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Preparation of Cr hard coatings by ion beam assisted electron beam vapor deposition on Ni and Cu substrates
Surface & Coatings Technology 201 (2007) 5190 – 5193 www.elsevier.com/locate/surfcoat
Preparation of Cr hard coatings by ion beam assisted electron beam vapor deposition on Ni and Cu substrates Yuefei Zhang ⁎, Qiang Chen, Zhengduo Wang, Guangqiu Zhang, Yuanjing Ge Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Beijing 102600, China Available online 10 August 2006
Abstract Hard Cr coatings on Ni and Cu surface were deposited by means of ion beam assisted deposition (IBAD) for the replacement of electroplated chromium in order to avoid the problems of polluting water and hydrogen-embrittlement films from the electroplating. The morphology and mechanical properties of films were examined using SEM, AFM, optical metallographic microscopy, microhardness tester, scratch tester and profile meter. Results showed that adding Al interlayer can improve the adhesion and decrease the residual stress between the coating and substrate due to atom diffusion between the interfaces. The thickness of the interlayer is about 100–200 nm. It concludes that the microhardness of the IBAD samples can reach or even surpass that of the electroplated. It is feasible to obtain Cr hard coatings on some metal substrates by IBAD with optimized equipment and processing. © 2006 Elsevier B.V. All rights reserved. PACS: 68.55Jk; 81.15Jj Keywords: Hard coatings; Ion beam assisted deposition; Interlayer; Adhesion
1. Introduction Electron beam physical vapor deposition is an effective method for dense coatings, high thermal efficiency, and relatively high deposition rates [1,2]. Additional benefits can be gained in this process with the simultaneous usage of ion source-assisted deposition. Ion bombardment of the substrate provides a better control of the deposition process with good adhesion of the films, morphology and chemical composition. By controlling the current density and energy of ions, the porous, columnar, textured and even epitaxial coatings may be obtained [3–6]. Chromium plating provides excellent hardness (typically 700– 1000 Vickers), bright appearance with no discoloration, and resistance to corrosive environments; it is easily applied and has a low cost. However, chromium plating suffers from low cathode efficiency, poor metal distribution. It is also a worker and environment unfriendly process. In particular the hexavalent chromium is a carcinogen and a designated hazardous air pollutant [7]. With a view to the disadvantages such as environmental pollution and high cost in the preparation of gravure plate by chromium plating, hard Cr coatings on Ni and Cu surface were ⁎ Corresponding author. Tel.: +86 10 6026 1099; fax: +86 10 6026 1108. E-mail address: [email protected] (Y. Zhang). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.138
deposited by means of ion beam assisted electron beam vapor deposition (IBAD) for the replacement of electroplated chromium so as to avoid the problems of polluting and hydrogen-embrittlement from the electroplating. 2. Experimental A vacuum chamber equipped with a turbomolecular pump and a 6 kW electron beam evaporator, which with a 270° beam
Fig. 1. Microstructure image of Ni/Al/Cr cross-section morphology.
Y. Zhang et al. / Surface & Coatings Technology 201 (2007) 5190–5193
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Fig. 2. Typical AFM images of the as-deposited Cr coatings by ion beam assisted deposition on Ni substrate. (a) With Al interlayer (b) without Al interlayer.
deflection by a permanent magnet and 10 kV acceleration voltage, was used for the deposition of metal films. The measurement of the density of ionic and electronic current was carried on the ion detector in which the ion energy ranged from tens to several hundred eVs. The substrates were pure nickel and copper, which are normal materials for printing of gravure plate. The induced coupled plasma (ICP) ion source is applied during the substrates pretreated and deposition with a substrate temperature of approximately 100–150 °C. The substrates were 30 mm in diameter, 2 mm-thick disks with surface finished with ultrasonic cleanout. The deposition process was as follows: Al metal was evaporated with simultaneous argon bombardment, getting a pure Al metallic interlayer with the thickness of about 100–200 nm. Subsequently, Cr metal was deposited with the thickness from 5 μm up to 10 μm on the top of this layer. The morphology was examined using SEM (JSM—6301F) with energy dispersive spectroscopy analysis (EDS), AFM (DI Nanoscope III) and optical metallographic microscopy. The microhardness was measured using a Vickers microhardness
tester at 0.01 N loading with time of 10 s. Scratch adhesion testing was performed using a 125 mm radius hemispherical diamond tip ramped up to a maximum force of 20 N over a scratch length of 8 mm at a rate of 10 N/min. The acoustic emission was monitored during the scratch to detect a delamination/fracture event. 3. Results and discussion 3.1. The effect of interlayer In order to improve the adhesion between substrate materials normally films intermediate layer was formed by ion beam assisted deposition [8]. In our experiment the Al, Ti, Ni and NiCr thin films were deposited as an interlayer respectively. The results show that Al interlayer improved observably the adhesion, microstructure and mechanical properties with the thickness of 100–200 nm. With the energetic ion bombarding, the interface composition was mixed which improved the
Fig. 3. Typical AFM images of the as-deposited Cr coatings on Ni substrate with Al interlayer. (a) With ion beam assisted (b) without ion beam assisted.
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Y. Zhang et al. / Surface & Coatings Technology 201 (2007) 5190–5193
adhesion and decreased the residual stress between the coating and substrate. It is found that the thickness of interlayer must be restrained under 100–200 nm, otherwise the residual stress will increase between underlayer and toplayer and cause a brittle fracture. Fig. 1 shows the cross-section of SEM morphology of Ni/Al/ Cr by ICP ion source-assisted deposition with the thickness of 5.6 μm. As seen in Fig. 1, the thickness of Al interlayer is about 200 nm. The interface between underlayer and toplayer is compact with the composition mixing at interface by SEM– EDS analysis. These films are consisted of fine grains with a layer-by-layer growth structure as especially observed in the cross-section morphology. Each film has smooth surface in the observation by SEM from the top view. The intermediate Al layer not only enhanced the adhesion between substrate and coating but also improved the growth mode of the coating. Fig. 2 shows the typical AFM images that illustrate how the microstructure of a Cr coating can be influenced by controlling the interlayer. Fig. 2(a) shows the Cr coating with about 150 nm of Al interlayer. The grains are small and compact with uniform size and conformation. Fig. 2(b) shows the Cr coating without Al interlayer. The grains are large and separated with irregular size and conformation. By adding Al metallic interlayer at the interface, in which there will be fresh and clean surface, it will enhance the atom and cluster mobility and lead to an enhanced nucleation and thus a decisive role in film growth. 3.2. The effect of ion bombarding Fig. 3 shows AFM images of as-deposition Cr coatings on Ni substrate with Al interlayer. The images illustrate how the microstructure of Cr coatings can be influenced by controlling the ion beam bombarding. Fig. 3(a) shows the Cr coating with ion beam assisted deposition. The grains are smaller and the surface is smooth. Fig. 3(b) shows the Cr coating without ion beam assisted deposition. The grains are large and the surface is rough. With no ion bombardment the voids and micropores of sizes were larger than that of the atom–atom existing interaction ranges. It can be seen from Fig. 3(a) that the films no longer
Fig. 5. Adhesion of the coatings by several different deposition processing.
grow in porous columnar microstructure as Fig. 3(b) shows but growing in densely packed structures as a instead. Energetic particle bombardment simultaneously with thin film deposition removes overhanging atoms and causes voids to remain open till they are filled by new depositing atoms. 3.3. The microhardness of coatings Hardness is an important characteristic of appropriate gravure plate wear-resistant coatings. Fig. 4 illustrates the variation of microstructure with the different deposition processing. It is clear that ion beam assisted deposition coatings were much harder than substrates and even exceeded that of the plating Ni/Cr sample. In metal and alloy films, the hardness is related to grain size according to the Hall–Petch relation [9]: H ¼ H0 þ kd −1=2 where H is the hardness, H0 is the intrinsic hardness of a single crystal, d is the grain size, k is a constant which depends upon the metal or alloy composition. The deposited coatings showing a decrease in grain size were relevant to that ion assisted deposition and reduce the fraction of grain boundary voids, thus strengthening their hardness. Adhesion is a critical issue for wear-resistant coating selection. In many applications, high loads and high temperatures result in failure at the coating–substrate interface. Fig. 5 shows the adhesion of coating on substrate by scratch adhesion testing. It can be seen from Fig. 5 that ion beam assisted deposition Ni/Al/Cr and Cu/Cr coatings have a high adhesion. One clear advantage to ion beam assisted deposition processing is the ability to conduct deposition at low temperature due to the energy imparted to the system by the ion beam. 4. Conclusions
Fig. 4. Microhardness of the coatings by several different deposition processing.
Cr hard coatings have been deposited on Cu and Ni substrates by IBAD processing. Al interlayer with the thickness of about 100–200 nm can provide reasonably a good adhesion and decrease the residual stress between the substrate and Cr
Y. Zhang et al. / Surface & Coatings Technology 201 (2007) 5190–5193
coatings. With the energetic ion bombarding, the interface composition was mixed into the matrix which improved the adhesion and decreased the residual stress in which the films grown in densely packed structures. The ion beam assisted deposition coatings were much harder and more adhesional. It even exceeded that of the plating Ni/Cr sample. The ion beam assisted deposition processing can conduct a deposition of high quality films at low temperature due to the energy imparted to the system by the ion beam. It is feasible to obtain Cr hard coatings on some metal substrates to replace electroplated chromium process. Acknowledgement The authors thank the Beijing Institute of Graphic Communication key research fund for financial support.
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References [1] C. Hopf, W. Jacob, A. von Keudell, J. Appl. Phys. 97 (2005) 094904. [2] S. Mohan, MGhanashyam Krishna, Vacuum 46 (1995) 645. [3] J.L. Andujar, F.J. Pino, M.C. Polo, E. Bertran, Diamond Relat. Mater. 10 (2001) 1175. [4] G.K. Hubler, J.A. Sprahue, Surf. Coat. Technol. 81 (1996) 29. [5] K.-F. Chiu, Z.H. Barbe, Thin Solid Films 358 (2000) 264. [6] A. Lyutovich, K. Maile, A. Gusko, Kh. Ashurov, Surf. Coat. Technol. 151–152 (2002) 105. [7] B. Navinsek, P. Panjan, I. Milosev, Surf. Coat. Technol. 116–119 (1999) 476. [8] Chang Q. Sun, Y.Q. Fu, B.B. Yan, J.H. Hsieh, S.P. Lau, X.W. Sun, B.K. Tay, J. Appl. Phys. 91 (2002) 2051. [9] M. Catherine, James Cotell, K. Hirvonen, Surf. Coat. Technol. 81 (1996) 118.
Preparation of Cr hard coatings by ion beam assisted electron beam vapor deposition on Ni and Cu substrates Yuefei Zhang ⁎, Qiang Chen, Zhengduo Wang, Guangqiu Zhang, Yuanjing Ge Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Beijing 102600, China Available online 10 August 2006
Abstract Hard Cr coatings on Ni and Cu surface were deposited by means of ion beam assisted deposition (IBAD) for the replacement of electroplated chromium in order to avoid the problems of polluting water and hydrogen-embrittlement films from the electroplating. The morphology and mechanical properties of films were examined using SEM, AFM, optical metallographic microscopy, microhardness tester, scratch tester and profile meter. Results showed that adding Al interlayer can improve the adhesion and decrease the residual stress between the coating and substrate due to atom diffusion between the interfaces. The thickness of the interlayer is about 100–200 nm. It concludes that the microhardness of the IBAD samples can reach or even surpass that of the electroplated. It is feasible to obtain Cr hard coatings on some metal substrates by IBAD with optimized equipment and processing. © 2006 Elsevier B.V. All rights reserved. PACS: 68.55Jk; 81.15Jj Keywords: Hard coatings; Ion beam assisted deposition; Interlayer; Adhesion
1. Introduction Electron beam physical vapor deposition is an effective method for dense coatings, high thermal efficiency, and relatively high deposition rates [1,2]. Additional benefits can be gained in this process with the simultaneous usage of ion source-assisted deposition. Ion bombardment of the substrate provides a better control of the deposition process with good adhesion of the films, morphology and chemical composition. By controlling the current density and energy of ions, the porous, columnar, textured and even epitaxial coatings may be obtained [3–6]. Chromium plating provides excellent hardness (typically 700– 1000 Vickers), bright appearance with no discoloration, and resistance to corrosive environments; it is easily applied and has a low cost. However, chromium plating suffers from low cathode efficiency, poor metal distribution. It is also a worker and environment unfriendly process. In particular the hexavalent chromium is a carcinogen and a designated hazardous air pollutant [7]. With a view to the disadvantages such as environmental pollution and high cost in the preparation of gravure plate by chromium plating, hard Cr coatings on Ni and Cu surface were ⁎ Corresponding author. Tel.: +86 10 6026 1099; fax: +86 10 6026 1108. E-mail address: [email protected] (Y. Zhang). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.138
deposited by means of ion beam assisted electron beam vapor deposition (IBAD) for the replacement of electroplated chromium so as to avoid the problems of polluting and hydrogen-embrittlement from the electroplating. 2. Experimental A vacuum chamber equipped with a turbomolecular pump and a 6 kW electron beam evaporator, which with a 270° beam
Fig. 1. Microstructure image of Ni/Al/Cr cross-section morphology.
Y. Zhang et al. / Surface & Coatings Technology 201 (2007) 5190–5193
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Fig. 2. Typical AFM images of the as-deposited Cr coatings by ion beam assisted deposition on Ni substrate. (a) With Al interlayer (b) without Al interlayer.
deflection by a permanent magnet and 10 kV acceleration voltage, was used for the deposition of metal films. The measurement of the density of ionic and electronic current was carried on the ion detector in which the ion energy ranged from tens to several hundred eVs. The substrates were pure nickel and copper, which are normal materials for printing of gravure plate. The induced coupled plasma (ICP) ion source is applied during the substrates pretreated and deposition with a substrate temperature of approximately 100–150 °C. The substrates were 30 mm in diameter, 2 mm-thick disks with surface finished with ultrasonic cleanout. The deposition process was as follows: Al metal was evaporated with simultaneous argon bombardment, getting a pure Al metallic interlayer with the thickness of about 100–200 nm. Subsequently, Cr metal was deposited with the thickness from 5 μm up to 10 μm on the top of this layer. The morphology was examined using SEM (JSM—6301F) with energy dispersive spectroscopy analysis (EDS), AFM (DI Nanoscope III) and optical metallographic microscopy. The microhardness was measured using a Vickers microhardness
tester at 0.01 N loading with time of 10 s. Scratch adhesion testing was performed using a 125 mm radius hemispherical diamond tip ramped up to a maximum force of 20 N over a scratch length of 8 mm at a rate of 10 N/min. The acoustic emission was monitored during the scratch to detect a delamination/fracture event. 3. Results and discussion 3.1. The effect of interlayer In order to improve the adhesion between substrate materials normally films intermediate layer was formed by ion beam assisted deposition [8]. In our experiment the Al, Ti, Ni and NiCr thin films were deposited as an interlayer respectively. The results show that Al interlayer improved observably the adhesion, microstructure and mechanical properties with the thickness of 100–200 nm. With the energetic ion bombarding, the interface composition was mixed which improved the
Fig. 3. Typical AFM images of the as-deposited Cr coatings on Ni substrate with Al interlayer. (a) With ion beam assisted (b) without ion beam assisted.
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Y. Zhang et al. / Surface & Coatings Technology 201 (2007) 5190–5193
adhesion and decreased the residual stress between the coating and substrate. It is found that the thickness of interlayer must be restrained under 100–200 nm, otherwise the residual stress will increase between underlayer and toplayer and cause a brittle fracture. Fig. 1 shows the cross-section of SEM morphology of Ni/Al/ Cr by ICP ion source-assisted deposition with the thickness of 5.6 μm. As seen in Fig. 1, the thickness of Al interlayer is about 200 nm. The interface between underlayer and toplayer is compact with the composition mixing at interface by SEM– EDS analysis. These films are consisted of fine grains with a layer-by-layer growth structure as especially observed in the cross-section morphology. Each film has smooth surface in the observation by SEM from the top view. The intermediate Al layer not only enhanced the adhesion between substrate and coating but also improved the growth mode of the coating. Fig. 2 shows the typical AFM images that illustrate how the microstructure of a Cr coating can be influenced by controlling the interlayer. Fig. 2(a) shows the Cr coating with about 150 nm of Al interlayer. The grains are small and compact with uniform size and conformation. Fig. 2(b) shows the Cr coating without Al interlayer. The grains are large and separated with irregular size and conformation. By adding Al metallic interlayer at the interface, in which there will be fresh and clean surface, it will enhance the atom and cluster mobility and lead to an enhanced nucleation and thus a decisive role in film growth. 3.2. The effect of ion bombarding Fig. 3 shows AFM images of as-deposition Cr coatings on Ni substrate with Al interlayer. The images illustrate how the microstructure of Cr coatings can be influenced by controlling the ion beam bombarding. Fig. 3(a) shows the Cr coating with ion beam assisted deposition. The grains are smaller and the surface is smooth. Fig. 3(b) shows the Cr coating without ion beam assisted deposition. The grains are large and the surface is rough. With no ion bombardment the voids and micropores of sizes were larger than that of the atom–atom existing interaction ranges. It can be seen from Fig. 3(a) that the films no longer
Fig. 5. Adhesion of the coatings by several different deposition processing.
grow in porous columnar microstructure as Fig. 3(b) shows but growing in densely packed structures as a instead. Energetic particle bombardment simultaneously with thin film deposition removes overhanging atoms and causes voids to remain open till they are filled by new depositing atoms. 3.3. The microhardness of coatings Hardness is an important characteristic of appropriate gravure plate wear-resistant coatings. Fig. 4 illustrates the variation of microstructure with the different deposition processing. It is clear that ion beam assisted deposition coatings were much harder than substrates and even exceeded that of the plating Ni/Cr sample. In metal and alloy films, the hardness is related to grain size according to the Hall–Petch relation [9]: H ¼ H0 þ kd −1=2 where H is the hardness, H0 is the intrinsic hardness of a single crystal, d is the grain size, k is a constant which depends upon the metal or alloy composition. The deposited coatings showing a decrease in grain size were relevant to that ion assisted deposition and reduce the fraction of grain boundary voids, thus strengthening their hardness. Adhesion is a critical issue for wear-resistant coating selection. In many applications, high loads and high temperatures result in failure at the coating–substrate interface. Fig. 5 shows the adhesion of coating on substrate by scratch adhesion testing. It can be seen from Fig. 5 that ion beam assisted deposition Ni/Al/Cr and Cu/Cr coatings have a high adhesion. One clear advantage to ion beam assisted deposition processing is the ability to conduct deposition at low temperature due to the energy imparted to the system by the ion beam. 4. Conclusions
Fig. 4. Microhardness of the coatings by several different deposition processing.
Cr hard coatings have been deposited on Cu and Ni substrates by IBAD processing. Al interlayer with the thickness of about 100–200 nm can provide reasonably a good adhesion and decrease the residual stress between the substrate and Cr
Y. Zhang et al. / Surface & Coatings Technology 201 (2007) 5190–5193
coatings. With the energetic ion bombarding, the interface composition was mixed into the matrix which improved the adhesion and decreased the residual stress in which the films grown in densely packed structures. The ion beam assisted deposition coatings were much harder and more adhesional. It even exceeded that of the plating Ni/Cr sample. The ion beam assisted deposition processing can conduct a deposition of high quality films at low temperature due to the energy imparted to the system by the ion beam. It is feasible to obtain Cr hard coatings on some metal substrates to replace electroplated chromium process. Acknowledgement The authors thank the Beijing Institute of Graphic Communication key research fund for financial support.
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References [1] C. Hopf, W. Jacob, A. von Keudell, J. Appl. Phys. 97 (2005) 094904. [2] S. Mohan, MGhanashyam Krishna, Vacuum 46 (1995) 645. [3] J.L. Andujar, F.J. Pino, M.C. Polo, E. Bertran, Diamond Relat. Mater. 10 (2001) 1175. [4] G.K. Hubler, J.A. Sprahue, Surf. Coat. Technol. 81 (1996) 29. [5] K.-F. Chiu, Z.H. Barbe, Thin Solid Films 358 (2000) 264. [6] A. Lyutovich, K. Maile, A. Gusko, Kh. Ashurov, Surf. Coat. Technol. 151–152 (2002) 105. [7] B. Navinsek, P. Panjan, I. Milosev, Surf. Coat. Technol. 116–119 (1999) 476. [8] Chang Q. Sun, Y.Q. Fu, B.B. Yan, J.H. Hsieh, S.P. Lau, X.W. Sun, B.K. Tay, J. Appl. Phys. 91 (2002) 2051. [9] M. Catherine, James Cotell, K. Hirvonen, Surf. Coat. Technol. 81 (1996) 118.