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Crack propagation behavior of TiN coatings by laser thermal shock experiments
Applied Surface Science 258 (2012) 8752–8757
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Crack propagation behavior of TiN coatings by laser thermal shock experiments Youngkue Choi a , Seol Jeon a , Min-seok Jeon b , Hyun-Gyoo Shin b , Ho Hwan Chun c , Youn-seoung Lee d , Heesoo Lee a,∗ a
School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea Material Testing Center, Korea Testing Laboratory, Seoul 152-848, Republic of Korea Department of Naval Architecture & Ocean Engineering, Pusan National University, Busan 609-735, Republic of Korea d Department of Information Communication Engineering, Hanbat National University, Daejon 305-719, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 5 March 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Crack propagation Laser ablation Focused ion beam Transmission electron microscopy
a b s t r a c t The crack propagation behavior of TiN coatings, deposited onto 304 stainless steel substrates by arc ion plating technique, related to a laser thermal shock experiment has been investigated using focused ion beam (FIB) and transmission electron microscopy (TEM). The ablated regions of TiN coatings by laser ablation system have been investigated under various conditions of pulse energies and number of laser pulses. The intercolumnar cracks were predominant cracking mode following laser thermal shock tests and the cracks initiated at coating surface and propagated in a direction perpendicular to the substrate under low loads conditions. Over and above those cracks, the cracks originated from coating-substrate interface began to appear with increasing laser pulse energy. The cracks from the interface also spread out transversely through the weak region of the columnar grains by repetitive laser shock. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Hard coatings have been applied to provide protective layers for hard metal tools, dies and many mechanical parts to improve their performance and lifetime because of their remarkable physical and chemical properties, such as high hardness, excellent corrosion resistance, and chemical stability [1,2]. However, hard coatings may result in failures, i.e., delamination, crack or spalling on the local regions of coatings owing to the high temperature and cyclic loads in a given environment. Thus it is crucial to understand the thermal degradation behavior of coatings under the actual working conditions for a reliable assessment of hard coating materials. Conventional furnace heating process which has been widely used for a thermal shock test is far from actual working environments of hard coatings; not only the entire area of a specimen suffers from thermal loads but it is also overexposed to heat. On the other hand, laser thermal shock experiment can locally apply thermomechanical loads separately from friction loads on the surface of coatings in a short period of time [3,4]. Despite of these advantages of laser thermal shock experiment, it is difficult to observe the cross sections of coatings since the laser ablated area is too limited. In recent years, the focused ion beam (FIB) miller based techniques are commonly being used to observe coating-substrate deformation mechanisms following nanoindentations [5–8]. The
∗ Corresponding author. Tel.: +82 51 510 2388; fax: +82 51 512 0528. E-mail address: [email protected] (H. Lee). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.086
FIB technique, as referred in many references [9–11], can be used to readily image and prepare cross section of coatings by observing locally damaged regions. It is also relatively rapid to prepare transmission electron microscope (TEM) specimens compared to conventional techniques. The present study aims to conduct laser thermal shock experiments on the surface of the relatively thin (∼0.5 m) TiN coatings with a beam spot size diameter of 200 m and then observe the surface morphology and cross section images to understand the crack propagation behavior under various conditions of pulsed laser by using focused ion beam miller and transmission electron microscope. 2. Experimental procedure Titanium nitride (TiN) coatings, ∼0.5 m in thickness, used in this experiment was deposited onto 304 stainless steel substrates by arc ion plating technique at 450 ◦ C deposition temperature. Prior to the TiN coatings deposition, the substrate surface was sputtercleaned using Ar ions under −800 V bias voltage for 10 min to remove the contaminant layer and to improve the adhesion of TiN films. Arc and sputter current were maintained as 50 A and 1.0 A, respectively [12]. Laser thermal shock experiments were performed by means of Nd-YAG laser ablation system (LSX-213) which could control laser beam size and pulse energy. The maximum laser pulse energy (100%) is about 4.7 mJ and it has homogeneous flat top energy profile and uniform density across all laser beam size. Details of the experimental conditions for laser pulse energy are shown in Table 1.
Y. Choi et al. / Applied Surface Science 258 (2012) 8752–8757
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Table 1 Output of laser pulse energy of laser ablation system (LSX-213). Energy level (%)
10
Energy (mJ)
0.45 0.90 1.35 1.86 2.29 2.81 3.26 3.71
20
30
40
50
60
70
80
90
100
4.14 4.74
To observe the morphological changes on TiN coating layer induced by the pulse energy, specimens were subjected to ablate for single laser pulse over a range of energy from 10 to 100% with a 200 m beam size. Also, the number of laser pulses increased from 1 to 15 while pulse energy was fixed as 100% to observe the microstructure of coating layer according to the cyclic loads. The surface morphologies of ablated region of specimens were investigated by scanning electron microscopy (SEM, Hitachi S4800). Cross-sections of both TiN film and substrate were imaged using a dual beam focused ion beam (Nova 200 NanoLab, FEI) for characterization of the microstructure on the center of laser ablated regions. Cross sectional TEM specimens were prepared from three specimens, after ablated by single laser pulse with 30%, 70% and twice laser pulse with 100%, using the FIB techniques. These specimens were then characterized in a field emission transmission electron microscope (Tecnai G2 F30 S-Twin, FEI). Details of preparation of cross-sections using the FIB and the use of the FIB for TEM specimen is described in detail elsewhere [9–11]. 3. Results and discussion Fig. 1 shows the surface morphologies of TiN coatings following laser thermal shock experiments discussed above. The laser ablated region was gradually deteriorated with hydrodynamic features like resolidified droplets of materials [13]. The pitted areas of coating surface, marked by an arrow, seem to be resulting from melting of defects such as pore and organic compound occurred during coating deposition process. Fig. 2 shows a FIB image of TiN coating surface ablated by single laser pulse with 70% of laser pulse energy after FIB milling. Although
Fig. 2. SEM image of TiN specimen surface after FIB milling based TEM foils lift out, following laser ablation with 70% of laser pulse energy.
damaged region diameter is approximately 200 m, cutting region in this experiment is only about 15 m. Since for a wide range of FIB cut process is laborious and time-consuming, it is a difficult to observe the overall damaged region. The centre of the damaged region was imaged by SEM and the FIB cuts were then done at cracks appearing as the widest in SEM to avoid misinterpretation [3], and thus, two FIB cuts at least per laser spot were performed. FIB cross section images of TiN coating applied under different stress modes are shown in Figs. 3 and 4. The TiN coating is approximately 0.5 m in thickness. As seen in Fig. 3(a), it is observed that the vacant region near the coating surface resulted from melting of pore or organic components. The micro cracks were initiated near the coating surface and propagated perpendicularly to the substrate in relatively lower loads. At higher loads (>50% of laser pulse energy), the size of pores at the coating surface has been gradually increased and the cracks began to be observed at the substratecoating interface as they are shown in Fig. 3(b)–(d).
Fig. 1. SEM images of TiN coating surface following laser ablation: (a, e) 10%, (b, f) 50%, (c, g) 80%, and (d, h) 100% (×2) of laser pulse energy. The pitted areas of coating surface are marked by arrows.
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Fig. 3. FIB images of cross section of the TiN coating following laser ablation by single laser pulse with 40%, (b) 50%, (c) 60%, and (d) 70% of laser pulse energy.
Fig. 4 shows cross sections of TiN coatings with the cyclic loads applied at 100% laser pulse energy. Fig. 4(a) shows the cracks initiated at coating-substrate interface propagated upwards to the surface of coating after 1 laser pulse action. For twice laser ablation, the cracks horizontally spread out running across the columnar grains by an additional impact on the previous formed cracks. The change of crack propagation direction from perpendicular to horizontal is considered due to repetitive laser ablation in a short time. After continuous laser pulses action, delamination or spalling were observed at the coating-substrate
interface region. Finally, coating layer was entirely melted and substrate was impacted and deformed by laser ablation as shown in Fig. 4(d). To further understand the microstructure induced by the laser pulse energy, bright field cross-sectional TEM analysis was conducted. Three TEM specimens were prepared at the center of damaged region from each specimen by using FIB. The introduction of FIB milling for preparation of the specimens make the production of wide, electron transparent specimens from site-specific regions [14,15].
Fig. 4. FIB images of cross section of the TiN coating following laser ablation at 100% laser pulse energy for (a) 1, (b) 2, (c) 3, and (d) 5 laser ablation times.
Y. Choi et al. / Applied Surface Science 258 (2012) 8752–8757
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Fig. 5. Bright field TEM images of intercolumnar cracks between columnar grains after laser ablation for (a, d) 30%, (b, e) 70% and (c, f) 100% (×2) of laser pulse energy.
As seen in Fig. 5(a), (b), and (c), TiN columnar grains are approximately 10–20 nm in width and the cracks were running along the columnar grains. The depth of columnar cracks increased with increasing of laser energy, marked as double-headed arrows, and their length were approximately 100, 120, and 150 nm, respectively. In Fig. 5(d), (e), and (f), TiN coating layers were deformed principally by shear cracking, marked C, between the columnar grain boundaries. This mechanism allow this cracking mode to
appear, so it is thought that the intercolumnar cracks formed by laser thermal shock are similar to the cracks in indented region by nanoindentation [16,17]. Figs. 6–8 show TEM bright field images of ablated region for single laser pulse with 30%, 70% and twice laser pulses with 100% of laser energy, respectively. The dark section along the top of coating is residue of a platinum layer that was introduced in the FIB prior to thinning for protecting the top edge of the specimen [9]. The
Fig. 6. Bright field cross-section TEM images of (a) whole specimen after 30% laser ablation, (b) at higher magnification, of the area indicated by the white square, shown in (a), (c) at higher magnification, of the area indicated by the white square, shown in (b).
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Fig. 7. Bright field cross-section TEM images of (a) whole specimen after 70% laser ablation, (b) at higher magnification, of the area indicated by the white circle, shown in (a), (c) of the area indicated by the white square, shown in (a).
stainless steel substrate and Ti sub-layer which was introduced during deposition process for improving adhesion strength of coating layer were labeled in the image. As laser energy increased, as mentioned in FIB images, the number of pores nearby the surface of TiN coating layer increased as seen in Figs. 6, 7 and 8(a). It is observed at higher magnification that columnar cracks, marked C, were originated from the top of TiN coating surface and their direction were perpendicular to the substrate at 30% and 70% of laser pulse energy as seen in Figs. 6(c) and 7(b), respectively. The grains having relatively dark color that are generated from highly diffraction display the agglomerated crystalline defects, in the grain marked R [18].
In Fig. 7(c), the cracks originated from coating-substrate interface were appeared at 70% laser energy. From the above observation, cracks propagation of the TiN coating changed before and after a certain laser energy. Under the lower pulse energy, the crack propagation was mainly occurred from the surface of TiN coating to the coating-substrate interface. With increasing laser pulse energy, the cracks occurred from coating-substrate interface begun to appear. After twice laser pulses with 100% of laser pulse energy, delamination, marked D, about 50 nm in width and 300 nm in length at the interface between the TiN layer and Ti sublayer can be obviously seen in Fig. 8(b). The number and size of pores, marked P, increased compared to other
Fig. 8. Bright field cross-section TEM images of (a) whole specimen after 100% (×2) laser ablation, (b) at higher magnification, of the area indicated by the white circle, shown in (a), (c) of the area indicated by the white square, shown in (a).
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specimens and propagated downwards to substrate as seen in Fig. 8(c). 4. Conclusions The crack propagation behavior of TiN coatings under the laser thermal shock experiments was investigated using FIB milling and imaging, and transmission electron microscopy. FIB and TEM images show that the intercolumnar cracks were predominant cracking mode following laser thermal shock tests and the cracks were propagated in a direction perpendicular to the substrate under lower laser energy conditions. As increased laser energy, not only the cracks initiated at the top of coating surface but also the cracks originated from coating-substrate interface begun to appear. The horizontal cracks were also observed across the columnar grains under the cyclic high loading. Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (M-2009–01-0028), Republic of Korea, and partially supported by grants-in-aid for the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NO. 2011-0030658) through GCRC-SOP. References [1] S. Boelens, H. Veltrop, Hard coatings of TiN, (TiHf)N and (TiNb)N deposited by random and steered arc evaporation, Surface and Coatings Technology 33 (1987) 63. [2] T. Ikeda, H. Satoh, Phase formation and characterization of hard coatings in the Ti–Al–N system prepared by the cathodic arc ion plating, Thin Solid Films 195 (1991) 99. [3] G. Kirchhoff, Th. Göbel, H.-A. Bahr, H. Balke, K. Wetzig, K. Bartsch, Damage analysis for thermally cycled (Ti,Al)N coatings-estimation of strength and interface fracture toughness, Surface and Coatings Technology 179 (2004) 39–46.
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[4] K. Klotz, H.-A. Bahr, H. Balke, Th. Gobel, S. Menzel, U. Bahr, G. Kirchhoff, K. Wetzig, Creep analysis and laser-induced cracking of (Ti,Al)N coatings, Thin Solid Films 413 (2002) 131. [5] J.M. Cairney, P.R. Munroe, M. Hoffman, The application of focused ion beam technology to the characterization of coatings, Surface and Coatings Technology 198 (2005) 165–168. [6] J.M. Cairney, R. Tsukano, M.J. Hoffman, M. Yang, Degradation of TiN coating under cyclic loading, Acta Materialia 52 (2004) 3229–3237. [7] S. Bhowmick, Z.-H. Xie, M. Hoffman, V. Jayaram, S.K. Biswas, Nature of contact deformation of TiN films on steel, Journal of Materials Research 19 (2004) 2616–2624. [8] V. Jayaram, S. Bhowmick, Z.-H. Xie, S. Math, M. Hoffman, S.K. Biswas, Contact deformation of TiN coatings on metallic substrates, Materials Science and Engineering A 423 (2006) 8–13. [9] Richard Wirth, Focused ion beam (FIB) combined with SEM and TEM: advanced analytical tools for studies of chemical composition, microstructure and crystal structure in geomaterials on a nanometer scale, Chemical Geology 261 (2009) 227–229. [10] D.R.P Singh, N. Chawla, Y.L. Shen, Focused ion beam (FIB) tomography of nanoindentation damage in nanoscale metal/ceramic multilayers, Materials Characterization 61 (2010) 481–488. [11] F. Elfallagh, B.J. Inkson, 3D analysis of crack morphologies in silicate glass using FIB tomography, Journal of the European Ceramic Society 29 (2009) 47–52. [12] S.Y. Yoon, J.K. Kim, K.H. Kim, A comparative study on tribological behavior of TiN and TiAlN coatings prepared by arc ion plating technique, Surface and Coatings Technology 161 (2002) 237–242. [13] M.S. Trtica, B.M. Gakovic, L.T. Petkovska, V.F. Tarasenko, A.V. Fedenev, E.I. Lipatov, M.A. Shulepov, Surface modifications of TiN coating by the pulsed TEA CO2 and KrCl laser, Applied Surface Science 225 (2004) 362–371. [14] J.M. Cairney, P.R. Munroe, D.J. Sordelet, Microstructural analysis of a FeAl/quasicrystal-based composite prepared using a focused ion beam miller, Journal of Microscopy 201 (2001) 201–211. [15] J.M. Cairney, R.D. Smith, P.R. Munroe, Transmission electron microscope specimen preparation of metal matrix composites using the focused ion beam miller, Microscopy and Microanalysis 6 (2000) 452–462. [16] L.W. Ma, J.M. Cairney, M.J. Hoffman, P.R. Munroe, Characterization of TiN thin films subjected to nanoindentation using focused ion beam milling, Applied Surface Science 237 (2004) 631–635. [17] L.W. Ma, J.M. Cairney, M.J. Hoffman, P.R. Munroe, Deformation mechanisms operating during nanoindentation of TiN coatings on steel substrates, Surface and Coatings Technology 192 (2005) 11–18. [18] J.M. Cairney, S.G. Harris, P.R. Munroe, E.D. Doyle, Transmission electron microscopy of TiN and TiAlN thin films using specimens prepared by focused ion beam milling, Surface and Coatings Technology 183 (2004) 239–246.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Crack propagation behavior of TiN coatings by laser thermal shock experiments Youngkue Choi a , Seol Jeon a , Min-seok Jeon b , Hyun-Gyoo Shin b , Ho Hwan Chun c , Youn-seoung Lee d , Heesoo Lee a,∗ a
School of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea Material Testing Center, Korea Testing Laboratory, Seoul 152-848, Republic of Korea Department of Naval Architecture & Ocean Engineering, Pusan National University, Busan 609-735, Republic of Korea d Department of Information Communication Engineering, Hanbat National University, Daejon 305-719, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 5 March 2012 Accepted 16 May 2012 Available online 24 May 2012 Keywords: Crack propagation Laser ablation Focused ion beam Transmission electron microscopy
a b s t r a c t The crack propagation behavior of TiN coatings, deposited onto 304 stainless steel substrates by arc ion plating technique, related to a laser thermal shock experiment has been investigated using focused ion beam (FIB) and transmission electron microscopy (TEM). The ablated regions of TiN coatings by laser ablation system have been investigated under various conditions of pulse energies and number of laser pulses. The intercolumnar cracks were predominant cracking mode following laser thermal shock tests and the cracks initiated at coating surface and propagated in a direction perpendicular to the substrate under low loads conditions. Over and above those cracks, the cracks originated from coating-substrate interface began to appear with increasing laser pulse energy. The cracks from the interface also spread out transversely through the weak region of the columnar grains by repetitive laser shock. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Hard coatings have been applied to provide protective layers for hard metal tools, dies and many mechanical parts to improve their performance and lifetime because of their remarkable physical and chemical properties, such as high hardness, excellent corrosion resistance, and chemical stability [1,2]. However, hard coatings may result in failures, i.e., delamination, crack or spalling on the local regions of coatings owing to the high temperature and cyclic loads in a given environment. Thus it is crucial to understand the thermal degradation behavior of coatings under the actual working conditions for a reliable assessment of hard coating materials. Conventional furnace heating process which has been widely used for a thermal shock test is far from actual working environments of hard coatings; not only the entire area of a specimen suffers from thermal loads but it is also overexposed to heat. On the other hand, laser thermal shock experiment can locally apply thermomechanical loads separately from friction loads on the surface of coatings in a short period of time [3,4]. Despite of these advantages of laser thermal shock experiment, it is difficult to observe the cross sections of coatings since the laser ablated area is too limited. In recent years, the focused ion beam (FIB) miller based techniques are commonly being used to observe coating-substrate deformation mechanisms following nanoindentations [5–8]. The
∗ Corresponding author. Tel.: +82 51 510 2388; fax: +82 51 512 0528. E-mail address: [email protected] (H. Lee). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.086
FIB technique, as referred in many references [9–11], can be used to readily image and prepare cross section of coatings by observing locally damaged regions. It is also relatively rapid to prepare transmission electron microscope (TEM) specimens compared to conventional techniques. The present study aims to conduct laser thermal shock experiments on the surface of the relatively thin (∼0.5 m) TiN coatings with a beam spot size diameter of 200 m and then observe the surface morphology and cross section images to understand the crack propagation behavior under various conditions of pulsed laser by using focused ion beam miller and transmission electron microscope. 2. Experimental procedure Titanium nitride (TiN) coatings, ∼0.5 m in thickness, used in this experiment was deposited onto 304 stainless steel substrates by arc ion plating technique at 450 ◦ C deposition temperature. Prior to the TiN coatings deposition, the substrate surface was sputtercleaned using Ar ions under −800 V bias voltage for 10 min to remove the contaminant layer and to improve the adhesion of TiN films. Arc and sputter current were maintained as 50 A and 1.0 A, respectively [12]. Laser thermal shock experiments were performed by means of Nd-YAG laser ablation system (LSX-213) which could control laser beam size and pulse energy. The maximum laser pulse energy (100%) is about 4.7 mJ and it has homogeneous flat top energy profile and uniform density across all laser beam size. Details of the experimental conditions for laser pulse energy are shown in Table 1.
Y. Choi et al. / Applied Surface Science 258 (2012) 8752–8757
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Table 1 Output of laser pulse energy of laser ablation system (LSX-213). Energy level (%)
10
Energy (mJ)
0.45 0.90 1.35 1.86 2.29 2.81 3.26 3.71
20
30
40
50
60
70
80
90
100
4.14 4.74
To observe the morphological changes on TiN coating layer induced by the pulse energy, specimens were subjected to ablate for single laser pulse over a range of energy from 10 to 100% with a 200 m beam size. Also, the number of laser pulses increased from 1 to 15 while pulse energy was fixed as 100% to observe the microstructure of coating layer according to the cyclic loads. The surface morphologies of ablated region of specimens were investigated by scanning electron microscopy (SEM, Hitachi S4800). Cross-sections of both TiN film and substrate were imaged using a dual beam focused ion beam (Nova 200 NanoLab, FEI) for characterization of the microstructure on the center of laser ablated regions. Cross sectional TEM specimens were prepared from three specimens, after ablated by single laser pulse with 30%, 70% and twice laser pulse with 100%, using the FIB techniques. These specimens were then characterized in a field emission transmission electron microscope (Tecnai G2 F30 S-Twin, FEI). Details of preparation of cross-sections using the FIB and the use of the FIB for TEM specimen is described in detail elsewhere [9–11]. 3. Results and discussion Fig. 1 shows the surface morphologies of TiN coatings following laser thermal shock experiments discussed above. The laser ablated region was gradually deteriorated with hydrodynamic features like resolidified droplets of materials [13]. The pitted areas of coating surface, marked by an arrow, seem to be resulting from melting of defects such as pore and organic compound occurred during coating deposition process. Fig. 2 shows a FIB image of TiN coating surface ablated by single laser pulse with 70% of laser pulse energy after FIB milling. Although
Fig. 2. SEM image of TiN specimen surface after FIB milling based TEM foils lift out, following laser ablation with 70% of laser pulse energy.
damaged region diameter is approximately 200 m, cutting region in this experiment is only about 15 m. Since for a wide range of FIB cut process is laborious and time-consuming, it is a difficult to observe the overall damaged region. The centre of the damaged region was imaged by SEM and the FIB cuts were then done at cracks appearing as the widest in SEM to avoid misinterpretation [3], and thus, two FIB cuts at least per laser spot were performed. FIB cross section images of TiN coating applied under different stress modes are shown in Figs. 3 and 4. The TiN coating is approximately 0.5 m in thickness. As seen in Fig. 3(a), it is observed that the vacant region near the coating surface resulted from melting of pore or organic components. The micro cracks were initiated near the coating surface and propagated perpendicularly to the substrate in relatively lower loads. At higher loads (>50% of laser pulse energy), the size of pores at the coating surface has been gradually increased and the cracks began to be observed at the substratecoating interface as they are shown in Fig. 3(b)–(d).
Fig. 1. SEM images of TiN coating surface following laser ablation: (a, e) 10%, (b, f) 50%, (c, g) 80%, and (d, h) 100% (×2) of laser pulse energy. The pitted areas of coating surface are marked by arrows.
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Fig. 3. FIB images of cross section of the TiN coating following laser ablation by single laser pulse with 40%, (b) 50%, (c) 60%, and (d) 70% of laser pulse energy.
Fig. 4 shows cross sections of TiN coatings with the cyclic loads applied at 100% laser pulse energy. Fig. 4(a) shows the cracks initiated at coating-substrate interface propagated upwards to the surface of coating after 1 laser pulse action. For twice laser ablation, the cracks horizontally spread out running across the columnar grains by an additional impact on the previous formed cracks. The change of crack propagation direction from perpendicular to horizontal is considered due to repetitive laser ablation in a short time. After continuous laser pulses action, delamination or spalling were observed at the coating-substrate
interface region. Finally, coating layer was entirely melted and substrate was impacted and deformed by laser ablation as shown in Fig. 4(d). To further understand the microstructure induced by the laser pulse energy, bright field cross-sectional TEM analysis was conducted. Three TEM specimens were prepared at the center of damaged region from each specimen by using FIB. The introduction of FIB milling for preparation of the specimens make the production of wide, electron transparent specimens from site-specific regions [14,15].
Fig. 4. FIB images of cross section of the TiN coating following laser ablation at 100% laser pulse energy for (a) 1, (b) 2, (c) 3, and (d) 5 laser ablation times.
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Fig. 5. Bright field TEM images of intercolumnar cracks between columnar grains after laser ablation for (a, d) 30%, (b, e) 70% and (c, f) 100% (×2) of laser pulse energy.
As seen in Fig. 5(a), (b), and (c), TiN columnar grains are approximately 10–20 nm in width and the cracks were running along the columnar grains. The depth of columnar cracks increased with increasing of laser energy, marked as double-headed arrows, and their length were approximately 100, 120, and 150 nm, respectively. In Fig. 5(d), (e), and (f), TiN coating layers were deformed principally by shear cracking, marked C, between the columnar grain boundaries. This mechanism allow this cracking mode to
appear, so it is thought that the intercolumnar cracks formed by laser thermal shock are similar to the cracks in indented region by nanoindentation [16,17]. Figs. 6–8 show TEM bright field images of ablated region for single laser pulse with 30%, 70% and twice laser pulses with 100% of laser energy, respectively. The dark section along the top of coating is residue of a platinum layer that was introduced in the FIB prior to thinning for protecting the top edge of the specimen [9]. The
Fig. 6. Bright field cross-section TEM images of (a) whole specimen after 30% laser ablation, (b) at higher magnification, of the area indicated by the white square, shown in (a), (c) at higher magnification, of the area indicated by the white square, shown in (b).
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Fig. 7. Bright field cross-section TEM images of (a) whole specimen after 70% laser ablation, (b) at higher magnification, of the area indicated by the white circle, shown in (a), (c) of the area indicated by the white square, shown in (a).
stainless steel substrate and Ti sub-layer which was introduced during deposition process for improving adhesion strength of coating layer were labeled in the image. As laser energy increased, as mentioned in FIB images, the number of pores nearby the surface of TiN coating layer increased as seen in Figs. 6, 7 and 8(a). It is observed at higher magnification that columnar cracks, marked C, were originated from the top of TiN coating surface and their direction were perpendicular to the substrate at 30% and 70% of laser pulse energy as seen in Figs. 6(c) and 7(b), respectively. The grains having relatively dark color that are generated from highly diffraction display the agglomerated crystalline defects, in the grain marked R [18].
In Fig. 7(c), the cracks originated from coating-substrate interface were appeared at 70% laser energy. From the above observation, cracks propagation of the TiN coating changed before and after a certain laser energy. Under the lower pulse energy, the crack propagation was mainly occurred from the surface of TiN coating to the coating-substrate interface. With increasing laser pulse energy, the cracks occurred from coating-substrate interface begun to appear. After twice laser pulses with 100% of laser pulse energy, delamination, marked D, about 50 nm in width and 300 nm in length at the interface between the TiN layer and Ti sublayer can be obviously seen in Fig. 8(b). The number and size of pores, marked P, increased compared to other
Fig. 8. Bright field cross-section TEM images of (a) whole specimen after 100% (×2) laser ablation, (b) at higher magnification, of the area indicated by the white circle, shown in (a), (c) of the area indicated by the white square, shown in (a).
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specimens and propagated downwards to substrate as seen in Fig. 8(c). 4. Conclusions The crack propagation behavior of TiN coatings under the laser thermal shock experiments was investigated using FIB milling and imaging, and transmission electron microscopy. FIB and TEM images show that the intercolumnar cracks were predominant cracking mode following laser thermal shock tests and the cracks were propagated in a direction perpendicular to the substrate under lower laser energy conditions. As increased laser energy, not only the cracks initiated at the top of coating surface but also the cracks originated from coating-substrate interface begun to appear. The horizontal cracks were also observed across the columnar grains under the cyclic high loading. Acknowledgements This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (M-2009–01-0028), Republic of Korea, and partially supported by grants-in-aid for the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NO. 2011-0030658) through GCRC-SOP. References [1] S. Boelens, H. Veltrop, Hard coatings of TiN, (TiHf)N and (TiNb)N deposited by random and steered arc evaporation, Surface and Coatings Technology 33 (1987) 63. [2] T. Ikeda, H. Satoh, Phase formation and characterization of hard coatings in the Ti–Al–N system prepared by the cathodic arc ion plating, Thin Solid Films 195 (1991) 99. [3] G. Kirchhoff, Th. Göbel, H.-A. Bahr, H. Balke, K. Wetzig, K. Bartsch, Damage analysis for thermally cycled (Ti,Al)N coatings-estimation of strength and interface fracture toughness, Surface and Coatings Technology 179 (2004) 39–46.
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