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CrN multilayer coatings prepared by magnetron sputtering
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Surface & Coatings Technology 202 (2007) 815 – 819 www.elsevier.com/locate/surfcoat
TEM investigation of TiAlN/CrN multilayer coatings prepared by magnetron sputtering M. Panjan ⁎, S. Šturm, P. Panjan, M. Čekada Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Available online 2 June 2007
Abstract Multilayer coatings TiAlN/CrN were deposited by reactive magnetron sputtering. Thickness of the individual layers varied from 2 nm to 25 nm while the total thickness of the coating was ∼ 5 μm. Coatings in cross-section were investigated using conventional, scanning and high-resolution transmission electron microscopy (TEM). Conventional TEM studies revealed a columnar microstructure. Chemical analysis of individual layers was preformed by high-angle annular dark-field scanning TEM (HAADF-STEM). The layers were well separated and no substantial intermixing was observed. High resolution TEM and electron diffraction studies showed that TiAlN and CrN layers are crystalline with B1 NaCl-type crystal structure. Coherent interfaces between TiAlN/CrN were observed, which can be attributed to a small mismatch between lattice parameters. In some areas between steel substrate (bcc-Fe) and the coating epitaxial relationship {001}Fe||{001}coating and b100NFe||b110Ncoating was observed. In different areas of the coating TiAlN layers appeared to be less crystalline than CrN. © 2007 Elsevier B.V. All rights reserved. Keywords: TiAlN/CrN; Multilayer; Magnetron sputtering; TEM; STEM; HRTEM
1. Introduction Combination of different materials in a form of multilayers produces coatings with superior properties than in the form of a single layer. Especially good properties are found for nitride materials with a few nanometer thick layers. These multilayers, also termed superlattices, exhibit higher hardness, better toughness, wear and oxidation resistance than constituent materials deposited in a single layer. In the past years a variety of different nitride multilayer coatings have been produced, for example: TiN/CrN [1,2], TiN/NbN [3,4], AlN/VN [5], TiAlN/ CrN [6,7]. All of these coatings reach maximum hardness in a narrow range of bilayer thickness of ∼ 3–10 nm. The maximum hardness is always significantly higher than the hardness of the materials in each layer. Several explanations have been proposed for the origin of this effect including dislocation blocking by layer interfaces, Hall–Petch strengthening, strain effects at layer interfaces, and the supermodulus effect [8]. One of the most studied materials in the hard coating technology are TiAlN and CrN. Properties of TiAlN depend on ⁎ Corresponding author. Tel.: + 386 1 477 3276; fax: +386 1 251 9385. E-mail address: [email protected] (M. Panjan). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.084
the content of aluminum [9]. At the approximate 1:1 ratio of titanium and aluminum atoms it is possible to get hardness higher than 30 GPa [10]. TiAlN has very good oxidation resistance compared to TiN [6]. CrN has lower hardness (∼18 GPa) but it is tougher and thermally more stable [11]. Combination of these two materials therefore yields a coating with good mechanical, wear and oxidation properties [6,12]. The maximum hardness of TiAlN/CrN superlattice was shown to be about 39 GPa for a bilayer period of 6 nm [6]. Our aim was to investigate microstructure, crystal structure and interfaces of TiAlN/CrN coating using cross-sectional transmission electron microscopy (TEM). TEM is the only technique with sufficient spatial resolution to resolve nanometersize layers in superlattices. Conventional TEM gives us detailed information on microstructure, including grain size, orientation and texture as well as on the types and distribution of defects (dislocations, stacking faults). On the other hand, high-resolution TEM (HRTEM) offers information on atomic scale with possibility to locally determine crystal structure and to study interface characteristics (coherency, morphology, epitaxial relationships between the layers). In spite of an immense number of papers published on PVD coatings TEM investigations of such structures are still relatively rare and detailed structure of PVD coatings on
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M. Panjan et al. / Surface & Coatings Technology 202 (2007) 815–819
the atomic level is not yet completely understood. Therefore, in the present study we used various TEM techniques in order to characterize and understand the structure/composition relationship in the TiAlN/CrN coating at the atomic level.
analyzed using bright field (BF) and dark field (DF) imaging, selected area electron diffraction (SAED) and high-resolution imaging. X-ray diffractometer with Bragg-Brentano geometry (Bruker D4 Endeavour) was used to check the texture and phases present in samples.
2. Experimental details 3. Results and discussion TiAlN/CrN coatings were deposited using CC800 (CemeCon) sputtering system. Deposition system has four unbalanced magnetron sources placed in the corners of the rectangle. Two segmental TiAl targets were placed on one side of the deposition chamber and two Cr targets on the other side. By two-fold rotation of the samples around the targets, multilayers were prepared. Details of the deposition system are described elsewhere [13]. Coatings were deposited on tool steel (AISI D2) discs. Substrates were ground, polished and ultrasonically cleaned. Prior to the coating deposition the substrates were RF ion etched for 85 min (maximum RF power was 2 kW). At the beginning of the deposition, the power on TiAl targets changed with a ramp of 200 W/min to the end power of 8 kW, and on Cr targets by 300 W/min to the end power of 3 kW. As a result the first deposited layers in coating were thinner. Total thickness of the coating was ∼5 μm. The deposition time of the coatings was 135 min. Flow rates of N2, Ar and Kr were 140, 140 and 80 ml/min, respectively. The substrate temperature was 450 °C and DC bias − 100 V. The parameters for TiAlN deposition were based on the protocol for commercial TiAlN with approximate atomic ratio of Ti:Al = 50:50. Specimens for cross-sectional TEM were prepared using the procedure of Helmerson and Sundgren [14]. Samples were investigated by JEOL 2010F TEM/STEM ultra-high resolution microscope, operated at 200 keV and equipped with a HAADF detector. Spherical aberration of the objective lens was Cs =0.5 mm. The HAADF collection angle was set between 100 and 200 mrad. HAADF STEM provides acquisition of incoherent images of crystalline materials with strong compositional sensitivity (i.e. Z-contrast imaging) [15]. Microstructure and crystal structure were
3.1. Microstructural analysis Microstructure of the coatings was analyzed with conventional TEM using bright field and dark field imaging. Fig. 1 shows BF and DF images of the TiAlN/CrN multilayer on steel substrate. Dark layers on BF image represent CrN and bright layers TiAlN. Thickness of the first layers next to the substrate is not constant but it increases due to increasing power on Cr and TiAl targets at the beginning of the deposition. The first layers of TiAlN and CrN are ∼ 2 nm and ∼ 7 nm thick, respectively. After the power on the targets became constant, the layers had approximately the same thickness (25 nm). DF image (Fig. 1b) reveals columnar microstructure of the coating. The columnar microstructure is typical for the coatings deposited at low gas pressures and low temperatures compared to melting point. On Fig. 1b we can see many different columnar grains nucleating on the substrate while the substrate appears to be single grain. Columnar grains grow perpendicularly to the substrate. They do not extend through entire thickness of the coating but they nucleate at the substrate as well as throughout the coating. In the grains which are orientated at strongly diffracting conditions strain contrast is seen (e.g. grain labeled C). Strain contrast is a consequence of residual stress, a common feature in the coatings deposited by sputtering. Microstructure of the coating appears not to be affected by the change in thickness of the layers, e.g. columnar grains exist in the thinner as well as in thicker layers. However, Nordin and Ericson [16] showed that microstructure of TiN/TaN multilayer depends on bilayer period. For bilayer periods thinner than
Fig. 1. Cross-sectional (a) BF and (b) DF image of TiAlN/CrN on steel substrate and corresponding diffraction pattern. Darker layers on BF image represent CrN and brighter layers TiAlN. Both layers have B1 NaCl-type structure. DF image reveals columnar microstructure of the coating.
M. Panjan et al. / Surface & Coatings Technology 202 (2007) 815–819
42 nm they obtained a columnar structure whereas for thicker periods the structure was more fine grained. 3.2. Crystal structure analysis Crystal structure of Ti1 − xAlxN depends on the content of aluminum. Up to x b 0.6 of aluminum Ti1 − xAlxN forms B1 NaCl-type structure (cubic phase), between 0.6 ≤ x ≤ 0.7 two phases coexist, one with NaCl-type structure and the other with AlN wurtzite structure (hexagonal phase). At x ≥ 0.7 only wurtzite structure forms [9,17]. CrNx crystallizes in a cubic B1 NaCl-type structure for x = 0.8–1.0 and in hexagonal Cr2N phase for x = 0.4–0.7 [18]. In superlattices other metastable phases can be stabilized over certain range of thicknesses due to the so-called template effect [19]. For example, Yashar et al. [2] showed that in Cr0.6N/TiN superlattice with a thickness of the layers below 10 nm, cubic Cr0.6N existed while in multilayers with 50 nm thick layers hexagonal Cr0.6N phase was stable. The nature of the coating crystallinity was analyzed using selected area diffraction, high resolution TEM and X-ray diffraction. On Fig. 1a the diffraction pattern is shown along with BF image. The diffraction rings correspond to the diffraction planes of the NaCl-type structure. The diffraction patterns taken from different areas of the coating showed only cubic phase and no other phases were observed. The same results were obtained by XRD. The diffraction rings corresponding to TiAlN and CrN could not be separated because difference in the lattice parameters was too small. Lattice parameter of TiAlN depends on the content of aluminum; it decreases with
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increasing Al content because substitution of smaller Al atoms with Ti atoms in TiN lattice reduces the size of the unit cell. In the case of pure TiN (x = 0) the lattice parameter is ∼0.423 nm and for x = 0.5 the lattice parameter is ∼0.419 nm [9,20]. Cubic CrN has a lattice parameter a = 0.414 nm and is almost independent of nitrogen content [21]. The lattice mismatch between stoichiometric TiAlN and CrN is ∼ 1%. From XRD measurements it was not possible to separate the lattice parameters of TiAlN and CrN but an average lattice parameter was approximately 0.417 nm. 3.3. Analysis of interfaces The same crystal structure of TiAlN and CrN and small mismatch between the lattices gives a good condition for epitaxial growth. Epitaxial growth is seen on the high resolution TEM image on Fig. 2a. Image shows the first TiAlN and CrN layers inside one columnar grain with steel substrate below. Interfaces between TiAlN and CrN are coherent. Although the interface between the steel and the coating appears distorted it is believed that in this case the distortion is only a Moiré pattern and that the interface is in fact coherent. Explanation for existence of Moiré pattern could be as follows. Ion etching of the steel substrate creates surface which is rough on atomic scale. When atoms of the coating material are deposited on such surface this causes an overlap of the steel and coating crystal structures. If such interface is viewed perpendicularly this gives rise to Moiré patterns. Additionally, small deviations from perpendicular view on perfectly flat interface also produce Moiré patterns.
Fig. 2. (a) High resolution image of TiAlN/CrN multilayer coating on steel substrate. Enlarged areas of coating and steel indicate epitaxial relationship between them. (b) Schematic diagram of CrN and Fe unit cells and epitaxial relationship between them: {001}Fe||{001}CrN and b100NFe||b110NCrN. Nearest Cr atoms in B1 NaCl-type CrN unit cell grow on corners of bcc-Fe unit cell.
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Both the coating and the substrate on Fig. 2a are in zone axis which indicates epitaxial relationship between them. From enlarged areas on Fig. 2a it can be seen that b110N directions in the coating (B1 NaCl) are parallel to the b100N directions of the steel substrate (bcc-Fe), whereas {001} planes of both coating and substrate are parallel to each other: epitaxial relationship is b100NFe||b110Ncoating and {001}Fe||{001}coating. This confirms that the substrate/coating interface has to be coherent because on incoherent interface planes of substrate and coating would not necessarily be parallel. The lattice parameter of iron is aFe ∼ 0.287 nm which is very similar to the distance between the second nearest neighbor atomspin pffiffiffi ffiffiffi the structure of CrN and TiAlN: aFe caCrN = 2caTiAlN = 2, e.g. for CrN the distance is dCr–Cr ∼ 0.293 nm (mismatch between aFe and dCr–Cr is ∼ 2%). Therefore it is very likely that the nearest metal atoms in CrN or TiAlN unit cell grow on the corners of bcc-Fe unit cell. Schematic diagram of this epitaxial relationship is shown on Fig. 2b. The {001} planes of the CrN unit cell are parallel to the {001} planes of the Fe unit cell, while the CrN unit cell is rotated for 45° around the interface normal. This kind of epitaxial relationship has also been observed between the layers of bcc-metal and NaCl-type nitride superlattices, e.g. Mo/NbN and W/NbN [22]. The energy of a coherent interface is smaller than energy of an incoherent interface therefore in those grains where steel substrate is favorably orientated TiAlN/CrN layers grow in the manner shown in Fig. 2b. Additionally, {001} planes of NaCl structure are the closest packed planes and therefore the energy needed for growth on this plane is the smallest. XRD measurements of the coating also showed the
strongest peaks to be (002) thus the coating had predominantly (002) texture. However, for non-favorable orientations of steel grains the interface between the steel and the coating can be incoherent. Coherent growth is also observed in the thicker layers (∼ 20 nm). Fig. 3 shows HRTEM image of a small columnar grain (width ∼ 15 nm) in the b110N zone axis. Bright layers correspond to TiAlN and dark layers to CrN. Inside the columnar grain epitaxy is seen. Images B–F on Fig. 3 show magnified parts of the coating inside the columnar grain with corresponding Fast Fourier Transformation (FFT) diagrams. Epitaxial relationship between the layers is clearly seen although it appears that TiAlN layers (C and F areas) are more distorted than CrN layers (B and D areas). Stronger distortion of TiAlN layers compared to CrN layers is even more pronounced in neighboring grains which are not orientated along the low index zone axis. This characteristic was observed in different areas of the coating. Possible explanation could be related with distortion of crystal structure. Alling et al. [23] calculated the displacement of atoms from the equilibrium positions in relaxed TiAlN supercell with random distributions of Ti and Al atoms on the metal sublattice. They showed that nitrogen atoms that were surrounded along the x axis by two metallic atoms of different type had the largest displacements, 1–2% of the average bond length. At the same time, the nitrogen atoms which were surrounded along the x axis by two chemically equivalent atoms showed only small displacements (0.1–0.3% of the average bond length). The metal atoms, which predominantly form HRTEM image, also showed only small
Fig. 3. High resolution image of a columnar grain in b110N zone axis. Inside the columnar grain epitaxy is seen. Dark layers are CrN and white layers TiAlN. Images A–F show magnified parts of the coating with corresponding FFT diagrams. In area A some dislocations in CrN layer are present.
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columnar structure in thin layers near the substrate as well as in the thicker layers. HAADF-STEM image analysis in the regions with thick and thin layers showed strong interfaces and no visible intermixing. TiAlN and CrN layers had B1 NaCl-type crystal structure and no other phases were observed. The same crystal structure and small mismatch between the TiAlN and CrN lattices provided good conditions for coherent growth inside the columnar grains. High resolution images showed that TiAlN layers appeared to be more distorted than CrN. In some areas epitaxial relationship between the steel substrate and the coating was observed. Epitaxial relationship was {001}Fe||{001}coating and b100NFe||b110Ncoating. Acknowledgements This work was sponsored by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia. References Fig. 4. High-angle annular dark-field STEM image of TiAlN/CrN layers next to steel substrate and in the center of coating. Bright areas represent CrN and dark areas TiAlN. Layers appear to be well differentiated.
displacements (∼ 0.5% of the average bond length). Therefore, the random distribution of metal atoms in TiAlN unit cell probably plays only a minor role in the observed distortion. The other reason for the distortion is probably related to strong point and/or line defects which more significantly distort the crystal structure. However, it is not clear why the concentration of defects in TiAlN would be higher than in CrN. On image A of Fig. 3 individual dislocations in the CrN layer can be seen. 3.4. Chemical analysis The HAADF-STEM images (Z-contrast images) provide a clear contrast in atomic number of elements present in layers. Fig. 4 shows HAADF STEM image of the layer structure near the substrate and in the central region of the coating. Bright areas correspond to the CrN and dark areas to the TiAlN layers. In both cases the contrast between CrN and TiAlN layers is sharp indicating that the layers are well differentiated with no visible intermixing. 4. Summary Multilayer coatings with variable bilayer period were prepared by magnetron sputtering. Microstructure, crystal structure and chemical composition were investigated using TEM. A comparison between thin layers near the substrate (2–10 nm) and thicker layers (20–25 nm) was made. Coatings had a
[1] M. Nordin, M. Larsson, S. Hogmark, Surf. Coat. Technol. 106 (1998) 234. [2] P. Yashar, X. Chu, S.A. Barnett, J. Rechner, Y.Y. Wang, M.S. Wong, W.D. Sproul, Appl. Phys. Lett. 72 (8) (1998) 987. [3] J.O. Kim, J.D. Achenbach, M. Shinn, S.A. Barnett, J. Mater. Res. 7 (3) (1992) 2248. [4] H.C. Barshilia, K.S. Rajam, D.V. Sridhara Rao, Surf. Coat. Technol. 200 (2006) 4586. [5] L. Geyang, L. Jijuan, T. Jaiwan, H. Zenghu, G. Mingyuan, J. Appl. Phys. 95 (1) (2004) 92. [6] H.C. Barshilia, M.S. Parkash, A. Jain, K.S. Rajam, Vacuum 77 (2005) 169. [7] D.B. Lewis, I. Wadsworth, W.D. Münz, R. Kuzel, V. Valvoda, Surf. Coat. Technol. 116–119 (1999) 284. [8] P.C. Yashar, W.D. Sproul, Vacuum 55 (1999) 179. [9] Ay. Kimura, Hi. Hasegawa, Ku. Yamada, Te. Suzuki, Surf. Coat. Technol. 120121 (1999) 438. [10] K. Kutschej, P.H. Mayrhofer, M. Kathrein, P. Polcik, R. Tessadri, C. Mitterer, Surf. Coat. Technol. 200 (2005) 2358. [11] I. Milošev, H.H. Strehblow, B. Navinšek, Surf. Coat. Technol. 74–75 (1995) 897. [12] I. Wadsworth, I.J. Smith, L.A. Donohue, W.D. Münz, Surf. Coat. Technol. 94–95 (1997) 315. [13] M. Panjan, T. Peterman, M. Čekada, P. Panjan, Plasma Process Polym 4 (S1) (2007) S921. [14] U. Helmersson, J.E. Sundgren, J. Electron. Micro. Technol. 4 (1986) 361. [15] S.J. Pennycook, D.E. Jesson, Ultramicroscopy 37 (1991) 14. [16] M. Nordin, F. Ericson, Thin Solid Films 385 (2001) 174. [17] Mi. Zhou, Y. Makino, M. Nose, K. Nogi, Thin Solid Films 339 (1999) 203. [18] P.M. Fabis, R.A. Cooke, S. McDonough, J. Vac. Sci. Technol., A 8 (1990) 3809. [19] G. Li, J. Lao, J. Tian, Z. Han, M. Gu, J. Appl. Phys. 95 (1) (2004) 92. [20] P.H. Mayrhofer, D. Music, J.M. Schneider, J. Appl. Phys. 100 (2006) 094906. [21] H. Era, Y. Ide, A. Nino, K. Kishitake, Surf. Coat. Technol. 194 (2005) 265. [22] A. Madan, W. Yun-yu, S.A. Barnett, C. Engström, H. Ljungcrantz, L. Hultman, M. Grimsditch, J. Appl. Phys. 84 (2) (1998) 776. [23] B. Alling, A.V. Ruban, A. Karimi, O.E. Peil, S.I. Simak, L. Hultman, I.A. Abrikosov, Phys. Rev. B 75 (2007) 045123.
Surface & Coatings Technology 202 (2007) 815 – 819 www.elsevier.com/locate/surfcoat
TEM investigation of TiAlN/CrN multilayer coatings prepared by magnetron sputtering M. Panjan ⁎, S. Šturm, P. Panjan, M. Čekada Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Available online 2 June 2007
Abstract Multilayer coatings TiAlN/CrN were deposited by reactive magnetron sputtering. Thickness of the individual layers varied from 2 nm to 25 nm while the total thickness of the coating was ∼ 5 μm. Coatings in cross-section were investigated using conventional, scanning and high-resolution transmission electron microscopy (TEM). Conventional TEM studies revealed a columnar microstructure. Chemical analysis of individual layers was preformed by high-angle annular dark-field scanning TEM (HAADF-STEM). The layers were well separated and no substantial intermixing was observed. High resolution TEM and electron diffraction studies showed that TiAlN and CrN layers are crystalline with B1 NaCl-type crystal structure. Coherent interfaces between TiAlN/CrN were observed, which can be attributed to a small mismatch between lattice parameters. In some areas between steel substrate (bcc-Fe) and the coating epitaxial relationship {001}Fe||{001}coating and b100NFe||b110Ncoating was observed. In different areas of the coating TiAlN layers appeared to be less crystalline than CrN. © 2007 Elsevier B.V. All rights reserved. Keywords: TiAlN/CrN; Multilayer; Magnetron sputtering; TEM; STEM; HRTEM
1. Introduction Combination of different materials in a form of multilayers produces coatings with superior properties than in the form of a single layer. Especially good properties are found for nitride materials with a few nanometer thick layers. These multilayers, also termed superlattices, exhibit higher hardness, better toughness, wear and oxidation resistance than constituent materials deposited in a single layer. In the past years a variety of different nitride multilayer coatings have been produced, for example: TiN/CrN [1,2], TiN/NbN [3,4], AlN/VN [5], TiAlN/ CrN [6,7]. All of these coatings reach maximum hardness in a narrow range of bilayer thickness of ∼ 3–10 nm. The maximum hardness is always significantly higher than the hardness of the materials in each layer. Several explanations have been proposed for the origin of this effect including dislocation blocking by layer interfaces, Hall–Petch strengthening, strain effects at layer interfaces, and the supermodulus effect [8]. One of the most studied materials in the hard coating technology are TiAlN and CrN. Properties of TiAlN depend on ⁎ Corresponding author. Tel.: + 386 1 477 3276; fax: +386 1 251 9385. E-mail address: [email protected] (M. Panjan). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.084
the content of aluminum [9]. At the approximate 1:1 ratio of titanium and aluminum atoms it is possible to get hardness higher than 30 GPa [10]. TiAlN has very good oxidation resistance compared to TiN [6]. CrN has lower hardness (∼18 GPa) but it is tougher and thermally more stable [11]. Combination of these two materials therefore yields a coating with good mechanical, wear and oxidation properties [6,12]. The maximum hardness of TiAlN/CrN superlattice was shown to be about 39 GPa for a bilayer period of 6 nm [6]. Our aim was to investigate microstructure, crystal structure and interfaces of TiAlN/CrN coating using cross-sectional transmission electron microscopy (TEM). TEM is the only technique with sufficient spatial resolution to resolve nanometersize layers in superlattices. Conventional TEM gives us detailed information on microstructure, including grain size, orientation and texture as well as on the types and distribution of defects (dislocations, stacking faults). On the other hand, high-resolution TEM (HRTEM) offers information on atomic scale with possibility to locally determine crystal structure and to study interface characteristics (coherency, morphology, epitaxial relationships between the layers). In spite of an immense number of papers published on PVD coatings TEM investigations of such structures are still relatively rare and detailed structure of PVD coatings on
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M. Panjan et al. / Surface & Coatings Technology 202 (2007) 815–819
the atomic level is not yet completely understood. Therefore, in the present study we used various TEM techniques in order to characterize and understand the structure/composition relationship in the TiAlN/CrN coating at the atomic level.
analyzed using bright field (BF) and dark field (DF) imaging, selected area electron diffraction (SAED) and high-resolution imaging. X-ray diffractometer with Bragg-Brentano geometry (Bruker D4 Endeavour) was used to check the texture and phases present in samples.
2. Experimental details 3. Results and discussion TiAlN/CrN coatings were deposited using CC800 (CemeCon) sputtering system. Deposition system has four unbalanced magnetron sources placed in the corners of the rectangle. Two segmental TiAl targets were placed on one side of the deposition chamber and two Cr targets on the other side. By two-fold rotation of the samples around the targets, multilayers were prepared. Details of the deposition system are described elsewhere [13]. Coatings were deposited on tool steel (AISI D2) discs. Substrates were ground, polished and ultrasonically cleaned. Prior to the coating deposition the substrates were RF ion etched for 85 min (maximum RF power was 2 kW). At the beginning of the deposition, the power on TiAl targets changed with a ramp of 200 W/min to the end power of 8 kW, and on Cr targets by 300 W/min to the end power of 3 kW. As a result the first deposited layers in coating were thinner. Total thickness of the coating was ∼5 μm. The deposition time of the coatings was 135 min. Flow rates of N2, Ar and Kr were 140, 140 and 80 ml/min, respectively. The substrate temperature was 450 °C and DC bias − 100 V. The parameters for TiAlN deposition were based on the protocol for commercial TiAlN with approximate atomic ratio of Ti:Al = 50:50. Specimens for cross-sectional TEM were prepared using the procedure of Helmerson and Sundgren [14]. Samples were investigated by JEOL 2010F TEM/STEM ultra-high resolution microscope, operated at 200 keV and equipped with a HAADF detector. Spherical aberration of the objective lens was Cs =0.5 mm. The HAADF collection angle was set between 100 and 200 mrad. HAADF STEM provides acquisition of incoherent images of crystalline materials with strong compositional sensitivity (i.e. Z-contrast imaging) [15]. Microstructure and crystal structure were
3.1. Microstructural analysis Microstructure of the coatings was analyzed with conventional TEM using bright field and dark field imaging. Fig. 1 shows BF and DF images of the TiAlN/CrN multilayer on steel substrate. Dark layers on BF image represent CrN and bright layers TiAlN. Thickness of the first layers next to the substrate is not constant but it increases due to increasing power on Cr and TiAl targets at the beginning of the deposition. The first layers of TiAlN and CrN are ∼ 2 nm and ∼ 7 nm thick, respectively. After the power on the targets became constant, the layers had approximately the same thickness (25 nm). DF image (Fig. 1b) reveals columnar microstructure of the coating. The columnar microstructure is typical for the coatings deposited at low gas pressures and low temperatures compared to melting point. On Fig. 1b we can see many different columnar grains nucleating on the substrate while the substrate appears to be single grain. Columnar grains grow perpendicularly to the substrate. They do not extend through entire thickness of the coating but they nucleate at the substrate as well as throughout the coating. In the grains which are orientated at strongly diffracting conditions strain contrast is seen (e.g. grain labeled C). Strain contrast is a consequence of residual stress, a common feature in the coatings deposited by sputtering. Microstructure of the coating appears not to be affected by the change in thickness of the layers, e.g. columnar grains exist in the thinner as well as in thicker layers. However, Nordin and Ericson [16] showed that microstructure of TiN/TaN multilayer depends on bilayer period. For bilayer periods thinner than
Fig. 1. Cross-sectional (a) BF and (b) DF image of TiAlN/CrN on steel substrate and corresponding diffraction pattern. Darker layers on BF image represent CrN and brighter layers TiAlN. Both layers have B1 NaCl-type structure. DF image reveals columnar microstructure of the coating.
M. Panjan et al. / Surface & Coatings Technology 202 (2007) 815–819
42 nm they obtained a columnar structure whereas for thicker periods the structure was more fine grained. 3.2. Crystal structure analysis Crystal structure of Ti1 − xAlxN depends on the content of aluminum. Up to x b 0.6 of aluminum Ti1 − xAlxN forms B1 NaCl-type structure (cubic phase), between 0.6 ≤ x ≤ 0.7 two phases coexist, one with NaCl-type structure and the other with AlN wurtzite structure (hexagonal phase). At x ≥ 0.7 only wurtzite structure forms [9,17]. CrNx crystallizes in a cubic B1 NaCl-type structure for x = 0.8–1.0 and in hexagonal Cr2N phase for x = 0.4–0.7 [18]. In superlattices other metastable phases can be stabilized over certain range of thicknesses due to the so-called template effect [19]. For example, Yashar et al. [2] showed that in Cr0.6N/TiN superlattice with a thickness of the layers below 10 nm, cubic Cr0.6N existed while in multilayers with 50 nm thick layers hexagonal Cr0.6N phase was stable. The nature of the coating crystallinity was analyzed using selected area diffraction, high resolution TEM and X-ray diffraction. On Fig. 1a the diffraction pattern is shown along with BF image. The diffraction rings correspond to the diffraction planes of the NaCl-type structure. The diffraction patterns taken from different areas of the coating showed only cubic phase and no other phases were observed. The same results were obtained by XRD. The diffraction rings corresponding to TiAlN and CrN could not be separated because difference in the lattice parameters was too small. Lattice parameter of TiAlN depends on the content of aluminum; it decreases with
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increasing Al content because substitution of smaller Al atoms with Ti atoms in TiN lattice reduces the size of the unit cell. In the case of pure TiN (x = 0) the lattice parameter is ∼0.423 nm and for x = 0.5 the lattice parameter is ∼0.419 nm [9,20]. Cubic CrN has a lattice parameter a = 0.414 nm and is almost independent of nitrogen content [21]. The lattice mismatch between stoichiometric TiAlN and CrN is ∼ 1%. From XRD measurements it was not possible to separate the lattice parameters of TiAlN and CrN but an average lattice parameter was approximately 0.417 nm. 3.3. Analysis of interfaces The same crystal structure of TiAlN and CrN and small mismatch between the lattices gives a good condition for epitaxial growth. Epitaxial growth is seen on the high resolution TEM image on Fig. 2a. Image shows the first TiAlN and CrN layers inside one columnar grain with steel substrate below. Interfaces between TiAlN and CrN are coherent. Although the interface between the steel and the coating appears distorted it is believed that in this case the distortion is only a Moiré pattern and that the interface is in fact coherent. Explanation for existence of Moiré pattern could be as follows. Ion etching of the steel substrate creates surface which is rough on atomic scale. When atoms of the coating material are deposited on such surface this causes an overlap of the steel and coating crystal structures. If such interface is viewed perpendicularly this gives rise to Moiré patterns. Additionally, small deviations from perpendicular view on perfectly flat interface also produce Moiré patterns.
Fig. 2. (a) High resolution image of TiAlN/CrN multilayer coating on steel substrate. Enlarged areas of coating and steel indicate epitaxial relationship between them. (b) Schematic diagram of CrN and Fe unit cells and epitaxial relationship between them: {001}Fe||{001}CrN and b100NFe||b110NCrN. Nearest Cr atoms in B1 NaCl-type CrN unit cell grow on corners of bcc-Fe unit cell.
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Both the coating and the substrate on Fig. 2a are in zone axis which indicates epitaxial relationship between them. From enlarged areas on Fig. 2a it can be seen that b110N directions in the coating (B1 NaCl) are parallel to the b100N directions of the steel substrate (bcc-Fe), whereas {001} planes of both coating and substrate are parallel to each other: epitaxial relationship is b100NFe||b110Ncoating and {001}Fe||{001}coating. This confirms that the substrate/coating interface has to be coherent because on incoherent interface planes of substrate and coating would not necessarily be parallel. The lattice parameter of iron is aFe ∼ 0.287 nm which is very similar to the distance between the second nearest neighbor atomspin pffiffiffi ffiffiffi the structure of CrN and TiAlN: aFe caCrN = 2caTiAlN = 2, e.g. for CrN the distance is dCr–Cr ∼ 0.293 nm (mismatch between aFe and dCr–Cr is ∼ 2%). Therefore it is very likely that the nearest metal atoms in CrN or TiAlN unit cell grow on the corners of bcc-Fe unit cell. Schematic diagram of this epitaxial relationship is shown on Fig. 2b. The {001} planes of the CrN unit cell are parallel to the {001} planes of the Fe unit cell, while the CrN unit cell is rotated for 45° around the interface normal. This kind of epitaxial relationship has also been observed between the layers of bcc-metal and NaCl-type nitride superlattices, e.g. Mo/NbN and W/NbN [22]. The energy of a coherent interface is smaller than energy of an incoherent interface therefore in those grains where steel substrate is favorably orientated TiAlN/CrN layers grow in the manner shown in Fig. 2b. Additionally, {001} planes of NaCl structure are the closest packed planes and therefore the energy needed for growth on this plane is the smallest. XRD measurements of the coating also showed the
strongest peaks to be (002) thus the coating had predominantly (002) texture. However, for non-favorable orientations of steel grains the interface between the steel and the coating can be incoherent. Coherent growth is also observed in the thicker layers (∼ 20 nm). Fig. 3 shows HRTEM image of a small columnar grain (width ∼ 15 nm) in the b110N zone axis. Bright layers correspond to TiAlN and dark layers to CrN. Inside the columnar grain epitaxy is seen. Images B–F on Fig. 3 show magnified parts of the coating inside the columnar grain with corresponding Fast Fourier Transformation (FFT) diagrams. Epitaxial relationship between the layers is clearly seen although it appears that TiAlN layers (C and F areas) are more distorted than CrN layers (B and D areas). Stronger distortion of TiAlN layers compared to CrN layers is even more pronounced in neighboring grains which are not orientated along the low index zone axis. This characteristic was observed in different areas of the coating. Possible explanation could be related with distortion of crystal structure. Alling et al. [23] calculated the displacement of atoms from the equilibrium positions in relaxed TiAlN supercell with random distributions of Ti and Al atoms on the metal sublattice. They showed that nitrogen atoms that were surrounded along the x axis by two metallic atoms of different type had the largest displacements, 1–2% of the average bond length. At the same time, the nitrogen atoms which were surrounded along the x axis by two chemically equivalent atoms showed only small displacements (0.1–0.3% of the average bond length). The metal atoms, which predominantly form HRTEM image, also showed only small
Fig. 3. High resolution image of a columnar grain in b110N zone axis. Inside the columnar grain epitaxy is seen. Dark layers are CrN and white layers TiAlN. Images A–F show magnified parts of the coating with corresponding FFT diagrams. In area A some dislocations in CrN layer are present.
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columnar structure in thin layers near the substrate as well as in the thicker layers. HAADF-STEM image analysis in the regions with thick and thin layers showed strong interfaces and no visible intermixing. TiAlN and CrN layers had B1 NaCl-type crystal structure and no other phases were observed. The same crystal structure and small mismatch between the TiAlN and CrN lattices provided good conditions for coherent growth inside the columnar grains. High resolution images showed that TiAlN layers appeared to be more distorted than CrN. In some areas epitaxial relationship between the steel substrate and the coating was observed. Epitaxial relationship was {001}Fe||{001}coating and b100NFe||b110Ncoating. Acknowledgements This work was sponsored by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia. References Fig. 4. High-angle annular dark-field STEM image of TiAlN/CrN layers next to steel substrate and in the center of coating. Bright areas represent CrN and dark areas TiAlN. Layers appear to be well differentiated.
displacements (∼ 0.5% of the average bond length). Therefore, the random distribution of metal atoms in TiAlN unit cell probably plays only a minor role in the observed distortion. The other reason for the distortion is probably related to strong point and/or line defects which more significantly distort the crystal structure. However, it is not clear why the concentration of defects in TiAlN would be higher than in CrN. On image A of Fig. 3 individual dislocations in the CrN layer can be seen. 3.4. Chemical analysis The HAADF-STEM images (Z-contrast images) provide a clear contrast in atomic number of elements present in layers. Fig. 4 shows HAADF STEM image of the layer structure near the substrate and in the central region of the coating. Bright areas correspond to the CrN and dark areas to the TiAlN layers. In both cases the contrast between CrN and TiAlN layers is sharp indicating that the layers are well differentiated with no visible intermixing. 4. Summary Multilayer coatings with variable bilayer period were prepared by magnetron sputtering. Microstructure, crystal structure and chemical composition were investigated using TEM. A comparison between thin layers near the substrate (2–10 nm) and thicker layers (20–25 nm) was made. Coatings had a
[1] M. Nordin, M. Larsson, S. Hogmark, Surf. Coat. Technol. 106 (1998) 234. [2] P. Yashar, X. Chu, S.A. Barnett, J. Rechner, Y.Y. Wang, M.S. Wong, W.D. Sproul, Appl. Phys. Lett. 72 (8) (1998) 987. [3] J.O. Kim, J.D. Achenbach, M. Shinn, S.A. Barnett, J. Mater. Res. 7 (3) (1992) 2248. [4] H.C. Barshilia, K.S. Rajam, D.V. Sridhara Rao, Surf. Coat. Technol. 200 (2006) 4586. [5] L. Geyang, L. Jijuan, T. Jaiwan, H. Zenghu, G. Mingyuan, J. Appl. Phys. 95 (1) (2004) 92. [6] H.C. Barshilia, M.S. Parkash, A. Jain, K.S. Rajam, Vacuum 77 (2005) 169. [7] D.B. Lewis, I. Wadsworth, W.D. Münz, R. Kuzel, V. Valvoda, Surf. Coat. Technol. 116–119 (1999) 284. [8] P.C. Yashar, W.D. Sproul, Vacuum 55 (1999) 179. [9] Ay. Kimura, Hi. Hasegawa, Ku. Yamada, Te. Suzuki, Surf. Coat. Technol. 120121 (1999) 438. [10] K. Kutschej, P.H. Mayrhofer, M. Kathrein, P. Polcik, R. Tessadri, C. Mitterer, Surf. Coat. Technol. 200 (2005) 2358. [11] I. Milošev, H.H. Strehblow, B. Navinšek, Surf. Coat. Technol. 74–75 (1995) 897. [12] I. Wadsworth, I.J. Smith, L.A. Donohue, W.D. Münz, Surf. Coat. Technol. 94–95 (1997) 315. [13] M. Panjan, T. Peterman, M. Čekada, P. Panjan, Plasma Process Polym 4 (S1) (2007) S921. [14] U. Helmersson, J.E. Sundgren, J. Electron. Micro. Technol. 4 (1986) 361. [15] S.J. Pennycook, D.E. Jesson, Ultramicroscopy 37 (1991) 14. [16] M. Nordin, F. Ericson, Thin Solid Films 385 (2001) 174. [17] Mi. Zhou, Y. Makino, M. Nose, K. Nogi, Thin Solid Films 339 (1999) 203. [18] P.M. Fabis, R.A. Cooke, S. McDonough, J. Vac. Sci. Technol., A 8 (1990) 3809. [19] G. Li, J. Lao, J. Tian, Z. Han, M. Gu, J. Appl. Phys. 95 (1) (2004) 92. [20] P.H. Mayrhofer, D. Music, J.M. Schneider, J. Appl. Phys. 100 (2006) 094906. [21] H. Era, Y. Ide, A. Nino, K. Kishitake, Surf. Coat. Technol. 194 (2005) 265. [22] A. Madan, W. Yun-yu, S.A. Barnett, C. Engström, H. Ljungcrantz, L. Hultman, M. Grimsditch, J. Appl. Phys. 84 (2) (1998) 776. [23] B. Alling, A.V. Ruban, A. Karimi, O.E. Peil, S.I. Simak, L. Hultman, I.A. Abrikosov, Phys. Rev. B 75 (2007) 045123.