Oxidation Behavior of Ta–Al Multilayer Coatings

Abstract

Ta–Al multilayer coatings were fabricated through cyclical gradient concentration deposition by direct current magnetron co-sputtering. The as-deposited coatings presented a multilayer structure in the growth direction. The oxidation behavior of the Ta–Al multilayer coatings was explored. The results specified that Ta-rich Ta–Al multilayer coatings demonstrated a restricted oxidation depth after annealing at 600 °C in 1% O₂–99% Ar for up to 100 h. This was attributed to the preferential oxidation of Al, the formation of amorphous Al-oxide sublayers, and the maintenance of a multilayer structure. By contrast, Ta₂O₅ formed after exhausting Al in the oxidation process in an ambient atmosphere at 600 °C which exhibited a crystalline Ta₂O₅-amorphous Al-oxide multilayer structure.

1. Introduction

Al-based inter-metallics have been used for high-temperature applications because of their high melting points, strength, and oxidation resistance [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. For example, Ru–Al alloys have been applied for jet engine components, bond coats for thermal barrier coatings, corrosion- and oxidation-protective coatings, and electrodes [1,2], whereas Ta–Al alloys have been applied for sulfidation and oxidation-protective coatings [3,4,5], heater materials [12,13,14], and electromagnetic shielding [15]. A previous paper [16] evaluated the oxidation behavior of Ru–Al multilayer coatings in 1% O₂–99% Ar at 400–800 °C. In our co-sputtering system [16,17,18,19,20,21], the plasma sources focused on a circular track but not the center of the substrate-holder; thus, a multilayer coating with cyclical gradient concentration formed, as the substrate-holder was rotated in a low speed of 1–7 rpm. Such multilayer coatings were constructed by alloy sublayers with continuous variation in compositions, but not monolithic sublayers of distinct elements. Additionally, the inward diffusion of oxygen during annealing resulted in the coating forming an internally oxidized multilayer structure comprising alternating oxygen-rich and oxygen-deficient sublayers. The execution of internal or external oxidation was affected by the annealing atmosphere and the diffusion of the active elements [17,22,23]. The internal and external oxidation of the Ru–Al multilayer coatings exhibited restricted oxidation depths and overcoats, respectively, which were attributed to the formation of continuous Al₂O₃ sublayers and scales [16]. The multilayer structure with O-saturated Al₂O₃ oxide sublayers benefited to inhibit further oxidation. Moreover, the Ru–Al multilayer films subjected to an ambient atmosphere at 800 °C exhibited a limited oxidation depth [24]. Considering the merits of oxidation resistance and cost reduction, the feasibility of Ta–Al to replace the Ru–Al multilayer coatings in high-temperature applications was evaluated. The oxidation behavior of the Ta–Al multilayer coatings with cyclical gradient concentrations prepared using direct current (DC) magnetron co-sputtering was investigated at 400–600 °C.

2. Materials and Methods

The Ta–Al coatings with a Ti interlayer were fabricated onto silicon wafers by DC magnetron co-sputtering. A previous study [18] illustrated the co-sputtering equipment and the experimental details such as the base pressure, working pressure, and Ar flow. The substrate holder was heated to 400 °C and rotated at 1 rpm. The Ti interlayer was deposited with a DC power of 200 W for 7 min. Then, Ta and Al were co-sputtered onto the Ti interlayer using various sputter powers for 35 min. The deposition rates of Ta and Al at a DC power of 200 W were 9.1 and 21.3 nm/min, respectively. The powers applied on Ta and Al targets were denoted as P_Ta and P_Al, respectively. The total powers of P_Ta and P_Al were fixed at 300 W. The annealing experiments were performed in 1% O₂–99% Ar at 400–600 °C for 0.5–100 h, or in air at 600 °C for 1–100 h.

A chemical composition analysis of the samples was conducted using a field-emission electron probe microanalyzer (FE-EPMA; JXA-8500F, JEOL, Akishima, Japan). The thickness evaluation of the coatings was performed using a field emission scanning electron microscope (FE-SEM; S4800, Hitachi, Tokyo, Japan). A conventional X-ray diffractometer (XRD; X’Pert PRO MPD, PANalytical, Almelo, The Netherlands) was adopted to identify the phases using the grazing incidence technique with an incidence angle of 1°. The nanostructure was examined using a transmission electron microscope (TEM; JEM-2010F, JEOL, Tokyo, Japan). The TEM samples with Pt protective layers were prepared by applying a focused ion beam system (FEI Nova 200, Hillsboro, OR, USA) which was transferred to Cu grids. An energy-dispersive spectrometry (EDS; Inca x-sight, Oxford Instruments, Tokyo, Japan) equipped with TEM was used to determine the local chemical compositions. The chemical composition depth profiles were evaluated using an Auger electron spectroscopy (AES; PHI700, ULVAC-PHI, Kanagawa, Japan). The sputter depths in the AES results were correlated with the thicknesses determined by the cross-sectional SEM images.

3. Results and Discussion

3.1. As-Deposited Ta–Al Coatings

Table 1. Sputtering parameters, chemical compositions, and thicknesses of the Ta–Al coatings.

Sample	Power		Chemical Composition (at.%)			Thickness (nm)
	PTa (W)	PAl (W)	Ta	Al	O
Ta0.33Al0.67	100	200	31.7 ± 0.4	64.5 ± 0.2	3.8 ± 0.2	1110
Ta0.61Al0.39	150	150	58.3 ± 0.4	37.3 ± 0.2	4.4 ± 0.4	884
Ta0.81Al0.19	200	100	76.8 ± 0.3	18.2 ± 0.1	5.0 ± 0.2	820

lists the sputtering parameters, chemical compositions, and thicknesses of the as-deposited Ta–Al coatings. These samples are denoted as Ta_0.33Al_0.67, Ta_0.61Al_0.39, and Ta_0.81Al_0.19 coatings. Figure 1 depicts these coatings exhibiting evident multilayer structures. The sputtering time was set at 35 min, and the total powers of P_Ta and P_Al were fixed at 300 W; therefore, the deposition rate increased with an increase in the power ratio of the Al target. Figure 2 exhibits the XRD patterns of the as-deposited and 1% O₂–99% Ar-annealed Ta–Al coatings. The as-deposited Ta_0.33Al_0.67 coatings exhibited a tetragonal TaAl₃ (ICDD 03–065–2665) phase and a cubic Al (ICDD 00–004–0787) phase, whereas the Ta_0.61Al_0.39 and Ta_0.81Al_0.19 coatings were composed of TaAl₃ and tetragonal β-Ta (ICDD 00–025–1280) phases. The β-Ta phase is metastable and is observed for the sputtered coatings [25,26,27]. The TaAl₃ has been reported to be the preferred aluminide phase in the Ta–Al system [14,28,29] with a formation enthalpy of −36 kJ/mol in the temperature interval of 1050–1350 K [29]. The Ta-rich and Al-rich Ta–Al films prepared at room temperature by Chung et al. [14] had TaAl₃–Ta and TaAl₃–Al phases, respectively, which exhibited polycrystalline structures. The Ta-rich films showed a remarkable thermal stability after having been annealed in a vacuum at 450–650 °C, whereas the Al-rich films exhibited a phase transformation at 450 °C [14]. Because the Ta–Al coatings in this study were prepared at 400 °C, no amorphous parts were observed in the as-deposited films.

3.2. Oxidation of Ta–Al Coatings in 1% O₂–99% Ar

The as-deposited Ta_0.33Al_0.67 coatings comprised Al and TaAl₃ phases with a strong Al(220) reflection, whereas the coatings annealed at 400 °C exhibited a strong TaAl₃(112) reflection, and no Al phase was shown by the coatings annealed at 500 °C (Figure 2a). Moreover, the Ta_0.33Al_0.67 coatings annealed at 600 °C exhibited reflections of TaAl₃, TaAl₂ (ICDD 00–050–1279), and Al₂O₃ phases, accompanied with a granular structure, as shown in a cross-sectional SEM image (Figure 3). Because the standard Gibbs free energy levels for the formation of the metal oxides Al₂O₃ and Ta₂O₅ at 600 °C are −934,815 and −663,572 J/(mol of O₂) [30], respectively, Al₂O₃ is formed preferentially when the Ta–Al coatings are oxidized. The XRD patterns of the Ta_0.61Al_0.39 and Ta_0.81Al_0.19 coatings annealed in 1% O₂–99% Ar at 400–600 °C for 30 min exhibited reflections similar to those of the as-deposited coatings (Figure 2b,c). A cubic α-Ta (110) reflection (ICDD 00–004–0788) became clear for the Ta_0.61Al_0.39 coating that had been annealed at 600 °C, whereas one extra broadened reflection, which was attributed to surface oxidation, was observed for the Ta_0.81Al_0.19 coating that had been annealed at 600 °C. The annealed Ta_0.61Al_0.39 and Ta_0.81Al_0.19 coatings maintained multilayered structures (Figure 3); however, the two outermost stacking periods extended their widths. The thicknesses of the Ti interlayers increased after annealing which was attributed to the interdiffusion of the Ti interlayer and Si substrate [24].

The cross-sectional TEM image (Figure 4a) of the Ta_0.61Al_0.39 coating annealed at 600 °C for 30 min exhibited a remaining multilayer structure. The EDS analysis verified that the inward diffusion of O tended to accumulate in the outmost stacking periods. The black sublayers (labelled as 1a, 2a, and 3a in Figure 4a) were dominated by Ta, whereas the white sublayers were enriched with Al and O. The selected area diffraction pattern (SADP) of the O-diffused region shows α-Ta and TaAl₃ rings without an evident oxide phase, suggesting that the Al oxide was amorphous. A high-resolution TEM image (Figure 4b) illustrates that the black sublayers were the α-Ta phase, whereas the crystalline TaAl₃ domains were identified between the Ta-rich sublayers and the amorphous Al-oxide sublayers. Figure 5 shows the AES results of the 600 °C and 4 h annealed Ta_0.61Al_0.39 coatings. The O diffusion depth was restricted to two or three of the stacking periods. The deviation of the AES analyzed atomic concentration from that examined by EPMA could be attributed to the preferential sputtering effects which were also reported for the Ru–Al system [31,32]. Moreover, the fluctuation of the profiles in the deeper region seemed unclear, accompanied with a large sampling interval because of a sputter etching rate of 6.76 nm/min. Figure 6 exhibits the XRD patterns of the Ta_0.61Al_0.39 coatings annealed for 0.5–100 h. The XRD patterns of the Ta_0.61Al_0.39 coatings that were annealed for 30 min exhibited coexisting α-Ta and β-Ta phases, whereas the α-Ta phase was dominant for the films with annealing times exceeding 4 h; these results implied that the metastable β-Ta in the shallow depth region transformed to the stable α-Ta, because the XRD patterns were recorded using the grazing incidence technique. The XRD patterns of the 4, 12, and 24 h annealed Ta_0.61Al_0.39 coatings exhibited similar results. By contrast, amorphous oxide signals at two theta angles less than 30° can be observed for the Ta_0.61Al_0.39 coatings annealed for 50 and 100 h. Figure 7a shows the cross-sectional TEM image of a Ta_0.61Al_0.39 coating annealed for 100 h; the O signals detected by EDS were observed at the outmost nine stacking periods, accompanied with expanded period widths. The high-resolution TEM image (Figure 7b) illustrates that the black sublayers were an α-Ta phase, whereas the crystalline TaAl₃ domains were dispersed in the amorphous Al-oxide matrix, which formed the white sublayers. Because Al and Ta are insoluble in equilibrium states [33,34], and the Ta–Al coatings were fabricated through a cyclical gradient concentration deposition, the Ta-rich sublayers behaved as a diffusion barrier for Al at 600 °C. Thus, internal oxidation became the main oxidation mechanism, and the preferentially formed Al-oxide sublayers restricted oxidation. Therefore, the Ta_0.61Al_0.39 coatings that had been annealed up to 100 h exhibited thermal stability and oxidation resistance. By contrast, no Ta-rich sublayers formed during the sublayer stacking of the Ta_0.33Al_0.67 coatings. Thus, granular Al₂O₃ formed during annealing at 600 °C, and the multilayer structure was destroyed and partially detached.

3.3. Oxidation of Ta–Al Coatings in Air

The Ta_0.63Al_0.37 coatings (60.8 at.% Al + 35.6 at.% Ta + 3.6 at.% O) were prepared using the same sputter parameters for fabricating Ta_0.61Al_0.39 coatings mentioned in Section 3.2. The two coatings belonged to two batches and exhibited a slight deviation in chemical compositions. Figure 8 shows the XRD patterns of the Ta_0.63Al_0.37 coatings annealed in air at 600 °C for 1–100 h. The Ta_0.63Al_0.37 coatings annealed for 1 h exhibited an XRD pattern comprising TaAl₃ and α-Ta phases, which was similar to those of the Ta_0.61Al_0.39 coatings annealed in 1% O₂–99% Ar at 600 °C for 4–24 h (Figure 6). The Ta_0.63Al_0.37 coatings annealed in air for 4 h demonstrated an amorphous and an orthorhombic Ta₂O₅ (ICDD 00–025–0922) phase. Moreover, the annealed Ta_0.63Al_0.37 coatings exhibited a Ta₂O₅ dominant structure after extending the annealing time to 12–100 h. The coating thickness increased from 792 to 924, 1098, 1220, 1565, 1687, and 1725 nm, while increasing the annealing time from 0 to 1, 4, 12, 24, 50, and 100 h by examining the cross-sectional SEM images (Figure 9). The 100 h annealed sample was well adhered on the substrate. Figure 10a,b displays the AES results of the Ta_0.63Al_0.37 coatings annealed in air at 600 °C for 1 and 12 h, respectively. The O diffused into a depth of 720 and 1220 nm (the entire annealed coating) for the 1 and 12 h annealed Ta_0.63Al_0.37 coatings, respectively. The variation of the O profile was consistent with the variation of the Al profile for the 1 h-annealed coatings, which indicated that the internal oxidation was conducted by the preferential formation of Al-oxide. However, the AES results of a 12 h annealed Ta_0.63Al_0.37 coating were classified into two parts. In the interior region, the variations of the O and Al profiles were consistent. By contrast, in the outer region, including six stacking layers, the positions with local maximum O levels were accompanied with high Ta contents, which implied that Ta-oxide formed after the inward diffusion of O, exceeding a demanded stoichiometric level for Al₂O₃.

Figure 11a shows the cross-sectional TEM image and SADPs of a Ta_0.63Al_0.37 coating after annealing in air at 600 °C for 12 h. The SADP of the outer region exhibited spots of an orthorhombic Ta₂O₅ phase which implied that the outer region comprised crystalline Ta₂O₅ and amorphous Al-oxide phases. By contrast, the SADP of the interior region exhibited a diffused ring of β-Ta phase. Figure 11b shows the EDS analysis results, which exhibit a decrease trend along the depth direction for the O content. The black sublayers exhibited high Ta and low Al compositions related to those of the white sublayers. Figure 11c displays a high-resolution TEM image in the outer region, indicated in Figure 11a, which shows that the black sublayer is crystalline Ta₂O₅ and the white sublayers are amorphous. Figure 11d illustrates a high-resolution TEM image in the deeper interior region, the sublayers 23 and 24, which shows that the black sublayers comprise lattice fringes of TaAl₃ and β-Ta phases, and the white sublayers are amorphous.

4. Conclusions

The Ta–Al multilayer coatings were prepared by direct current magnetron co-sputtering through cyclical gradient concentration deposition which formed alternatively stacked Ta- and Al-rich sublayers. The preferentially formed oxides were amorphous Al₂O₃, and then orthorhombic Ta₂O₅ at 600 °C. The insolubility between Ta and Al resulted in the Ta-rich sublayers obstructing the diffusion of Al, whereas the Al-rich sublayers formed amorphous Al-oxide sublayers after annealing in O₂-containing atmospheres which restricted further oxidation; these effects caused the Ta-rich Ta–Al multilayer coatings to exhibit thermal stability and oxidation resistance when they were annealed up to 100 h at 600 °C in a 1% O₂–99% Ar atmosphere. Moreover, the multilayer structure of the Ta_0.63Al_0.37 coatings was maintained after annealing up to 100 h at 600 °C in air. The Ta–Al multilayer coatings exhibited the potential to replace the Ru–Al coatings utilized under oxygen containing atmospheres for high-temperature purposes such as bond coats in thermal barrier systems; therefore, the feature study should focus on the bond coat characteristics of the Ta–Al multilayer coatings, including the formation of a thermally grown oxide layer on the bond coats and the restricted oxidation behavior of multilayer coatings during the deposition of thermal barrier coatings.