Formation mechanisms of Ti2AlC MAX phase on SiC-4H using magnetron sputtering and post-annealing

Accepted Manuscript Formation mechanisms of Ti2AlC MAX phase on SiC-4H using magnetron sputtering and post-annealing

J. Nicolaï, C. Furgeaud, B.W. Fonrose, C. Bail, M.F. Beaufort PII: DOI: Reference:

S0264-1275(18)30137-0 doi:10.1016/j.matdes.2018.02.046 JMADE 3713

To appear in:

Materials & Design

Received date: Revised date: Accepted date:

24 December 2017 12 February 2018 15 February 2018

Please cite this article as: J. Nicolaï, C. Furgeaud, B.W. Fonrose, C. Bail, M.F. Beaufort , Formation mechanisms of Ti2AlC MAX phase on SiC-4H using magnetron sputtering and post-annealing. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi:10.1016/j.matdes.2018.02.046

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ACCEPTED MANUSCRIPT Formation mechanisms of Ti2AlC MAX phase on SiC-4H using magnetron sputtering and post-annealing J. Nicolaï,1) C. Furgeaud,1) B. W. Fonrose,2) C. Bail,1) and M.F. Beaufort1) 1

Institut Pprime, UPR 3346, Université de Poitiers, SP2MI-Boulevard 3, Téléport 2-BP 30179, 86962 Futuroscope

Chasseneuil Cedex, France CEMES-CNRS-UPR 8011, Université de Toulouse, 31055 Toulouse, France

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Abstract

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In the present work we focus on the mechanisms involved in Ti2 AlC MAX phase thin-film formation. The TiAl2 thin-film was deposited by magnetron sputtering on a SiC-4H [0001] substrate. Samples were annealed at various temperatures (700800°C) for various times and analysed by XRD and TEM. The epitaxial Ti2 AlC phase was formed as follows: [0001]MAX //

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[0001]SiC and (11-20)MAX//(11-20)SiC which is in a good agreement with thermodynamic considerations. The presence of TiC structures at the interface indicates that the formation of this structure is necessary to obtain Ti2 AlC. Moreover, the formation

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of a liquid AlSi alloy was highlighted due to the interdiffusion of Al and Si respectively from TiAl 2 and SiC during TiC formation. Finally, we assume that, during the cooling, the AlSi alloy separates and Al diffuses to the surface of the TiAl2 layer leading to the formation of an Al-rich layer. The remaining Si reacts with Ti from TiAl2 to form a Ti5 Si3 layer following

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this epitaxial relation: [0001]Ti2AlC // [0001]Ti5Si3 and (11-20)Ti2AlC//(3-210)Ti5Si3 . These mechanisms lead to the stacking of four different layers. Between 700 and 800 °C, the nature of the formation mechanism is not time-dependent. However, the

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kinetics of the reactions are both temperature and time dependent.

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Keywords: MAX phase, thin film, epitaxy, TEM, Magnetron Sputtering

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Introduction

M n+1 AXn phases (n=1-3) are in a large class of nanolaminated materials. M is an early transition metal element, A is an A -

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group element and X is either C or N [1-4]. For n=1, 2, 3 the MAX phases are called 211, 312 and 413 respectively due to the periodic arrangement of the structure: MX octahedrons with layers of the A element in a hexagonal structure. This particular arrangement gives the MAX phases a unique combination of metal and ceramic properties, opening the way to a large field of applications [5-7]. The Ti3 SiC2 and Ti2 AlC MAX phases have been extensively studied due to these excellent properties which include irradiation resistance [8-9]. Several different techniques have been used to synthesize bulk MAX phases . Of these techniques, the most common is hot isostatic pres sing (HIP) [10]; however, various techniques have been developed for the growth of MAX phase thin films using magnetron sputtering technology, either from elemental targets or from compound targets on various substrates [11-14]. As shown on our recent papers , the co-deposition of Ti and Al on SiC substrates leads, under specific annealing conditions [15-16], to the formation of Ti3 SiC2 films. The use of Al is known to be helpful to the

ACCEPTED MANUSCRIPT synthesis of Ti3 SiC2 , although the mechanism is not well known [17-18]. In the present study, we focus on the mechanisms involved in MAX phase formation at low temperatures. We demonstrate that a Ti 2 AlC thin layer is formed. Ti2 AlC formation was studied by XRD, STEM-HAADF, EDS and HRTEM as a function of the annealing temperature and time. The growth mechanism of the MAX phase is discussed by considering thermodynamic and kinetic effects.

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Method

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SiC-4H substrates, single crystal (0001), n-type, supplied by TANKEBLUE, were used in this study. Al and Ti were co -

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deposited (300 nm thick) at room temperature onto SiC substrates by magnetron sputtering using pure Al (99,999%) and Ti (99,995%) targets in a high vacuum system (residual pressure <2.10-7 Pa). The working pressure of Ar was 0.15 Pa

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corresponding to 3 sccm. The DC power supplied supplied to the Al and Ti targets was 158W and 300W respectively in order to obtain stoichiometry phase TiAl2 . Before deposition, the substrate was in situ cleaned by an etching at 60V for 600s. The

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samples were annealed in a vacuum lamp furnace (heating rate close to 20°C.s -1 ). The structural investigations were performed using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). Diffraction experiments were

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conducted on a D8 Brucker AXS diffractometer operating in the Bragg -Brentano geometry under atmosphere environment. The diffractometer operates with a Cu tube and the Kβ radiation is absorbed by a Ni filter in order to obtain a pure Cu Kα

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radiation (λ = 0.15418 nm). The LynxEye detector is used for ω-2θ scans with a slight offset to avoid SiC reflections (ω-

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θ=0.2°). Conventional, High-Resolution TEM and Scanning Transmission Electron Microscopy (STEM) images were carried out using a JEOL 2200 FS (Schottky-FEG, 200 kV), using an Energy Dispersive x-ray Spectroscopy (EDS) detector (Bruker

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Quantax X flash 6) and analytic single tilt-holder. The Al, Ti and Si atomic compositions were studied. Due to carbon contamination induced by STEM, the carbon was detected but not quantified. TEM samples were prepared by Focused Ion

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Beam (FIB) using an FEI-HELIOS dual-beam along the [11-20] direction using the standard lift-out method [19].

Results

TEM analyses show that the fresh deposited layer is 300 nm thick, with a global TiAl2 stoichiometry, composed of nanocrystalline grains. This assumption is confirmed by the homogeneous contrast of the layer in HAADF/BF-STEM images (without chemical or diffraction contrast) and by the presence of only one well-defined row in the associated diffraction pattern (not shown here). Fig. 1 shows X-ray diffractograms of TiAl2 on SiC annealed at 700 and 800°C for 10 to 60 min to

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ACCEPTED MANUSCRIPT produce the MAX phase. These diffractograms show two very intense peaks at 13.05° and 39.73° corresponding to (0002) and (0006) reflections of the Ti2 AlC structure. Only (000l-type peaks of the Ti2 AlC structure are present in these diffractograms. All the crystallites of the films have a growth direction oriented along the [0001] axis. The (0002) diffraction planes of the MAX phase present a full width at half-maximum (FWHM) of 0.5–1° indicating good crystallinity of the film. One peak is also present at 38.98° corresponding to the (116) reflection of the TiAl2 structure as described by Braun [20]. Only the (116) reflection of the TiAl2 structure is observed, which is expected as this reflection is theoretically the most

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intense one, as calculated with JEMS ©. Moreover, we can observe a peak at 39.15° due to SiC reflection.

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These peaks are observed for all the annealing conditions: times and temperatures. The intensity ratio of the peaks (I0002 /I116 ), plotted Fig. 1(c), has the same behaviour for 700°C and 800°C. The overall trend is an increase of the ratio as a function of

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annealing time. This indicates a relative increase in the amount of the Ti2 AlC phase compared to the TiAl2 phase with annealing time. This is more significant at higher annealing temperature. Due to the similarity of the XRD diffractograms , for

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the different annealing times, we assume that the mechanisms involved in MAX phase formation are similar for 700°C and

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800°C so we will just focus on the 800°C samples.

Figure 1: X-ray diffractograms of TiAl 2 (300nm of thickness) onto (0001) 4H-SiC annealed at 700 (a) and 800°C (b) from 10 to 60 min. Intensity ratio of peak Ti 2 AlC (0002) and TiAl 2 (116) for 700 and 800°C annealing temperature (c).

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ACCEPTED MANUSCRIPT Fig. 2 shows STEM micrographs of the sample annealed at 800°C for 60min. Four major layers can be identified: SiC-4H substrate; a very-well crystallized thin film; a hemispherical shaped, non-continuous layer; and a thicker layer composed of several grains. For the sake of simplicity these layers will be labelled 1, 2, 3 and 4 as shown on Fig. 2(c). The STEM-BF and STEM-HAADF micrographs (Fig. 2(a) and 2(b)) of this sample show that layer 4 is polycrystalline and chemically homogeneous. Between the deposited Pt and layer 4 a very-thin rough, chemically different layer can be seen in both BF and HAADF micrographs. This layer stacking is similar in the 700 and 800°C samples and for the different annealing times.

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However, the thickness ratio (layer 2/layer 4) increases as a function of the annealing time. Indeed, the MAX phase thickness

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increases from ∼10nm (for 10min) to ∼27nm (for 60min) while the TiAl2 thickness decreases from ∼295nm (for 10min) to

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∼278nm (for 60min) as measured on HRTEM micrographs.

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Figure 2: General view of one area obtained by BF-STEM (a) and HAADF-STEM (b). HAADF-STEM micrograph (c) highlighting the layer stacking. White arrow indicates the growth direction [0001]. Blue circles correspond to Alrich precipitates on the surface of the sample. Red dot lines represent interfaces between layers 1/2 – 2/3 and 3/4.

EDS cartographies for Al, Ti and Si, has been carried out on the same area as in Fig. 2(c). (Fig. 3). As shown on these maps

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the presence of Si is observed in layers 1 and 3; Ti in layers 2, 3 and 4; and Al in layers 2 and 4. The EDS results confirmed that layer 2 is the Ti2 AlC MAX phase with a small incorporation of Si atoms. Indeed, the ratio Ti/Al is very close to 2 as shown on the EDS profile in Fig. 3. Nevertheless, some traces of Si are also detected. This is in a good agreement with the known possibility of including a few Si atoms in the stable Ti2 AlC structure [21]. The composition of layer 3 is close to Ti5 Si3 . Layer 4, has a stoichiometry close to TiAl2 (33%Ti-66%Al). Finally, on top of layer 4 a thin Al-rich layer with some bigger Al-rich grains can be observed. The interface between each layer seems to be well defined. For example, the Si profile sharply decreases and increases at the interface between layers 1 and 2 (SiC and Ti2 AlC) and between layers 2 and 3.

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Figure 3 : EDS analysis of Al, Si and Ti. The corresponding elementary profile obtained along the withe arrows . The black dot lines limited the different layers described Fig. 2.

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HRTEM micrographs (Fig. 4) and associated FFT show that the first layer corresponds to the Ti 2 AlC MAX phase, which is consistent with the XRD results. This layer presents an epitaxial relationship with SiC [0001]MAX // [0001]SiC and (11-

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20)MAX//(11-20)SiC. HRTEM reveals a perturbed zone, containing a TiC layer at the interface between SiC and Ti2 AlC. The presence of TiC is also confirmed by the FFT which exhibits (220) planes of TiC structure. All the other diffracted spots can

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be attributed to the (116) family of planes of the TiAl2 structure. The micrograph in Fig. 4(c) is localized at the third layer and highlights a crystalline phase, corresponding to the Ti5 Si3 structure, as indexed on FFT Fig. 4(d). This layer exhibits an

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orientation relationship with the Ti2 AlC as follows: [0001]Ti2AlC // [0001]Ti5Si3 and (11-20)Ti2AlC//(3-210)SiC.

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Discussion

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Figure 4: HRTEM micrographs ([11-20] zone axis) of SiC/Ti 2 AlC interface (a), Ti 2 AlC/Ti 5 Si3 interface ([3-2-10]) (b) and respective associated Fast-Fourier-Transform (b and c) of 800°C-60min annealed sample. Red square corresponds to the (116) reflexion of TiAl 2 structure. Yellow arrows indicate the (1-100) and (0001) directions of SiC and Ti 2 AlC structures.

The growth mechanism of the Ti2 AlC MAX phase can be described as a multi-step process described in following reactions: 2 TiAl2(s) + SiC(s) → Ti2 AlC(s) + (Al3 Si)l The first step of this reaction consists of the formation of TiC: SiC(s) + Ti(s) → TiC(s) + Si(s) by considering that this reaction is thermodynamically favoured (∆𝐺 0 = −107 kJ) [22-23].

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ACCEPTED MANUSCRIPT This is in very good agreement with the observation of the TiC layer between the SiC and MAX phase layers. Moreover, simulations show that Al can be spontaneously intercalated into TiC leading to Ti 2 AlC [24]. Indeed, it is well-known that at low temperatures the 211 MAX phase is favoured [1]. In fact, two isomorphic MAX phases can be considered due to the elements used: Ti2 AlC and Ti2 SiC. The Ti2 SiC MAX phase is not thermodynamically stable [25], which therefore leads to the formation of the Ti2 AlC MAX phase [26]. This thermodynamic consideration is reinforced

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by EDS results, showing the absence of Si in the structure (Fig. 3).

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As shown, for these low temperatures, we obtain only the Ti2 AlC MAX phase, for all the annealing times. Thus, we assume

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that the formation mechanism is not time-dependant. Finally, the increase of annealing time and/or temperature (from 700 to 800°C) has the same effect: the thin film MAX phase thickness increases. This is observed on different STEM micrographs

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where the ratio layer 2/layer 4 (Ti2 AlC/TiAl2 ) increases for all the different formation conditions . This is in a good agreement with the evolution of the reflection peak intensity ratio observed in the XRD results.

misfit f equal to 𝑓 =

𝑎 𝑆𝑖𝐶 −𝑎 𝑀𝐴𝑋 𝑎 𝑆𝑖𝐶

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In addition, the epitaxial relation between SiC and Ti2 AlC: [0001]MAX // [0001]SiC and (11-20)MAX//(11-20)SiC leads to a = −0.7% considering a MAX = 0.30750 nm [27] and a SiC = 0.3050 nm [28]. We assume that

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this misfit is negligible. Furthermore, MAX phase peaks observed in the XRD diffractograms are perfectly centred on their

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theoretical position which can be related to an absence of residual stress. This epitaxial relation corresponds to a minimization of the residual stress and elastic strain leading to a stable system during the growth. The study of such residual

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stress needs a more accurate study such as in situ specimen curvature measurement. The evolution of substrate curvature gives access to the stress using the Stoney equation [29]. Recently Ghidelli et. al have developed a new method to measure ex

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situ residual stress using nanoindentation [30].

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Furthermore, due to the high concentration of Al, a liquid AlSi alloy is formed between the MAX phase and the TiAl2 . During this time, the nano-crystalline grains of the TiAl2 grow to form micro-crystalline TiAl2 grains approximately 400 nm in size as observed by STEM. The growth of the TiAl2 should lead to an increase in the intensity of the diffraction peaks. However, the consumption of part of the Ti and Al to form the Ti2 AlC will lead to a decrease of these peaks. This gives rise to an overall evolution of the intensity of the diffraction peaks which is non-monotonic. We assume that this fact could be responsible for the complex evolution of the intensity peaks ratio observed in Fig. 1(c). Although the annealing was carried out using a vacuum lamp furnace, the temperature ramp-down is relatively slow. During the cooling of the samples , the AlSi alloy separates due to the poor miscibility of the Al and the Si at very low temperatures (below 580°C) [31]. Due to this separation, Si atoms can react with Ti from the TiAl2 layer leading to the formation of a

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ACCEPTED MANUSCRIPT Ti5 Si3 . Fig. 2(a) shows that Ti5 Si3 grains are bigger between two TiAl2 grains. We also observe, on the surface of the sample at the interface of two TiAl2 grains, an Al-rich precipitate due to the diffusion of Al atoms through TiAl 2 grain boundaries (indicated by the blue label on Figure 2). The end of the EDS profile (~350nm) (Fig. 3) shows a decrease of Ti and an increase of Al contents supporting our assumption.

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Conclusion

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In summary, we have demonstrated the possibility of forming a Ti2 AlC MAX phase thin film on (0001) 4H-SiC using a two-

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step method: magnetron sputtering deposition followed by annealing. The composition of the TiAl2 thin film deposited and the low annealing temperatures used (700-800°C) lead to the formation of an epitaxial Ti2 AlC MAX phase thin film:

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[0001]MAX // [0001]SiC and (11-20)MAX//(11-20)SiC. The interface between the SiC and the MAX phase exhibits TiC grains which indicate that the formation of this structure is a prerequisite to from the Ti2 AlC MAX phase. Al can then be easily

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incorporated in the TiC structure to form the Ti2 AlC layer. The presence of grain boundaries in the TiAl2 layer leads to the diffusion of Al to the surface. The remaining Si reacts with Ti from TiAl2 to form Ti5 Si3 grains. These mechanisms remain

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identical for the range of temperatures and annealing times studied. We have thus shown that we can obtain very-good

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Ti2 AlC thin films on SiC-4H and we have also shown that the thickness of this MAX phase can be controlled by the

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annealing time.

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Acknowledgments

The authors would like to acknowledge T. Cabioch’ for the helpful discussions concerning DRX experiments, P. Guérin for

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his technical support with the magnetron sputtering and G. Amiard for the TEM thin foil preparation. The authors would also like to thank S. Donnelly and C. Jany for her help. This work partially pertains to the French Government program “Investissements d’Avenir” (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01). This work has been partially supported by « Nouvelle Aquitaine » Region and by European Structural and Investment Funds (ERDF reference: P -2016BAFE-94/95).

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ACCEPTED MANUSCRIPT Figure captions Figure 1: X-ray diffractograms of TiAl2 (300nm of thickness) onto (0001) 4H-SiC annealed at 700 (a) and 800°C (b) from 10 to 60 min. Intensity ratio of peak Ti2 AlC (0002) and TiAl2 (116) for 700 and 800°C annealing temperature (c).

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Figure 4: General view of one area obtained by BF-STEM (a) and HAADF-STEM (b). HAADF-STEM micrograph (c) highlighting the layer stacking. White arrow indicates the growth direction [0001]. Blue circles correspond to Al-rich precipitates on the surface of the sample. Red dot lines represent interfaces between layers 1/2 – 2/3 and 3/4.

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Figure 3: EDS analysis of Al, Si and Ti. The corresponding elementary profile obtained along the withe arrows. The black dot lines limited the different layers described Fig. 2.

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Figure 4: HRTEM micrographs ([11-20] zone axis) of SiC/Ti2 AlC interface (a), Ti2 AlC/Ti5 Si3 interface ([3-2-10]) (b) and respective associated Fast-Fourier-Transform (b and c) of 800°C-60min annealed sample. Red square corresponds to the (116) reflexion of TiAl2 structure. Yellow arrows indicate the (1100) and (0001) directions of SiC and Ti2 AlC structures.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

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A two-step process to obtain an epitaxial Ti2 AlC thin film onto SiC substrate is proposed The mechanisms involved in Ti2 AlC phase formation are described for 800°C annealing temperature Interdiffusion between TiAl2 nanocrytalline film and SiC monocrystalline during annealing is enhanced Thickness of Ti2 AlC phase can be controlled by annealing time or temperature

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