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Tribological study of mechanically milled graphite nanoparticles codeposited in electroless Ni-P coatings
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Tribological study of mechanically milled graphite nanoparticles codeposited in electroless Ni-P coatings I.S. Thakur a, V.S. Pandey b, P.S. Rao a, S. Tyagi b, Deepam Goyal a,⇑ a b
Department of Mechanical Engineering, National Institute of Technical Teachers Training and Research, Chandigarh, India Central Scientific Instruments Organisation, CSIR-CSIO, Chandigarh, India
The present study focusses to enhance the wear resistance of electroless Ni-P coatings with the codeposition of selflubricating nano graphite particles synthesized by mechanical milling using a high energy planetary ball mill. The coatings were developed on an aluminum substrate and nano graphite particles were co-deposited (4 g/l) into the Ni-P matrix using alkaline hypophosphite reduced electroless bath. Changes in properties after heat treatment at 220 °C for 4 h in vacuum have been correlated with morphological and microstructural changes shown by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) respectively. Energy dispersive analysis of X-ray (EDAX) determined the quality and elemental composition of coatings. Wear resistance, coefficient of friction (pin on disc) and microhardness of electroless Ni-P-graphite nanocomposite coatings were calculated and contrasted with Ni-P coatings. The outcomes reveal that the size of synthesized graphite nanoparticles is 20 nm. Microhardness and wear resistance of electroless coatings have improved after heat treatment. Excellent wear resistance has been observed at the cost of some hardness in Ni-Pgraphite nanocomposite coatings over Ni-P coatings. 1. Introduction Electroless nickel coating is one of the favorable surface engineering technology that is commonly used in the automotive, aviation and chemical industries due to its inherent properties like a high hardness, uniform deposition, and excellent wear and corrosion resistance etc. [1,2]. To improve the mechanical and tribological properties, various micron-sized second phase hard/soft particles have been successfully reinforced into Ni-P matrix [3–8]. The transition of deposits from amorphous to the crystalline structure during heat treatment is a function of time, temperature, heating rate and phosphorus content [9]. Recent efforts have been made to incorporate nanoparticles like TiO2, Al2O3, CNTs, WC, Au and diamond etc. into Ni-P matrix to further enhance the mechanical and tribological properties [10–15]. It is well known that graphite particles have been used in many industries for the development of composite coatings due to its good self-lubricating and antisticking properties, chemical inert⇑ Corresponding author. E-mail address: Goyal, D. ([email protected])
ness and low coefficient of friction. Izzard and Dennis have successfully reinforced micron-sized graphite particles into a Ni-P matrix; however, the studies on incorporation of nano-sized graphite particles into Ni-P alloy coatings are not reported in the literature [16]. In the current study, an attempt has been made to explore the self-lubricating properties of nano-sized graphite particles for improving wear resistance of electroless (EL) coatings. To develop EL Ni-P-graphite nanocomposite coatings, the embedded second phase nano-sized graphite particles were first synthesized by mechanical alloying method for 20 h and then co-deposited with Ni-P on an aluminum substrate. Further, coatings were studied for surface morphology, phase identification, microhardness and wear resistance in both as-coated and heat-treated (HT) conditions.
2. Experimental 2.1. Synthesis of nano graphite particle Graphite nanoparticles were synthesized by the mechanical milling using a high-energy planetary ball mill (Retsch, PM
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100) with a ball-to-powder ratio of 10:1 at a speed of 200 rpm. A grinding jar and agate balls of 10 mm and 20 mm diameter were used as milling media. A graphite powder of 50 mm size was used as a starting charge for milling. In grinding work, 500 ml jar was utilized to mill the 20 g graphite powder. The milling of graphite particles was carried out for 20 h, with periodic milling time of 2 h. An interval of 15 min was given to prevent the overheating of milling media.
2.2. Preparation of electroless coatings Electroless Ni-P alloy coatings and Ni-P-graphite nanocomposite coatings were deposited on an aluminum substrate for 30 min. The bath composition for EL deposition is shown in Table 1. Before coating, the aluminum substrate was polished with emery papers of 320, 400 and 600 no. respectively. Polished substrate (Flat = 20 20 2.5 mm; pin = £ 6.3 mm, 32 mm long) was degreased by dipping into acetone for 30 s and etched by dipping into dilute HCl for 30 s. The substrate was sensitized by immersing in 0.1% SnCl2 solution for 120 s and activated by immersing in 0.01% PdCl2 solution for 30 s. Following each step of surface preparation and pretreatment, the substrate was rinsed in distilled water and dried air. The EL coating was developed by immersing the activated substrate into the electroless bath at temperature 90 ± 2 °C and pH 9 ± 0.2. Ammonia was added continuously to sustain the pH of the bath. The detailed experimental procedure is illustrated in Fig. 1. To obtain nanocomposite coatings, a deposit of electroless nickel was plated for 10 min before adding the nano graphite (20 nm) particles into the bath and stirred continuously for their suspension. The thickness (mm) of the coating was calculated by using Eq. (1): t¼
W 104 qA
ð1Þ
where ‘q’ deposits density (7.75 g/cm3), ‘W’ is weight gain (g), and ‘A’ is the deposition surface area (cm2). The rate of deposition (mm/h) was calculated as the coating thickness per unit deposition time [17]. Both plain and nanocomposite coatings were heat treated in vacuum at 220 °C for 4 h to enhance their mechanical properties.
2.3. Characterization Phase identification of graphite particles and EL coatings in ascoated and HT conditions were done by X-ray diffraction (XRD) using Bruker AXS D8 diffractometer with Cu-Ka radiation. The crystallite size of the milled graphite was computed using
TABLE 1
Bath composition for EL coatings. Chemical compound/Element
Quantity (g/l)
Nickel sulfate Sodium hypophosphite Trisodium citrate Ammonium sulfate Ammonium chloride Graphite*
33 20 84 25 25 4
* Used only for nanocomposite coatings.
FIGURE 1
Experimental set up for EL coatings.
Scherrer’s formula. Field emission scanning electron microscopy (FESEM, QUANTA FEG 200 FEI) and energy dispersive analysis of X-ray (EDAX) characterizes the morphology and element composition of coatings respectively.
2.4. Hardness and wear tests The microhardness of Ni-P plain and Ni-P-graphite nanocomposite coatings in as-coated and HT condition were measured with Vickers hardness indenter (Mitutoyo Hardness Testing Machine, HM) utilizing a load of 50 gf for 10 s. The microhardness was measured as a mean value of three measurements. The wear study of coated and uncoated aluminum substrates was conducted by using disc or pin tester (Ducom, TR-20LE) in dry conditions. Pin substrate was slide against steel disc (61 HRC) rotating at 120 rpm, for 2653 counts, under 5 N load and 60 mm wear track diameter. Linear sliding speed and sliding distance of substrate were come out to be 0.38 m/s and 500 m respectively. The loss in wear volume loss (mm3) was calculated by using the equation:
2 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
Metal Powder Report
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ph 3d 2 Loss in pin volume ¼ þ h2 6 4
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! ð2Þ
h i 2 1=2 where h ¼ r r 2 d4 and ‘d’ = wear scar diameter (mm), ‘r’ = pin end radius (mm), ‘h’ = wear scar depth (mm).
3. Results and discussion The thickness of EL coatings was computed to be 6.11 mm and the rate of deposition was 12.23 mm/h.
3.1. Surface morphology and composition The hemispherical globules of Ni-P and Ni-P-graphite coatings (shown in Fig. 2) indicate that the coating on the substrate was initiated at the catalytic region, then having the lateral development followed by vertical development of deposits. The agglomerations of deposits begin during vertical growth, leading to the formation of hemispherical shape globules. Heat treatment refines the grain structure of both plain and nanocomposite coatings. The coatings become more uniform and pore-free. It was found that the morphology of electroless coatings depends upon heat treatment temperature as well as the time of heating. The EDAX analysis reveals the elemental composition and purity of Ni-P and Ni-P-graphite coatings as depicted in Fig. 3 and their numerical values are given in Table 2. The presence of oxygen in nanocomposite coating may be due to atmospheric
oxygen in the surrounding. The atomic % of Ni was 91.39% in a plain coating which gets reduced to 73.59% in nanocomposite coating, while its wt. % reduced from 95.26% to 91.17%. A little reduction in wt. % of Ni can be attributed to its much higher density than graphite.
3.2. XRD analysis The XRD behaviors of as-received and milled graphite are given in Fig. 4(a). The XRD pattern of as received graphite particles reveal a large bump distributed in a wide range of diffraction angle (2h). Low-intensity peaks at a diffraction angle of 26.8° and 77.91° (JCPDS Ref. No. 00-002-0456) indicates the amorphous nature of as-received graphite. While milled graphite particles show intense peaks at a diffraction angle of 26.61° and 72.57°. The intense peaks indicate the crystalline phase of milled graphite particles. The crystallite size of graphite particles was calculated to be 20 nm. It was observed that graphite particles were trapped between extremely kinetic balls and a vial’s internal surface, causing repeated rewelding, deformation and fragmentation. The motion of mechanical milling rely on the transfer of energy from the ball to the powder throughout the milling process. The major variables involved in energy transfer are size of balls, size distribution of balls, speed of milling, milling temperature and milling time [18]. The X-ray pattern of Al substrate and EL coatings is shown in Fig. 4(b). Each coating shows a predominant peak at 2h = 44.5° which conforms to nickel (JCPDS Ref. No. 00-001-1260). A
FIGURE 2
FESEM micrographs of electroless coatings (a) Ni-P (b) Ni-P (HT) (c) Ni-P-graphite (d) Ni-P-graphite (HT). 3 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
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FIGURE 3
EDAX (a) Ni-P coating (b) Ni-P-graphite coating.
TABLE 2
Element composition for Ni-P and Ni-P-graphite coatings. Coating
Element
Wt. %
At. %
Ni-P
Ni P
95.26 4.74
91.39 8.61
Ni-P-graphite
Ni C P O
91.17 4.95 3.21 0.67
73.59 19.53 4.91 1.98
broadening of peak shows the amorphous nature of the coating, owing to nickel crystal lattice distortion by phosphorus atoms. Apart from a single broad peak, three more peaks were also observed at diffraction angles of 38.23°, 64.9° and 78.03°
showing the reflection of Al (JCPDS Ref. No. 00-002-1109) from the substrate material. El Ni-P-graphite nanocomposite coatings show one additional peak of Ni at a diffraction angle 51.6°. The intensity of peak at diffraction angle of 44.5° is much higher in heat-treated conditions for plain as well as nanocomposite coatings. Thus heat treatment improved the height to breadth ratio of a broad XRD peak at Ni (1 1 1) for both coatings. It shows that grains of Ni-P get refined due to heat treatment and its phase structure tends to make a shift towards the crystalline phase.
3.3. Hardness study In as-coated environment, the microhardness of electroless Ni-P coating was found to be 317 HV50 (Fig. 5). Thus, coating increases the hardness of the aluminum substrate by more than
FIGURE 4
XRD pattern (a) as-received and milled graphite powder and (b) Ni-P and Ni-P-graphite in as-coated and HT environment. 4 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
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FIGURE 5
Hardness of EL coatings in as-coated and HT conditions.
4 times. Heat treatment at 220 °C for 4 h increases the hardness up to 395 HV50. However, the hardness of Ni-P-graphite nanocomposite coatings was 248 HV50. The uniform dispersion of smooth graphite particles in the NiP matrix is the reason for the reduction in hardness of nanocomposite coating. The hardness of Ni-P-graphite further increases to 339 HV50 after heat treatment. The heat treatment refines the grain structure and improves the plastic deformation of deposits that resulted in increased hardness of plain as well as nanocomposite coatings.
3.4. Tribological study The wear resistance behavior of Al substrate and EL coatings is shown in Fig. 6(a). The wear of Al was high at the beginning and continues to fluctuate till 600 s after that wear rate slightly decreased. This can occur due to the development of a protective film of debris [19]. As coated Ni-P alloy coating has shown better wear performance over Al substrate. The initial wear of Ni-Pgraphite nanocomposite coating was greater than plain Ni-P alloy coating, but once a protective layer of nano graphite particles has formed, it reduced the adhesion between the mating surfaces and ultimately resulted in less wear than that of plain Ni-P alloy coating. Ni-P-graphite nanocomposite coating has shown a steady wear performance than Al substrate and plain coating; this may be due to the formation of the selflubricating nano graphite layer. Heat-treated Ni-P alloy coating and Ni-P-graphite nanocomposite coating showed negative wear at the beginning that may be due to thermal expansion of the substrate. In HT plain and nanocomposite coatings wear started after 75 s and 120 s respectively. As time progressed nanocomposite coating has shown much excellent wear resistance than that of as coated plain coating due to the formation of a self-lubricating layer of nano graphite particles. The significant enhancement in wear resistance of heattreated coatings can be recognized to increased plastic defor-
FIGURE 6
Wear graph for Al substrate and EL coatings (a) Wear vs. time (b) Co-efficient of friction vs. time.
mation and hardness, which acts as a barrier against material removal [20]. The graphs of co-efficient of friction w.r.t. time is shown in Fig. 6(b). The coefficient of friction was almost same for Al substrate and as coated EL coatings; however, it decreases in nanocomposite coatings which resulted in their low wear rate. The outcomes of wear volume loss are presented in Table 3. The values of wear scar diameter and loss in wear volume are significantly less with the coated substrates, especially for nanocomposite coatings in both as-coated and HT conditions. The formation of self-lubricating nano graphite layer turns out to be the major parameter for the excellent wear performance of nanocomposite coatings. 5
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TABLE 3
Results of wear volume loss. Substrate
Pin end radius (mm)
Wear scar diameter (mm)
Wear scar depth (mm)
Wear volume loss (mm3)
Al Ni-P as coated Ni-P-Cg as coated Ni-P (heat treated) Ni-P-Cg (heat treated)
3.15 3.15 3.15 3.15 3.15
2 1.5 1.3 0.9 0.7
0.1629 0.0905 0.0678 0.0323 0.0195
0.2582 0.0804 0.0451 0.0103 0.0038
4. Conclusion
References
Graphite nanoparticles (20 nm) are synthesized by mechanical milling. The deposition of Ni-P alloy coating and Ni-P-graphite nanocomposite coating shows hemispherical globules on the surface of the substrate. Phase analysis of EL coatings by XRD confirms the presence of nickel and EDAX reveals the purity of coatings. The hardness of nanocomposite coatings is less than plain coatings. Heat treatment (220 °C for 4 h) refines the grain structure of coatings and increases the plastic deformation of deposits, thus enhance the hardness property. The frictional coefficient heightens with the heat treatment of coatings. The Ni-P-graphite nanocomposite coatings demonstrate excellent wear resistance over Ni-P coatings in both as coated and HT conditions due to the self-lubricating property of graphite particles.
[1] R.C. Agarwala, V. Agarwala, Sadhana 28 (3–4) (2003) 475–493. [2] Y.S. Huang, X.T. Zeng, I. Annergren, F.M. Liu, Surf. Coat. Technol. 167 (2–3) (2003) 207–211. [3] R.C. Agarwala, V. Agarwala, R. Sharma, Synth. React. Inorg. Met-Org. Nano-Met. Chem. 36 (6) (2006) 493–515. [4] S. Alirezaei, S.M. Monirvaghefi, M. Salehi, A. Saatchi, Wear 262 (7–8) (2007) 978–985. [5] J.N. Balaraju, T.S. Narayanan, S.K. Seshadri, J. App. Electrochem. 33 (9) (2003) 807–816l. [6] S.M. Vaghefi, A. Saatchi, M. EbrahimianHoseinabadi, Surf. Coat. Technol. 168 (2–3) (2003) 259–262. [7] Z.A. Hamid, S.A. El Badry, A.A. Aal, Surf. Coat. Technol. 201 (12) (2007) 5948– 5953. [8] E. Pena-Munoz, P. Bercot, A. Grosjean, M. Rezrazi, J. Pagetti, Surf. Coat. Technol. 107 (2–3) (1998) 85–93. [9] M.W. Mahoney, P.J. Dynes, Scr. Metall. 19 (4) (1985) 539–542. [10] P. Makkar, R.C. Agarwala, V. Agarwala, Adv. Powder Technol. 25 (5) (2014) 1653–1660. [11] G.H. Zhou, H.Y. Ding, F. Zhou, Y. Zhang, J. Iron Steel Res. Int. 15 (1) (2008) 65–69. [12] W.X. Chen, J.P. Tu, H.Y. Gan, Z.D. Xu, Q.G. Wang, J.Y. Lee, Z.L. Liu, X.B. Zhang, Surf. Coat. Technol. 160 (1) (2002) 68–73. [13] Y.Y. Liu, J. Yu, H. Huang, B.H. Xu, X.L. Liu, Y. Gao, X.L. Dong, Surf. Coat. Technol. 201 (16–17) (2007) 72467251. [14] H. Ma, Z. Liu, F. Tian, H. Sun, J. Alloys Compds. 450 (1–2) (2008) 348–351. [15] H. Xu, Z. Yang, M.K. Li, Y.L. Shi, Y. Huang, H.L. Li, Surf. Coat. Technol. 191 (2–3) (2005) 161–165. [16] M. Izzard, J.K. Dennis, Trans. IMF 65 (1) (1987) 8589. [17] A. Sharma, A.K. Singh, J. Mater. Eng. Perform. 22 (1) (2013) 176–183. [18] T.P. Yadav, R.M. Yadav, D.P. Singh, Nanosci. Nanotechnol. 2 (3) (2012) 2248. [19] S.J. Harris, M.P. Overs, A.J. Gould, Wear 106 (1–3) (1985) 35–52. [20] I. Apachitei, J. Duszczyk, Surf. Coat. Technol. 132 (1) (2000) 89–98.
5. Research directions In the future, the tribological properties can be optimized for different weight proportions of graphite nanoparticles reinforced in Ni-P coatings. In addition to this, the optimum value of wear and microhardness of nanocomposite coatings can be obtained by heat treatment (annealing) at a higher temperature (above 300 °C) and by adding hard particles like oxides and carbides with graphite nanoparticles.
Acknowledgement The authors are grateful for providing research facilities to The Director, CSIR-Central Scientific Instruments Organisation, Chandigarh.
6 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
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metal-powder.net
SPECIAL FEATURE
Tribological study of mechanically milled graphite nanoparticles codeposited in electroless Ni-P coatings I.S. Thakur a, V.S. Pandey b, P.S. Rao a, S. Tyagi b, Deepam Goyal a,⇑ a b
Department of Mechanical Engineering, National Institute of Technical Teachers Training and Research, Chandigarh, India Central Scientific Instruments Organisation, CSIR-CSIO, Chandigarh, India
The present study focusses to enhance the wear resistance of electroless Ni-P coatings with the codeposition of selflubricating nano graphite particles synthesized by mechanical milling using a high energy planetary ball mill. The coatings were developed on an aluminum substrate and nano graphite particles were co-deposited (4 g/l) into the Ni-P matrix using alkaline hypophosphite reduced electroless bath. Changes in properties after heat treatment at 220 °C for 4 h in vacuum have been correlated with morphological and microstructural changes shown by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) respectively. Energy dispersive analysis of X-ray (EDAX) determined the quality and elemental composition of coatings. Wear resistance, coefficient of friction (pin on disc) and microhardness of electroless Ni-P-graphite nanocomposite coatings were calculated and contrasted with Ni-P coatings. The outcomes reveal that the size of synthesized graphite nanoparticles is 20 nm. Microhardness and wear resistance of electroless coatings have improved after heat treatment. Excellent wear resistance has been observed at the cost of some hardness in Ni-Pgraphite nanocomposite coatings over Ni-P coatings. 1. Introduction Electroless nickel coating is one of the favorable surface engineering technology that is commonly used in the automotive, aviation and chemical industries due to its inherent properties like a high hardness, uniform deposition, and excellent wear and corrosion resistance etc. [1,2]. To improve the mechanical and tribological properties, various micron-sized second phase hard/soft particles have been successfully reinforced into Ni-P matrix [3–8]. The transition of deposits from amorphous to the crystalline structure during heat treatment is a function of time, temperature, heating rate and phosphorus content [9]. Recent efforts have been made to incorporate nanoparticles like TiO2, Al2O3, CNTs, WC, Au and diamond etc. into Ni-P matrix to further enhance the mechanical and tribological properties [10–15]. It is well known that graphite particles have been used in many industries for the development of composite coatings due to its good self-lubricating and antisticking properties, chemical inert⇑ Corresponding author. E-mail address: Goyal, D. ([email protected])
ness and low coefficient of friction. Izzard and Dennis have successfully reinforced micron-sized graphite particles into a Ni-P matrix; however, the studies on incorporation of nano-sized graphite particles into Ni-P alloy coatings are not reported in the literature [16]. In the current study, an attempt has been made to explore the self-lubricating properties of nano-sized graphite particles for improving wear resistance of electroless (EL) coatings. To develop EL Ni-P-graphite nanocomposite coatings, the embedded second phase nano-sized graphite particles were first synthesized by mechanical alloying method for 20 h and then co-deposited with Ni-P on an aluminum substrate. Further, coatings were studied for surface morphology, phase identification, microhardness and wear resistance in both as-coated and heat-treated (HT) conditions.
2. Experimental 2.1. Synthesis of nano graphite particle Graphite nanoparticles were synthesized by the mechanical milling using a high-energy planetary ball mill (Retsch, PM
0026-0657/Ó 2020 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.mprp.2019.12.001
Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
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100) with a ball-to-powder ratio of 10:1 at a speed of 200 rpm. A grinding jar and agate balls of 10 mm and 20 mm diameter were used as milling media. A graphite powder of 50 mm size was used as a starting charge for milling. In grinding work, 500 ml jar was utilized to mill the 20 g graphite powder. The milling of graphite particles was carried out for 20 h, with periodic milling time of 2 h. An interval of 15 min was given to prevent the overheating of milling media.
2.2. Preparation of electroless coatings Electroless Ni-P alloy coatings and Ni-P-graphite nanocomposite coatings were deposited on an aluminum substrate for 30 min. The bath composition for EL deposition is shown in Table 1. Before coating, the aluminum substrate was polished with emery papers of 320, 400 and 600 no. respectively. Polished substrate (Flat = 20 20 2.5 mm; pin = £ 6.3 mm, 32 mm long) was degreased by dipping into acetone for 30 s and etched by dipping into dilute HCl for 30 s. The substrate was sensitized by immersing in 0.1% SnCl2 solution for 120 s and activated by immersing in 0.01% PdCl2 solution for 30 s. Following each step of surface preparation and pretreatment, the substrate was rinsed in distilled water and dried air. The EL coating was developed by immersing the activated substrate into the electroless bath at temperature 90 ± 2 °C and pH 9 ± 0.2. Ammonia was added continuously to sustain the pH of the bath. The detailed experimental procedure is illustrated in Fig. 1. To obtain nanocomposite coatings, a deposit of electroless nickel was plated for 10 min before adding the nano graphite (20 nm) particles into the bath and stirred continuously for their suspension. The thickness (mm) of the coating was calculated by using Eq. (1): t¼
W 104 qA
ð1Þ
where ‘q’ deposits density (7.75 g/cm3), ‘W’ is weight gain (g), and ‘A’ is the deposition surface area (cm2). The rate of deposition (mm/h) was calculated as the coating thickness per unit deposition time [17]. Both plain and nanocomposite coatings were heat treated in vacuum at 220 °C for 4 h to enhance their mechanical properties.
2.3. Characterization Phase identification of graphite particles and EL coatings in ascoated and HT conditions were done by X-ray diffraction (XRD) using Bruker AXS D8 diffractometer with Cu-Ka radiation. The crystallite size of the milled graphite was computed using
TABLE 1
Bath composition for EL coatings. Chemical compound/Element
Quantity (g/l)
Nickel sulfate Sodium hypophosphite Trisodium citrate Ammonium sulfate Ammonium chloride Graphite*
33 20 84 25 25 4
* Used only for nanocomposite coatings.
FIGURE 1
Experimental set up for EL coatings.
Scherrer’s formula. Field emission scanning electron microscopy (FESEM, QUANTA FEG 200 FEI) and energy dispersive analysis of X-ray (EDAX) characterizes the morphology and element composition of coatings respectively.
2.4. Hardness and wear tests The microhardness of Ni-P plain and Ni-P-graphite nanocomposite coatings in as-coated and HT condition were measured with Vickers hardness indenter (Mitutoyo Hardness Testing Machine, HM) utilizing a load of 50 gf for 10 s. The microhardness was measured as a mean value of three measurements. The wear study of coated and uncoated aluminum substrates was conducted by using disc or pin tester (Ducom, TR-20LE) in dry conditions. Pin substrate was slide against steel disc (61 HRC) rotating at 120 rpm, for 2653 counts, under 5 N load and 60 mm wear track diameter. Linear sliding speed and sliding distance of substrate were come out to be 0.38 m/s and 500 m respectively. The loss in wear volume loss (mm3) was calculated by using the equation:
2 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
Metal Powder Report
d
Volume xx, Number xx
ph 3d 2 Loss in pin volume ¼ þ h2 6 4
d
SPECIAL FEATURE
xxxx 2020
! ð2Þ
h i 2 1=2 where h ¼ r r 2 d4 and ‘d’ = wear scar diameter (mm), ‘r’ = pin end radius (mm), ‘h’ = wear scar depth (mm).
3. Results and discussion The thickness of EL coatings was computed to be 6.11 mm and the rate of deposition was 12.23 mm/h.
3.1. Surface morphology and composition The hemispherical globules of Ni-P and Ni-P-graphite coatings (shown in Fig. 2) indicate that the coating on the substrate was initiated at the catalytic region, then having the lateral development followed by vertical development of deposits. The agglomerations of deposits begin during vertical growth, leading to the formation of hemispherical shape globules. Heat treatment refines the grain structure of both plain and nanocomposite coatings. The coatings become more uniform and pore-free. It was found that the morphology of electroless coatings depends upon heat treatment temperature as well as the time of heating. The EDAX analysis reveals the elemental composition and purity of Ni-P and Ni-P-graphite coatings as depicted in Fig. 3 and their numerical values are given in Table 2. The presence of oxygen in nanocomposite coating may be due to atmospheric
oxygen in the surrounding. The atomic % of Ni was 91.39% in a plain coating which gets reduced to 73.59% in nanocomposite coating, while its wt. % reduced from 95.26% to 91.17%. A little reduction in wt. % of Ni can be attributed to its much higher density than graphite.
3.2. XRD analysis The XRD behaviors of as-received and milled graphite are given in Fig. 4(a). The XRD pattern of as received graphite particles reveal a large bump distributed in a wide range of diffraction angle (2h). Low-intensity peaks at a diffraction angle of 26.8° and 77.91° (JCPDS Ref. No. 00-002-0456) indicates the amorphous nature of as-received graphite. While milled graphite particles show intense peaks at a diffraction angle of 26.61° and 72.57°. The intense peaks indicate the crystalline phase of milled graphite particles. The crystallite size of graphite particles was calculated to be 20 nm. It was observed that graphite particles were trapped between extremely kinetic balls and a vial’s internal surface, causing repeated rewelding, deformation and fragmentation. The motion of mechanical milling rely on the transfer of energy from the ball to the powder throughout the milling process. The major variables involved in energy transfer are size of balls, size distribution of balls, speed of milling, milling temperature and milling time [18]. The X-ray pattern of Al substrate and EL coatings is shown in Fig. 4(b). Each coating shows a predominant peak at 2h = 44.5° which conforms to nickel (JCPDS Ref. No. 00-001-1260). A
FIGURE 2
FESEM micrographs of electroless coatings (a) Ni-P (b) Ni-P (HT) (c) Ni-P-graphite (d) Ni-P-graphite (HT). 3 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
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FIGURE 3
EDAX (a) Ni-P coating (b) Ni-P-graphite coating.
TABLE 2
Element composition for Ni-P and Ni-P-graphite coatings. Coating
Element
Wt. %
At. %
Ni-P
Ni P
95.26 4.74
91.39 8.61
Ni-P-graphite
Ni C P O
91.17 4.95 3.21 0.67
73.59 19.53 4.91 1.98
broadening of peak shows the amorphous nature of the coating, owing to nickel crystal lattice distortion by phosphorus atoms. Apart from a single broad peak, three more peaks were also observed at diffraction angles of 38.23°, 64.9° and 78.03°
showing the reflection of Al (JCPDS Ref. No. 00-002-1109) from the substrate material. El Ni-P-graphite nanocomposite coatings show one additional peak of Ni at a diffraction angle 51.6°. The intensity of peak at diffraction angle of 44.5° is much higher in heat-treated conditions for plain as well as nanocomposite coatings. Thus heat treatment improved the height to breadth ratio of a broad XRD peak at Ni (1 1 1) for both coatings. It shows that grains of Ni-P get refined due to heat treatment and its phase structure tends to make a shift towards the crystalline phase.
3.3. Hardness study In as-coated environment, the microhardness of electroless Ni-P coating was found to be 317 HV50 (Fig. 5). Thus, coating increases the hardness of the aluminum substrate by more than
FIGURE 4
XRD pattern (a) as-received and milled graphite powder and (b) Ni-P and Ni-P-graphite in as-coated and HT environment. 4 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
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FIGURE 5
Hardness of EL coatings in as-coated and HT conditions.
4 times. Heat treatment at 220 °C for 4 h increases the hardness up to 395 HV50. However, the hardness of Ni-P-graphite nanocomposite coatings was 248 HV50. The uniform dispersion of smooth graphite particles in the NiP matrix is the reason for the reduction in hardness of nanocomposite coating. The hardness of Ni-P-graphite further increases to 339 HV50 after heat treatment. The heat treatment refines the grain structure and improves the plastic deformation of deposits that resulted in increased hardness of plain as well as nanocomposite coatings.
3.4. Tribological study The wear resistance behavior of Al substrate and EL coatings is shown in Fig. 6(a). The wear of Al was high at the beginning and continues to fluctuate till 600 s after that wear rate slightly decreased. This can occur due to the development of a protective film of debris [19]. As coated Ni-P alloy coating has shown better wear performance over Al substrate. The initial wear of Ni-Pgraphite nanocomposite coating was greater than plain Ni-P alloy coating, but once a protective layer of nano graphite particles has formed, it reduced the adhesion between the mating surfaces and ultimately resulted in less wear than that of plain Ni-P alloy coating. Ni-P-graphite nanocomposite coating has shown a steady wear performance than Al substrate and plain coating; this may be due to the formation of the selflubricating nano graphite layer. Heat-treated Ni-P alloy coating and Ni-P-graphite nanocomposite coating showed negative wear at the beginning that may be due to thermal expansion of the substrate. In HT plain and nanocomposite coatings wear started after 75 s and 120 s respectively. As time progressed nanocomposite coating has shown much excellent wear resistance than that of as coated plain coating due to the formation of a self-lubricating layer of nano graphite particles. The significant enhancement in wear resistance of heattreated coatings can be recognized to increased plastic defor-
FIGURE 6
Wear graph for Al substrate and EL coatings (a) Wear vs. time (b) Co-efficient of friction vs. time.
mation and hardness, which acts as a barrier against material removal [20]. The graphs of co-efficient of friction w.r.t. time is shown in Fig. 6(b). The coefficient of friction was almost same for Al substrate and as coated EL coatings; however, it decreases in nanocomposite coatings which resulted in their low wear rate. The outcomes of wear volume loss are presented in Table 3. The values of wear scar diameter and loss in wear volume are significantly less with the coated substrates, especially for nanocomposite coatings in both as-coated and HT conditions. The formation of self-lubricating nano graphite layer turns out to be the major parameter for the excellent wear performance of nanocomposite coatings. 5
Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001
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TABLE 3
Results of wear volume loss. Substrate
Pin end radius (mm)
Wear scar diameter (mm)
Wear scar depth (mm)
Wear volume loss (mm3)
Al Ni-P as coated Ni-P-Cg as coated Ni-P (heat treated) Ni-P-Cg (heat treated)
3.15 3.15 3.15 3.15 3.15
2 1.5 1.3 0.9 0.7
0.1629 0.0905 0.0678 0.0323 0.0195
0.2582 0.0804 0.0451 0.0103 0.0038
4. Conclusion
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Graphite nanoparticles (20 nm) are synthesized by mechanical milling. The deposition of Ni-P alloy coating and Ni-P-graphite nanocomposite coating shows hemispherical globules on the surface of the substrate. Phase analysis of EL coatings by XRD confirms the presence of nickel and EDAX reveals the purity of coatings. The hardness of nanocomposite coatings is less than plain coatings. Heat treatment (220 °C for 4 h) refines the grain structure of coatings and increases the plastic deformation of deposits, thus enhance the hardness property. The frictional coefficient heightens with the heat treatment of coatings. The Ni-P-graphite nanocomposite coatings demonstrate excellent wear resistance over Ni-P coatings in both as coated and HT conditions due to the self-lubricating property of graphite particles.
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5. Research directions In the future, the tribological properties can be optimized for different weight proportions of graphite nanoparticles reinforced in Ni-P coatings. In addition to this, the optimum value of wear and microhardness of nanocomposite coatings can be obtained by heat treatment (annealing) at a higher temperature (above 300 °C) and by adding hard particles like oxides and carbides with graphite nanoparticles.
Acknowledgement The authors are grateful for providing research facilities to The Director, CSIR-Central Scientific Instruments Organisation, Chandigarh.
6 Please cite this article in press as: I.S. Thakur et al., Met. Powder Rep. (2020), https://doi.org/10.1016/j.mprp.2019.12.001