics of the modified layer the processes of the carbon and nitrogen diffusion, oxidation and anode dissolution of the workpieces occur simultaneously. These processes determine the carbon and nitrogen concentrations in the modified layer and structure obtained.
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(4) The maximum hardness reaching up to 930 HV is observed at a depth of 20e60 mm which is probably associated with the highest total concentration of nitrogen and carbon atoms forming a supersaturated solution in austenite at the treatment temperature and a martensite phase after quenching. (5) Anode PEN/C of steel enables surface roughness reduction (Ra) from 1.013 to 0.054 mm due to the anode dissolution. (6) The electrolyte composition (5% ammonia, 5% acetone, 10e12.5% ammonium chloride) and processing mode (800 C, 5e10 min) of low carbon steels are developed. In this case, a hardened surface layer up to 0.2 mm with microhardness 930 HV can be obtained and with the decrease in the roughness of 13 times. Wear tests showed that wear resistance of the samples considerably improved after nitrocarburising.
Conflict of interest
[11]
[12]
[13]
[14]
[15]
[16]
[17] [18]
The authors declare that they have no conflict of interest. Acknowledgments This work was financially supported by the Russian Science Foundation (Contract No. 15-13-10018) to the Nekrasov Kostroma State University.
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nitrocarburized surface layer on AISI 1020 steel by electrolytic plasma processing in an urea electrolyte, J. Mater. Res. Technol. 2 (3) (2013) 213e220. http://www.sciencedirect.com/science/article/pii/S2238785413000574. H. Pang, G.-L. Zhang, X.Q. Wang, G.-H. Lv, H. Chen, S.-Z. Yang, Mechanical performances of carbonitriding films on cast iron by plasma electrolytic carbonitriding, Chin. Phys. Lett. 28 (11) (2011) 118103. A.R. Rastkar, B. Shokri, Surface modification and wear test of carbon steel by plasma electrolytic nitrocarburizing, Surf. Interface Anal. 44 (2012) 342e351, http://dx.doi.org/10.1002/sia.3808. N. Afsar Kazerooniy, M.E. Bahrololoom, M.H. Shariat, F. Mahzoon, T. Jozaghi, Effect of Ringer's solution on wear and friction of stainless steel 316L after plasma electrolytic nitrocarburising at low voltages, J. Mater. Sci. Technol. 27 (10) (2011) 906e912. Y.F. Jiang, Y.F. Bao, K. Yang, Effect of C/N concentration fluctuation on formation of plasma electrolytic carbonitriding coating on Q235, J. Iron Steel Res. 19 (11) (2012) 39e45. L.C. Kumruoglu, A. Ozel, Plasma electrolytic saturation of 316 L stainless steel in an aqueous electrolyte containing urea and ammonium nitrate, Mater. Technol. 47 (3) (2013) 307e310. Pat. 461 161 USSR, (C23C 9/10). Method of Thermochemical Treatment of Metals//B.R. Lazarenko, V.N. Duradzhy, A.A. Factorovich, I.V. Bryantsev. 25.02.1975. Pat. 922 177 USSR, (C23C 9/16). Electrolyte for Treatment of Steel Products// V.N. Duradzhy, A.K. Tovarkov. 23.04.1982. S.A. Kusmanov, Yu.V. Kusmanova, A.R. Naumov, P.N. Belkin, Features of anode plasma electrolytic nitrocarburising of low carbon steel, Surf. Coat. Technol. 272 (2015) 149e157. A.L. Yerokhin, A. Leyland, C. Tsotsos, A.D. Wilson, X. Nie, A. Matthews, Duplex surface treatments combining plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition, Surf. Coat. Technol. 142e144 (2001) 1129e1136. C. Tsotsos, A.L. Yerokhin, A.D. Wilson, A. Leyland, A. Matthews, Tribological evaluation of AISI 304 stainless steel duplex treated by plasma electrolytic nitrocarburising and diamond-like carbon coating, Wear 253 (9e10) (2002) 986e993. F. Mahzoon, S.A. Behgozin, M.E. Bahrololoom, S. Javadpour, Study the fatiguewear behavior of a plasma electrolytic nitrocarburized (PEN/C) 316L stainless steel, J. Mater. Eng. Perform. 21 (8) (2012) 1751e1756. P.N. Belkin, I.G. Dyakov, A.V. Zhirov, S.A. Kusmanov, T.L. Mukhacheva, Effect of compositions of active electrolytes on properties of anodic carburization, Prot. Met. Phys. Chem. 46 (6) (2010) 715e720. P. Belkin, B. Krit, I. Dyakov, V. Vostrikov, T. Mukhacheva, Anode saturation with nitrogen and carbon in aqueous solution of carbamide-bearing electrolytes, Met. Sci. Heat. Treat. 52 (1e2) (2010) 20e24. M. Skakov, E. Batyrbekov, L. Zhurerova, M. Scheffler, Microstructure and microhardness changes of 30CrMnSiA steel modified surface layers by electrolyte-plasma processing, Appl. Mech. Mater. 423e426 (2013) 824e827. М. Skakov, S. Kurbanbekov, M. Scheffler, Influence of regimes electrolytic plasma cementation on the mechanical properties of steel 12Cr18Ni10Ti, Key Eng. Mater. 531e532 (2013) 173e177. P.N. Belkin, E.A. Pasinkovskij, Heat treatment and case hardening of steels subjected to heat in electrolytic solutions, Met. Sci. Heat. Treat. 31 (5e6) (1989) 331e337. S.A. Kusmanov, S.Yu. Shadrin, P.N. Belkin, Carbon transfer from aqueous electrolytes to steel by anode plasma electrolytic carburizing, Surf. Coat. Technol. 258 (2014) 727e733. S.A. Kusmanov, P.N. Belkin, I.G. D'yakov, A.V. Zhirov, T.L. Mukhacheva, A.R. Naumov, Influence of oxide layer on carbon diffusion during anode plasma electrolytic carburizing, Prot. Met. Phys. Chem. 50 (2014) 223e229. S.A. Kusmanov, A.A. Smirnov, Yu.V. Kusmanova, P.N. Belkin, Anode plasma electrolytic nitrohardening of medium carbon steel, Surf. Coat. Technol. 269 (2015) 308e313. P. Belkin, S. Kusmanov, A. Naumov, Yu. Parkaeva, Anodic plasma electrolytic nitrocarburizing of low-carbon steel, Adv. Mater. Res. 704 (2013) 31e36.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Modification of steel surface by plasma electrolytic saturation with nitrogen and carbon S.A. Kusmanov*, Yu.V. Kusmanova, A.A. Smirnov, P.N. Belkin Nekrasov Kostroma State University, 1 May, 14, Kostroma 156961, Russia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Aqueous solution (12.5% NH4Cl, 5% ammonia, 5% acetone) is proposed for PEN/C steels. Microhardness of steel (0.2% C) is 930 HV due to PEN/C for 5e10 min at 800 C. Anode PEN/C of low carbon steel decreases its roughness (Ra) from 1.013 to 0.054 mm. Anode PEN/C decreases friction coefficient of low carbon steel from 0.191 to 0.169 Anode PEN/C decreases wear loss of low carbon steel from 13.5 mg to 1.0 mg.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 September 2015 Received in revised form 15 February 2016 Accepted 13 March 2016 Available online 24 March 2016
The effect of the electrolyte composition with ammonia, acetone, and ammonium chloride on the structure and properties of low carbon steel was studied in anode plasma electrolytic nitrocarburising. An X-ray diffractometer, a scanning electron microscopy (SEM) and an optical microscope were used to characterize the phase composition of the modified layer and its surface morphology. Surface roughness was studied with a profilometereprofilograph. The hardness of the treated and untreated samples was measured using a microhardness tester. The sources of nitrogen and carbon are shown to be the products of evaporation and thermal decomposition of the electrolyte components. It is established that the influence of concentration of ammonia, acetone, and ammonium chloride on the size of the structural components of the hardened layer is explained by the competition of the anode dissolution, hightemperature oxidation and diffusion of the saturating component. The electrolyte composition (10 e12.5% ammonium chloride, 5% acetone, 5% ammonia) and processing mode (800 C, 5e10 min) of low carbon steels allowing to obtain the hardened surface layer up to 0.2 mm with microhardness 930 HV and with decrease in the roughness (Ra) from 1.013 to 0.054 mm are proposed. The anode plasma electrolytic nitricarburising is able to decrease friction coefficient of the treated low carbon steel from 0.191 to 0.169 and wear rate from 13.5 mg to 1.0 mg. © 2016 Elsevier B.V. All rights reserved.
Keywords: Coatings Heat treatment Diffusion Hardness Microstructure Wear
1. Introduction * Corresponding author. E-mail addresses: [email protected] (S.A. Kusmanov), yulia.kusmanova@ yandex.ru (Yu.V. Kusmanova), [email protected] (A.A. Smirnov), belkinp@ yandex.ru (P.N. Belkin). http://dx.doi.org/10.1016/j.matchemphys.2016.03.011 0254-0584/© 2016 Elsevier B.V. All rights reserved.
Plasma electrolytic nitrocarburising (PEN/C) of steels is carried out in the solutions containing components generating diffusing
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carbon and nitrogen. Therefore, other proprietary ingredients may be added in the electrolyte primarily for adjustment of its electrical conductivity. There are also components that are capable of donating nitrogen and carbon simultaneously. These substances include carbamide [1], monoethanolamine [2], and acetonitrile [3]. The carbamide-base electrolytes can be served as only nitrogen source at low temperature. Nitriding of high-speed steel at 550 C during 7 min enables to obtain a layer with maximum microhardness of 1250 HV [4]. Increase in the processing time leads to an unjustified increase in roughness [5]. The cathode nitriding of structural steel 34CrNi1Mo in the carbamide electrolyte results in the surface hardness of 600 HV [6]. Electrolyte compositions used for the steels nitrocarburising process and its results are shown in Table 1. The PEN/C of low carbon steel promotes the increase in their microhardness to 930 HV. The medium carbon or austenitic stainless steels after their PEN/C are characterized by higher values of microhardness due to the formation of compounds of nickel and chromium. The nitrocarburising of AISI 304 stainless steel with a diamond-like carbon coating is of special concern [19]. The duplex treatment combining the PEN/C with a plasma-immersion ion-assisted deposition applied to AISI 316 stainless steel using a modified aqueous solution of urea as the treatment electrolyte can provide both a very low friction coefficient and excellent wear resistance [1]. Despite the sharp increase in roughness from 0.002 to 0.15 mm the average friction coefficients of the treated samples were slightly lower than that of the substrate. The sample treated at 250 V during 1 min demonstrated a much higher wear resistance than the untreated one. Ploughing occurred on the untreated substrate and on other samples treated under different conditions. Additional increase in wear resistance of AISI 304 stainless steel can be obtained using diamond-like carbon (DLC) coating on the PEN/C pre-treated substrate [19]. This treatment results in a simultaneous reduction of the friction coefficient and of the wear rate due to changes in the wear mechanism from adhesion/abrasion to asperity deformation and polishing. The primary observed wear mechanism observed is one of mild asperity deformation and polishing and, in extreme
165
cases, localised delamination of the DLC coating. The volumetric wear coefficient was almost constant for loads up to 15 N, particularly when sliding against WCeCo, where material loss from the ball counterface is less severe than with bearing steel 52100, despite the higher contact pressure at an equivalent load. No failure of the DLC coating was observed for 10 and 15 N loads against WCeCo and at up to 25 N against bearing steel 52100 steel [20]. It is shown that the PEN/C of the samples made of steel 316 L in the solution based on urea containing NH4Cl, Na2CO3, and other additives results in higher resistance with superimposed cyclic stress and contact pressure [21]. The weight loss of samples decreases from 58 mg for untreated steel to 35 mg for nitrocarburised one. The corrosion resistance of the nitrocarburized samples was increased in relation to the untreated sample. The corrosion current densities and corrosion potential voltages of untreated and treated samples are determined on the base the classic Tafel analysis by extrapolating the linear portions of the potential vs. log current plot back to their intersection [10]. AISI 1020 steel disk specimens were treated in the aqueous solution of urea and sodium carbonate. The corrosion potential of sample nitrocarburized at 240 V (715 C, 5 min) increased from 795 mV (for the untreated substrate) to 545 mV. Moreover, the corrosion current densities for treated samples were lower than those for the untreated ones. Similar results are obtained for the anode PEN/C of medium carbon steel [22]. In conclusion, we present the results of rapid anodic hardening of austenite stainless steel 12Cr18Ni10Ti through saturation with nitrogen and carbon in chlorideecarbamide electrolyte with subsequent hardening [23]. Treatment at 850 С for 10 min makes it possible to obtain a hardened surface area up to 80 mm thick with microhardness of 450 HV. Intercrystalline corrosion test of nitrocarburized steel does not reveal characteristic failures along the grain boundaries. Corrosive failures with depths approximately equal to their widths are detected on specimens, i.e., corrosion spots. The depth of the corrosion spots on the specimens treated for 5 min at all temperatures did not exceed 30 mm. The average velocity of corrosion probably did not exceed the corresponding value
Table 1 Examples of the PEN/C. Nomenclature: U e voltage, T e sample temperature, d e layer thickness, t e treatment time, dc e duty cycle. Electrolyte composition
U (V)/T ( C)
Material and sizes sample (mm)
d (mm)
65% carbamide, 27% water и 8% sodium carbonate
200/700
0.02
Ethanolamine in water
150, 50 Hz
AISI 316L ∅ 25 3 Q235 8 6 1.5
Formamide-ethanolamine aqueous solution Carbamide and sodium carbonate
150 240/715
Acetamide, glycerol and sodium chloride
350; dc 50%; 100 Hz
Saturated solution of carbamide
220
10% ammonium chloride, 3% sodium carbonate, carbamide Monoethanolamine and potassium chloride
150/400 220
Carbamide 1 kg/l, potassium hydroxide 10 g/l
300
5e10% nitric acid, 5e15% ammonium chloride, 10e15% glycerol 20% ammonium chloride, 17% ammonium borftoride, 15% glycerol 10% carbamide и 10% ammonium chloride
/850 /860
10% acetonitrile, 10% ammonium chloride
/850
/850
Q235 AISI 1020 ∅ 25 5 Cast iron 15 10 1 AISI 1045 ∅ 20 2 316L 20 13 1 Q235 50 10 2 316 L 10 10 1,5 Steel (0.1% C) Steel ∅ 10 Steel ∅ 10 Steel ∅ 10
(0.4% C) 18 (0.2% C) 15 (0.2% C) 15
t (min)
HV
Research group P. Taheri et al. [7]
0.05 0.07 0.09 0.05 0.23
0.75 1.25 2.00 0.5 5
750
D.J. Shen et al. [8]
730 720 930
J. Li et al. [9] M.K. Zarchi et al. [10]
0.02
1.0
550
H. Pang et al. [11]
0.1
9
1280
10
783
N. Afsar Kazerooniy et al. [13]
3
779
Y.F. Jiang et al. [14]
30
550
L.C. Kumruoglu et al. [15]
0.19
4
700
B.R. Lazarenko et al. [16]
0.46
5
950
V.N. Duradzhy et al. [17]
0.18
10
740
Kusmanov et al. [18]
0.18
10
740
Kusmanov et al. [3]
0.065 0.7
A.R. Rastkar et al. [12]
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Fig. 1. X-ray diffraction patterns of surface layers before (a) and after anode PEN/C in a solution of ammonia (5%), acetone (5%) and ammonium chloride (10%) for 5 min at 800 С (b). In the diffraction pattern are shown interplanar distances (10e10 m).
Fig. 2. SEM image of cross-section of steel surface after anode PEN/C in a solution of ammonia (5%), acetone (5%) and ammonium chloride (10%) for 5 min at 800 С (1 e oxide layer, 2 e nitrocarburised layer, 3 e martensite layer, 4 e diffusion layer, 5 e initial pearlite-ferrite structure).
for reference specimens. The most common component for saturation of steel with carbon is glycerol. The cathode carburizing in this electrolyte at 850e900 С enables to increase the hardness of the steel 30CrMnSiA to 750 HV [24] and steel 12Cr18Ni10Ti to 485 HV [25]. It is established that the highest nitrogen concentration (7 wt.%) is attainable on the steel surface when using the electrolyte with ammonia under plasma electrolytic nitriding [26]. The highest carbon concentration (1.27 wt.%) is reached on the steel surface using the electrolyte with acetone [27]. It might be expected that the high volatility of these compounds provides the maximum concentration of the nitrogen and carbon in the vapour-gas envelope (VGE). This paper considers anode PEN/C of low-carbon steel in aqueous solution containing ammonia and acetone. 2. Material and methods 2.1. PEN/C treatment Cylindrical samples (∅ 10 15 mm) of low carbon steel (0.2% C) were nitrocarburised in a cylindrical working chamber with an
Fig. 3. EDX nitrogen and carbon distributions in the surface layers of samples treated by anode PEN/C in a solution of ammonia (5%), acetone (5%) and ammonium chloride (10%) for 5 min at 800 С (1 e oxide layer, 2 enitrocarburised layer, 3 e martensite layer, 4 e diffusion layer, 5 e initial pearlite-ferrite structure).
axially symmetric electrolyte flow supplied through a nozzle located at the bottom of the chamber [27]. In the upper part of the chamber, the electrolyte was overflowing into the sump and was further pumped through a heat exchanger at a rate of 3 l/min. The volume flow rate was measured with a RMF-0.16 GUZ flowmeter (accuracy of ±2.5%). The solution temperature was measured using thermocouple placed at the bottom of the chamber. The electrolyte temperature was maintained at 20 ± 1 C. The samples were connected as the positive output of the power supply. The chamber was connected as the negative output of the power supply. After switching the voltage, the samples were immersed in the electrolyte at a depth equal to their height. The voltage was measured using an LM-1 voltmeter (accuracy ±0.5%). The current was probed with an MS8221 multimeter. The sample temperature was measured with another MS8221 multimeter and M89-K1 thermocouple accuracy to 2% over a temperature range of 400e1000 C. Temperature measurements were performed using thermocouple fixed in a hole made in the samples at a distance of 2 mm from the heated surface. Rise of temperature is carried out by increase in voltage. The treatment time varied from 2 to 10 min. The treatment temperature varied from 650 to 800 C with an increase
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length of 50 m, an inner diameter of 0.32 mm, and a liquid stationary phase layer 0.5 mm thick containing polyethylene glycolmodified nitrotereftalat. The initial temperature of the column was 60 C after an isothermal exposure for 6 min. The column temperature was then increased to 132 C at a rate of 12 C/min and further to 180 C at a rate of 25 C/min. The temperature of the capillary evaporator was maintained at 160 C, and the detector temperature was maintained at 200 C. Nitrogen was used as the carrier gas with the flow rate of 30 ml/min and the pressure of 30 kPa. The flow rates of the hydrogen and the air were 20 and 200 ml/min, respectively. The volume of gas sample used for analysis was 1.0 ml. Fig. 4. Chromatogram of the VGE during anode PEN/C of steel in an electrolyte with ammonia (5%), acetone (5%) and ammonium chloride (10%) at 800 C (1 e acetone, 2 e acetonitrile).
in voltage of DC power supply from 140 to 220 V. After PEN/C all samples were quenched in the electrolyte (hardening). Aqueous solution of ammonia NH3, acetone (CH3)2CO and ammonium chloride NH4Cl was used as the working electrolyte. The concentration of the electrolyte components ranged from 2 to 15 wt.%. The solution concentration was regulated by measuring its density with a densimeter (accuracy ±0.001 g/cm3). 2.2. Surface characterization The phase composition of the surface layers after PEN/C was investigated with the use of an ARL X'tra x-ray diffractometer (Thermo Fisher Scientific) with Cu Ka radiation at a simple scanning in the theta-2theta-mode and scanning rate of 2 /min. Quanta 3D 200i scanning electron microscopy (FEI Company) (SEM) with an energy-dispersive spectroscope and a silicon drift detector (Apollo X, Amptek Inc.) was used to observe the structure of the surface layer of the samples and for the subsequent elemental microanalysis after PEN/C. In addition, the thickness of the diffusion layers on cross section of samples was measured under an EU METAM CM 21 optical microscope after polishing and etching with the use of a 4% nitric acid solution in ethanol for 5e10 s. The microhardness of the sample surface layer after PEN/C was measured on a PMT-3M apparatus at a loading of 50 g. The surface roughness before and after PEN/C was investigated using a 130-model profilometereprofilograph (Proton-MIET). A pin-on-disc tribometer was applied to evaluate friction coefficient of the untreated and treated samples at lubricated conditions (engine oil “LITOL” contained petroleum oil with viscosity of 60e75 mm2/s at 50 C, lithium soap of 12-hydroxy acid and antioxidant additive) with 315 N normal load, 0.47 m/s sliding speed, and 1000 m sliding distance with hardened steel (50 HRC) disk. The weight loss is measured by an electronic balance with accuracy ±0.0001 g. 2.3. Chromatographic analysis of VGE The VGE composition was determined with the use of the chromatographic method. A sample of the gas-vapour mixture from the working chamber used to perform the PEN/C process was collected in a gas collector and passed through double-distilled water. This aqueous solution was subjected to chromatographic analysis with the use of a Crystal 2000 M gas chromatograph (Chromatec) with a flame ionisation detector (frequency 25 Hz) and a Zebron ZB-FFAP (Phenomenex) silica capillary column with a
3. Results and discussion 3.1. X-ray diffraction Fig. 1 presents the X-ray diffraction (XRD) patterns of the untreated steel and treated steel samples at 800 C temperatures for 5 min. The X-ray study shows that the Fe3O4 oxide, Fe4N and Fe2-3N nitrides, ferrite, martensite, and austenite phases are detected after PEN/C. It suggests the diffusion of carbon and nitrogen into the steel surface and the high-temperature oxidation of surface. 3.2. Microstructure and EDX-analysis of the surface layer According to metallographic photos, the structure of the treated steel surface is composed of the following layers (Fig. 2): e e e e
oxide layer including the Fe3O4 oxide phase, nitrocarburised layer including the Fe2-3N nitride phase, martensite layer (nitrides, martensite and remained austenite), diffusion layer (solid solution of nitrogen and/or carbon depending on the treatment temperature and cooling conditions), and initial pearlite-ferrite structure.
According to the results of EDX-analysis the nitrogen concentration reaches its maximum value in the nitrocarburised layer and further decreases into the sample surface (Fig. 3). The carbon concentration is found to reduce near the edge of the surface layer. This is probably due to the displacement of carbon atoms by diffusing nitrogen. 3.3. Formation of the diffusion layers The resulting structure is conditioned by the simultaneous occurrence of several boundary processes at the sample e VGE interface: the electrochemical dissolution of the anode material, the oxidation of the surface steel by water vapour and oxygen, and also the adsorption of compounds of nitrogen and carbon. The oxide layer formation is associated with the occurrence of hightemperature oxidation on the surface of the sample and the formation of iron oxide [28]. Sources of the diffusing atoms are the electrolyte components and their fragments formed after the substance transfer from electrolyte to the VGE and further to the sample [3]. Acetone and acetonitrile are found in the VGE by means of chromatographic method during PEN/C at 800 С (Fig. 4). The obtained data suggest the following description of the transport mechanism of saturating components. During PEN/C acetone is evaporated in the VGE where it is subjected to thermal decomposition to form CO and radical CH3: (CH3)2CO / 2CH3 þ CO
(1)
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Fig. 5. Interfaces of oxide layer (1), nitrocarburised layer (2), martensite layer (3) and diffusion layer (4) vs treatment temperature (a) and time (b), concentration of ammonia (c), acetone (d) and ammonium chloride (e) at: a) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 5 min; b) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 800 C; c) 5% (CH3)2CO, 10% NH4Cl, 5 min, 800 C; d) 5% NH3, 10% NH4Cl, 5 min, 800 C; e) 5% NH3, 5% (CH3)2CO, 5 min, 800 C.
Carbon monoxide and radical CH3 are adsorbed on the sample as a source of carbon. The synthesis of acetonitrile can be explained by reacting acetone with ammonia: (CH3)2CO þ NH3 / CH3CN þ CH4 þ H2O
NH3 þH2 O/NH4 OHðNH3 $H2 OÞ/NH4 þ þOH
(3)
Ammonia is also adsorbed on the sample and decomposed to atomic nitrogen.
(2)
Thermodynamic possibility of this reaction is confirmed by a decrease in the Gibbs energy (484.34 kJ/mol). Acetonitrile is adsorbed and subjected to thermal decomposition to form the atomic nitrogen and carbon on the sample surface [3]. Methane is adsorbed on the sample as a source of carbon. Ammonia was found after dissolving the gas mixture (from VGE) in water with the use of the potentiometric method for the presence of ammonium ions NHþ 4:
3.4. Thickness of the diffusion layers Fig. 5 shows the effect of the processing condition on the thicknesses of all layers of modified samples. The oxide layer is formed due to the oxidation of the surface steel by water vapour and oxygen therefore the thickness of this layer increases with the rise of the temperature, treatment time and concentration of ammonia promoting iron oxidation. In the latter case the oxide layer growth due to the decrease in current density and anode dissolution rate from 2.09 to 1.27 А/cm2 and 4.4 to 3.8 mg/
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169
Fig. 6. Microhardness distribution in the modified layer after anode PEN/C for different treatment temperature (a) and time (b), concentration of ammonia (c), acetone (d) and ammonium chloride (e) at: a) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 5 min; b) 5% NH3, 5% (CH3)2CO, 10% NH4Cl, 800 C; c) 5% (CH3)2CO, 10% NH4Cl, 5 min, 800 C; d) 5% NH3, 10% NH4Cl, 5 min, 800 C; e) 5% NH3, 5% (CH3)2CO, 5 min, 800 C.
(cm2 min) respectively. The increase in the ammonium chloride concentration facilitates the anode dissolution that affects the oxide layer thickness. The intensification of the anode dissolution results in a faster transport of iron atoms from the sample into the VGE and inhibition of the oxide layer formation. When the concentration of the ammonium chloride in the solution increases from 7.5% to 12.5%, the growth of the thicknesses of the nitrocarburised, martensite and diffusion layers thickness is observed due to the decrease in the thickness of the oxide layer (Fig. 5e). The oxide layer thickness is decreased due to the increase in current density and anode dissolution rate from 1.18 to 2.00 А/cm2 and 2.3e10.0 mg/(cm2 min) respectively. Acetone does not affect the oxide layer thickness (Fig. 5d) unlike glycerol promoting to iron oxidation [27].
The nitrocarburised layer is detected only at the treatment temperature not lower than 700 C and the ammonia concentration over 2 wt.%. Hence, this layer formation is affected primarily by the nitrogen diffusion therefore its thickness increases with the rise in temperature, processing time, and ammonia concentration. The concentration of acetone in solution lower 5 wt.% does not affect the external layer thickness but this layer formation is inhibited when the acetone concentration is higher than 5 wt.% (Fig. 5d). This may be associated with the decrease of the nitrogen sources on the surface with a concomitant increase in the carbon sources. This also explains the growth of the thickness of the martensite layer with increasing concentration of acetone in the solution. The thickness of the martensite layer also rises with the temperature and treatment time increase suggesting the diffusion
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nature of its formation. This layer growth is promoted by the increase in the acetone concentration to a greater extent and to the concentration of ammonia in a lesser degree (Fig. 5c and d). Consequently, in this case the carbon diffusion is manifested more prominently than nitrogen diffusion. The diffusion layer is known to grow with the rise of the temperature and time treatment. In this case, the effect of the electrolyte composition on this layer thickness is complex. The oxide layer is found to slow down the nitrogen [29] and carbon [18] diffusion therefore the decrease in the oxide layer thickness leads to the rising of the other layers thickness when the ammonium chloride concentration increases to 10 wt.% (Fig. 5e). The diffusion layer thickness grows thinner when the ammonium chloride concentration is more than 10% and the ammonia or acetone concentration is more than 5%. This fact appears to reflect a complex competition between adsorption and diffusion. Fig. 7. Wear behaviour of untreated and treated samples in a solution of ammonia (5%), acetone (5%) and ammonium chloride (12.5%) for 5 min at 800 С.
3.5. Microhardness of the diffusion layers Fig. 6 shows the distribution of microhardness in the modified layer. This result is associated with the phase composition of the modified layers. The nitrocarburised layer comprising nitrides and retained austenite has a lower hardness and apparently a higher ductility. It can be expected that such a layer will provide an increase in the wear resistance of the material in the presence of a martensite sublayer. The maximum value of microhardness is reached in the zone of the highest concentration of martensite. The martensite layer microhardness rises with the increase in treatment temperature from 650 to 800 C (Fig. 6a). Variation of the treatment time does not affect the maximum values of microhardness but influences the width of the hardened zone (Fig. 6b). The highest value of microhardness (930 HV) is reached at all concentrations of electrolyte components except low content of ammonia (Fig. 6c) and acetone (Fig. 6d) and high content of ammonium chloride (Fig. 6e). The hardness obtained is greater than that observed after nitrocarburised in urea electrolyte [30]. The hardened layer thickness associated with the depth of carbon and nitrogen penetrating is affected by the treatment temperature, treatment time and electrolyte component concentrations. 3.6. Surface roughness and wear behaviour The data in Table 2 demonstrate the results of surface roughness measurements of the untreated and treated samples with different treatment temperature, processing time and electrolyte component concentrations. It is shown that after anode PEN/C surface roughness in most cases is reduced by one order of magnitude
Table 2 The surface roughness and anode dissolution rate of the untreated and treated samples. Nomenclature: C1 is the concentration of the ammonia, C2 is the concentration of the acetone, C3 is the concentration of the ammonium chloride, T is the treatment temperature, t is the treatment time, m is the weight loss of the sample, and Ra is the arithmetic average surface roughness. C1 (wt.%)
C2 (wt.%)
1 5 5 2 5 5 3 5 5 4 5 5 5 5 5 6 2 5 7 6.5 5 8 5 2 9 5 5 Before treatment
C3 (wt.%)
T ( C)
t (min)
m (g)
Ra (mm)
10 10 10 10 10 10 10 10 15
700 750 800 800 800 800 800 800 800
5 5 5 2 10 5 5 5 5
0.1100 0.0934 0.1144 0.0572 0.1653 0.1217 0.1058 0.1233 0.2756
0.164 0.054 0.117 0.056 0.075 0.085 0.100 0.131 0.054 1.013
which is associated with the electrochemical dissolution of the sample material. The wear behaviour of untreated and treated samples is shown in Fig. 7. It can be seen from the figure that the wear rate of the nitrocarburised samples is one order of magnitude lower than that of the untreated steel. The nitrocarburised samples exhibited a lower friction coefficient compared to untreated ones: 0.169 and 0.191 respectively. Hence, the obtained structure improves the tribological behaviour.
4. Conclusions (1) The structure of the modified layer of low carbon steel treated by anode PEN/C in an aqueous solution of ammonia, acetone and ammonium chloride is detected. External part of layer comprises iron oxides formed due to high-temperature oxidizing with water vapour. The increase in concentration of nitrogen and carbon is assumed to lead to the formation of dispersed iron nitrides and carbon, surrounded by a solid solution of nitrogen and carbon. The nitrocarburised layer has a relatively low hardness, but usually plays a positive role in friction pairs, providing good wear resistance which is confirmed by wear tests. Under this part of layer martensitic sublayer is located. This phase and remained austenite are formed owing to diffusion of nitrogen and carbon followed by rapid cooling in the electrolyte. Further, a solid solution of carbon and nitrogen in iron take place. (2) The mechanism of nitrogen and carbon liberation from electrolyte under study is proposed. Acetone and ammonia are supposed to evaporate in the VGE with the following thermal decomposition on the sample surface. Acetone also reacts with ammonia to form acetonitrile found in the VGE through the chromatographic method and methane. Acetonitrile and methane decompose in VGE as well. (3) According to study of influence of the processing regimes and electrolyte composition on the structural characteristics of the modified layer the processes of the carbon and nitrogen diffusion, oxidation and anode dissolution of the workpieces occur simultaneously. These processes determine the carbon and nitrogen concentrations in the modified layer and structure obtained.
S.A. Kusmanov et al. / Materials Chemistry and Physics 175 (2016) 164e171
(4) The maximum hardness reaching up to 930 HV is observed at a depth of 20e60 mm which is probably associated with the highest total concentration of nitrogen and carbon atoms forming a supersaturated solution in austenite at the treatment temperature and a martensite phase after quenching. (5) Anode PEN/C of steel enables surface roughness reduction (Ra) from 1.013 to 0.054 mm due to the anode dissolution. (6) The electrolyte composition (5% ammonia, 5% acetone, 10e12.5% ammonium chloride) and processing mode (800 C, 5e10 min) of low carbon steels are developed. In this case, a hardened surface layer up to 0.2 mm with microhardness 930 HV can be obtained and with the decrease in the roughness of 13 times. Wear tests showed that wear resistance of the samples considerably improved after nitrocarburising.
Conflict of interest
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The authors declare that they have no conflict of interest. Acknowledgments This work was financially supported by the Russian Science Foundation (Contract No. 15-13-10018) to the Nekrasov Kostroma State University.
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