Accepted Manuscript Electrodeposited Ni/SiC composite coating on graphite for high temperature solar thermal applications M. Sindhuja, V. Sudha, S. Harinipriya, Ramani Venugopal, Belal Usmani PII: DOI: Reference:
S2589-2991(18)30013-2 https://doi.org/10.1016/j.mset.2018.05.003 MSET 10
To appear in:
Materials Science for Energy Technologies
Received Date: Revised Date: Accepted Date:
3 April 2018 21 May 2018 22 May 2018
Please cite this article as: M. Sindhuja, V. Sudha, S. Harinipriya, R. Venugopal, B. Usmani, Electrodeposited Ni/ SiC composite coating on graphite for high temperature solar thermal applications, Materials Science for Energy Technologies (2018), doi: https://doi.org/10.1016/j.mset.2018.05.003
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Electrodeposited Ni/SiC composite coating on graphite for high temperature solar thermal applications M. Sindhujaa, V. Sudhab, S. Harinipriyaa*, Ramani Venugopalc, Belal Usmanid a Electrochemical Systems Laboratory, SRM Research Institute, SRM Institute of Science and Technology, Kattankulathur, Chennai, India, 603203 b Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, Chennai, India, 603203 c Materials Science and Engineering Division, Bhabha Atomic Research Center, Mumbai, India d Center for Energy, Indian Institute of Technology Jodhpur, Rajasthan, India, 342011 * Corresponding Author, Electrochemical Systems Laboratory, SRM Research Institute, SRM Institute of Science and Technology, Kattankulathur, Chennai, India, 603203 Abstract Ni/SiC nanocomposite coatings were electrodeposited on graphite (of different shapes) under galvanostatic condition. The effect of the shape of graphite on the morphological, composition, structural properties of the composite coatings, thermal stability and anti-oxidative behaviour at high temperatures were studied. The major composition of the composite coating is silicon carbide (Si5C3, CSi), nickel silicon (Ni0.92Si0.08) and graphite (C). Porous and fine grain structure is formed with cracks. Uniform distributions of SiC nanoparticle were observed. Circular graphite possessed higher adhesion and thickness of the coatings in comparison with rod and square shaped graphite substrate. Thermal analysis of the coated samples indicates the weight loss of SiC coated graphite as 16.5% (rod), 30% (square), 2.5% (Circular). Uncoated graphite had a weight loss of 32.5%. The thermal oxidation of the coated sample in air is least for circular geometry in comparison to rod and square. Thus SiC coated circular graphite can be utilized for high temperature applications such as solar thermal absorbers.
Keywords: SiC; graphite; high temperature coatings; solar thermal; anti-oxidation; geometry. email:
[email protected];
1. Introduction Graphite possesses high mechanical strength, thermal conductivity and low thermal expansion. At high temperatures graphite possess fracture toughness. Hence, graphite is extensively used in aerospace industries like brake disk, nose tips, engine components, heat exchanger, diesel engine components, refractory material, hot press die components, crucibles etc[1]. On the other hand, in oxidative environment, graphite gets oxidized at 450oC. Therefore, there exists a need to develop oxidation protection coatings on to graphite materials for their application at high temperature. Since these coatings should be compatible with base carbon materials, SiC has been established to be the best solution as oxidation protection coating[1, 2]. Composite coatings with incorporation of inert particles into metal matrix have various important applications such as wear resistance [3,4], dry lubrication[5,6], anticorrosion [7,8] and dispersion hardening [9,10]. The inert particles can be hard oxide or carbide particles such as SiC, Al2O3, TiO2, WC, SiO2 or diamond, a solid lubricant, such as PTFE, graphite or MoS 2, or even liquid-containing microcapsules[11]. Ni-matrix electrocomposite having dispersed in fine ceramic particles such as SiC[12,13], Al2O3[12,14], Cr2O3[15], TiO2[16], WC[17], or diamond[18] improved the mechanical properties such as wear resistance, hardness and strength of the ceramics. Ni/SiC composite coating is one of the extensively studied composite coatings. Researchers have focused and studied several conditions and its effect on the coatings such as agitation rate [19], temperature [19,20], additives [21-22], composition and pH of the electrolyte [19,23], surface properties of SiC particles [24-26], density [19, 20, 27], size [13, 28], concentration in the bath [20,23,28] and structure [29-31]. The incorporation of inert particles in to the deposit has been attributed to a simple mechanical entrapment [32], or electrophoretic force acting on electrically charged particles [33]. So far to the best of our knowledge, no studies were done on the Ni/SiC composite coatings on graphite for high temperature solar thermal applications. Thus the aim of
the present study is to understand the effect of different shape of graphite on the (i) morphology, structure and composition of the Ni/SiCcomposite coatings, (ii) thermal stability of the coatings upto 7000C and (iii) Anti-oxidative behavior of the coatings at high temperatures such as 15000C. 2. Experimental Method The composition of the electrolyte bath and operating parameters for electrodeposition are shown in Table 1. Nanopowder of SiC with <100 nm particles size were used. As SiC nanoparticles are hydrophobic in nature and difficult to hydrate, prior to electrodeposition, 55.1gL-1 of SiC nanopowder was dispersed in nickel watts solution and continuously stirred for 7h till slurry consistency is attained. The cathodes were of different shape of graphite such as rod, square and circular. Prior to electrodeposition cathodes were washed with distilled water and acetone. Platinum coil was used as anode and all depositions were carried out in galvanostatic conditions at room temperature. A Bruker D8 Advance X-ray diffractometer (XRD) with Cu target (λ=1.5406A˚) monochromated radiation sources, operating at 40.0 kV and 40.0 mA, was used to determine the phase and preferred orientation of the composite coatings. XRD data were collected in the 2θ ranging from 20° to 80° with a step size of 0.02°. The surface morphology of the composite coatings were analyzed by Carl Zeiss EVO 18 special edition operated at 20kV scanning electron microscopy (SEM). The chemical compositions of the composite coatings were determined using the Oxford Energy Dispersive X-ray spectroscopy (EDS) attachment to the SEM. Thermal analyses of the coatings in N2 atmosphere is carried out using Perkin –Elmer Thermogravimetric analyser (TGA), workable temperature upto 9000C. Thermal oxidation is done in air in a muffle furnace operating upto 18000C
3. Results and discussion 3.1 X-ray diffraction Analysis The influence of different shape and geometry of graphite on the phase and structure of electrodeposited composite film were analyzed by XRD. Fig 1, depicts the XRD pattern of composite coating on graphite (a) rod, (b) square, and (c) circular. It can be seen form the diffraction patterns that the structure and phase of SiC coatings changes with change in the geometry of the graphite substrate. This change in structure and phase of the composite coatings is further associated with the variation in the thickness depending upon the geometry of the graphite. In case of rod shape geometry (Fig.1 a), the phases present in the composite coatings are (i) cubic structure (CSi) with reflections of (111), (220), (311) and space group of F-43m (216) [34] (ii) NiSi cubic structure with reflections of (111), (200), (220) and space group Fm3m (225) [35] (iii) graphite hexagonal structure with reflections of (002), (100), (004), (110) and space group P63/mmc (194) [36]. However, in case of square and circular shaped graphite substrate (Figs.1 b & c), phase of the composite coatings constitutes Si5C3 structure with reflections of (110), (123) and space group P213 (198) [36].We noticed that, individual Ni phase is not seen in the composite in all three geometries of the graphite. It may be due to the high intensity reflection of graphite and SiC phases. Although a slight X-offset of the graphite phase is observed in rod geometry, it is consistent with Square geometry of graphite. SiC reflection intensity is high for circular geometry of graphite as compared to square geometry (Figs. 1 b & c), whereas graphite reflection intensity is high for square geometry than circular geometry. This may be attributed to thick SiC coatings on circular than square graphite substrates. Thus the following order is seen for Ni and SiC coatings on the graphite. Circular > Square > Rod
: SiC coatings
Rod > Square Circular
: Ni coatings
3.2 Scanning Electron Microscopy Analysis Scanning electron microscopy (SEM) is used to study the morphology of electrodeposited Ni/SiC composite coatings on different shape of graphite substrate. Fig 2 shows the SEM of electrodeposited Ni/SiC composite coatings on (a) rod, (b) square, (c) circular shape of graphite substrates. Fig 2 demonstrates that in case of rod and square shaped graphite substrate (Fig.2a & b) highly porous composite coatings with fine grain size were formed with cracks whereas less porous structure with fine grain size were formed on circular graphite substrate. This indicates more
adhesion
of
composite
coatings
with
circular
substrate
of
graphite.
Thus
Circular>rod=square is the order elucidated from SEM analysis for the fine grain size and adhesion of the coatings on graphite whereas porosity of Ni/SiC coatings follows reverse trend. This dense composite coating on circular graphite substrate observed via SEM made it thermally stable upto 7000C in N2 atmosphere and anti-oxidative upto 16000C for 15h as discussed in sections 3.5 and 3.6. 3.3 Energy Dispersive X-ray spectroscopy (EDS) analysis Fig 3 shows the EDS spectrum of electrodeposited Ni/SiC composite coatings on (a) rod, (b) square and (c) circular geometry of graphite substrate respectively. Elemental distributions of composite coatings on different shape geometry of graphite are shown in Table 2. It can be seen from Table 2 that the silicon content increases in the order Circular > Square > Rod However, the Ni content is in the order Rod> Square > Circular The above mentioned trend is further supported by XRD analysis. This trend may be attributed to the uniform distribution of SiC nanoparticles in the composite coatings. The presence of higher Si content in the coatings results in the formation of glass SiO2 upon oxidation at very high
temperatures such as 15000C. Formation of a layer of SiO2 on the surface prevents further oxidation of the sample and hence the coated graphite becomes thermally stable in inert as well as reactive oxygen environment. As the composition of Si is very high for SiC coated circular graphite substrate, it is anticipated to possess higher thermal stability (cf. Sections 3.5 and 3.6) when compared to rod and square substrates. Thus the conclusions from EDS analysis is helpful in explaining the anti-oxidative behavior of Ni/SiC coated graphite as demonstrated in following discussion. 3.4 Surface roughness Analysis Surface roughness of a coating has tremendous influence on its functionality[37]. Roughness affects not only surface properties–such as hydrophobicity[38], optical and plasmonic behaviour[39], adhesion[40-42], friction and casimir forces[43]- but also “bulk” properties such as fracture toughness and fatigue resistance[44]. In this study, surface roughness of electrodeposited Ni/nanoSiC and Ni/bulkSiC composite coatings on different geometry (circular, rod, square) of graphite is calculated. Tapping mode is used to image the coatings by Atomic Force Microscopy (AFM) to avoid adhesion and shear forces between the tip and the deposited layer [45]. Gwyddion software has been used for evaluation of parameters from AFM images. In the following, two types of surface roughness values are calculated through the software: i.e., average surface roughness Ra and root mean square (rms) surface roughness Rq. The average surface roughness can be defined as[46],
Ra
1 L Y ( x) dx L 0
(1)
where Ra is average surface roughness, Y is total area of scan, and L represents total number of point which can be taken for the calculation of the surface roughness. Similarly, the rms surface roughness can be defined as[47],
Ra
1 L {Y ( x)}2 dx L 0
(2)
Figs.4 and 5 shows the AFM images of Ni/bulk SiC and Ni/nanoSiC composite coatings on different geometry of Graphite. The calculated surface roughness values using gwyiddon software is shown in Tables 4 and 5. From Table 4, it can be seen that the circular graphite substrate have lower surface roughness compared to rod and square. Also, from Table 5, it is observed the circular graphite substrate have lower surface roughness value compared to rod and square. Thus, Circular > rod = square is the order elucidated from AFM analysis which is consistent with SEM results. The surface roughness values of Ni/ bulk SiC coatings on different geometry of graphite is higher compared to Ni/ nano SiC coatings on graphite. Low surface roughness on circular graphite is mainly due to the dense thick coating on the substrate as supported by SEM analysis (cf. Section 3.2) and may be attributed to the high Si content than Ni in the coating as demonstrated by the EDS analysis (cf. Section 3.3). Thus AFM analysis also supports smooth coating on circular graphite incomparison with the rod and square. This dense smooth coating on the surface makes Ni/SiC coated circular graphite more thermally stable as discussed in sections 3.5 and 3.6. 3.5 Thermal analysis in N2 atmosphere The thermogravimetric analysis of the coated and uncoated samples of graphite in all geometries had been tested (Fig 6). From the thermal analysis it is observed that Ni coated graphite undergoes oxidation at a rapid rate in N2 atmosphere with 85% weight loss within 2500C. In case
of Ni/SiC coated graphite, circular geometry possessed least weight loss of 2.5% and stable upto 7000C, rod geometry exhibited weight loss of 16% whereas square geometry contributed a weight loss of 30%. Uncoated graphite showed a weight loss of 32.5% almost similar to Ni/SiC coated square shaped graphite substrate. Thus from thermogravimetric analysis we can infer the following order for the thermal stability of the samples analyzed SiC-gra(Circular) > SiC-gra (Rod)> SiC-gra(Square)> Graphite> Gra-Ni Due to uniform and dense coating on circular geometry as supported by SEM, EDS and AFM analysis, it is more stable upto 7000C. These results are in concurrence with the observations made by the structural, morphological studies of the Ni/SiC coatings on circular graphite as explained in the previous sections. 3.6 Thermal Oxidation of the samples in air Fig.7 presents the isothermal oxidation curves of the Ni/SiC coated and uncoated samples in air at 13270C. As shown in Fig.7, it is obvious that Ni/SiC coated circular graphite sample exhibit a totally different anti-oxidation behavior. Throughout the oxidation process, the weight loss of Ni/SiC coated graphite increases linearly with oxidation time. After 15 h of oxidation, the weight loss of the coated graphite is up to (i) 0.45% (circular), (ii) 2.4% (rod), (iii) 6.4% (square) and (iv) 12.9% (uncoated), which suggests that the coating is not dense and the oxygen can directly attack the graphite substrate in the case of rod, square and uncoated graphite. In contrast, the Ni/SiC coated circular graphite exhibits excellent oxidation resistance in air at 13270C. After 15h of oxidation, the weight loss of the coated sample is 0.45%, indicating a better oxidation protection of the coating. In addition, high temperature oxidation test is also carried out in air at 15270 C (see Fig.8). For the Ni/SiC coated circular graphite oxidized at 15270C, the weight loss as a function of time is
still linear and after 15 h oxidation, the weight loss is around 1.20%, which further confirms the highly efficient protection ability of the coating. With respect to the Ni/SiC coated graphite rod, the oxidation process is fast and enormous weight loss is observed (14.25%) indicating that the as-prepared coating has very less oxidation resistance at high temperatures. Therefore, based on the above results, it can be concluded that Ni/SiC coated circular graphite has better oxidation resistance than uncoated graphite. The oxidation of Ni/SiC coated rod and square shaped graphite could be attributed to the formation of SiO2 glass due to the micro-cracks formed on the coating surfaces. In addition, the defects such as cracks and voids formed on the surface of the coating will act as diffusion pathway for oxygen to attack the graphite substrate at high temperature. It is clear from SEM, EDS and AFM studies that the Ni/SiC coating on rod and square shaped graphite is not totally dense and continuous, The defects formed in the thinner region of the coating acts as the diffusion channels for oxygen to attack the substrate and thus results in oxidation of the coated sample at high temperatures. 4. Conclusion Ni/SiC nanocomposite coatings were deposited on different shape of graphite substrate employing electrodeposition technique under galvanostatic condition. The major content in the composite coatings were silicon carbide (Si5C3, CSi), nickel silicon (Ni0.92Si0.08) and graphite (C). Porous and fine grain structure was formed with cracks. Uniform distributions of SiC nanoparticle were observed. The Si content increases in the order Square > Circular > Rod, however, the Ni content is in the order Rod> Square > Circular. The thickness, adhesion and finesse of the grains were in the order circular > rod = square. Thermal oxidation of the Ni/SiC coated and uncoated graphite
revealed that in circular
geometry, the SiC coating is very dense and hence the oxidation is very minimal whereas on the rod and square geometry due to the presence of cracks and voids on the surface of the coatings,
thermal oxidation is spontaneous. Thus circular graphite substrate coated with Ni/SiC is resistant to thermal shocks and oxidation in air at high temperatures, thereby revealing the ability of these coatings to be utilized in high temperature applications such as concentrated solar power absorbers operating in the range of 2500 to 12000C.
Acknowledgements: The authors gratefully acknowledge the funding by Board of Research for Nuclear Sciences (BRNS), Government of India. The authors acknowledge the experimental facilities provided by DST-FIST, Department of Chemistry. The authors gratefully acknowledge the constructive comments and suggestions by the reviewers that improved the manuscript substantially.
Table 1: The electrolyte bath and composition of Ni/SiC coatings on graphite Bath composition and electrodeposition Quantity process parameters Rod shape Square shape Circular shape -1 -1 NiSO4. 6H2O 31.2 gL 31.2 gL 31.2 gL-1 NiCl2.6H2O 7.5 gL-1 7.5 gL-1 7.5 gL-1 H3BO3 10 gL-1 10 gL-1 10 gL-1 SiC (nanopowder, <100nm particle size)
55.1 gL-1
55.1 gL-1
55.1 gL-1
Temperature pH
Room temp. 6
Room temp. 6
Room temp. 6
Current density
0.04 A/cm2
0.10 A/cm2
0.05A/cm2
Magnetic Stirring speed
200
400
500
Deposition time
3600
3600
3600
Table 2 Elemental composition of electrodeposited Ni/SiC composite coatings on different shape geometry of graphite Element C O Si S Cl Ni
Weight (%) Square 26.0 16.97 47.03 1.43 0.69 17.28
Rod 28.94 17.88 32.09 1.44 0.50 19.16
Circular 23.81 47.67 8.84 3.04 8.23
Table 3:Surface roughness parameters of Ni/bulkSiCcomposite coatings on graphite Ni/bulk SiCcoatings
Average surface roughness
Root mean surface surface
on different graphite
Ra(nm)
roughness Rq(nm)
Circular
158
196
Rod
259
286
geometry
Square
244
282
Table 4: Surface roughness parametersof Ni/nanoSiCcomposite coatings on graphite Ni/nanoSiCcoatings
Average surface roughness
Root mean surface surface
on different graphite
Ra(nm)
roughness Rq(nm)
Circular
138
175
Rod(hcd)
164
195
Rod(lcd)
95
117
Square
166
198
geometry
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* CSi Ni
(002)
(110)
(004)
(111)
(200)
(311)
(220)
*
*
(111) (100)
Si5C3
(220)
Intensity (a.u.)
Intensity (a.u.)
C
(002)
Si0.08 0.92
C
(110)
30
40 50 60 2-Theta-scale
70
(100)
80
20
30
(102)
(103)
40 50 60 2-Theta-scale (b)
(a)
Si5C3
(110)
Intensity (a.u.)
*
20
(110)
(004) (101)
C
(002)
(110)
(004)
(101)
(100)
20
30
(102)
(103)
(123)
40 50 60 2-Theta-Scale
70
80
(C) Fig.1. X-ray diffraction pattern of electrodeposited Ni/SiC composite coating on (a) rod, (b) square, (c)circular shape of graphite structure
(123)
70
80
Fig.2. SEM micrographs of electrodeposited Ni/SiC composite coatings on (a) rod, (b) square, (c) circular shape geometry of graphite
(a)
(c)
Fig.3. Energy dispersive X-ray spectroscopy (EDS) spectrum of electrodeposited Ni/SiC composite coatings on (a) rod, (b) square and (c) circular shape geometry of graphite.
(b)
(a)
(b)
(c)
Fig.4: AFM images of Ni/bulk SiC composite coatings on a)graphite-circular b)Graphiterod c)Graphite-square
(a)
(b)
(c)
(d)
Fig.5: AFM images of Ni/nanoSiC composite coatings on a)graphite-circular b)Graphiterod(hcd) c)Graphite-rod(lcd) d)Graphite-square
Fig 6: TGA weight loss analysis of graphite and SiC coated graphite (Circular, rod and square) and Ni coated Graphite
14 0
12
1327 C uncoated graphite
10
w ei 8 gh t 6 los s 4 %
SiC coated Square
SiC coated rod
2
SiC coated circular 0 0
2
4
6
8
Time (hrs)
10
12
14
Fig 7 : Thermal oxidation of SiC coated and uncoated graphite at 13270C in air inside muffle furnace
18 0
1527 C
16 14
w ei gh t los s %
12
uncoated graphite
10 8
SiC coated Square SiC coated rod
6 4
SiC coated circular
2 0 0
2
4
6
8
10
12
14
16
Time (hrs)
Fig 8: Thermal oxidation of SiC coated and uncoated graphite at 15270C in air inside muffle furnace
Graphical abstract Electrodeposited Ni/SiC composite coating on graphite for high temperature solar thermal applications M. Sindhujaa, V. Sudhab, S. Harinipriyaa*, Ramani Venugopalc, Belal Usmanid a*
Corresponding Author, Electrochemical Systems Laboratory, SRM Research Institute, SRM Institute of Science and Technology, Kattankulathur, Chennai, India, 603203 b Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, Chennai, India, 603203 c Scientist, Materials Science and Engineering Division, Bhabha Atomic Research Center, Mumbai, India d Center for Energy, Indian Institute of Technology Jodhpur, Rajasthan, India, 342011
Circular > Square > Rod Rod > Square Circular
: SiC coatings : Ni coatings