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Al multilayer thin films
Intermetallics 16 (2008) 1061–1065
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Intermetallic phase formation in nanometric Ni/Al multilayer thin films J. Noro, A.S. Ramos*, M.T. Vieira ˆnica, Faculdade de Cieˆncias e Tecnologia, Universidade de Coimbra, R. Luı´s Reis Santos, 3030-788 Coimbra, Portugal ICEMS, Departamento de Engenharia Meca
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2008 Received in revised form 9 June 2008 Accepted 13 June 2008 Available online 26 July 2008
Ni/Al multilayer films were deposited by dc magnetron sputtering from pure nickel and aluminium targets. Films with periods between 5 and 140 nm were produced. All the multilayers were tailored in order to have an overall atomic composition close to 50Al:50Ni. The as-deposited films are constituted by alternate nickel and aluminium layers that upon thermal annealing react with each other to form intermetallic compounds. During deposition an amorphous phase is formed at the interfaces, which is more evident for low periods where the interfaces constitute a large portion of the multilayers. For equiatomic chemical compositions and periods up to 140 nm, it has been shown that after annealing at increasing temperatures the ultimate phase is B2-NiAl. However, the structural evolution towards equilibrium strongly depends on the multilayer period. The presence of Al-rich intermediate phases, namely NiAl3 and Ni2Al3, is observed for the higher periods. The intermediate periods seem to be the most promising for future use in joining applications. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nickel aluminides, based on Ni3Al A. Nanostructured intermetallics B. Phase transformation C. Thin films C. Vapour deposition
1. Introduction Advanced aluminides, including the ordered phases TiAl, NiAl and Ni3Al, are potential candidates for practical applications, as bulk or thin films. NiAl with a high melting temperature is very attractive due to its low density, good corrosion resistance and high electrical and thermal conductivities [1]. NiAl is strongly ordered and according to the Ni–Al phase diagram the B2 structure is stable for large deviations from stoichiometry. Intermetallic compounds, such as aluminides, particularly as thin films, can be formed by thermal annealing of alternate nanolayers of metals which react exothermically. These metals have medium/high energy of mixing and when in adiabatic conditions the reaction could be self-propagating resulting in atomic diffusion that is normal to the wave propagation direction. The self-propagating nature of the Ni–Al system has potential utility in joining applications. In fact Ni/Al multilayer foils have been used as local heat sources for soldering and brazing [2,3]. However, before using the Ni/Al multilayers for joining purposes, a previous characterisation is always required. Although thin film reactions in the Ni–Al system have been extensively characterised, there is no consensus regarding the first phase to form. Several studies have indicated that the first crystalline phase to form in Ni/Al diffusion couples is NiAl3 [4,5]. Actually, a phase selection in favour of the most Al-rich phases, which are usually found to be tri-aluminides, such as NiAl3, TiAl3 and NbAl3, has been found upon solid-state reactions in Me/Al systems
* Corresponding author. Tel.: þ351 239790700; fax: þ351 239790701. E-mail address: sofi[email protected] (A.S. Ramos). 0966-9795/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2008.06.002
[6]. In contrast, the thermodynamics indicates NiAl as the preferred phase. In fact, the NiAl standard heat of formation at 25 C is 58.7 kJ mol1 while the value for NiAl3 is 37.6 kJ mol1 [7]. More recently, depending on the overall chemical composition and modulation period of the multilayer, and on the processing history, other initial phases such as NiAl [8–10] or a metastable Ni2Al9phase [8,11,12] were also observed. According to the literature, Ni/ Al multilayers have mostly been deposited by sputtering or electron beam evaporation [8,10]. Folding and cold rolling of Al and Ni foils have also been applied as an alternative to synthesize Ni/Al multilayers [13–15]. Whatever the case, special care has to be made when studying phase evolution in the Ni–Al system since Ni/Al diffusion couples and multilayers react at rather low temperatures. Furthermore, the B2-NiAl phase is difficult to identify by X-ray diffraction alone, being easy to incorrectly assign NiAl3 as the first phase to form. This study concerns the structural evolution upon thermal annealing of Ni/Al multilayer films deposited by magnetron sputtering with the aim of finding the most promising Ni/Al multilayer thin films for future use in joining applications. Ni/Al multilayers with different modulation periods were studied focusing on the structural changes during in situ X-ray diffraction at increasing temperatures. The most promising multilayer thin films for joining applications should be selected. The idea in the future is to obtain sound joints using these multilayers without the need of solder or brazing alloys. Joining experiments on g-TiAl alloys by diffusion bonding using Ti/Al multilayer thin films as filler material were performed with success taking advantage of the improved diffusivity and reactivity of the nanometric layers [16]. High quality joints were achieved indicating that these multilayers are
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promising filler materials for diffusion bonding at lower temperatures and shorter holding times [17]. 2. Experimental details 2.1. Deposition technique Ni/Al multilayer films were deposited by magnetron sputtering at 0.3 Pa argon (99.9999%) pressure from pure Ni and Al targets (99.98 and 99.999%, respectively) onto a rotating substrate holder. To achieve the desired overall chemical composition, the Al and Ni targets operated at 4.4 102 and 3.3 102 W mm2, respectively. Prior to deposition, an ultimate vacuum pressure lower than 3 104 Pa was reached and the substrate’s surface was cleaned with an ion gun. To avoid heating during deposition, the stainless steel and Si substrates were mounted on a thick copper block, which acted as a heat sink. Modulation periods, L, ranging from 5 to 140 nm were studied. To guarantee a good adhesion to the substrate, Al was always the bottom layer, while the top layer was either Al or Ni. The total thickness of the films was approximately 2–2.5 mm. 2.2. Characterisation techniques The as-deposited and thermal annealed films were analysed by electron probe microanalysis (EPMA) in a Cameca SX 50 equipment. Scanning electron microscopy (SEM) was used for the morphological characterisation of the films. The multilayer films’ crosssections were observed in secondary electron (SE) mode using an FEI QUANTA 400 F field emission gun scanning electron microscope. The cross-sections were prepared by cutting the coated substrates from the backside without attaining the film. Then the samples were fractured by applying an impact force on the uncoated side of the samples in order to preserve the morphology of the multilayer films. X-ray diffraction (XRD) measurements using Co Ka radiation were carried out in a conventional Philips diffractometer with Bragg-Brentano geometry for phase identification. XRD studies at elevated temperatures were performed on the same diffractometer using a hot stage enclosed in a chamber under vacuum (<103 Pa). To prevent oxidation the experiments were carried out in hydrogenated argon atmosphere at a pressure close to 1 Pa. The samples were heated at a rate of 1 C s1 to the desired temperatures, where isothermal XRD scans were acquired after a 10 min stabilising period. The acquisition time for each XRD scan is close to 22 min. The initial (100 C) and final heating temperatures and the temperature range between two consecutive measurements (25 C) were previously defined according to the sketch shown in Fig. 1. The final heating temperature was 300 C, except for the highest period multilayer that was heated up to 550 C. In all cases a final XRD scan was acquired after cooling to room temperature (Fig. 1). In order to check if phase formation occurred isothermally during the XRD acquisitions, a multilayer with intermediate period was maintained at 110 C (below the detection of the first structural changes) for more than 300 min during which several XRD scans were acquired and no phase formation was detected. Moreover, a hot XRD experiment was conducted acquiring two XRD scans for some temperatures and the results are coincident revealing that no phase formation occurred during the course of the XRD scans. Therefore, although possible the isothermal phase formation during the hot XRD experiments does not seem to take place. 3. Results and discussion 3.1. As-deposited films The Ni/Al multilayer films were designed to have an overall atomic composition close to 50Al:50Ni with various modulation
Fig. 1. Thermal cycle used in the hot XRD experiments.
periods. Although a slight Al enrichment is observed by EPMA (52– 54 at.% of Al), the chemical compositions still lie in the NiAl phase domain. In order to obtain an Al:Ni atomic ratio close to 1:1, the alternating layers of Al and Ni must have an Al to Ni thickness ratio of roughly 3:2. The cross-section images of the multilayer films observed by high-resolution SEM are shown in Fig. 2. The images presented allow the alternating layers of Al and Ni to be distinguished. For the highest period multilayer it is possible to confirm that, as desired, the Al layers are thicker than the Ni ones, while for the 30 nm period film the Al:Ni thickness ratio does not seem compatible with the near equiatomic chemical composition measured by EPMA, revealing that intermixing occurred during the deposition process. According to the SEM images presented, the Ni/ Al multilayer films exhibit cross-section morphology typical of zone T as predicted by Thornton’s model [18]. The classification of J.A. Thornton provides an overview of the relationship between the morphology of sputtered films and the most prominent deposition parameters. Zone T corresponds to a transition state between columnar and featureless morphologies. In the case of the films under study the type T morphology is quite compact, almost featureless. However, as the period decreases the films exhibit a stronger tendency to a columnar structure. X-ray diffractograms of the as-deposited multilayer films are presented in Fig. 3. Apart from the stainless steel substrate diffraction peaks, the XRD diffractograms only reveal the presence of (111) diffraction peaks of Ni and Al, indicating a preferential orientation of the close-packed planes in the growth direction. In fact, multilayer films of fcc elements, such as Ni and Al, often are orientated along the C111D direction normal to the plane of the film [8]. It should be noted that after deposition no crystalline reaction products are detected by XRD, indicating that a well-succeeded methodology was used to keep the substrate temperature low. With decreasing multilayer period, the XRD peaks exhibit decreasing intensities suggesting the presence of a misorientation. The presence of amorphous interfaces would explain the XRD diffractogram of the 5 nm period film. In fact, the X-ray diffractogram of the shortest period multilayer presents a possible amorphous broad peak at low 2-q as shown at higher resolution in the inset of Fig. 3. In this case the substrate peak is the most intense. These results are consistent with the presence of an amorphous phase at the interfaces, since for short periods a large portion of the multilayer is composed of interfacial regions. As the period increases the role of the interfaces is attenuated and the presence of the amorphous phase is no longer evident in the XRD diffractograms due to the poor detection limit of this technique. According to Barmak
J. Noro et al. / Intermetallics 16 (2008) 1061–1065
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Fig. 2. Cross-sectional SEM micrographs of as-deposited multilayer films with (a) L ¼ 140 nm and (b) and (c) L ¼ 30 nm. Al-rich phase (black layers) and Ni-rich phase (grey layers).
et al. [11], Ni/Al sputtered multilayer thin films are incoherent with disordered or amorphous interfaces. Therefore, the as-deposited Ni/Al multilayer films are formed by alternate Ni- and Al-rich layers, probably with amorphous interfaces. 3.2. Structural evolution The chemical composition of the heat treated Ni/Al films is similar to that measured after deposition and according to the EPMA results the oxygen content remain around 1 at.%. During in situ heat treatment small shifts in the substrate peak positions are observed which are related to the thermal expansion. The structural evolution of the film with the lowest period is shown in Fig. 4. The crystallization of the as-deposited amorphous phase into NiAl starts at around 150 C as evidenced by the presence of a diffraction peak at around 36 , while according to the decrease of the 45 Al peak the reaction between Al and Ni occurs at a slightly higher temperature. At the highest temperature presented only the (100) and (110) diffraction peaks of the NiAl intermetallic phase are identified along with those from the substrate. In the multilayer film with L ¼ 30 nm the Al peak disappears at 125 C and new peaks could be detected in the XRD diffractogram corresponding to the NiAl3 phase namely the (101) and (111) diffraction peaks indexed in Fig. 5. The NiAl3 is a complex orthorhombic D020 structure possessing 20 atoms per unit cell. The NiAl equilibrium phase starts to grow upon further annealing and at 225 C there is no sign of the NiAl3 intermediate phase. Early research showed in essence that Al3Ni is the first phase to form in some Al/Ni diffusion couples and multilayers [5,6]. Later, other phases, namely NiAl, have been reported as the first to form upon annealing [8,9]. It has been shown that the initial phase and the subsequent phase sequencing depend on the deposition process,
overall chemical composition and multilayer period. A phase selection in favour of the most Al-rich phases has also been found upon solid-state reaction in other metal/Al systems, such as Ti–Al [19]. However, previous authors’ works detected g-TiAl as the first phase to form in Ti/Al multilayer films with equiatomic chemical compositions and periods up to 20 nm [20]. Upon annealing a multilayer film with L ¼ 140 nm another intermediate phase indexed as Ni2Al3 appears besides the NiAl3
Fig. 3. XRD diffractograms of as-deposited Ni/Al multilayer films. An inset of the XRD diffractogram of the 5 nm period multilayer is included on the top right corner.
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Fig. 4. Hot-stage diffractograms for L ¼ 5 nm Ni/Al film. The stainless steel substrate peaks are labelled with ‘‘S’’ and the major intermetallic peaks are identified.
phase already detected for L ¼ 30 nm, as illustrated by the XRD diffractograms presented in Fig. 6. For the highest period, first the reaction between Al and Ni results in NiAl3 but the initial Al-rich phase is still present. At 250 C the Al peak disappears and a second phase is detected which corresponds to the Ni2Al3 phase according to the diffraction peaks arising at 29.4 and 52.8 . Then, the NiAl3 phase progressively transforms into NiAl, whereas the Ni2Al3 hexagonal phase only disappears at 500 C. At the final annealing temperature the NiAl equilibrium phase is the only one detected by XRD, like already observed for the 5 and 30 nm period multilayers (see highest temperature X-ray diffractogram of Figs. 4–6). Whatever the period is the reaction between Al and Ni starts at temperatures as low as 125 or 150 C. Atomic mobility is feasible at such temperatures due to the enhancement of the diffusion process resulting from the nanometric character of the multilayer films. Grain boundary diffusion increases as the grain size decreases, while the nanometric periods provide short diffusion paths, both allowing the reaction to occur at lower temperatures. To summarise the following crystalline phase evolution sequences are proposed for the multilayer films under study.
Fig. 6. Hot-stage diffractograms for L ¼ 140 nm Ni/Al film. The stainless steel substrate peaks are labelled with ‘‘S’’ and the major intermetallic peaks are identified.
K [ 5 nm Al D Ni / Al D Ni D NiAl / NiAl K [ 30 nm Al D Ni / Ni D NiAl3 / Ni D NiAl3 D NiAl / NiAl K [ 140 nm Al D Ni / Al D Ni D NiAl3 / NiAl3 D Ni2 Al3 D Ni / Ni2 Al3 D NiAl D Ni / NiAl Annealing short period multilayer films allows the equilibrium phase NiAl to be directly produced without forming intermediate crystalline phases. The metastable phase NiAl3 is the first to form in 30 and 140 nm period multilayers, while it does not form for L ¼ 5 nm probably because in short periods the intermixed layer consumes most of the Al whose concentration drops too low for the Al-rich NiAl3 phase to nucleate. The by-pass of the richer Al phase in short period multilayers was already found by Blobaum et al. [21] with Ni/Al multilayer foils designed to produce Ni2Al3 as the final product. It should be noted that whatever the period is the final product is NiAl and the phase transitions promoted during heat treatment are irreversible as confirmed by the XRD scans performed after cooling down to room temperature. In fact, the X-ray diffractograms acquired at room temperature after the thermal cycle match those obtained at high heat treatment temperature, revealing in all cases the NiAl equilibrium phase. According to the results a decrease in the released enthalpy with decreasing modulation period is expected due to intermixing and to the formation of an amorphous phase during deposition which reduce the driving-force for self-propagation. On the other hand, in the highest period multilayer the phase evolution sequence will reduce the released enthalpy due to the formation of intermediate aluminides with heats of formation lower than that of the NiAl ultimate phase. Therefore, the multilayer thin films with intermediate periods close to 30–40 nm or slightly below seem to be the most promising for use in future joining applications, especially if after ignition the reaction proceeds in a self-propagating mode fast enough to overcome the formation of the NiAl3 phase. 4. Summary
Fig. 5. Hot-stage diffractograms for L ¼ 30 nm Ni/Al film. The stainless steel substrate peaks are labelled with ‘‘S’’ and the major intermetallic peaks are identified.
The structural evolution with temperature of Ni/Al multilayers deposited by magnetron sputtering was studied. The as-deposited films with a layered structure and almost equiatomic chemical
J. Noro et al. / Intermetallics 16 (2008) 1061–1065
composition evolve during annealing towards the NiAl equilibrium phase. However, the in situ XRD diffractograms reveal that the structural evolution depends on the multilayer period. For short periods (L ¼ 5 nm) the films composed of a mixture of amorphous and Ni- and Al-rich phases crystallize and transform into NiAl at temperatures below 275 C. The multilayers with 30 nm period also evolve to NiAl at rather low temperatures but an NiAl3 intermediate phase is formed. For the highest period a phase formation sequence starting with NiAl3 and the subsequent formation of phases with increasing Ni content was observed. At 500 C the Ni2Al3 intermediate phase vanishes and NiAl is the only phase present. Although with different structural evolutions, up to 140 nm period and according to their overall chemical composition all the sputtered films studied converted into B2-NiAl. The Ni/Al multilayers with intermediate period show potential for joining applications by using the self-propagating nature of these films. Well-succeeded joining experiments on g-TiAl alloys were already performed by diffusion bonding using Ti/Al multilayer films. Acknowledgements This work was supported by Fundaça˜o para a Cieˆncia e a Tecnologia, BPD/6771/2001, and FEDER.
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References [1] Miracle DB. Acta Metall Mater 1993;41:646. [2] Wang J, Besnoin E, Duckham A, Spey SJ, Reiss ME, Knio OM, et al. J Appl Phys 2004;95:248. [3] Swiston AJ, Besnoin E, Duckham A, Knio OM, Weihs TP, Hufnagel TC. Acta Mater 2005;53:3713. [4] Rothhaar U, Oechsner H, Scheib M, Mu¨ller R. Phys Rev B 2000;61:974. [5] Ma E, Nicolet M-A, Nathan M. J. Appl Phys 1989;65:2703. [6] Colgan EG. Mater Sci Rep 1990;5:1. [7] Rzyman K, Moser Z. Prog Mater Sci 2004;49:581. [8] Edelstein AS, Everett RK, Richard GY, Qadri SB, Altman EI, Foley JC, et al. J Appl Phys 1994;76:7850. [9] Michaelsen C, Lucadamo G, Barmak K. J Appl Phys 1996;80:6689. [10] Jeske T, Seibt M, Schmitz G. Mater Sci Eng 2003;A353:105. [11] Barmak K, Michaelsen C, Lucadamo G. J Mater Res 1997;12:133. [12] Gavens AJ, Van Haeerden D, Mann AB, Reiss ME, Weihs TP. J Appl Phys 2000; 87:1255. [13] Sauvage X, Dinda GP, Wilde G. Scr. Mater. 2007;56:181. [14] Battezzati L, Antoine C, Fracchia F. Intermetallics 1995;3:67. [15] Sieber H, Park JS, Weissmueller J, Perepezko JH. Acta Mater 2001;49:1139. [16] Ramos AS, Vieira MT, Duarte LI, Vieira MF, Viana F, Calinas R. Intermetallics 2006;14:1157. [17] Duarte LI, Ramos AS, Vieira MF, Viana F, Vieira MT, Koçak M. Intermetallics 2006;14:1151. [18] Thornton JA. J Vac Sci Technol 1974;11:666. [19] Wo¨hlert S, Bormann R. J Appl Phys 1999;85:825. [20] Ramos AS, Vieira MT. Surf Coat Technol 2005;200:326. [21] Blobaum KJ, Van Heerden D, Gavens AJ, Weihs TP. Acta Mater 2003;51:3871.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Intermetallic phase formation in nanometric Ni/Al multilayer thin films J. Noro, A.S. Ramos*, M.T. Vieira ˆnica, Faculdade de Cieˆncias e Tecnologia, Universidade de Coimbra, R. Luı´s Reis Santos, 3030-788 Coimbra, Portugal ICEMS, Departamento de Engenharia Meca
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 May 2008 Received in revised form 9 June 2008 Accepted 13 June 2008 Available online 26 July 2008
Ni/Al multilayer films were deposited by dc magnetron sputtering from pure nickel and aluminium targets. Films with periods between 5 and 140 nm were produced. All the multilayers were tailored in order to have an overall atomic composition close to 50Al:50Ni. The as-deposited films are constituted by alternate nickel and aluminium layers that upon thermal annealing react with each other to form intermetallic compounds. During deposition an amorphous phase is formed at the interfaces, which is more evident for low periods where the interfaces constitute a large portion of the multilayers. For equiatomic chemical compositions and periods up to 140 nm, it has been shown that after annealing at increasing temperatures the ultimate phase is B2-NiAl. However, the structural evolution towards equilibrium strongly depends on the multilayer period. The presence of Al-rich intermediate phases, namely NiAl3 and Ni2Al3, is observed for the higher periods. The intermediate periods seem to be the most promising for future use in joining applications. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nickel aluminides, based on Ni3Al A. Nanostructured intermetallics B. Phase transformation C. Thin films C. Vapour deposition
1. Introduction Advanced aluminides, including the ordered phases TiAl, NiAl and Ni3Al, are potential candidates for practical applications, as bulk or thin films. NiAl with a high melting temperature is very attractive due to its low density, good corrosion resistance and high electrical and thermal conductivities [1]. NiAl is strongly ordered and according to the Ni–Al phase diagram the B2 structure is stable for large deviations from stoichiometry. Intermetallic compounds, such as aluminides, particularly as thin films, can be formed by thermal annealing of alternate nanolayers of metals which react exothermically. These metals have medium/high energy of mixing and when in adiabatic conditions the reaction could be self-propagating resulting in atomic diffusion that is normal to the wave propagation direction. The self-propagating nature of the Ni–Al system has potential utility in joining applications. In fact Ni/Al multilayer foils have been used as local heat sources for soldering and brazing [2,3]. However, before using the Ni/Al multilayers for joining purposes, a previous characterisation is always required. Although thin film reactions in the Ni–Al system have been extensively characterised, there is no consensus regarding the first phase to form. Several studies have indicated that the first crystalline phase to form in Ni/Al diffusion couples is NiAl3 [4,5]. Actually, a phase selection in favour of the most Al-rich phases, which are usually found to be tri-aluminides, such as NiAl3, TiAl3 and NbAl3, has been found upon solid-state reactions in Me/Al systems
* Corresponding author. Tel.: þ351 239790700; fax: þ351 239790701. E-mail address: sofi[email protected] (A.S. Ramos). 0966-9795/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2008.06.002
[6]. In contrast, the thermodynamics indicates NiAl as the preferred phase. In fact, the NiAl standard heat of formation at 25 C is 58.7 kJ mol1 while the value for NiAl3 is 37.6 kJ mol1 [7]. More recently, depending on the overall chemical composition and modulation period of the multilayer, and on the processing history, other initial phases such as NiAl [8–10] or a metastable Ni2Al9phase [8,11,12] were also observed. According to the literature, Ni/ Al multilayers have mostly been deposited by sputtering or electron beam evaporation [8,10]. Folding and cold rolling of Al and Ni foils have also been applied as an alternative to synthesize Ni/Al multilayers [13–15]. Whatever the case, special care has to be made when studying phase evolution in the Ni–Al system since Ni/Al diffusion couples and multilayers react at rather low temperatures. Furthermore, the B2-NiAl phase is difficult to identify by X-ray diffraction alone, being easy to incorrectly assign NiAl3 as the first phase to form. This study concerns the structural evolution upon thermal annealing of Ni/Al multilayer films deposited by magnetron sputtering with the aim of finding the most promising Ni/Al multilayer thin films for future use in joining applications. Ni/Al multilayers with different modulation periods were studied focusing on the structural changes during in situ X-ray diffraction at increasing temperatures. The most promising multilayer thin films for joining applications should be selected. The idea in the future is to obtain sound joints using these multilayers without the need of solder or brazing alloys. Joining experiments on g-TiAl alloys by diffusion bonding using Ti/Al multilayer thin films as filler material were performed with success taking advantage of the improved diffusivity and reactivity of the nanometric layers [16]. High quality joints were achieved indicating that these multilayers are
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promising filler materials for diffusion bonding at lower temperatures and shorter holding times [17]. 2. Experimental details 2.1. Deposition technique Ni/Al multilayer films were deposited by magnetron sputtering at 0.3 Pa argon (99.9999%) pressure from pure Ni and Al targets (99.98 and 99.999%, respectively) onto a rotating substrate holder. To achieve the desired overall chemical composition, the Al and Ni targets operated at 4.4 102 and 3.3 102 W mm2, respectively. Prior to deposition, an ultimate vacuum pressure lower than 3 104 Pa was reached and the substrate’s surface was cleaned with an ion gun. To avoid heating during deposition, the stainless steel and Si substrates were mounted on a thick copper block, which acted as a heat sink. Modulation periods, L, ranging from 5 to 140 nm were studied. To guarantee a good adhesion to the substrate, Al was always the bottom layer, while the top layer was either Al or Ni. The total thickness of the films was approximately 2–2.5 mm. 2.2. Characterisation techniques The as-deposited and thermal annealed films were analysed by electron probe microanalysis (EPMA) in a Cameca SX 50 equipment. Scanning electron microscopy (SEM) was used for the morphological characterisation of the films. The multilayer films’ crosssections were observed in secondary electron (SE) mode using an FEI QUANTA 400 F field emission gun scanning electron microscope. The cross-sections were prepared by cutting the coated substrates from the backside without attaining the film. Then the samples were fractured by applying an impact force on the uncoated side of the samples in order to preserve the morphology of the multilayer films. X-ray diffraction (XRD) measurements using Co Ka radiation were carried out in a conventional Philips diffractometer with Bragg-Brentano geometry for phase identification. XRD studies at elevated temperatures were performed on the same diffractometer using a hot stage enclosed in a chamber under vacuum (<103 Pa). To prevent oxidation the experiments were carried out in hydrogenated argon atmosphere at a pressure close to 1 Pa. The samples were heated at a rate of 1 C s1 to the desired temperatures, where isothermal XRD scans were acquired after a 10 min stabilising period. The acquisition time for each XRD scan is close to 22 min. The initial (100 C) and final heating temperatures and the temperature range between two consecutive measurements (25 C) were previously defined according to the sketch shown in Fig. 1. The final heating temperature was 300 C, except for the highest period multilayer that was heated up to 550 C. In all cases a final XRD scan was acquired after cooling to room temperature (Fig. 1). In order to check if phase formation occurred isothermally during the XRD acquisitions, a multilayer with intermediate period was maintained at 110 C (below the detection of the first structural changes) for more than 300 min during which several XRD scans were acquired and no phase formation was detected. Moreover, a hot XRD experiment was conducted acquiring two XRD scans for some temperatures and the results are coincident revealing that no phase formation occurred during the course of the XRD scans. Therefore, although possible the isothermal phase formation during the hot XRD experiments does not seem to take place. 3. Results and discussion 3.1. As-deposited films The Ni/Al multilayer films were designed to have an overall atomic composition close to 50Al:50Ni with various modulation
Fig. 1. Thermal cycle used in the hot XRD experiments.
periods. Although a slight Al enrichment is observed by EPMA (52– 54 at.% of Al), the chemical compositions still lie in the NiAl phase domain. In order to obtain an Al:Ni atomic ratio close to 1:1, the alternating layers of Al and Ni must have an Al to Ni thickness ratio of roughly 3:2. The cross-section images of the multilayer films observed by high-resolution SEM are shown in Fig. 2. The images presented allow the alternating layers of Al and Ni to be distinguished. For the highest period multilayer it is possible to confirm that, as desired, the Al layers are thicker than the Ni ones, while for the 30 nm period film the Al:Ni thickness ratio does not seem compatible with the near equiatomic chemical composition measured by EPMA, revealing that intermixing occurred during the deposition process. According to the SEM images presented, the Ni/ Al multilayer films exhibit cross-section morphology typical of zone T as predicted by Thornton’s model [18]. The classification of J.A. Thornton provides an overview of the relationship between the morphology of sputtered films and the most prominent deposition parameters. Zone T corresponds to a transition state between columnar and featureless morphologies. In the case of the films under study the type T morphology is quite compact, almost featureless. However, as the period decreases the films exhibit a stronger tendency to a columnar structure. X-ray diffractograms of the as-deposited multilayer films are presented in Fig. 3. Apart from the stainless steel substrate diffraction peaks, the XRD diffractograms only reveal the presence of (111) diffraction peaks of Ni and Al, indicating a preferential orientation of the close-packed planes in the growth direction. In fact, multilayer films of fcc elements, such as Ni and Al, often are orientated along the C111D direction normal to the plane of the film [8]. It should be noted that after deposition no crystalline reaction products are detected by XRD, indicating that a well-succeeded methodology was used to keep the substrate temperature low. With decreasing multilayer period, the XRD peaks exhibit decreasing intensities suggesting the presence of a misorientation. The presence of amorphous interfaces would explain the XRD diffractogram of the 5 nm period film. In fact, the X-ray diffractogram of the shortest period multilayer presents a possible amorphous broad peak at low 2-q as shown at higher resolution in the inset of Fig. 3. In this case the substrate peak is the most intense. These results are consistent with the presence of an amorphous phase at the interfaces, since for short periods a large portion of the multilayer is composed of interfacial regions. As the period increases the role of the interfaces is attenuated and the presence of the amorphous phase is no longer evident in the XRD diffractograms due to the poor detection limit of this technique. According to Barmak
J. Noro et al. / Intermetallics 16 (2008) 1061–1065
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Fig. 2. Cross-sectional SEM micrographs of as-deposited multilayer films with (a) L ¼ 140 nm and (b) and (c) L ¼ 30 nm. Al-rich phase (black layers) and Ni-rich phase (grey layers).
et al. [11], Ni/Al sputtered multilayer thin films are incoherent with disordered or amorphous interfaces. Therefore, the as-deposited Ni/Al multilayer films are formed by alternate Ni- and Al-rich layers, probably with amorphous interfaces. 3.2. Structural evolution The chemical composition of the heat treated Ni/Al films is similar to that measured after deposition and according to the EPMA results the oxygen content remain around 1 at.%. During in situ heat treatment small shifts in the substrate peak positions are observed which are related to the thermal expansion. The structural evolution of the film with the lowest period is shown in Fig. 4. The crystallization of the as-deposited amorphous phase into NiAl starts at around 150 C as evidenced by the presence of a diffraction peak at around 36 , while according to the decrease of the 45 Al peak the reaction between Al and Ni occurs at a slightly higher temperature. At the highest temperature presented only the (100) and (110) diffraction peaks of the NiAl intermetallic phase are identified along with those from the substrate. In the multilayer film with L ¼ 30 nm the Al peak disappears at 125 C and new peaks could be detected in the XRD diffractogram corresponding to the NiAl3 phase namely the (101) and (111) diffraction peaks indexed in Fig. 5. The NiAl3 is a complex orthorhombic D020 structure possessing 20 atoms per unit cell. The NiAl equilibrium phase starts to grow upon further annealing and at 225 C there is no sign of the NiAl3 intermediate phase. Early research showed in essence that Al3Ni is the first phase to form in some Al/Ni diffusion couples and multilayers [5,6]. Later, other phases, namely NiAl, have been reported as the first to form upon annealing [8,9]. It has been shown that the initial phase and the subsequent phase sequencing depend on the deposition process,
overall chemical composition and multilayer period. A phase selection in favour of the most Al-rich phases has also been found upon solid-state reaction in other metal/Al systems, such as Ti–Al [19]. However, previous authors’ works detected g-TiAl as the first phase to form in Ti/Al multilayer films with equiatomic chemical compositions and periods up to 20 nm [20]. Upon annealing a multilayer film with L ¼ 140 nm another intermediate phase indexed as Ni2Al3 appears besides the NiAl3
Fig. 3. XRD diffractograms of as-deposited Ni/Al multilayer films. An inset of the XRD diffractogram of the 5 nm period multilayer is included on the top right corner.
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Fig. 4. Hot-stage diffractograms for L ¼ 5 nm Ni/Al film. The stainless steel substrate peaks are labelled with ‘‘S’’ and the major intermetallic peaks are identified.
phase already detected for L ¼ 30 nm, as illustrated by the XRD diffractograms presented in Fig. 6. For the highest period, first the reaction between Al and Ni results in NiAl3 but the initial Al-rich phase is still present. At 250 C the Al peak disappears and a second phase is detected which corresponds to the Ni2Al3 phase according to the diffraction peaks arising at 29.4 and 52.8 . Then, the NiAl3 phase progressively transforms into NiAl, whereas the Ni2Al3 hexagonal phase only disappears at 500 C. At the final annealing temperature the NiAl equilibrium phase is the only one detected by XRD, like already observed for the 5 and 30 nm period multilayers (see highest temperature X-ray diffractogram of Figs. 4–6). Whatever the period is the reaction between Al and Ni starts at temperatures as low as 125 or 150 C. Atomic mobility is feasible at such temperatures due to the enhancement of the diffusion process resulting from the nanometric character of the multilayer films. Grain boundary diffusion increases as the grain size decreases, while the nanometric periods provide short diffusion paths, both allowing the reaction to occur at lower temperatures. To summarise the following crystalline phase evolution sequences are proposed for the multilayer films under study.
Fig. 6. Hot-stage diffractograms for L ¼ 140 nm Ni/Al film. The stainless steel substrate peaks are labelled with ‘‘S’’ and the major intermetallic peaks are identified.
K [ 5 nm Al D Ni / Al D Ni D NiAl / NiAl K [ 30 nm Al D Ni / Ni D NiAl3 / Ni D NiAl3 D NiAl / NiAl K [ 140 nm Al D Ni / Al D Ni D NiAl3 / NiAl3 D Ni2 Al3 D Ni / Ni2 Al3 D NiAl D Ni / NiAl Annealing short period multilayer films allows the equilibrium phase NiAl to be directly produced without forming intermediate crystalline phases. The metastable phase NiAl3 is the first to form in 30 and 140 nm period multilayers, while it does not form for L ¼ 5 nm probably because in short periods the intermixed layer consumes most of the Al whose concentration drops too low for the Al-rich NiAl3 phase to nucleate. The by-pass of the richer Al phase in short period multilayers was already found by Blobaum et al. [21] with Ni/Al multilayer foils designed to produce Ni2Al3 as the final product. It should be noted that whatever the period is the final product is NiAl and the phase transitions promoted during heat treatment are irreversible as confirmed by the XRD scans performed after cooling down to room temperature. In fact, the X-ray diffractograms acquired at room temperature after the thermal cycle match those obtained at high heat treatment temperature, revealing in all cases the NiAl equilibrium phase. According to the results a decrease in the released enthalpy with decreasing modulation period is expected due to intermixing and to the formation of an amorphous phase during deposition which reduce the driving-force for self-propagation. On the other hand, in the highest period multilayer the phase evolution sequence will reduce the released enthalpy due to the formation of intermediate aluminides with heats of formation lower than that of the NiAl ultimate phase. Therefore, the multilayer thin films with intermediate periods close to 30–40 nm or slightly below seem to be the most promising for use in future joining applications, especially if after ignition the reaction proceeds in a self-propagating mode fast enough to overcome the formation of the NiAl3 phase. 4. Summary
Fig. 5. Hot-stage diffractograms for L ¼ 30 nm Ni/Al film. The stainless steel substrate peaks are labelled with ‘‘S’’ and the major intermetallic peaks are identified.
The structural evolution with temperature of Ni/Al multilayers deposited by magnetron sputtering was studied. The as-deposited films with a layered structure and almost equiatomic chemical
J. Noro et al. / Intermetallics 16 (2008) 1061–1065
composition evolve during annealing towards the NiAl equilibrium phase. However, the in situ XRD diffractograms reveal that the structural evolution depends on the multilayer period. For short periods (L ¼ 5 nm) the films composed of a mixture of amorphous and Ni- and Al-rich phases crystallize and transform into NiAl at temperatures below 275 C. The multilayers with 30 nm period also evolve to NiAl at rather low temperatures but an NiAl3 intermediate phase is formed. For the highest period a phase formation sequence starting with NiAl3 and the subsequent formation of phases with increasing Ni content was observed. At 500 C the Ni2Al3 intermediate phase vanishes and NiAl is the only phase present. Although with different structural evolutions, up to 140 nm period and according to their overall chemical composition all the sputtered films studied converted into B2-NiAl. The Ni/Al multilayers with intermediate period show potential for joining applications by using the self-propagating nature of these films. Well-succeeded joining experiments on g-TiAl alloys were already performed by diffusion bonding using Ti/Al multilayer films. Acknowledgements This work was supported by Fundaça˜o para a Cieˆncia e a Tecnologia, BPD/6771/2001, and FEDER.
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