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In-situ oxide-free titanium nitride coating by conventional plasma spraying with improved properties
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
In-situ oxide-free titanium nitride coating by conventional plasma spraying with improved properties Rohit Gupta, Aminul Islam, Krishna Kant Pandey, Shreshtha Ranjan, Ravi Kumar Singh, Biswajyoti Mukherjee, Anup Kumar Keshri∗ Plasma Spray Coating Laboratory, Metallurgical and Materials Engineering, Indian Institute of Technology Patna, Bihta, Bihar, 801106, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium nitride Shroud plasma spraying Mechanical properties Tribological properties Corrosion behaviour
We report in situ fabrication of oxide-free titanium nitride (TiN) coating by conventional plasma spraying technique. Complete oxide free TiN coating without using low pressure or vacuum environment was achieved by using the N2 shroud with plasma spraying. X-Ray diffraction and Transmission electron microscope confirmed the absence of any traces of oxide in the coating. Coating showed exceptionally higher mechanical properties (H: ∼18 GPa; E: ∼317 GPa). Outstanding reduction of ∼15 times in wear rate and ∼4 times in corrosion rate was observed compared to bare substrate (i.e. Ti-6Al-4V).
1. Introduction Titanium (Ti) based alloy is extensively used in aerospace, marine and medical industries due to high specific strength, relatively high corrosion resistance and unique biocompatibility [1]. However, these alloys suffer from limited hardness, wear and corrosion resistance which limits theirapplications in engineering sectors [2]. This has led researchers to develop a hard protective layer over Ti alloy using different surface modification techniques. Earlier, surface nitriding was used asa surface modification technique for protecting the underlying Ti surface [3]. However, the limited penetration depth of nitrogen (Ni) into Ti surface led to thinner (10–50 μm) and in homogeneous TiN stratum [3]. Therefore, en route to meet the industrial stature, researcher sought towards directly depositing TiN films to protect the underlying surface. Various deposition techniques, including chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used to deposit TiN films [4,5]. However, their inability to deposit coatings with substantial thickness (> 10 μm) coupled with the low efficiency, complications involved and expensive nature could not be considered as industrially advantageous [6]. It is known that the industrial efficiency of TiN films depends on the film thickness. As an example, TiN films with thickness lower than 12 μm is said to suffer from osmotic blistering [6]. This has led scientists to seek towards technique that can deposit thick TiN coatings that can meet the industrial standards. In this regard, atmospheric plasma spraying (APS) has already established itself as a benevolent technique with the capability to
∗
fabricate thick coating efficiently. Furthermore, the high temperature involved in plasma spray technique leads to ultrafast kinetically favourable reaction between Ti powder and nitrogen gas to form TiN, a technique also acknowledged as reactive plasma spraying (RPS) [6]. In this regard, Bacci and co-workers attempted to develop thick TiN coating by spraying Ti powders in a nitrogen atmosphere [7]. The deposited coating not only consisted of TiN, but also large amount of unreacted Ti phase. Since then, considerable number of research has already taken place to effectively deposit TiN using RPS [6]. However, all the coatings deposited using APS still consisted unwanted phase, mostly oxides of Ti, which will degrade the properties of the coating. Therefore, in-order to tackle the issue of oxide formation, researchers have adopted reactive plasma spraying of Ti in very low pressure or vacuum otherwise known as vacuum plasma spray (VPS) [8]. This technique aided in the deposition of complete oxide free TiN coating with very few non-stoichiometric nitride phases. However, the involvement of very low pressure chamber is expensive and is not advantageous for large scale fabrication in industries. Therefore, it is of critical importance to fabricate oxide free TiN coating using a facile method with a vision to scale up the productivity and push the boundaries in industrialization. Therefore, we propose a strategy to fabricate complete oxide free TiN coating using RPS technique and just by using an inert atmosphere shroud. The effect of an inert atmosphere shroud during RPS on the phase composition of the coating was studied. Also, the key properties related to mechanical, tribological and corrosion properties of the coatings were also studied.
Corresponding author. E-mail address: [email protected] (A.K. Keshri).
https://doi.org/10.1016/j.ceramint.2019.03.063 Received 2 March 2019; Received in revised form 8 March 2019; Accepted 10 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Rohit Gupta, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.03.063
Ceramics International xxx (xxxx) xxx–xxx
R. Gupta, et al.
2. Experimental procedure Titanium powder (Trixotech Adv. Mat. Pvt Ltd.; Particle size: 20–50 μm; Purity: 99.9%) was used as feedstock. High purity nitrogen gas(Praxair; Purity: 99.9%) was used both for primary and inert shroud gas. Coatings were deposited at optimized parameters (Table S1) using a plasma spraying system (9 MB plasma gun, Oerlikon Metco, USA) overTi6Al4V substrate. A custom shroud with N2 flow (30 psi) was mounted in front of the gun. More details about the shroud is available in our previously published article [9]. The coatings were synthesized at two different conditions, i.e., (i) N2 as primary gas and without shroud (TiN-PN) and (ii) N2 as primary gas with shroud (TiN-PSN). Morphology of the powder and cross-section of the coating was investigated using a Field Emission Scanning Electron Microscope (FESEM) (Zeiss, Sigma HD, USA). Phases were analysed by X-ray diffraction (TTRAX III, Rigaku, Japan) using CuKα radiation of wavelength 1.54 Å and using a scanning rate of 2°/min. Further, High Resolution Transmission Electron Microscopy (HRTEM)(FEI Tecnai, USA) operating at 200 kV was employed to verify the phase of the coating. Mechanical properties were measured using an instrumented microindentation and scratch tester (Microtest MTR3, Spain) at a load of 2 N; 15s dual time, while the loading rate kept constant at 25 mN/s.The wear test was carried out at 250 rpm and at normal loads of 80 N for 3600s with a stationary sample and rotating tungsten carbide (WC) ball (dia: 10 mm). The offset value from the centre was fixed at ∼4 mm for the all the wear test. Corrosion test was performed in 3.5 wt % NaCl solution using Potentiostat/Galvanostat (Gamry instruments, USA), with exposure area: 0.25 cm2. Open circuit potential (OCP) test was accomplished for 3600s to achieve a stabilized potential.
Fig. 2. (a) HRTEM image and (b) SAED pattern of TiN-PSN coating.
minor TiN0.3 could be due to incomplete decomposition and inadequate reaction between Ti powder and N2. Interestingly, it is still fascinating to achieve complete oxide free coating without using low pressure or vacuum environment. However, it is to be noted that XRD often fails to detect the presence of minor phase (> 2%) in an alloy [10]. Therefore, HR-TEM was used to confirm the results obtained by XRD. Fig. 2a shows the HRTEM image of TiN-PSN coating while Fig. 2b also shows selected-area electron-diffraction (SAED) pattern of the area shown in Fig. 1a. The pattern shows lattice spacing of 0.243, 0.211, 0.149, 0.127 and 0.121 nm, which corresponds to (111), (200), (220), (311), (222) plane of cubic TiN. Few non-stoichiometric TiN, i.e., TiN0.3 (102), TiN0.3 (101) and Ti2N (110) were also observed in the pattern, while no trace of TiO2 was found in the coating. This confirms that the coating fabricated using shroud is free of any oxide. Table S2 list out some works done by researchers on RPS to deposit TiN coatings. For comparison, the result obtained in current study is also included in the table. It is seen in table that suppressing the oxide content is not possible in RPS. However, this study successfully demonstrated the complete suppression of oxide formation. The formation of pure TiN phase in TiN-PSN coating can be attributed to the combination of N2 as primary gas and use of N2 gas shroud during plasma spraying. Firstly, the absence of pure Ti proves that reaction has taken place between Ti and N2 during plasma spraying. This result is consistent with literature. Secondly, it is anticipated the presence of N2 gas as shroud might create a drain of N2 rich environment that expurgated the contact of atmospheric air with Ti and prevented the formation of oxides. Now, mechanical study was performed to see the viability of these coatings. It is clearly visible from Fig. 3a and b that penetration depth as well as indent area of TiN-PS and TiN-PSN coating has significantly reduced compared to Ti6Al4V substrate, which indicated a drastic increase in the mechanical properties of the coating. The measured hardness values of bare Ti6Al4V, TiN-PN and TiN-PSN coating were 3.22 ± 0.108 GPa, 13.91 ± 1.18 GPa, 17.65 ± 1.04 GPa respectively. Similarly, elastic modulus of bare Ti6Al4V, TiN-PN and TiN-PSN coating were found to be 144.17 ± 2.40 GPa, 212.69 ± 8.95 GPa, 316.68 ± 9.28 GPa respectively. However, upon comparing TiN-PN and TiN-PSN coating, TiN-PSN coating displays the highest hardness and elastic modulus among two. It is fascinating that the hardness and elastic modulus values of TiN-PSN coating achieved in this work is among the highest for coating deposited using RPS (Table S2) and are comparable to coatings deposited using PVD/CVD technique [5]. These optimal hardness and elastic modulus of TiN-PSN coating is due to the formation of pure TiN formation. It is anticipated that in case of TiNPSN coating, higher nitrogen might have infused with Ti matrix which might have resulted in more ionic bond (Ti-N) formation, which eventually strengthened the coating. Fig. 4a and b illustrates the tribological property of bare substrate and the coatings. The wear weight loss is depicted using a bar plot (Fig. 4a), while the image of their wear tracks is shown as inset and the coefficient of friction (COF) are shown in Fig. 4b. The 3D optical profilometer images of the wear tracks are also presented in Fig. S3. The wear weight loss of bare Ti6Al4V, TiN-PN and TiN-PSN was measured
3. Results and discussions Fig. 1a-bdisplays the cross sectional images of the TiN-PN and TiNPSNcoatings respectively. Both the coatings exhibit uniform thickness of ∼400 μm and are well adhered to the substrate. Fig. 1cshows the XRD pattern of the starting Tipowder, Ti6Al4V substrate and TiN-PN and TiN-PSNcoating respectively. Ti powder and Ti6Al4Vsubstrate exhibits pure α-Ti phase. TheTiN-PN and TiN-PSN coating shows dominant peaks of cubic TiN, which confirms the formation of cubic TiN.However, apart from TiN peaks, some smaller peaks at 2θ = 27.4°, 36.0°, 41.2° and 54.3˚are also observed in TiN-PN coating. The presence of these peaks indicated the formation of TiO2 phase in TiN-PN coating. The TiN-PN coating is composed of ∼72% TiN and ∼28% TiO2 phase. Interestingly, the TiN-PSN coating showed the formation of only TiN phase with small non-stoichiometric TiN0.3 phase. The formation of
Fig. 1. FESEM images of (a) TiN-PN and (b) TiN-PSN coating (c) XRD pattern of feedstock powder, substrate and coatings. 2
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Fig. 3. (a) Load vs displacement curves and (b) optical micrograph showing the indent over the substrate and coatings.
attributed to the high strengthionic bonds between Ti and N in titanium nitride. This strong bonding between them prevents the removal of electrons even in highly aggressive solvents [12–14]. Moreover, the surface passivation of titanium nitride provides excellent resistance to corrosion, which considerably increases their corrosion resistance. This significant increase in mechanical properties and wear resistance and subsequent decrement of corrosion rate proves that TiN-PSN coating has the potential to deliver admirable results in industries.
using Equation S(4) and values were found to be 262.9 ± 10.5, 78.3 ± 7.3, 20.4 ± 5.2 mg respectively. A remarkable reduction of wear rate was also observed in TiN-PSN (0.167 ± 0.04 × 10−3 mm3/ N-m) coating compared to TiN-PN (0.684 ± 0.03 × 10−3 mm3/N-m) coating and bare substrate (2.580 ± 0.05 × 10−3 mm3/N-m) (Fig. 4c). These observations agree with Archard's law for abrasive sliding wear, which states that harder material accomplish lower wear rate [11]. The wear generation of wear debris will be lesser for coating with higher hardness (TiN-PSN). This will ultimately reduce the three body wear which and as expected, theTiN-PSN coating also demonstrated the best wear resistance with lowest COF (0.36 ± 0.04). The typical potentiodynamic polarization curves of bare Ti6Al4Vand coatings were shown in Fig. 5. The Icorr values of the bare Ti6Al4V, TiN-PN and TiN-PSN are 3250 nA, 173 nA, 129 nA respectively while the Ecorr value of TiN-PSN towards higher potential (V). Further, corrosion rate of bare Ti6Al4V, TiN-PN, TiN-PSN coating was found to be 1.114 mpy, 0.339 mpy, 0.253 mpy respectively. All these parameters indicate that the TiN-PSN coating reduces the corrosion rate of Ti6Al4V.The reason of this high corrosion resistance could be
4. Conclusion In summary, in situ TiN coating was successfully deposited over Ti6Al4V alloy by conventional plasma spraying equipped with a N2 shroud. Use of a N2 shroud prevented formation of any other phase apart from TiN. The TiN coating demonstrated superior mechanical, tribological properties and corrosion resistance.
Fig. 4. Tribology properties of Ti6Al4Vsubstrate and TiN-PS, TiN-PSN coatings. (a) Weight loss; inset shows digital image of the wear track, (b) Coefficient of friction and (c) Wear rate of the substrate and coatings. 3
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References [1] C. Leyens, M. Peters, Titanium and Titanium Alloys: Fundamentals and Applications, Wiley-VCH, Germany, 2003. [2] A.M. Kamat, S.M. Copley, J.A. Todd, Effect of processing parameters on microstructure during laser sustained plasma (LSP) nitriding of commercially-pure titanium, Acta Mater. 107 (2016) 72–82. [3] J. Liu, S. Suslov, A. Vellore, Z. Ren, A. Amanov, Y.S. Pyun, A. Martini, Y. Dong, C. Ye, Surface nanocrystallization by ultrasonic nano-crystal surface modification and its effect on gas nitriding of Ti6Al4V alloy, Mater. Sci. Eng. 24 (2018) 335–343. [4] J.H. Yang, M.F. Cheng, X.D. Luo, T.H. Zhang, Surface properties and microstructure of implanted TiN films using MEVVA ion source, Mater. SciEngg. A 445–446 (2007) 558–562. [5] S. Wu, S. Wu, G. Zhang, W. Zhang, Hardness and elastic modulus of titanium nitride coatings prepared by PIRAC method, Surf. Rev. Lett. 25 (2018). [6] J. Fathi, S. Mohsenian, B. Shokri, Plasma spray deposition enhancement of titanium nitride layer using nitrogen-contained plasma irradiation of titanium substrate as a pre-spraying method, J. Therm. Spray Technol. 27 (2018) 1187–1193. [7] T. Bacci, L. Bertamini, F. Ferrari, F.P. Galliano, E. Galvanetto, Reactive plasma spraying of titanium in nitrogen containing plasma gas, Mater. Sci. Eng., A 283 (2000) 189–195. [8] H. Proudhon, J. Savkova, S. Basseville, V. Guipont, M. Jeandin, G. Cailletaud, Experimental and numerical wear studies of porous Reactive Plasma Sprayed Ti–6Al–4V/TiN composite coating, Wear 311 (2014) 159–166. [9] O.S. Asiq Rahman, B. Mukherjee, A. Islam, A.K. Keshri, Instant tuning of superhydrophilic to robust superhydrophobicand self-cleaning metallic coating: simple, direct, one-step and Scalable technique, ACS Appl. Mater. Interfaces 11 (2019) 4616–4624. [10] N. Ravet, Y. Chouinard, J.F. Magnan, S. Besner, M. Gauthier, M. Armand, Electroactivity of natural and synthetic triphylite, J. Power Sources 97–98 (2001) 503–507. [11] E. Hornbogen, The role of fracture toughness in the wear of metals, Wear 33 (1975) 251–259. [12] I.M. Pohrelyuk, V.M. Fedirko, O.V. Tkachuk, R.V. Proskurnyak, Corrosion Resistance of Ti–6Al–4V Alloy with Nitride Coatings in Ringer's Solution vol. 66, (2013), pp. 392–398. [13] A.A. Adesina, Z.M. Gasem, A.M. Kumar, Corrosion resistance behavior of singlelayer cathodic arc PVD nitride-base coatings in 1M HCl and 3.5 pctNaCl solutions, Metall. Mater. Trans. B 48 (2017) 1321–1332. [14] S. Datta, M. Das, B.K. Balla, S. Bodhak, V.K. Murugesan, Mechanical, wear, corrosion and biological properties of arc deposited titanium nitride coatings, Surf. Coating. Technol. 344 (2018) 214–222.
Fig. 5. Tafel plot of the bare Ti6Al4V and TiN-PS, TiN-PSN coatings.
Acknowledgements The authors acknowledge the financial support from Indian Institute of Technology Patna, Bihar. K.K.P and A.K.K acknowledge the financial support from RESPOND-ISRO, Government of India, Grant No. ISRO/ RES/3/735/16-17. A. I and A.K·K acknowledge the financial support from DST, Government of India, Grant No. DST/TSG/AMT/2015/264. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.03.063.
4
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
In-situ oxide-free titanium nitride coating by conventional plasma spraying with improved properties Rohit Gupta, Aminul Islam, Krishna Kant Pandey, Shreshtha Ranjan, Ravi Kumar Singh, Biswajyoti Mukherjee, Anup Kumar Keshri∗ Plasma Spray Coating Laboratory, Metallurgical and Materials Engineering, Indian Institute of Technology Patna, Bihta, Bihar, 801106, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium nitride Shroud plasma spraying Mechanical properties Tribological properties Corrosion behaviour
We report in situ fabrication of oxide-free titanium nitride (TiN) coating by conventional plasma spraying technique. Complete oxide free TiN coating without using low pressure or vacuum environment was achieved by using the N2 shroud with plasma spraying. X-Ray diffraction and Transmission electron microscope confirmed the absence of any traces of oxide in the coating. Coating showed exceptionally higher mechanical properties (H: ∼18 GPa; E: ∼317 GPa). Outstanding reduction of ∼15 times in wear rate and ∼4 times in corrosion rate was observed compared to bare substrate (i.e. Ti-6Al-4V).
1. Introduction Titanium (Ti) based alloy is extensively used in aerospace, marine and medical industries due to high specific strength, relatively high corrosion resistance and unique biocompatibility [1]. However, these alloys suffer from limited hardness, wear and corrosion resistance which limits theirapplications in engineering sectors [2]. This has led researchers to develop a hard protective layer over Ti alloy using different surface modification techniques. Earlier, surface nitriding was used asa surface modification technique for protecting the underlying Ti surface [3]. However, the limited penetration depth of nitrogen (Ni) into Ti surface led to thinner (10–50 μm) and in homogeneous TiN stratum [3]. Therefore, en route to meet the industrial stature, researcher sought towards directly depositing TiN films to protect the underlying surface. Various deposition techniques, including chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used to deposit TiN films [4,5]. However, their inability to deposit coatings with substantial thickness (> 10 μm) coupled with the low efficiency, complications involved and expensive nature could not be considered as industrially advantageous [6]. It is known that the industrial efficiency of TiN films depends on the film thickness. As an example, TiN films with thickness lower than 12 μm is said to suffer from osmotic blistering [6]. This has led scientists to seek towards technique that can deposit thick TiN coatings that can meet the industrial standards. In this regard, atmospheric plasma spraying (APS) has already established itself as a benevolent technique with the capability to
∗
fabricate thick coating efficiently. Furthermore, the high temperature involved in plasma spray technique leads to ultrafast kinetically favourable reaction between Ti powder and nitrogen gas to form TiN, a technique also acknowledged as reactive plasma spraying (RPS) [6]. In this regard, Bacci and co-workers attempted to develop thick TiN coating by spraying Ti powders in a nitrogen atmosphere [7]. The deposited coating not only consisted of TiN, but also large amount of unreacted Ti phase. Since then, considerable number of research has already taken place to effectively deposit TiN using RPS [6]. However, all the coatings deposited using APS still consisted unwanted phase, mostly oxides of Ti, which will degrade the properties of the coating. Therefore, in-order to tackle the issue of oxide formation, researchers have adopted reactive plasma spraying of Ti in very low pressure or vacuum otherwise known as vacuum plasma spray (VPS) [8]. This technique aided in the deposition of complete oxide free TiN coating with very few non-stoichiometric nitride phases. However, the involvement of very low pressure chamber is expensive and is not advantageous for large scale fabrication in industries. Therefore, it is of critical importance to fabricate oxide free TiN coating using a facile method with a vision to scale up the productivity and push the boundaries in industrialization. Therefore, we propose a strategy to fabricate complete oxide free TiN coating using RPS technique and just by using an inert atmosphere shroud. The effect of an inert atmosphere shroud during RPS on the phase composition of the coating was studied. Also, the key properties related to mechanical, tribological and corrosion properties of the coatings were also studied.
Corresponding author. E-mail address: [email protected] (A.K. Keshri).
https://doi.org/10.1016/j.ceramint.2019.03.063 Received 2 March 2019; Received in revised form 8 March 2019; Accepted 10 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Rohit Gupta, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.03.063
Ceramics International xxx (xxxx) xxx–xxx
R. Gupta, et al.
2. Experimental procedure Titanium powder (Trixotech Adv. Mat. Pvt Ltd.; Particle size: 20–50 μm; Purity: 99.9%) was used as feedstock. High purity nitrogen gas(Praxair; Purity: 99.9%) was used both for primary and inert shroud gas. Coatings were deposited at optimized parameters (Table S1) using a plasma spraying system (9 MB plasma gun, Oerlikon Metco, USA) overTi6Al4V substrate. A custom shroud with N2 flow (30 psi) was mounted in front of the gun. More details about the shroud is available in our previously published article [9]. The coatings were synthesized at two different conditions, i.e., (i) N2 as primary gas and without shroud (TiN-PN) and (ii) N2 as primary gas with shroud (TiN-PSN). Morphology of the powder and cross-section of the coating was investigated using a Field Emission Scanning Electron Microscope (FESEM) (Zeiss, Sigma HD, USA). Phases were analysed by X-ray diffraction (TTRAX III, Rigaku, Japan) using CuKα radiation of wavelength 1.54 Å and using a scanning rate of 2°/min. Further, High Resolution Transmission Electron Microscopy (HRTEM)(FEI Tecnai, USA) operating at 200 kV was employed to verify the phase of the coating. Mechanical properties were measured using an instrumented microindentation and scratch tester (Microtest MTR3, Spain) at a load of 2 N; 15s dual time, while the loading rate kept constant at 25 mN/s.The wear test was carried out at 250 rpm and at normal loads of 80 N for 3600s with a stationary sample and rotating tungsten carbide (WC) ball (dia: 10 mm). The offset value from the centre was fixed at ∼4 mm for the all the wear test. Corrosion test was performed in 3.5 wt % NaCl solution using Potentiostat/Galvanostat (Gamry instruments, USA), with exposure area: 0.25 cm2. Open circuit potential (OCP) test was accomplished for 3600s to achieve a stabilized potential.
Fig. 2. (a) HRTEM image and (b) SAED pattern of TiN-PSN coating.
minor TiN0.3 could be due to incomplete decomposition and inadequate reaction between Ti powder and N2. Interestingly, it is still fascinating to achieve complete oxide free coating without using low pressure or vacuum environment. However, it is to be noted that XRD often fails to detect the presence of minor phase (> 2%) in an alloy [10]. Therefore, HR-TEM was used to confirm the results obtained by XRD. Fig. 2a shows the HRTEM image of TiN-PSN coating while Fig. 2b also shows selected-area electron-diffraction (SAED) pattern of the area shown in Fig. 1a. The pattern shows lattice spacing of 0.243, 0.211, 0.149, 0.127 and 0.121 nm, which corresponds to (111), (200), (220), (311), (222) plane of cubic TiN. Few non-stoichiometric TiN, i.e., TiN0.3 (102), TiN0.3 (101) and Ti2N (110) were also observed in the pattern, while no trace of TiO2 was found in the coating. This confirms that the coating fabricated using shroud is free of any oxide. Table S2 list out some works done by researchers on RPS to deposit TiN coatings. For comparison, the result obtained in current study is also included in the table. It is seen in table that suppressing the oxide content is not possible in RPS. However, this study successfully demonstrated the complete suppression of oxide formation. The formation of pure TiN phase in TiN-PSN coating can be attributed to the combination of N2 as primary gas and use of N2 gas shroud during plasma spraying. Firstly, the absence of pure Ti proves that reaction has taken place between Ti and N2 during plasma spraying. This result is consistent with literature. Secondly, it is anticipated the presence of N2 gas as shroud might create a drain of N2 rich environment that expurgated the contact of atmospheric air with Ti and prevented the formation of oxides. Now, mechanical study was performed to see the viability of these coatings. It is clearly visible from Fig. 3a and b that penetration depth as well as indent area of TiN-PS and TiN-PSN coating has significantly reduced compared to Ti6Al4V substrate, which indicated a drastic increase in the mechanical properties of the coating. The measured hardness values of bare Ti6Al4V, TiN-PN and TiN-PSN coating were 3.22 ± 0.108 GPa, 13.91 ± 1.18 GPa, 17.65 ± 1.04 GPa respectively. Similarly, elastic modulus of bare Ti6Al4V, TiN-PN and TiN-PSN coating were found to be 144.17 ± 2.40 GPa, 212.69 ± 8.95 GPa, 316.68 ± 9.28 GPa respectively. However, upon comparing TiN-PN and TiN-PSN coating, TiN-PSN coating displays the highest hardness and elastic modulus among two. It is fascinating that the hardness and elastic modulus values of TiN-PSN coating achieved in this work is among the highest for coating deposited using RPS (Table S2) and are comparable to coatings deposited using PVD/CVD technique [5]. These optimal hardness and elastic modulus of TiN-PSN coating is due to the formation of pure TiN formation. It is anticipated that in case of TiNPSN coating, higher nitrogen might have infused with Ti matrix which might have resulted in more ionic bond (Ti-N) formation, which eventually strengthened the coating. Fig. 4a and b illustrates the tribological property of bare substrate and the coatings. The wear weight loss is depicted using a bar plot (Fig. 4a), while the image of their wear tracks is shown as inset and the coefficient of friction (COF) are shown in Fig. 4b. The 3D optical profilometer images of the wear tracks are also presented in Fig. S3. The wear weight loss of bare Ti6Al4V, TiN-PN and TiN-PSN was measured
3. Results and discussions Fig. 1a-bdisplays the cross sectional images of the TiN-PN and TiNPSNcoatings respectively. Both the coatings exhibit uniform thickness of ∼400 μm and are well adhered to the substrate. Fig. 1cshows the XRD pattern of the starting Tipowder, Ti6Al4V substrate and TiN-PN and TiN-PSNcoating respectively. Ti powder and Ti6Al4Vsubstrate exhibits pure α-Ti phase. TheTiN-PN and TiN-PSN coating shows dominant peaks of cubic TiN, which confirms the formation of cubic TiN.However, apart from TiN peaks, some smaller peaks at 2θ = 27.4°, 36.0°, 41.2° and 54.3˚are also observed in TiN-PN coating. The presence of these peaks indicated the formation of TiO2 phase in TiN-PN coating. The TiN-PN coating is composed of ∼72% TiN and ∼28% TiO2 phase. Interestingly, the TiN-PSN coating showed the formation of only TiN phase with small non-stoichiometric TiN0.3 phase. The formation of
Fig. 1. FESEM images of (a) TiN-PN and (b) TiN-PSN coating (c) XRD pattern of feedstock powder, substrate and coatings. 2
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Fig. 3. (a) Load vs displacement curves and (b) optical micrograph showing the indent over the substrate and coatings.
attributed to the high strengthionic bonds between Ti and N in titanium nitride. This strong bonding between them prevents the removal of electrons even in highly aggressive solvents [12–14]. Moreover, the surface passivation of titanium nitride provides excellent resistance to corrosion, which considerably increases their corrosion resistance. This significant increase in mechanical properties and wear resistance and subsequent decrement of corrosion rate proves that TiN-PSN coating has the potential to deliver admirable results in industries.
using Equation S(4) and values were found to be 262.9 ± 10.5, 78.3 ± 7.3, 20.4 ± 5.2 mg respectively. A remarkable reduction of wear rate was also observed in TiN-PSN (0.167 ± 0.04 × 10−3 mm3/ N-m) coating compared to TiN-PN (0.684 ± 0.03 × 10−3 mm3/N-m) coating and bare substrate (2.580 ± 0.05 × 10−3 mm3/N-m) (Fig. 4c). These observations agree with Archard's law for abrasive sliding wear, which states that harder material accomplish lower wear rate [11]. The wear generation of wear debris will be lesser for coating with higher hardness (TiN-PSN). This will ultimately reduce the three body wear which and as expected, theTiN-PSN coating also demonstrated the best wear resistance with lowest COF (0.36 ± 0.04). The typical potentiodynamic polarization curves of bare Ti6Al4Vand coatings were shown in Fig. 5. The Icorr values of the bare Ti6Al4V, TiN-PN and TiN-PSN are 3250 nA, 173 nA, 129 nA respectively while the Ecorr value of TiN-PSN towards higher potential (V). Further, corrosion rate of bare Ti6Al4V, TiN-PN, TiN-PSN coating was found to be 1.114 mpy, 0.339 mpy, 0.253 mpy respectively. All these parameters indicate that the TiN-PSN coating reduces the corrosion rate of Ti6Al4V.The reason of this high corrosion resistance could be
4. Conclusion In summary, in situ TiN coating was successfully deposited over Ti6Al4V alloy by conventional plasma spraying equipped with a N2 shroud. Use of a N2 shroud prevented formation of any other phase apart from TiN. The TiN coating demonstrated superior mechanical, tribological properties and corrosion resistance.
Fig. 4. Tribology properties of Ti6Al4Vsubstrate and TiN-PS, TiN-PSN coatings. (a) Weight loss; inset shows digital image of the wear track, (b) Coefficient of friction and (c) Wear rate of the substrate and coatings. 3
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Fig. 5. Tafel plot of the bare Ti6Al4V and TiN-PS, TiN-PSN coatings.
Acknowledgements The authors acknowledge the financial support from Indian Institute of Technology Patna, Bihar. K.K.P and A.K.K acknowledge the financial support from RESPOND-ISRO, Government of India, Grant No. ISRO/ RES/3/735/16-17. A. I and A.K·K acknowledge the financial support from DST, Government of India, Grant No. DST/TSG/AMT/2015/264. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.03.063.
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