Hardness and wear behaviour of multiple component coating on Ti-6Al-4V substrate by laser application

Journal Pre-proof Hardness and wear behaviour of multiple component coating on Ti-6Al-4V substrate by laser application Dipanjan Dey, Kalinga Simant Bal, Anitesh Kumar Singh, Asimava Roy Choudhury

PII:

S0030-4026(19)31453-6

DOI:

https://doi.org/10.1016/j.ijleo.2019.163555

Reference:

IJLEO 163555

To appear in:

Optik

Received Date:

9 July 2019

Accepted Date:

7 October 2019

Please cite this article as: Dey D, Bal KS, Singh AK, Choudhury AR, Hardness and wear behaviour of multiple component coating on Ti-6Al-4V substrate by laser application, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163555

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Hardness and wear behaviour of multiple component coating on Ti-6Al-4V substrate by laser application Dipanjan Dey1, Kalinga Simant Bal2, Anitesh Kumar Singh3, Asimava Roy Choudhury4,* 1,2,3,4

Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, West

*

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Bengal 721302, India Corresponding author: Prof. Asimava Roy Choudhury

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E-mail address: [email protected]

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Graphical abstract

Highlights

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Coating of B4C and SiC on Ti-6Al-4V using laser → formation of ceramic phases

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Ceramic phases (TiB2, TiC, TiSi2 and V8C7) → improves hardness and wear resistance

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Index = [(Volumetric wear rate)Coating/(Hardness)Coating] proposed

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(Index = 2.5 × 10−8 mm3/N-m/HV)Present study < (Index = 4.08 × 10−8 mm3/N − m/ HV)Reported literature

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(Scratch hardness)Coating > (Scratch hardness)Substrate → improved functionality of Ti-6Al4V

Abstract

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Different material compositions provide different hardness and wear resistance to Ti-6Al-4V substrate against the external load. In order to normalize the effect of different characteristics properties of the

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material added to the coating, the ratio of (volumetric wear rate)Coating to (hardness)Coating has been proposed as an ‘index’ in the present study for comparison between the reported studies on laser cladding

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of Ti-6Al-4V alloy. The lower ratio of (volumetric wear rate)Coating to (hardness)Coating would correspond to the higher wear resistance of the coating. According to the data reported until now on laser cladding of Ti-6Al-4V alloy, the minimum (volumetric wear rate)Coating to (hardness)Coating has been found to be

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4.08 × 10−8 mm3/N-m/HV (calculated from the reported data). In the present study, an attempt has been made to further reduce the ‘index’ by in-situ synthesis of ceramic particles in composite coating using

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B4C and SiC as precursor on Ti-6Al-4V alloy by laser cladding. It was observed that (a) (hardness)Coating ≈ 6.5 (hardness)Substrate, (b) (volumetric wear rate)Coating ≈ 13 (volumetric wear rate)Substrate and (c) (scratch hardness)Coating ≈ 1.83 (scratch hardness)Substrate. The ratio of (volumetric wear rate)Coating to (hardness)Coating

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for the present study was found to be lower (= 2.5 × 10−8 mm3/N-m/HV) as compared to that of the previously reported literature on laser cladding of Ti-6Al-4V alloy. The improvement in tribological property might have been possible due to the reaction of B4C and SiC with Ti-6Al-4V substrate forming a

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composite coating containing ceramic phases such as TiB2, TiC, TiSi2 and V8C7.

Keywords: Ti-6Al-4V; Laser; Coating; Scratch hardness; Volumetric wear rate; Hardness.

1. Introduction

High strength to weight ratio has led Ti-6Al-4V to be used extensively in aerospace, bio-medical industries and marine industry etc. [1–4]; however, its inferior tribological property has limited

its use in surface applications [5–8]. The lower wear resistance of Ti-6Al-4V substrate [9] has been reported to improve by developing hard and wear resistant coating on the surface [10]. Developing coating by laser is an efficient technique for surface modification because bulk substrate properties get affected insignificantly, and only the surface of the substrate is modified [11]. Laser surface coating can be done in two ways (a) in-situ and (b) ex-situ [12]. Due to the diffusion of hard ceramics particle in the metal matrix in the in-situ synthesis reaction, the coating formed through in-situ synthesis has been reported to show better performance under

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external loading as compared to the coating formed through ex-situ synthesis [13]. The laser has been used to trigger self-propagating high-temperature synthesis (SHS) reaction between the coating powders [14]. In the SHS reaction, heat is generated during the exothermic reaction,

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which causes the synthesis of the coating [15]. Development of coating by SHS reaction helps to fabricate the coating with minimum laser heat input, and also results in uniform coating [12, 14–

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17].

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Various powders like titanium with boron [5], Ti6Al4V alloy powder with boron nitride powder [8], Cr3C2 powder [11], B4C with TiO2 powder [18], Al, TiO2 and hBN powder [19] and Fe3Al,

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TiB2 with Al2O3 powder [20] etc have been used to improve the surface properties of Ti-6Al-4V. Nickel-based alloy powder (Ni-Cr-B-Si) has been tried as a coating on the top of the Ti-6Al-4V substrate [21] and the effect of laser scan speed on the microstructural evolution and wear

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properties were studied. At low scan speed (5 mm/s), nickel-based alloy powder reacted with the Ti-6Al-4V substrate to form NiTi, Ti2Ni, TiC and TiB2 in the coating. At high scan speed (20 mm/s), nickel-based matrix reinforced with CrB, Cr3C2 and TiC were formed in the coating, and was reported to have higher hardness and wear resistance among other scan speeds [22]. In another study carried out by Liu et al. [22], laser injection of a hard ceramic element like WC on

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Ti-6Al-4V substrate resulted in excellent fracture toughness due to the formation of reaction product TiC and W in the coating along with WC.

Due to the higher hardness of the TiB2, boron-based coating powder also has been tried in-situ synthesis of TiB2 on the coating [23]. To improve the surface properties of the Ti-6Al-4V plate, Bai et al. [23] had developed cladding of B4C and Ni-Cr-B-Si alloy powder on Ti-6Al-4V substrate. B4C and Ni-Cr-B-Si alloy powder reacted with Ti-6Al-4Vsubstrate to produce TiB2

and TiN, which increased the hardness and wear resistance. Makuch et al. [24] carried out the insitu synthesis of TiB2, TiB and TiC coating on pure titanium using boron and graphite as the precursor, and observed an improvement in the hardness (= 1500 HV) and the wear resistance property.

Zhou et al. [25] developed in-situ synthesized Ti5Si3 and Al3Ni2 on the top of the Ti-6Al-4V substrate by using preplaced powders (Ni-Cr-40Al-10Si and Ni-Cr-40Al-20Si) with the help of

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laser. Zhou et al. [25] observed that wear resistance of the coating made by NiCr-40Al-20Si powder was twice as compared to that of the substrate. Weng et al. [26] cladded Co42 and SiC powder mixture on the Ti-6Al-4V substrate and observed formation of Ti5Si3/TiC reinforced Co-

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based composite coatings. Weng et al. [26] observed that in-situ formed TiC, Ti5Si3, CoTi, CoTi2, NiTi, Cr7C3 and TiB increased the hardness of the clad substrate up to 1700 HV. TiN and

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TiAl3 coating was formed by using Aluminum powder as coating powder in a nitrogen environment with the help of laser [27]. Titanium gets converted to TiN when nitrogen was

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supplied in the melt pool [27]. Hardness and wear property were significantly improved due to the in-situ formation of TiN and TiAl3 [27]. A further effect of h-BN with Ni60 alloy powder

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was studied on Ti-6Al-4V substrate [28]. It was observed [28] that both the hardness and wear resistance improved upon increasing the quantity of h-BN due to the formation of TiB2.

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Problem Definition

A considerable amount of work has been reported in the field of laser-assisted coating of Ti-6Al4V with wear-resistant materials like TiB2, TiC, TiN and Ti3Si5 etc; however, most of these coatings are monolithic. Further, reports on such investigations generally present only

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indentation hardness data. In the present work, it is proposed to develop a multi-component coating on Ti-6Al-4V and analyze its wear and hardness property. The study of the hardness profile of such a multi-component coating would provide data on the utility of such a coating. Further, the study of the overall hardness behaviour of the coating is carried out by conducting both the indentation and the scratch hardness tests together with wear results from a ball and disc test.

2. Experimentation

2.1 Material

Ti-6Al-4V sample of dimension ((length = 100 mm) x (breadth = 50 mm) x (thickness = 6 mm)) was used as the substrate material. Before the laser cladding experiment, grit blasting of the substrate surface was carried out by alumina grits (average size = 250 μm) to generate a favourable surface texture on the substrate for better adhesion between the precursor powder and

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the substrate. B4C powder (of average particle size between 25 to 35 μm) and SiC powder (of average particle size between 30 to 50 μm) were used as the precursor powders in the coating.

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SEM images for both of the powders have been shown in the Fig. 1 (a, b).

Fig. 1. SEM images of powders (a) B4C (b) SiC.

Measured quantities of these powders with 1:1 ratio were mixed with polyvinyl alcohol (4 % aq.) to make a viscous slurry paste. To avoid agglomeration, the slurry was stirred using a Spinot

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magnetic stirrer for 2 hours, and then ultrasonically vibrated in Toshcon (Model S11-1) vibrator for a few minutes. The substrate surface was cleaned with acetone prior to the application of slurry. A 50 μm thick coating of the slurry was applied on the substrate surface and the slurry was levelled in the RK Control coater coating machine. The slurry coated substrate was dried on the TARSON 5040 hot plate to remove the moisture content.

2.2 Laser Run

Laser coating was carried out on the IPG YLR-2000 Ytterbium fiber laser having a wavelength of 1.064 μm and maximum output power of 2 kW. 1850 W laser power at 1200 mm/min scanning speed was set as process parameters. Schematic view of experimental set up has been shown on Fig. 2. To prevent atmospheric contamination during the laser run, slurry coated substrate was kept inside an argon flushed box as shown in Fig. 2. The argon flushed box is made of Perspex sheet which allows the incident laser beam to pass through and irradiate the

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slurry coated substrate. The focal spot diameter of the incident laser beam on the slurry coated

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substrate was 5 mm.

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Fig. 2. Schematic view of the experimental set-up.

2.3 Processing of laser clad sample

The laser clad samples were cut by wire-EDM and polished using 1200#, 1500#, and 2500# SiC grit abrasive paper, followed by diamond polishing. The polished samples were etched by Keller's reagent [29]. Microstructural analysis and compositional analysis was carried out using ZEISS EVO 18 Research scanning electron microscope and EDAX AMETEK energy-dispersive

X-ray spectroscopy (EDS) analyzer respectively. The phase analysis of the laser clad samples was carried through X-ray diffraction analysis in PANalytical EMPYREAN diffractometer by using CrKα target source for scan angle from 20° to 160°. Hardness test was carried out from the top of the coating (clad) toward the substrate in OMNITECH MVH-S-AUTO Vickers microhardness tester machine.

Wear tests were repeated thrice on all samples in DUCOM (TR-201-M3) ball-on-disk testing

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machine using a tungsten carbide ball rotating at 300 rpm under 2 kg load for 30 minutes. Wear rate was calculated using the following equation 3 [30] 𝑑𝑣

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𝑤̇ =

2𝜋𝑅𝑁𝑇𝐹 𝑑

𝑑

2𝑟

4

𝑑

𝑑

𝑟 2 sin−1 (2𝑟)−( 4 )√(4𝑟 2 −𝑑 2 ) 𝑁𝑇𝐹

equation 2 equation 3

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𝑤̇ =

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𝑑𝑣 = 2𝜋R [𝑟 2 sin−1 ( ) − ( ) √(4𝑟 2 − 𝑑 2 )]

equation 1

Here, 𝑤̇ = wear rate (mm3/N-m), dv = volume loss (mm3) after the wear test, R = radius of

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curvature of the wear track (mm), r = radius of curvature of the cross-section of the wear grove (mm), N = rotational speed of the ball during the wear test (RPM), T = duration of the wear test

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(sec) and F = applied load during the wear test (N).

Coefficient of friction was calculated from the relation of obtained frictional force and applied load. The cross-sectional profiles of the wear tracks were taken by non-contact type profilometer. Scratch test was carried out on Anton Paar machine by gradual loading from 5 N to 30 N for a length of 4 mm with scratch indenter scanning velocity of 2 mm/min and data collection at the

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rate of 50 Hz. Depth of penetration, applied load and acoustic emission were noted along the length of scratch during the scratch test. To investigate the mechanical resistance property of the coating, scratch hardness [31] was determined from the following relation as shown by equation 4 [32,33].

Scratch hardness (MPa) =

8𝐹𝑛 𝜋𝑑 2

equation 4

Here, Fn= Normal load (N) and d = scratch width (mm). 3. Results and Discussion

3.1 Microstructural and XRD analysis of the coating

Due to the incidence of laser on the slurry coated substrate, the very high localized temperature

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was generated; and consequently, several inter-atomic reactions took place between the powders and substrate. As a result of these reactions, several phases and microstructures of different

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composition were formed in the coating. The microstructure of the coating has been analyzed in

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SEM, the images of which have been shown in Fig 3.

Fig. 3 (a-c). Microstructures formed in the coating and (d) its EDS analysis.

It could be observed from Fig. 3 (a) that the thickness of the coating is about 50 μm thick. Fig. 3 (a) shows that due to the dilution between the powders and substrate, a different type of microstructures has been formed in the coating. At the top of the coating (marked by A in the

Fig. 3 (a) and magnified in the Fig. 3 (b)), finer grains could have formed due to the faster cooling rate. The microstructure of the coating mainly consists of whisker-shaped structure particle (marked by 1 in the Fig. 3 (b)) and equiaxed-shaped structure particle (marked by 2 in the Fig. 3 (b)). Just after the interface, i.e., at the heat affected substrate, acicular martensitic microstructure (Fig. 3 (c)) was observed due to rapid cooling through conduction heat loss. If the cooling rate is more than 200 °C/s, the martensite transformation can occur without diffusion,

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and thus acicular martensitic structure was formed in the heat-affected zone [20].

The composition of the microstructure formed was determined through EDS and is shown in Fig. 3 (d). EDS analysis of the whisker-shaped microstructure (marked by 1) shows that it is enriched

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with boron and carbon, while the equiaxed-shape microstructure (marked by 2) was found to be enriched with titanium and carbon. Particle 3 (as shown in Fig. 3 (b)) was observed to be

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titanium- and boron- enriched particles. To identify the phase of the elements formed on the

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coating, XRD analysis was carried and the phases identified have been shown in Fig 4.

Fig. 4. XRD analysis of coating.

XRD analysis (Fig 4) shows that titanium silicate (TiSi2), titanium carbide (TiC), boron silicate (B4Si), silicon carbide (SiC), titanium boride (TiB2), boron carbide (B4C) and vanadium carbide

(V8C7) were present in the coating. Presence of vanadium and carbon in particle 1 and 3 (Fig. 3 (b)) was observed from the EDS analysis (Fig. 3 (d)). Non-equilibrium reaction (due to rapid cooling) occurring between the powders in the molten pool during laser scanning could have lead to the formation of V8C7. It has been reported that Gibbs free energy for V8C7 at 2000 °C and 2500 °C is -70 kJ/mol and -80 kJ/mol [34], thus V8C7 is likely to be formed at this temperature range.

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Formation of TiC and TiSi2 could be analyzed through the Gibbs free energy. When titanium reacts with silicon carbide, several phases like TiC, Ti3Si, Ti5Si3, Ti5Si4, TiSi and TiSi2 could be formed [26]. Out of these phases, the formation of TiC and TiSi2 is most likely to occur because

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the Gibbs free energy for this transformation is highly negative [26]. Thus, TiC and TiSi2 could be formed spontaneously in the coating prior to the formation of other phases. Further, it was

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found that all of the particles of B4C and SiC did not take participate in the reaction in the melt pool. B4C (2760°) [35] and SiC (2730°C) [36] have a high melting point as compared to the Ti-

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6Al-4V substrate (= 1200 °C) [37], and thus all of the B4C and SiC could not melt and remains unreacted in the molten pool. So, at high temperature, carbon starts to react with other elements

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and form diffused carbide particles at the periphery of the B4C and SiC. Thus, some of the B4C

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and SiC remained unreacted in the coating as shown in Fig. 5.

Fig. 5. SEM image of unfused B4C and SiC.

3.2 Hardness test of the coating Hardness result of the coating, heat affected substrate and the unaffected substrate has been plotted in Fig 6. Hardness of the coating was found to increase almost seven times as compare to that of the substrate material which could be due to the presence of hard ceramic particles of TiC, B4C, SiC, TiB2 and V8C7 in the coating. Maximum hardness of 2060 HV was achieved in the coating. Higher hardness up to 50 µm coating thickness was retained. After 50 µm of the coating

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thickness, the intermediate layer (heat affected substrate, shown in Fig. 3 (c)) was formed as discussed in section 3.1. Due to the dilution of substrate material in the intermediate layer,

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hardness tends to reduce. Approximate hardness of the formed particles for 100 gf load has been

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listed in Table 1 [38][39][40].

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Fig. 6. Hardness plot of coating, heat affected substrate and unaffected substrate.

Table 1. Reported [38–40] hardness of the formed elements of the coating. Element

B4C

SiC

TiB2

TiC

TiSi2

V8C7

Vickers Hardness (HV0.1)

3560

2960

2550

2750

970

2350

It could be observed from Table 1 that, except for TiSi2, all the other particles have hardness more than 2500 HV. B4C has the highest hardness among all the particles formed. TiB2, TiC and

V8C7 have formed in-situ, and thus, are distributed uniformly in the coating. As a result of the formation of these hard precipitates, the average hardness of the coating was more than 2000 HV. It is important to mention that hardness of the coating at some places was found to be more

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than 3500 HV as shown in Fig. 7.

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Fig. 7. Hardness indent on the unreacted B4C and SiC.

Due to the presence of unreacted B4C and SiC, higher hardness values have been obtained in the coating. These unreacted B4C and SiC are favoured to improve the mechanical properties of the

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coating. Apart from the unreacted particles, the maximum hardness of the coating was obtained at the top section. This phenomenon occurred due to two reasons. Firstly, the high cooling rate at the top resulted in the formation of finer grain; and according to the Hall-Petch effect, finer grain leads to high hardness [41]. Secondly, due to the minimum dilution of powders with the substrate

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at the top portion of the coating, higher hardness could be achieved.

3.3 Wear performance Wear track depth and width were determined for both of the substrate and the coating by noncontact

type

surface

profilometer

(MICRO-EPSILON

displacement sensor) and the result have been shown in Fig. 8.

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ILD2300-2

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The wear rates of the substrate and the coating were calculated using equation 1. It could be observed from the Fig. 8 (a) that the width and depth of the substrate wear profiles are 1.8 mm and 0.12 mm respectively, whereas the width and the depth of coating wear profile are 0.8 mm and 0.032 mm respectively (Fig. 8 (b)). Hence, the wear dimension of the substrate was higher as compared to that of the coating. This shows that an improvement in the wear resistance of the Ti6Al-4V substrate has occurred due to the laser cladding of B4C and SiC. Wear rates of the substrate and coating have been found as 7.31 × 10-4 mm3/N-m and 5.102 × 10-5 mm3/N-m

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respectively, so, the wear rate has been improved by fourteen times.

High hardness of the coating leads to an increase in wear performance characteristics [11]. Thus,

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the formation of the composite coating by in-situ synthesized hard ceramic particles (precipitates) and the excellent metallurgical bond between the coating and substrate has led to

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an improvement in the wear resistant of Ti-6Al-4V substrate. Ratio of wear rate to hardness gives an idea about the hardness as well as tribological properties of the coating. Low ratio of

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wear rate to hardness will lead to high hardness and low wear rate. This ratio for the present study was calculated and found to be 2.47 × 10−8 mm3/N-m/HV which implies that the coating

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has high hardness as well as low wear rate. Thus, it could be inferred that although the coating has high hardness, it has good tribological performance.

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In earlier section 3.2, it has been discussed that some of the B4C and SiC were trapped in the coating matrix. Presence of these hard ceramic particles in the coating has also resulted in increase in the wear resistance of the coating. It has been reported [34] that the presence of V8C7 increases the wear resistance. So, in the present study, the in-situ developed V8C7 in the coating is also responsible for increasing the wear resistance. SEM analysis was carried out on the wear

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track of the substrate and has been shown in Fig. 9. Regular flow lines due to plastic deformation could be observed on the wear track of the substrate.

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Fig. 9. SEM image of wear track of substrate.

Fig. 10. (a) SEM image and (b) BSD image of wear track on coating.

Fig. 10 (a) shows that the coating has partially worn out after the wear test. In Fig. 10 (a), at the

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region A (marked by a circle), the substrate was exposed while most of the portion of the coating remained intact after the wear test. Region C and region D in Fig. 10 (a) shows the presence of the B4C and SiC in the coating after the wear test. Fig. 10 (b) shows the backscattered emission (BSD) image of the wear track. From the BSD analysis (Fig. 10 (b)), it has been identified that the grey portion in BSD image is the carbide-enriched particle. So, this in-situ developed carbide-enriched particle could withstand the external load during the wear test. The black region consists of B4C, which shows that the bonding between the diffused B4C and other elements in the coating is strong enough to survive the external loading during the wear test. So, it can be

stated that high wear resistance rate has been mainly achieved because of the presence of hard ceramic particle in the composite coating. 3.4 Scratch test

Scratch test results for the substrate and the coating have been shown pictorially in Fig. 11. From Fig. 11, it could be observed that the coating showed higher scratch resistance as compared to the substrate. As the applied load goes on increasing, scratch width as well as the depth of

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penetration of indenter for both the substrate and the coating increases. However, the coating has a smaller scratch width in each zone as compared to the substrate. Scratch hardness result for

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both of the samples has been shown in Fig. 12.

Fig. 11. Pictorial presentation of the scratch test result of substrate and coating.

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Fig. 12. Scratch hardness of substrate and coating for 5 N to 30 N loading.

It could be observed from Fig. 12 that on an average, the scratch hardness of the coating was found to be twice the scratch hardness of the substrate for all the cases. It was also observed (Fig. 12) that scratch hardness of the coating did not fall at maximum loading which shows that the

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coating did not peel off at even at maximum loading. To investigate this fact, the study of acoustic emission study was carried out.

Acoustic emission test is the measurement of sound that emits due to the fracture of the material during the scratch test [42]. So, when the material suddenly peels off, consequently sudden breaking sound is generated which records a high-intensity acoustic emission peak at the detector. So, the acoustic emission peak is proportional to the amount of the material removed in a single failure [42]. During the scratch test, the correlation between the amount of coating

detachment and corresponding acoustic emission peak has been reported by previous literature [41, 42]. Acoustic emission test was carried out on the coating and the results have been shown

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in Fig. 13.

Fig. 13. Acoustic emission and depth penetration of the coating sample during the scratch test.

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It could be observed from the acoustic emission analysis of the scratch test on the coating (Fig. 13) that high-intensity peaks were not detected throughout the scratch operation. This indicates that sudden peeling off of material from the coating has not occurred during the scratch test. Along with the analysis of the acoustic emission peak, the depth of penetration was also recorded during the scratch test as shown in Fig. 13. Depth of penetration was observed to increase

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gradually due to the increment of gradually applied loading. As the scratch hardness remained almost the same throughout the test, thus the depth of penetration, as well as width, would increase according to the increment of applied loading. However, it was found out that the maximum depth of penetration was about 42 µm which is less than to the coating thickness (= 50 µm, Fig. 3 (a)). Thus, acoustic emission, as well as the depth of penetration, shows that coating could sustain loading up to 30 N.

Scratch test basically assesses the adhesion property of a material [43]. So, excellent scratch resistance was found out as the coating has excellent adhesion property with the substrate. Further, to investigate the scratch failure mechanism, the microstructural analysis was carried out through SEM and EDS analysis on both of the substrate and the coating. Optical microscopic images and corresponding SEM images of both the coating and substrate have been shown in

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Fig. 14, Fig. 15 and Fig. 16.

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Fig. 14. Optical images of the scratch profiles of both the substrate and coating.

Fig. 14 shows the prominence of the scratch line made on the substrate surface, while the scratch line was found to be intermittent on the coating surface. Different types of failure mechanism were observed during the scratch test. SEM image of the substrate profile (Fig. 15) showed the micro-ploughing failure mechanism for removing the material on the scratch track. Continuous loading causes ductile material to be plastically deformed leading to pushing of the material

aside without detachment from the track [45]. Due to the plastic deformation, flow lines were

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observed along with the scratch indenter movement on the track.

Fig. 15. SEM image of the worn profile of the substrate after the scratch test.

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SEM images of the different zone of the coating sample have been shown in Fig. 16 (a-e). Zone 1 is the zone where initial loading was applied. The micro-chipping phenomenon could be observed in zone 1 (Fig. 16 (a)). Initially, the load was varied from 5 to 10 N which could not

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penetrate the coating surface significantly, so only a few μm from the top surface got worn in the form of micro-chip [45]. Formation of micro-steps was observed in the following zone 2. Mechanism of formation of micro-steps has been shown in the SEM image (Fig 16. (b)) and the failure mechanism of the zone 2 has been analyzed by four steps (Fig. 16 (b)). As the coating

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materials consist of several hard ceramic elements, brittle nature of the coating was found. Due to the brittleness of the coating, when the front of the indenter gave compressive stress on the layer (1st step), one layer was removed up and on further compressive stress, a crack was initiated at the junction of the layer, as shown in the 2nd steps in the Fig. 16 (b). Following the crack, the layer was completely removed and rafted up on the next layer (3rd step). When indenter moved on the rafted layer, it gave compressive load on that layer by the bottom portion of the indenter and micro-steps were generated (4th step).

Track widening was observed in zone 3. In this experiment, conical shape indenter was used and thus, increment in depth and track width was observed. As load and depth of penetration of the indenter increased in the zone 3, a thick layer was removed and could not be suppressed like zone 2, and thus removed layers were pushed away generating widened track. This kind of failure could have happened to minimize the stored elastic energy by the compressive stress generated in front of the indenter [44, 45]. When the load is further increased, a thicker layer is confronted by the indenter load, as a result, the generated crack was profound enough; and that

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even after removing the layer, micro-cracks were present on the track. The tensile stress generated at the edge of the diamond stylus produced crack which propagated from the surface of coating towards the substrate [42]. At the end of zone 4, the interface layer appeared, thus

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substrate material started to move, showing flow line like characteristic. The indention depth of zone 5 is 42 μm, which is the near-interface layer between the coating and the substrate. Due to

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the dilution from the substrate in this layer, it is ductile in nature (like the substrate), and thus ductile failure mechanism was observed. Flow lines on track reveal the direction of the flow of

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plastic deformation. At the end of the scratch test, the material removed at the front of the

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indenter, accumulated at the end of the track as shown in Fig. 16 (e).

of ro -p re lP ur na Jo Fig. 16. Scratch analysis of the coating scratch track at different loads.

4. Conclusions

After the analysis of the performance characteristics of the developed coating following conclusions could be drawn. 1) Both mechanical property and tribological properties were improved by laser coating of B4C and SiC powders on the Ti-6Al-4V substrate.

substrate improved the surface properties of the substrate.

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2) Triggered by the laser, in-situ synthesized TiB2, TiC, SiC and TiSi2, V8C7 on Ti-6Al-4V

3) Strong metallurgical bond was established between the in-situ synthesized coating

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materials. Formation of hard ceramic elements and presence of B4C and SiC in the metal matrix increased the hardness of the substrate.

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4) The fast cooling rate during laser coating favoured the formation of hard and wear

properties of the coating.

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resistant in-situ synthesized V8C7 which improved the hardness as well as the adhesion 5) The ratio of the wear and the hardness for the present work was 2.47 × 10-8 mm3/N-

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m/HV which was much less compared to other available literature. 6) Scratch hardness of the coating was approximately 4.556 GPa which was almost double compared to that of the substrate, thus scratch resistance of the coating was improved

ur na

significantly.

Acknowledgement

We are very grateful to all the faculty members, technical staffs and research scholars of Department of Mechanical Engineering, Department of Metallurgical & Materials Engineering, and Central Research Facility, I.I.T. Kharagpur

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for extending their support in carrying out various experiments. Authors would like to thank Prof. Debalay Chakrabarti, Department of Metallurgical and Materials Engineering, IIT Kharagpur for allowing the authors to use the scratch testing facility at his laboratory. Technical supports from Mr Miska Shreekant, Mr Pranab Karmakar and Mr Swarup Roy throughout the entire study are gratefully acknowledged.

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