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Interfacial reactions during IBAD and their effects on the adhesion of Cr–N coatings on steel
Surface & Coatings Technology 191 (2005) 149 – 154 www.elsevier.com/locate/surfcoat
Interfacial reactions during IBAD and their effects on the adhesion of Cr–N coatings on steel Lin-Hai Tian a,b,*, Bin Tang b, Dao-Xin Liu c, Xiao-Dong Zhu a, Jia-Wen He a a
State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong University, Xian 710049,China b Taiyuan University of Technology, Taiyuan 030024, China c Northwestern Polytechnical University, Xian 710072, China Received 29 April 2003; accepted in revised form 10 March 2004
Abstract Different energies of nitrogen ion intermixing to modify the interfaces of Ion Beam Assisted Deposition (IBAD) Cr – N films were studied on AISI 52100 and SAE 1045 steel substrates. Cyclic rolling contact has been applied on the coatings to fracture the interface at the weakest sites in order to reveal the interfacial reaction products. The fractured interfacial regions after rolling contact loading were observed by atomic force microscopy (AFM) and the compositions were analyzed with glow discharge optical emission spectrometry (GDOES), scanning electron microscopy (SEM) and EDAX. XPS was also applied to detect the carbon state. It was found that the carbon content in the interface subject to 40 keV ion intermixing reached 12.6 at.%, and for 20 keV, it was about 7 at.%, which is close to that in the substrate of AISI 52100. The different sputtering rates of carbide and matrix lead to enrichment of carbon in the interface. The carbide decomposes by thermal spikes caused by the ion bombardment and transforms into graphite, which is confirmed by XPS. The bonding strength evaluated by the contact fatigue method indicates that as the graphite is involved in the interface the adhesion between the coating and the substrate is decreased. The analysis of the fatigue origin at the delamination site confirms the graphite enrichment. D 2004 Published by Elsevier B.V. Keywords: Dynamic ion intermixing; Interfacial reaction; Carbon enrichment; Adhesion
1. Introduction When the adhesion between the coating layer and the substrate depends purely on mechanical interlocking, the bonding strength will not be high. The adhesion free of chemical reaction depends mainly on surface morphology, contamination and the internal stress of the coating layer. Chemical reactions at the interface are an important way to increase the bonding strength. The formation of intermetallic compound may also be helpful for the joint strength as in welding. However, not all interfacial reactions are beneficial; some of the reaction products may lead to brittleness of the interface layer and result in premature spalling of the coating. For instance, as TiN is deposited on stainless steel, the impact of nitrogen ions on the substrate surface can form a super saturation of nitrogen. When TiN * Corresponding author. State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong University, Xian 710049, China. Tel.: +86-29-826-68696; fax: +82-29-826-63453. E-mail address: [email protected] (L.-H. Tian). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.03.028
is coated on the top of this layer, the supersaturated nitrogen will release from the matrix and the pores forming in the interface reduce the adhesion [1]. If a hard layer is deposited on a carburized or nitrided surface by plasmaassisted chemical vapor deposition, a ‘‘dark’’ layer might be observed in the interface. Low concentration of carbon or nitrogen with high oxygen results in the color contrast after etching, and this dark and brittle layer in the interface reduces the bonding between the coating and substrate [2]. Another example is DLC coating on the surface of tungsten carbide, which is a conventional procedure to improve the endurance life of cutting tools. The catalytic effect of Co in the matrix of tungsten carbide can lead the SP3 structure in DLC transforming into SP2 of the graphite state. Delamination occurs at the low-strength graphite sites, and the adhesion is deteriorated [3]. One way to reduce the reaction is to remove a part of the Co from the matrix, single or double etching has been employed [4]. Nevertheless, the fraction of Co must be kept to a minimum level in order to sustain the cutting load. To reach the optimized status of the interface, it is necessary to evaluate the reaction
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products in a quantitative way. Regarding microanalysis of the constituents, ion sputtering is usually employed for the detection through the depth. As the substrate surface is intensively etched, the reaction products on the roughened surface cannot be quantitatively evaluated by ion thinning. A useful way is to fracture the interface layer of the coating at the weakest sites. The bonding strength test, either scratch or indentation, is presumed to detach the coating at the interface, yet the practice is not as expected; the reaction products in the interface are difficult to reveal with these tests. Impact loading is effective to fracture the workpiece along the weak sites. An example was the examination of P segregation at the grain boundaries by impact fracture of the sample inside of the Auger chamber [5]. At present, it is difficult to crack the interface of a thin film by impact loading; the rolling contact method developed in this group can be more effective [6]. The shear stress amplitude of the contact fatigue loading can lead to cleavage at the weakest sites and the reaction products can be inspected. In addition, the shear stress which results in the delamination of the coating layer can be employed as a criterion to evaluate the effect of the reaction product on the bonding strength. It is generally accepted that the increase of impact energy of ion bombarding during Ion Beam Assisted Deposition
(IBAD) can improve ion mixing and increase the bonding strength. However, our experiments showed that, as the impact energy exceeded a critical level, the adhesion dropped rapidly, and the effect became prominent as the carbon content of the steel substrate was increased [7].
2. Experimental Cr – N films were synthesized by Ion Beam Assisted Deposition (IBAD). The substrates were AISI 52100 (HRC61) and SAE 1045 steel. The samples were ground and polished to an average surface roughness less than 0.02 Am and then cleaned with acetone in an ultrasonic container. Prior to deposition, the samples were sputter-etched with argon ions for approximately 10 min, then the interface was prepared using dynamic intermixing. The high energy nitrogen ions are bombarding as the chromium atoms arrive the surface of the substrate. The ion impact forces the Cr atom to intermix with Fe and two energy levels of 40 and 20 keV were applied. The ion current was 17 AA/cm2 and the nitrogen flux was 4 sccm. The Cr sputtering ion current was 210 AA/cm2 and energy was 2.4 keV. The mixing time was 20 min. Then, Cr– N films were deposited with N ion bombarding energy 4 keV, ion current
Fig. 1. AFM images of the interface prepared by different nitrogen ion beam dynamic intermixing: (a) 40 keV, AISI 52100 substrate, Ra=3.7986 nA; (b) 20 keV, AISI 52100 substrate, Ra=0.4614 nA; (c) 40 keV, SAE 1045 substrate, Ra=4.7933 nA; (d) 20 keV, SAE 1045 substrate, Ra=1.2477 nA.
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Fig. 2. GDOES depth profiles of Cr – N coatings prepared by (a) 40 and (b) 20 keV ion intermixing.
4 AA/cm2 and N2 flux 10 sccm; in the mean time, the Cr target was sputtered at 2.4 keV and 210 AA/cm2. The film thickness for testing of the interface morphology and toughness Ra was 150– 200 and 700 – 800 nm for mechanical evaluation. The spherical rolling contact method was employed to detach the coating; the cracking will initiate at the weakest sites at the interface by the shear stress. If delamination does not occur after 5 million cycles, the shear stress range calculated from the loading force is taken as the magnitude of the bonding strength. The method has been described in detail elsewhere [6]. Atomic force microscopy (AFM) was used to observe the interface morphology and roughness, and expressed by electric current in nA scale. The constituent profiles through the depth were studied by glow discharge optical emission spectrometry (GDOES). The morphology and composition at the spalled sites after rolling contact fatigue were studied by scanning electron microscopy (SEM) and EDAX. In order to detect the state of carbon, XPS was also employed.
Fig. 3. Carbon concentrations near interface of 40 and 20 keV intermixing.
3. Results 3.1. Interface morphology and roughness The interfaces prepared by dynamic intermixing on AISI 52100 and SAE 1045 steel substrates are shown in Fig. 1. AFM results indicate that, for both substrates, the interface bombarded by 40 keV energy intermixing is rougher than that by 20 keV. With the same bombarding energy, the roughness of SAE 1045 steel is less than that of AISI 52100 substrate. 3.2. Interface composition and structure The GDOES depth profiles of the Cr –N films deposited on AISI 52100 substrate by 40 and 20 keV energies ion dynamic intermixing are shown in Fig. 2. The carbon concentration has little change through the depth and is close to that in the substrate when the intermixing energy is 20 keV. When the mixing energy is increased to 40 keV, the carbon content is increased significantly around the interface. Fig. 3 shows the magnified scale to compare the
Fig. 4. Effect of dynamic intermixing energy on bonding strength of coating on different substrates.
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Fig. 5. (a) Low and (b) high magnified images of Cr – N films by 40 keV intermixing on AISI 52100 substrate after rolling contact fatigue test.
carbon contents in the interface region; the maximum C% reaches 12.6 at.% for 40 keV bombardment. 3.3. Adhesion The adhesions of Cr –N films on different substrates with two intermixing performances are shown in Fig. 4. The results show that the Cr –N coating by 20 keV dynamic recoiling provides better bonding strength than that by 40 keV for both substrates. In fact, the bonding strengths of the coatings by 20 keV intermixing reached the maximum value of the capacity limit of the rolling contact machine. When 40 keV is used for intermixing, the bonding strength of the Cr – N coating on SAE 1045 steel is twice as much as that on AISI 52100 substrate. Adhesion tests reveal that high energy intermixing leads to deteriorate the interface particularly on AISI 52100 substrate, which contains higher carbon than SAE 1045 steel. In order to investigate the nature of the drastic drop of the adhesion of Cr –N coating on AISI 52100 substrate, the
interface on the substrate revealed by delamination of Cr– N coating after rolling contact fatigue was examined. Fig. 5a shows the SEM morphology of the detached area, and Fig. 5b shows the magnified image. The figure suggests that the interfacial crack initiates at the center of Fig. 5b, and the growth of the crack leads to delamination after a number of loading cycles. The crack origin examined by EDAX showed that the carbon content reached 11.46 at.%, which is much higher than that in the AISI 52100 substrate (5.17 at.%). The detached patch was also checked by XPS as in Fig. 6. The fatigue origin is mainly composed of carbon. Besides of O by contamination, only small amounts of Cr and N are involved. It is noticed that the substrate constituent Fe was not detected in Fig. 6. The dynamic recoiling covers Cr on the top and the intermixing layer is of high strength; hence, the fatigue crack initiates between the mixing layer and the coating. Fig. 7 shows the magnification of XPS C1s spectrum at the binding energy of 285.35 eV. The C1s binding energy of graphite is 284.3 eV. The slight difference
Fig. 6. XPS spectra of rolling contact fatigue crack origin.
L.-H. Tian et al. / Surface & Coatings Technology 191 (2005) 149–154
Fig. 7. XPS C1s spectra at the fatigue crack origin.
between the two peaks is very likely due to contamination and measurement error. The other possible peak in the vicinity is C1s of Cr2C3 (282.8 eV), which is further away from that of graphite [8]. This confirms that the carbon is existed as graphite in the fatigue origin.
4. Discussion When Cr – N is coated on the hardened steel substrate, the cyclic shear stress initiating the crack at the interface is lower than the yield strength of the substrate. The coating delamination by high cycle contact loading is characterized in brittleness; the interfacial layer free of plastic deformation will be easy to examine if any reaction products are involved. The fatigue damage under cyclic loading leads to cracking at the weakest sites. The soft graphite acts as pores in the interface and increases stress concentration under loading. The crack initiates at the graphite then propagates leading to brittle fracture; the enrichment of graphite can be revealed by microanalysis. In addition, the maximum shear stress amplitude at the interface Dsc is taken as a measure of the bonding strength. For hard coating and hardened substrate system, this value will not be affected by noninterfacial factors such as film thickness and the hardnesses of either substrate or coating; it is only sensitive to the interfacial factors as roughness, interface composition and internal stress [9]. Therefore, the comparison of Dsc values in different states is of help to sort out the chemical reaction at the interface. The selections of AISI 52100 and SAE 1045 were emphasized on different fractions of carbide (Fe3C). Fe3C is metastable and can be decomposed into Fe and graphite as that in the cast iron. The Gibbs free energy of the decomposition is [10]
DGT0 ¼ 5400 þ 3:36T
153
Fe3C would decompose into graphite when the temperature is higher than 1607 K. Nitrogen ion bombardment with Cr dynamic recoiling on the substrate surface is characterized in cascade collision. The thermal spikes by ions impingement at the surface result in high temperature, and Fe3C can be thermodynamically decomposed into Fe and graphite at the collision spots. In addition, the surface material will be sputtered away during ion bombardment and the sputtering rates of Fe and Cr are nearly one order higher than that of C. If Ar ions of 500 eV are used for bombarding, the sputtering rates of Fe and Cr are 1.10 and 1.18, yet only 0.12 for carbon [11]. The high temperature thermal spikes by ion bombardment provide the dynamic energy for carbon decomposition, and the preferential sputtering makes the carbon enrich in the interface. As a result, a large fraction of graphite is formed in the interface. This procedure is associated with bombarding energy. When the bombardment is decreased from 40 to 20 keV, the carbide decomposition and preferential sputtering are not so significant, and so does the graphite enrichment in the interface. It is generally accepted that, in order to intensify the atomic mixing and increase the implantation depth, highenergy bombarding is favorable. However, as the preferential sputtering and carbon decomposition take place, the bombarding energy should be carefully selected to keep from reactions which deteriorate the adhesion. The surface of SAE 1045 steel substrate is easier to be sputtered by ion bombardment; the roughness of the interface on SAE 1045 steel is slightly higher than that on AISI 52100 substrate. As in Figs. 1 and 4, with the increase of interface roughness, the adhesion of 40 keV dynamic ion intermixing is improved, yet the increase of interface roughness does not make significant improvement of adhesion for 20 keV intermixing. This fact suggests that if the bonding strength is low, the mechanical interlocking can be of important help. Once chemical reaction is involved such as graphite enrichment in this study; it will play a predominant role on the adhesion of the coating.
5. Conclusions 1. When a hard coating is deposited on the hardened substrate, the rolling contact fatigue loading can detach the coating at the weakest sites with brittle fracture; the interfacial products are revealed. The microanalysis confirms that contact fatigue is an effective way to evaluate interfacial reaction. 2. Different energies of ion intermixing during IBAD for Cr – N coating indicate that high-energy ion bombardment might deteriorate its adhesion on the steel substrate, particularly on the high carbon steel. 3. Different sputtering rates make carbide enrichment on the steel surface and thermal spikes of bombardment decomposes the carbide into graphite. The concentration
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of the graphite in the interface is the main reason of poor adhesion.
Acknowledgements This research is supported by China Natural Science Foundation (No. 5017104) and Shanxi Province Natural Science Foundation (No. 20001043).
References [1] C. Quaeyyhagens, J.D. Haen, L.M. Stals, Van M. Stappen, F. Bodart, G. Terwagne, Surf. Coat. Technol. 61 (1993) 227.
[2] W.-S. Baek, S.-C. Kwon, J.-Y. Lee, S.-R. Lee, J.-J. Rha, K.-S. Nam, Thin Solid Films 323 (1998) 146. [3] R. Haubner, W.D. Schubert, B. Lux, Int. J. Refract. Met. Hard Mater. 16 (3) (1998) 177. [4] Z. Kesong, Met. Powder Rep. 52 (12) (1997) 45. [5] S.-F. Chen, T.S. Lui, L.-H. Chen, Metall. Mater. Trans., A 25 (3) (1994) 557. [6] J.-W. He, K.-W. Xu, N.-S. Hu, Surf. Coat. Technol. 97 (1997) 295. [7] B. Tang, X.-Z. Zhu, N.-S. Hu, J.-W. He, Trans. Nonferr. Met. Soc. China 10 (1) (2000) 64. [8] C.-D. Wagner, et al., Handbook of X-ray Photoelectron Spectroscopy, Perkin – Elmer, Minnesota, 1979. [9] J.-W. He, B.C. Hendrix, N.-S. Hu, K.-W. Xu, T. Bell, Y. Sun, K. Mao, Surf. Eng. 12 (1996) 49. [10] L. Shi, Alloy Thermodynamics, Mechanical Industry Press, Beijing, 1992, p. 338. [11] B.-Q. Chen, Ion Plating and Sputtering Technology, National Defence Industry Press, Beijing, 1990, p. 90.
Interfacial reactions during IBAD and their effects on the adhesion of Cr–N coatings on steel Lin-Hai Tian a,b,*, Bin Tang b, Dao-Xin Liu c, Xiao-Dong Zhu a, Jia-Wen He a a
State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong University, Xian 710049,China b Taiyuan University of Technology, Taiyuan 030024, China c Northwestern Polytechnical University, Xian 710072, China Received 29 April 2003; accepted in revised form 10 March 2004
Abstract Different energies of nitrogen ion intermixing to modify the interfaces of Ion Beam Assisted Deposition (IBAD) Cr – N films were studied on AISI 52100 and SAE 1045 steel substrates. Cyclic rolling contact has been applied on the coatings to fracture the interface at the weakest sites in order to reveal the interfacial reaction products. The fractured interfacial regions after rolling contact loading were observed by atomic force microscopy (AFM) and the compositions were analyzed with glow discharge optical emission spectrometry (GDOES), scanning electron microscopy (SEM) and EDAX. XPS was also applied to detect the carbon state. It was found that the carbon content in the interface subject to 40 keV ion intermixing reached 12.6 at.%, and for 20 keV, it was about 7 at.%, which is close to that in the substrate of AISI 52100. The different sputtering rates of carbide and matrix lead to enrichment of carbon in the interface. The carbide decomposes by thermal spikes caused by the ion bombardment and transforms into graphite, which is confirmed by XPS. The bonding strength evaluated by the contact fatigue method indicates that as the graphite is involved in the interface the adhesion between the coating and the substrate is decreased. The analysis of the fatigue origin at the delamination site confirms the graphite enrichment. D 2004 Published by Elsevier B.V. Keywords: Dynamic ion intermixing; Interfacial reaction; Carbon enrichment; Adhesion
1. Introduction When the adhesion between the coating layer and the substrate depends purely on mechanical interlocking, the bonding strength will not be high. The adhesion free of chemical reaction depends mainly on surface morphology, contamination and the internal stress of the coating layer. Chemical reactions at the interface are an important way to increase the bonding strength. The formation of intermetallic compound may also be helpful for the joint strength as in welding. However, not all interfacial reactions are beneficial; some of the reaction products may lead to brittleness of the interface layer and result in premature spalling of the coating. For instance, as TiN is deposited on stainless steel, the impact of nitrogen ions on the substrate surface can form a super saturation of nitrogen. When TiN * Corresponding author. State Key Laboratory for Mechanical Behaviour of Materials, Xi’an Jiaotong University, Xian 710049, China. Tel.: +86-29-826-68696; fax: +82-29-826-63453. E-mail address: [email protected] (L.-H. Tian). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.03.028
is coated on the top of this layer, the supersaturated nitrogen will release from the matrix and the pores forming in the interface reduce the adhesion [1]. If a hard layer is deposited on a carburized or nitrided surface by plasmaassisted chemical vapor deposition, a ‘‘dark’’ layer might be observed in the interface. Low concentration of carbon or nitrogen with high oxygen results in the color contrast after etching, and this dark and brittle layer in the interface reduces the bonding between the coating and substrate [2]. Another example is DLC coating on the surface of tungsten carbide, which is a conventional procedure to improve the endurance life of cutting tools. The catalytic effect of Co in the matrix of tungsten carbide can lead the SP3 structure in DLC transforming into SP2 of the graphite state. Delamination occurs at the low-strength graphite sites, and the adhesion is deteriorated [3]. One way to reduce the reaction is to remove a part of the Co from the matrix, single or double etching has been employed [4]. Nevertheless, the fraction of Co must be kept to a minimum level in order to sustain the cutting load. To reach the optimized status of the interface, it is necessary to evaluate the reaction
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products in a quantitative way. Regarding microanalysis of the constituents, ion sputtering is usually employed for the detection through the depth. As the substrate surface is intensively etched, the reaction products on the roughened surface cannot be quantitatively evaluated by ion thinning. A useful way is to fracture the interface layer of the coating at the weakest sites. The bonding strength test, either scratch or indentation, is presumed to detach the coating at the interface, yet the practice is not as expected; the reaction products in the interface are difficult to reveal with these tests. Impact loading is effective to fracture the workpiece along the weak sites. An example was the examination of P segregation at the grain boundaries by impact fracture of the sample inside of the Auger chamber [5]. At present, it is difficult to crack the interface of a thin film by impact loading; the rolling contact method developed in this group can be more effective [6]. The shear stress amplitude of the contact fatigue loading can lead to cleavage at the weakest sites and the reaction products can be inspected. In addition, the shear stress which results in the delamination of the coating layer can be employed as a criterion to evaluate the effect of the reaction product on the bonding strength. It is generally accepted that the increase of impact energy of ion bombarding during Ion Beam Assisted Deposition
(IBAD) can improve ion mixing and increase the bonding strength. However, our experiments showed that, as the impact energy exceeded a critical level, the adhesion dropped rapidly, and the effect became prominent as the carbon content of the steel substrate was increased [7].
2. Experimental Cr – N films were synthesized by Ion Beam Assisted Deposition (IBAD). The substrates were AISI 52100 (HRC61) and SAE 1045 steel. The samples were ground and polished to an average surface roughness less than 0.02 Am and then cleaned with acetone in an ultrasonic container. Prior to deposition, the samples were sputter-etched with argon ions for approximately 10 min, then the interface was prepared using dynamic intermixing. The high energy nitrogen ions are bombarding as the chromium atoms arrive the surface of the substrate. The ion impact forces the Cr atom to intermix with Fe and two energy levels of 40 and 20 keV were applied. The ion current was 17 AA/cm2 and the nitrogen flux was 4 sccm. The Cr sputtering ion current was 210 AA/cm2 and energy was 2.4 keV. The mixing time was 20 min. Then, Cr– N films were deposited with N ion bombarding energy 4 keV, ion current
Fig. 1. AFM images of the interface prepared by different nitrogen ion beam dynamic intermixing: (a) 40 keV, AISI 52100 substrate, Ra=3.7986 nA; (b) 20 keV, AISI 52100 substrate, Ra=0.4614 nA; (c) 40 keV, SAE 1045 substrate, Ra=4.7933 nA; (d) 20 keV, SAE 1045 substrate, Ra=1.2477 nA.
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Fig. 2. GDOES depth profiles of Cr – N coatings prepared by (a) 40 and (b) 20 keV ion intermixing.
4 AA/cm2 and N2 flux 10 sccm; in the mean time, the Cr target was sputtered at 2.4 keV and 210 AA/cm2. The film thickness for testing of the interface morphology and toughness Ra was 150– 200 and 700 – 800 nm for mechanical evaluation. The spherical rolling contact method was employed to detach the coating; the cracking will initiate at the weakest sites at the interface by the shear stress. If delamination does not occur after 5 million cycles, the shear stress range calculated from the loading force is taken as the magnitude of the bonding strength. The method has been described in detail elsewhere [6]. Atomic force microscopy (AFM) was used to observe the interface morphology and roughness, and expressed by electric current in nA scale. The constituent profiles through the depth were studied by glow discharge optical emission spectrometry (GDOES). The morphology and composition at the spalled sites after rolling contact fatigue were studied by scanning electron microscopy (SEM) and EDAX. In order to detect the state of carbon, XPS was also employed.
Fig. 3. Carbon concentrations near interface of 40 and 20 keV intermixing.
3. Results 3.1. Interface morphology and roughness The interfaces prepared by dynamic intermixing on AISI 52100 and SAE 1045 steel substrates are shown in Fig. 1. AFM results indicate that, for both substrates, the interface bombarded by 40 keV energy intermixing is rougher than that by 20 keV. With the same bombarding energy, the roughness of SAE 1045 steel is less than that of AISI 52100 substrate. 3.2. Interface composition and structure The GDOES depth profiles of the Cr –N films deposited on AISI 52100 substrate by 40 and 20 keV energies ion dynamic intermixing are shown in Fig. 2. The carbon concentration has little change through the depth and is close to that in the substrate when the intermixing energy is 20 keV. When the mixing energy is increased to 40 keV, the carbon content is increased significantly around the interface. Fig. 3 shows the magnified scale to compare the
Fig. 4. Effect of dynamic intermixing energy on bonding strength of coating on different substrates.
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Fig. 5. (a) Low and (b) high magnified images of Cr – N films by 40 keV intermixing on AISI 52100 substrate after rolling contact fatigue test.
carbon contents in the interface region; the maximum C% reaches 12.6 at.% for 40 keV bombardment. 3.3. Adhesion The adhesions of Cr –N films on different substrates with two intermixing performances are shown in Fig. 4. The results show that the Cr –N coating by 20 keV dynamic recoiling provides better bonding strength than that by 40 keV for both substrates. In fact, the bonding strengths of the coatings by 20 keV intermixing reached the maximum value of the capacity limit of the rolling contact machine. When 40 keV is used for intermixing, the bonding strength of the Cr – N coating on SAE 1045 steel is twice as much as that on AISI 52100 substrate. Adhesion tests reveal that high energy intermixing leads to deteriorate the interface particularly on AISI 52100 substrate, which contains higher carbon than SAE 1045 steel. In order to investigate the nature of the drastic drop of the adhesion of Cr –N coating on AISI 52100 substrate, the
interface on the substrate revealed by delamination of Cr– N coating after rolling contact fatigue was examined. Fig. 5a shows the SEM morphology of the detached area, and Fig. 5b shows the magnified image. The figure suggests that the interfacial crack initiates at the center of Fig. 5b, and the growth of the crack leads to delamination after a number of loading cycles. The crack origin examined by EDAX showed that the carbon content reached 11.46 at.%, which is much higher than that in the AISI 52100 substrate (5.17 at.%). The detached patch was also checked by XPS as in Fig. 6. The fatigue origin is mainly composed of carbon. Besides of O by contamination, only small amounts of Cr and N are involved. It is noticed that the substrate constituent Fe was not detected in Fig. 6. The dynamic recoiling covers Cr on the top and the intermixing layer is of high strength; hence, the fatigue crack initiates between the mixing layer and the coating. Fig. 7 shows the magnification of XPS C1s spectrum at the binding energy of 285.35 eV. The C1s binding energy of graphite is 284.3 eV. The slight difference
Fig. 6. XPS spectra of rolling contact fatigue crack origin.
L.-H. Tian et al. / Surface & Coatings Technology 191 (2005) 149–154
Fig. 7. XPS C1s spectra at the fatigue crack origin.
between the two peaks is very likely due to contamination and measurement error. The other possible peak in the vicinity is C1s of Cr2C3 (282.8 eV), which is further away from that of graphite [8]. This confirms that the carbon is existed as graphite in the fatigue origin.
4. Discussion When Cr – N is coated on the hardened steel substrate, the cyclic shear stress initiating the crack at the interface is lower than the yield strength of the substrate. The coating delamination by high cycle contact loading is characterized in brittleness; the interfacial layer free of plastic deformation will be easy to examine if any reaction products are involved. The fatigue damage under cyclic loading leads to cracking at the weakest sites. The soft graphite acts as pores in the interface and increases stress concentration under loading. The crack initiates at the graphite then propagates leading to brittle fracture; the enrichment of graphite can be revealed by microanalysis. In addition, the maximum shear stress amplitude at the interface Dsc is taken as a measure of the bonding strength. For hard coating and hardened substrate system, this value will not be affected by noninterfacial factors such as film thickness and the hardnesses of either substrate or coating; it is only sensitive to the interfacial factors as roughness, interface composition and internal stress [9]. Therefore, the comparison of Dsc values in different states is of help to sort out the chemical reaction at the interface. The selections of AISI 52100 and SAE 1045 were emphasized on different fractions of carbide (Fe3C). Fe3C is metastable and can be decomposed into Fe and graphite as that in the cast iron. The Gibbs free energy of the decomposition is [10]
DGT0 ¼ 5400 þ 3:36T
153
Fe3C would decompose into graphite when the temperature is higher than 1607 K. Nitrogen ion bombardment with Cr dynamic recoiling on the substrate surface is characterized in cascade collision. The thermal spikes by ions impingement at the surface result in high temperature, and Fe3C can be thermodynamically decomposed into Fe and graphite at the collision spots. In addition, the surface material will be sputtered away during ion bombardment and the sputtering rates of Fe and Cr are nearly one order higher than that of C. If Ar ions of 500 eV are used for bombarding, the sputtering rates of Fe and Cr are 1.10 and 1.18, yet only 0.12 for carbon [11]. The high temperature thermal spikes by ion bombardment provide the dynamic energy for carbon decomposition, and the preferential sputtering makes the carbon enrich in the interface. As a result, a large fraction of graphite is formed in the interface. This procedure is associated with bombarding energy. When the bombardment is decreased from 40 to 20 keV, the carbide decomposition and preferential sputtering are not so significant, and so does the graphite enrichment in the interface. It is generally accepted that, in order to intensify the atomic mixing and increase the implantation depth, highenergy bombarding is favorable. However, as the preferential sputtering and carbon decomposition take place, the bombarding energy should be carefully selected to keep from reactions which deteriorate the adhesion. The surface of SAE 1045 steel substrate is easier to be sputtered by ion bombardment; the roughness of the interface on SAE 1045 steel is slightly higher than that on AISI 52100 substrate. As in Figs. 1 and 4, with the increase of interface roughness, the adhesion of 40 keV dynamic ion intermixing is improved, yet the increase of interface roughness does not make significant improvement of adhesion for 20 keV intermixing. This fact suggests that if the bonding strength is low, the mechanical interlocking can be of important help. Once chemical reaction is involved such as graphite enrichment in this study; it will play a predominant role on the adhesion of the coating.
5. Conclusions 1. When a hard coating is deposited on the hardened substrate, the rolling contact fatigue loading can detach the coating at the weakest sites with brittle fracture; the interfacial products are revealed. The microanalysis confirms that contact fatigue is an effective way to evaluate interfacial reaction. 2. Different energies of ion intermixing during IBAD for Cr – N coating indicate that high-energy ion bombardment might deteriorate its adhesion on the steel substrate, particularly on the high carbon steel. 3. Different sputtering rates make carbide enrichment on the steel surface and thermal spikes of bombardment decomposes the carbide into graphite. The concentration
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of the graphite in the interface is the main reason of poor adhesion.
Acknowledgements This research is supported by China Natural Science Foundation (No. 5017104) and Shanxi Province Natural Science Foundation (No. 20001043).
References [1] C. Quaeyyhagens, J.D. Haen, L.M. Stals, Van M. Stappen, F. Bodart, G. Terwagne, Surf. Coat. Technol. 61 (1993) 227.
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