Microstructure, mechanical and high temperature tribological behaviour of graphene nanoplatelets reinforced plasma sprayed titanium nitride coating

Journal Pre-proof Microstructure, Mechanical and High Temperature Tribological Behaviour of Graphene Nanoplatelets reinforced Plasma Sprayed Titanium Nitride Coating Shreshtha Ranjan, Biswajyoti Mukherjee, Aminul Islam, Krishna Kant Pandey, Rohit Gupta, Anup Kumar Keshri

PII:

S0955-2219(19)30714-9

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.10.043

Reference:

JECS 12805

To appear in:

Journal of the European Ceramic Society

Received Date:

20 July 2019

Revised Date:

9 October 2019

Accepted Date:

20 October 2019

Please cite this article as: Ranjan S, Mukherjee B, Islam A, Pandey KK, Gupta R, Keshri AK, Microstructure, Mechanical and High Temperature Tribological Behaviour of Graphene Nanoplatelets reinforced Plasma Sprayed Titanium Nitride Coating, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.10.043

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Microstructure, Mechanical and High Temperature Tribological Behaviour of Graphene Nanoplatelets reinforced Plasma Sprayed Titanium Nitride Coating Shreshtha Ranjan, Biswajyoti Mukherjee, Aminul Islam, Krishna Kant Pandey, Rohit Gupta, Anup Kumar Keshri* Plasma Spray Coating Laboratory, Metallurgical and Materials Engineering Indian Institute of Technology Patna, Bihar, India - 801106 *

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Corresponding author: Ph: +91-612-3028184. Email address: [email protected] (A.K. Keshri)

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Abstract

In the present study, graphene nanoplatelets (GNPs: 1-2 wt. %) reinforced TiN coating were

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successfully fabricated over titanium alloy using a reactive shroud plasma spraying technique. All coatings were completely oxide free, while the addition of GNPs suppressed the non-

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stoichiometric TiN0.3 phase. Improvement of 19%, 18% and 300% in hardness, elastic modulus and fracture toughness was achieved by mere addition of 2 wt. % GNP. The addition of GNP in

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TiN also reduced the wear volume loss and the wear rate of the coatings for the entire range of temperature (293-873K). Moreover, GNPs also manifested the coefficient of friction (COF) of the coating. Post wear characterization revealed that the presence of GNP throughout the wear

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track even at 873K. The multi-layer structure of GNPs assisted in long term lubricity to the surface and increased the wear resistance of the coating.

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Keywords: Titanium Nitride, Graphene Nanoplatelets, Shroud plasma spray, Mechanical property, High temperature tribological behavior

1. Introduction Titanium nitride (TiN) has established itself as one of the most encouraging hard coating candidates, serving the industrial needs since last two decades. Their high strength to weight ratio, higher hardness (>15 GPa) and melting point (~3273K) makes them a perfect candidate to

protect underlying surfaces [1-3]. However, in the current scenario, sectors like the aerospace, power generation and metal working industries demand material that are capable enough to protect running assemblies that are constantly exposed to continuous sliding motion at elevated temperatures (>773K) [4]. Therefore, in order to cope with these ever-increasing demands, it is essential for TiN to maintain its structural integrity while offering a low coefficient of friction (COF) at high temperatures. Although, ample research has been made for studying the room temperature tribological behavior of TiN coatings, the literatures involving high temperature

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tribological behavior is still scarce [5-6]. Mitchell and co-workers were among the first to investigate the tribological properties of thin (3

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μm) TiN coating at temperatures ranging from room temperature (293K) to 773K [7]. Their investigation showed that the thin TiN coating considerably reduced the COF and wear upto

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573K. However, at 773K, the progressive wear resulted in thinning of the coating and subsequently increased the COF and wear of the surface suggesting that a thick coating is a necessary mandate for achieving enhanced tribological behavior at high temperature. In this

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regard, our recent study focused in developing a scalable and rapid protocol to synthesize oxide free thick TiN coating using a modified reactive plasma spraying technique [8]. This hard-to-

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achieve TiN coating mechanism was attributed to commendatory brisk reaction taking place between Ti powder and nitrogen gas in extremely high temperature of plasma plume [8]. The fabricated coatings also displayed promising room temperature tribological behavior. Similar

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observation has also been made by other groups during the synthesis of TiN coating by reactive plasma spraying [9-10]. They fabricated a thin layer of TiN coating having a higher hardness value (1319-1325 HV) [9-10].

However, one prime factor that has been neglected throughout is the dependence of brittleness of

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TiN on its tribological properties. In general, the wear rate of a material is inversely proportional to the toughness of the material. Hence, the lower fracture toughness (~2 MPa.m0.5) of TiN limits its applications as the toughness is one of a crucial factor in defining the reliability and safe operation of critical components [11]. Hence, despite having higher hardness, the intrinsic brittleness of TiN restricts its anti-wear performance. Previous works have tried to address this issue by reinforcing nanofillers viz., Al2O3, ZrO2, TiO2 etc. in ceramic coating [12-15]. Although, these nanofillers provided slight improvement in fracture toughness, it is believed that

the intrinsic brittleness of these nanofillers could have adverse effect on the tribological properties of the coating. A prospective solution to overcome this perplexity is the addition of higher aspect ratio nanofillers which includes carbon black, carbon nanotubes or graphene nanoplatelets. In the recent past, these reinforcements have proved themselves as an effective lubricating as well as strengthening agent due to their stacked lamellar structure and high strength [16-17]. Henceforth, only a meager addition of these nano-fillers offers significant enhancement in strengthening and

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lubrication of the matrix, which otherwise is not possible with the introduction of higher percentage of brittle ceramics as reinforcements. A previous work of ours has shown that the

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addition of 8 wt. % CNT in Al2O3 coating provided improved wear resistance of 76% at high temperature (873K) [18]. Raman spectra of the wear track confirmed the presence of defected

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CNT even after exposing the coating to such high temperature. The enhancement in the wear resistance of coatings was attributed to the uniform coverage by protective tribo-film on the wear surface as well as the higher fracture toughness of the coating offered by the introduction of

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CNT. However, in recent past, GNP is been explored as a better substitute for CNT, both as strengthening and lubricating agent [19-21]. The planer 2D structure of GNP compared to the

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rolled 1D structure of CNT translates to much higher aspect ratio of the reinforcement [22]. Porwal et al. investigated the wear resistance behavior of GNP reinforced SiO2 coating at room temperature and observed an increase in ~8.5% times in wear resistance after the addition of the

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reinforcement [23]. This enhancement in wear resistance of the coating was primarily accredited to the 2D structure of GNP that provided lubrication during the wear. However, the study did not mention anything about the retention and post structure of the reinforcement. The retention of GNP during and after wear is a crucial factor in determining the overall applicability of these coatings in long run applications. Moreover, to the best of our knowledge, not a single article

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exists in the literature that report the high temperature tribological behavior of GNPs reinforced TiN coating.

Hence, motivated by the above investigations and also the scarcity of literatures in current topic, this study aims to study the complete mechanical behavior as well as high temperature tribological behavior of GNPs reinforced TiN coating. Three different compositions with GNPs reinforcement viz. TiN, TiN-1wt. % GNP and TiN-2 wt. % GNP coatings will be fabricated

using modified reactive plasma spraying. Hardness, elastic modulus, fracture toughness and high temperature tribological behavior of the coatings will be performed. The tribological properties of the coatings will be analyzed from RT temperature to 873K. The novelty of the present work lies in two folds, i.e., (i) role of GNPs in TiN as well as its effect on microstructural and mechanical properties and (ii) tribological performance of the composite coating at high temperature. It is expected that lubrication behavior of GNPs along with a stable cubic phase formation of TiN matrix could lead to enhanced wear resistance at elevated temperature.

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2. Experimental Details 2.1. Powder Preparation

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Titanium powder (Ti) (Particle size: 20-40 μm; Purity: 99.9%) was procured from Trixotech Advanced Materials Pvt. Ltd, India. The GNPs (Size: 5μm; xGNP-M-5 was obtained from XG

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Sciences, Lansing, MI, USA. The as-received GNPs were ultrasonicated in a bath ultrasonicator (in acetone) for 30 minutes. The GNPs (1-2 wt. %) were then mixed with Ti powder using a

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planetary ball mill (PM-100, Retsch, Germany) for 2 hours at 400 rpm using zirconia ball. The

and Ti2G respectively.

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compositions: Ti, Ti-1 wt. % GNP and Ti- 2 wt. % GNP shall henceforth be referred as Ti, Ti1G

2.2. Fabrication of Plasma Sprayed Coating

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The powders were sprayed on titanium alloy (Dimension: 100 mm × 20 mm × 3 mm) using a plasma spraying system (9 MB plasma gun, OerlikonMetco, USA). Prior to deposition, the substrates were cleaned in an ultrasonic bath (distilled water and ethanol). The substrates were then grit blasted with alumina grit (20 grit size) to generate surface roughness.Nitrogen and Hydrogen were used as primary and secondary gas respectively, whereas Argon was used as

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carrier gas for powder feeder. An inert atmosphere shroud was attached to the plasma gun and high purity Argon (Ar) was fed to the shroud. A stand-off distance of 80 mm was managed throughout the coating process in order to shorten the heating time and increase the substrate temperature. These two factors also play a key role in accelerating the reaction between the nitrogen atmosphere and the in-flight Ti powder particles [24]. The optimized plasma parameters to synthesize three different coatings are provided in Table-1. Table 1: Optimized plasma parameters for fabrication of TiN, TiN1G and TiN2G coatings

Plasma Spray Parameters

Value 524

Voltage (V)

67

Primary gas flow, Nitrogen (SCFH)

110

Secondary gas flow, Hydrogen (SCFH)

02

Shroud gas Pressure, Nitrogen (psi)

30

Stand-off distance from the substrate (mm)

80

Powder feed rate (g/min)

10

Substrate preheat temperature (°C)

250

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Current (A)

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Figure 1 shows the schematic of the experimental setup for the shroud attachment for atmospheric plasma spraying of TiN coatings. The shroud attachment in front of gun nozzle

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envelops the plasma flame and cut down its exposure with the surrounding environment, hence

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minimizing the oxidation of the powder during spraying.

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Figure-1. Schematic of the experimental setup for the shroud attachment for atmospheric plasma spraying

2.3. Microstructural and phase Characterizations The morphology of the initial powders, cross-section, fracture surface and wear track of the coatings were investigated using a Field Emission Scanning Electron Microscope (FE-SEM) (Zeiss, Sigma HD, UK) equipped with EDS (OXFORDX-Max, UK) at an operating voltage of 5 kV. X-ray diffraction (TTRAX III, Rigaku, Japan) was carried out to evaluate the phase of the

powder and coatings using Cu-Kα radiation (λ = 0.154 nm). Raman spectroscopy (Renishaw, Model 3900S, UK) was employed to validate the retention and structural changes in GNP. An excitation laser wavelength of 514 nm of spectral resolution was used to generate Raman spectra. Further, High Resolution Transmission Electron Microscopy (HRTEM) (FEI Tecnai, USA) was carried out to verify the structure of the GNPs. Helium gas pycnometer (Ultrapyc, Model 1200e, Quantachrome Instruments, USA) operating at an outlet gas pressure of 0.34 bars in a 0.25 cm3 cylindrical sample cell was used to calculate the apparent density of the coatings.

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2.3. Mechanical properties of the coatings Micro-hardness, elastic modulus and fracture toughness the coatings were measured using an

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instrumented micro-indentation and scratch tester (Microtest, Model MTR 3, Spain). The hardness (H) and reduced elastic modulus ( ) of the coatings were measured using a load of 2N

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with holding time of 10s.The actual elastic modulus of the coatings was calculated using the Equation 1 [25].

and

represents the Poisson's ratio of indenter tip (Vicker’s) (0.22) and the coating

(0.25), whereas,

and

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Where,

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(1)

represents the elastic modulus of indenter and coating respectively.

A load of 30N was used to generate cracks and the fracture toughness (KIC) of the coatings were

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calculated using the Anstis equation (Equation 2) [26].

(2)

Here, H and E denotes hardness and elastic modulus (in GPa) of the coating respectively. P is the

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applied load (in N) for fracture and C is the radial crack length (in µm) at the same load. The total 10 number of indentations were performed and the average values of H and E was reported with standard deviations.

2.4 Tribological properties of the coatings A ball on disc tribometer (Ducom, Model: TR-20LE-CHM800, India) was used to assess the wear properties, i.e., wear volume loss, wear rate and coefficient of friction of the coatings. Wear tests were carried out at 250 rpm and at constant normal load of 80 N at different temperature

(293K, 473K, 673K and 873K) for 60 min. Tungsten carbide (WC) ball (Dia: 10 mm) was used as the counter-body to slide against the coating. The coefficient of friction was measured for all the coatings at all temperature throughout the tests. The depth and width of the wear tracks was calculated using a Nano Map-3D optical profiler (AEP Technology, USA). The wear volume loss was investigated by multiplying the cross-sectional area and depth of the wear track. The specific wear rates (mm3N-1 m-1) were calculated as total volume loss per unit contact area per revolution using Equation 3, which is the classical Archard’s equation.

Where,

(mm3) is the volume loss during wear,

(3)

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Wear Rate (WR) =

(N) is the normal load, and L (m) is the total

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sliding distance.

3. Results and Discussions

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3.1 Microstructural and phase analysis of plasma sprayed coating

Figure 2a-c illustrates the FE-SEM images of Ti, balled milled Ti-1G and Ti-2G powders

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respectively. The average size of these powders lies in the range of 20-40 µm. It is seen from Figure 1b and c that the GNPs are uniformly distributed throughout the Ti matrix and are free from any agglomeration. It is believed that the simultaneous ball milling and ultrasonication of

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the powder has led uniform dispersion of GNPs over the Ti matrix. The presence of transparent GNPs can also be seen over Ti matrix (Figure 2d).GNPs, in general possess higher stress transferwhich could greatly enhance the reinforcement efficacy of the composite coatings [19, 27]. It can be envisaged that these non-agglomerated and uniformly distributed GNPs could have a positive influence in proficient stress transfer between the matrix and the reinforcement in the

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coating.

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Figure 2:FESEM images of (a) Ti (b) Ti1G and (c) Ti2G powder used for fabricating the plasma

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sprayed coatings. Yellow arrows represent the distribution of GNPs throughout the Ti matrix. (d) High magnification FE-SEM image of Ti2G powder showing the presence of transparent GNP

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over Ti matrix.

Figure 3a-c illustrates the cross-sectional FESEM images of TiN, TiN1G and TiN2G coatings fabricated using the shroud plasma spraying. All the three coatings show similar thickness (~400

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μm) and are well integrated with the substrate. It is observed that the pores in the coating have gradually reduced with the addition of GNP, which hints towards better densification of the coating after the addition of the reinforcement. The relative density of TiN, TiN1G and TiN2G coatings were measured to be 85.6±1.9%, 91.8 ± 0.9% and 93.3 ± 0.8% respectively, which

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proves that the addition of GNP has indeed led to the densification of the coatings. Since, GNPs possess very high thermal conductivity of ~3000 W/m-K [28], and were uniformly distributed throughout the matrix, it is anticipated that the addition of GNPs have dissipated the heat evenly throughout the matrix. This resulted in uniform melting of the powders during plasma spraying which led to the better densification of the GNP reinforced coatings.

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Figure 3:Cross-sectional FESEM images of (a) TiN (b) Ti1G and (c) TiN2G coatings over

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titanium alloy substrate.

The XRD of feedstock Ti powder and TiN, TiN1G and TiN2G coatings are shown in Figure 4a. As expected, the Ti powder exhibits peaks corresponding to α-Ti phase. On the other hand, the TiN, TiN1G and TiN2G coating exhibits dominant peaks corresponding to cubic TiN phase. The

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formation of TiN phase was achieved by the reaction of Ti powder and the primary gas (N2) at high temperature. The diffusion of N into the Ti particleoccurs in-situ during plasma spraying at temperature higher than 2273K, leading to the formation of TiN [29]. The reaction between Ti and N2 is rapid, instantaneous and is accomplished before the splat reaches the substrate.However, it is to be noted that, in addition to the cubic TiN phase, a minor nonstoichiometric TiN0.3 peak at 39.6° was also observed in the TiN coating. The formation of TiN0.3 is due to the diffusion of N to the surface of Ti particles instead of volumetric diffusion, leading to a rather non-stoichiometric reaction. Many researchers have observed the formation of similar

non-stoichiometric in reactive plasma spraying [29-33]. However, surprisingly, the minor nonstoichiometric (TiN0.3) peak reduces in intensity with the addition of 1 wt. % GNP and completely disappears after the addition of 2 wt. % GNP (Also shown in Figure 4b). It is worth mentioning here that as per author’s knowledge, this is the only study that have achieved complete oxide and non-stoichiometric phase free TiN coating using a modified reactive plasma spraying just by the addition of a proportionate amount of GNPs.The complete disappearance of TiN0.3 in TiN2G coating might be accredited to the higher thermal conductivity of GNPs, which

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has helped in uniform dissipation of heat throughout the matrix, making the complete reaction between Ti and N possible. Moreover, the addition of carbonaceous reinforcement such as CNT or GNP is said to lower down the in-flight particle velocity of the composite powder [34]. As a

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result, there is an adequate time for N to diffuse completely into Ti particle before solidification,

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thus aiding the eliminatingof non-stoichiometric TiN0.3 in Ti2G coating.

Additionally, upon carefully observing the spectra in Figure 4b, it is also observed that a peak corresponding to TiC (111) appears at 36.2° for the Ti1G coating and the intensity of the peak

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further increases with increase in GNP content (TiN2G). The formation of the TiC phase is due to the thermodynamically favorable reaction between Ti and carbon (GNP) during spraying of

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the powder at high temperature. Arvieu et al. reported the formation ofTiC at intermediate temperature in Ti-carbon composites [35]. It is to be noted that the formation of only minor amount of TiC, compared to that of TiN phase is due to the more favorable reaction between Ti

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and N2 (ΔH= -80.47±0.27kcal/mole) when compared to the reaction between Ti and C (ΔH= 43.85±0.39 kcal/mole) at high temperature [36]. It is anticipated that this minor amount of TiC

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phase might play an important role in determining the final properties of the coating.

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Figure 4: XRD pattern of (a) Ti powder, TiN, TiN1G and TiN2G coating (b) magnified image

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of region between 35°- 41° of TiN, TiN1G and TiN2G coating.

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However, it is to be noted that although XRD is an ideal tool in identification and differentiation of several phases in a multi-phase system, it sometimes fails to detect any minor phases (>2%) in the system [37]. Hence, HR-TEM was performed to further verify the results obtained using

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XRD. Figure 5 illustrates the low magnification HR-TEM image of the TiN2G coating showing the presence of GNP within the TiN matrix, while the inset shows the corresponding selected area electron diffraction pattern (SAED) of the embodied area. The pattern displayed a lattice spacing of 0.34 nm, resembling to the (002) plane of the planer GNP. Lattice spacing of 0.243, 0.211, 0.149, 0.127 and 0.121 nm corresponding to (111), (200), (220), (311) and (222) plane of

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cubic TiN was also observed in the pattern (ICDD Ref No.: 01-087-0632). In addition to that, a unique lattice spacing of 0.249 nm was also observed. It is believed that the simultaneous ball milling and ultrasonication of the powder has led uniform dispersion of GNPs over the Ti matrix. In addition, phase of TiC resembling to (111) was also observed (ICDD Ref No.: 98-015-9871). Importantly, no other phases were observed, which determines that the coating is composed of mainly phase with a minor amount of TiC.

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Figure 5: Low magnification HR-TEM image of TiN2G coating and its corresponding SAED

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pattern (inset).

3.2 Structural Integrity of GNPs after Plasma Spraying

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It is well known that GNPs starts getting oxidized at ~573Kand completely burns above 873K, when exposed to oxygen rich environment [38]. However, in this current work, shroud plasma spraying has been used in an attempt to cut down the direct contact between the GNPsand surrounding atmosphere. In order to get an insight on the retention of GNPs,Raman spectroscopy was carried out on the GNP reinforced TiN coatings. Figure 6 illustratesthe Raman spectra of

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Ti1G and Ti2G powders and corresponding coatings. The spectrum exhibits distinct D and G bands at 1352 cm-1 and 1577 cm-1 respectively for both the powders and the coatings [39].This indicates that the GNPs have been retained in the coatings even after exposing it in the harsh environment duringplasma spraying. It is also worth mentioning that no shift in the D and G peaks were observed in either direction. This confirms that the GNPs did not undergo any tensile or compressive stress during the impact with the substrate at a velocity close to Mach 1.In one of our earlier study, a substantial shift in the D peak towards lower wavelength was observed for high velocity oxy fuel (HVOF) sprayed aluminum (Al)-CNT coating [40]. The CNTs, in their

study appeared to be highly damaged when examined using electron microscopy. However, the velocity involved in HVOF is much higher (~3 Mach) compared to that of plasma spraying. Therefore, it is anticipated that in this present study, the velocity involved in plasma spraying is not sufficient to substantially damage the GNPs. Additionally, the quality of the GNP after plasma spraying was further accessed by reviewing the ID/IG ratio of both the coating with respect to the corresponding powder [38]. The ID/IG ratio increased marginally from 0.14 to 0.20 for the TiN1G and from 0.13 to 0.21 for the TiN2G coating. This slight increment in the ID/IG

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only marginal with no major damage to their structural integrity.

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ratio for both the coatings indicate that the defects introduced in GNP during plasma spraying is

Figure 6:Raman spectroscopy of Ti-1G, Ti-2G powder and their corresponding coating. 3.2 Mechanical properties of the plasma sprayed coating 3.2.1 Hardness and Elastic Modulus

Figure 7 shows the load vs. displacement (L-D) curves of TiN, TiN1G and TiN2G coatings obtained at an applied load of 2N. From the L-D curves, it is clear that the penetration depth for TiN2G is the lowest for the same applied load, followed by TiN1G and TiN coating. The hardness values of TiN coating was measured to be 17.02 ± 0.9 GPa which increased by 12% (18.99 ± 1.1GPa) after the addition of mere 1 wt. % GNP. The hardness of the coating further increased by 19% (20.25 ± 1.3 GPa) when the content of GNP was increased to 2 wt. %. This clearly signifies that the hardness of the TiN coating has increased after the addition of GNP.In

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general, the hardness of a plasma sprayed coating is significantly lower than its bulk counterpart due to the splats like morphology which tends to slide over one another on application of external load. One plausible explanation for the improvement of hardness after the addition of

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GNP is the resistance of splat movement during localized indentation. Figure 8a shows an intact GNP between two splats, a phenomenon known as ‘splat sandwiching’. The extremely high

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thermal conductivity coefficients (λ) mismatch between GNP (3000-6000 W/m-K) and TiN (19 W/m-K) [41] induces localized heating between them which gives rise to a strong GNP-TiC

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interface (Figure 8a). Islam et al. also observed similar phenomenon in spark plasma sintered (SPS) CNT-TiC composite which resulted in increase in the hardness [42]. Moreover, it is also

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anticipated that the 2D planer structure of GNP translates to much higher area of interaction,

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which have helped in increasing the hardness of the GNP reinforced coatings.

Figure 7: Load vs. displacement (L-D) curve of TiN, TiN1G and TiN2G coating at 2N load

The elastic modulus of the TiN coating was measured to be 314.08 ± 7.8 GPa, which is comparable to the values available in literature [43]. Improvement of 10% was observed upon the addition of 1 wt. % GNP (TiN1G), while the improvement was 18% upon increasing the GNP content to 2 wt. % (TiN2G). This increase in the elastic modulus after the addition of GNP can be attributed primarily to the following three factors: (i) very high strength and elastic modulus of the reinforcement, i.e., GNP (~1 TPa) [19] (ii) uniform distribution of GNP

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throughout the TiN matrix and (iii) strong interfacial bonding between the reinforcement and matrix.A strong interfacial bonding generally resists the selective deformation of the matrix by effectively transferring the load from the matrix to the reinforcement. Figure 8b presents the

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fracture surface of the TiN2G coating showing the protruded GNPs from the TiN matrix. The length of the protruded GNP is around 500 nm, which is much lesser than that of the GNP (~5

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µm) used in this study. Shorter pull-out length of the GNP clearly indicates a strong bonding between GNP and TiN. Debrupa et al. also observed similar CNT pull-out from hydroxyapatite

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(HAP) matrix which led to 20% increase in the elastic modulus of the composite [44]. Moreover, upon carefully observing the edges of GNP in Figure 8b, shearing and tearing of GNP can be

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observed. This indicates that in some cases, the GNPs rips apart instead of detaching itself from the TiN matrix, indicating strong interfacial bonding between GNP and TiC, which eventually

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have a positive effect in the elastic modulus of the coating.

Figure 8: Fractured surface of TiN2G coating showing (a) GNP sandwiching between two splats and (b) GNP pull-out from TiN matrix. 3.2.2 Fracture toughness of the coatings The fracture toughness of the coatings was evaluated by indentation cracking method. The fracture toughness of the TiN coating was measured to be 1.3± 0.2 MPam0.5. However, this value is slightly lower than the values reported in the literature (~1.9 MPam0.5) for sintered TiN [11]. One probable reason for the fracture toughness to be on the lower side is the higher porosity of

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plasma sprayed TiN coating (~12%) compared to that of the bulk sintered TiN (1-4%). However, the fracture toughness of the TiN coating drastically increased by 133% from 1.3± 0.2 MPam 0.5

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for TiN to 3.1 ± 0.2 MPa m0.5 after the addition of mere 1 wt. % GNP. Further, an exceptional increase in 300% was observed in the fracture toughness (5.2 ± 0.3 MPa m0.5) of the coating after

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the addition of 2 wt. % GNP. It is interesting to note that the fracture toughness obtained in this study after the addition of 2 wt. % GNP is the highest among the values previously reported in the literatures [11,45-46]. This drastic improvement in the fracture toughness after the inclusion

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of mere amount GNP can be attributed to the simultaneous effect of the following major factors:

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(i) crack bridging, (ii) crack arrest and (iii) strong interface between GNP and TiN.

Figure 9a shows the high magnification FE-SEM image of the crack in TiN2G coating generated during indentation which provides the visual confirmation of simultaneous crack bridging and

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crack arrest by GNP. It can be clearly seen form Figure 9a that the GNP helps in bridging two splats together. It is anticipated that the crack can propagate easily through TiN which is intrinsically brittle. However, when a crack comes in the vicinity of GNP, much higher fracture energy is required for the crack tip to pass through the strong interface of GNP/TiN. This prevents the crack from wideningandgets arrested when it encounters another GNP in its path

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which eventually increases the fracture toughness of the coating.

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Figure 9: (a) High magnification FE-SEM image showing simultaneous crack bridging and

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crack arrest by GNP, (b) HR-TRM image showing the interface between GNP and TiN. Sample

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used in both the cases is TiN2G.

In addition to crack bridging and crack arrest, the nature of interface between the reinforcement

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and matrix also play a major part in defining the fracture toughness of a coating. Figure 9b presents a high magnification HR-TEM image of TiN2G coating showing the interaction between GNP and TiC. The formation of a secondary TiC phase can be observed in some area

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between GNP and TiN. The presence of TiC has previously been seen in the XRD spectra of the GNP reinforced coatings. The formation of TiC can mainly be attributed to the excellent wettability of GNP and Ti [47]. The reaction between GNP and Ti (Ti + C = TiC) can lead to the precipitation of carbide (TiC) at the interface [46]. The plasma sprayed GNP contains few broken or defective sites, as evidenced by Raman spectra, acts as reactive site between the

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reinforcement and TiN. This secondary carbide layer at the interface can eliminate the crevices and can form anchors or interlocks at the GNP/TiN interface to increase the fracture toughness of the coatings. In addition to the TiC precipitates, an amorphous interface of ~0.5 nm thickness can also be observed between GNP and TiN. The presence of this amorphous layer acts like ashock absorber at the GNP/TiN interface by introducing a viscoplastic resistance which in turn lowers the interfacial stress between the reinforcement and matrix [34,48]. This TiC layer helps in

reducing the stress concentration against applied loading which in turn improves the fracture toughness of the GNP reinforced coatings.

4.Wear behavior of the coatings 4.1 Wear Volume Loss and Wear Rate Figure 10a-f shows the 3D optical profilometer image of the wear track of TiN, TiN1G and TiN2G coating. For the sake of convenience, only the image of the wear track generated at 298K

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and 873K are shown here. At 298K, the depth of wear track of the TiN was measured as ~113 µm which reduced to 94 µm after the addition of 1 wt. % GNP. The lowest wear depth was

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recorded for the coating fabricated using 2 wt. % GNP (66 µm). Similar trend in wear depth was observed for the coatings tested at higher temperature, i.e., 873K. The wear depth for TiN

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coating was 22 µm, which reduced to 16 µm and 11 µm after the addition of 1 wt. % and 2 wt. % GNP respectively.

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Similarly, the track width of the TiN, TiN1G and TiN2G coatings were ~2.3, ~1.8 and ~1.5mm respectively at 298K and ~1.4, ~1.1 and ~1.0mm at 873K for all the three coating. Similar trends

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were also observed at the intermediate temperatures, i.e., 473K and 673K for both wear depth and wear width of the wear track.This clearly indicates that the addition of GNPs has aided in the reduction in both depth and width of the wear track for the entire range of test temperature

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(293K-873K).

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Figure 10: 3D optical profile of the wear track of (a-b) TiN, (c-d) TiN1G and (e-f) TiN2G coating at RT (298K) and 873K

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Now, utilizing the wear depth and width of the coatings, the wear volume loss and wear rate was calculated. Figure 10a shows the wear volume loss of the TiN, TiN1G and TiN2G coatings at four different temperatures, i.e., 298K, 473K, 673K and 873K. It is observed that at 298K, the addition of GNP has drastically reduced the wear volume loss by 62% from 8.3mm3 to 3.2mm3.It is interesting to note that, the above trend is also similar for the tests performed at all the temperatures. Figure 11b demonstrates the bar diagram depicting the wear rate of all the coatings at different temperatures.The wear rate of TiN at 298K was recorded at (276 ± 22) × 10-6 mm3 N1

m-1 for TiN coating which reduced to (180 ± 20) × 10−6 mm3 N−1 m−1 after the addition of 1 wt.

% GNP. However, TiN2G yielded the lowest wear rate of (107 ± 14) × 10−6 mm3N−1 m−1, translating to an overall reduction of 61% in the wear rate of the coating at 298K after the addition of 2 wt. % GNP. Similar decrease in trend in the wear rate was observed for all coatings operated at different temperatures as seen in Figure 11b.This clearly demonstrates that the addition of GNPs has a remarkable effect in reducing the wear volume loss and wear rate of the

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coatings operating in room temperature (293K) as well as in high temperature (873K).

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Figure 11:(a) Wear volume loss and (b) Wear rate of TiN, TiN1G and TiN2G coating at 298K, 473K, 673K and 873K

It can be recalled that the TiN1G and TiN2G coatings have much higher hardness and toughness

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compared to the TiN coating. The superior mechanical properties might result in lower wear volume loss and wear rate of the TiN1G and TiN2G coatings compared to TiN coating. Figure 12a shows the low magnification FE-SEM image of the wear track of TiN coating tested at 873K while Figure 12b shows the magnified area of the marked area in Figure 12a. Fine debris can be observed in the wear track of TiN indicating high material loss during the tribology test.The low

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hardness and toughness of the TiN permits easy withdrawal of wear debris (grains) during wear by ploughing the grains from the surfaces as the wear test progresses. This wear debris leads to a three body wear mechanism which increases the grain removal rate leading to higher wear volume loss and wear rate [49-50]. The easy withdrawal of the grains also lead to a relatively high roughness (Sq, root mean square height =2.38 µm) of the wear track as shown in Figure 12c.

On the other hand, the superior hardness and toughness of GNP reinforced coatings could help in increasing the wear resistance of the coating. Figure 12d presents the low magnification FE-SEM image of the wear track of TiN2G coating tested at 873K. Figure 12e which is the magnified view of the marked area represented in Figure 12d shows no visible grain pull-out during wear. It is anticipated that the GNPs holds the splats together and resists grain pull-out during wear, thereby decreasing the formation of wear debris in the wear track. Surface smoothening was also observed (Inset in Figure 12e) which represents almost no formation of wear debris on the wear

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track over prolonged duration of time. The wear track surface is much smoother (Sq=1.24 µm) than that of the worn TiN surface (Sq=2.38 µm) as shown in Figure 12f. The surface smoothening is attributed to high abrasion resistance of the GNP reinforced coating which is

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clearly observed by the lowest wear volume loss and wear rate for all temperatures.

Figure 12: (a) Low magnification FE-SEM image of wear track in TiN coating tested at 893K, (b) magnified image of the wear track of TiN showing ploughing of wear debris and (c) corresponding roughness of the wear track. (d) Low magnification FE-SEM image of wear track in TiN2G coating tested at 893K, (b) magnified image of the wear track of TiN2G surface smoothening during the wear and (c) corresponding roughness of the wear track.

3.4 Coefficient of Friction of the coatings Figure 13 shows the bar graph depicting the coefficient of friction (COF) of TiN, TiN1G and TiN2G coatings measured at 298K, 473K, 673K and 873K. The COF were measured at 80N and the average COF values of the coatings were obtained from the best-fit line in the steady value of the graphs for 3600s. The COF of the TiN coating at 293K was measured to be 0.37±0.06, which decreased by 3% to 0.36±0.03 for TiN1G which further reduced by 8% to 0.34±0.02 for TiN2G coating. The high COF for the TiN coating could be attributed to the chafing of hard wear debris

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between the ball and surface. All coatings showed similar reduction in COF for all temperature and TiN2G displayed the lowest COF among the three coatings. The lowest COF was recorded

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for the TiN2G coating operating at 873K (0.17±0.04). Hence, it is clear that the addition of GNP has definitely played a role in reducing the COF of the coatings operating at all temperature, i.e.,

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from RT (293K) to 873K.

Figure 13: Coefficient of friction of TiN, TiN1G and TiN2G coating at various temperatures (298-873K)

The lubricating capability of graphene materials is well known throughout the scientific community [51-52]. The load used in this study to evaluate the tribological properties of the coatings in high (80N). Hence, Raman spectra of the tracks were collected in order to verify the structural changes in GNP after the tribological study. Figure 14 shows the Raman spectra of the wear track of TiN2G coating operated at all different temperatures. The presence of D and G peaks hints towards the presence of GNPs on the wear track. Interestingly, D and G peaks were observed on all the spectra which suggest that GNPs were present even on the track operating at

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temperature as high as 873K. However, upon analyzing the ID/IG ratio, it is observed that the ID/IG ratio of the coatings lies between 0.8-0.9 which is much higher than the GNP used in this study (0.13). Since, D peak arises due to the defect density present in graphene structure,

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increase in ID/IG ratio suggest the structural damage in GNPs after the wear.

Figure 14: Raman spectra on wear track of TiN2G coating at different temperature. The GNPs present in the wear track provides the necessary graphitic lubrication which is the reason behind the reduction of COF. Figure 15 presents a high magnification image of the wear

track of TiN2G coating showing the presence of GNP after wear. The GNP looks to be sheared in the wear direction suggesting tearing off during the wear process. This is in accordance to the higher defects in the worn surface presented by the Raman spectra.In addition to the GNP attached to surface, the torn fragments of sheared GNP are known to travel in the wear direction during the process, further reducing the COF. The multilayer nature of GNPs (Figure 15) is also known to have a positive effect in reducing the COF of the reinforced coating. The presence of multilayer nature is known to provide consistencyin the COF by reducing ‘puckering effect’ [52-

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53]. The multilayer GNP also counteract any imperfect morphological feature, thereby proving a smooth lubricated surface between the ball and surface [53], leading to lower COF in GNP

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reinforced TiN coating.

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Figure 15: High magnification FE-SEM image of a GNP after wear test showing sheared edges. Numbers over the GNP represents different layers. The presence of the lubricating agent was validated using SEM EDAX as shown in Figure 16. It is clearly seen that a continuous layer of carbon is present throughout the wear track operating in both room temperature (Figure 16a) and high temperature (873K) (Figure 16b). A continuous lubricating film is essential in maintaining low COF throughout the entire cycle of wear process. This reconfirms that not only the bulk GNP has contributed to the low COF, but also the

fragmented GNP that spreads throughout the entire wear track as course of time. These GNPs also play a part in smoothening the wear track (Figure 16b) which in turn reduces the COF by filling up any morphological imperfections in the track. Hence, it is seen that the addition of GNP in TiN not only increases the wear resistance of the coating but also reduces the COF from

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RT (293K) to temperature as high as 873K.

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Figure 16: Line EDAX performed over wear track of TiN2G coatings at (a) 298K and (b) 873K.

Conclusions

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Oxide and non-stoichiometric phase free TiN reinforced GNPs (1-2 wt. %) coatings were fabricated using reactive shroud plasma spraying. The TiN1G and TiN2G coatings showed the presence of a secondary TiC phase. The GNPs survived the high temperature and did not display

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much structural defects post plasma spraying. The addition of GNPs increased the hardness, elastic modulus and fracture toughness by 19%, 18% and 300% respectively. The GNP reinforced coatings also exhibited higher wear resistance in term of wear volume loss and wear rate compared to TiN coating for entire range of temperature (293K-873K). The addition of

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GNPs also reduced the COF of the coatings which was consistent for both RT and high temperature. It is envisaged that these TiN-GNP coatings could open a new venture in the development of hard coatings for wholesome industrial applications.

Acknowledgements Authors of this paper, Shreshtha Ranjan and Anup Kumar Keshri acknowledge Indian Institute of Technology Patna for the financial support for carrying out this work. Further, Authors, also

acknowledges the financial support from Department of Science and Technology (DST), Government of India (GoI), India, Grant No. DST/ TSG/AMT/2015/149.The authors acknowledge Dr. Seema Sharma for providing Raman Spectroscopy in Centre for Nanoscience, IIT Kanpur.

Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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