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Cu, Cu-SiC functionally graded coating for protection against corrosion and wear
Surface & Coatings Technology 374 (2019) 833–844
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Cu, Cu-SiC functionally graded coating for protection against corrosion and wear Swastika Banthiaa, Srijan Senguptab,c, Siddhartha Dasa,b, Karabi Dasa,b,
T
⁎
a
School of Nano Science and Technology, Indian Institute of Technology Kharagpur, Kharagpur, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, India c Now at Department of Mechanical Engineering, Madanpalle Institute of Technology and Science, Madanpalle, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrodeposition Functionally graded coating SiC nanoparticle Wear Corrosion Coating
This work presents the performance of the Copper (Cu) based functionally graded coatings (FGCs) when subjected to corrosion in the 3.5 wt% NaCl aqueous solution and sliding wear under the different loads (2, 5, 8 and 10 N). The Cu FGC, with three layers of Cu (each 20 μm thick), has a gradual decrement in the crystallite size from bottom to the top surface. The Cu, Cu-SiC FGC consists of three-layered Cu FGC (each Cu layer 12 μm thick) followed by two layers of Cu-SiC nanocomposite (each 12 μm thick) with a steep increase in the amount of reinforced SiC nanoparticles from 2 to 7 vol% towards the top. A comparison is made among the equally thick (60 μm) electrodeposited FGCs, i.e., Cu FGC and Cu, Cu-SiC FGC, single-layered Cu coating with the finest microstructure (smallest crystallite size) and Cu-SiC nanocomposite coating with 7 vol% SiC. A drastic reduction in the corrosion and wear rate is observed in the Cu and Cu, Cu-SiC FGCs when compared with the same of single-layered Cu and Cu-SiC nanocomposite coatings, respectively. It is also observed that the Cu, Cu-SiC FGC has higher corrosion resistance than the Cu FGC. The Cu, Cu-SiC FGC is found to be more wear resistant than the Cu FGC at high load, while the latter shows lower specific wear rate at low loads.
1. Introduction Copper (Cu) is often used as an economical material in electronic devices and heat exchangers due to the combination of high electrical and thermal conductivity along with ductility at a certain temperature and pressure conditions [1]. It is also used in building industry, machinery and transportation sector because of machinability and corrosion resistant nature at ambient conditions [2]. However, the applications of Cu in the pure form are limited because of low tensile strength, low resistance to wear and high corrosion rate in the harsh environment. The surface of Cu can be tailored by electrodepositing Cu based composite coatings reinforced with micron-sized Max phase and nanosized SiC, SiO2, CeO2, CNT and Al2O3 particles [3–14]. These electrodeposited composite coatings with improved resistance against corrosion and wear are a suitable alternate to the conventional materials for various applications. However, the failure at the interface of hard nanocomposite coating and soft substrate due to the mismatch of
mechanical properties can be overcome by the codeposition of an alloy coating as an underlayer beneath the nanocomposite coating [15]. Also, the undesirable, high residual stress is generated in the coating which can be decreased by introducing a gradient of microstructure in terms of grain size and volume fraction of reinforcement [16,17]. It is demonstrated that multilayers in a film with a gradient in the microstructure has a high interface bending strength, strain and modulus which doubles its performance for mechanical, structural and functional applications [18]. A microstructurally graded Fe with a variation of grain size from nanocrystalline to microcrystalline range is reported to have a combination of toughness and strength with good resistance against corrosion [19]. Several FGC1 based on Ni-Al2O3, Ni‑nickel coated ZrO2 and Zn-Ni-Al2O3 with high resistance against wear and corrosion have been electrodeposited by varying different experimental parameters such as frequency, current density and duty cycle [20–23]. It is reported that the Ni-Co/SiC nanocomposite FGC is more wear resistant in nature as compared to the conventional uniform
Abbreviations: FGC, Functionally graded coating; PRED, Pulse reverse electrodeposition; CCD, Cathodic Current Density; ACD, Anodic Current Density; OCP, Open circuit potential; FESEM, Field emission scanning electron microscope; EDS, Energy dispersive spectroscope; Ecorr, Corrosion potential; Icorr, Corrosion current density; mpy, Miles per year ⁎ Corresponding author at: Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India. E-mail address: [email protected] (K. Das). 1 FGC, Functionally graded coating https://doi.org/10.1016/j.surfcoat.2019.06.050 Received 11 February 2019; Received in revised form 14 June 2019; Accepted 17 June 2019 Available online 26 June 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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was used to study the surface profile of the wear track in order to determine the wear track depth. The morphological characterization of the coating after corrosion and wear track was done using FESEM4 (FEI Quanta FEG 250, USA and Merlin, Carl Zeiss Microscope GmbH, Germany). The elemental mapping of the wear track and coating after corrosion was done using an EDS5 (Bruker). An electrochemical corrosion test of C50A3, C100A3, C200A3, C200A3S350, C200A3S450, Cu FGC and Cu, Cu-SiC FGC was done at 25 ± 2 °C in 3.5 wt% NaCl aqueous corrosive medium using Autolab PGSTAT-30. A three electrode setup was made using a platinum rod, standard Ag|AgCl electrode and an electrodeposited sample as a counter, reference and working electrode, respectively. The OCP6 was determined by immersing the samples in the solution for 1800 s and after that the corrosion test was carried-out at a step potential of 1 mV/s in the potential range of −0.29 to 0.22 V for all the coatings with respect to the reference electrode. After the corrosion test, the samples were cleaned in acetone and examined under FESEM. The NOVA 1.10 software was used to calculate the corrosion parameters, i.e., Ecorr,7 Icorr,8 polarization resistance and corrosion rate by inserting the values of equivalent weight and density of the sample in the Tafel extrapolation method where, the surface area of the sample was 1 cm2 [26]. The DUCOM TR-208-M1 ball-on-disc machine was used in the present investigation. The sliding wear of sample, i.e., uncoated, annealed Cu, electrodeposited C200A3, Cu FGC, C200A3S450 and Cu, CuSiC FGC was performed. It was done at a constant load of 2, 5, 8 and 10 N for 900 s when the disc, i.e., the sample was rotated at 10 rpm against a fixed, counter ball of hardened steel (hardness of 64–66 HRC) with 3 mm radius as shown in Fig. 2a. A schematic representation of the parameters used to calculate the volume loss after wear is shown in Fig. 2b. Eq. (1) is reported to calculate the volume loss (V loss) of wear track [27].
nanocomposite coatings due to the combined effect of grain size and reinforced particle content gradation [24]. However, reports on the Cu based functionally graded material are very few in number. Recently, Cu FGC and Cu, Cu-SiC FGC are reported to possess lower residual stress as compared to the single-layered Cu and Cu-SiC nanocomposite coatings [25]. The uniqueness of Cu FGC is that a Cu coating with coarse microstructure is electrodeposited on an annealed Cu (coarse microstructure) such that there is an insignificant change in the properties at the interface between the annealed Cu substrate and first layer of Cu coating. Then, a step-wise increment in the current density parameter results in gradual refinement in the crystallite size of electrodeposited Cu. Further, the Cu-SiC nanocomposite coatings are electrodeposited on the Cu FGC such that there is a steep increment in the incorporation of nano-sized SiC nanoparticles along the thickness (towards the outer end). The structural variation along the thickness of the electrodeposited Cu FGC and Cu, Cu-SiC FGC can effectively play a role in providing protection in harsh environment. This work is an extension of our previous work [25] with an aim to project the Cu based FGCs as a protective coating when they are subjected to the corrosive medium, i.e., 3.5 wt% NaCl aqueous solution and sliding wear under different loads (2, 5, 8 and 10 N). 2. Experimental procedure 2.1. Sample preparation A Cu sample of 2 cm × 2 cm × 3 mm in dimension was annealed for an hour at 400 °C and then air-cooled. The steps followed in polishing and cleaning the sample were carried-out as per the literature [25]. 2.2. Electrodeposition A 60 μm thick, single-layered Cu coating (Fig. 1a) was electrodeposited from an aqueous bath (Bath 1), prepared by adding 0.8 M CuSO4.5H2O and 1 M H2SO4. The pH of the Bath was 0.12 + 0.02. The coatings were deposited at CCDs of 50, 100 and 200 mA/cm2 and the corresponding ACDs were kept as one-third of the CCDs (Table 1). The duty cycle and frequency parameter were fixed as 50% and 50 Hz, respectively for all the deposits [25]. Similar parameters were used to deposit a Cu FGC (Fig. 1b) which comprised of three layers of Cu coatings (each 20 μm thick). A Cu FGC was deposited on an annealed Cu substrate and there was a gradual change in the microstructure from coarse (bottom) to fine (top) towards the outer surface. A 60 μm thick, single-layered Cu-SiC nanocomposite coating (Fig. 1c) was electrodeposited from Bath 2 which was prepared by addition of 10 g/ L of SiC (~ 45–55 nm, Alfa Aesar, β-phase) and 1 mM N-Cetyl-N,N,N trimethyl ammonium bromide (CTAB) to Bath 1. 2 and 7 vol% of SiC nanoparticles were incorporated in the Cu matrix by varying the plating bath agitation at 350 and 450 rpm, respectively where other deposition parameters were kept constant (Table 1). Cu, Cu-SiC FGC (Fig. 1d) comprised of three layers of Cu coatings followed by two layers of CuSiC coatings which were deposited from Bath 1 and 2, respectively where each layer was 12 μm thick. The microstructure became fine along the thickness and the amount of reinforcement was increased from 2 to 7 vol% towards the outer surface. The galvanostatic PRED2 was done using Autolab PGSTAT 302 N (Metrohm, Herisau, Switzerland). The nomenclature used for the electrodeposited coatings in this work is shown in Table 2.
Vloss = 2πr (R2 sin−1 (w/2R) − (w/4) (4R2 − w 2)0.5)
where, r: average radius of the wear track (here, value of r = 196 mm), R: radius of the counter ball and w: width of the wear track. It was assumed in Eq. (1) that the shape of the wear track was semicircular even after the removal of the load. Therefore, the depth of the wear track (d in Fig. 2b) was represented in terms of width of the wear track and radius of the counter ball in Eq. (1). The value of d (from Fig. 2b) was calculated using the Pythagoras theorem and shown in Eq. (2).
d = R ± ((4R2 − w 2)0.5)/2
4
The non-contact mode of 3D OSP3 (Contour GT, Bruker, Germany)
3
(2)
However, the annealed Cu being a highly ductile material, undergoes plastic deformation where, the wear debris was assumed to be accumulated as pile-up at the edges on release of load i.e., removal of the counter ball. Hence, in the present investigation, the shape of the wear track of uncoated, annealed Cu, C200A3 and C200A3S450 was assumed to be elliptical, semi-circular and v shaped, respectively (Fig. 3). The Eq. (1) is true only when it is assumed that the wear track shape is spherical. However, in the present investigation, it is found that the wear track does not always remain spherical and may change to elliptical or even triangular. Therefore, the values of wear track width and depth have been measured independently from FESEM and 3D OSP, respectively. On replacing the value of (4R2 − w2)0.5 in Eq. (1) with the value of d in Eq. (2), the modified volume loss (VM loss) (in mm3) was calculated using Eq. (3).
2.3. Characterization
2
(1)
FESEM, Field emission scanning electron microscope. EDS, Energy dispersive spectroscope. 6 OCP, Open circuit potential. 7 Ecorr, Corrosion potential. 8 Icorr, corrosion current density. 5
PRED, Pulse reverse electrodeposition OSP, Optical surface profilometer. 834
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Fig. 1. A schematic representation of electrodeposited Cu based (a) single-layered Cu coating, (b) Cu FGC, (c) single-layered Cu-SiC nanocomposite coating and (d) Cu, Cu-SiC FGC.
where, π = 3.412, D is the diameter of the wear track (in mm), n is the revolutions per meter (in rpm), and t is the total time (in seconds). Hence, the modified volume loss was normalized by the load applied and the total sliding distance and expressed as specific wear rate (W) in Eq. (5).
Table 1 Electrodeposition parameters [25]. Electrodeposition parameters Coating
Cu Cu-SiC nanocomposite
Cathodic current time = Relaxation time = Anodic current time = 50 ms Anodic Current Density (ACD) (in mA/cm2) = CCD/3 Cathodic Current Density (CCD) (in 50, 100 and mA/cm2) 200 Bath agitation (in rpm) at 350 and 450 CCD = 200 mA/cm2
W = VM loss/(N × S)
(5)
where, N: applied load in N, S: sliding distance in m and VM modified volume loss in mm3.
loss:
3. Results and discussion Table 2 Nomenclature for electrodeposited coatings [25].
3.1. Corrosion test in 3.5 wt% NaCl aqueous solution
Electrodeposited coating
Nomenclature
Cu coating deposited at CCD = 50 mA/cm2 Cu coating deposited at CCD = 100 mA/cm2 Cu coating deposited at CCD = 200 mA/cm2 Cu- 2 vol% SiC nanocomposite coating at CCD = 200 mA/cm2 and 350 bath agitation Cu- 7 vol% SiC nanocomposite coating at CCD = 200 mA/cm2 and 450 bath agitation
C50A3 C100A3 C200A3 C200A3S350
VMloss = 2πr (R2 sin−1 (w/2R) − (w/2) × (R − d))
3.1.1. Single-layered Cu coatings and Cu FGC Fig. 4 shows the potentiodynamic polarization curves of the electrodeposited single-layered Cu coatings (Fig. 4a-c) and Cu FGC (Fig. 4d). It is observed that there is a significant positive shift in the corrosion potential of Cu FGC as compared to the single-layered Cu coatings and the corrosion current density of Cu FGC is in between the values of C200A3 and C100A3. The electrochemical parameters are calculated from the potentiodynamic polarization curve (Fig. 4) using the Tafel extrapolation method (Table 3). In the single-layered Cu coatings, the crystallite size decreases with an increase in the deposition current density [25]. It is reported that the fine crystallite size creates an addition in the crystallite boundary density and results in an increment in the corrosion rate of the coating [28]. Therefore, on increasing the current density, there is an increase in the corrosion rate for single-layered Cu coatings (Fig. 4a-c). The Cu
C200A3S450
(3)
The total sliding distance (S) travelled by the counter ball during sliding wear was calculated according to the Eq. (4) and expressed in meters (m).
Sliding distance (S) = (π × D × n × t)/60000
(4) 835
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Fig. 2. A schematic representation of the (a) ball-on-disk sliding wear test in the present investigation and (b) parameters used to calculate the volume loss after wear.
Fig. 3. A Schematic illustrations of wear track width and depth (a) and (b) elliptical for annealed Cu under low and high load value, respectively, (c) semi-circular for C200A3 (d) C200A3S450 in the present investigation.
Table 3 Electrochemical parameters calculated using the Tafel extrapolation method. Sample
Icorr (μA/cm2)
Ecorr (V) vs Ag|AgCl
Corrosion rate (mpy)
Thickness removal per year (μm)
Polarization resistance (kΩ)
C50A3 C100A3 C200A3 Cu FGC
5.65 8.31 22.5 9.70
−0.18 −0.18 −0.18 −0.16
0.18 0.23 0.67 0.28
4.50 5.80 16.25 7.0
2.14 1.87 0.74 1.68
mpy, miles per year, i.e., 1 mpy = 25 μm per year.
in the case of Cu FGC, the corrosion progresses from top to the bottom layer where the crystallite size gradually increases and simultaneously the corrosion rate decreases. Thus, the corrosion rate of the Cu FGC is a less than that of the C200A3, but higher than that of the C100A3. Fig. 5 shows the FESEM images and elemental mapping of the single-layered Cu coatings (C50A3, C100A3, and C200A3) and Cu FGC after corrosion. It appears that the surface of C50A3 after corrosion (Fig. 5a) is smoother than before corrosion [25]. However, the morphology is irregular due to dissolution of Cu ions from the high energy sites during corrosion (Fig. 5b). In C100A3 (Fig. 5c), the corroded surface is uneven and Fig. 5d shows small pits. C200A3 is made of high index planes and after corrosion, big pits are observed (Fig. 5e and f) because of the high corrosion rate at high index planes (high energy planes). In Fig. 5g and h, the morphology of the Cu FGC appears
Fig. 4. Potentiodynamic polarization curves of the electrodeposited singlelayered Cu coating (a) C50A3, (b) C100A3, (c) C200A3 and (d) Cu FGC.
FGC has the same C200A3 layer in the exterior but, has a positive shift of Ecorr value and lower Icorr as compared to C200A3 and thus, it has less corrosion rate than C200A3 (Table 3). The corrosion rate of the Cu FGC is more than C50A3 and C100A3 but less than C200A3. This is because 836
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Fig. 5. FESEM images (a, c, e and g) at 100× and (b, d, f, and h) 500× of the electrodeposited C50A3, C100A3, C200A3 and Cu FGC after corrosion and (i-l) shows the elemental mapping of Cu (red color), Cl (yellow color) and O (green color) corresponding to (b, d, f, and h, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
smoother than C200A3 even after corrosion due to its compact structure [25]. It is also reported that the grain refining of the deposit obtained by electrodeposition does not always diminish its resistance to corrosion, since the increase of energy due to the larger grain contour area can be overcome by other effects such as a lesser roughness of the coating [29]. Thus, Cu FGC has higher corrosion resistance than C200A3. 3.1.2. Single-layered Cu-SiC nanocomposite coatings and Cu, Cu-SiC FGC Fig. 6 shows the potentiodynamic polarization curves of the electrodeposited single-layered Cu-SiC nanocomposite coatings and Cu, CuSiC FGC. The electrochemical parameters are calculated using the Tafel extrapolation method (Table 4). It is observed that the corrosion rate of C200A3S450 is significantly lower than C200A3S350 but slightly higher than the Cu, Cu-SiC FGC. It is observed from the Tables 3 and 4 that the ECorr values of the electrodeposited single-layered Cu coatings and Cu FGC are around −0.18 and −0.16 V, respectively. While for single-layered Cu-SiC nanocomposite coatings and Cu, Cu-SiC FGC, the values are observed to change from −0.12 to −0.13 V and − 0.08 V, respectively. This difference is related to the volume percentage of incorporated SiC nanoparticles in the coating.
Fig. 6. Potentiodynamic polarization curves of electrodeposited single-layered Cu-SiC nanocomposite coating (a) C200A3S350, (b) C200A3S450 and (c) Cu, Cu-SiC FGC.
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Table 4 Electrochemical parameters calculated using the Tafel extrapolation method. Sample
ICorr (μA/cm2)
ECorr (V) vs Ag|AgCl
Corrosion rate (mpy)
Polarization resistance (kΩ)
C200A3S350 C200A3S450 Cu, Cu-SiC FGC
4.80 3.60 2.62
−0.13 −0.12 −0.08
0.15 0.09 0.04
2.31 3.23 5.20
The corrosion rate of electrodeposited single-layered Cu is more than the Cu-SiC nanocomposite coatings because of the following reasons:
of Cu takes place around the SiC particles (Fig. 8b and d).
(a) The uniform dispersion of nano-sized particles reduce the effective net area exposed to the corrosive medium and act as a physical barrier because of their extremely low chemical reactivity. Thus, the chemical inertia of the nanocomposite coating increases and enhances the passivation of the matrix. This results in a reduction of the corrosion rate of the nanocomposite [30,31]. (b) The valence electrons are restricted in the region of inert nanoparticle-metal interface. This results in an increase in the work function and builds a high hindrance to the electrons in the nanocomposite coatings and prevents them to take part in the corrosion reactions [32]. (c) The uniform dispersion of SiC nanoparticles in the Cu matrix forms many micro-galvanic cells which provides anodic polarization and inhibits localized corrosion [33,34]. (d) It is reported that the resistance of nanocomposite deposits is slightly higher than those of the pure copper in 3.5 wt% NaCl aqueous medium due to the compact microstructure [35].
3.2.1. Specific wear rate The specific wear rate at various loads for the uncoated, annealed Cu, C200A3, Cu FGC, C200A3S450 and Cu, Cu-SiC FGC is compared (Fig. 9). It is observed that in the uncoated, annealed Cu, C200A3 and Cu FGC, the specific wear rate does not vary significantly up to 8 N and there is a significant increase at 10 N. However, in single-layered Cu-SiC nanocomposite coating and Cu, Cu-SiC FGC, the trend is reversed as compared to Cu coatings. It is observed that the addition of SiC nanoparticles do not always improve the wear rate rather, it depends on the applied load and application. It is explained by the detailed characterization of the wear tracks after completion of wear.
3.2. Ball-on-disk sliding wear test
3.2.2. Characterization of the wear track after sliding wear 3.2.2.1. Uncoated, annealed Cu. Fig. 10 shows the FESEM images of wear tracks from the uncoated, annealed Cu at 2, 5, 8 and 10 N loads, respectively. It is observed that the wear track width increases with increase in the load from 2 to 10 N with prominent sliding marks in the wear track along the sliding direction (Fig. 10a, c, e and g). It is also observed that with increase in load from 2 to 10 N, the wear track depth increases and significant amount of pile-up is observed in the depth profile (Fig. 10b, d, f and h). This gives a glimpse that on removing the counter ball, the wear track profile appears as shown in Fig. 3a and b at low and high load, respectively. The explanation of this behaviour is stated on the basis of heavy plastic deformation with accumulation of deformed copper at the edges of wear track in the form of pile-up. This is due to the abrasive wear mechanism where ploughing action of hard counter surface removes the material from the soft, annealed Cu surface (1.2 GPa). The specific wear rate (volumetric wear normalized by the load applied and the distance covered) is found to be constant (0.015 mm3 N−1 m−1) up to 8 N load and then suddenly increased to (0.0267 mm3 N−1 m−1) at 10 N (Fig. 9). This observation can be explained on the basis that the strain hardened Cu (in the form of pile-up) is brittle in nature and at 10 N load, this hard Cu gets worn out. This can be validated by comparing the height of the piled-up Cu at 8 and 10 N load. If it had been only the ploughing action, the height of the Cu pileup should increase with the increasing load as the extent of the plastic deformation is high. From 2 to 8 N load, the height of the Cu pile-up increases but, at 10 N load, it is found to be lower than 8 N load which indicates that wear-out of hard and brittle (deformed) Cu is from the pile-up.
The incorporation of 7 vol% SiC nanoparticle in Cu matrix (C200A3S450) has resulted in lower corrosion rate than Cu – 2 vol% SiC (C200A3S350) because of uniform dispersion of high amount of reinforcement which impedes the initiation of the corrosion potential of the coatings. For both electrodeposited single-layered Cu-SiC nanocomposite coating (C200A3S450) and Cu, Cu-SiC FGC, the top surface composition is similar and therefore, the corrosion rate is expected to be identical. However, the corrosion rate of Cu, Cu-SiC FGC is found to be lower than that of C200A3S450. This is because the corrosion process does not only take place at the top surface (Cu- 7 vol% SiC) rather, proceeds throughout the thickness where the microstructure is changing after every 12 μm. In Cu, Cu-SiC FGC, the Cu near to SiC nanoparticles dissolutes preferentially at the top surface due to galvanic corrosion (Case A in Fig. 7) and when the corrosive medium channelizes vertically down (from top to second layer), the net area of Cu (anode) starts increasing and the relative percentage of SiC (cathode) decreases (Case B and C in Fig. 7). As soon as the top layer of Cu FGC is exposed (Case D in Fig. 7), the Cu areas beneath the SiC nanoparticles undergo uniform corrosion and those Cu sites are much smaller as compared to that available at the top surface (Cu- 7 vol% SiC layer). Hence, it can be stated that the five-layered Cu, Cu-SiC FGC is under a conjoint influence of both galvanic and uniform corrosion as each layer is 12 μm thick. Since, the rate of uniform corrosion is lower than the galvanic corrosion, the net corrosion rate of the Cu, Cu-SiC FGC is less than C200A3S450 where only galvanic corrosion takes place throughout the coating (60 μm thick). This phenomenon is shown with the help of schematic (Fig. 7). Fig. 8 shows the morphology and elemental mapping of the electrodeposited single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450) and Cu, Cu-SiC FGC after corrosion. The surface is rough and the presence of pits is observed in Fig. 8a while, in Fig. 8c, the appearance of surface is smoother and compact than C200A3S450. It is due to the higher surface roughness of as deposited C200A3S450 than Cu, Cu-SiC FGC [25]. The elemental mapping shows that the dissolution
3.2.2.2. Single-layered Cu coating (C200A3). Fig. 11 shows the FESEM and 3D OSP images of the wear tracks with depth profile from the 60 μm thick electrodeposited single-layered Cu coating (C200A3) at 2, 5, 8 and 10 N load. It is observed that the wear track width increases with increase in load from 2 to 10 N but the increment is less than the uncoated, annealed Cu (Fig. 11a, c, e and g). It is also observed from the depth profile that there is no pile-up at the edge and the wear depth does not vary significantly with increase in load (Fig. 11b, d, f and h) unlike that in Fig. 10. The shape of the wear track after removal of the ball is observed to be spherical (Fig. 3c). It shows that the wear takes 838
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Fig. 7. A schematic representation of the cross-sectional view of corrosion in Cu, Cu-SiC FGC.
Fig. 8. FESEM images of (a) C200A3S450 and (c) Cu, Cu-SiC FGC after corrosion and (b and d) elemental mapping of Cu (red color) and Si (green color) corresponding to (a and c, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 839
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and there is less energy available to wear the top hard layer. It is observed that there is an increase in the wear track width with increase in load to 8 N and at the same time, the prevention of the flow of material to edges, which is similar to the observations in the case of soft Cu and C200A3, respectively. At 10 N load, the wear track depth (Fig. 12h) is higher than that observed in C200A3 (Fig. 11h) which indicates that the counter ball has reached the second layer of Cu FGC (less hard than the top layer) but, the increase in wear track width is less than C200A3 (Fig. 11g). Thus, at 10 N load in Cu FGC, the energy is spent to wear the outer hard layer and specific wear rate is high (Fig. 9). The hardness of 2.7 GPa and three-layered microstructure graded structure of Cu FGC is observed to be responsible for the improvement in wear rate (0.0024 ± 0.0003 mm3 N−1 m−1 at 2, 5 and 8 N and 0.0048 mm3 N−1 m−1 at 10 N load) as compared to C200A3. In soft, annealed Cu, the main mechanism is ploughing in which the deformed material is accumulated along the wear track edges whereas in electrodeposited, hard Cu (C200A3), wear takes place because of material loss. In the case of Cu FGC, both of the mechanisms take place simultaneously. The Cu FGC is made-up of hard exterior (fine microstructured hard Cu) and soft interior (coarse microstructured soft Cu) due to which the contact load gets distributed over larger volume and the soft interior under the entire hard surface deforms plastically.
Fig. 9. The specific wear rate versus load of electrodeposited C200A3, Cu FGC, C200A3S450, Cu, Cu-SiC FGC and uncoated, annealed Cu.
place only by the removal of material during sliding wear. The electrodeposited single-layered Cu (C200A3) has small crystallite size (25 nm) and the morphology is near to spherical with a hardness of 2.5 GPa due to the presence of high twin density [25]. Since, the C200A3 deposit is full of deformation twins that enhance the yield stress and thus, the pile-up effect is not significant. The specific wear rate (0.011mm3 N−1 m−1) as shown in Fig. 9 is observed to be independent of the load and is less than uncoated, annealed Cu.
3.2.2.4. Single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450). Fig. 13 shows the wear depth profile with 3D OSP and FESEM images along with the elemental mapping of the wear tracks from the electrodeposited single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450) at 2, 5, 8 and 10 N load. Fig. 13a and d and Fig. 13b and e show that the wear track depth
3.2.2.3. Cu FGC. Fig. 12 shows the FESEM and 3D OSP images of the wear tracks with depth profile from Cu FGC at 2, 5, 8 and 10 N load. It is observed that the wear track width increases with load (Fig. 12a, c, e and g). The wear track depth (Fig. 12b, d and f) is less than C200A3 as part of the energy is spent for plastic deformation of the soft interior
Fig. 10. (a, c, e and g) FESEM and (b, d, f and h) 3D OSP images of wear track with depth profile from the uncoated, annealed Cu at 2, 5, 8 and 10 N load, respectively. 840
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Fig. 11. (a, c, e and g) FESEM and (b, d, f and h) 3D OSP images of wear track with depth profile from the single-layered Cu coating (C200A3) at 2, 5, 8 and 10 N load, respectively.
accumulate at the edge of the wear track. Hence, the track edges are uneven and this phenomenon is shown with the help of schematic (Fig. 15).
and width, respectively increases till 5 N load. Fig. 13 (g and j) show that the wear track depth does not increase significantly but width increase at 8 and 10 N loads (Fig. 13h and k). The elemental mapping of the wear tracks is shown in Fig. 13c, f, i and l corresponding to Fig. 13b, e, h and k, respectively at different loads where, loose SiC particles are seen along the edges of track. The C200A3S450 has a hardness of 3.4 GPa due to the uniform dispersion of SiC nanoparticles in the Cu matrix through-out the thickness (60 μm) and surface roughness of 1.8 μm [25]. The SiC nanoparticle is harder than the counter hardened steel ball due to which it resists wear. Therefore, the wear is restricted only to the ductile Cu matrix and the shape of wear track profile is tapered (Fig. 13a, d g, and h). However, the specific wear rate at 2 N is higher for C200A3S450 than C200A3 because of high surface roughness. It can be explained from Fig. 14, where same load is applied on both C200A3 and C200A3S450 but, the contact points of the counter ball are less in C200A3S450 unlike C200A3 and thus, the total pressure is more in C200A3S450. This results in the wearing-away of surface irregularity to form a smooth surface and wear-debris is added to the volume loss and ultimately, the specific wear rate increases. In C200A3 and Cu FGC, the as deposited surfaces are smooth, the sharp edges are absent and during the sliding wear, only material loss takes place from smooth surface. Therefore, until the surface becomes smooth the C200A3S450 undergoes wear in form of elimination of surface irregularities and in C200A3 and Cu FGC this does not take place as the deposits are smooth prior to wear. Hence, initially the wear rate of C200A3S450 is more than C200A3 till the irregularities vanish. Once both the samples (C200A3 and C200A3S450) become smooth, C200A3S450 wears at a lower rate than C200A3 and justifies the statement made in Section 3.2.1. Further, as the SiC nanoparticles are distributed through-out the matrix, they act as inhibitor towards the sliding motion of the ball. Therefore, the ball plastically deforms the Cu matrix and the loose SiC nanoparticles flow away from the wear track along with the Cu and
3.2.2.5. Cu, Cu-SiC FGC. Fig. 16 shows the wear depth profile with 3D OSP and FESEM image along with the elemental mapping of the wear tracks from five-layered Cu, Cu-SiC FGC at 2, 5, 8 and 10 N load. The wear track depth penetrates through the first to fourth layer (each layer 12 μm thick) with increase in load (Fig. 16a, d, g and j). However, the increase in the wear track width (22 ± 3 μm) is independent of load as shown in Fig. 16b, e, h and k. This is due to the smoother (0.9 μm) surface, higher hardness (3.8 GPa) and more compact structure of Cu, Cu-SiC FGC than Cu - 7 vol% SiC nanocomposite coating (C200A3S450) [25]. The outer layer (Cu-7 vol% SiC) in Cu, Cu-SiC FGC is hard due to the uniformly dispersed SiC in the Cu matrix followed by Cu - 2 vol% SiC layer and these two nanocomposite layers are followed by a Cu FGC. During wear, the top layer undergoes the same effect as described for C200A3S450. Apart from that, part of the energy is utilised to deform the underneath pure Cu layers (Cu FGC). Therefore, the Cu, CuSiC FGC has lower specific wear rate than C200A3S450. The elemental mapping to the wear tracks is shown in Fig. 16c, f, i and l corresponding to Fig. 16b, e, h and k, respectively at different loads where SiC loose particles are seen along the edges of track. 4. Conclusions Cu is generally used in electrical and structural applications under ambient conditions. Under high loads and in the presence of high concentration of chlorine, the applications of pure bulk Cu are restricted. The performance of pure bulk Cu in the above mentioned harsh conditions can be improved by electrodepositing Cu based coatings. In this work, the wear and corrosion properties of the single-layered and functionally graded Cu based coating are evaluated in order to find its 841
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Fig. 12. (a, c, e and g) FESEM and (b, d, f and h) 3D OSP images of wear track with depth profile from Cu FGC at 2, 5, 8 and 10 N load, respectively.
Fig. 13. (a, d, g and j) wear depth profile with 3D OSP and (b, e, h and k) FESEM images, (c, f, i and l) elemental mapping of Cu and Si corresponding to (b, e, h, and k, respectively) of the wear track from single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450) at different loads.
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Fig. 14. Schematic illustration of (a) C200A3 (b) and (c) C200A3S450 at low and high load.
Fig. 15. Schematics on SiC nanoparticles pulled-out along the edges of the wear track during sliding wear.
Fig. 16. (a, d, g and j) wear depth profile with 3D OSP and (b, e, h and k) FESEM images, (c, f, i and l) elemental mapping of Cu and Si corresponding to (b, e, h, and k, respectively) of the wear track from Cu, Cu-SiC FGC at different loads.
• Cu FGC and Cu, Cu-SiC FGC have improved corrosion resistance than C200A3 and C200A3S450, respectively. • A modified volume loss formula is proposed in terms of wear track
suitability as potential protective coatings in the sectors of electrical, construction, pipelines and marine. The following conclusions can be drawn from the present study.
• The corrosion rate of single-layered Cu coatings increases with increase in the deposition current density. • The corrosion rate of single-layered Cu-SiC nanocomposite coating
• •
(C200A3S350) is lower than the single-layered Cu coatings and the rate decreases further with increase in the amount of uniformly dispersed SiC nanoparticles in the Cu matrix (C200A3S450). 843
width and depth which are measured independently from FESEM and 3D OSP, respectively. Cu FGC and Cu, Cu-SiC FGC have lower wear rate than C200A3 and C200A3S450, respectively. For low load based applications, it is recommended to use Cu FGC while for high load Cu, Cu-SiC FGC should be used.
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.06.050.
[16]
Notes [17]
The authors declare no competing financial interest. [18]
Declaration of interest None.
[19]
Contributions
[20]
All authors have contributed equally. [21]
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Cu, Cu-SiC functionally graded coating for protection against corrosion and wear Swastika Banthiaa, Srijan Senguptab,c, Siddhartha Dasa,b, Karabi Dasa,b,
T
⁎
a
School of Nano Science and Technology, Indian Institute of Technology Kharagpur, Kharagpur, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, India c Now at Department of Mechanical Engineering, Madanpalle Institute of Technology and Science, Madanpalle, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrodeposition Functionally graded coating SiC nanoparticle Wear Corrosion Coating
This work presents the performance of the Copper (Cu) based functionally graded coatings (FGCs) when subjected to corrosion in the 3.5 wt% NaCl aqueous solution and sliding wear under the different loads (2, 5, 8 and 10 N). The Cu FGC, with three layers of Cu (each 20 μm thick), has a gradual decrement in the crystallite size from bottom to the top surface. The Cu, Cu-SiC FGC consists of three-layered Cu FGC (each Cu layer 12 μm thick) followed by two layers of Cu-SiC nanocomposite (each 12 μm thick) with a steep increase in the amount of reinforced SiC nanoparticles from 2 to 7 vol% towards the top. A comparison is made among the equally thick (60 μm) electrodeposited FGCs, i.e., Cu FGC and Cu, Cu-SiC FGC, single-layered Cu coating with the finest microstructure (smallest crystallite size) and Cu-SiC nanocomposite coating with 7 vol% SiC. A drastic reduction in the corrosion and wear rate is observed in the Cu and Cu, Cu-SiC FGCs when compared with the same of single-layered Cu and Cu-SiC nanocomposite coatings, respectively. It is also observed that the Cu, Cu-SiC FGC has higher corrosion resistance than the Cu FGC. The Cu, Cu-SiC FGC is found to be more wear resistant than the Cu FGC at high load, while the latter shows lower specific wear rate at low loads.
1. Introduction Copper (Cu) is often used as an economical material in electronic devices and heat exchangers due to the combination of high electrical and thermal conductivity along with ductility at a certain temperature and pressure conditions [1]. It is also used in building industry, machinery and transportation sector because of machinability and corrosion resistant nature at ambient conditions [2]. However, the applications of Cu in the pure form are limited because of low tensile strength, low resistance to wear and high corrosion rate in the harsh environment. The surface of Cu can be tailored by electrodepositing Cu based composite coatings reinforced with micron-sized Max phase and nanosized SiC, SiO2, CeO2, CNT and Al2O3 particles [3–14]. These electrodeposited composite coatings with improved resistance against corrosion and wear are a suitable alternate to the conventional materials for various applications. However, the failure at the interface of hard nanocomposite coating and soft substrate due to the mismatch of
mechanical properties can be overcome by the codeposition of an alloy coating as an underlayer beneath the nanocomposite coating [15]. Also, the undesirable, high residual stress is generated in the coating which can be decreased by introducing a gradient of microstructure in terms of grain size and volume fraction of reinforcement [16,17]. It is demonstrated that multilayers in a film with a gradient in the microstructure has a high interface bending strength, strain and modulus which doubles its performance for mechanical, structural and functional applications [18]. A microstructurally graded Fe with a variation of grain size from nanocrystalline to microcrystalline range is reported to have a combination of toughness and strength with good resistance against corrosion [19]. Several FGC1 based on Ni-Al2O3, Ni‑nickel coated ZrO2 and Zn-Ni-Al2O3 with high resistance against wear and corrosion have been electrodeposited by varying different experimental parameters such as frequency, current density and duty cycle [20–23]. It is reported that the Ni-Co/SiC nanocomposite FGC is more wear resistant in nature as compared to the conventional uniform
Abbreviations: FGC, Functionally graded coating; PRED, Pulse reverse electrodeposition; CCD, Cathodic Current Density; ACD, Anodic Current Density; OCP, Open circuit potential; FESEM, Field emission scanning electron microscope; EDS, Energy dispersive spectroscope; Ecorr, Corrosion potential; Icorr, Corrosion current density; mpy, Miles per year ⁎ Corresponding author at: Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India. E-mail address: [email protected] (K. Das). 1 FGC, Functionally graded coating https://doi.org/10.1016/j.surfcoat.2019.06.050 Received 11 February 2019; Received in revised form 14 June 2019; Accepted 17 June 2019 Available online 26 June 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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was used to study the surface profile of the wear track in order to determine the wear track depth. The morphological characterization of the coating after corrosion and wear track was done using FESEM4 (FEI Quanta FEG 250, USA and Merlin, Carl Zeiss Microscope GmbH, Germany). The elemental mapping of the wear track and coating after corrosion was done using an EDS5 (Bruker). An electrochemical corrosion test of C50A3, C100A3, C200A3, C200A3S350, C200A3S450, Cu FGC and Cu, Cu-SiC FGC was done at 25 ± 2 °C in 3.5 wt% NaCl aqueous corrosive medium using Autolab PGSTAT-30. A three electrode setup was made using a platinum rod, standard Ag|AgCl electrode and an electrodeposited sample as a counter, reference and working electrode, respectively. The OCP6 was determined by immersing the samples in the solution for 1800 s and after that the corrosion test was carried-out at a step potential of 1 mV/s in the potential range of −0.29 to 0.22 V for all the coatings with respect to the reference electrode. After the corrosion test, the samples were cleaned in acetone and examined under FESEM. The NOVA 1.10 software was used to calculate the corrosion parameters, i.e., Ecorr,7 Icorr,8 polarization resistance and corrosion rate by inserting the values of equivalent weight and density of the sample in the Tafel extrapolation method where, the surface area of the sample was 1 cm2 [26]. The DUCOM TR-208-M1 ball-on-disc machine was used in the present investigation. The sliding wear of sample, i.e., uncoated, annealed Cu, electrodeposited C200A3, Cu FGC, C200A3S450 and Cu, CuSiC FGC was performed. It was done at a constant load of 2, 5, 8 and 10 N for 900 s when the disc, i.e., the sample was rotated at 10 rpm against a fixed, counter ball of hardened steel (hardness of 64–66 HRC) with 3 mm radius as shown in Fig. 2a. A schematic representation of the parameters used to calculate the volume loss after wear is shown in Fig. 2b. Eq. (1) is reported to calculate the volume loss (V loss) of wear track [27].
nanocomposite coatings due to the combined effect of grain size and reinforced particle content gradation [24]. However, reports on the Cu based functionally graded material are very few in number. Recently, Cu FGC and Cu, Cu-SiC FGC are reported to possess lower residual stress as compared to the single-layered Cu and Cu-SiC nanocomposite coatings [25]. The uniqueness of Cu FGC is that a Cu coating with coarse microstructure is electrodeposited on an annealed Cu (coarse microstructure) such that there is an insignificant change in the properties at the interface between the annealed Cu substrate and first layer of Cu coating. Then, a step-wise increment in the current density parameter results in gradual refinement in the crystallite size of electrodeposited Cu. Further, the Cu-SiC nanocomposite coatings are electrodeposited on the Cu FGC such that there is a steep increment in the incorporation of nano-sized SiC nanoparticles along the thickness (towards the outer end). The structural variation along the thickness of the electrodeposited Cu FGC and Cu, Cu-SiC FGC can effectively play a role in providing protection in harsh environment. This work is an extension of our previous work [25] with an aim to project the Cu based FGCs as a protective coating when they are subjected to the corrosive medium, i.e., 3.5 wt% NaCl aqueous solution and sliding wear under different loads (2, 5, 8 and 10 N). 2. Experimental procedure 2.1. Sample preparation A Cu sample of 2 cm × 2 cm × 3 mm in dimension was annealed for an hour at 400 °C and then air-cooled. The steps followed in polishing and cleaning the sample were carried-out as per the literature [25]. 2.2. Electrodeposition A 60 μm thick, single-layered Cu coating (Fig. 1a) was electrodeposited from an aqueous bath (Bath 1), prepared by adding 0.8 M CuSO4.5H2O and 1 M H2SO4. The pH of the Bath was 0.12 + 0.02. The coatings were deposited at CCDs of 50, 100 and 200 mA/cm2 and the corresponding ACDs were kept as one-third of the CCDs (Table 1). The duty cycle and frequency parameter were fixed as 50% and 50 Hz, respectively for all the deposits [25]. Similar parameters were used to deposit a Cu FGC (Fig. 1b) which comprised of three layers of Cu coatings (each 20 μm thick). A Cu FGC was deposited on an annealed Cu substrate and there was a gradual change in the microstructure from coarse (bottom) to fine (top) towards the outer surface. A 60 μm thick, single-layered Cu-SiC nanocomposite coating (Fig. 1c) was electrodeposited from Bath 2 which was prepared by addition of 10 g/ L of SiC (~ 45–55 nm, Alfa Aesar, β-phase) and 1 mM N-Cetyl-N,N,N trimethyl ammonium bromide (CTAB) to Bath 1. 2 and 7 vol% of SiC nanoparticles were incorporated in the Cu matrix by varying the plating bath agitation at 350 and 450 rpm, respectively where other deposition parameters were kept constant (Table 1). Cu, Cu-SiC FGC (Fig. 1d) comprised of three layers of Cu coatings followed by two layers of CuSiC coatings which were deposited from Bath 1 and 2, respectively where each layer was 12 μm thick. The microstructure became fine along the thickness and the amount of reinforcement was increased from 2 to 7 vol% towards the outer surface. The galvanostatic PRED2 was done using Autolab PGSTAT 302 N (Metrohm, Herisau, Switzerland). The nomenclature used for the electrodeposited coatings in this work is shown in Table 2.
Vloss = 2πr (R2 sin−1 (w/2R) − (w/4) (4R2 − w 2)0.5)
where, r: average radius of the wear track (here, value of r = 196 mm), R: radius of the counter ball and w: width of the wear track. It was assumed in Eq. (1) that the shape of the wear track was semicircular even after the removal of the load. Therefore, the depth of the wear track (d in Fig. 2b) was represented in terms of width of the wear track and radius of the counter ball in Eq. (1). The value of d (from Fig. 2b) was calculated using the Pythagoras theorem and shown in Eq. (2).
d = R ± ((4R2 − w 2)0.5)/2
4
The non-contact mode of 3D OSP3 (Contour GT, Bruker, Germany)
3
(2)
However, the annealed Cu being a highly ductile material, undergoes plastic deformation where, the wear debris was assumed to be accumulated as pile-up at the edges on release of load i.e., removal of the counter ball. Hence, in the present investigation, the shape of the wear track of uncoated, annealed Cu, C200A3 and C200A3S450 was assumed to be elliptical, semi-circular and v shaped, respectively (Fig. 3). The Eq. (1) is true only when it is assumed that the wear track shape is spherical. However, in the present investigation, it is found that the wear track does not always remain spherical and may change to elliptical or even triangular. Therefore, the values of wear track width and depth have been measured independently from FESEM and 3D OSP, respectively. On replacing the value of (4R2 − w2)0.5 in Eq. (1) with the value of d in Eq. (2), the modified volume loss (VM loss) (in mm3) was calculated using Eq. (3).
2.3. Characterization
2
(1)
FESEM, Field emission scanning electron microscope. EDS, Energy dispersive spectroscope. 6 OCP, Open circuit potential. 7 Ecorr, Corrosion potential. 8 Icorr, corrosion current density. 5
PRED, Pulse reverse electrodeposition OSP, Optical surface profilometer. 834
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Fig. 1. A schematic representation of electrodeposited Cu based (a) single-layered Cu coating, (b) Cu FGC, (c) single-layered Cu-SiC nanocomposite coating and (d) Cu, Cu-SiC FGC.
where, π = 3.412, D is the diameter of the wear track (in mm), n is the revolutions per meter (in rpm), and t is the total time (in seconds). Hence, the modified volume loss was normalized by the load applied and the total sliding distance and expressed as specific wear rate (W) in Eq. (5).
Table 1 Electrodeposition parameters [25]. Electrodeposition parameters Coating
Cu Cu-SiC nanocomposite
Cathodic current time = Relaxation time = Anodic current time = 50 ms Anodic Current Density (ACD) (in mA/cm2) = CCD/3 Cathodic Current Density (CCD) (in 50, 100 and mA/cm2) 200 Bath agitation (in rpm) at 350 and 450 CCD = 200 mA/cm2
W = VM loss/(N × S)
(5)
where, N: applied load in N, S: sliding distance in m and VM modified volume loss in mm3.
loss:
3. Results and discussion Table 2 Nomenclature for electrodeposited coatings [25].
3.1. Corrosion test in 3.5 wt% NaCl aqueous solution
Electrodeposited coating
Nomenclature
Cu coating deposited at CCD = 50 mA/cm2 Cu coating deposited at CCD = 100 mA/cm2 Cu coating deposited at CCD = 200 mA/cm2 Cu- 2 vol% SiC nanocomposite coating at CCD = 200 mA/cm2 and 350 bath agitation Cu- 7 vol% SiC nanocomposite coating at CCD = 200 mA/cm2 and 450 bath agitation
C50A3 C100A3 C200A3 C200A3S350
VMloss = 2πr (R2 sin−1 (w/2R) − (w/2) × (R − d))
3.1.1. Single-layered Cu coatings and Cu FGC Fig. 4 shows the potentiodynamic polarization curves of the electrodeposited single-layered Cu coatings (Fig. 4a-c) and Cu FGC (Fig. 4d). It is observed that there is a significant positive shift in the corrosion potential of Cu FGC as compared to the single-layered Cu coatings and the corrosion current density of Cu FGC is in between the values of C200A3 and C100A3. The electrochemical parameters are calculated from the potentiodynamic polarization curve (Fig. 4) using the Tafel extrapolation method (Table 3). In the single-layered Cu coatings, the crystallite size decreases with an increase in the deposition current density [25]. It is reported that the fine crystallite size creates an addition in the crystallite boundary density and results in an increment in the corrosion rate of the coating [28]. Therefore, on increasing the current density, there is an increase in the corrosion rate for single-layered Cu coatings (Fig. 4a-c). The Cu
C200A3S450
(3)
The total sliding distance (S) travelled by the counter ball during sliding wear was calculated according to the Eq. (4) and expressed in meters (m).
Sliding distance (S) = (π × D × n × t)/60000
(4) 835
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Fig. 2. A schematic representation of the (a) ball-on-disk sliding wear test in the present investigation and (b) parameters used to calculate the volume loss after wear.
Fig. 3. A Schematic illustrations of wear track width and depth (a) and (b) elliptical for annealed Cu under low and high load value, respectively, (c) semi-circular for C200A3 (d) C200A3S450 in the present investigation.
Table 3 Electrochemical parameters calculated using the Tafel extrapolation method. Sample
Icorr (μA/cm2)
Ecorr (V) vs Ag|AgCl
Corrosion rate (mpy)
Thickness removal per year (μm)
Polarization resistance (kΩ)
C50A3 C100A3 C200A3 Cu FGC
5.65 8.31 22.5 9.70
−0.18 −0.18 −0.18 −0.16
0.18 0.23 0.67 0.28
4.50 5.80 16.25 7.0
2.14 1.87 0.74 1.68
mpy, miles per year, i.e., 1 mpy = 25 μm per year.
in the case of Cu FGC, the corrosion progresses from top to the bottom layer where the crystallite size gradually increases and simultaneously the corrosion rate decreases. Thus, the corrosion rate of the Cu FGC is a less than that of the C200A3, but higher than that of the C100A3. Fig. 5 shows the FESEM images and elemental mapping of the single-layered Cu coatings (C50A3, C100A3, and C200A3) and Cu FGC after corrosion. It appears that the surface of C50A3 after corrosion (Fig. 5a) is smoother than before corrosion [25]. However, the morphology is irregular due to dissolution of Cu ions from the high energy sites during corrosion (Fig. 5b). In C100A3 (Fig. 5c), the corroded surface is uneven and Fig. 5d shows small pits. C200A3 is made of high index planes and after corrosion, big pits are observed (Fig. 5e and f) because of the high corrosion rate at high index planes (high energy planes). In Fig. 5g and h, the morphology of the Cu FGC appears
Fig. 4. Potentiodynamic polarization curves of the electrodeposited singlelayered Cu coating (a) C50A3, (b) C100A3, (c) C200A3 and (d) Cu FGC.
FGC has the same C200A3 layer in the exterior but, has a positive shift of Ecorr value and lower Icorr as compared to C200A3 and thus, it has less corrosion rate than C200A3 (Table 3). The corrosion rate of the Cu FGC is more than C50A3 and C100A3 but less than C200A3. This is because 836
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Fig. 5. FESEM images (a, c, e and g) at 100× and (b, d, f, and h) 500× of the electrodeposited C50A3, C100A3, C200A3 and Cu FGC after corrosion and (i-l) shows the elemental mapping of Cu (red color), Cl (yellow color) and O (green color) corresponding to (b, d, f, and h, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
smoother than C200A3 even after corrosion due to its compact structure [25]. It is also reported that the grain refining of the deposit obtained by electrodeposition does not always diminish its resistance to corrosion, since the increase of energy due to the larger grain contour area can be overcome by other effects such as a lesser roughness of the coating [29]. Thus, Cu FGC has higher corrosion resistance than C200A3. 3.1.2. Single-layered Cu-SiC nanocomposite coatings and Cu, Cu-SiC FGC Fig. 6 shows the potentiodynamic polarization curves of the electrodeposited single-layered Cu-SiC nanocomposite coatings and Cu, CuSiC FGC. The electrochemical parameters are calculated using the Tafel extrapolation method (Table 4). It is observed that the corrosion rate of C200A3S450 is significantly lower than C200A3S350 but slightly higher than the Cu, Cu-SiC FGC. It is observed from the Tables 3 and 4 that the ECorr values of the electrodeposited single-layered Cu coatings and Cu FGC are around −0.18 and −0.16 V, respectively. While for single-layered Cu-SiC nanocomposite coatings and Cu, Cu-SiC FGC, the values are observed to change from −0.12 to −0.13 V and − 0.08 V, respectively. This difference is related to the volume percentage of incorporated SiC nanoparticles in the coating.
Fig. 6. Potentiodynamic polarization curves of electrodeposited single-layered Cu-SiC nanocomposite coating (a) C200A3S350, (b) C200A3S450 and (c) Cu, Cu-SiC FGC.
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Table 4 Electrochemical parameters calculated using the Tafel extrapolation method. Sample
ICorr (μA/cm2)
ECorr (V) vs Ag|AgCl
Corrosion rate (mpy)
Polarization resistance (kΩ)
C200A3S350 C200A3S450 Cu, Cu-SiC FGC
4.80 3.60 2.62
−0.13 −0.12 −0.08
0.15 0.09 0.04
2.31 3.23 5.20
The corrosion rate of electrodeposited single-layered Cu is more than the Cu-SiC nanocomposite coatings because of the following reasons:
of Cu takes place around the SiC particles (Fig. 8b and d).
(a) The uniform dispersion of nano-sized particles reduce the effective net area exposed to the corrosive medium and act as a physical barrier because of their extremely low chemical reactivity. Thus, the chemical inertia of the nanocomposite coating increases and enhances the passivation of the matrix. This results in a reduction of the corrosion rate of the nanocomposite [30,31]. (b) The valence electrons are restricted in the region of inert nanoparticle-metal interface. This results in an increase in the work function and builds a high hindrance to the electrons in the nanocomposite coatings and prevents them to take part in the corrosion reactions [32]. (c) The uniform dispersion of SiC nanoparticles in the Cu matrix forms many micro-galvanic cells which provides anodic polarization and inhibits localized corrosion [33,34]. (d) It is reported that the resistance of nanocomposite deposits is slightly higher than those of the pure copper in 3.5 wt% NaCl aqueous medium due to the compact microstructure [35].
3.2.1. Specific wear rate The specific wear rate at various loads for the uncoated, annealed Cu, C200A3, Cu FGC, C200A3S450 and Cu, Cu-SiC FGC is compared (Fig. 9). It is observed that in the uncoated, annealed Cu, C200A3 and Cu FGC, the specific wear rate does not vary significantly up to 8 N and there is a significant increase at 10 N. However, in single-layered Cu-SiC nanocomposite coating and Cu, Cu-SiC FGC, the trend is reversed as compared to Cu coatings. It is observed that the addition of SiC nanoparticles do not always improve the wear rate rather, it depends on the applied load and application. It is explained by the detailed characterization of the wear tracks after completion of wear.
3.2. Ball-on-disk sliding wear test
3.2.2. Characterization of the wear track after sliding wear 3.2.2.1. Uncoated, annealed Cu. Fig. 10 shows the FESEM images of wear tracks from the uncoated, annealed Cu at 2, 5, 8 and 10 N loads, respectively. It is observed that the wear track width increases with increase in the load from 2 to 10 N with prominent sliding marks in the wear track along the sliding direction (Fig. 10a, c, e and g). It is also observed that with increase in load from 2 to 10 N, the wear track depth increases and significant amount of pile-up is observed in the depth profile (Fig. 10b, d, f and h). This gives a glimpse that on removing the counter ball, the wear track profile appears as shown in Fig. 3a and b at low and high load, respectively. The explanation of this behaviour is stated on the basis of heavy plastic deformation with accumulation of deformed copper at the edges of wear track in the form of pile-up. This is due to the abrasive wear mechanism where ploughing action of hard counter surface removes the material from the soft, annealed Cu surface (1.2 GPa). The specific wear rate (volumetric wear normalized by the load applied and the distance covered) is found to be constant (0.015 mm3 N−1 m−1) up to 8 N load and then suddenly increased to (0.0267 mm3 N−1 m−1) at 10 N (Fig. 9). This observation can be explained on the basis that the strain hardened Cu (in the form of pile-up) is brittle in nature and at 10 N load, this hard Cu gets worn out. This can be validated by comparing the height of the piled-up Cu at 8 and 10 N load. If it had been only the ploughing action, the height of the Cu pileup should increase with the increasing load as the extent of the plastic deformation is high. From 2 to 8 N load, the height of the Cu pile-up increases but, at 10 N load, it is found to be lower than 8 N load which indicates that wear-out of hard and brittle (deformed) Cu is from the pile-up.
The incorporation of 7 vol% SiC nanoparticle in Cu matrix (C200A3S450) has resulted in lower corrosion rate than Cu – 2 vol% SiC (C200A3S350) because of uniform dispersion of high amount of reinforcement which impedes the initiation of the corrosion potential of the coatings. For both electrodeposited single-layered Cu-SiC nanocomposite coating (C200A3S450) and Cu, Cu-SiC FGC, the top surface composition is similar and therefore, the corrosion rate is expected to be identical. However, the corrosion rate of Cu, Cu-SiC FGC is found to be lower than that of C200A3S450. This is because the corrosion process does not only take place at the top surface (Cu- 7 vol% SiC) rather, proceeds throughout the thickness where the microstructure is changing after every 12 μm. In Cu, Cu-SiC FGC, the Cu near to SiC nanoparticles dissolutes preferentially at the top surface due to galvanic corrosion (Case A in Fig. 7) and when the corrosive medium channelizes vertically down (from top to second layer), the net area of Cu (anode) starts increasing and the relative percentage of SiC (cathode) decreases (Case B and C in Fig. 7). As soon as the top layer of Cu FGC is exposed (Case D in Fig. 7), the Cu areas beneath the SiC nanoparticles undergo uniform corrosion and those Cu sites are much smaller as compared to that available at the top surface (Cu- 7 vol% SiC layer). Hence, it can be stated that the five-layered Cu, Cu-SiC FGC is under a conjoint influence of both galvanic and uniform corrosion as each layer is 12 μm thick. Since, the rate of uniform corrosion is lower than the galvanic corrosion, the net corrosion rate of the Cu, Cu-SiC FGC is less than C200A3S450 where only galvanic corrosion takes place throughout the coating (60 μm thick). This phenomenon is shown with the help of schematic (Fig. 7). Fig. 8 shows the morphology and elemental mapping of the electrodeposited single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450) and Cu, Cu-SiC FGC after corrosion. The surface is rough and the presence of pits is observed in Fig. 8a while, in Fig. 8c, the appearance of surface is smoother and compact than C200A3S450. It is due to the higher surface roughness of as deposited C200A3S450 than Cu, Cu-SiC FGC [25]. The elemental mapping shows that the dissolution
3.2.2.2. Single-layered Cu coating (C200A3). Fig. 11 shows the FESEM and 3D OSP images of the wear tracks with depth profile from the 60 μm thick electrodeposited single-layered Cu coating (C200A3) at 2, 5, 8 and 10 N load. It is observed that the wear track width increases with increase in load from 2 to 10 N but the increment is less than the uncoated, annealed Cu (Fig. 11a, c, e and g). It is also observed from the depth profile that there is no pile-up at the edge and the wear depth does not vary significantly with increase in load (Fig. 11b, d, f and h) unlike that in Fig. 10. The shape of the wear track after removal of the ball is observed to be spherical (Fig. 3c). It shows that the wear takes 838
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Fig. 7. A schematic representation of the cross-sectional view of corrosion in Cu, Cu-SiC FGC.
Fig. 8. FESEM images of (a) C200A3S450 and (c) Cu, Cu-SiC FGC after corrosion and (b and d) elemental mapping of Cu (red color) and Si (green color) corresponding to (a and c, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 839
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and there is less energy available to wear the top hard layer. It is observed that there is an increase in the wear track width with increase in load to 8 N and at the same time, the prevention of the flow of material to edges, which is similar to the observations in the case of soft Cu and C200A3, respectively. At 10 N load, the wear track depth (Fig. 12h) is higher than that observed in C200A3 (Fig. 11h) which indicates that the counter ball has reached the second layer of Cu FGC (less hard than the top layer) but, the increase in wear track width is less than C200A3 (Fig. 11g). Thus, at 10 N load in Cu FGC, the energy is spent to wear the outer hard layer and specific wear rate is high (Fig. 9). The hardness of 2.7 GPa and three-layered microstructure graded structure of Cu FGC is observed to be responsible for the improvement in wear rate (0.0024 ± 0.0003 mm3 N−1 m−1 at 2, 5 and 8 N and 0.0048 mm3 N−1 m−1 at 10 N load) as compared to C200A3. In soft, annealed Cu, the main mechanism is ploughing in which the deformed material is accumulated along the wear track edges whereas in electrodeposited, hard Cu (C200A3), wear takes place because of material loss. In the case of Cu FGC, both of the mechanisms take place simultaneously. The Cu FGC is made-up of hard exterior (fine microstructured hard Cu) and soft interior (coarse microstructured soft Cu) due to which the contact load gets distributed over larger volume and the soft interior under the entire hard surface deforms plastically.
Fig. 9. The specific wear rate versus load of electrodeposited C200A3, Cu FGC, C200A3S450, Cu, Cu-SiC FGC and uncoated, annealed Cu.
place only by the removal of material during sliding wear. The electrodeposited single-layered Cu (C200A3) has small crystallite size (25 nm) and the morphology is near to spherical with a hardness of 2.5 GPa due to the presence of high twin density [25]. Since, the C200A3 deposit is full of deformation twins that enhance the yield stress and thus, the pile-up effect is not significant. The specific wear rate (0.011mm3 N−1 m−1) as shown in Fig. 9 is observed to be independent of the load and is less than uncoated, annealed Cu.
3.2.2.4. Single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450). Fig. 13 shows the wear depth profile with 3D OSP and FESEM images along with the elemental mapping of the wear tracks from the electrodeposited single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450) at 2, 5, 8 and 10 N load. Fig. 13a and d and Fig. 13b and e show that the wear track depth
3.2.2.3. Cu FGC. Fig. 12 shows the FESEM and 3D OSP images of the wear tracks with depth profile from Cu FGC at 2, 5, 8 and 10 N load. It is observed that the wear track width increases with load (Fig. 12a, c, e and g). The wear track depth (Fig. 12b, d and f) is less than C200A3 as part of the energy is spent for plastic deformation of the soft interior
Fig. 10. (a, c, e and g) FESEM and (b, d, f and h) 3D OSP images of wear track with depth profile from the uncoated, annealed Cu at 2, 5, 8 and 10 N load, respectively. 840
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Fig. 11. (a, c, e and g) FESEM and (b, d, f and h) 3D OSP images of wear track with depth profile from the single-layered Cu coating (C200A3) at 2, 5, 8 and 10 N load, respectively.
accumulate at the edge of the wear track. Hence, the track edges are uneven and this phenomenon is shown with the help of schematic (Fig. 15).
and width, respectively increases till 5 N load. Fig. 13 (g and j) show that the wear track depth does not increase significantly but width increase at 8 and 10 N loads (Fig. 13h and k). The elemental mapping of the wear tracks is shown in Fig. 13c, f, i and l corresponding to Fig. 13b, e, h and k, respectively at different loads where, loose SiC particles are seen along the edges of track. The C200A3S450 has a hardness of 3.4 GPa due to the uniform dispersion of SiC nanoparticles in the Cu matrix through-out the thickness (60 μm) and surface roughness of 1.8 μm [25]. The SiC nanoparticle is harder than the counter hardened steel ball due to which it resists wear. Therefore, the wear is restricted only to the ductile Cu matrix and the shape of wear track profile is tapered (Fig. 13a, d g, and h). However, the specific wear rate at 2 N is higher for C200A3S450 than C200A3 because of high surface roughness. It can be explained from Fig. 14, where same load is applied on both C200A3 and C200A3S450 but, the contact points of the counter ball are less in C200A3S450 unlike C200A3 and thus, the total pressure is more in C200A3S450. This results in the wearing-away of surface irregularity to form a smooth surface and wear-debris is added to the volume loss and ultimately, the specific wear rate increases. In C200A3 and Cu FGC, the as deposited surfaces are smooth, the sharp edges are absent and during the sliding wear, only material loss takes place from smooth surface. Therefore, until the surface becomes smooth the C200A3S450 undergoes wear in form of elimination of surface irregularities and in C200A3 and Cu FGC this does not take place as the deposits are smooth prior to wear. Hence, initially the wear rate of C200A3S450 is more than C200A3 till the irregularities vanish. Once both the samples (C200A3 and C200A3S450) become smooth, C200A3S450 wears at a lower rate than C200A3 and justifies the statement made in Section 3.2.1. Further, as the SiC nanoparticles are distributed through-out the matrix, they act as inhibitor towards the sliding motion of the ball. Therefore, the ball plastically deforms the Cu matrix and the loose SiC nanoparticles flow away from the wear track along with the Cu and
3.2.2.5. Cu, Cu-SiC FGC. Fig. 16 shows the wear depth profile with 3D OSP and FESEM image along with the elemental mapping of the wear tracks from five-layered Cu, Cu-SiC FGC at 2, 5, 8 and 10 N load. The wear track depth penetrates through the first to fourth layer (each layer 12 μm thick) with increase in load (Fig. 16a, d, g and j). However, the increase in the wear track width (22 ± 3 μm) is independent of load as shown in Fig. 16b, e, h and k. This is due to the smoother (0.9 μm) surface, higher hardness (3.8 GPa) and more compact structure of Cu, Cu-SiC FGC than Cu - 7 vol% SiC nanocomposite coating (C200A3S450) [25]. The outer layer (Cu-7 vol% SiC) in Cu, Cu-SiC FGC is hard due to the uniformly dispersed SiC in the Cu matrix followed by Cu - 2 vol% SiC layer and these two nanocomposite layers are followed by a Cu FGC. During wear, the top layer undergoes the same effect as described for C200A3S450. Apart from that, part of the energy is utilised to deform the underneath pure Cu layers (Cu FGC). Therefore, the Cu, CuSiC FGC has lower specific wear rate than C200A3S450. The elemental mapping to the wear tracks is shown in Fig. 16c, f, i and l corresponding to Fig. 16b, e, h and k, respectively at different loads where SiC loose particles are seen along the edges of track. 4. Conclusions Cu is generally used in electrical and structural applications under ambient conditions. Under high loads and in the presence of high concentration of chlorine, the applications of pure bulk Cu are restricted. The performance of pure bulk Cu in the above mentioned harsh conditions can be improved by electrodepositing Cu based coatings. In this work, the wear and corrosion properties of the single-layered and functionally graded Cu based coating are evaluated in order to find its 841
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Fig. 12. (a, c, e and g) FESEM and (b, d, f and h) 3D OSP images of wear track with depth profile from Cu FGC at 2, 5, 8 and 10 N load, respectively.
Fig. 13. (a, d, g and j) wear depth profile with 3D OSP and (b, e, h and k) FESEM images, (c, f, i and l) elemental mapping of Cu and Si corresponding to (b, e, h, and k, respectively) of the wear track from single-layered Cu-7 vol% SiC nanocomposite coating (C200A3S450) at different loads.
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Fig. 14. Schematic illustration of (a) C200A3 (b) and (c) C200A3S450 at low and high load.
Fig. 15. Schematics on SiC nanoparticles pulled-out along the edges of the wear track during sliding wear.
Fig. 16. (a, d, g and j) wear depth profile with 3D OSP and (b, e, h and k) FESEM images, (c, f, i and l) elemental mapping of Cu and Si corresponding to (b, e, h, and k, respectively) of the wear track from Cu, Cu-SiC FGC at different loads.
• Cu FGC and Cu, Cu-SiC FGC have improved corrosion resistance than C200A3 and C200A3S450, respectively. • A modified volume loss formula is proposed in terms of wear track
suitability as potential protective coatings in the sectors of electrical, construction, pipelines and marine. The following conclusions can be drawn from the present study.
• The corrosion rate of single-layered Cu coatings increases with increase in the deposition current density. • The corrosion rate of single-layered Cu-SiC nanocomposite coating
• •
(C200A3S350) is lower than the single-layered Cu coatings and the rate decreases further with increase in the amount of uniformly dispersed SiC nanoparticles in the Cu matrix (C200A3S450). 843
width and depth which are measured independently from FESEM and 3D OSP, respectively. Cu FGC and Cu, Cu-SiC FGC have lower wear rate than C200A3 and C200A3S450, respectively. For low load based applications, it is recommended to use Cu FGC while for high load Cu, Cu-SiC FGC should be used.
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.06.050.
[16]
Notes [17]
The authors declare no competing financial interest. [18]
Declaration of interest None.
[19]
Contributions
[20]
All authors have contributed equally. [21]
References
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