ES Materials & Manufacturing, 2018, 2, 2-8
Published online: 29 Oct 2018
Received 30 Aug 2018, Accepted 28 Oct 2018
Antony Joseph, Mugilan Narayanasamy, Balakrishnan Kirubasankar and Subramania Angaiah*
Electro-Materials Research Laboratory, Centre for Nanoscience and Technology,
Pondicherry University, Puducherry - 605014, India
MoS2 nanosheets were prepared by a facile hydrothermal method and its morphology and structural properties were characterized by FE-SEM and XRD analysis. MoS2 nanosheets reinforced Ni composite coatings were prepared by a simple electrodeposition method using various concentrations of MoS2 in Watts nickel plating bath. Surface morphology, microstructure and crystal orientation of pure Ni and Ni- MoS2 composite coatings were characterized by FE-SEM and XRD analysis. Microhardness and wear resistance of pure Ni coating were improved by the addition of MoS2 nanosheets in the Ni matrix to use as solid lubricant.
Table of Content
Ni-MoS2 nanocomposite coating shows higher microhardness than other Ni based composite coatings.
Keywords: Electrodeposition, Ni-MoS2 composite Coating, MoS2 Nanosheets, Liquid exfoliation, Microhardness.
Electrodeposition is a simple and efficient method to produce metallic coatings with improved surface properties such as hardness, wear resistance and corrosion resistance.1 Nickel is known to be a good electrodeposited metal because of its high mechanical properties and compatibility with most of the substrate materials. Electrodeposited Ni has high density, excellent wear and anti-corrosion resistance and potential substitute for hard chromium coatings. As the grain size of the Ni reduces, the strength and strain hardening rate also increases. Electrodeposited nickel and nickel composites are emerging as a good replacement for environmentally harmful and carcinogenic hexavalent hard chrome coating.2–6 Electrodeposited metal matrix nanocomposite coatings exhibit enhanced material properties such as microhardness, wear and corrosion resistance than the pure metal coatings. These enhanced properties depend mainly on the nature of deposited nanofillers and the distribution of nanofillers in the metallic matrix. However, it is difficult to achieve a stable homogenous dispersion of deposited fillers in the metallic matrix. Several researchers have deposited different nanofillers such as transition metal oxides, rare earth metal oxides, and carbon-based materials embedded nickel composite coatings to enhance the microhardness, wear, and corrosion resistance.7–16 Venkatesh et al. studied the mechanical and corrosion properties of Ni-Graphene nanocomposite coating and improved microhardness and corrosion resistance than pure Ni coating.17
After the successful exfoliation of graphene, 2D materials have been received much attention. Transition metal dichalcogenides (TMDs) such as MoS2, MoSe2, WS2, Wse2, WTe2 etc are important 2D materials and alternatives for graphene in semiconducting applications due to their large intrinsic bandgap.18 Molybdenum disulfide (MoS2) has Mo layer sandwiched between two layers of S (S-Mo-S) and stacked into a 3D arrangement by weak Van der Waals forces. Such layered arrangements allow the MoS2 layers to easily shear between the basal planes and are responsible for the excellent lubricity of MoS2 and thereby utilized as a solid lubricant. The in-plane stability of graphene is responsible for its extraordinary mechanical properties which are utilized in metal matrix composites. This in-plane stable structure is common for all other 2D materials. But other 2D materials, especially MoS2 is underutilized for reinforcement purposes. Previous Researches have shown that 2D MoS2 possess high mechanical properties than their bulk form. Castellano-Gomez et al. measured the elastic properties of freely suspended few-layer MoS2 nanosheets (5 to 25 layers) in a bending test performed using AFM. The Young’s modulus value for MoS2 nanosheets obtained from the experiment is E = 0.33 ± 0.07 TPa, which is very high and only one third lower than exfoliated graphene (E = 0.8–1.0 TPa), which is higher than graphene oxide (0.2 TPa) and bulk MoS2 (0.24 TPa). These results showed that few-layers MoS2 can be effectively used as a reinforcement for metal matrix composites and as an alternative for graphene.19 Recently, He et al. prepared MoS2 nanoparticles embedded Ni-P composite coating and studied its mechanical and wear resistance properties.20 But, there are no report on MoS2 nanosheets embedded nickel composite coatings. Hence, in the present investigation, we prepared few layers of MoS2 nanosheets by hydrothermal method and codeposited with Ni matrix by simple electrodeposition technique. Its microhardness and wear resistance are studied in detail and compared with MoS2 nanoparticles embedded Ni composite and pure Ni coatings.
Chemicals and reagents - All the chemicals and reagents were extra pure. Ammonium molybdate (NH4)6⸱Mo7O24), thiourea (CH4N2S), boric acid and sodium dodecyl sulphate were obtained from Merck, India. Nickel sulphate and nickel chloride were purchased from Hi-media, India. Deionized water is used throughout the process. All chemical and reagents were of analytical grade
Synthesis of MoS2 nanosheets - MoS2 nanosheets were prepared by a facile hydrothermal method using ammonium molybdate ((NH4)6⸱Mo7O24) and thiourea as the Mo and S precursors, respectively. 4.96 g of (NH4)6⸱Mo7O24 and 9.12 g of thiourea were dissolved in 150 ml deionized water and stirred for 30 min. This solution was transferred into a 200 mL Teflon-lined stainless steel autoclave, sealed and is heated in an oven at 220 oC for 24 h and then allowed to cool down to room temperature. The product was then collected, centrifuged, washed with water and ethanol and dried in vacuum oven at 70 oC for 12 h.
Characterization of MoS2 nanosheets - The surface morphology of prepared MoS2 nanosheets were analyzed by FE-SEM (JSM, JEOL Model: 7600F) analysis. X-ray diffraction analysis was carried out to confirm the formation of few layer MoS2. XRD pattern was recorded using X-ray diffractometer (Rigaku, Ultima IV, Japan) with Cu-K radiation ( = 1.54 Ao) and its 2 value ranging from 10o to 80o.
Electrodeposition of Ni-MoS2 composite coatings - Electrodeposition of pure Ni and Ni-MoS2 nanocomposite coatings were carried out by direct current on mild steel substrates (2.5 × 2.5 cm2) using Watt’s nickel bath. The bath composition and electrodeposition parameters are given in Table 1. Nickel metal and mild steel substrate were used as the anode and cathode, respectively. Prior to plating, the mild steel substrates were mechanically polished using different grade emery papers, degreased with acetone, cleaned with soap solution and rinsed with distilled water. They were then dipped in 10 % H2SO4 solution for 10 sec and immediately transferred to the plating bath. MoS2 nanosheets were dispersed in the plating bath to obtain Ni-MoS2 composite coating. Deposition was carried out by the addition of different concentrations of MoS2 in the bath and also by varying the stirring rate of the bath. Prior to electrodeposition, the bath solution was stirred using magnetic stirrer at 500 rpm for 24 h and subsequently ultrasonicated for 15 min to ensure an uniform dispersion of nanosheets in the plating bath.
Characterization of Ni-MoS2 composite coatings - The XRD and FE-SEM studies were done on pure Ni and Ni-MoS2 composite coating with an optimized concentration of MoS2 and stirring rate of 1 g/L and 400 rpm, respectively. The surface morphology for pure Ni and Ni-MoS2 composite coating were analyzed using Field Emission Scanning Electron Microscope (JSM, JEOL Model: 7600F). XRD pattern of the coatings were recorded using X-ray diffractometer (Rigaku, Ultima IV, Japan) with = 1.54 Ao and 2 ranging from 10o to 80o to study the phase purity and crystal structure. The average crystallite size of nickel and nickel composite coatings were calculated using the following Scherrer equation (Eq. 1);
where D is the average crystallite size, K is the shape factor (typically 0.89), is the wavelength of incident X-ray radiation, is the full width half maxima (FWHM) or the peak width (radians) at half of the maximum intensity and is the Bragg’s angle (degree). The preferred crystal orientation of the coatings were calculated from the diffractogram data using the relative texture coefficient (RTC) relation given in Eq. 2.
where, and are the diffraction intensity of (hkl) planes of the coating and the standard Ni powder sample, respectively.
Williamson-Hall method from the line broadening in XRD pattern was used to confirm the crystallite size of Ni and to identify the lattice strain during electrodeposition.21 Broadening of X-ray diffraction peaks is mainly due to three factors: instrumental broadening, crystallite size, and lattice strain. In this method, it is assumed that the X-ray diffraction peak is a convolution of Lorentzian curve (influence of grain size) and Gaussian curve (broadening due to the strain).22 Whereas, instrumental broadening error is not considered. Williamson and Hall suggested a relation for the FWHM of the profile which can be expressed as follows;
where, D and are volume weighted average crystallite size and lattice strain, respectively. Williamson-Hall plot is drawn between 4sin at the x-axis and at the y-axis. The slope and y-intercept of the plot give the values of lattice strain and crystallite size, respectively.
Microhardness of the coatings were analyzed using Vickers microhardness tester (Wilson Wolpert, Germany) at a load of 0.5 Kg for a dwell time of 20 sec. Friction measurements were carried out by employing pin (8 mm diameter) on disc (high carbon high chromium steel) tribometer (Ducom instruments, India) at 5 N load with a sliding velocity of 0.5 m/s.
Characterization of MoS2 nanosheets - The XRD pattern of prepared MoS2 nanosheets is shown in Figure 1(a). The four distinct and clear peaks having planes (002), (100), (103) and (110) can be indexed to the standard powder diffraction file of hexagonal phase of bulk MoS2 (JCPDS 37-1492). No characteristic other impurity peaks are found indicating that the prepared MoS2 nanosheets are highly pure. The broad peaks suggest that the crystallite size was at the nanolevel. The most prominent peak having the (002) plane shows a shift towards the lower angle with a 2θ value of 14.050 which is attributed to the lattice expansion and introduction of strain.23 The interlayer distance has been calculated using the Bragg’s equation and is found to be 6.30 Å which is greater than the bulk MoS2 (6.15 Å) having lattice parameters, a = 3.15 Å and c = 12.30 Å.24 The increase in interlayer spacing confirms the formation of few layers of MoS2 nanosheets. FE-SEM image reveals that MoS2 is exfoliated into few layers of nanosheets [Figure 1(b)].
Figure 1 (a) XRD pattern of MoS2, (b) FE-SEM photograph of few layer of MoS2 nanosheets
Effect of concentration of MoS2 - Figure 2(a) represents the effect of concentration of MoS2 nanosheets on codeposition of MoS2. The Ni-MoS2 nanocomposite coatings are prepared by varying the concentrations of MoS2 in the plating bath (0.5, 1.0 and 1.5 g/L) at constant current density of 2.5 A/dm2 with stirring rate of 300 rpm. The amount of codeposition of MoS2 is found to be increased with increase in MoS2 concentration. A maximum codeposition of 15 wt % is observed for the bath having 1 g/L concentration of MoS2 nanosheets. The maximum codeposition of MoS2 in the nickel matrix refers to the adsorption on the cathode surface which can be explained by Guglielmi’s two-step adsorption model.25,26 At higher concentration (1.5 g/L), the amount of MoS2 codeposition slightly reduced due to saturation in adsorption on the cathode surface.
Effect of stirring rate - The relationship between the weight percentage (wt %) of codeposited MoS2 and the stirring rate is shown in Figure 2(b). With increase in stirring rate, the codeposition of MoS2 increases and reached a maximum value of 17 wt % at 400 rpm. Further, increase in agitation rate reduces the codeposition due to collision factor. Another reason may be the increased streaming velocity of the bath sweep away the loosely bound particles from the cathode surface and make the rate of particle removal higher than the rate of adsorption in the cathode surface which results in a decrease in the amount of codeposited MoS2 in the nickel matrix.22 Thus, the stirring rate for maximum amount of codeposition of MoS2 is optimized and is found to be 400 rpm.
Figure 2 (a) Effect of concentration of MoS2 on codeposition of MoS2, (b) Influence of stirring rates on codeposition of MoS2 nanosheets
FE-SEM studies - Surface morphologies of the pure Ni and Ni-MoS2 composite coatings are analyzed using FE-SEM and the results are shown in Figure 3. The surface of pure Ni shows uniform structure with higher grain size. Ni-MoS2 coating has uniform deposition with smaller grain. The addition of MoS2 in the nickel matrix has changed the microstructure of the coating. The addition of MoS2 hinders the crystal growth and increases the nucleation sites for the reduction of Ni ions which result in reduced grain size.9,20
Figure 3 FE-SEM images of (a) pure Ni, (b) Ni-MoS2 composite coating.
X-ray diffraction studies - The X-ray diffraction diagrams of pure Ni and Ni-MoS2 composite coatings are shown in Figure 4(a). Three major peaks are visible for pure Ni as well as Ni-MoS2 coatings. However, some differences in the intensity of (111), (200) and (220) peaks are observed in the composite coating. Two peaks of MoS2 are visible which can be attributed to (002) and (006) planes of hexagonal MoS2. The peaks of MoS2 are short due to high relative intensity of Ni peaks. The average crystallite size of Ni deposit was calculated using Scherrer formula (Eq. 1) and is shown in Table 2. From the table, it is evident that the incorporation of MoS2 has reduced the crystallite size of Ni in the composite coating.
The preferred crystal orientation of Ni was calculated using RTC values (Eq. 2) and the results are shown as bar graph in Figure 4(b). The preferred crystal orientation for the pure Ni coating is (220) with the maximum texture coefficient (TC) value of 41%, whereas for Ni-MoS2 coating, the preferred crystal orientation is (111) plane with the maximum TC value of 55%. The incorporation of MoS2 in the Ni matrix inhibit the grain growth of Ni along its preferred direction and change the preferred orientation to different plane.27
Figure 4 (a) XRD patterns of pure Ni and Ni-MoS2 composite coatings, (b) RTC of pure Ni and Ni-MoS2 composite coatings
Figure 5 shows the Williamson-Hall plots of pure Ni and Ni-MoS2 composite coatings. The values of lattice strain and crystallite size are calculated from the plot and are given in Table 2. From the table, it can be seen that the average crystallite size of nickel has reduced with the addition of MoS2 in the Ni matrix. There is an increase in crystallite size obtained from Williamson-Hall method compared to crystallite size calculated using Scherrer’s formula. This is due to the negligence of instrumental broadening error. The lattice strain calculated from the Williamson-Hall method is found to decrease for Ni-MoS2 composite coating compared to pure Ni coating. This shows that there is a stress relaxation when MoS2 is added into the Ni matrix. The MoS2 nanosheets fill the defects and grain boundaries which result in grain boundary relaxation and release of stress.
Figure 5 Williamson-Hall plot for pure Ni and Ni-MoS2 composite coatings
Microhardness - The Vickers microhardness of pure Ni and Ni-MoS2 composite coatings obtained at various concentrations of MoS2 in the Ni matrix are shown in Figure 6 (a). The microhardness of pure Ni is found to be 297 HV. It is tend to increase with the addition of MoS2 in the Ni matrix. At the concentration of 1 g/L of MoS2, it shows the maximum microhardness value of 722 HV. This value is higher than the microhardness value of other electrodeposited Ni composite coatings is shown in Figure 6 (b).17,28–35 At higher concentration of 1.5 g/L of MoS2, the microhardness tend to decrease due to decrease in MoS2 content in the Ni matrix. The increase in microhardness can be attributed to the increase in codeposition of MoS2. The rise in MoS2 content in the Ni matrix has resulted in decrease in grain size. The MoS2 nanosheets adsorbed on the growth centers of Ni which inhibited the growth of nuclei by blocking the surface of growing Ni and thus increasing the rate of nucleation and decreasing the grain size of nickel. More the amount of MoS2 nanosheets, more the number of grains and grain.20 This grain refinement strengthening the material by blocking the dislocation motion and grain boundary sliding.36 The enhancement in microhardness is due to grain refinement that can be explained by the Hall-Petch relation. The experimental result confirms the validation of Hall-Petch effect. But, the validation of Hall-Petch relation and previous studies confirm that grain size has reduced with increase in codeposition of MoS2.9,22
Figure 6 (a)Microhardness of pure Ni and Ni-MoS2 composite coatings at different concentration of MoS2 (0.5, 1.0 and 1.5 g/L) and (b) comparison of microhardness of Ni-MoS2 composite coating with other reported Ni-composite coatings.
Wear resistance – The wear weight loss of pure Ni and Ni-MoS2 composite coatings are calculated by measuring weight of the test samples before and after the wear test. The weight loss was found to be 5.2 mg for pure Ni and 2.8 mg for Ni-MoS2 composite coating. The graph between coefficient of friction versus wear test time for electrodeposited pure Ni and Ni-MoS2 composite coating is shown in Figure 7 (a). The friction coefficient exhibits a lower value for Ni-MoS2 coating compared to pure Ni. For pure Ni coating, the friction coefficient shows a steady increase at first, then comes down to a lower value and keeps stable at ~0.5 with prolong wear test time. Ni-MoS2 coating has a gradual increase to a friction coefficient value of ~0.4, but shows a fluctuation thereafter and again picks up to a steady value of ~0.4. The wear loss and friction coefficient is calculated for Ni and Ni-MoS2 composite coating is shown in Figure 7b. The Ni-MoS2 shows a higher wear resistance then the Ni coating. Hence the friction coefficient value of Ni-MoS2 composite coating is lower than the other electrodeposited Ni composite coatings is shown in Figure 7c.8,14,20,34,35,37 Moreover the weight loss of Ni-MoS2 composite coating is lower compared with literature report.14,20,37 The addition of MoS2 nanosheets in the composite coating reduces the coefficient of friction and increases the wear resistance by sliding between the van der Waal’s layers and act as a solid lubricant. In addition to friction coefficient, the grain refinement and microhardness also contribute to improve the wear resistance.38,39
Figure 7 (a) Friction coefficient vs wear test time for pure Ni and Ni-MoS2 composite coatings, (b) Wear weight loss and friction coefficient of pure Ni and Ni-MoS2 composite coatings and (c) Comparison of friction coefficient of Ni-MoS2 composite coating with other reported Ni-composite coatings
MoS2 nanosheets were successfully synthesized by a facile hydrothermal method and confirmed its formation by XRD and FE-SEM analysis. MoS2 nanosheets were codeposited in the Ni matrix using direct current electrodeposition method and the maximum codeposition of MoS2 was obtained at a concentration of 1 g/L with the stirring rate of 400 rpm. The XRD and FE-SEM studies on the coatings confirmed that the addition of MoS2 inhibited the crystal growth and thereby changed the preferred crystal orientation and reduced crystallite size and lattice strain. The Ni-MoS2 nanocomposite coating exhibited a maximum microhardness (722 HV) due to grain refinement. Friction measurements produced better wear resistance for Ni-MoS2 composite coating compared to pure Ni coating. These results revealed that Ni-MoS2 composite coating can be used as a hard and wear resistant coating for solid lubricant applications.
The authors gratefully acknowledge the Central Instrumentation Facility (CIF), Pondicherry University for providing the instrumentation facilities.
1. S. Ramalingam, K. Balakrishnan and A. Subramania, Trans. IMF, 2015, 93, 262–266.
2. C. Ma, S. C. Wang, L. P. Wang, F. C. Walsh and R. J. K. Wood, Surf. Coat. Technol., 2013, 235, 495–505.
3. K. M. Zadeh, R. A. Shakoor and A. B. Radwan, Int. J. Electrochem. Sci., 2016, 11, 7020–7030.
4. K. A. Kumar, G. P. Kalaignan and V. S. Muralidharan, Appl. Surf. Sci., 2012, 259, 231–237.
5. N. P. Wasekar and G. Sundararajan, Wear, 2015, 342–343, 340–348.
6. N. P. Wasekar, S. M. Latha, M. Ramakrishna, D. S. Rao and G. Sundararajan, Mater. Des., 2016, 112, 140–150.
7. P. Baghery, M. Farzam, A. B. Mousavi and M. Hosseini, Surf. Coat. Technol., 2010, 204, 3804–3810.
8. A. Bigos, P. Indyka, A. Chojnacka, A. Drewienkiewicz, S. Zimowski, M. Kot and M. J. Szczerba, J. Electroanal. Chem., 2018, 813, 39–51.
9. T. Borkar and S. Harimkar, Surf. Eng., 2011, 27, 524–530.
10. H. Jin, Y. Wang, Y. Wang and H. Yang, Rare Met., 2017.
11. S. Khabazian and S. Sanjabi, Appl. Surf. Sci., 2011, 257, 5850–5856.
12. C. T. J. Low, J. O. Bello, J. A. Wharton, R. J. K. Wood, K. R. Stokes and F. C. Walsh, Surf. Coat. Technol., 2010, 205, 1856–1863.
13. N. S. Qu, D. Zhu and K. C. Chan, Scr. Mater., 2006, 54, 1421–1425.
14. Z. Xue, W. Lei, Y. Wang, H. Qian and Q. Li, Surf. Coat. Technol., 2017, 325, 417–428.
15. Y. Zhang, X. Leng, X. Wang, P. Ou and W. Zhang, Metallogr. Microstruct. Anal., 2017.
16. S. Ramalingam, K. Balakrishnan, S. Shanmugasamy and A. Subramania, Surf. Eng., 2017, 33, 369–374.
17. C. M. P. Kumar, T. V Venkatesha and R. Shabadi, Mater. Res. Bull., 2013, 48, 1477–1483.
18. B. Kirubasankar, V. Murugadoss and S. Angaiah, RSC Adv., 2017, 7, 5853–5862.
19. A. Castellanos-gomez, M. Poot, G. A. Steele and H. S. J. Van Der Zant, Adv. Mater., 2012, 24, 772–775.
20. Y. He, S. C. Wang, F. C. Walsh, Y. Chiu and P. A. S. Reed, Surf. Coat. Technol., 2016, 307, 926–934.
21. S. Ott, P. G. Sanders, A. Borbe, J. R. Weertman and T. Unga, Acta Mater., 1998, 46, 3693–3699.
22. R. Sen, S. Das and K. Das, Surf. Coat. Technol., 2011, 205, 3847–3855.
23. M. Chhowalla and G. A. J. Amaratunga, Nature, 2000, 407, 164–167.
24. N. Wakabayashi, Phys. Rev. B, 1975, 12, 659–663.
25. N. Guglielmi, J. Electrochem. Soc., 1972, 119, 1009–1012.
26. S. Ramalingam, V. S. Muralidharan and A. Subramania, J. Solid State Electrochem., 2009, 13, 1777–1783.
27. A. G. Mccormack, M. J. Pomeroy and V. J. Cunnane, J. Electrochem. Soc., 2003, 150, 356–361.
28. L. Shi, C. Sun, P. Gao, F. Zhou and W. Liu, Appl. Surf. Sci., 2006, 252, 3591–3599.
29. S. Khabazian and S. Sanjabi, Appl. Surf. Sci., 2011, 257, 5850–5856.
30. Q. Niu, G. Liu and C. Ran, IOP Mater. Sci. Eng., 2018, 301, 012001.
31. M. W. Khalil, T. A. Salah Eldin, H. B. Hassan, K. El-Sayed and Z. Abdel Hamid, Surf. Coatings Technol., 2015, 275, 98–111.
32. H. Jin, Y. Y. Wang, Y. T. Wang and H. B. Yang, Rare Met., 2018, 37, 148–153.
33. G. Parida, D. Chaira and A. Basu, Trans. Indian Inst. Met., 2013, 66, 5–11.
34. J. Chen, J. Li, D. Xiong, Y. He, Y. Ji and Y. Qin, Appl. Surf. Sci., 2016, 361, 49–56.
35. P. Jha, R. K. Gautam and R. Tyagi, Friction, 2017, 5, 437–446.
36. H. Li, Y. He, T. He, Y. Fan, Q. Yang and Y. Zhan, Ceram. Int., 2016, 42, 18380–18392.
37. M. F. Cardinal, P. A. Castro, J. Baxi, H. Liang and F. J. Williams, Surf. Coatings Technol., 2009, 204, 85–90.
38. S. Shanmugasamy, K. Balakrishnan, A. Subasri, S. Ramalingam and A. Subramania, J. Rare Earths, 2018, 36, 1319-1325.
39. Y. Fan, Y. He, P. Luo, T. Shi and H. Li, J. Electrochem. Soc., 2015, 162, D270–D274.