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SEM study of defects in PVD hard coatings using focused ion beam milling
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Surface & Coatings Technology 202 (2008) 2302 – 2305 www.elsevier.com/locate/surfcoat
SEM study of defects in PVD hard coatings using focused ion beam milling P. Panjan a,⁎, D. Kek Merl a , F. Zupanič b , M. Čekada a , M. Panjan a a
b
Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia Available online 3 October 2007
Abstract Hard coatings CrN, TiAlN and multilayer CrN/TiAlN were prepared on different substrates (HSS, D2 tool steels, Al-alloy) by thermoionic arc ion plating and by sputtering. The defects incorporated into the coating were studied by four techniques: top view conventional and field-emission SEM, cross-section SEM, AFM and stylus profilometry. As a specifically useful tool to study internal structure of the defect, we applied focused ion beam milling system, which is built in a conventional scanning electron microscope. By ion beam milling we prepared cross-sections through the defects. © 2007 Elsevier B.V. All rights reserved. Keywords: PVD hard coating; Focused ion milling; SEM; AFM; 3D stylus profilometry
1. Introduction The growth defect phenomena are known in all coating technologies (CVD, evaporation, sputtering and electroplating) [1–7]. The defects are non-uniformly distributed, while their form, size and density of defects depend on the substrate type, its pretreatment, and on deposition conditions. On PVD hard coatings at least four types of defects can be distinguished [7]: (a) big, shallow craters with a diameter of 5–40 μm (macrodefects), (b) cone structures with diameter from 1 μm to several μm (microdefects), (c) dish-like holes arising from wrenching of the cone structure and (d) pin-holes extending through the whole coating. Besides these defects we observed on the SEM crosssectional fracture a lot of submicrometer sized (less than micrometer) particles built in the coating. Such defects are drawbacks in application of hard coatings, because they can cause local loss of adhesion, delamination, sticking of workpiece material, higher friction, voids, pitting corrosion and gas permeation. Therefore it is very important to minimize the concentration of defects. In order to do this we have to know their origin.
⁎ Corresponding author. Tel.: +386 1 477 3278; fax: +386 1 251 9385. E-mail address: [email protected] (P. Panjan). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.09.033
We found that some defects can appear already during mechanical pretreatment (grinding, polishing, blasting) due to the pitting effect [7]. The result are caves with dimensions corresponding to those of the carbide inclusions in selected tool steel. Such caves cannot be covered completely by a relatively thin hard coating due to shadowing effect. It is usually assumed that the larger defects are related to the cleaning and drying problems as well as to the cleanliness of the batching room [2,3]. Namely, a problem represent the particles with microscopic dimension, which remain on the substrate surface after ultrasonic cleaning and drying, or those which fall onto the substrate surface during batching. The presence of foreign particles at the substrate surface before the coating starts to grow is the cause for flakes or cones growth [3]. During deposition process they are coated and due to the high level of residual stress on cones part of them can spontaneously spall off forming pin-holes in the coating [8]. One part of defects definitely originates in the contamination of the hard coating with foreign particles during the deposition process [3]. We found that the micro- and submicroparticles, which flake from the vacuum chamber components during deposition, are incorporated into the growing coating and form cone-like defects on the surface [7]. The number of such defects can be reduced by frequent blasting of fixtures and other components of the vacuum chamber. Some defects originate from
P. Panjan et al. / Surface & Coatings Technology 202 (2008) 2302–2305
the evaporation or sputter source due to arcing [1,2]. All impurities on targets and substrates increase the arc tendency during etching or deposition steps. The microdroplets which form during arcing, incorporate into the growing coating. We found a correlation between the roughness and frequency of arcs [7]. In our previous work [7], field-emission scanning electron microscopy was used for study of the defect morphology in planar surface view and cross-sectional fracture view. The aim of this work was to study the defects using SEM in combination with focused ion beam milling and EDX analysis. In this way we can obtain the insight into the internal structure of the defect. 2. Experimental 2.1. Deposition techniques The BAI 730 M (Balzers) deposition system with thermoionic arc was used for deposition of CrN, while the magnetron sputtering system CC800 (CemeCon) was used for deposition of TiAlN and CrN/TiAlN multilayer hard coatings. Three types of substrates were used: a powder metallurgical high speed steel (ASP30); a cold work tool steel (D2) and an aluminium alloy (7075). The substrates were polished, ultrasonically cleaned and dried in hot air. Prior to coating deposition they were cleaned by ion etching. In BAI 730 deposition system DC bias voltage −200 V was used, while the etching time was 15 min. In CC800 deposition system the RF bias with maximum power 2 kW was used, while the etching time was 85 min. During deposition the bias voltage and the substrate temperature were −125 V and 450 °C, valid for both apparatus. 2.2. Growth defect characterization Focused ion beam (FIB) workstation [9] was used to prepare cross-section through the defects. We used FIB integrated in FEI QUANTA 200 3D microscope. Ion beam was used to remove precise sections of material (close to the defect) from the specimen surface by sputtering. The initial trough (with dimension of app. 8 × 5 × 4 μm) was milled at a high beam current (20 nA), while the energy of ions was 30 keV. Then the cross-sectional was polished with a lower beam current (3 nA). After polishing the specimen was tilted and cross-section face was imaged by electrons. Then the specimen was put back in initial position and the next slice of coating material with the thickness of about 0.5 μm was removed by ion milling. The picture of the new cross-section was taken again by electrons. This procedure was repeated until the selected volume of the defect was removed. A field-emission scanning electron microscope (SIRION 400 NC, FEI) was used for study of the coating microstructure and defect morphology in planar surface view and cross-sectional fracture view. The surface morphology of the substrates was also examined by atomic force microscope (Solver PRO) and 3D stylus profilometer (Taylor Hobson Talysurf). The vertical resolution of our 3D-profilometer was a few nm, while the lateral resolution was limited to 1 μm.
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3. Results and discussion Conventional and field-emission SEM are the most common techniques for the study of defect morphology. Fig. 1a shows SEM image of typical defects embedded in the TiAlN coating deposited on D2 tool steel. We can see that they are of various shapes and sizes. However, the applicability of SEM microsope is limited due to relatively low depth resolution. In order to obtain the 3D-image of defects we performed AFM analysis (Fig. 1c). In spite of very good depth and lateral resolution the use of AFM microscope (Fig. 1c) is limited due to the small scan area (of about 50 × 50 μm). In contrast to a planar view SEM micrograph, which shows a lot of defects of various sizes, on AFM image only micrometer-sized defects are visible. Therefore it is not appropriate for the study of larger defects in hard coatings, because its density is rather low. On the other hand the 3D stylus profilometry was performed over a scan area of over 10 × 10 mm and more (Fig. 1d). Thus we obtained a 3D-image of the coating surface on large scanning area with all the micrometer-sized details. From 3D-image we estimated the surface density and height distribution of the defects. At TiAlN coatings deposited on three types of substrates we counted the following average number of peaks per mm2: 200 peaks higher than 0.5 μm, 70 peaks higher than 1 μm, and 20 peaks higher than 2 μm. The number of craters is much smaller. Appoximately 25 of them are deeper than 0.5 μm, 10 craters are deeper than 1 μm, while only 5 of them are deeper than 2 μm. The height and depth distributions of defects are independent on substrate type, while the average diameter is larger for D2 tool steel than for ASP30. Further information about structure of defects was obtained from SEM cross-sectional fracture view (Fig. 1b). From the contour of the CrN/TiAlN multilayer structure deposited on D2 tool steel it is evident where individual foreign particles (with typical dimension of few tens of nm) are built in the coating. However, it is a mere coincidence to prepare the fracture which would propagate through the selected defects. We have to be aware that all these techniques (except crosssection SEM) do not allow to examine the internal structure of the defects. The detailed study of defect microstructure and composition was done by SEM in combination with a focused ion beam milling. After removing of precise sections of material close to the defect by ion milling and after polishing the specimen was tilted and cross-section face was imaged by electrons. The consecutive serial sectioning of thin slices and imaging of crosssection gives us the insight into the defect internal structure (Figs. 2 and 3). EDX analysis was used to determine the composition of inclusions. The applicability of this new technique was tested on several samples. Fig. 2 presents two types of defects in the CrN coating deposited on Al-based substrate (a thin nickel interlayer was also deposited in order to improve the corrosion resistance of substrate). The first defect (Fig. 2a) has the form of a cone, while the second one (Fig. 2d) resembles a pinhole. The question was weather these defects extend through the whole coating or not, and what is the origin for their nucleation. After consecutive serial sectioning we found that the first defect started to grow in
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P. Panjan et al. / Surface & Coatings Technology 202 (2008) 2302–2305
Fig. 1. Comparison of different analytical techniques to study defects on hard coatings (TiAlN, TiAlN/CrN on D2 steel): a) SEM (top view), b) SEM (cross-section), c) AFM, d) 3D stylus profilometry.
the middle of the coating due to the incorporation of a foreign particle. Fig. 2c shows that the region under the cone is not completely filled with material. The second defect is extending through the whole coating and originates in a small hole in the
substrate (Fig. 2f). It is well known that PVD processes have a poor ability to cover a small hole due to shadowing effect. Fig. 2d–f clearly shows that a small crater on the substrate surface cannot be covered completely and that a pinhole extending
Fig. 2. SEM micrographs of two defect types in the CrN hard coating deposited on Al-alloy substrate: (a–c) protruding grain, (d–f) hole. Consecutive slices are presented for both defect types.
P. Panjan et al. / Surface & Coatings Technology 202 (2008) 2302–2305
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Fig. 3. SEM micrograph of a defect in the TiAlN hard coating deposited on ASP30 steel substrate. Consecutive slices are presented (a–c).
through the whole coating was formed. We found that corrosion takes place on such defects, while solution can reach the base material [10]. The defect in TiAlN hard coating deposited on sintered high speed steel (ASP30) substrate has also the form of a cone (Fig. 3). The defect extended through the whole coating. Individual foreign particles are visible at the interface with the substrate. Such obstacles cause geometrical shadowing during deposition resulting a cone. After ion milling an EDX compositional analysis was performed. The particle was proved to be iron based, which probably means that it originates from the vacuum chamber. 4. Conclusions In order to obtain the complete information about defect size and distribution it is recommended to combine different analytical techniques. In this paper four different techniques were used to study micro- and macrodefects on PVD hard coatings. The emphasis was on SEM in combination with focused ion beam milling. We demonstrated that this technique can give us the useful information about internal structure of the defects. By consecutive serial sectioning of thin slices and imaging of crosssection we found that defects in the form of cones are either the result of the presence of foreign particles at the substrate surface, or a consequence of incroporation of small particles which flake from the vacuum chamber components during deposition.
Therefore we can conclude that a careful cleaning of the substrates as well as frequent cleaning of vacuum chamber and fixture components helps to reduce the defect density. Acknowledgements This work was supported by the Slovenian Research Agency (project M2-0125). References [1] W.-D. Münz, D.B. Lewis, S. Creasey, T. Hurkmans, T. Trinh, W.v. Ijzendorn, Vacuum 46 (1995) 323. [2] C. Mitterer, O. Heuzè, V.-H. Derflinger, Surf. Coat. Technol. 89 (1997) 233. [3] J. Vetter, M. Stuber, S. Ulrich, Surf. Coat. Technol. 168 (2003) 169. [4] H.W. Wang, M.M. Stack, S.B. Lyon, P. Hovsepian, W.-D. Münz, Surf. Coat. Technol. 126 (2000) 279. [5] H.A. Jehn, Surf. Coat. Technol. 125 (2000) 212. [6] D.B. Lewis, S.J. Creasey, C. Wüstefeld, A.P. Ehiasarian, P.Eh. Hovespian, Thin Solid Films 503 (2006) 143. [7] M. Čekada, P. Panjan, D. Kek-Merl, M. Panjan, G. Kapun, Vacuum 82 (2008) 252. [8] U. Wiklund, J. Gunnars, S. Hogmark, Wear 232 (1999) 262. [9] J.M. Cairney, P.R. Munroe, M. Hoffman, Surf. Coat. Technol. 198 (2005) 165. [10] D. Kek Merl, P. Panjan, M. Panjan, M. Cekada, Plasma Process. Polym. 4 (2007) 5613.
Surface & Coatings Technology 202 (2008) 2302 – 2305 www.elsevier.com/locate/surfcoat
SEM study of defects in PVD hard coatings using focused ion beam milling P. Panjan a,⁎, D. Kek Merl a , F. Zupanič b , M. Čekada a , M. Panjan a a
b
Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia Available online 3 October 2007
Abstract Hard coatings CrN, TiAlN and multilayer CrN/TiAlN were prepared on different substrates (HSS, D2 tool steels, Al-alloy) by thermoionic arc ion plating and by sputtering. The defects incorporated into the coating were studied by four techniques: top view conventional and field-emission SEM, cross-section SEM, AFM and stylus profilometry. As a specifically useful tool to study internal structure of the defect, we applied focused ion beam milling system, which is built in a conventional scanning electron microscope. By ion beam milling we prepared cross-sections through the defects. © 2007 Elsevier B.V. All rights reserved. Keywords: PVD hard coating; Focused ion milling; SEM; AFM; 3D stylus profilometry
1. Introduction The growth defect phenomena are known in all coating technologies (CVD, evaporation, sputtering and electroplating) [1–7]. The defects are non-uniformly distributed, while their form, size and density of defects depend on the substrate type, its pretreatment, and on deposition conditions. On PVD hard coatings at least four types of defects can be distinguished [7]: (a) big, shallow craters with a diameter of 5–40 μm (macrodefects), (b) cone structures with diameter from 1 μm to several μm (microdefects), (c) dish-like holes arising from wrenching of the cone structure and (d) pin-holes extending through the whole coating. Besides these defects we observed on the SEM crosssectional fracture a lot of submicrometer sized (less than micrometer) particles built in the coating. Such defects are drawbacks in application of hard coatings, because they can cause local loss of adhesion, delamination, sticking of workpiece material, higher friction, voids, pitting corrosion and gas permeation. Therefore it is very important to minimize the concentration of defects. In order to do this we have to know their origin.
⁎ Corresponding author. Tel.: +386 1 477 3278; fax: +386 1 251 9385. E-mail address: [email protected] (P. Panjan). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.09.033
We found that some defects can appear already during mechanical pretreatment (grinding, polishing, blasting) due to the pitting effect [7]. The result are caves with dimensions corresponding to those of the carbide inclusions in selected tool steel. Such caves cannot be covered completely by a relatively thin hard coating due to shadowing effect. It is usually assumed that the larger defects are related to the cleaning and drying problems as well as to the cleanliness of the batching room [2,3]. Namely, a problem represent the particles with microscopic dimension, which remain on the substrate surface after ultrasonic cleaning and drying, or those which fall onto the substrate surface during batching. The presence of foreign particles at the substrate surface before the coating starts to grow is the cause for flakes or cones growth [3]. During deposition process they are coated and due to the high level of residual stress on cones part of them can spontaneously spall off forming pin-holes in the coating [8]. One part of defects definitely originates in the contamination of the hard coating with foreign particles during the deposition process [3]. We found that the micro- and submicroparticles, which flake from the vacuum chamber components during deposition, are incorporated into the growing coating and form cone-like defects on the surface [7]. The number of such defects can be reduced by frequent blasting of fixtures and other components of the vacuum chamber. Some defects originate from
P. Panjan et al. / Surface & Coatings Technology 202 (2008) 2302–2305
the evaporation or sputter source due to arcing [1,2]. All impurities on targets and substrates increase the arc tendency during etching or deposition steps. The microdroplets which form during arcing, incorporate into the growing coating. We found a correlation between the roughness and frequency of arcs [7]. In our previous work [7], field-emission scanning electron microscopy was used for study of the defect morphology in planar surface view and cross-sectional fracture view. The aim of this work was to study the defects using SEM in combination with focused ion beam milling and EDX analysis. In this way we can obtain the insight into the internal structure of the defect. 2. Experimental 2.1. Deposition techniques The BAI 730 M (Balzers) deposition system with thermoionic arc was used for deposition of CrN, while the magnetron sputtering system CC800 (CemeCon) was used for deposition of TiAlN and CrN/TiAlN multilayer hard coatings. Three types of substrates were used: a powder metallurgical high speed steel (ASP30); a cold work tool steel (D2) and an aluminium alloy (7075). The substrates were polished, ultrasonically cleaned and dried in hot air. Prior to coating deposition they were cleaned by ion etching. In BAI 730 deposition system DC bias voltage −200 V was used, while the etching time was 15 min. In CC800 deposition system the RF bias with maximum power 2 kW was used, while the etching time was 85 min. During deposition the bias voltage and the substrate temperature were −125 V and 450 °C, valid for both apparatus. 2.2. Growth defect characterization Focused ion beam (FIB) workstation [9] was used to prepare cross-section through the defects. We used FIB integrated in FEI QUANTA 200 3D microscope. Ion beam was used to remove precise sections of material (close to the defect) from the specimen surface by sputtering. The initial trough (with dimension of app. 8 × 5 × 4 μm) was milled at a high beam current (20 nA), while the energy of ions was 30 keV. Then the cross-sectional was polished with a lower beam current (3 nA). After polishing the specimen was tilted and cross-section face was imaged by electrons. Then the specimen was put back in initial position and the next slice of coating material with the thickness of about 0.5 μm was removed by ion milling. The picture of the new cross-section was taken again by electrons. This procedure was repeated until the selected volume of the defect was removed. A field-emission scanning electron microscope (SIRION 400 NC, FEI) was used for study of the coating microstructure and defect morphology in planar surface view and cross-sectional fracture view. The surface morphology of the substrates was also examined by atomic force microscope (Solver PRO) and 3D stylus profilometer (Taylor Hobson Talysurf). The vertical resolution of our 3D-profilometer was a few nm, while the lateral resolution was limited to 1 μm.
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3. Results and discussion Conventional and field-emission SEM are the most common techniques for the study of defect morphology. Fig. 1a shows SEM image of typical defects embedded in the TiAlN coating deposited on D2 tool steel. We can see that they are of various shapes and sizes. However, the applicability of SEM microsope is limited due to relatively low depth resolution. In order to obtain the 3D-image of defects we performed AFM analysis (Fig. 1c). In spite of very good depth and lateral resolution the use of AFM microscope (Fig. 1c) is limited due to the small scan area (of about 50 × 50 μm). In contrast to a planar view SEM micrograph, which shows a lot of defects of various sizes, on AFM image only micrometer-sized defects are visible. Therefore it is not appropriate for the study of larger defects in hard coatings, because its density is rather low. On the other hand the 3D stylus profilometry was performed over a scan area of over 10 × 10 mm and more (Fig. 1d). Thus we obtained a 3D-image of the coating surface on large scanning area with all the micrometer-sized details. From 3D-image we estimated the surface density and height distribution of the defects. At TiAlN coatings deposited on three types of substrates we counted the following average number of peaks per mm2: 200 peaks higher than 0.5 μm, 70 peaks higher than 1 μm, and 20 peaks higher than 2 μm. The number of craters is much smaller. Appoximately 25 of them are deeper than 0.5 μm, 10 craters are deeper than 1 μm, while only 5 of them are deeper than 2 μm. The height and depth distributions of defects are independent on substrate type, while the average diameter is larger for D2 tool steel than for ASP30. Further information about structure of defects was obtained from SEM cross-sectional fracture view (Fig. 1b). From the contour of the CrN/TiAlN multilayer structure deposited on D2 tool steel it is evident where individual foreign particles (with typical dimension of few tens of nm) are built in the coating. However, it is a mere coincidence to prepare the fracture which would propagate through the selected defects. We have to be aware that all these techniques (except crosssection SEM) do not allow to examine the internal structure of the defects. The detailed study of defect microstructure and composition was done by SEM in combination with a focused ion beam milling. After removing of precise sections of material close to the defect by ion milling and after polishing the specimen was tilted and cross-section face was imaged by electrons. The consecutive serial sectioning of thin slices and imaging of crosssection gives us the insight into the defect internal structure (Figs. 2 and 3). EDX analysis was used to determine the composition of inclusions. The applicability of this new technique was tested on several samples. Fig. 2 presents two types of defects in the CrN coating deposited on Al-based substrate (a thin nickel interlayer was also deposited in order to improve the corrosion resistance of substrate). The first defect (Fig. 2a) has the form of a cone, while the second one (Fig. 2d) resembles a pinhole. The question was weather these defects extend through the whole coating or not, and what is the origin for their nucleation. After consecutive serial sectioning we found that the first defect started to grow in
2304
P. Panjan et al. / Surface & Coatings Technology 202 (2008) 2302–2305
Fig. 1. Comparison of different analytical techniques to study defects on hard coatings (TiAlN, TiAlN/CrN on D2 steel): a) SEM (top view), b) SEM (cross-section), c) AFM, d) 3D stylus profilometry.
the middle of the coating due to the incorporation of a foreign particle. Fig. 2c shows that the region under the cone is not completely filled with material. The second defect is extending through the whole coating and originates in a small hole in the
substrate (Fig. 2f). It is well known that PVD processes have a poor ability to cover a small hole due to shadowing effect. Fig. 2d–f clearly shows that a small crater on the substrate surface cannot be covered completely and that a pinhole extending
Fig. 2. SEM micrographs of two defect types in the CrN hard coating deposited on Al-alloy substrate: (a–c) protruding grain, (d–f) hole. Consecutive slices are presented for both defect types.
P. Panjan et al. / Surface & Coatings Technology 202 (2008) 2302–2305
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Fig. 3. SEM micrograph of a defect in the TiAlN hard coating deposited on ASP30 steel substrate. Consecutive slices are presented (a–c).
through the whole coating was formed. We found that corrosion takes place on such defects, while solution can reach the base material [10]. The defect in TiAlN hard coating deposited on sintered high speed steel (ASP30) substrate has also the form of a cone (Fig. 3). The defect extended through the whole coating. Individual foreign particles are visible at the interface with the substrate. Such obstacles cause geometrical shadowing during deposition resulting a cone. After ion milling an EDX compositional analysis was performed. The particle was proved to be iron based, which probably means that it originates from the vacuum chamber. 4. Conclusions In order to obtain the complete information about defect size and distribution it is recommended to combine different analytical techniques. In this paper four different techniques were used to study micro- and macrodefects on PVD hard coatings. The emphasis was on SEM in combination with focused ion beam milling. We demonstrated that this technique can give us the useful information about internal structure of the defects. By consecutive serial sectioning of thin slices and imaging of crosssection we found that defects in the form of cones are either the result of the presence of foreign particles at the substrate surface, or a consequence of incroporation of small particles which flake from the vacuum chamber components during deposition.
Therefore we can conclude that a careful cleaning of the substrates as well as frequent cleaning of vacuum chamber and fixture components helps to reduce the defect density. Acknowledgements This work was supported by the Slovenian Research Agency (project M2-0125). References [1] W.-D. Münz, D.B. Lewis, S. Creasey, T. Hurkmans, T. Trinh, W.v. Ijzendorn, Vacuum 46 (1995) 323. [2] C. Mitterer, O. Heuzè, V.-H. Derflinger, Surf. Coat. Technol. 89 (1997) 233. [3] J. Vetter, M. Stuber, S. Ulrich, Surf. Coat. Technol. 168 (2003) 169. [4] H.W. Wang, M.M. Stack, S.B. Lyon, P. Hovsepian, W.-D. Münz, Surf. Coat. Technol. 126 (2000) 279. [5] H.A. Jehn, Surf. Coat. Technol. 125 (2000) 212. [6] D.B. Lewis, S.J. Creasey, C. Wüstefeld, A.P. Ehiasarian, P.Eh. Hovespian, Thin Solid Films 503 (2006) 143. [7] M. Čekada, P. Panjan, D. Kek-Merl, M. Panjan, G. Kapun, Vacuum 82 (2008) 252. [8] U. Wiklund, J. Gunnars, S. Hogmark, Wear 232 (1999) 262. [9] J.M. Cairney, P.R. Munroe, M. Hoffman, Surf. Coat. Technol. 198 (2005) 165. [10] D. Kek Merl, P. Panjan, M. Panjan, M. Cekada, Plasma Process. Polym. 4 (2007) 5613.