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Effect of solid lubricant particles on room and elevated temperature tribological properties of Ni–SiC composite coating
Surface & Coatings Technology 254 (2014) 252–259
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of solid lubricant particles on room and elevated temperature tribological properties of Ni–SiC composite coating M. Fazel a,⁎, M.R. Garsivaz Jazi a, S. Bahramzadeh b, S.R. Bakhshi c, M. Ramazani c a b c
Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran Department of Materials Engineering, University of Tehran, Tehran, Iran Department of Materials Engineering, Malek Ashtar University of Technology, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 25 February 2014 Accepted in revised form 12 June 2014 Available online 22 June 2014 Keywords: Composite coating Electrodeposition Ni–SiC Solid lubricant particles High temperature tribological behavior
a b s t r a c t In this article, the Ni–SiC, Ni–SiC–MoS2 and Ni–SiC–graphite composite coatings were prepared from a sulfamate bath. Both mechanical and ultrasonic stirrers were used simultaneously during the electrodeposition process. Tribological properties of coatings were evaluated from 25 °C to 300 °C. Based on the results, the friction coefficient of Ni–SiC composite coating at room temperature is very stable during the wear, but this stability decreases with increasing the test temperature. The incorporation of MoS2 and graphite lubricant particles in the coating reduced the strong adhesive wear and the un-stability of friction coefficient at high temperatures. However, about 15 and 32% reductions were observed in high temperature friction coefficient values of coatings containing MoS2 and Gr particle coatings, respectively. However, the Ni–SiC–Gr composite coating showed the best friction and wear behavior at all temperatures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction New metallic materials with better surface properties are constantly required for driving the technological advancement. The co-deposition of finely dispersed reinforcement particles in a metallic matrix allows the deposition of a metal matrix composite coating with excellent surface characteristics [1,2]. Among the various procedures that have been developed to prepare composite coatings, the electro-codeposition technique with remarkable characteristics such as simplicity, low working temperatures, and cost effectiveness has been a rapid development in the recent decades [3,4]. In the past few years, Ni–SiC composite coatings have been successfully employed as functional coatings to increase the corrosion and wear resistance in friction parts, engine cylinders, casting molds, heat exchangers, etc. [5–7]. Liquid lubricants such as oil and grease were usually used to control the friction in these applications. But in some cases, for example at high temperatures or vacuum performance conditions, the liquid lubricants couldn't meet the higher demands of advancing technology and their poor performance leads to severe damages of the coating. However, due to the working condition of these composite coatings in many functions, the solid lubricants are required to improve the coating performance in a variety of situations: corrosion, wear, friction, lubrication, extreme
⁎ Corresponding author. Tel.: +98 910 3144002. E-mail address: [email protected] (M. Fazel).
http://dx.doi.org/10.1016/j.surfcoat.2014.06.027 0257-8972/© 2014 Elsevier B.V. All rights reserved.
temperatures and in many other adverse operating conditions [8]. Many authors reported that the incorporation of solid lubricant particles (such as PTFE [1], h-BN [9], CNT [10], MoS2 [11,12], WS2 [13] and graphite [6]) in Ni-based composite coatings, can improve their tribological behavior. For example, Carpenter et al. [10] illustrated that CNT containing coatings exhibit up to a 68% reduction in wear rate compared to pure nickel coating. Guo et al. [1] showed that the hardness of the Rare Earth–Ni–W–P–SiC–PTFE coating is lower than that of the Rare Earth–Ni–W–P–SiC coating, but its mass loss is much smaller than that of the latter. On the other hand, high temperature tribological behavior of Ni-base composite coatings was investigated by some of the authors. It has been reported by Lekka et al. [14] that the presence of micron size SiC particles in the Ni matrix leads to a 51% increase of the microhardness and 63% decrease of the wear coefficient at 300 °C while it doesn't offer any improvement at the wear resistance at room temperature. In another research, León et al. [9] found that a mild adhesive wear mechanism occurred for the Ni–P–BN (hexagonal) coating tested at room temperature. Whereas for samples worn at higher temperature a mixed adhesive and fatigue wear mechanism, accompanied by a large plastic deformation of coatings and high coating transfer to the ball was reported. However, graphite (Gr) and MoS2 solid lubricants, because of their layered structure, good thermal stability until about 400 °C, low cost and easy to incorporate in Ni matrix by electroplating, can be used in many performance conditions [12,15]. Graphite is one of the best choices for lubrication in regular atmospheres. The adsorption of water reduces the bonding energy between
M. Fazel et al. / Surface & Coatings Technology 254 (2014) 252–259
the hexagonal planes of the Gr to a lower level than the adhesion energy between a substrate and the graphite. Because water vapor is a requirement for lubrication, graphite is not effective in vacuum. In an oxidative atmosphere graphite is effective at high temperatures up to 450 °C continuously and can withstand much higher temperature peaks. Just unlike Gr, MoS2 doesn't need to adsorb water vapor for playing the lubrication role. So, MoS2 is effective in high vacuum conditions as well whereas graphite does not. The temperature limitation of MoS2 at about 400 °C in the air is restricted by oxidation. On the whole, the codeposition of MoS2 and Gr particles in Ni-based composite coatings can reduce the friction coefficient significantly. These particles due to formation a transfer lubricant film between the coating and other contacting surface, are useful solid lubricants where liquid lubrication cannot be used [6,12]. In this work, the Ni–SiC, Ni–SiC–Gr and Ni–SiC–MoS2 composite coatings were carried out from a sulfamate bath. Both mechanical and ultrasonic stirrers were used simultaneously during the plating to achieve a good dispersion of reinforcing particles in the bath. The morphology of coatings was examined by a scanning electron microscope (SEM). The wear behavior of coatings was evaluated at 25, 100, 200 and 300 °C by the pin-on-disk method and the influence of adding solid lubricant particles on tribological properties of Ni–SiC composite coating at various temperatures was investigated.
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Table 2 Wear test conditions. Parameter
Value
Normal load Sliding distance Wear track radius Pin Ball diameter Velocity Test temperature
15 N 500 m 25 mm Alumina ball 7 mm 0.1 m/s 25, 100, 200 & 300 °C
carbonate. The composition of sulfamate plating bath and electroplating parameters are shown in Table 1. For the production of composite coatings, 12 g/l of SiC powders (b10 μm; Trading Company, China) was added to the electroplating bath. An amount of 12 g/l of Gr and MoS2 powders (b 10 μm in diameter) was also added for coatings containing lubricant particles. Both mechanical (400 rpm) and ultrasonic (200 W, 20 kHz) stirrings were used simultaneously during the coating to disperse particles and achieve better distribution and homogeneity in the solution. The deposition time was 1 h to obtain a 40–50 μm thickness of coating. Before the wear tests, the uncoated surfaces of samples were copper plated to inhibit the oxidation of them in particular at higher temperatures, and their effect on the weight loss results.
2. Material and methods 2.2. Coating characterization 2.1. Specimen's production Ni–SiC, Ni–SiC–MoS2 and Ni–SiC–Gr composite coatings were deposited on ASTM A53 steel disks (d = 65 mm and t = 7 mm). A pure nickel plate was used as an anode. The substrates have been pretreated before the electroplating process, since only one of the surfaces of the samples has been coated and the others which are exposed to the solution were masked. To create a good adhesion between substrate and coating, the sandblast operation was performed on the target surface. Next, the samples are degreased using TURKO degreaser solution for 60 min at 65 °C and then acid pickling in a H2SO4 solution for 2 min at room temperature has been done. After degreasing and acid pickling pretreatment steps, the substrates were cleaned with distilled water. The sulfamate bath was used for the composite plating. After the formulation of electrolyte and before the use, the following treatments were done for purifying a freshly prepared nickel plating solution. First, sufficient nickel carbonate was added to bring the pH above 5.2. About 1–2 ml/l of 30% hydrogen peroxide was added to the bath, agitated briefly and allowed to settle for 90 min. Then, treatment with 2 g/l activated carbon was done to remove the organic impurities and the solution was filtered after a day. Finally, if the pH was out of the range (4.2–4.5), it was adjusted by the addition of boric acid or nickel
Ni(SO3NH2) H3BO3 NiCl SiC particles (2–3 μm) Additive SLOTONIK M Additive BFL Current density Temperature pH
3. Results and discussion 3.1. Microhardness and thickness The HV0.1 microhardness and thickness of coatings as well as the measured values have been reported in Table 3. Increasing
Table 3 The microhardness and thickness of coatings.
Table 1 Composition and condition of plating bath. Composition
Surface morphology of coatings from the surface of specimens was examined by SEM (Philips XL 30). Optical microscopic images were used to evaluate the coating thickness. Microhardness values (HV0.1) were determined from five measurements of the cross section of coatings. The X-ray diffraction profile was determined using a Bruker D8 ADVANCE diffractometer, employing Cu Kα radiation in the 2θ range of 10–100°. Wear tests were carried out by a high temperature pin-on-disk tribometer under a normal load of 15 N and at four temperatures from 25 °C to 300 °C according to Table 2. Each experiment was done at least two times. Coefficient of friction vs. sliding distance was plotted at various temperatures by tribometer software. Weight loss values were measured by a balance (Sartorius model) with an accuracy of one ten-thousandth of a gram. The weight loss of alumina pin was negligible during the wear tests. The worn surface morphologies were evaluated by FE-SEM (Hitachi S4160) to determine the wear mechanism.
Quantity 350 g/l 15 g/l 35 g/l 12 g/l 0.5 cm3/l 2.5 cm3/l 5 A/dm2 50–55 °C 4.0–4.6
Coating
Pure Ni Ni–SiC Ni–SiC–Gr Ni–SiC–MoS2
Parameter
Thickness (μm) Microhardness (HV0.1) Thickness (μm) Microhardness (HV0.1) Thickness (μm) Microhardness (HV0.1) Thickness (μm) Microhardness (HV0.1)
Measured values 1
2
3
4
5
Ave.
40 268 42.8 478 47 460 25.5 334
39 270 44 472 48 465 28.2 397
41 268 43.5 464 42 470 31.4 445
36.8 274 45.4 460 43 463 35 320
39.1 271 43 466 42 478 41 368
39.2 270 43.7 468 44.4 467 32.2 373
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M. Fazel et al. / Surface & Coatings Technology 254 (2014) 252–259
the microhardness of Ni-based composite coating in comparison to the pure nickel coating can be found in the effects of changing in three factors; texture, grain size and distribution of reinforcing particles [16–19]. As shown in Fig. 1-a, the XRD patterns of the samples confirm the presence of SiC, MoS2 and graphite phases into the Ni matrix. In addition, all the coatings have shown (111), (200), (220), (311) and (222) diffraction lines of Ni matrix. The preferred crystalline orientation of coatings was estimated using the term Relative Texture Coefficient (RTC) which is defined as:
RTC ¼
Iex:ðhklÞ =IsðhklÞ 1 Xn I =I i¼1 ex:ðhklÞ sðhklÞ n
where Iex.(hkl) and Is(hkl) are the intensities of the (hkl) diffraction lines measured in the diffractogram of the coating and the standard Ni
powder sample. The preferred orientation of a plane was indicated by values of RTC greater than 1 [20]. According to Fig. 1-b, it can be said that the addition of particles significantly influenced the preferred orientation of Ni–SiC composite coatings. It should be noted that (111) diffraction line at θ = 22.25° is the most intense reflection in the standard XRD pattern from randomly oriented polycrystalline nickel (no. 04-0850). For Ni–SiC coating, the peak intensities of (111) and (200) planes become weak, while the ones of (220), (311) and (222) turn strong. However, the texture of the Ni–SiC–Gr coating changed to the (111) preferred orientation. The coating containing MoS2 particles also showed a mixed preferred orientation of (111) and (200) planes. According to the results no significant decrease in the microhardness of Ni–SiC–Gr coating was observed. It may be due to the successful co-deposition of graphite particles on sample surfaces with a good homogeneity. According to Yeh's model [21], the transmission of the particles into the substrate is the control process of electro-co-
(a)
(b)
Fig. 1. (a) XRD pattern and (b) Relative Texture Coefficient (RTC) for diffraction lines, of Ni–SiC, Ni–SiC-Gr and Ni–SiC–MoS2 composite coatings.
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Fig. 2. SEM micrograph and EDS analysis of the top surfaces of: (a) Ni–SiC, (b) Ni–SiC–MoS2 and (c) Ni–SiC–Gr composite coatings.
deposition. The agglomeration of particles can be occurred in the solution and the transmission of large particles is very difficult. Ultrasonic dispersion is not only able to prevent the particle sedimentation, but
also able to hinder their agglomeration because of its high-pressure waves and intensive vibrations. Mechanical stirring can also assist to achieve the better uniformity of suspension and make the transmission
(a)
(b)
100 um
200 um
(c)
100 um
Fig. 3. The distribution of particles throughout the Ni matrix in: (a) Ni–SiC, (b) Ni–SiC–MoS2 and (c) Ni–SiC–Gr coatings.
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of reinforced particles easier. So, a very good distribution of hard and lubricant particles in the Ni matrix causes no drop in the microhardness of Ni–SiC–Gr composite coating. On the other hand, it can be said that the microhardness measurements related to Ni–SiC–MoS2 composite coating have a considerable error. It's because of the irregular porous sponge-like structure of the coating and local aggregation of reinforcement particles in the MoS2 containing coatings (see below). 3.2. Microstructure SEM micrographs from the top surface of Ni–SiC, Ni–SiC–Gr and Ni– SiC–MoS2 coatings are shown in Figs. 2 and 3. As seen in the figure, top surface images and EDS analysis of Ni–SiC composite coating show a compact co-deposition of SiC particles in Ni matrix. However, the embedding solid lubricant particles in the coating cause to change in the surface morphology and reduce the compaction, compared with Ni–SiC coating. However, it can be observed that the crystallite size of the Ni matrix is larger in Ni–SiC–MoS2 coating that could partly explain why the microhardness values of this coating is lower compared to the coating containing Gr particles (apart from the inherent low hardness of MoS2, Ni matrix texture, non-uniformity in the thickness of Ni–SiC– MoS2 coating). Furthermore, the results of EDS analysis from the coatings (Fig. 2) confirm the co-deposition of MoS2 and Gr particles with Ni matrix. According to Fig. 3, SiC and Gr particles were homogenously distributed throughout the Ni matrix. The volume fraction of SiC particles in the Ni–SiC and Ni–SiC–Gr composite coatings was calculated to be approximately 19% and 18%, respectively. This is the other important reason for the relatively high microhardness values of these coatings. The volume fraction of graphite particles in Ni–SiC–Gr coating was also about 11%. However, the local aggregation of particles was observed in Ni–SiC–MoS2 composite coating (Fig. 3-b). As MoS2 particles are deposited on the surface of the coating, due to their semiconducting properties, the effective cathode area was increased resulting in the non-uniformity in the current density, coating thickness and distribution of particles. Similar behavior was reported by other authors for coatings containing MoS2 and WS2 particles [11–13].
Fig. 5. The friction coefficient vs. sliding distance curves for Ni–SiC–MoS2 composite coating at various temperatures.
The wear behavior of the coatings was evaluated from 25 to 300 °C by the pin-on-disk method and friction coefficient was plotted as a function of the sliding distance for three coatings at various temperatures (Figs. 4–6). The friction coefficient of Ni–SiC composite coating at room and 100 °C temperatures is very stable during the test. However, increasing the test temperature reduces the stability. This fact has been illustrated in Fig. 4. As shown in the figure, the stability of friction coefficient has reached from the highest level at 25 °C to the lowest at 300 °C. Fig. 7-a shows the average values of friction coefficient vs. temperature for three coatings. As this graph illustrates, the change in
the friction coefficient of Ni–SiC coating with increasing the test temperature up to 100 °C is not significant. But when the temperature increases to 200 °C and 300 °C, the friction coefficient suddenly shifts to high values. In other words, increasing the temperature has caused 17%, 80% and 67% increases in the friction coefficient at 100 °C, 200 °C and 300 °C, respectively, compared to the value (0.36) at 25 °C. Similar results are also observed for wear weight loss values (Fig. 7-b). This is due the reduced strength and increased ductility of Ni matrix at higher temperatures. Figs. 8 and 9 show the SEM images of the worn surfaces of coatings at 25 °C and 200 °C that illustrate the influence of wear temperature on the wear resistance of coatings. As shown, the amount of wear of Ni–SiC coating has strongly increased by increasing the temperature. The width of wear tracks has risen from 0.6 at 25 °C to 1.6 mm at 200 °C. It can be said that the wear mechanism of Ni–SiC coating worn at various temperatures is spalling. During the wear test and at lower temperatures, the embedded SiC particles increase the strength and reduce the amount of plastic deformation during the wear. The lower delamination of the coating could clarify the smoother friction coefficient vs. time observed on the coatings worn at near room temperatures. But according to Fig. 9, at higher temperatures due to the reduced strength and increase ductility of the Ni matrix, much more plastic deformation of Ni matrix occurred and the delamination of the coating from the surface was highly increased. According to Fig. 10, the displaced material forms ridges adjacent to grooves which can be removed by the subsequent passage of abrasive particles. So, it can be said that the densified wear particles play the most important role in the wear process as a third body and aggravate the formation of cracks. These cracks then freely propagate locally around the wear groove, resulting in additional coating removal by spalling (Fig. 10). At these temperatures the strong spalling was seen on the worn surfaces. So, the wear resistance of Ni–SiC coating has decreased significantly.
Fig. 4. The friction coefficient vs. sliding distance curves for Ni–SiC composite coating at various temperatures.
Fig. 6. The friction coefficient vs. sliding distance curves for Ni–SiC–Gr composite coating at various temperatures.
3.3. Friction and wear behavior
M. Fazel et al. / Surface & Coatings Technology 254 (2014) 252–259
(a)
257
(b)
Fig. 7. (a) The mean friction coefficient values and (b) the wear weight loss values of three coatings at any temperature.
The sharp jumps in friction coefficient at 300 °C could also be due to the gradual propagation of cracks (Fig. 10) which is caused by the periodic delamination of the coating accompanied by strong adhesive wear. Therefore, the friction coefficient and weight loss values of Ni–SiC composite coating strongly increased by increasing the temperature. Similar results were obtained by Lekka et al. [22]. MoS2 and Gr solid lubricant particles were added in the solution to modify the high temperature tribological performance of Ni–SiC coating. As shown in Fig. 5, the stability of friction coefficient at high temperature has increased by adding MoS2 solid lubricant particles in the coating because of their lubrication properties at high temperature. The percentages of increasing the friction coefficient at 100 °C, 200 °C and 300 °C rather than at room temperature are about 2%, 25% and 23%, respectively. In other words, the incorporation of MoS2 particles caused a 10–15% reduction in friction coefficient values of Ni–SiC coating at high temperatures. Similar results were obtained for weight loss measurements. According to SEM images from the worn surfaces of Ni–SiC–MoS2 coating (Figs. 8 and 9), it can be said that the MoS2 solid lubricant particles could partly prevent the local severe frictions and detachment of layers at high temperatures. But the high magnified
image from the worn surface at 300 °C (Fig. 10) shows that the damaging effects of strong wear are still obvious. Non-uniformity in thickness is the most important reason of the less than expected tribological performance of this coating. The stability of friction coefficient was observed at higher test temperatures in Ni–SiC–Gr composite coating, too (Fig. 6). In addition unlike the other coatings, no evident increase in the friction coefficient of Ni–SiC–Gr composite coating was observed at high temperatures. In the beginning of wear, the partial separation of Gr solid lubricant particles from the coating occurred [23]. These particles transfer onto the opposing surface and form a transfer lubricant film (Figs. 9 and 10). The build-up of a lamellar structure graphite film between the two surfaces leads to reduce in friction coefficient and its fluctuations [23,24]. The friction coefficient values have increased about 8%, 22% and 16% at 100 °C, 200 °C and 300 °C rather than at room temperature, respectively. The smooth worn surface in the Ni–SiC coating containing Gr particles was more comparatively compact that induced more effective lubrication, which illustrates that a mild adhesive wear was dominated at all temperatures. As shown in Fig. 7-b, the weight loss values (lower than 5 mg at all wear temperatures) confirm what was
Fig. 8. Low and high magnification SEM micrographs of the worn surfaces at room temperature.
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1.00 mm
1.00 mm
1.00 mm
120 µm
120 µm
120 µm
Fig. 9. Low and high magnification SEM micrographs of the worn surfaces at 200 °C.
50 µm
50 µm
50 µm
Fig. 10. High magnification SEM micrographs of the worn surfaces at 300 °C in SE and BSE modes.
M. Fazel et al. / Surface & Coatings Technology 254 (2014) 252–259
mentioned. The high wear resistance accompanied by a low friction coefficient of the Ni–SiC–Gr coating at room and elevated temperatures will produce wide applications such as casting molds, engines, cylinders and other friction parts.
loss values of Ni–SiC–Gr coating were lower than 5 mg at all wear temperatures. Conflict of interest There is no conflict of interest.
4. Conclusions The Ni–SiC, Ni–SiC–MoS2 and Ni–SiC–Gr composite coatings were prepared by electro-co-deposition from a Ni sulfamate plating bath containing reinforcement particles in suspension under DC conditions. The results from the tribological behavior of coatings illustrated that the friction coefficient of Ni-SiC composite coatings at room temperatures is very stable during the test. But, this stability decreases with increasing the wear temperature. Increasing the wear temperature has caused to 17%, 80% and 67% increases in the friction coefficient at 100, 200 and 300 °C, respectively, rather than at 25 °C. The wear mechanism of Ni–SiC coating worn at various temperatures is spalling. The lower delamination of the coating could clarify the smoother friction coefficient vs. time observed on the coatings worn at near room temperatures. But at higher temperatures due to the reduced strength and increase ductility of the Ni matrix, much more plastic deformation of Ni matrix occurred and the delamination of the coating from the surface was highly increased. The stability of friction coefficient at high temperature increased by adding MoS2 and graphite solid lubricant particles in the coating. For Ni–SiC–MoS2 composite coating, the MoS2 solid lubricant particles could partly prevent the local severe frictions and detachment of layers, but the damaging effects of strong wear are still obvious at 200 and 300 °C. The presence of these particles in the Ni–SiC coating caused about a 10–15% reduction in friction coefficient values at high temperatures. The incorporation of graphite particles in the coating induced more effective lubrication and reduced the strong adhesive wear. So, more than a 30% reduction was observed in friction coefficient values of graphite containing coating at high temperatures. The wear weight
259
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Effect of solid lubricant particles on room and elevated temperature tribological properties of Ni–SiC composite coating M. Fazel a,⁎, M.R. Garsivaz Jazi a, S. Bahramzadeh b, S.R. Bakhshi c, M. Ramazani c a b c
Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran Department of Materials Engineering, University of Tehran, Tehran, Iran Department of Materials Engineering, Malek Ashtar University of Technology, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 25 February 2014 Accepted in revised form 12 June 2014 Available online 22 June 2014 Keywords: Composite coating Electrodeposition Ni–SiC Solid lubricant particles High temperature tribological behavior
a b s t r a c t In this article, the Ni–SiC, Ni–SiC–MoS2 and Ni–SiC–graphite composite coatings were prepared from a sulfamate bath. Both mechanical and ultrasonic stirrers were used simultaneously during the electrodeposition process. Tribological properties of coatings were evaluated from 25 °C to 300 °C. Based on the results, the friction coefficient of Ni–SiC composite coating at room temperature is very stable during the wear, but this stability decreases with increasing the test temperature. The incorporation of MoS2 and graphite lubricant particles in the coating reduced the strong adhesive wear and the un-stability of friction coefficient at high temperatures. However, about 15 and 32% reductions were observed in high temperature friction coefficient values of coatings containing MoS2 and Gr particle coatings, respectively. However, the Ni–SiC–Gr composite coating showed the best friction and wear behavior at all temperatures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction New metallic materials with better surface properties are constantly required for driving the technological advancement. The co-deposition of finely dispersed reinforcement particles in a metallic matrix allows the deposition of a metal matrix composite coating with excellent surface characteristics [1,2]. Among the various procedures that have been developed to prepare composite coatings, the electro-codeposition technique with remarkable characteristics such as simplicity, low working temperatures, and cost effectiveness has been a rapid development in the recent decades [3,4]. In the past few years, Ni–SiC composite coatings have been successfully employed as functional coatings to increase the corrosion and wear resistance in friction parts, engine cylinders, casting molds, heat exchangers, etc. [5–7]. Liquid lubricants such as oil and grease were usually used to control the friction in these applications. But in some cases, for example at high temperatures or vacuum performance conditions, the liquid lubricants couldn't meet the higher demands of advancing technology and their poor performance leads to severe damages of the coating. However, due to the working condition of these composite coatings in many functions, the solid lubricants are required to improve the coating performance in a variety of situations: corrosion, wear, friction, lubrication, extreme
⁎ Corresponding author. Tel.: +98 910 3144002. E-mail address: [email protected] (M. Fazel).
http://dx.doi.org/10.1016/j.surfcoat.2014.06.027 0257-8972/© 2014 Elsevier B.V. All rights reserved.
temperatures and in many other adverse operating conditions [8]. Many authors reported that the incorporation of solid lubricant particles (such as PTFE [1], h-BN [9], CNT [10], MoS2 [11,12], WS2 [13] and graphite [6]) in Ni-based composite coatings, can improve their tribological behavior. For example, Carpenter et al. [10] illustrated that CNT containing coatings exhibit up to a 68% reduction in wear rate compared to pure nickel coating. Guo et al. [1] showed that the hardness of the Rare Earth–Ni–W–P–SiC–PTFE coating is lower than that of the Rare Earth–Ni–W–P–SiC coating, but its mass loss is much smaller than that of the latter. On the other hand, high temperature tribological behavior of Ni-base composite coatings was investigated by some of the authors. It has been reported by Lekka et al. [14] that the presence of micron size SiC particles in the Ni matrix leads to a 51% increase of the microhardness and 63% decrease of the wear coefficient at 300 °C while it doesn't offer any improvement at the wear resistance at room temperature. In another research, León et al. [9] found that a mild adhesive wear mechanism occurred for the Ni–P–BN (hexagonal) coating tested at room temperature. Whereas for samples worn at higher temperature a mixed adhesive and fatigue wear mechanism, accompanied by a large plastic deformation of coatings and high coating transfer to the ball was reported. However, graphite (Gr) and MoS2 solid lubricants, because of their layered structure, good thermal stability until about 400 °C, low cost and easy to incorporate in Ni matrix by electroplating, can be used in many performance conditions [12,15]. Graphite is one of the best choices for lubrication in regular atmospheres. The adsorption of water reduces the bonding energy between
M. Fazel et al. / Surface & Coatings Technology 254 (2014) 252–259
the hexagonal planes of the Gr to a lower level than the adhesion energy between a substrate and the graphite. Because water vapor is a requirement for lubrication, graphite is not effective in vacuum. In an oxidative atmosphere graphite is effective at high temperatures up to 450 °C continuously and can withstand much higher temperature peaks. Just unlike Gr, MoS2 doesn't need to adsorb water vapor for playing the lubrication role. So, MoS2 is effective in high vacuum conditions as well whereas graphite does not. The temperature limitation of MoS2 at about 400 °C in the air is restricted by oxidation. On the whole, the codeposition of MoS2 and Gr particles in Ni-based composite coatings can reduce the friction coefficient significantly. These particles due to formation a transfer lubricant film between the coating and other contacting surface, are useful solid lubricants where liquid lubrication cannot be used [6,12]. In this work, the Ni–SiC, Ni–SiC–Gr and Ni–SiC–MoS2 composite coatings were carried out from a sulfamate bath. Both mechanical and ultrasonic stirrers were used simultaneously during the plating to achieve a good dispersion of reinforcing particles in the bath. The morphology of coatings was examined by a scanning electron microscope (SEM). The wear behavior of coatings was evaluated at 25, 100, 200 and 300 °C by the pin-on-disk method and the influence of adding solid lubricant particles on tribological properties of Ni–SiC composite coating at various temperatures was investigated.
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Table 2 Wear test conditions. Parameter
Value
Normal load Sliding distance Wear track radius Pin Ball diameter Velocity Test temperature
15 N 500 m 25 mm Alumina ball 7 mm 0.1 m/s 25, 100, 200 & 300 °C
carbonate. The composition of sulfamate plating bath and electroplating parameters are shown in Table 1. For the production of composite coatings, 12 g/l of SiC powders (b10 μm; Trading Company, China) was added to the electroplating bath. An amount of 12 g/l of Gr and MoS2 powders (b 10 μm in diameter) was also added for coatings containing lubricant particles. Both mechanical (400 rpm) and ultrasonic (200 W, 20 kHz) stirrings were used simultaneously during the coating to disperse particles and achieve better distribution and homogeneity in the solution. The deposition time was 1 h to obtain a 40–50 μm thickness of coating. Before the wear tests, the uncoated surfaces of samples were copper plated to inhibit the oxidation of them in particular at higher temperatures, and their effect on the weight loss results.
2. Material and methods 2.2. Coating characterization 2.1. Specimen's production Ni–SiC, Ni–SiC–MoS2 and Ni–SiC–Gr composite coatings were deposited on ASTM A53 steel disks (d = 65 mm and t = 7 mm). A pure nickel plate was used as an anode. The substrates have been pretreated before the electroplating process, since only one of the surfaces of the samples has been coated and the others which are exposed to the solution were masked. To create a good adhesion between substrate and coating, the sandblast operation was performed on the target surface. Next, the samples are degreased using TURKO degreaser solution for 60 min at 65 °C and then acid pickling in a H2SO4 solution for 2 min at room temperature has been done. After degreasing and acid pickling pretreatment steps, the substrates were cleaned with distilled water. The sulfamate bath was used for the composite plating. After the formulation of electrolyte and before the use, the following treatments were done for purifying a freshly prepared nickel plating solution. First, sufficient nickel carbonate was added to bring the pH above 5.2. About 1–2 ml/l of 30% hydrogen peroxide was added to the bath, agitated briefly and allowed to settle for 90 min. Then, treatment with 2 g/l activated carbon was done to remove the organic impurities and the solution was filtered after a day. Finally, if the pH was out of the range (4.2–4.5), it was adjusted by the addition of boric acid or nickel
Ni(SO3NH2) H3BO3 NiCl SiC particles (2–3 μm) Additive SLOTONIK M Additive BFL Current density Temperature pH
3. Results and discussion 3.1. Microhardness and thickness The HV0.1 microhardness and thickness of coatings as well as the measured values have been reported in Table 3. Increasing
Table 3 The microhardness and thickness of coatings.
Table 1 Composition and condition of plating bath. Composition
Surface morphology of coatings from the surface of specimens was examined by SEM (Philips XL 30). Optical microscopic images were used to evaluate the coating thickness. Microhardness values (HV0.1) were determined from five measurements of the cross section of coatings. The X-ray diffraction profile was determined using a Bruker D8 ADVANCE diffractometer, employing Cu Kα radiation in the 2θ range of 10–100°. Wear tests were carried out by a high temperature pin-on-disk tribometer under a normal load of 15 N and at four temperatures from 25 °C to 300 °C according to Table 2. Each experiment was done at least two times. Coefficient of friction vs. sliding distance was plotted at various temperatures by tribometer software. Weight loss values were measured by a balance (Sartorius model) with an accuracy of one ten-thousandth of a gram. The weight loss of alumina pin was negligible during the wear tests. The worn surface morphologies were evaluated by FE-SEM (Hitachi S4160) to determine the wear mechanism.
Quantity 350 g/l 15 g/l 35 g/l 12 g/l 0.5 cm3/l 2.5 cm3/l 5 A/dm2 50–55 °C 4.0–4.6
Coating
Pure Ni Ni–SiC Ni–SiC–Gr Ni–SiC–MoS2
Parameter
Thickness (μm) Microhardness (HV0.1) Thickness (μm) Microhardness (HV0.1) Thickness (μm) Microhardness (HV0.1) Thickness (μm) Microhardness (HV0.1)
Measured values 1
2
3
4
5
Ave.
40 268 42.8 478 47 460 25.5 334
39 270 44 472 48 465 28.2 397
41 268 43.5 464 42 470 31.4 445
36.8 274 45.4 460 43 463 35 320
39.1 271 43 466 42 478 41 368
39.2 270 43.7 468 44.4 467 32.2 373
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the microhardness of Ni-based composite coating in comparison to the pure nickel coating can be found in the effects of changing in three factors; texture, grain size and distribution of reinforcing particles [16–19]. As shown in Fig. 1-a, the XRD patterns of the samples confirm the presence of SiC, MoS2 and graphite phases into the Ni matrix. In addition, all the coatings have shown (111), (200), (220), (311) and (222) diffraction lines of Ni matrix. The preferred crystalline orientation of coatings was estimated using the term Relative Texture Coefficient (RTC) which is defined as:
RTC ¼
Iex:ðhklÞ =IsðhklÞ 1 Xn I =I i¼1 ex:ðhklÞ sðhklÞ n
where Iex.(hkl) and Is(hkl) are the intensities of the (hkl) diffraction lines measured in the diffractogram of the coating and the standard Ni
powder sample. The preferred orientation of a plane was indicated by values of RTC greater than 1 [20]. According to Fig. 1-b, it can be said that the addition of particles significantly influenced the preferred orientation of Ni–SiC composite coatings. It should be noted that (111) diffraction line at θ = 22.25° is the most intense reflection in the standard XRD pattern from randomly oriented polycrystalline nickel (no. 04-0850). For Ni–SiC coating, the peak intensities of (111) and (200) planes become weak, while the ones of (220), (311) and (222) turn strong. However, the texture of the Ni–SiC–Gr coating changed to the (111) preferred orientation. The coating containing MoS2 particles also showed a mixed preferred orientation of (111) and (200) planes. According to the results no significant decrease in the microhardness of Ni–SiC–Gr coating was observed. It may be due to the successful co-deposition of graphite particles on sample surfaces with a good homogeneity. According to Yeh's model [21], the transmission of the particles into the substrate is the control process of electro-co-
(a)
(b)
Fig. 1. (a) XRD pattern and (b) Relative Texture Coefficient (RTC) for diffraction lines, of Ni–SiC, Ni–SiC-Gr and Ni–SiC–MoS2 composite coatings.
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Fig. 2. SEM micrograph and EDS analysis of the top surfaces of: (a) Ni–SiC, (b) Ni–SiC–MoS2 and (c) Ni–SiC–Gr composite coatings.
deposition. The agglomeration of particles can be occurred in the solution and the transmission of large particles is very difficult. Ultrasonic dispersion is not only able to prevent the particle sedimentation, but
also able to hinder their agglomeration because of its high-pressure waves and intensive vibrations. Mechanical stirring can also assist to achieve the better uniformity of suspension and make the transmission
(a)
(b)
100 um
200 um
(c)
100 um
Fig. 3. The distribution of particles throughout the Ni matrix in: (a) Ni–SiC, (b) Ni–SiC–MoS2 and (c) Ni–SiC–Gr coatings.
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of reinforced particles easier. So, a very good distribution of hard and lubricant particles in the Ni matrix causes no drop in the microhardness of Ni–SiC–Gr composite coating. On the other hand, it can be said that the microhardness measurements related to Ni–SiC–MoS2 composite coating have a considerable error. It's because of the irregular porous sponge-like structure of the coating and local aggregation of reinforcement particles in the MoS2 containing coatings (see below). 3.2. Microstructure SEM micrographs from the top surface of Ni–SiC, Ni–SiC–Gr and Ni– SiC–MoS2 coatings are shown in Figs. 2 and 3. As seen in the figure, top surface images and EDS analysis of Ni–SiC composite coating show a compact co-deposition of SiC particles in Ni matrix. However, the embedding solid lubricant particles in the coating cause to change in the surface morphology and reduce the compaction, compared with Ni–SiC coating. However, it can be observed that the crystallite size of the Ni matrix is larger in Ni–SiC–MoS2 coating that could partly explain why the microhardness values of this coating is lower compared to the coating containing Gr particles (apart from the inherent low hardness of MoS2, Ni matrix texture, non-uniformity in the thickness of Ni–SiC– MoS2 coating). Furthermore, the results of EDS analysis from the coatings (Fig. 2) confirm the co-deposition of MoS2 and Gr particles with Ni matrix. According to Fig. 3, SiC and Gr particles were homogenously distributed throughout the Ni matrix. The volume fraction of SiC particles in the Ni–SiC and Ni–SiC–Gr composite coatings was calculated to be approximately 19% and 18%, respectively. This is the other important reason for the relatively high microhardness values of these coatings. The volume fraction of graphite particles in Ni–SiC–Gr coating was also about 11%. However, the local aggregation of particles was observed in Ni–SiC–MoS2 composite coating (Fig. 3-b). As MoS2 particles are deposited on the surface of the coating, due to their semiconducting properties, the effective cathode area was increased resulting in the non-uniformity in the current density, coating thickness and distribution of particles. Similar behavior was reported by other authors for coatings containing MoS2 and WS2 particles [11–13].
Fig. 5. The friction coefficient vs. sliding distance curves for Ni–SiC–MoS2 composite coating at various temperatures.
The wear behavior of the coatings was evaluated from 25 to 300 °C by the pin-on-disk method and friction coefficient was plotted as a function of the sliding distance for three coatings at various temperatures (Figs. 4–6). The friction coefficient of Ni–SiC composite coating at room and 100 °C temperatures is very stable during the test. However, increasing the test temperature reduces the stability. This fact has been illustrated in Fig. 4. As shown in the figure, the stability of friction coefficient has reached from the highest level at 25 °C to the lowest at 300 °C. Fig. 7-a shows the average values of friction coefficient vs. temperature for three coatings. As this graph illustrates, the change in
the friction coefficient of Ni–SiC coating with increasing the test temperature up to 100 °C is not significant. But when the temperature increases to 200 °C and 300 °C, the friction coefficient suddenly shifts to high values. In other words, increasing the temperature has caused 17%, 80% and 67% increases in the friction coefficient at 100 °C, 200 °C and 300 °C, respectively, compared to the value (0.36) at 25 °C. Similar results are also observed for wear weight loss values (Fig. 7-b). This is due the reduced strength and increased ductility of Ni matrix at higher temperatures. Figs. 8 and 9 show the SEM images of the worn surfaces of coatings at 25 °C and 200 °C that illustrate the influence of wear temperature on the wear resistance of coatings. As shown, the amount of wear of Ni–SiC coating has strongly increased by increasing the temperature. The width of wear tracks has risen from 0.6 at 25 °C to 1.6 mm at 200 °C. It can be said that the wear mechanism of Ni–SiC coating worn at various temperatures is spalling. During the wear test and at lower temperatures, the embedded SiC particles increase the strength and reduce the amount of plastic deformation during the wear. The lower delamination of the coating could clarify the smoother friction coefficient vs. time observed on the coatings worn at near room temperatures. But according to Fig. 9, at higher temperatures due to the reduced strength and increase ductility of the Ni matrix, much more plastic deformation of Ni matrix occurred and the delamination of the coating from the surface was highly increased. According to Fig. 10, the displaced material forms ridges adjacent to grooves which can be removed by the subsequent passage of abrasive particles. So, it can be said that the densified wear particles play the most important role in the wear process as a third body and aggravate the formation of cracks. These cracks then freely propagate locally around the wear groove, resulting in additional coating removal by spalling (Fig. 10). At these temperatures the strong spalling was seen on the worn surfaces. So, the wear resistance of Ni–SiC coating has decreased significantly.
Fig. 4. The friction coefficient vs. sliding distance curves for Ni–SiC composite coating at various temperatures.
Fig. 6. The friction coefficient vs. sliding distance curves for Ni–SiC–Gr composite coating at various temperatures.
3.3. Friction and wear behavior
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(a)
257
(b)
Fig. 7. (a) The mean friction coefficient values and (b) the wear weight loss values of three coatings at any temperature.
The sharp jumps in friction coefficient at 300 °C could also be due to the gradual propagation of cracks (Fig. 10) which is caused by the periodic delamination of the coating accompanied by strong adhesive wear. Therefore, the friction coefficient and weight loss values of Ni–SiC composite coating strongly increased by increasing the temperature. Similar results were obtained by Lekka et al. [22]. MoS2 and Gr solid lubricant particles were added in the solution to modify the high temperature tribological performance of Ni–SiC coating. As shown in Fig. 5, the stability of friction coefficient at high temperature has increased by adding MoS2 solid lubricant particles in the coating because of their lubrication properties at high temperature. The percentages of increasing the friction coefficient at 100 °C, 200 °C and 300 °C rather than at room temperature are about 2%, 25% and 23%, respectively. In other words, the incorporation of MoS2 particles caused a 10–15% reduction in friction coefficient values of Ni–SiC coating at high temperatures. Similar results were obtained for weight loss measurements. According to SEM images from the worn surfaces of Ni–SiC–MoS2 coating (Figs. 8 and 9), it can be said that the MoS2 solid lubricant particles could partly prevent the local severe frictions and detachment of layers at high temperatures. But the high magnified
image from the worn surface at 300 °C (Fig. 10) shows that the damaging effects of strong wear are still obvious. Non-uniformity in thickness is the most important reason of the less than expected tribological performance of this coating. The stability of friction coefficient was observed at higher test temperatures in Ni–SiC–Gr composite coating, too (Fig. 6). In addition unlike the other coatings, no evident increase in the friction coefficient of Ni–SiC–Gr composite coating was observed at high temperatures. In the beginning of wear, the partial separation of Gr solid lubricant particles from the coating occurred [23]. These particles transfer onto the opposing surface and form a transfer lubricant film (Figs. 9 and 10). The build-up of a lamellar structure graphite film between the two surfaces leads to reduce in friction coefficient and its fluctuations [23,24]. The friction coefficient values have increased about 8%, 22% and 16% at 100 °C, 200 °C and 300 °C rather than at room temperature, respectively. The smooth worn surface in the Ni–SiC coating containing Gr particles was more comparatively compact that induced more effective lubrication, which illustrates that a mild adhesive wear was dominated at all temperatures. As shown in Fig. 7-b, the weight loss values (lower than 5 mg at all wear temperatures) confirm what was
Fig. 8. Low and high magnification SEM micrographs of the worn surfaces at room temperature.
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1.00 mm
1.00 mm
1.00 mm
120 µm
120 µm
120 µm
Fig. 9. Low and high magnification SEM micrographs of the worn surfaces at 200 °C.
50 µm
50 µm
50 µm
Fig. 10. High magnification SEM micrographs of the worn surfaces at 300 °C in SE and BSE modes.
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mentioned. The high wear resistance accompanied by a low friction coefficient of the Ni–SiC–Gr coating at room and elevated temperatures will produce wide applications such as casting molds, engines, cylinders and other friction parts.
loss values of Ni–SiC–Gr coating were lower than 5 mg at all wear temperatures. Conflict of interest There is no conflict of interest.
4. Conclusions The Ni–SiC, Ni–SiC–MoS2 and Ni–SiC–Gr composite coatings were prepared by electro-co-deposition from a Ni sulfamate plating bath containing reinforcement particles in suspension under DC conditions. The results from the tribological behavior of coatings illustrated that the friction coefficient of Ni-SiC composite coatings at room temperatures is very stable during the test. But, this stability decreases with increasing the wear temperature. Increasing the wear temperature has caused to 17%, 80% and 67% increases in the friction coefficient at 100, 200 and 300 °C, respectively, rather than at 25 °C. The wear mechanism of Ni–SiC coating worn at various temperatures is spalling. The lower delamination of the coating could clarify the smoother friction coefficient vs. time observed on the coatings worn at near room temperatures. But at higher temperatures due to the reduced strength and increase ductility of the Ni matrix, much more plastic deformation of Ni matrix occurred and the delamination of the coating from the surface was highly increased. The stability of friction coefficient at high temperature increased by adding MoS2 and graphite solid lubricant particles in the coating. For Ni–SiC–MoS2 composite coating, the MoS2 solid lubricant particles could partly prevent the local severe frictions and detachment of layers, but the damaging effects of strong wear are still obvious at 200 and 300 °C. The presence of these particles in the Ni–SiC coating caused about a 10–15% reduction in friction coefficient values at high temperatures. The incorporation of graphite particles in the coating induced more effective lubrication and reduced the strong adhesive wear. So, more than a 30% reduction was observed in friction coefficient values of graphite containing coating at high temperatures. The wear weight
259
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