- Email: [email protected]
Wear properties of niobium carbide coatings performed by pack method on AISI 1040 steel
Thin Solid Films 483 (2005) 152 – 157 www.elsevier.com/locate/tsf
Wear properties of niobium carbide coatings performed by pack method on AISI 1040 steel Ugur SenT Sakarya University, Technical Education Faculty, Department of Metal Education, 54187 Sakarya, Turkey Received 25 August 2003; accepted in revised form 5 January 2005 Available online 10 February 2005
Abstract A series of experiments was performed to evaluate tribological properties of niobium carbide coated AISI 1040 steel. In coating process, thermo-reactive diffusion treatment by pack method was performed at the temperatures of 800, 900 and 1000 8C for 1–4 h, respectively. Depending on coating process time and temperature, the thickness of niobium carbide layer formed on substrate ranged from 3.4F0.5 to 12F2 Am. The hardness of niobium carbide layers on the steel sample is 1792 HV. The presence of carbides (e.g. NbC, Nb2C) formed on the surface of coated AISI 1040 steel was confirmed by X-ray diffraction analysis. Dry wear tests for uncoated and coated AISI 1040 steel against AISI D2 steel were carried out on pin-on-disk configuration and at sliding speed range of 0.5, 1, 2 and 5 m/s, and under 15 N and 30 N loads. The results showed that the average coefficient of friction for coated and uncoated AISI 1040 steel were 0.3 and 0.5, respectively. The specific wear rates for uncoated and coated steel are 4.4710-5 mm3/N.m to 4.2910-4 and 4.3710-7 to 3.5510-5 mm3/N.m. D 2005 Elsevier B.V. All rights reserved. Keywords: Deposition process; Carbides; Tribology; Steel
1. Introduction Mechanical components and tools are facing higher performance requirements. The use of surface coatings opens up to the possibility for material design in which the specific properties are located where they are most needed. The substrate material can be designed for strength and toughness, while the coating is responsible for the resistance to wear, thermal loads and corrosion. Surface treatments offer remarkable choices for a wide range of tribological applications where the control of friction and wear are of primary concern [1–3]. The thermo-reactive deposition/diffusion process (TRD) is a method of coating steel with a hard, wear-resistant layer of carbides, nitrides, or carbonitrides. In the TRD process, the carbon and nitrogen in the steel substrate diffuse into a
T Tel.: +90 532 574 7993; fax: +90 264 346 0262. E-mail address: [email protected]. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.01.008
deposited layer with a carbide-forming element (CFE) or nitride forming element (NFE) such as vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. The diffused carbon or nitrogen react with the CFE and NFE in the deposited coating so as to form a dense and metallurgically bonded carbide or nitride coating at the substrate surface [4–6]. The distinct boundary line is attributable to the difference in physical and chemical properties between the carbide and the steel. As the layers are metallurgically bonded to the substrate, carbide coated steel maintain the excellent properties inherent in the carbides: high hardness, excellent wear, seizure, corrosion, and oxidation resistance. Thus the process has a wide range of practical application [7]. Niobium carbide (NbC) coating presents a number of characteristic potentially interesting for its use in wear applications. Adding to high hardness [8], high toughness and Young’s modulus, the material has an extremely high melting temperature (3873 8C), which is a good indication for its high temperature environments [9]. The wear and
U. Sen / Thin Solid Films 483 (2005) 152–157
friction properties of niobium carbide coated steel have not yet been studied extensively. Having said that there are many reports in the literature related to niobium carbide coating techniques and its structural and wear properties [10–17]. Da.Costa et al. [10] studied the phase transition in the Nb based coatings. Miller et al. [11] examined high temperature oxidation of Nb based coatings. Tsubouchi et al. [15] studied the wear properties of NbC coating in the rolling operation. Arai et al. [16] studied practical use of NbC coated steel in industrial applications such as surface finishing, sand crushing, metal cutting application, etc.; wear resistance of NbC coated steel by TRD method showed excellent wear resistance. In comparison to other coating process, previous studies showed that NbC coating has higher wear resistance than that of borided, hardened and nitrided steel, being nearly equal to VC, TiN, TiC and TiCN coated steel [16,17]. Therefore it is concluded that the TRD method could be preferred to the other process from the point of view of cost, simplicity and high performance. The main objective of this study was to investigate tribological performance of niobium carbide coated AISI 1040 plain carbon steel, which is one of the cheapest medium carbon steel [18]. Wear tests were carried out for uncoated and niobium carbide coated AISI 1040 steel against AISI D2 steel disk. The coating process was carried out using TRD method. Finally the tribological properties of uncoated and coated AISI 1040 steel at different sliding speeds and loads were obtained and compared.
2. Experimental details A pin sample was prepared by coating the substrate material, AISI 1040 plain carbon steel, essentially contained: 0.42 wt.% C; 0.27 wt.% Si; 0.768 wt.% Mn; 0.120 wt.% Cr; 0.086 wt.% Ni; 0.036 wt.% Mo; 0.018 wt.% Al; 0.009 wt.% P and 0.048 wt.% S. Pin samples were prepared as cylindrical shape dimensions of 30 mm in length, 6 mm in diameter and flat ended shape. Thermoreactive diffusion treatment was performed in a solid medium consisting of ferro niobium, alumina and ammonium chloride at temperatures of 800, 900 and 1000 8C for 1–4 h. Niobium carbide coating was carried out in an electrical resistance furnace under atmospheric pressure and then left to cool in open air. Optical and SEM study was performed on polished cross-sections of niobium carbide coated test samples using Olympus B O71 optical microscopy and JEOL JSM-5410 instrument. The thickness of coating layer was measured with 0.1 Am accuracy by means of optical micrometer attached to the optical microscopy up to 5 measurements. The XRD analysis of the niobium carbide layer formed on the substrate was performed using a Philips diffractometer. Cu Ka radiation with a wavelength of 1.5418 2 was used
153
over a 2u range of 10–1108. Micro-hardness measurements of carbide layers were carried out on a Leitz Vickers indenter with a load of 25 g. Wear test of the niobium carbide coated and uncoated steel sample were performed via AISI D2 steel disk (780 HV) by the configuration of a flat end of pin vs. disk for 1 km, under the load of 15 N and 30 N at the sliding speeds of 0.5, 1, 2 and 5 m/s in the atmospheric condition (62% relative humidity). The wear was measured from mass loss. The specific wear rate was calculated from the equation of Wa=DG/(d.M.S), where DG is the worn mass, d is density of sample, S is the sliding distance and M is the wear load [19,20].
3. Result and discussion Optical microscope and scanning electron microscopy cross-sectional examinations of niobium carbide coated AISI 1040 steel surfaces revealed that niobium carbide formed on the surface of the substrate has uniform thickness all over the surfaces. Coating layer is dense, smooth and compact (Figs. 1, 2). Depending on coating process time and temperature, the measured average thickness values of niobium carbide layer formed on substrate ranged from 3.4F0.5 to 12F2 Am. The microhardness measurements of niobium carbide layer surfaces on coated steel samples were measured at least five times and average values were recorded. The hardness of the carbide layer and uncoated steel were 1792 HV and 330 HV, respectively. Using SEM micrograph, two distinct regions were identified on the cross-sections of niobium carbide coated steel; these regions are: (i) niobium carbide layer and (ii) steel matrix as seen in Fig. 2. The XRD analysis confirmed that, the carbide layer consists of NbC and Nb2C phases (Fig. 3).
Fig. 1. Optical micrograph of cross-section of niobium carbide coated AISI 1040 steel at 900 8C for 2 h.
154
U. Sen / Thin Solid Films 483 (2005) 152–157
Coefficent of friction, µ
1,0 Niobium carbide coated steel/15 N Niobium carbide coated steel/30 N Uncoated steel/15 N Uncoated steel/30 N
0,8
0,6
0,4
0,2
0,0 0
1
2
Fig. 2. SEM back scattered electron image of cross-section of niobium carbide coated AISI 1040 steel at 900 8C for 2 h.
3 4 Sliding speed, m/s
5
6
Fig. 4. Coefficient of friction of niobium carbide coated and uncoated AISI 1040 steel according to sliding speed.
Fig. 4 presents the coefficient of friction of niobium carbide coated and uncoated AISI 1040 steel with sliding speed under 15 N and 30 N load values. In this figure, the coefficient of friction of niobium carbide coated AISI 1040 steel is about 75% of that of uncoated steel. Furthermore, the coefficient of friction of uncoated and coated steel decrease with increase in sliding speed and load values. Thus, friction coefficient values exhibit significant load and speed dependency. This could be explained by the increase in sliding speed, which causes increase in the temperature of the contact area of pin and disk, and oxidation takes place on the contact area [21,22]. The variation of the specific wear rate with sliding speed is indicated in Fig. 5 for both niobium carbide coated and uncoated steel. It is clear from Fig. 5 that for the range of speeds and loads of this investigation, below 2 m/s sliding speed, the niobium carbide coated steel exhibit specific wear rate values up to 10 times lower values than that of the uncoated. Above 2 m/s speed this value is varying between 10–982 times lower than the uncoated steel. This result is in agreement with Venkataraman and Sundararajan [23] who concluded that the wear loss absolutely depends on the increasing temperature between
pins and counterface and explained that when the sliding speed exceeds the 2.5 m/s, oxidized layer was scraped and severe wear was shown on the steel sample under the lower and moderate pressure. It is believed that these results are related to the change in wear regime above 2 m/ s sliding speed. It is also clear from this figure that apart from coated steel under 30 N loads, the specific wear rates for coated and uncoated steel follow a profile of decreasing, then increasing with the increase in load and speed values. Fig. 6 presents the optical micrographs of the worn surfaces for uncoated steel. The wear scar of steel sample at 0.5 m/s sliding speed is essentially composed of grooves running along the sliding direction; see Fig. 6(a) and (c). On the other hand, the wear scar of steel sample at 5 m/s sliding speed has gray smeared film, see Fig. 6(b) and (d). Fig. 7 presents the micrographs of worn coated steel surfaces. In this figure, all the worn surfaces have gray colored patches at all sliding speeds. This might indicate the formation of oxidation, because of temperature incre-
1e-3 1
Specific wear rate, mm3/N.m
1-NbC 2-Nb2C
1
Intensity
3-α-Fe
1 1 2 1 2
3
20
30
40
3
3
2
50
60
70
80
1 1 2 2 1
1 2
90
100
110
2θ, degree Fig. 3. X-ray diffraction pattern of niobium carbide coated AISI 1040 steel at 1000 8C for 2 h.
Niobium carbide coated steel/15 N Niobium carbide coated steel/30 N Uncoated steel/15 N Uncoated steel/30 N
1e-4
1e-5
1e-6
1e-7 0
1
2
3 4 Sliding speed, m/s
5
6
Fig. 5. Specific wear rate of niobium carbide coated and uncoated AISI 1040 steel according to sliding speed.
U. Sen / Thin Solid Films 483 (2005) 152–157
155
Fig. 6. The optical micrographs of the worn surfaces of uncoated steel, at sliding speed and load of (a) 0.5 m/s, 15 N, (b) 5 m/s, 15 N, (c) 0.5 m/s, 30 N and (d) 5 m/s, 30 N, respectively.
ment due to friction effect. These results are in a good agreement with the results obtained by Venkataraman and Sundararajan [23]. Finally, Lim and Ashby [21,22] have developed wear mechanism maps for steel. To estimate and evaluate our
study, we decided to show our result in the similar graph for coated and uncoated steel on the map, see Fig. 8. In this figure, hashed and white boxes correspond to the coated and uncoated results. These results were computed in the normalized form (F˜V˜ ). Eqs. (1) and (2) show the normalized
Fig. 7. The optical micrographs of the worn surfaces of niobium carbide coated steel, at sliding speed and load of (a) 0.5 m/s, 15 N, (b) 5 m/s, 15 N, (c) 0.5 m/s, 30 N and (d) 5 m/s, 30 N, respectively.
156
U. Sen / Thin Solid Films 483 (2005) 152–157
Normalized pressure, ˜F
1e+1 Uncoated steel, 15 N load Uncoated steel, 30 N load Coated steel, 15 N load Coated steel, 30 N load Lim-Ashby map data
1e+0 1e-1 1e-2 1e-3 1e-4 1e-5 1e-2
1e-1
1e+ 0
1e+1
1e+2
1e+3
1e+ 4
1e+ 5
Normalized velocity, ˜V Fig. 8. Wear mechanisms map for uncoated steels indicating the wear regimes and also the experimental conditions utilized the present study [21,22,26]. Lines represent data from Lim–Ashby map in Ref. [22].
low sliding speeds, niobium carbide coated and uncoated steel exhibit the highest coefficient of friction values. However, with increasing sliding speed, the coefficient of friction values of the coated and uncoated steel decrease. Niobium carbide coated steel exhibit a considerably lower specific wear rate than that of uncoated steel. Below 2 m/s sliding speed, the niobium carbide coated steel exhibit specific wear rate values up to 10 times lower than that of the uncoated. Above 2 m/s sliding speed, the values vary between 10–982 times lower than that of the uncoated steel. While the wear regime of the uncoated steel samples under the speed of 2 m/s is mild, its behavior is severe wear above 2 m/s sliding speed. On the other hand niobium carbide coated steel samples has mild wear regime at all the sliding speed and load values.
Acknowledgment pressure and velocity, respectively, used in the Lim–Ashby map. F˜ ¼
F Ao Ho
mro V˜ ¼ a
ð1Þ ð2Þ
where, A o is the nominal contact area of the wearing surface, H o is the hardness at room temperature, a is the thermal diffusivity and r o, is the radius of the critical nominal contact area. Using the appropriate values of niobium carbide coated steel samples r o =0.003 m, H=1792 kg/mm2, and a=6.78x106 m2/s, determined and calculated from Refs. [24,25], and uncoated steel are r o=0.003 m, H=330 kg/mm2, and a=105 m2/s [23]. It is clear from Fig. 8 that our results for coated and uncoated steel are starting from transition region and carrying within the severe region. This is in agreement with Venkataraman and Sundararajan [23]. For uncoated steel, this result is in agreement with the micrographs of the severe worn surfaces, see Fig. 6. On the other hand the micrograph results for coated steel is in disagreement, which showed a mild surface, see Fig. 7. It is believed that this disagreement is due to the difference in the hardness values for coated and uncoated steel, which is used in calculations. Furthermore, it is important to note that the Lim–Ashby map represents the uncoated steel, only.
4. Conclusions Niobium carbide coating can be produced on AISI 1040 steel by pack method. The coating layer has two different phases, which are NbC and Nb2C. In addition, niobium carbide coating layer is dense, compact and smooth. The coefficient of friction of niobium carbide coated AISI 1040 steel is about 75% of that of uncoated steel. At
The author gratefully acknowledges Professor Abdullah Mimaroglu for fruitful discussions. The author also thanks SA.U colleagues including Ozkan Kon and Omer Savas for their help on sample preparation.
References [1] K. Holmberg, A. Matthews, D. Dowson (Eds.), Coatings Tribology, Tribology Series, vol. 28, Elsevier, Amsterdam, 1990, p. 1. [2] I. Ozbek, C. Bindal, Surf. Coat. Technol. 154 (2002) 14. [3] C. Bindal, A.H. Ucisik, J. Aust. Ceram. Soc. 34 (1998) 287. [4] T. Arai, in: ASM International Handbook Committee, (Eds.), ASM Handbook, vol. 4, Heat treating, ASM International, Ohio,1991, p.448. [5] U. Sen, Mater. Des. 26 (2005) 167. [6] T. Arai, in: T.S. Sudarshan, D.G. Bhat, H. Hinderman (Eds.), Development of Carbide and nitride coatings by thermo-reactive deposition and diffusion, Proceedings of Third International Surface Modification Technologies, Neuchatel, Switzerland, August 23– September 1, 1989, p. 587. [7] T. Arai, J. Heat Treat. 1/2 (1979) 15. [8] P.J. Martin, A. Bendavid, Surf. Coat. Technol. 142 (2001) 7. [9] B. Sustarsic, M. Jenko, M. Godec, L. Kosec, Vacuum 71 (2003) 77. [10] A.R. Da. Costa, A. Craievich, R. Vilar, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 336 (2002) 215. [11] C.F. Miller, G.W. Simmons, R.P. Wei, Scr. Mater. 42 (2000) 227. [12] A. Bendavid, P.J. Martin, T.J. Kinder, E.W. Preston, Surf. Coat. Technol. 163/164 (2003) 347. [13] S. Duhalde, R. Colaco, F. Audebert, A. Perrone, A. Zocco, Appl. Phys., A 69 (1999) 569. [14] S. Hotta, Y. Itou, K. Saruki, T. Arai, Surf. Coat. Technol. 73 (1995) 5. [15] K. Tsubouchi, M. Akiyama, M. Tsumura, S. Amano, J. Eng. Tribology (Part J) 213 (1999) 473. [16] T. Arai, H. Fujita, Y. Sugimoto, Y. Ohta, in: B. Richard, Jr. Sisson (Eds.), Diffusion Carbide Coatings Formed in Molten Borax Systems (Reaction in Borax Bath and Properties of Carbide Coated Steel), Proceedings of ’85 ASM’s International Conference on Surface Modifications and Coatings (Materials Week ’85), Toronto, Ontario, Canada, October 14–16, 1985, p. 1. [17] K. Saruki, S. Hotta, H. Fujita, T. Arai, Thin Solid Films 181 (1989) 383. [18] G. Krauss, D. Hudok, in: ASM International Handbook committee, (Eds.), ASM Handbook, vol. 1, Properties and Selection: Irons,
U. Sen / Thin Solid Films 483 (2005) 152–157
[19]
[20] [21] [22]
Steels, and High-Performance Alloys, ASM International, Ohio, (1991) 151. F. Yilmaz, in: E.S. Kayali, M. Urgen, K. Sesen, C.H. Gur, L. Ovecoglu (Eds.), Friction and Wear, Proceedings of the 9th International Metallurgy and Materials Congress, Volume 1, Istanbul, Turkey, June 11–15, 1997, p. 229. D.M. Kennedy, M.S.J. Hashmi, J. Mater. Process. Technol. 77 (1998) 246. S.C. Lim, M.F. Ashby, J.H. Brunton, Acta Metall. 35 (1987) 1343. S.C. Lim, M.F. Ashby, Acta Metall. 35 (1) (1987) 1.
157
[23] B. Venkataraman, G. Sundararajan, Surf. Coat. Technol. 73 (1995) 177. [24] M.W. Barsoum, T. El-Raghy, W.D. Porter, J.C. Ho, S. Chackraborty, J. Appl. Phys. 88 (11) (2000) 6313. [25] O. Kubaschewski, C.B. Alcock, in: International Series on Materials Science and Technology Committee (Eds.), International Series on Materials Science and Technology, vol. 24, Pergamon Press, Oxford, 1983, p. 298. [26] S.C. Lim, Tribol. Int. 35 (2002) 717.
Wear properties of niobium carbide coatings performed by pack method on AISI 1040 steel Ugur SenT Sakarya University, Technical Education Faculty, Department of Metal Education, 54187 Sakarya, Turkey Received 25 August 2003; accepted in revised form 5 January 2005 Available online 10 February 2005
Abstract A series of experiments was performed to evaluate tribological properties of niobium carbide coated AISI 1040 steel. In coating process, thermo-reactive diffusion treatment by pack method was performed at the temperatures of 800, 900 and 1000 8C for 1–4 h, respectively. Depending on coating process time and temperature, the thickness of niobium carbide layer formed on substrate ranged from 3.4F0.5 to 12F2 Am. The hardness of niobium carbide layers on the steel sample is 1792 HV. The presence of carbides (e.g. NbC, Nb2C) formed on the surface of coated AISI 1040 steel was confirmed by X-ray diffraction analysis. Dry wear tests for uncoated and coated AISI 1040 steel against AISI D2 steel were carried out on pin-on-disk configuration and at sliding speed range of 0.5, 1, 2 and 5 m/s, and under 15 N and 30 N loads. The results showed that the average coefficient of friction for coated and uncoated AISI 1040 steel were 0.3 and 0.5, respectively. The specific wear rates for uncoated and coated steel are 4.4710-5 mm3/N.m to 4.2910-4 and 4.3710-7 to 3.5510-5 mm3/N.m. D 2005 Elsevier B.V. All rights reserved. Keywords: Deposition process; Carbides; Tribology; Steel
1. Introduction Mechanical components and tools are facing higher performance requirements. The use of surface coatings opens up to the possibility for material design in which the specific properties are located where they are most needed. The substrate material can be designed for strength and toughness, while the coating is responsible for the resistance to wear, thermal loads and corrosion. Surface treatments offer remarkable choices for a wide range of tribological applications where the control of friction and wear are of primary concern [1–3]. The thermo-reactive deposition/diffusion process (TRD) is a method of coating steel with a hard, wear-resistant layer of carbides, nitrides, or carbonitrides. In the TRD process, the carbon and nitrogen in the steel substrate diffuse into a
T Tel.: +90 532 574 7993; fax: +90 264 346 0262. E-mail address: [email protected]. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.01.008
deposited layer with a carbide-forming element (CFE) or nitride forming element (NFE) such as vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. The diffused carbon or nitrogen react with the CFE and NFE in the deposited coating so as to form a dense and metallurgically bonded carbide or nitride coating at the substrate surface [4–6]. The distinct boundary line is attributable to the difference in physical and chemical properties between the carbide and the steel. As the layers are metallurgically bonded to the substrate, carbide coated steel maintain the excellent properties inherent in the carbides: high hardness, excellent wear, seizure, corrosion, and oxidation resistance. Thus the process has a wide range of practical application [7]. Niobium carbide (NbC) coating presents a number of characteristic potentially interesting for its use in wear applications. Adding to high hardness [8], high toughness and Young’s modulus, the material has an extremely high melting temperature (3873 8C), which is a good indication for its high temperature environments [9]. The wear and
U. Sen / Thin Solid Films 483 (2005) 152–157
friction properties of niobium carbide coated steel have not yet been studied extensively. Having said that there are many reports in the literature related to niobium carbide coating techniques and its structural and wear properties [10–17]. Da.Costa et al. [10] studied the phase transition in the Nb based coatings. Miller et al. [11] examined high temperature oxidation of Nb based coatings. Tsubouchi et al. [15] studied the wear properties of NbC coating in the rolling operation. Arai et al. [16] studied practical use of NbC coated steel in industrial applications such as surface finishing, sand crushing, metal cutting application, etc.; wear resistance of NbC coated steel by TRD method showed excellent wear resistance. In comparison to other coating process, previous studies showed that NbC coating has higher wear resistance than that of borided, hardened and nitrided steel, being nearly equal to VC, TiN, TiC and TiCN coated steel [16,17]. Therefore it is concluded that the TRD method could be preferred to the other process from the point of view of cost, simplicity and high performance. The main objective of this study was to investigate tribological performance of niobium carbide coated AISI 1040 plain carbon steel, which is one of the cheapest medium carbon steel [18]. Wear tests were carried out for uncoated and niobium carbide coated AISI 1040 steel against AISI D2 steel disk. The coating process was carried out using TRD method. Finally the tribological properties of uncoated and coated AISI 1040 steel at different sliding speeds and loads were obtained and compared.
2. Experimental details A pin sample was prepared by coating the substrate material, AISI 1040 plain carbon steel, essentially contained: 0.42 wt.% C; 0.27 wt.% Si; 0.768 wt.% Mn; 0.120 wt.% Cr; 0.086 wt.% Ni; 0.036 wt.% Mo; 0.018 wt.% Al; 0.009 wt.% P and 0.048 wt.% S. Pin samples were prepared as cylindrical shape dimensions of 30 mm in length, 6 mm in diameter and flat ended shape. Thermoreactive diffusion treatment was performed in a solid medium consisting of ferro niobium, alumina and ammonium chloride at temperatures of 800, 900 and 1000 8C for 1–4 h. Niobium carbide coating was carried out in an electrical resistance furnace under atmospheric pressure and then left to cool in open air. Optical and SEM study was performed on polished cross-sections of niobium carbide coated test samples using Olympus B O71 optical microscopy and JEOL JSM-5410 instrument. The thickness of coating layer was measured with 0.1 Am accuracy by means of optical micrometer attached to the optical microscopy up to 5 measurements. The XRD analysis of the niobium carbide layer formed on the substrate was performed using a Philips diffractometer. Cu Ka radiation with a wavelength of 1.5418 2 was used
153
over a 2u range of 10–1108. Micro-hardness measurements of carbide layers were carried out on a Leitz Vickers indenter with a load of 25 g. Wear test of the niobium carbide coated and uncoated steel sample were performed via AISI D2 steel disk (780 HV) by the configuration of a flat end of pin vs. disk for 1 km, under the load of 15 N and 30 N at the sliding speeds of 0.5, 1, 2 and 5 m/s in the atmospheric condition (62% relative humidity). The wear was measured from mass loss. The specific wear rate was calculated from the equation of Wa=DG/(d.M.S), where DG is the worn mass, d is density of sample, S is the sliding distance and M is the wear load [19,20].
3. Result and discussion Optical microscope and scanning electron microscopy cross-sectional examinations of niobium carbide coated AISI 1040 steel surfaces revealed that niobium carbide formed on the surface of the substrate has uniform thickness all over the surfaces. Coating layer is dense, smooth and compact (Figs. 1, 2). Depending on coating process time and temperature, the measured average thickness values of niobium carbide layer formed on substrate ranged from 3.4F0.5 to 12F2 Am. The microhardness measurements of niobium carbide layer surfaces on coated steel samples were measured at least five times and average values were recorded. The hardness of the carbide layer and uncoated steel were 1792 HV and 330 HV, respectively. Using SEM micrograph, two distinct regions were identified on the cross-sections of niobium carbide coated steel; these regions are: (i) niobium carbide layer and (ii) steel matrix as seen in Fig. 2. The XRD analysis confirmed that, the carbide layer consists of NbC and Nb2C phases (Fig. 3).
Fig. 1. Optical micrograph of cross-section of niobium carbide coated AISI 1040 steel at 900 8C for 2 h.
154
U. Sen / Thin Solid Films 483 (2005) 152–157
Coefficent of friction, µ
1,0 Niobium carbide coated steel/15 N Niobium carbide coated steel/30 N Uncoated steel/15 N Uncoated steel/30 N
0,8
0,6
0,4
0,2
0,0 0
1
2
Fig. 2. SEM back scattered electron image of cross-section of niobium carbide coated AISI 1040 steel at 900 8C for 2 h.
3 4 Sliding speed, m/s
5
6
Fig. 4. Coefficient of friction of niobium carbide coated and uncoated AISI 1040 steel according to sliding speed.
Fig. 4 presents the coefficient of friction of niobium carbide coated and uncoated AISI 1040 steel with sliding speed under 15 N and 30 N load values. In this figure, the coefficient of friction of niobium carbide coated AISI 1040 steel is about 75% of that of uncoated steel. Furthermore, the coefficient of friction of uncoated and coated steel decrease with increase in sliding speed and load values. Thus, friction coefficient values exhibit significant load and speed dependency. This could be explained by the increase in sliding speed, which causes increase in the temperature of the contact area of pin and disk, and oxidation takes place on the contact area [21,22]. The variation of the specific wear rate with sliding speed is indicated in Fig. 5 for both niobium carbide coated and uncoated steel. It is clear from Fig. 5 that for the range of speeds and loads of this investigation, below 2 m/s sliding speed, the niobium carbide coated steel exhibit specific wear rate values up to 10 times lower values than that of the uncoated. Above 2 m/s speed this value is varying between 10–982 times lower than the uncoated steel. This result is in agreement with Venkataraman and Sundararajan [23] who concluded that the wear loss absolutely depends on the increasing temperature between
pins and counterface and explained that when the sliding speed exceeds the 2.5 m/s, oxidized layer was scraped and severe wear was shown on the steel sample under the lower and moderate pressure. It is believed that these results are related to the change in wear regime above 2 m/ s sliding speed. It is also clear from this figure that apart from coated steel under 30 N loads, the specific wear rates for coated and uncoated steel follow a profile of decreasing, then increasing with the increase in load and speed values. Fig. 6 presents the optical micrographs of the worn surfaces for uncoated steel. The wear scar of steel sample at 0.5 m/s sliding speed is essentially composed of grooves running along the sliding direction; see Fig. 6(a) and (c). On the other hand, the wear scar of steel sample at 5 m/s sliding speed has gray smeared film, see Fig. 6(b) and (d). Fig. 7 presents the micrographs of worn coated steel surfaces. In this figure, all the worn surfaces have gray colored patches at all sliding speeds. This might indicate the formation of oxidation, because of temperature incre-
1e-3 1
Specific wear rate, mm3/N.m
1-NbC 2-Nb2C
1
Intensity
3-α-Fe
1 1 2 1 2
3
20
30
40
3
3
2
50
60
70
80
1 1 2 2 1
1 2
90
100
110
2θ, degree Fig. 3. X-ray diffraction pattern of niobium carbide coated AISI 1040 steel at 1000 8C for 2 h.
Niobium carbide coated steel/15 N Niobium carbide coated steel/30 N Uncoated steel/15 N Uncoated steel/30 N
1e-4
1e-5
1e-6
1e-7 0
1
2
3 4 Sliding speed, m/s
5
6
Fig. 5. Specific wear rate of niobium carbide coated and uncoated AISI 1040 steel according to sliding speed.
U. Sen / Thin Solid Films 483 (2005) 152–157
155
Fig. 6. The optical micrographs of the worn surfaces of uncoated steel, at sliding speed and load of (a) 0.5 m/s, 15 N, (b) 5 m/s, 15 N, (c) 0.5 m/s, 30 N and (d) 5 m/s, 30 N, respectively.
ment due to friction effect. These results are in a good agreement with the results obtained by Venkataraman and Sundararajan [23]. Finally, Lim and Ashby [21,22] have developed wear mechanism maps for steel. To estimate and evaluate our
study, we decided to show our result in the similar graph for coated and uncoated steel on the map, see Fig. 8. In this figure, hashed and white boxes correspond to the coated and uncoated results. These results were computed in the normalized form (F˜V˜ ). Eqs. (1) and (2) show the normalized
Fig. 7. The optical micrographs of the worn surfaces of niobium carbide coated steel, at sliding speed and load of (a) 0.5 m/s, 15 N, (b) 5 m/s, 15 N, (c) 0.5 m/s, 30 N and (d) 5 m/s, 30 N, respectively.
156
U. Sen / Thin Solid Films 483 (2005) 152–157
Normalized pressure, ˜F
1e+1 Uncoated steel, 15 N load Uncoated steel, 30 N load Coated steel, 15 N load Coated steel, 30 N load Lim-Ashby map data
1e+0 1e-1 1e-2 1e-3 1e-4 1e-5 1e-2
1e-1
1e+ 0
1e+1
1e+2
1e+3
1e+ 4
1e+ 5
Normalized velocity, ˜V Fig. 8. Wear mechanisms map for uncoated steels indicating the wear regimes and also the experimental conditions utilized the present study [21,22,26]. Lines represent data from Lim–Ashby map in Ref. [22].
low sliding speeds, niobium carbide coated and uncoated steel exhibit the highest coefficient of friction values. However, with increasing sliding speed, the coefficient of friction values of the coated and uncoated steel decrease. Niobium carbide coated steel exhibit a considerably lower specific wear rate than that of uncoated steel. Below 2 m/s sliding speed, the niobium carbide coated steel exhibit specific wear rate values up to 10 times lower than that of the uncoated. Above 2 m/s sliding speed, the values vary between 10–982 times lower than that of the uncoated steel. While the wear regime of the uncoated steel samples under the speed of 2 m/s is mild, its behavior is severe wear above 2 m/s sliding speed. On the other hand niobium carbide coated steel samples has mild wear regime at all the sliding speed and load values.
Acknowledgment pressure and velocity, respectively, used in the Lim–Ashby map. F˜ ¼
F Ao Ho
mro V˜ ¼ a
ð1Þ ð2Þ
where, A o is the nominal contact area of the wearing surface, H o is the hardness at room temperature, a is the thermal diffusivity and r o, is the radius of the critical nominal contact area. Using the appropriate values of niobium carbide coated steel samples r o =0.003 m, H=1792 kg/mm2, and a=6.78x106 m2/s, determined and calculated from Refs. [24,25], and uncoated steel are r o=0.003 m, H=330 kg/mm2, and a=105 m2/s [23]. It is clear from Fig. 8 that our results for coated and uncoated steel are starting from transition region and carrying within the severe region. This is in agreement with Venkataraman and Sundararajan [23]. For uncoated steel, this result is in agreement with the micrographs of the severe worn surfaces, see Fig. 6. On the other hand the micrograph results for coated steel is in disagreement, which showed a mild surface, see Fig. 7. It is believed that this disagreement is due to the difference in the hardness values for coated and uncoated steel, which is used in calculations. Furthermore, it is important to note that the Lim–Ashby map represents the uncoated steel, only.
4. Conclusions Niobium carbide coating can be produced on AISI 1040 steel by pack method. The coating layer has two different phases, which are NbC and Nb2C. In addition, niobium carbide coating layer is dense, compact and smooth. The coefficient of friction of niobium carbide coated AISI 1040 steel is about 75% of that of uncoated steel. At
The author gratefully acknowledges Professor Abdullah Mimaroglu for fruitful discussions. The author also thanks SA.U colleagues including Ozkan Kon and Omer Savas for their help on sample preparation.
References [1] K. Holmberg, A. Matthews, D. Dowson (Eds.), Coatings Tribology, Tribology Series, vol. 28, Elsevier, Amsterdam, 1990, p. 1. [2] I. Ozbek, C. Bindal, Surf. Coat. Technol. 154 (2002) 14. [3] C. Bindal, A.H. Ucisik, J. Aust. Ceram. Soc. 34 (1998) 287. [4] T. Arai, in: ASM International Handbook Committee, (Eds.), ASM Handbook, vol. 4, Heat treating, ASM International, Ohio,1991, p.448. [5] U. Sen, Mater. Des. 26 (2005) 167. [6] T. Arai, in: T.S. Sudarshan, D.G. Bhat, H. Hinderman (Eds.), Development of Carbide and nitride coatings by thermo-reactive deposition and diffusion, Proceedings of Third International Surface Modification Technologies, Neuchatel, Switzerland, August 23– September 1, 1989, p. 587. [7] T. Arai, J. Heat Treat. 1/2 (1979) 15. [8] P.J. Martin, A. Bendavid, Surf. Coat. Technol. 142 (2001) 7. [9] B. Sustarsic, M. Jenko, M. Godec, L. Kosec, Vacuum 71 (2003) 77. [10] A.R. Da. Costa, A. Craievich, R. Vilar, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 336 (2002) 215. [11] C.F. Miller, G.W. Simmons, R.P. Wei, Scr. Mater. 42 (2000) 227. [12] A. Bendavid, P.J. Martin, T.J. Kinder, E.W. Preston, Surf. Coat. Technol. 163/164 (2003) 347. [13] S. Duhalde, R. Colaco, F. Audebert, A. Perrone, A. Zocco, Appl. Phys., A 69 (1999) 569. [14] S. Hotta, Y. Itou, K. Saruki, T. Arai, Surf. Coat. Technol. 73 (1995) 5. [15] K. Tsubouchi, M. Akiyama, M. Tsumura, S. Amano, J. Eng. Tribology (Part J) 213 (1999) 473. [16] T. Arai, H. Fujita, Y. Sugimoto, Y. Ohta, in: B. Richard, Jr. Sisson (Eds.), Diffusion Carbide Coatings Formed in Molten Borax Systems (Reaction in Borax Bath and Properties of Carbide Coated Steel), Proceedings of ’85 ASM’s International Conference on Surface Modifications and Coatings (Materials Week ’85), Toronto, Ontario, Canada, October 14–16, 1985, p. 1. [17] K. Saruki, S. Hotta, H. Fujita, T. Arai, Thin Solid Films 181 (1989) 383. [18] G. Krauss, D. Hudok, in: ASM International Handbook committee, (Eds.), ASM Handbook, vol. 1, Properties and Selection: Irons,
U. Sen / Thin Solid Films 483 (2005) 152–157
[19]
[20] [21] [22]
Steels, and High-Performance Alloys, ASM International, Ohio, (1991) 151. F. Yilmaz, in: E.S. Kayali, M. Urgen, K. Sesen, C.H. Gur, L. Ovecoglu (Eds.), Friction and Wear, Proceedings of the 9th International Metallurgy and Materials Congress, Volume 1, Istanbul, Turkey, June 11–15, 1997, p. 229. D.M. Kennedy, M.S.J. Hashmi, J. Mater. Process. Technol. 77 (1998) 246. S.C. Lim, M.F. Ashby, J.H. Brunton, Acta Metall. 35 (1987) 1343. S.C. Lim, M.F. Ashby, Acta Metall. 35 (1) (1987) 1.
157
[23] B. Venkataraman, G. Sundararajan, Surf. Coat. Technol. 73 (1995) 177. [24] M.W. Barsoum, T. El-Raghy, W.D. Porter, J.C. Ho, S. Chackraborty, J. Appl. Phys. 88 (11) (2000) 6313. [25] O. Kubaschewski, C.B. Alcock, in: International Series on Materials Science and Technology Committee (Eds.), International Series on Materials Science and Technology, vol. 24, Pergamon Press, Oxford, 1983, p. 298. [26] S.C. Lim, Tribol. Int. 35 (2002) 717.