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Effect of graphite addition on the microstructure, hardness and abrasive wear behavior of plasma sprayed NiCrBSi coatings
Materials Chemistry and Physics 175 (2016) 100e106
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of graphite addition on the microstructure, hardness and abrasive wear behavior of plasma sprayed NiCrBSi coatings S. Natarajan a, b, *, Edward Anand E.c, K.S. Akhilesh a, Ananya Rajagopal a, Preeti P. Nambiar a a b c
Department of Metallurgical & Materials Engineering, National Institute of Technology (NITT), Trichy, India Center of Excellence in Corrosion and Surface Engineering, NITT, India Department of Physics, M.Kumarasamy College of Engineering (Autonomous), Karur, India
h i g h l i g h t s Effect of graphite on microstructure, hardness & abrasive wear of NiCrBSi coatings investigated. XRD analysis reveals coatings are composed of g-Ni, CrB & Cr7C3. Graphite addition in thermal spray coatings improve wear resistance. NiCrBSie8 wt % C coating has excellent abrasion resistance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2015 Received in revised form 6 February 2016 Accepted 28 February 2016 Available online 19 March 2016
Plasma sprayed coatings have been considered as one of the important surface modification processes to improve the tribological properties of materials used in extreme conditions. NiCrBSi composite coatings on stainless steel are known to offer good abrasive wear resistance in service conditions. Graphite is the material which has many potential advantages when processed and used in different forms. This paper describes the effect of graphite addition on the microstructure; hardness and abrasive wear behavior of plasma sprayed NiCrBSi coatings. The mechanical and tribological properties of the coatings can be significantly improved by refinement of grain structure of the coatings. The XRD results show that the NiCrBSiegraphite composite coatings are mainly composed of g-Ni, CrB and Cr7C3. With addition of graphite, phases such as CrB, Cr7C3 emerge in composite coating. The study was conducted, using an abrasive wear test rig according to ASTM G65 on uncoated as well as coated SS304 samples at room temperature. The samples were analyzed for volume loss and wear rate with respect to increasing content of graphite in the coatings. The results suggest that addition of graphite in the coatings resulted in reduction of volume loss and wear rate significantly. The NiCrBSie8 wt %C composite coating presents excellent abrasion resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Alloys Plasma deposition Electron microscopy Mechanical testing Abrasion Microstructure
1. Introduction Abrasion, or abrasive wear, is a type of wear experienced in many industries, particularly in the fields of agriculture, mining and mineral processing [1,2]. Ni and Co based alloys that exhibit high strength, hardness and excellent wear and corrosion resistance have been recently employed as coating materials for applications
* Corresponding author. Department of Metallurgical & Materials Engineering, NITT, India. E-mail addresses: [email protected], [email protected] (S. Natarajan). http://dx.doi.org/10.1016/j.matchemphys.2016.02.076 0254-0584/© 2016 Elsevier B.V. All rights reserved.
in the industries [3,4]. Nickel based alloy coatings containing typically, a combination of NiCrBSi are used in mill rolls, high capacity pumps, piston extruders and glass moulding industries, where wear resistance combined with oxidation or hot corrosion resistance is required. Nickel provides ductility and enhances the corrosion resistance. Chromium provides good resistance to wear and corrosion. Boron enhances wear resistance and silicon reduces the eutectic melting point of the alloy and improves the self fluxing properties of the coating. Abrasion is a complicated phenomenon influenced by different factors such as the properties of the materials coming into contact with each other, the service conditions or the environment all play their part in abrasive wear [5]. Abrasive
S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106
wear behavior of a coating is determined by a number of test parameters like applied load, abrasive size, environment, the size, shape and distribution of hard phase precipitates, volume content of the hard phase, hardness of the matrix etc [6]. Many researchers have reported the improvement in hardness of Ni based alloys by incorporation of hard carbides such as WC, TiC and rare earth oxides such as cerium oxide and lanthanum oxide. The hard carbides such as WC and TiC increase the hardness of the coatings at the cost of toughness. Addition of rare earth elements improves hardness and toughness besides enhancing the corrosion and oxidation resistance of the coatings [7e17]. Refinement in microstructure, increase in micro-hardness and abrasive wear resistance of NiCrBSi flame sprayed coatings with the optimum addition of CeO2 (0.8 wt.%) has been reported by the Sharma et al. [18]. Ahmad khan et al. have reported that Ni powders, which belong to the NieBeSi, with chromium addition, increase the hardness of the coating by the formation of hard carbides with improved oxidation and corrosion resistance at elevated temperatures. Boron supports the formation of carbides by depressing the melting temperature, and addition of silicon promotes the self-fluxing properties [19]. Gil et al. reported that the nickel based self-fluxing thermal sprayed alloys have been used in many applications to protect machinery parts against wear and corrosion [20]. The NiCrBSi with addition of Ta in the coating exhibited higher fracture toughness, and higher abrasive and adhesive wear resistance than the NiCrBSi coating as suggested by Tu et al. [21]. Bolleli et al. have reported that HVOF coatings on stainless steel have improved the mechanical and tribological properties of the coatings [22]. Santana et al. suggested improved elastic modulus and hardness for the HVOF coatings [23]. Nicolas et al. has done a detailed study on the microstructures of metallic NiCrBSi Coatings manufactured via Hybrid Plasma Spray and In Situ Laser Remelting Process [24]. However, little literature is available on the study of graphite addition and the effect of graphite addition on the NiCrBSi plasma sprayed coating on microstructure, hardness and abrasive wear resistance. In the light of above an attempt has been made in this investigation to study the effect of addition of graphite in the NiCrBSi plasma sprayed coatings on microstructure, hardness and abrasive wear resistance.
101
roughness of the samples was measured using Mitutoyo Surftest equipment and the average surface roughness was reported in the Table 3.
2.3. X-ray diffraction studies X-ray Diffraction studies were conducted to analyze the coating powders using a Rigaku D/Tex Ultra diffractometer with a Cu Ka radiation (1.5405 Å). The scanning angle (2q) ranged from 20 to 80 with a step size of 0.02 and counting time of 2 s/step. The diffraction patterns obtained were indexed and compared with the Table 1 Chemical composition of the powder. Coating
NCB-4G NCB-6G NCB-8G
Chemical composition (wt%) Ni
Cr
B
Si
Fe
Graphite
69.03 67.59 66.15
14.93 14.62 14.31
2.97 2.91 2.85
3.99 3.91 3.82
3.91 3.82 3.74
4 6 8
Table 2 Micro hardness values of the samples. Sample
Avg. Vickers hardness (Hv)
Standard deviation
Uncoated NiCrBSi-4%Graphite NiCrBSi-6%Graphite NiCrBSi-8%Graphite
259.2 570.9 512.9 488.7
0.1237 0.1339 0.1857 0.1448
Table 3 Surface Roughness values of As-Coated specimen. Sample
Avg. Surface roughness
Standard deviation
NiCrBSi-4%Graphite NiCrBSi-6%Graphite NiCrBSi-8%Graphite
5.3292 5.2328 5.8926
0.3887 0.4939 0.2985
2. Materials and methods 2.1. Specimen preparation The NiCrBSiegraphite composite is composed of NiCrBSi powders and graphite powders. The chemical compositions of the NiCrBSi and graphite powder are shown in the Table 1. The particle sizes were in the range of 100e110 mm. The purity of graphite powders with sizes less than 250 mm was 98%. The composite spray powders were made by mixing the Ni-based alloy powders and the graphite powders in ball mill for 1 h. SS 304 was chosen as the substrate, and was pretreated by sand blasting. The plasma spray coating was done at Spraymet coating industries, Bangalore, India. The spray technological parameters were electric current of 500 A, voltage of 65e75 V and spray distance of 100e150 mm. The thickness of the spray coating was approximately 200 ± 15 mm with surface roughness of 0.9e1.5 mm. 2.2. Microhardness and surface roughness tests Vickers microhardness equipment (Matsuzawa MMT-X7 B type, Japan) was used to measure microhardness by performing indentations at a loading force of 100 gf and holding time of 15 s. At least 5 measurements were conducted and the ranges of microhardness values for the samples are shown in the Table 2. Surface
Fig. 1. Testing under progress.
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S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106
Fig. 2. a. Coated sample. b. Wear scar after abrasion test.
ICDD-PDF standard data available. 2.4. Two body abrasive wear tests Coated samples of 76 mm 25mm 4 mm size each were used for the wear studies. All the samples were thoroughly cleaned with acetone and weighed before the wear test. The tests were carried out on a two and three body abrasion testing unit supplied by DUCOM Instruments, Bangalore, India. The abrasive wear test setup is shown in Fig. 1. The equipment consists of a rotating wheel with a chlorobutyl rubber beading on its outer surface. The sample holder is designed in such a way that the sample to be tested is pressed against the rotating rubber wheel and also has provision for changing the applied load. The rubber wheel is set to rotate at constant speed of 200 rpm. Silica sand of grit size 60 micron is used as the abrasive medium and it is fed between the contact surfaces of the rubber wheel and the sample as shown in Fig. 1. The wear tests
Fig. 3. SEM image of coating with 4 wt% graphite.
were carried out for 5 min using a constant load of 54.54 N and 78.67 N. Weight loss was measured after each test. The wear of different samples was measured as the loss of weight in grams and then converted into the specific wear rate (mm3/mm). The samples before and after the wear test are shown in the Fig. 2a and b. The samples are analyzed for the influence of graphite addition on the coatings with respect to the wear rate.
2.5. Optical, scanning electron microscopy and elemental analysis The as deposited and the coatings after the abrasive wear tests were analyzed through Olympus LED microscope. SEM and EDS analysis of the coated and tested samples were done by Tescan Vega 3 Electron Microscope fitted with Oxford EDS analyzer.
Fig. 4. SEM image of coating with 6 wt% graphite.
S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106
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Fig. 5. EDS analysis of the coating with 4 wt% graphite.
3. Results and discussion 3.1. Microstructure of the NiCrBSi-Graphite coatings The SEM images of the coatings with 4wt% and 6wt% graphite are shown in the Figs. 3 and 4. The coatings were dense and showed typical microstructure of plasma spray coatings containing molten splats and a few unmelted particles indicating an appropriate selection of the spray parameters. The distribution of various elements analyzed through EDS analysis for the coatings with 4 and 8 wt% graphite additions are shown in Figs. 5 and 6 respectively. The NiCrBSi-graphite composite coatings are mainly composed of Ni, CrB and Cr7C3. These phases such as CrB and Cr7C3 emerge in the NiCrBSi composite coating during the spraying process. The NiCrBSi powders and the graphite powders were heated by the plasma flame, and then the particles are accelerated quickly and get solidified on the substrate SS304. The solid solubility of graphite/ carbon in NiCrBSi is very limited though the atomic radius of C (0.070 nm) is less when compared to Ni (0.135 nm), Cr (0.140 nm), Si (0.110 nm) and B (0.085 nm). As the concentrations of graphite
increase to certain degree in the coatings, phases of CrB and Cr7C3 may precipitate. The hard phases of CrB and Cr7C3 can improve the wear resistance of the composite coatings. Also a little graphite segregation is observed in the composite coating which indicates some unreacted graphite particles are retained in the composite coating as shown in the Fig. 7. According to the earlier observations by Wang et al. that the graphite particles are pushed by solid/liquid phase during solidification towards segregation at certain places in the matrix [14,16,17]. 3.2. Phase analysis by XRD The XRD patterns of the NiCrBSiegraphite composite coatings are shown in Fig. 8. It is observed that the characteristic reflections from g-Ni, Cr2B and Cr7C3 are seen prominently along with peaks of carbon. It is observed that these combinations of powders tend to form various phases such as Ni, Ni3B, Ni5Si2, Cr2B and Cr7C3 during the plasma spray process [18,25,26]. Using Metal Plus image analyzer software the quantity of each of the phases is analyzed and is given in the Table 4. Among these, the Cr2B and Cr7C3 phases are
Fig. 6. EDS analysis of the coating with 8 wt% graphite.
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S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106
hard which is also evident from the XRD and micro hardness values obtained for these coatings. 3.3. Effect of graphite content on tribological behavior and mechanisms of composite coatings Fig. 9 shows the wear mass loss of the NiCrBSi composite coatings as a function of graphite content. The wear mass losses of the composite coatings firstly decrease with respect to increase in graphite content. The wear mass loss of the NiCrBSi coating with 8wt% graphite in the coating has the lowest weight loss (54 mg) which is 40% less when compared with the 4 wt% graphite in NiCrBSi coating (90 mg). Fig. 10 shows the wear rate with respect to the applied loads. Again the coating with highest graphite content showed lowest wear rate. This may be attributed to the ductility of the NiCrBSi due to the addition of graphite in the coating. This is also evident from the microhardness values obtained. The addition of graphite in the coating decreases its microhardness to a certain extent but in turn improves its abrasion resistance. The optical images of the abraded samples are shown in the Figs. 11e13. The images reveal the lubricating effect of the graphite in the coatings. Coatings with 8wt% graphite showed the highest abrasion resistance. The graphite added in the coatings acts as intermediate lubricating agents and allows the abrasive sand particles to slip on Fig. 7. SEM image showing graphite segregation in the coatings.
Fig. 8. XRD pattern of the NiCrBSieGraphite Coatings.
S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106 Table 4 Phase analysis and quantification in Vol %. Sample
g-Ni
Cr3C7
Cr2B
Ni5Si2
Ni3B
NiCrBSi-4%Graphite NiCrBSi-6%Graphite NiCrBSi-8%Graphite
86.20 85.49 84.81
7.36 8.22 8.74
3.11 3.16 3.40
1.33 1.58 1.47
1.24 1.55 1.58
Fig. 11. Optical image of coating with 4 wt% graphite.
Fig. 9. Weight loss in the NiCrBSi composite coatings as a function of graphite content.
Fig. 12. Optical image of coating with 6 wt% graphite.
Fig. 10. Wear rate Vs Applied load.
the surface of the coating. This phenomenon is evident from the images (Figs. 11e13) which shows the graphite being crushed or elongated by the applied load and the sand particles. The wear phenomenon is mainly by grooving and ploughing. During the initials stages the abrasive sand in contact with the coating gives rise to the abrasive wear. But when the test progresses the graphite particles impart lubricating effect on the coating which changes the wear to adhesive nature. The Fig. 14 shows the SEM image of the coating revealing the adhesive nature of the wear by grooving and ploughing mechanism. It is also confirmed that the right amount of addition of graphite in the hard coatings such NiCrBSi can impart sufficient amount of lubricating effect during the service conditions thereby reducing the wear rate and improving the life of the coatings.
Fig. 13. Optical image of coating with 8 wt% graphite.
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S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106
Fig. 14. SEM image showing adhesive wear by grooving and ploughing.
4. Conclusions 1) NiCrBSi with graphite additions is prepared by plasma spray process. With addition of graphite phases such as CrB and Cr7C3 emerge in the coating during the plasma spray process. The micro hardness of the NiCrBSieGraphite composite coatings decreased with increased wt% of graphite. 2) The NiCrBSieGraphite composite coating exhibited better tribological properties. The wear mass loss of the composite coating with 8wt% graphite showed highest abrasion resistance. The composite coating showed slight wear. The wear mechanisms are initially abrasive in nature and then changed to adhesive wear with grooving and ploughing. References [1] T.S. Eyre, Wear characteristics of metals, Tribol. Int. 10 (1976) 203e212. [2] J.H. Tylczak, A. Oregon, Abrasive wear, in: P.J. Blau (Ed.), ASM Handbook, Friction, Lubrication, and Wear Technology, Vol. 18, ASM International, 1992, pp. 184e190. [3] B.A. Kushner, E.R. Novinski, Thermal spray coatings, in: P.J. Blau (Ed.), ASM Handbook, Friction, Lubrication, and Wear Technology, Vol. 18, ASM International, 1992, pp. 829e833. [4] P. Crook, Friction and wear of hardfacing alloy, in: P.J. Blau (Ed.), ASM Handbook, Friction, Lubrication, and Wear Technology, Vol. 18, ASM International, 1992, pp. 758e765. [5] K.H. Zum Gahr, Relation between abrasive wear rate and microstructure of metals, Proc. Int. Conf. Wear Mater., Rest. (1983) 266e274.
[6] T. Hejwowski, S. Szewczyk, A. Weron Aski, An investigation of the abrasive and erosive wear of flame-sprayed coatings, J. Mater. Process. Technol. 106 (2000) 54e57. [7] H. Wang, W. Xia, Y. Jin, A study on abrasive resistance of Ni-based coatings with a WC hard phase, Wear 195 (2006) 47e52. [8] H. Chen, C. Xu, J. Chen, H. Zhao, L. Zhang, Z. Wang, Microstructure and phase transformation of WC/Ni60B laser cladding coatings during dry sliding wear, Wear 264 (2008) 487e493. [9] A. Martin, J. Rodriguez, J.E. Fernandez, R. Vijande, Sliding wear behaviour of plasma sprayed WC-NiCrBSi coatings at different temperatures, Wear 251 (2001) 1017e1022. [10] Z. Tao, C. Xun, W. Shunxing, Z. Shian, Effect of CeO2 on microstructure and corrosive wear behavior of laser-cladded NirWC coating, Thin Solid Films 379 (2000) 128e132. [11] Q. Li, T.C. Lei, W.Z. Chen, Microstructural characterization of laser-clad TiCp reinforced NieCreBeSieC composite coatings on steel, Surf. Coatings Technol. 114 (1999) 278e284. [12] X. Li, Y. Mo, X. Yu, W. Liu, Influence of La2O3 on tribological properties of laser clad Ni-base alloy coating for ZL108, Lubr. Eng. 3 (2006) 103e104. [13] K. Wang, Q. Zhang, X. Wei, Rare-earth La2O3 modification of laser-clad coatings, J. Mater. Sci. 33 (1998) 3573e3577. [14] K. Wang, Q. Zhang, X. Wei, Y. Zhu, Effect of La2O3 on the microstructure and wear resistance of laser clad nickel-based alloy coatings, J. Tsinghua Univ. 39 (1999) 5e8. [15] K.L. Wang, Q.B. Zhang, M.L. Sun, X.G. Wei, Microstructural characteristics of laser clad coatings with rare earth metal elements, J. Mater. Process. Technol. 139 (2003) 448e452. [16] K.L. Wang, Q.B. Zhang, M.L. Sun, X.G. Wei, Y.M. Zhu, Microstructure and corrosion resistance of laser clad coatings with rare earth elements, Corros. Sci. 43 (2001) 255e267. [17] K.L. Wang, Q.B. Zhang, M.L. Sun, X.G. Wei, Y.M. Zhu, Rare earth elements modification of laser clad nickel based alloy coatings, Appl. Surf. Sci. 174 (2001) 191e200. [18] S.P. Sharma, D.K. Dwivedi, P.K. Jain, Effect of CeO2 addition on the microstructure, hardness, and abrasive wear behaviour of flame-sprayed Ni-based Coatings, Proc. Inst. Mech. Eng., Part J- J. Eng.Tribology 222 (7) (2008) 925e933. [19] D. Ahmadkhaniha, F. Mahboubi, Effects of plasma nitriding on properties of electroless Ni-B coating, Surf. Eng. 28 (2012) 195e198. [20] L. Gil, M.H. Staia, R. Guevara, E.S. Puchi Cabrera, D.B. Lewis, Microstructural characterisation of NiWCrBSiC alloy coating produced by HVOF thermal spraying, Surf. Eng. 22 (4) (2006) 304e313. [21] T. Yu, Q.L. Deng, J.F. Zheng, G. Dong, J.G. Yang, Microstructure and wear behaviour of laser clad NiCrBSiþTa composite coating, Surf. Eng. 28 (5) (2012) 357e363. [22] G. Bolelli, L. Lusvarghi, F. Pighetti Mantini, M. Barletta, F. Casadei, Microstructural and tribological characterisation of as sprayed and heat treated HVOF deposited Ni alloys, Surf. Eng. 23 (5) (2007) 355e372. [23] Y.Y. Santana La, J.G. Barbera Sosa, J. Caro, E.S. Puchi Cabrera, M.H. Staia, Mechanical properties and microstructure of WCe10Coe4Cr and WCe12Co thermal spray coatings deposited by HVOF, Surf. Eng. 24 (5) (2008) 374e382. [24] Nicolas Serres, Francoise Hlawka, Sophie Costil, Cecile Langlade, Frederique Machi, Microstructures of metallic NiCrBSi coatings manufactured via Hybrid plasma Spray and in Situ laser remelting process, J. Therm. Spray. Tech. 20 (1e2) (2011) 336e343. [25] S.P. Sharma, D.K. Dwivedi, P.K. Jain, Abrasive wear behaviour of continuously compacted thermal-sprayed Ni base alloy powder coatings in different conditions, Int. J. Surf. Sci. Eng. 2 (3/4) (2008) 240e251. [26] H.J. Kim, Y.J. Kim, Wear and corrosion resistance of PTA weld surfaced Ni and Co based alloy layers, Surf. Eng. 15 (1999) 495e501.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Effect of graphite addition on the microstructure, hardness and abrasive wear behavior of plasma sprayed NiCrBSi coatings S. Natarajan a, b, *, Edward Anand E.c, K.S. Akhilesh a, Ananya Rajagopal a, Preeti P. Nambiar a a b c
Department of Metallurgical & Materials Engineering, National Institute of Technology (NITT), Trichy, India Center of Excellence in Corrosion and Surface Engineering, NITT, India Department of Physics, M.Kumarasamy College of Engineering (Autonomous), Karur, India
h i g h l i g h t s Effect of graphite on microstructure, hardness & abrasive wear of NiCrBSi coatings investigated. XRD analysis reveals coatings are composed of g-Ni, CrB & Cr7C3. Graphite addition in thermal spray coatings improve wear resistance. NiCrBSie8 wt % C coating has excellent abrasion resistance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2015 Received in revised form 6 February 2016 Accepted 28 February 2016 Available online 19 March 2016
Plasma sprayed coatings have been considered as one of the important surface modification processes to improve the tribological properties of materials used in extreme conditions. NiCrBSi composite coatings on stainless steel are known to offer good abrasive wear resistance in service conditions. Graphite is the material which has many potential advantages when processed and used in different forms. This paper describes the effect of graphite addition on the microstructure; hardness and abrasive wear behavior of plasma sprayed NiCrBSi coatings. The mechanical and tribological properties of the coatings can be significantly improved by refinement of grain structure of the coatings. The XRD results show that the NiCrBSiegraphite composite coatings are mainly composed of g-Ni, CrB and Cr7C3. With addition of graphite, phases such as CrB, Cr7C3 emerge in composite coating. The study was conducted, using an abrasive wear test rig according to ASTM G65 on uncoated as well as coated SS304 samples at room temperature. The samples were analyzed for volume loss and wear rate with respect to increasing content of graphite in the coatings. The results suggest that addition of graphite in the coatings resulted in reduction of volume loss and wear rate significantly. The NiCrBSie8 wt %C composite coating presents excellent abrasion resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Alloys Plasma deposition Electron microscopy Mechanical testing Abrasion Microstructure
1. Introduction Abrasion, or abrasive wear, is a type of wear experienced in many industries, particularly in the fields of agriculture, mining and mineral processing [1,2]. Ni and Co based alloys that exhibit high strength, hardness and excellent wear and corrosion resistance have been recently employed as coating materials for applications
* Corresponding author. Department of Metallurgical & Materials Engineering, NITT, India. E-mail addresses: [email protected], [email protected] (S. Natarajan). http://dx.doi.org/10.1016/j.matchemphys.2016.02.076 0254-0584/© 2016 Elsevier B.V. All rights reserved.
in the industries [3,4]. Nickel based alloy coatings containing typically, a combination of NiCrBSi are used in mill rolls, high capacity pumps, piston extruders and glass moulding industries, where wear resistance combined with oxidation or hot corrosion resistance is required. Nickel provides ductility and enhances the corrosion resistance. Chromium provides good resistance to wear and corrosion. Boron enhances wear resistance and silicon reduces the eutectic melting point of the alloy and improves the self fluxing properties of the coating. Abrasion is a complicated phenomenon influenced by different factors such as the properties of the materials coming into contact with each other, the service conditions or the environment all play their part in abrasive wear [5]. Abrasive
S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106
wear behavior of a coating is determined by a number of test parameters like applied load, abrasive size, environment, the size, shape and distribution of hard phase precipitates, volume content of the hard phase, hardness of the matrix etc [6]. Many researchers have reported the improvement in hardness of Ni based alloys by incorporation of hard carbides such as WC, TiC and rare earth oxides such as cerium oxide and lanthanum oxide. The hard carbides such as WC and TiC increase the hardness of the coatings at the cost of toughness. Addition of rare earth elements improves hardness and toughness besides enhancing the corrosion and oxidation resistance of the coatings [7e17]. Refinement in microstructure, increase in micro-hardness and abrasive wear resistance of NiCrBSi flame sprayed coatings with the optimum addition of CeO2 (0.8 wt.%) has been reported by the Sharma et al. [18]. Ahmad khan et al. have reported that Ni powders, which belong to the NieBeSi, with chromium addition, increase the hardness of the coating by the formation of hard carbides with improved oxidation and corrosion resistance at elevated temperatures. Boron supports the formation of carbides by depressing the melting temperature, and addition of silicon promotes the self-fluxing properties [19]. Gil et al. reported that the nickel based self-fluxing thermal sprayed alloys have been used in many applications to protect machinery parts against wear and corrosion [20]. The NiCrBSi with addition of Ta in the coating exhibited higher fracture toughness, and higher abrasive and adhesive wear resistance than the NiCrBSi coating as suggested by Tu et al. [21]. Bolleli et al. have reported that HVOF coatings on stainless steel have improved the mechanical and tribological properties of the coatings [22]. Santana et al. suggested improved elastic modulus and hardness for the HVOF coatings [23]. Nicolas et al. has done a detailed study on the microstructures of metallic NiCrBSi Coatings manufactured via Hybrid Plasma Spray and In Situ Laser Remelting Process [24]. However, little literature is available on the study of graphite addition and the effect of graphite addition on the NiCrBSi plasma sprayed coating on microstructure, hardness and abrasive wear resistance. In the light of above an attempt has been made in this investigation to study the effect of addition of graphite in the NiCrBSi plasma sprayed coatings on microstructure, hardness and abrasive wear resistance.
101
roughness of the samples was measured using Mitutoyo Surftest equipment and the average surface roughness was reported in the Table 3.
2.3. X-ray diffraction studies X-ray Diffraction studies were conducted to analyze the coating powders using a Rigaku D/Tex Ultra diffractometer with a Cu Ka radiation (1.5405 Å). The scanning angle (2q) ranged from 20 to 80 with a step size of 0.02 and counting time of 2 s/step. The diffraction patterns obtained were indexed and compared with the Table 1 Chemical composition of the powder. Coating
NCB-4G NCB-6G NCB-8G
Chemical composition (wt%) Ni
Cr
B
Si
Fe
Graphite
69.03 67.59 66.15
14.93 14.62 14.31
2.97 2.91 2.85
3.99 3.91 3.82
3.91 3.82 3.74
4 6 8
Table 2 Micro hardness values of the samples. Sample
Avg. Vickers hardness (Hv)
Standard deviation
Uncoated NiCrBSi-4%Graphite NiCrBSi-6%Graphite NiCrBSi-8%Graphite
259.2 570.9 512.9 488.7
0.1237 0.1339 0.1857 0.1448
Table 3 Surface Roughness values of As-Coated specimen. Sample
Avg. Surface roughness
Standard deviation
NiCrBSi-4%Graphite NiCrBSi-6%Graphite NiCrBSi-8%Graphite
5.3292 5.2328 5.8926
0.3887 0.4939 0.2985
2. Materials and methods 2.1. Specimen preparation The NiCrBSiegraphite composite is composed of NiCrBSi powders and graphite powders. The chemical compositions of the NiCrBSi and graphite powder are shown in the Table 1. The particle sizes were in the range of 100e110 mm. The purity of graphite powders with sizes less than 250 mm was 98%. The composite spray powders were made by mixing the Ni-based alloy powders and the graphite powders in ball mill for 1 h. SS 304 was chosen as the substrate, and was pretreated by sand blasting. The plasma spray coating was done at Spraymet coating industries, Bangalore, India. The spray technological parameters were electric current of 500 A, voltage of 65e75 V and spray distance of 100e150 mm. The thickness of the spray coating was approximately 200 ± 15 mm with surface roughness of 0.9e1.5 mm. 2.2. Microhardness and surface roughness tests Vickers microhardness equipment (Matsuzawa MMT-X7 B type, Japan) was used to measure microhardness by performing indentations at a loading force of 100 gf and holding time of 15 s. At least 5 measurements were conducted and the ranges of microhardness values for the samples are shown in the Table 2. Surface
Fig. 1. Testing under progress.
102
S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106
Fig. 2. a. Coated sample. b. Wear scar after abrasion test.
ICDD-PDF standard data available. 2.4. Two body abrasive wear tests Coated samples of 76 mm 25mm 4 mm size each were used for the wear studies. All the samples were thoroughly cleaned with acetone and weighed before the wear test. The tests were carried out on a two and three body abrasion testing unit supplied by DUCOM Instruments, Bangalore, India. The abrasive wear test setup is shown in Fig. 1. The equipment consists of a rotating wheel with a chlorobutyl rubber beading on its outer surface. The sample holder is designed in such a way that the sample to be tested is pressed against the rotating rubber wheel and also has provision for changing the applied load. The rubber wheel is set to rotate at constant speed of 200 rpm. Silica sand of grit size 60 micron is used as the abrasive medium and it is fed between the contact surfaces of the rubber wheel and the sample as shown in Fig. 1. The wear tests
Fig. 3. SEM image of coating with 4 wt% graphite.
were carried out for 5 min using a constant load of 54.54 N and 78.67 N. Weight loss was measured after each test. The wear of different samples was measured as the loss of weight in grams and then converted into the specific wear rate (mm3/mm). The samples before and after the wear test are shown in the Fig. 2a and b. The samples are analyzed for the influence of graphite addition on the coatings with respect to the wear rate.
2.5. Optical, scanning electron microscopy and elemental analysis The as deposited and the coatings after the abrasive wear tests were analyzed through Olympus LED microscope. SEM and EDS analysis of the coated and tested samples were done by Tescan Vega 3 Electron Microscope fitted with Oxford EDS analyzer.
Fig. 4. SEM image of coating with 6 wt% graphite.
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Fig. 5. EDS analysis of the coating with 4 wt% graphite.
3. Results and discussion 3.1. Microstructure of the NiCrBSi-Graphite coatings The SEM images of the coatings with 4wt% and 6wt% graphite are shown in the Figs. 3 and 4. The coatings were dense and showed typical microstructure of plasma spray coatings containing molten splats and a few unmelted particles indicating an appropriate selection of the spray parameters. The distribution of various elements analyzed through EDS analysis for the coatings with 4 and 8 wt% graphite additions are shown in Figs. 5 and 6 respectively. The NiCrBSi-graphite composite coatings are mainly composed of Ni, CrB and Cr7C3. These phases such as CrB and Cr7C3 emerge in the NiCrBSi composite coating during the spraying process. The NiCrBSi powders and the graphite powders were heated by the plasma flame, and then the particles are accelerated quickly and get solidified on the substrate SS304. The solid solubility of graphite/ carbon in NiCrBSi is very limited though the atomic radius of C (0.070 nm) is less when compared to Ni (0.135 nm), Cr (0.140 nm), Si (0.110 nm) and B (0.085 nm). As the concentrations of graphite
increase to certain degree in the coatings, phases of CrB and Cr7C3 may precipitate. The hard phases of CrB and Cr7C3 can improve the wear resistance of the composite coatings. Also a little graphite segregation is observed in the composite coating which indicates some unreacted graphite particles are retained in the composite coating as shown in the Fig. 7. According to the earlier observations by Wang et al. that the graphite particles are pushed by solid/liquid phase during solidification towards segregation at certain places in the matrix [14,16,17]. 3.2. Phase analysis by XRD The XRD patterns of the NiCrBSiegraphite composite coatings are shown in Fig. 8. It is observed that the characteristic reflections from g-Ni, Cr2B and Cr7C3 are seen prominently along with peaks of carbon. It is observed that these combinations of powders tend to form various phases such as Ni, Ni3B, Ni5Si2, Cr2B and Cr7C3 during the plasma spray process [18,25,26]. Using Metal Plus image analyzer software the quantity of each of the phases is analyzed and is given in the Table 4. Among these, the Cr2B and Cr7C3 phases are
Fig. 6. EDS analysis of the coating with 8 wt% graphite.
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hard which is also evident from the XRD and micro hardness values obtained for these coatings. 3.3. Effect of graphite content on tribological behavior and mechanisms of composite coatings Fig. 9 shows the wear mass loss of the NiCrBSi composite coatings as a function of graphite content. The wear mass losses of the composite coatings firstly decrease with respect to increase in graphite content. The wear mass loss of the NiCrBSi coating with 8wt% graphite in the coating has the lowest weight loss (54 mg) which is 40% less when compared with the 4 wt% graphite in NiCrBSi coating (90 mg). Fig. 10 shows the wear rate with respect to the applied loads. Again the coating with highest graphite content showed lowest wear rate. This may be attributed to the ductility of the NiCrBSi due to the addition of graphite in the coating. This is also evident from the microhardness values obtained. The addition of graphite in the coating decreases its microhardness to a certain extent but in turn improves its abrasion resistance. The optical images of the abraded samples are shown in the Figs. 11e13. The images reveal the lubricating effect of the graphite in the coatings. Coatings with 8wt% graphite showed the highest abrasion resistance. The graphite added in the coatings acts as intermediate lubricating agents and allows the abrasive sand particles to slip on Fig. 7. SEM image showing graphite segregation in the coatings.
Fig. 8. XRD pattern of the NiCrBSieGraphite Coatings.
S. Natarajan et al. / Materials Chemistry and Physics 175 (2016) 100e106 Table 4 Phase analysis and quantification in Vol %. Sample
g-Ni
Cr3C7
Cr2B
Ni5Si2
Ni3B
NiCrBSi-4%Graphite NiCrBSi-6%Graphite NiCrBSi-8%Graphite
86.20 85.49 84.81
7.36 8.22 8.74
3.11 3.16 3.40
1.33 1.58 1.47
1.24 1.55 1.58
Fig. 11. Optical image of coating with 4 wt% graphite.
Fig. 9. Weight loss in the NiCrBSi composite coatings as a function of graphite content.
Fig. 12. Optical image of coating with 6 wt% graphite.
Fig. 10. Wear rate Vs Applied load.
the surface of the coating. This phenomenon is evident from the images (Figs. 11e13) which shows the graphite being crushed or elongated by the applied load and the sand particles. The wear phenomenon is mainly by grooving and ploughing. During the initials stages the abrasive sand in contact with the coating gives rise to the abrasive wear. But when the test progresses the graphite particles impart lubricating effect on the coating which changes the wear to adhesive nature. The Fig. 14 shows the SEM image of the coating revealing the adhesive nature of the wear by grooving and ploughing mechanism. It is also confirmed that the right amount of addition of graphite in the hard coatings such NiCrBSi can impart sufficient amount of lubricating effect during the service conditions thereby reducing the wear rate and improving the life of the coatings.
Fig. 13. Optical image of coating with 8 wt% graphite.
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Fig. 14. SEM image showing adhesive wear by grooving and ploughing.
4. Conclusions 1) NiCrBSi with graphite additions is prepared by plasma spray process. With addition of graphite phases such as CrB and Cr7C3 emerge in the coating during the plasma spray process. The micro hardness of the NiCrBSieGraphite composite coatings decreased with increased wt% of graphite. 2) The NiCrBSieGraphite composite coating exhibited better tribological properties. The wear mass loss of the composite coating with 8wt% graphite showed highest abrasion resistance. The composite coating showed slight wear. The wear mechanisms are initially abrasive in nature and then changed to adhesive wear with grooving and ploughing. References [1] T.S. Eyre, Wear characteristics of metals, Tribol. Int. 10 (1976) 203e212. [2] J.H. Tylczak, A. Oregon, Abrasive wear, in: P.J. Blau (Ed.), ASM Handbook, Friction, Lubrication, and Wear Technology, Vol. 18, ASM International, 1992, pp. 184e190. [3] B.A. Kushner, E.R. Novinski, Thermal spray coatings, in: P.J. Blau (Ed.), ASM Handbook, Friction, Lubrication, and Wear Technology, Vol. 18, ASM International, 1992, pp. 829e833. [4] P. Crook, Friction and wear of hardfacing alloy, in: P.J. Blau (Ed.), ASM Handbook, Friction, Lubrication, and Wear Technology, Vol. 18, ASM International, 1992, pp. 758e765. [5] K.H. Zum Gahr, Relation between abrasive wear rate and microstructure of metals, Proc. Int. Conf. Wear Mater., Rest. (1983) 266e274.
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