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Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications
Materials Today: Proceedings xxx (xxxx) xxx
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Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications Suneel Motru ⇑, Nabil Hussain, Zahid Ali Khan, Avinash Mechanical Engineering, PES University, Bengaluru 560085, India
a r t i c l e
i n f o
Article history: Received 4 August 2019 Accepted 17 September 2019 Available online xxxx Keywords: Adhesive wear Physical vapour deposition Titanium nitride Titanium carbide Low friction coatings Nanotribology
a b s t r a c t Adhesive wear is a significant material loss mechanism that leads to wide scale surface degradation of functional components especially in engineering industries like wind turbine roller bearings. This paper deals with improving the tribological properties of surfaces where adhesive wear is prominent. Adhesive wear is a common problem in gears, bearings, cutting tools, fittings, valves and earth moving equipment. For instance in a planetary gear arrangement, worn out debris from gears cause adhesive wear on the surface of roller bearings. Ceramic coatings are significant for protecting components and to improve the performance of surfaces subjected to adhesive wear over longer periods. Most of the ceramic coatings have high surface roughness owing to which they contribute to further adhesive wear of fine surfaces. In this paper, focus is on studying the performance of ion plated titanium nitride (TiN) and titanium carbide (TiC) PVD coatings against adhesive wear on EN 31 steel (substrate). Surfaces of samples coated with TiN, TiC and uncoated substrate of EN 31 steel are comparatively studied for adhesive wear behavior using wear tests as per ASTM standards. EN 31 steel is composed of 95% Fe, 0.9–1.1% C, 0.35% Mn, and 0.2% Si. Surface morphology and compositional analysis is inferred from Electron Microscopy (SEM and EDS) and surface roughness is estimated using 3-D confocal microscopy and surface profile measurement. The bond strength of coating is analysed from nanoscratch test; the coefficient of friction, material removal rate and hardness of surface is obtained from nano indentation Test. On experimental investigation, it is found that TiN and TiC ceramic coatings have excellent wear resistance, a low coefficient of friction and fine surface finish which are suitable for adhesive wear resistant applications. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction The tribological interaction of a component with the surroundings can lead to loss of material from the surface, the phenomenon commonly known as wear. Wear is the gradual elimination or plastic deformation of material by the detachment of particles forming debris. The wear occurrence is extremely undesirable and it leads to seizure of components. The worn out particle size varies from millimetres to nanometres. The solution to control wear is the design of operating conditions and choice of materials [1].
Abbreviation: TiN, Titanium Nitride; TiC, Titanium Carbide; SEM, Scanning Electron Microscopy; EDX, Energy Dispersive X-Ray Analysis; PVD, Physical Vapour Deposition; CVD, Chemical Vapour Deposition. ⇑ Corresponding author. E-mail address: [email protected] (S. Motru).
The different classifications of wear accountable for breakdown of engineering elements are adhesive wear, abrasive wear, micropitting, sliding wear, fretting, erosive wear and chemically assisted wear. Wear contributes to loss of material and causes expensive replacements of industrial components and failure of the equipment. This makes wear resistance a very critical material selection factor [2]. Adhesive wear is a kind of wear which occurs due to localized bonding between contacting solid surfaces leading to material loss from either surface. The contact load is sustained only by a small area when two mating surfaces are pressed against each other. By the use of hard coatings with low friction coefficient and lubricants, adhesive wear may be decreased [3]. Thin coatings and films are currently a distinctive attribute of numerous engineering applications. Recent PVD (Physical Vapour Deposition) and CVD (Chemical Vapour Deposition) methods offer the potential to
https://doi.org/10.1016/j.matpr.2019.09.098 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
2
S. Motru et al. / Materials Today: Proceedings xxx (xxxx) xxx
modify these coatings and films with diverse composition and properties providing a high surface finish with low friction [4]. The use of surface engineered coatings to improve the tribological properties of tools and machine elements is increasing continuously [5]. For adhesive wear resistant applications many coatings such as SiC, Al2O3, NiP, and MoC have been used. However to get very good surface finish associated with coating it is necessary to go for low friction and high surface finish coatings. For this high purity coating methods are implemented for applications where adhesive wear is predominant. Such applications include roller bearings in wind turbine, mining and metallurgy and underground drilling.
Then polishing is done using alumina paste on a velvet cloth on the grinder-polisher machine. Smooth mirror-like surface finish was obtained after polishing the specimens. Unpolished samples were heat treated to increase the bulk hardness. The heat treatment was done in muffle furnace of 3.5 kW rating, 230 V input and 1.6 A current. The specimen was heated and maintained at 8000 C for 20 min until red hot [14]. Then they were quenched in cold water of temperature 130 C for 2 min. These samples were then polished using alumina paste on velvet cloth in grinderpolisher.
2. Materials and methods
It is a type of indentation hardness test applied at nano level. In nanoindentation, the depression area may only be a few square nanometers due to minute loads and tip. It’s a diamond based indenter, formed into a pointed, symmetric three-sided Berkovich tip. A nanoindentation test involves noting down of force and displacement values as the material’s surface is scratched with the indenter which gives load vs displacement curve [15]. The ASTM standard for the nanoindentor test is ISO14577. The nanoscratch test is a complex method for the evaluation of adhesive and cohesive properties of thin coatings. All through the scratch test, an indenter made of diamond is pulled over the examined surface at a uniform velocity in a straight line [16]. The on-load probe depth and residual depth is obtained from the graph of displacement into surface vs scratch distance. The frictional force on the indenter can be noted throughout the experiment. The ASTM standard for the nanoscratch test is ISO14577.
Industrial applications typically use components such as gears, shafts, pinions, bearings, cutting tools and earth moving equipment which are made of EN 31 steel [6]. When subjected to high-speed and demanding applications such as drilling and meshing of gears, EN31 steel surface is damaged at an increased wear rate which leads to failure of the components [7]. The composition of EN 31 steel is seen from EDX results in Fig. 4(a). Nitride coatings are mostly used for their characteristics such as high bond strength and very good protection against surface wear and erosion. Especially, transition metal nitrides such as Titanium Nitride (TiN) are extensively used because of their exceptional intrinsic properties such as wear resistance, hardness (1800 VHN), low surface roughness (0.6 mm) [8], chemical stability and electrical conductivity (40,000 O 1 cm 1) [9]. Titanium Carbide (TiC) is a very hard (3200 VHN) ceramic material with superior tribological wear properties, good chemical inertness and conductivity (5555.55 O 1 cm 1) [10]. Carbide coatings also have an excellent hardness at high temperatures [11]. Ceramic coatings prepared by ion plating have long been recognized as an important method of resisting corrosion and minimizing wear [12]. Ion plating is defined as film deposition process at an atomic level in which the substrate is exposed to a flux of high energy ions adequate to trigger considerable sputtering prior to and during deposition. Ion plating provides good adhesion between film and the surface, providing fine surface finish. In this study cathodic arc process of Ion plating was used to deposit TiN/TiC coating on EN31 steel substrate. An electric arc is generated in a vacuum (1.4 10–4 mbar) and power (TiN – 80 KVA; TiC – 135 KVA) is applied between cathode and anode which is the wall of the vacuum chamber. This arc causes the Titanium ions to be released from the alloy (target). Nitrogen (TiN), acetylene gas (TiC) and argon is introduced in the vacuum chamber maintained at a temperature of 250–300 °C where nitrogen/acetylene ions bond with titanium ions to get deposited on the substrate as a thin film coating [13]. These coatings were carried out in LKBT, ModelDLDC 1100 (TiN) and Huicheng, Model-JTL 1212 (TiC).
3.2. Nanoindentation and nanoscratch test
3.3. Confocal microscopy Confocal microscopy is a laser technique which utilizes a laser light transiting across the confocal optical path for image visualization. This system is regarded as a flexible optical method for surface structure examination. The degree of roughness (Ra and Rz values) in the surface is measured over an arbitrary rectangular range using this method [17]. 3.4. SEM analysis Scanning Electron Microscopy (SEM) is a dominant technique for the study of surface structures. It gives high-resolution imaging suitable for assessing numerous materials for surface fractures, defects and impurities. It is also used for particle classification, surface topography such as wear debris produced in the course of mechanical wear examination [18]. EDX analysis is done along with SEM which tells about the specimen composition. 4. Results and discussion
3. Experimental techniques
4.1. Hardness test
The following tests in accordance with ASTM standards are performed on uncoated, heat treated and TiN/TiC coated samples of EN 31 steel.
Two types of hardness tests were conducted: Surface Hardness Test and Nano hardness test.
3.1. Substrate preparation and heat treatment EN31 steel specimens of four numbers with a size of 80 mm 80 mm 6 mm were procured and laser cut to 20 mm 20 mm 6 mm dimension. Holes of 4 mm diameter were drilled in the laser cut specimen to hang in the vacuum PVD coating chamber. Prior to the coating process, specimens were initially ground against sandpaper of grit sizes 100, 200, 400, 600.
4.1.1. Hardness test The hardness tests were conducted using Brinell (30 kgf), Rockwell (150 kgf on C scale) and Vickers (0.5 kg for a dwell time of 10 s) testers. Before the heat treatment of EN31 steel substrate, Brinell hardness was obtained as 191 BHN and Rockwell hardness as 14.2 HRC and after the heat treatment it was 311 BHN and 54.3 HRC respectively. The Vickers hardness pre heat treatment and post heat treatment was obtained as 207 VHN and 613 VHN respectively. The observed hardness values on coated substrate
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
S. Motru et al. / Materials Today: Proceedings xxx (xxxx) xxx
may have recorded unusual as indenters pierced through the 1 lm coating thickness. 4.1.2. Nano hardness test The nano hardness test was performed on all the four samples under varying loads at a temperature of 27.3 °C. From this test, load vs displacement graph was obtained as shown in Fig. 1. Various properties such as elastic modulus and nanohardness were obtained from the graph. The hardness values show that TiN is the hardest among the four samples. All the nano indentation tests were done till a maximum depth of 500 nm. Five trials of this test were done on each sample and an average value was procured. Slopes of the graphs shown in Fig. 1 give (a) Hardness–1.52 GPa, modulus–154.9 GPa (b) hardness–1.86 GPa, modulus-162.1 GPa (c) Hardness–0.58 GPa, modulus–16.8 GPa (d) Hardness–2.39 GPa, modulus–98.1 GPa. 4.2. Nanoscratch test Two types of scratch tests were conducted: progressive load scratch and constant load scratch
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treated specimen showed a critical load as 193 mN while the friction coefficient, the lateral force on the indenter and the depth of scratch was obtained as 0.61, 160 mN and 842 nm respectively. The mean critical load on TiC specimen was 70 mN. At this critical load, the friction coefficient obtained was 0.08, the lateral force on the indenter was 5.3 mN and the depth was 1822 nm. The critical load on TiN specimen was 241 mN. At this critical load, the friction coefficient obtained was 1.12, the lateral force on the indenter was 269.5 mN and the depth was 2447 nm. The maximum critical load was observed for TiN and the least coefficient of friction was obtained for TiC. 4.2.2. Constant load scratch test Constant Load scratch tests were conducted on coated samples to study the wear track properties. The maximum wear track deformation on TiC specimen was 132 mm2. The width of the groove formed was 36.2 lm and its height was 847 nm. The maximum wear track deformation obtained on TiN specimen was 124 mm2. The width of the groove formed was 17.6 lm and its height was 197 nm. 4.3. 3D confocal test
4.2.1. Progressive load scratch test Progressive load nano scratch tests were conducted on all the four samples to study the effects of critical load. On the EN 31 steel sample the mean critical load was 206 mN at which the friction coefficient obtained was 0.83, the lateral force on the indenter was 191.6 mN and the depth of scratch was 1633 nm. The heat
The 3-D confocal test was conducted on all the four samples to study the surface roughness as shown in Fig. 2. The average roughness (Ra) values and maximum roughness depth (Rz) were examined on all the samples and scratch dimensions on the coated samples. Under the confocal microscope, the scratches on TiN
Fig. 1. Load on sample vs displacement into surface for (a) uncoated (b) heat treated (c) TiC (d) TiN.
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
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Fig. 2. 3-D confocal test on (a) uncoated (b) heat treated (c) TiC (d) TiN coated EN31 samples.
and TiC coatings were observed. On the TiC coated specimen, the maximum length of scratch was 473 lm, maximum height was 1.05 lm and the width of scratch at the centre was 13 lm and at the end was 18 lm. On the TiN coated specimen, the maximum length of scratch was 483.3 lm, maximum height was 1.24 lm and the width of scratch at the center was 12 lm and at the end was 16 lm.
4.4. SEM and EDX analysis The samples observed under the electron microscope are shown in Fig. 3. From the figure, we can analyze that TiN coating has a very smooth finish. Under similar load, scratch on the uncoated specimen is wider than TiC. More wear debris has accumulated around the wear track on the EN 31 sample when compared to
Fig. 3. SEM images of (a) uncoated-Scale-1 cm = 10 mm, Magnification-4.5 kX (b) heat treated-Scale-1 cm = 20 mm, Magnification-2 kX (c) TiC Scale-1 cm = 5 mm, Magnification-9 kX (d) TiN Scale-1 cm = 3 mm, Magnification-15 kX. Scratch width for (a) EN 31 – 2.70 lm and (c) TiC – 72 nm respectively.
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
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a) EN31 steel substrate
c) TiC coating on EN31 steel substrate
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b) EN31 heat treated steel substrate
d) TiN coating on EN31 steel substrate
Fig. 4. EDX images of (a) uncoated (b) heat treated (c) TiC (d) TiN coated specimen Compositions of various samples (a) Fe-91.57%, C-0.99%, Si-0.75%, Cr-1.61%, Mn-5.08% (b) Fe-67.32%, C-1.43%, Si-0.75%, Cr-1.61%, Mn-11.11%, F-14.07% (c) Ti-31.19%, C-35.42%, O-33.39% (d) Ti-84.60%, N-15.40%.
TiC. The EDX compositional analysis was also done as shown in Fig. 4.
5. Conclusions In this work, from the hardness test results it can be concluded that TiN is the hardest and it will not fail under substantial loads. The heat treated specimen also showed an improved hardness over the EN 31 sample. From the progressive load scratch test results it is confirmed that TiN has the maximum critical load and its coefficient of friction is also on the lower side. From the constant load scratch test observations, TiN has lesser wear track deformation and the groove depth is also smaller under similar loads. In the 3-D confocal test, the progressive load scratches were measured and it showed that the scratch on TiN has a lesser depth. The Ra and Rz values of TiN were found to be the smallest, which also tell us that it has the least surface roughness. However, metallography images indicated that heat treated samples have high hardness and good wear properties. In summary, it is concluded that even at nano level TiN has a very good amount of wear resistance and tribological properties. It can withstand very high loads and can enhance the lifespan of various engineering components. Owing to the high electrical conductivity of TiN, it can also be used in various other applications in the future.
Acknowledgement The authors would like to thank the management of PES institutions and the research associates of PMR Lab, Department of Mechanical Engineering, PES University Bangalore for providing the laboratory facilities to carry out this research. The authors are extremely thankful for the support rendered by Mr Hanumanth Reddy, Kammaje Industries, Bangalore and CMTI, Bangalore for offering their technical expertise in experimentation for the successful completion of this study.
References [1] Karuppiah Kaliprasanth, Temperature Influence on Abrasive Resistance of Quenched and Tempered Steels, Hogskolan IHalmstad, 2017-06-20. p. 60. [2] Abin Baburaj, K.B. Shishupal Singh Chaudhary, Rajesh K. Khatirkarand, Sanjay G. Sapate, Abrasive wear behaviour of heat treated En31 steel, ISIJ Int. 53 (8) (2013) 1471–1478. [3] J. Dutta Majumdar, I. Manna, Laser surface engineering of titanium and its alloys for improved wear, corrosion and high-temperature oxidation resistance, Laser Surf. Eng. (2015) 483–521. ˇ ep, Vojteˇch Sléha Properties and [4] Ivan Mrkvica, Miroslav Neslušan, Robert C comparison of PVD coatings, DOI: 10.17559/TV-20140509105317. [5] Vishakha S. Pahade, Pankaj S. Chavan, Vaishali P. Baisane, A review paper on Vapour deposition coating, Int. J. Eng. Appl. Sci. (IJEAS) 3 (6) (2016), ISSN: 2394-3661. [6] A. Greco, S. Sheng, J. Keller, A. Erdemir, Material wear and fatigue in wind turbine systems, Wear (2013), https://doi.org/10.1016/j.wear.2013.01.060. [7] J. Woolman, R.A. Mottram, The Mechanical and Physical Properties of the British Standard En Steels (B.S. 970-1955), vol. 2, Pergamon Press, Oxford. [8] P. Harlin, P. Carlsson, U. Bexell, M. Olsson, Influence of surface roughness of PVD coatings on tribological performance in sliding contacts, Surf. Coat. Technol. 201 (7) (2006). [9] Eleonora Santecchia, A.M.S. Hamouda, Farayi Musharavati, Erfan Zalnezhad, Marcello Cabibbo, Stefano Spigarelli, Wear resistance investigation of titanium nitride-based coatings, Ceram. Int. (2015) 10349–10379. [10] F. Sergejev, M. Antonov, A. Gregor, I. Hussainova, P. Kulu, J. Kübarsepp, Investigation of the surface fatigue of carbide composites and PVD hard coatings, 28 April, 2008. [11] G. Mah, C.W. Nordin, J.F. Fuller, Structure and properties of sputtered titanium carbide and titanium nitride coatings, J. Vacuum Sci. Technol. (2001). [12] M. Sayer, K. Sreenivas, Ceramic thin films: fabrication and applications, Science 247 (4946) (1990). [13] D. William, B.L.R.L. Sproul, Physical vapor deposition tool coatings, Surf. Coat. Technol. 81 (1) (1996) 1–7. [14] Vimal B. Patel, Smit D. Parikh, Parin R. Patel, Mohit S. Mishra, Investigating the heat treatment parameters of En-31 using Taguchi method., Int. J. Innov. Sci. Eng. Technol. 5 (5) (2018). [15] Christopher A. Schuh, Nanoindentation studies of materials, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, vol. 9, pp. 32–40, May 2006. [16] J. Tomastik J.R. Ctvrtlik, Nanoscratch test – A tool for evaluation of cohesive and adhesive properties of thin films and coatings, EPJ web of conferences, 2013. [17] Tomáš Bezák, Martin Kusy´, Michal Eliáš, Michal Kopcˇek, surface roughness determination using laser scanning confocal microscope zeisslsm 700, Metal 17 (2013) 05. [18] C.T.K.-H. Stadtländer, J. Diaz. Scanning Electron Microscopy and Transmission Electron Microscopy of Mollicutes: Challenges and Opportunities. January 2017.
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications Suneel Motru ⇑, Nabil Hussain, Zahid Ali Khan, Avinash Mechanical Engineering, PES University, Bengaluru 560085, India
a r t i c l e
i n f o
Article history: Received 4 August 2019 Accepted 17 September 2019 Available online xxxx Keywords: Adhesive wear Physical vapour deposition Titanium nitride Titanium carbide Low friction coatings Nanotribology
a b s t r a c t Adhesive wear is a significant material loss mechanism that leads to wide scale surface degradation of functional components especially in engineering industries like wind turbine roller bearings. This paper deals with improving the tribological properties of surfaces where adhesive wear is prominent. Adhesive wear is a common problem in gears, bearings, cutting tools, fittings, valves and earth moving equipment. For instance in a planetary gear arrangement, worn out debris from gears cause adhesive wear on the surface of roller bearings. Ceramic coatings are significant for protecting components and to improve the performance of surfaces subjected to adhesive wear over longer periods. Most of the ceramic coatings have high surface roughness owing to which they contribute to further adhesive wear of fine surfaces. In this paper, focus is on studying the performance of ion plated titanium nitride (TiN) and titanium carbide (TiC) PVD coatings against adhesive wear on EN 31 steel (substrate). Surfaces of samples coated with TiN, TiC and uncoated substrate of EN 31 steel are comparatively studied for adhesive wear behavior using wear tests as per ASTM standards. EN 31 steel is composed of 95% Fe, 0.9–1.1% C, 0.35% Mn, and 0.2% Si. Surface morphology and compositional analysis is inferred from Electron Microscopy (SEM and EDS) and surface roughness is estimated using 3-D confocal microscopy and surface profile measurement. The bond strength of coating is analysed from nanoscratch test; the coefficient of friction, material removal rate and hardness of surface is obtained from nano indentation Test. On experimental investigation, it is found that TiN and TiC ceramic coatings have excellent wear resistance, a low coefficient of friction and fine surface finish which are suitable for adhesive wear resistant applications. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction The tribological interaction of a component with the surroundings can lead to loss of material from the surface, the phenomenon commonly known as wear. Wear is the gradual elimination or plastic deformation of material by the detachment of particles forming debris. The wear occurrence is extremely undesirable and it leads to seizure of components. The worn out particle size varies from millimetres to nanometres. The solution to control wear is the design of operating conditions and choice of materials [1].
Abbreviation: TiN, Titanium Nitride; TiC, Titanium Carbide; SEM, Scanning Electron Microscopy; EDX, Energy Dispersive X-Ray Analysis; PVD, Physical Vapour Deposition; CVD, Chemical Vapour Deposition. ⇑ Corresponding author. E-mail address: [email protected] (S. Motru).
The different classifications of wear accountable for breakdown of engineering elements are adhesive wear, abrasive wear, micropitting, sliding wear, fretting, erosive wear and chemically assisted wear. Wear contributes to loss of material and causes expensive replacements of industrial components and failure of the equipment. This makes wear resistance a very critical material selection factor [2]. Adhesive wear is a kind of wear which occurs due to localized bonding between contacting solid surfaces leading to material loss from either surface. The contact load is sustained only by a small area when two mating surfaces are pressed against each other. By the use of hard coatings with low friction coefficient and lubricants, adhesive wear may be decreased [3]. Thin coatings and films are currently a distinctive attribute of numerous engineering applications. Recent PVD (Physical Vapour Deposition) and CVD (Chemical Vapour Deposition) methods offer the potential to
https://doi.org/10.1016/j.matpr.2019.09.098 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
2
S. Motru et al. / Materials Today: Proceedings xxx (xxxx) xxx
modify these coatings and films with diverse composition and properties providing a high surface finish with low friction [4]. The use of surface engineered coatings to improve the tribological properties of tools and machine elements is increasing continuously [5]. For adhesive wear resistant applications many coatings such as SiC, Al2O3, NiP, and MoC have been used. However to get very good surface finish associated with coating it is necessary to go for low friction and high surface finish coatings. For this high purity coating methods are implemented for applications where adhesive wear is predominant. Such applications include roller bearings in wind turbine, mining and metallurgy and underground drilling.
Then polishing is done using alumina paste on a velvet cloth on the grinder-polisher machine. Smooth mirror-like surface finish was obtained after polishing the specimens. Unpolished samples were heat treated to increase the bulk hardness. The heat treatment was done in muffle furnace of 3.5 kW rating, 230 V input and 1.6 A current. The specimen was heated and maintained at 8000 C for 20 min until red hot [14]. Then they were quenched in cold water of temperature 130 C for 2 min. These samples were then polished using alumina paste on velvet cloth in grinderpolisher.
2. Materials and methods
It is a type of indentation hardness test applied at nano level. In nanoindentation, the depression area may only be a few square nanometers due to minute loads and tip. It’s a diamond based indenter, formed into a pointed, symmetric three-sided Berkovich tip. A nanoindentation test involves noting down of force and displacement values as the material’s surface is scratched with the indenter which gives load vs displacement curve [15]. The ASTM standard for the nanoindentor test is ISO14577. The nanoscratch test is a complex method for the evaluation of adhesive and cohesive properties of thin coatings. All through the scratch test, an indenter made of diamond is pulled over the examined surface at a uniform velocity in a straight line [16]. The on-load probe depth and residual depth is obtained from the graph of displacement into surface vs scratch distance. The frictional force on the indenter can be noted throughout the experiment. The ASTM standard for the nanoscratch test is ISO14577.
Industrial applications typically use components such as gears, shafts, pinions, bearings, cutting tools and earth moving equipment which are made of EN 31 steel [6]. When subjected to high-speed and demanding applications such as drilling and meshing of gears, EN31 steel surface is damaged at an increased wear rate which leads to failure of the components [7]. The composition of EN 31 steel is seen from EDX results in Fig. 4(a). Nitride coatings are mostly used for their characteristics such as high bond strength and very good protection against surface wear and erosion. Especially, transition metal nitrides such as Titanium Nitride (TiN) are extensively used because of their exceptional intrinsic properties such as wear resistance, hardness (1800 VHN), low surface roughness (0.6 mm) [8], chemical stability and electrical conductivity (40,000 O 1 cm 1) [9]. Titanium Carbide (TiC) is a very hard (3200 VHN) ceramic material with superior tribological wear properties, good chemical inertness and conductivity (5555.55 O 1 cm 1) [10]. Carbide coatings also have an excellent hardness at high temperatures [11]. Ceramic coatings prepared by ion plating have long been recognized as an important method of resisting corrosion and minimizing wear [12]. Ion plating is defined as film deposition process at an atomic level in which the substrate is exposed to a flux of high energy ions adequate to trigger considerable sputtering prior to and during deposition. Ion plating provides good adhesion between film and the surface, providing fine surface finish. In this study cathodic arc process of Ion plating was used to deposit TiN/TiC coating on EN31 steel substrate. An electric arc is generated in a vacuum (1.4 10–4 mbar) and power (TiN – 80 KVA; TiC – 135 KVA) is applied between cathode and anode which is the wall of the vacuum chamber. This arc causes the Titanium ions to be released from the alloy (target). Nitrogen (TiN), acetylene gas (TiC) and argon is introduced in the vacuum chamber maintained at a temperature of 250–300 °C where nitrogen/acetylene ions bond with titanium ions to get deposited on the substrate as a thin film coating [13]. These coatings were carried out in LKBT, ModelDLDC 1100 (TiN) and Huicheng, Model-JTL 1212 (TiC).
3.2. Nanoindentation and nanoscratch test
3.3. Confocal microscopy Confocal microscopy is a laser technique which utilizes a laser light transiting across the confocal optical path for image visualization. This system is regarded as a flexible optical method for surface structure examination. The degree of roughness (Ra and Rz values) in the surface is measured over an arbitrary rectangular range using this method [17]. 3.4. SEM analysis Scanning Electron Microscopy (SEM) is a dominant technique for the study of surface structures. It gives high-resolution imaging suitable for assessing numerous materials for surface fractures, defects and impurities. It is also used for particle classification, surface topography such as wear debris produced in the course of mechanical wear examination [18]. EDX analysis is done along with SEM which tells about the specimen composition. 4. Results and discussion
3. Experimental techniques
4.1. Hardness test
The following tests in accordance with ASTM standards are performed on uncoated, heat treated and TiN/TiC coated samples of EN 31 steel.
Two types of hardness tests were conducted: Surface Hardness Test and Nano hardness test.
3.1. Substrate preparation and heat treatment EN31 steel specimens of four numbers with a size of 80 mm 80 mm 6 mm were procured and laser cut to 20 mm 20 mm 6 mm dimension. Holes of 4 mm diameter were drilled in the laser cut specimen to hang in the vacuum PVD coating chamber. Prior to the coating process, specimens were initially ground against sandpaper of grit sizes 100, 200, 400, 600.
4.1.1. Hardness test The hardness tests were conducted using Brinell (30 kgf), Rockwell (150 kgf on C scale) and Vickers (0.5 kg for a dwell time of 10 s) testers. Before the heat treatment of EN31 steel substrate, Brinell hardness was obtained as 191 BHN and Rockwell hardness as 14.2 HRC and after the heat treatment it was 311 BHN and 54.3 HRC respectively. The Vickers hardness pre heat treatment and post heat treatment was obtained as 207 VHN and 613 VHN respectively. The observed hardness values on coated substrate
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
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may have recorded unusual as indenters pierced through the 1 lm coating thickness. 4.1.2. Nano hardness test The nano hardness test was performed on all the four samples under varying loads at a temperature of 27.3 °C. From this test, load vs displacement graph was obtained as shown in Fig. 1. Various properties such as elastic modulus and nanohardness were obtained from the graph. The hardness values show that TiN is the hardest among the four samples. All the nano indentation tests were done till a maximum depth of 500 nm. Five trials of this test were done on each sample and an average value was procured. Slopes of the graphs shown in Fig. 1 give (a) Hardness–1.52 GPa, modulus–154.9 GPa (b) hardness–1.86 GPa, modulus-162.1 GPa (c) Hardness–0.58 GPa, modulus–16.8 GPa (d) Hardness–2.39 GPa, modulus–98.1 GPa. 4.2. Nanoscratch test Two types of scratch tests were conducted: progressive load scratch and constant load scratch
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treated specimen showed a critical load as 193 mN while the friction coefficient, the lateral force on the indenter and the depth of scratch was obtained as 0.61, 160 mN and 842 nm respectively. The mean critical load on TiC specimen was 70 mN. At this critical load, the friction coefficient obtained was 0.08, the lateral force on the indenter was 5.3 mN and the depth was 1822 nm. The critical load on TiN specimen was 241 mN. At this critical load, the friction coefficient obtained was 1.12, the lateral force on the indenter was 269.5 mN and the depth was 2447 nm. The maximum critical load was observed for TiN and the least coefficient of friction was obtained for TiC. 4.2.2. Constant load scratch test Constant Load scratch tests were conducted on coated samples to study the wear track properties. The maximum wear track deformation on TiC specimen was 132 mm2. The width of the groove formed was 36.2 lm and its height was 847 nm. The maximum wear track deformation obtained on TiN specimen was 124 mm2. The width of the groove formed was 17.6 lm and its height was 197 nm. 4.3. 3D confocal test
4.2.1. Progressive load scratch test Progressive load nano scratch tests were conducted on all the four samples to study the effects of critical load. On the EN 31 steel sample the mean critical load was 206 mN at which the friction coefficient obtained was 0.83, the lateral force on the indenter was 191.6 mN and the depth of scratch was 1633 nm. The heat
The 3-D confocal test was conducted on all the four samples to study the surface roughness as shown in Fig. 2. The average roughness (Ra) values and maximum roughness depth (Rz) were examined on all the samples and scratch dimensions on the coated samples. Under the confocal microscope, the scratches on TiN
Fig. 1. Load on sample vs displacement into surface for (a) uncoated (b) heat treated (c) TiC (d) TiN.
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
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Fig. 2. 3-D confocal test on (a) uncoated (b) heat treated (c) TiC (d) TiN coated EN31 samples.
and TiC coatings were observed. On the TiC coated specimen, the maximum length of scratch was 473 lm, maximum height was 1.05 lm and the width of scratch at the centre was 13 lm and at the end was 18 lm. On the TiN coated specimen, the maximum length of scratch was 483.3 lm, maximum height was 1.24 lm and the width of scratch at the center was 12 lm and at the end was 16 lm.
4.4. SEM and EDX analysis The samples observed under the electron microscope are shown in Fig. 3. From the figure, we can analyze that TiN coating has a very smooth finish. Under similar load, scratch on the uncoated specimen is wider than TiC. More wear debris has accumulated around the wear track on the EN 31 sample when compared to
Fig. 3. SEM images of (a) uncoated-Scale-1 cm = 10 mm, Magnification-4.5 kX (b) heat treated-Scale-1 cm = 20 mm, Magnification-2 kX (c) TiC Scale-1 cm = 5 mm, Magnification-9 kX (d) TiN Scale-1 cm = 3 mm, Magnification-15 kX. Scratch width for (a) EN 31 – 2.70 lm and (c) TiC – 72 nm respectively.
Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098
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a) EN31 steel substrate
c) TiC coating on EN31 steel substrate
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b) EN31 heat treated steel substrate
d) TiN coating on EN31 steel substrate
Fig. 4. EDX images of (a) uncoated (b) heat treated (c) TiC (d) TiN coated specimen Compositions of various samples (a) Fe-91.57%, C-0.99%, Si-0.75%, Cr-1.61%, Mn-5.08% (b) Fe-67.32%, C-1.43%, Si-0.75%, Cr-1.61%, Mn-11.11%, F-14.07% (c) Ti-31.19%, C-35.42%, O-33.39% (d) Ti-84.60%, N-15.40%.
TiC. The EDX compositional analysis was also done as shown in Fig. 4.
5. Conclusions In this work, from the hardness test results it can be concluded that TiN is the hardest and it will not fail under substantial loads. The heat treated specimen also showed an improved hardness over the EN 31 sample. From the progressive load scratch test results it is confirmed that TiN has the maximum critical load and its coefficient of friction is also on the lower side. From the constant load scratch test observations, TiN has lesser wear track deformation and the groove depth is also smaller under similar loads. In the 3-D confocal test, the progressive load scratches were measured and it showed that the scratch on TiN has a lesser depth. The Ra and Rz values of TiN were found to be the smallest, which also tell us that it has the least surface roughness. However, metallography images indicated that heat treated samples have high hardness and good wear properties. In summary, it is concluded that even at nano level TiN has a very good amount of wear resistance and tribological properties. It can withstand very high loads and can enhance the lifespan of various engineering components. Owing to the high electrical conductivity of TiN, it can also be used in various other applications in the future.
Acknowledgement The authors would like to thank the management of PES institutions and the research associates of PMR Lab, Department of Mechanical Engineering, PES University Bangalore for providing the laboratory facilities to carry out this research. The authors are extremely thankful for the support rendered by Mr Hanumanth Reddy, Kammaje Industries, Bangalore and CMTI, Bangalore for offering their technical expertise in experimentation for the successful completion of this study.
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Please cite this article as: S. Motru, N. Hussain, Z. Ali Khan et al., Tribological studies of high surface finish ceramic coatings for low friction and adhesive wear resistant applications, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.098