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Microstructural investigations of polycrystalline Ti2AlN prepared by physical vapor deposition of Ti-AlN multilayers
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Microstructural investigations of polycrystalline Ti2AlN prepared by physical vapor deposition of Ti-AlN multilayers Lukas Grönera,⁎, Lutz Kirsteb, Sabine Oesera, Alexander Fromma, Marco Wirtha, Frank Meyera, Frank Burmeistera, Chris Eberla a b
Fraunhofer-Institut für Werkstoffmechanik IWM, Woehlerstrasse 11, 79108 Freiburg, Germany Fraunhofer-Institut für Angewandte Festkörperphysik IAF, Tullastrasse 72, 79108 Freiburg, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: MAX phase Ti2AlN PVD Multilayer deposition Microstructure
Ti2AlN is a prominent ternary nitride and belongs to the class of nanolaminated Mn + 1AXn phase materials which combine metallic and ceramic properties. In this work we report on the successful synthesis of polycrystalline Ti2AlN thin films with a preferential (000l) orientation on a polycrystalline Al2O3 substrate by depositing multiple Ti-AlN double layers and applying a subsequent annealing step. Investigations with scanning electron microscopy (SEM), X-ray diffraction (XRD), electron back scatter diffraction (EBSD) and Raman spectroscopy reveal a successful transformation of the multilayer system into a polycrystalline and dense Ti2AlN coating with a thickness of 2.7 μm. The observed grains are plate-like shaped with an in-plane size of about 100 to 300 nm and a thickness of 30 to 60 nm. Furthermore EBSD measurements proof that these macroscopic grains have a preferred orientation in the [000l] direction. We believe that a (000l)-textured microstructure will lead to new applications for protective coatings on polycrystalline substrates.
1. Introduction
wear, corrosion and indiffusion of oxygen and hydrogen. Especially Al containing MAX phases featuring a (000l) texture are most stable at high temperatures when compared to coatings with different textures. Since Al atoms diffuse preferentially in the Al basal planes [19], a (000l) texture impedes the desorption of Al atoms from the sample surface and thus decelerates Al depletion and void formation. Further anisotropies regarding the diffusion of hydrogen and oxygen within textured MAX phase materials were recently discovered by Colonna and Elsässer [13]. Particularly the diffusion of hydrogen is reduced by an order of magnitude for a diffusion perpendicular to the basal planes when compared to a diffusion along the basal planes. When it comes to quality and ease-of-use [20,2], the synthesis and application of MAXphase materials in the form of thin films and coatings by means of PVDtechniques is still under investigation and can be done by different methods which were summarized e.g. by Eklund et al. in [5]. The best crystallinity of Ti2AlN MAX phase PVD-coatings was achieved by heteroepitaxial deposition [21] or topotaxial growth [22] on single crystalline substrates at high temperatures. These coatings are typically very thin (~ 300 nm in [12]) and are difficult to transfer to applications. Thicker MAX phase coatings with a preferred orientation on polycrystalline or amorphous substrates have been synthesized by a deposition of amorphous Ti and AlN multilayers with a subsequent annealing step which led to interdiffusion of the layers and formation of
Mn + 1AXn phases are known to have a high oxidation resistance as well as a high thermal and electrical conductivity. Furthermore, they are damage tolerant, ductile and machineable [1]. This combination of metallic and ceramic properties opens up a wide field for applications, e.g. as protective coatings [2–6]. The general formula, Mn + 1AXn, describes a material consisting of an early transition metal (M), mostly a group IIIA- or IVA-element (A) and nitrogen and/or carbon (X) with the stoichiometry of n = 1,2,3 [7]. These MAX phases crystallize in a hexagonal lattice within the space group D46h (P63/mmc) in which the octahedral Mn + 1Xn layers are separated by atomic monolayers of pure A-atoms. Ti2AlN is a ternary nitride and a prominent member of the nanolaminated MAX phases. The alternating mixture of metallic Al-Al and covalent Ti2N bonds but weakly bonded Ti-Al allow for an interesting set of properties. However, in order to fully exploit the properties of the MAX phase materials with respect to possible applications, their intrinsic anisotropy due to the 2D nanolaminated lattice structure also has to be considered and implemented in the coating. Various anisotropic material properties of MAX phases have already been investigated by other authors (e.g. conductance [8,9], corrosion [10–12], diffusion [13,14], dielectric [15] and mechanical properties [16–18]). Therein it is reported that a (000l) textured surface is less prone to
⁎
Corresponding author. E-mail address: [email protected] (L. Gröner).
http://dx.doi.org/10.1016/j.surfcoat.2017.09.042 Received 29 June 2017; Received in revised form 15 September 2017; Accepted 18 September 2017 0257-8972/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Gröner, L., Surface & Coatings Technology (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.09.042
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Fig. 1. XRD diffraction pattern of the as-deposited Ti-AlN multilayer and annealed Ti2AlN coating on polycrystalline Al2O3.
camera. The EBSD patterns were recorded at 20 kV at a working distance of 10 mm and sample tilt of 70° with respect to the horizontal. Prior to the measurement, the sample surface was polished with a 0.1 μm oxide polishing suspension. The crystallographic orientation and phase composition of the coatings were investigated by X-ray diffractometry (XRD) using a Panalytical Empyrean in Bragg-Brentano geometry and Cu Kα1 radiation with a 2-bounce Ge 220 monochromator. The samples were irradiated with primary X-rays using a line focus. The diffracted X-rays were detected using a PIXcel-3D detector with a 1 mm slit for the phase analysis and a 10 mm slit for the pole figures. Furthermore confocal Raman spectroscopy (InVia Renishaw) in backscatter geometry was performed. The Raman spectrometer is equipped with a frequency doubled Nd:YAG-laser (λexc = 532 nm) and a 100 fold objective which allows to focus the incident laser to ~1 μm in diameter at a power of 6.5 ± 1.0 mW. The first-order Raman modes were measured in the Z(Y, X + Y)Z configuration, whereas the Z-direction is parallel to the sample surface normal.
Ti2AlN. This multilayer approach was described by Grieseler et al. [23]. Similar deposition techniques for Ti2AlN and Ti2AlC were recently reported by Cabioch et al. [24] and Tang et al. [25]. The latter observed that their Ti2AlC coatings exhibited a distinct texture with a preferential (000l) orientation. In this study, we also used a multilayer approach to synthesize Ti2AlN MAX phase coatings on polycrystalline Al2O3 substrates.
1.1. Experimental details Ti2AlN was synthesized as representative for MAX-phase materials on polished polycrystalline Al2O3-substrates. The deposition was carried out in a custom build industrial sized magnetron sputter chamber (FHR SV400/S3). The chamber was equipped with an internal radiation heater. The base pressure was 2 ∗ 10− 7 mbar. Prior to the deposition, the targets were operated in a metallic sputter mode for 10 min to remove residual contaminants on the target surface. Afterwards, the substrates were plasma etched for 10 min at a bias voltage of 320 V and a pressure of 2 ∗ 10− 2 mbar. Then, an Al base layer was deposited at 200 V and RF-bias power of 200 W followed by a preheating of the substrates at 500 °C for 10 min. Alternating Ti and AlN layers were deposited by DC and reactive HF sputtering from elemental Ti and Al targets (size 457 × 88.9 × 10 mm3) respectively by commuting the substrate in front of the targets. The surface power density during the DC sputtering of Ti and the HF sputtering of Al was 2.46 W/cm2. Argon (flow: 50 sccm, purity 5N) was continuously used as a process gas whereas nitrogen (flow: 7 sccm, purity 6N) was used as reactive gas while sputtering from the Al target to form AlN. During the deposition the pressure was kept constant at 8 ∗ 10− 3 mbar. In between the deposition of the AlN and Ti layers, a getter step was introduced to avoid a TiN formation due to remaining traces of nitrogen gas during deposition. The overall stoichiometry was set to 2:1:1 for Ti:Al:N. After the deposition of 75 Ti-AlN double layers, the samples were annealed inside the sputter chamber (~ 5 ∗ 10− 7 mbar) for 30 min at 700 °C with a radiation heater from the backside. Then another 75 Ti-AlN double layers were deposited on top to increase the total thickness of the coating to approximately 3 μm. The annealing step was repeated for 1 h. The motivation for two separate annealing steps during the deposition process is a desired reduction of the number of thermal stress induced cracks in the coating. Results corroborating this hypothesis are not shown in this publication due to space limitations. In order to examine the coating's microstructure and composition before the annealing step, an as-deposited sample was removed from the sputter chamber after the deposition of the first set of 75 double layers. For comparison, a second, fully annealed sample was prepared in another deposition process. Then, both samples were examined with a scanning electron microscope (FE-SEM, Zeiss SmartSEM, Supra 40VP) equipped with an energy dispersive X-ray spectrometer (Genesis EDAX). To investigate the grain dependent orientation, electron back scatter diffraction (EBSD) measurements were performed with an EDAX DigiView
2. Results A XRD phase analysis was performed and demonstrates the successful synthesis of the Ti2AlN MAX phase. Fig. 1 shows in comparison a 2Θ-Θ-scan in the range of 10–80° of the as-deposited and annealed sample. Fig. 1 (b) shows the 2Θ-range from 36 to 44° in more detail. Therein, besides the α-Al2O3 substrate peaks, the as-deposited sample exhibits only few broad peaks between 38 and 40° which originate probably from a Al 111 reflection at 2Θ = 38.2° and amorphic Ti-Al intermetallic phases. The observed diffraction peaks of the annealed sample can be unambiguously assigned to Ti2AlN (powder diffraction file PDF 04-019-0884) and polycrystalline Al2O3 (PDF 04-015-8608). No competing or metastable phases like TiN, Ti2N or Ti3AlN, which were reported earlier in [26] or [27], were detected. The high intensities of the 000l peaks are assigned to a preferential texture wherein the Al basal planes are oriented parallel to the substrate surface. Lower intensity peaks can be assigned to the (1016) and (1019) net planes. In order to clarify a possible peak overlap at ~39.6° resulting from the (0006) and (1013) net planes, a pole figure was recorded. The pole figure of the (0006) net plane is depicted in Fig. 2. The pole figure possesses a global maximum in the center and a local maximum at χ ≈ 57°. This dependency indicates a preferred (000l) orientation where the basal planes of the Ti2AlN crystal are aligned parallel to the substrate surface and the previous deposited multilayers. A minor contribution to the peak at 2Θ = 39.6° originates from the 1013 reflection since the local maximum at χ ≈ 57° is quite close to the expected angle of χ ≈ 60°. The difference might stem from intrinsic stresses. Due to the homogenous intensities at χ ≈ 57° for Φ = 0–360° no in-plane orientation of the film can be observed. To obtain spatially resolved information about the crystallographic orientation of the single grains, additional EBSD scans were performed. 2
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Fig. 2. 0006 Pole figure of the annealed sample at 2Θ = 39.6° (0° < χ < 80°).
Fig. 3. EBSD map of the polished Ti2AlN surface.
grains have a thickness ranging from 30 nm to 60 nm. Furthermore there is no evidence that the grains of the polycrystalline Al2O3 substrate, with a size of 400 nm–600 nm, have any influence on the coatings growth and orientation. The shapes of the Ti2AlN grains seem to be uniform throughout the entire layer. The total thickness of the coating was measured to 2.7 μm. The elemental composition of the coating was measured on both the as-deposited and the annealed sample by EDX. The results are given in Table 1. The composition of the annealed sample matches the desired Ti2AlN stoichiometry within the measurement uncertainties of ± 3at%. However, the annealing seems to reduce the nitrogen content probably due to desorption from the surface after dissociation of AlN. A reason for an increase of the titanium content might be both the intermixing of the terminating AlN surface layer with Ti and the simultaneous evaporation of the Al content. The Raman spectrum of the annealed sample is depicted in Fig. 5 and features three distinct peaks, which were first described by Presser et al. [28]. for bulk Ti2AlN-samples. The peaks were assigned to vibrational modes of Ti2AlN. Two peaks of the E2g mode (ω1, ω2) and one A1g (ω4) mode are visible. The E1g (ω3) mode is not detectable in the applied configuration. Besides the typical MAX-phase peaks additional Raman modes with very low intensities appear at 190 cm− 1 and between 500 cm− 1 and 600 cm− 1. In Fig. 5 the Raman spectrum of an
With EBSD, see Fig. 3, domains of 100 nm–300 nm size with a pronounced orientation in the [0001] direction can be observed. In between the grains, some areas could not be assigned clearly to a certain orientation, probably due to insufficient polishing or surface contaminations. The SEM images in Fig. 4 (a) and (c) depict the surface of the asdeposited and the annealed sample, whereas Fig. 4 (b) and (d) depict the fracture cross-section of the as-deposited and the annealed sample. The as-deposited sample exhibits a rough surface with columnar structure. In some columns, the stacking of the Ti and AlN single layers can be observed. SEM-investigations of the annealed sample reveal a change in the microstructure resulting in a polycrystalline and dense coating without the typical columnar morphology. In Fig. 4 (c) the lateral size of the grains can be estimated to a size ranging from 100 to 300 nm. The lateral dimensions correlate quite well with the observed grain sizes in the EBSD images (see Fig. 4). Furthermore, the surface is partially covered with some randomly distributed clusters of small spherulitic particles. These particles are located at grain boundaries and on top of single grains. In the SEM image of the fracture cross-section, Fig. 4 (d), the grains appear plate-like and are mainly oriented parallel to the substrate surface. Despite of the two step deposition process, with the first set of 75 double layers effectively being annealed twice, no difference in the morphology is visible along the cross-section. All
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Fig. 4. SEM images of the (a) as-deposited coating surface (top view), (b) as-deposited fracture cross-section of the coating (side view); (c) annealed coating surface (top view), (d) annealed cross-section of the coating (side view).
Table 1 Elemental composition measured by EDX.
Table 2 Results of the mixed Gaussian-Lorentzian fitting analysis with the centers of the peak positions and linewidths. Numbers in brackets depict literature values [28].
Atomic composition [at%]
As-deposited Annealed
Ti
Al
N
45.3 48.6
27.4 27.1
27.3 24.3
Vib. mode
Center [cm− 1]
Linewidth [cm− 1]
ω1 ω2 ω3 ω4
150.1 (149.7) 234.5 (234,2) - (-) 364.3 (365.3)
5.5 (5.2) 9.8 (9.3) - (-) 16.8 (15.4)
(E2g) (E2g) (E1g) (A1g)
crystallinity of the thin film can be clearly seen.
3. Discussion The experimental investigations clearly reveal that an insitu annealing of a sputter-deposited Ti-AlN-multilayer coating induces a well ordered Ti2AlN MAX phase with preferential orientation even on polycrystalline Al2O3 substrates. The XRD and EBSD measurements confirm a preferential (000l) orientation of the plate-like grains observed with a mainly single phase morphology. Especially the successful detection of an EBSD pattern demonstrates the high crystalline quality of the synthesized films, also when compared to previous literature results for MAX phase coatings synthesized on polycrystalline or amorphous substrates [23,25,29]. EBSD maps of MAX-phases have only been published for bulk materials [11,16,30–32], where larger grain sizes simplify the detecion of EBSD patterns. The diameter of the platelike grains, which can be estimated by the SEM images and the EBSD maps, lies in the range of 100 nm–300 nm. The grain thickness can only be estimated by the SEM images to 30 nm–60 nm. However, this interpretation has to be taken with care due to a possible delamination in the Al/Ti2N interfaces during the preparation of the fracture surface. This delamination was already observed in bulk materials and is caused
Fig. 5. Gaussian-Lorentzian fitting (red line) of measured Raman spectra (black crosses) from an annealed sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
annealed Ti2AlN sample was fitted with a mixed Gaussian-Lorentzian routine with a linear background subtraction. The values for the calculated peak positions and the linewidths of the characteristic Ti2AlN Raman modes are depicted in Table 2. The numbers in brackets are taken from literature values [28] for comparison. The excellent 4
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References
by the weak Ti-Al bonding in the MAX phase lattice [1]. Further TEM images could clarfy this question. The difference in the grain dimensions, when comparing the lateral and transversal direction of the platelike grains, could originate from a higher growth rate in the lateral direction [2]. To further increase the grain size we assume that the main issue is the proper stoichiometry of the multilayered stack. In the present study the stoichiometry, measured by EDX is close to the desired 2:1:1 values. However, we assume an error of ± 3 at% in the atomic contents due to the principal limitations of EDX for light elements like nitrogen. The Raman measurements also revealed a good crystalline structure with sharp individual peaks and narrow linewidths, which are very close to the values for bulk materials, which were prepared by hot isostatic pressing [28,33]. These bulk samples developed a polycrystalline microstructure with no preferential orientation and an average grain size of 100 μm. In the present Raman analysis only a few minor deviations to the fit cannot be matched to the Ti2AlN vibrational modes. These modes can be referred to surface oxides and/or TiC which might form as exclusions on the coatings surface [34]. Possibly these peaks correspond to the spherulitic particles which were found in SEM-investigations on the sample surface. Similar clustering of small particles around larger surface features was also observed by Frodelius et al. [10] and was assigned to surface oxides. A possible explanation for the formation of a texture was recently given by Tang et al. [25]. They assumed a competetive growth, where the face with the lowest growth rate determines the orientation of the grains. They also argued that the (000l) surfaces have the lowest surface energy and therefore tend to terminate the grain surface. However, in the work of Yang et al. [29], a homogenous film of Ti-Al-N was deposited by magnetron sputtering from a Ti- and TiAl targets using N2 as reactive gas. After a subsequent annealing at 700 °C, they could not detect a preferred orientation in the [000l] direction. So we assume that also the elemental ordering in the as-deposited coating of AlN and Ti, which is due to the multilayer deposition, plays a crucial role in the induction of a preferential texture of the MAX phase coating. We assume that during the synthesis, there is a competetive growth of grains with randomly distributed orientations. But only these grains, which find a spatial homogeneous elemental composition parallel to the fast growing direction, which is the c direction, can expand unhindered. Therefore the multilayered deposition might adjust the texture.
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3.1. Conclusion and outlook In this work, we have investigated the synthesis of Ti2AlN by a multilayer deposition of Ti and AlN on polycrystalline Al2O3 with a subsequent annealing step. We found that the microstructure of the coating is polycrystalline, almost single phase, dense and has a preferred orientation in the [000l] direction, whereas the basal planes are parallel to the substrate surface. We conclude in context of existing literature that the multilayered deposition induces the texture. Further investigations need to be done in order to elucidate how the details of the multilayer deposition, e.g. the double layer thickness, influences the evolving microstructure. Being able to synthesize a textured MAX phase coating on e.g. ferritic stainless steels would widen the possibilities for the use of MAX phase coatings as protective layers, e.g. on steel interconnectors in solid oxide fuel cells (SOFC) [35]. Due to the lattice anisotropy the (000l) texture should further reduce the effective diffusion and the corrosion.
Acknowledgment Financial support by the Baden-Württemberg-Stiftung gGmbH in the context of "CleanTech" (project CT-6 "LamiMat") is gratefully acknowledged.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructural investigations of polycrystalline Ti2AlN prepared by physical vapor deposition of Ti-AlN multilayers Lukas Grönera,⁎, Lutz Kirsteb, Sabine Oesera, Alexander Fromma, Marco Wirtha, Frank Meyera, Frank Burmeistera, Chris Eberla a b
Fraunhofer-Institut für Werkstoffmechanik IWM, Woehlerstrasse 11, 79108 Freiburg, Germany Fraunhofer-Institut für Angewandte Festkörperphysik IAF, Tullastrasse 72, 79108 Freiburg, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: MAX phase Ti2AlN PVD Multilayer deposition Microstructure
Ti2AlN is a prominent ternary nitride and belongs to the class of nanolaminated Mn + 1AXn phase materials which combine metallic and ceramic properties. In this work we report on the successful synthesis of polycrystalline Ti2AlN thin films with a preferential (000l) orientation on a polycrystalline Al2O3 substrate by depositing multiple Ti-AlN double layers and applying a subsequent annealing step. Investigations with scanning electron microscopy (SEM), X-ray diffraction (XRD), electron back scatter diffraction (EBSD) and Raman spectroscopy reveal a successful transformation of the multilayer system into a polycrystalline and dense Ti2AlN coating with a thickness of 2.7 μm. The observed grains are plate-like shaped with an in-plane size of about 100 to 300 nm and a thickness of 30 to 60 nm. Furthermore EBSD measurements proof that these macroscopic grains have a preferred orientation in the [000l] direction. We believe that a (000l)-textured microstructure will lead to new applications for protective coatings on polycrystalline substrates.
1. Introduction
wear, corrosion and indiffusion of oxygen and hydrogen. Especially Al containing MAX phases featuring a (000l) texture are most stable at high temperatures when compared to coatings with different textures. Since Al atoms diffuse preferentially in the Al basal planes [19], a (000l) texture impedes the desorption of Al atoms from the sample surface and thus decelerates Al depletion and void formation. Further anisotropies regarding the diffusion of hydrogen and oxygen within textured MAX phase materials were recently discovered by Colonna and Elsässer [13]. Particularly the diffusion of hydrogen is reduced by an order of magnitude for a diffusion perpendicular to the basal planes when compared to a diffusion along the basal planes. When it comes to quality and ease-of-use [20,2], the synthesis and application of MAXphase materials in the form of thin films and coatings by means of PVDtechniques is still under investigation and can be done by different methods which were summarized e.g. by Eklund et al. in [5]. The best crystallinity of Ti2AlN MAX phase PVD-coatings was achieved by heteroepitaxial deposition [21] or topotaxial growth [22] on single crystalline substrates at high temperatures. These coatings are typically very thin (~ 300 nm in [12]) and are difficult to transfer to applications. Thicker MAX phase coatings with a preferred orientation on polycrystalline or amorphous substrates have been synthesized by a deposition of amorphous Ti and AlN multilayers with a subsequent annealing step which led to interdiffusion of the layers and formation of
Mn + 1AXn phases are known to have a high oxidation resistance as well as a high thermal and electrical conductivity. Furthermore, they are damage tolerant, ductile and machineable [1]. This combination of metallic and ceramic properties opens up a wide field for applications, e.g. as protective coatings [2–6]. The general formula, Mn + 1AXn, describes a material consisting of an early transition metal (M), mostly a group IIIA- or IVA-element (A) and nitrogen and/or carbon (X) with the stoichiometry of n = 1,2,3 [7]. These MAX phases crystallize in a hexagonal lattice within the space group D46h (P63/mmc) in which the octahedral Mn + 1Xn layers are separated by atomic monolayers of pure A-atoms. Ti2AlN is a ternary nitride and a prominent member of the nanolaminated MAX phases. The alternating mixture of metallic Al-Al and covalent Ti2N bonds but weakly bonded Ti-Al allow for an interesting set of properties. However, in order to fully exploit the properties of the MAX phase materials with respect to possible applications, their intrinsic anisotropy due to the 2D nanolaminated lattice structure also has to be considered and implemented in the coating. Various anisotropic material properties of MAX phases have already been investigated by other authors (e.g. conductance [8,9], corrosion [10–12], diffusion [13,14], dielectric [15] and mechanical properties [16–18]). Therein it is reported that a (000l) textured surface is less prone to
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Corresponding author. E-mail address: [email protected] (L. Gröner).
http://dx.doi.org/10.1016/j.surfcoat.2017.09.042 Received 29 June 2017; Received in revised form 15 September 2017; Accepted 18 September 2017 0257-8972/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Gröner, L., Surface & Coatings Technology (2017), http://dx.doi.org/10.1016/j.surfcoat.2017.09.042
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Fig. 1. XRD diffraction pattern of the as-deposited Ti-AlN multilayer and annealed Ti2AlN coating on polycrystalline Al2O3.
camera. The EBSD patterns were recorded at 20 kV at a working distance of 10 mm and sample tilt of 70° with respect to the horizontal. Prior to the measurement, the sample surface was polished with a 0.1 μm oxide polishing suspension. The crystallographic orientation and phase composition of the coatings were investigated by X-ray diffractometry (XRD) using a Panalytical Empyrean in Bragg-Brentano geometry and Cu Kα1 radiation with a 2-bounce Ge 220 monochromator. The samples were irradiated with primary X-rays using a line focus. The diffracted X-rays were detected using a PIXcel-3D detector with a 1 mm slit for the phase analysis and a 10 mm slit for the pole figures. Furthermore confocal Raman spectroscopy (InVia Renishaw) in backscatter geometry was performed. The Raman spectrometer is equipped with a frequency doubled Nd:YAG-laser (λexc = 532 nm) and a 100 fold objective which allows to focus the incident laser to ~1 μm in diameter at a power of 6.5 ± 1.0 mW. The first-order Raman modes were measured in the Z(Y, X + Y)Z configuration, whereas the Z-direction is parallel to the sample surface normal.
Ti2AlN. This multilayer approach was described by Grieseler et al. [23]. Similar deposition techniques for Ti2AlN and Ti2AlC were recently reported by Cabioch et al. [24] and Tang et al. [25]. The latter observed that their Ti2AlC coatings exhibited a distinct texture with a preferential (000l) orientation. In this study, we also used a multilayer approach to synthesize Ti2AlN MAX phase coatings on polycrystalline Al2O3 substrates.
1.1. Experimental details Ti2AlN was synthesized as representative for MAX-phase materials on polished polycrystalline Al2O3-substrates. The deposition was carried out in a custom build industrial sized magnetron sputter chamber (FHR SV400/S3). The chamber was equipped with an internal radiation heater. The base pressure was 2 ∗ 10− 7 mbar. Prior to the deposition, the targets were operated in a metallic sputter mode for 10 min to remove residual contaminants on the target surface. Afterwards, the substrates were plasma etched for 10 min at a bias voltage of 320 V and a pressure of 2 ∗ 10− 2 mbar. Then, an Al base layer was deposited at 200 V and RF-bias power of 200 W followed by a preheating of the substrates at 500 °C for 10 min. Alternating Ti and AlN layers were deposited by DC and reactive HF sputtering from elemental Ti and Al targets (size 457 × 88.9 × 10 mm3) respectively by commuting the substrate in front of the targets. The surface power density during the DC sputtering of Ti and the HF sputtering of Al was 2.46 W/cm2. Argon (flow: 50 sccm, purity 5N) was continuously used as a process gas whereas nitrogen (flow: 7 sccm, purity 6N) was used as reactive gas while sputtering from the Al target to form AlN. During the deposition the pressure was kept constant at 8 ∗ 10− 3 mbar. In between the deposition of the AlN and Ti layers, a getter step was introduced to avoid a TiN formation due to remaining traces of nitrogen gas during deposition. The overall stoichiometry was set to 2:1:1 for Ti:Al:N. After the deposition of 75 Ti-AlN double layers, the samples were annealed inside the sputter chamber (~ 5 ∗ 10− 7 mbar) for 30 min at 700 °C with a radiation heater from the backside. Then another 75 Ti-AlN double layers were deposited on top to increase the total thickness of the coating to approximately 3 μm. The annealing step was repeated for 1 h. The motivation for two separate annealing steps during the deposition process is a desired reduction of the number of thermal stress induced cracks in the coating. Results corroborating this hypothesis are not shown in this publication due to space limitations. In order to examine the coating's microstructure and composition before the annealing step, an as-deposited sample was removed from the sputter chamber after the deposition of the first set of 75 double layers. For comparison, a second, fully annealed sample was prepared in another deposition process. Then, both samples were examined with a scanning electron microscope (FE-SEM, Zeiss SmartSEM, Supra 40VP) equipped with an energy dispersive X-ray spectrometer (Genesis EDAX). To investigate the grain dependent orientation, electron back scatter diffraction (EBSD) measurements were performed with an EDAX DigiView
2. Results A XRD phase analysis was performed and demonstrates the successful synthesis of the Ti2AlN MAX phase. Fig. 1 shows in comparison a 2Θ-Θ-scan in the range of 10–80° of the as-deposited and annealed sample. Fig. 1 (b) shows the 2Θ-range from 36 to 44° in more detail. Therein, besides the α-Al2O3 substrate peaks, the as-deposited sample exhibits only few broad peaks between 38 and 40° which originate probably from a Al 111 reflection at 2Θ = 38.2° and amorphic Ti-Al intermetallic phases. The observed diffraction peaks of the annealed sample can be unambiguously assigned to Ti2AlN (powder diffraction file PDF 04-019-0884) and polycrystalline Al2O3 (PDF 04-015-8608). No competing or metastable phases like TiN, Ti2N or Ti3AlN, which were reported earlier in [26] or [27], were detected. The high intensities of the 000l peaks are assigned to a preferential texture wherein the Al basal planes are oriented parallel to the substrate surface. Lower intensity peaks can be assigned to the (1016) and (1019) net planes. In order to clarify a possible peak overlap at ~39.6° resulting from the (0006) and (1013) net planes, a pole figure was recorded. The pole figure of the (0006) net plane is depicted in Fig. 2. The pole figure possesses a global maximum in the center and a local maximum at χ ≈ 57°. This dependency indicates a preferred (000l) orientation where the basal planes of the Ti2AlN crystal are aligned parallel to the substrate surface and the previous deposited multilayers. A minor contribution to the peak at 2Θ = 39.6° originates from the 1013 reflection since the local maximum at χ ≈ 57° is quite close to the expected angle of χ ≈ 60°. The difference might stem from intrinsic stresses. Due to the homogenous intensities at χ ≈ 57° for Φ = 0–360° no in-plane orientation of the film can be observed. To obtain spatially resolved information about the crystallographic orientation of the single grains, additional EBSD scans were performed. 2
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Fig. 2. 0006 Pole figure of the annealed sample at 2Θ = 39.6° (0° < χ < 80°).
Fig. 3. EBSD map of the polished Ti2AlN surface.
grains have a thickness ranging from 30 nm to 60 nm. Furthermore there is no evidence that the grains of the polycrystalline Al2O3 substrate, with a size of 400 nm–600 nm, have any influence on the coatings growth and orientation. The shapes of the Ti2AlN grains seem to be uniform throughout the entire layer. The total thickness of the coating was measured to 2.7 μm. The elemental composition of the coating was measured on both the as-deposited and the annealed sample by EDX. The results are given in Table 1. The composition of the annealed sample matches the desired Ti2AlN stoichiometry within the measurement uncertainties of ± 3at%. However, the annealing seems to reduce the nitrogen content probably due to desorption from the surface after dissociation of AlN. A reason for an increase of the titanium content might be both the intermixing of the terminating AlN surface layer with Ti and the simultaneous evaporation of the Al content. The Raman spectrum of the annealed sample is depicted in Fig. 5 and features three distinct peaks, which were first described by Presser et al. [28]. for bulk Ti2AlN-samples. The peaks were assigned to vibrational modes of Ti2AlN. Two peaks of the E2g mode (ω1, ω2) and one A1g (ω4) mode are visible. The E1g (ω3) mode is not detectable in the applied configuration. Besides the typical MAX-phase peaks additional Raman modes with very low intensities appear at 190 cm− 1 and between 500 cm− 1 and 600 cm− 1. In Fig. 5 the Raman spectrum of an
With EBSD, see Fig. 3, domains of 100 nm–300 nm size with a pronounced orientation in the [0001] direction can be observed. In between the grains, some areas could not be assigned clearly to a certain orientation, probably due to insufficient polishing or surface contaminations. The SEM images in Fig. 4 (a) and (c) depict the surface of the asdeposited and the annealed sample, whereas Fig. 4 (b) and (d) depict the fracture cross-section of the as-deposited and the annealed sample. The as-deposited sample exhibits a rough surface with columnar structure. In some columns, the stacking of the Ti and AlN single layers can be observed. SEM-investigations of the annealed sample reveal a change in the microstructure resulting in a polycrystalline and dense coating without the typical columnar morphology. In Fig. 4 (c) the lateral size of the grains can be estimated to a size ranging from 100 to 300 nm. The lateral dimensions correlate quite well with the observed grain sizes in the EBSD images (see Fig. 4). Furthermore, the surface is partially covered with some randomly distributed clusters of small spherulitic particles. These particles are located at grain boundaries and on top of single grains. In the SEM image of the fracture cross-section, Fig. 4 (d), the grains appear plate-like and are mainly oriented parallel to the substrate surface. Despite of the two step deposition process, with the first set of 75 double layers effectively being annealed twice, no difference in the morphology is visible along the cross-section. All
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Fig. 4. SEM images of the (a) as-deposited coating surface (top view), (b) as-deposited fracture cross-section of the coating (side view); (c) annealed coating surface (top view), (d) annealed cross-section of the coating (side view).
Table 1 Elemental composition measured by EDX.
Table 2 Results of the mixed Gaussian-Lorentzian fitting analysis with the centers of the peak positions and linewidths. Numbers in brackets depict literature values [28].
Atomic composition [at%]
As-deposited Annealed
Ti
Al
N
45.3 48.6
27.4 27.1
27.3 24.3
Vib. mode
Center [cm− 1]
Linewidth [cm− 1]
ω1 ω2 ω3 ω4
150.1 (149.7) 234.5 (234,2) - (-) 364.3 (365.3)
5.5 (5.2) 9.8 (9.3) - (-) 16.8 (15.4)
(E2g) (E2g) (E1g) (A1g)
crystallinity of the thin film can be clearly seen.
3. Discussion The experimental investigations clearly reveal that an insitu annealing of a sputter-deposited Ti-AlN-multilayer coating induces a well ordered Ti2AlN MAX phase with preferential orientation even on polycrystalline Al2O3 substrates. The XRD and EBSD measurements confirm a preferential (000l) orientation of the plate-like grains observed with a mainly single phase morphology. Especially the successful detection of an EBSD pattern demonstrates the high crystalline quality of the synthesized films, also when compared to previous literature results for MAX phase coatings synthesized on polycrystalline or amorphous substrates [23,25,29]. EBSD maps of MAX-phases have only been published for bulk materials [11,16,30–32], where larger grain sizes simplify the detecion of EBSD patterns. The diameter of the platelike grains, which can be estimated by the SEM images and the EBSD maps, lies in the range of 100 nm–300 nm. The grain thickness can only be estimated by the SEM images to 30 nm–60 nm. However, this interpretation has to be taken with care due to a possible delamination in the Al/Ti2N interfaces during the preparation of the fracture surface. This delamination was already observed in bulk materials and is caused
Fig. 5. Gaussian-Lorentzian fitting (red line) of measured Raman spectra (black crosses) from an annealed sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
annealed Ti2AlN sample was fitted with a mixed Gaussian-Lorentzian routine with a linear background subtraction. The values for the calculated peak positions and the linewidths of the characteristic Ti2AlN Raman modes are depicted in Table 2. The numbers in brackets are taken from literature values [28] for comparison. The excellent 4
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References
by the weak Ti-Al bonding in the MAX phase lattice [1]. Further TEM images could clarfy this question. The difference in the grain dimensions, when comparing the lateral and transversal direction of the platelike grains, could originate from a higher growth rate in the lateral direction [2]. To further increase the grain size we assume that the main issue is the proper stoichiometry of the multilayered stack. In the present study the stoichiometry, measured by EDX is close to the desired 2:1:1 values. However, we assume an error of ± 3 at% in the atomic contents due to the principal limitations of EDX for light elements like nitrogen. The Raman measurements also revealed a good crystalline structure with sharp individual peaks and narrow linewidths, which are very close to the values for bulk materials, which were prepared by hot isostatic pressing [28,33]. These bulk samples developed a polycrystalline microstructure with no preferential orientation and an average grain size of 100 μm. In the present Raman analysis only a few minor deviations to the fit cannot be matched to the Ti2AlN vibrational modes. These modes can be referred to surface oxides and/or TiC which might form as exclusions on the coatings surface [34]. Possibly these peaks correspond to the spherulitic particles which were found in SEM-investigations on the sample surface. Similar clustering of small particles around larger surface features was also observed by Frodelius et al. [10] and was assigned to surface oxides. A possible explanation for the formation of a texture was recently given by Tang et al. [25]. They assumed a competetive growth, where the face with the lowest growth rate determines the orientation of the grains. They also argued that the (000l) surfaces have the lowest surface energy and therefore tend to terminate the grain surface. However, in the work of Yang et al. [29], a homogenous film of Ti-Al-N was deposited by magnetron sputtering from a Ti- and TiAl targets using N2 as reactive gas. After a subsequent annealing at 700 °C, they could not detect a preferred orientation in the [000l] direction. So we assume that also the elemental ordering in the as-deposited coating of AlN and Ti, which is due to the multilayer deposition, plays a crucial role in the induction of a preferential texture of the MAX phase coating. We assume that during the synthesis, there is a competetive growth of grains with randomly distributed orientations. But only these grains, which find a spatial homogeneous elemental composition parallel to the fast growing direction, which is the c direction, can expand unhindered. Therefore the multilayered deposition might adjust the texture.
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3.1. Conclusion and outlook In this work, we have investigated the synthesis of Ti2AlN by a multilayer deposition of Ti and AlN on polycrystalline Al2O3 with a subsequent annealing step. We found that the microstructure of the coating is polycrystalline, almost single phase, dense and has a preferred orientation in the [000l] direction, whereas the basal planes are parallel to the substrate surface. We conclude in context of existing literature that the multilayered deposition induces the texture. Further investigations need to be done in order to elucidate how the details of the multilayer deposition, e.g. the double layer thickness, influences the evolving microstructure. Being able to synthesize a textured MAX phase coating on e.g. ferritic stainless steels would widen the possibilities for the use of MAX phase coatings as protective layers, e.g. on steel interconnectors in solid oxide fuel cells (SOFC) [35]. Due to the lattice anisotropy the (000l) texture should further reduce the effective diffusion and the corrosion.
Acknowledgment Financial support by the Baden-Württemberg-Stiftung gGmbH in the context of "CleanTech" (project CT-6 "LamiMat") is gratefully acknowledged.
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