Thermal Stability of MoNbTaVW High Entropy Alloy Thin Films

Refractory high entropy alloys are an interesting material class because of their high thermal stability, decent electrical conductivity, and promising mechanical properties at elevated temperature. In the present work, we report on the thermal stability of body-centered cubic MoNbTaVW solid solution thin films that were synthesized by cathodic arc deposition. After vacuum annealing up to 1600 °C, the morphology, chemical composition, crystal structure, and electrical conductivity, as well as the mechanical properties, were analyzed. The observed body-centered cubic MoNbTaVW solid solution phase is stable up to 1500 °C. The evolution of electrical and mechanical properties due to the annealing treatment is discussed based on the observed structural changes of the synthesized thin films.


Introduction
Metallic high entropy alloys (HEAs), also referred to as multi-principal element alloys, multi-component alloys or complex concentrated alloys, are a new material class which was first described in early 2000 [1][2][3][4][5][6]. HEAs are defined as an intermixture between 5 and 13 constituting elements in equimolar or near equimolar ratios. It is believed that in some HEAs the high mixing entropy favors the formation of face-centered cubic (fcc) or body-centered cubic (bcc) single-phase solid solutions rather than intermetallic or other complex phases, with the latter phases usually having a lower mixing entropy due to their reduced number of constituting elements. Especially, refractory HEAs have recently gained momentum as promising candidates for ultra high temperature applications due to the high melting temperatures of up to 3500 • C of the single elements. Their excellent properties such as high temperature strengths beyond 1000 • C [7][8][9][10], extended ductility at high temperature, thermal diffusion and high temperature conductivity [1,5] are promising for applications such as protective coatings for cutting tools, effective diffusion barriers in microelectronics [11], thermal barrier coatings for gas turbines [12] and thermoelectric elements [13] or interconnectors of solid oxide fuel cell [14]. Lee and co-workers conducted studies on the electrical properties and oxidation resitance of magnetron-sputtered refractory MoNbTaVW HEA thin films [15] and on the corrosion properties of MoNbTaVW and BCrMoNbTaVW HEA coatings [16]. Kim [17][18][19].
However, limited information regarding the thermal stability of refractory MoNbTaVW HEA thin films is available in literature. In the present study, we elucidate the thermal influence on the structure and properties of such thin films which were synthesized by cathodic arc deposition (CAD).

Materials and Methods
The films were deposited onto Al 2 O 3 (0001) single-crystal substrates which were placed in a custom-built deposition system with a base pressure below 1.3 × 10 −3 Pa. Stoichiometric Mo 0.20 Nb 0.20 Ta 0.20 V 0.20 W 0.20 arc cathodes with a diameter of 65 mm were powder metallurgically produced by Plansee Composite Materials GmbH, Lechbruck am See, Germany. The Ar pressure of 4.8 Pa and the current of 120 A during CAD were kept constant. The substrates were placed on a grounded substrate holder at a distance of 25 cm to the cathode surface which yielded a deposition rate of about 110 nm/min and a final film thickness of 1.1 µm. No external heating was applied. Annealing of the as-deposited MoNbTaVW films was performed in a vacuum furnace (HTM Reetz, Berlin, Germany) from 1000 to 1600 • C in steps of 100 • C and a holding time of 60 min. The pressure in the vacuum furnace increased from 5 × 10 −6 Pa to about 6 × 10 −2 Pa during annealing up to 1600 • C. To assess the film morphology, scanning electron microscopy (SEM) images of the as-deposited and annealed thin films were taken with a Tescan Clara (Brno, Czech Republic). The chemical composition of the films was obtained by energy dispersive X-ray spectroscopy (EDX). The film microstructure of the as-deposited and annealed films, was obtained by X-ray diffraction (XRD) using a Bruker-AXS D8 Advance diffractometer (Billerica, MA, USA) equipped with Cu-K α radiation and parallel optics. Changes in the electrical resistivity of the as-deposited and annealed thin films were evaluated by four-point probe measurements (Jandel resistivity test unit, Leighton Buzzard, UK). Hardness and Young's modulus were determined by means of nanoindentation using a UMIS II nanoindenter by Fischer-Cripps Laboratories (Sydney, Australia) with a Berkovich diamond tip (tip radius of 230 nm).

Results and Discussion
Possible changes in the film morphology after high temperature vacuum annealing were evaluated by SEM top-view images as shown in Figure 1. These images reveal a smooth thin film surface for the as-deposited film, which remained without significant changes up to an annealing temperature of 1300 • C. While visual observations of the cathodic arc plasma from the MoNbTaVW cathode revealed the emission of droplets due to the high plasma pressure in the cathode spot interacting with molten cathode material, only a limited number of droplets was incorporated into the deposited films. This is similar to previous works [20,21]. Most likely the vast majority of these droplets reached the growth front of the film in solid state strongly reducing their sticking probability. Starting at 1400 • C, a roughening of the thin film surface with the formation of a granular morphology is noticeable which becomes more pronounced with increasing annealing temperature. At 1500 • C and above, partial film delamination occurs. A reference annealing at 1500 • C in Ar atmosphere (atmospheric pressure) revealed the same roughening of the film surface but without signs of delamination.
Similarly to the film morphology, the chemical composition of the annealed thin films up to 1300 • C resembled the as-deposited thin film composition as determined by EDX (see Figure 2). The concentration of the constituting elements in as-deposited state was measured between 18 and 22 at.%, which represents the cathode composition. The concentration of V, the lightest element in the alloy, was measured between 18 and 19 at.% up to 1300 • C. Such a slight reduction in V occurs at an deposition angle of 0 • for CAD which was shown in a previous study [21]. Similar chemical compositions were reported on comparable co-sputtered MoNbTaW thin films [18,19] and bulk MoNbTaW and MoNbTaVW [8], where the overall metal ratio varied within 10 at.%. With further increase in annealing temperature at 1400 and 1500 • C, the V content decreases to 10 at.% which is compensated by higher concentrations of Mo, Nb and W. Only the concentration of Ta remains constant at about 21 at.%. The composition of the film annealed at 1600 • C was not measured due to severe film spalling as shown in Figure 1. However, the reduction in V content seems to be associated to the vacuum conditions during annealing. The reference annealing in Ar atmosphere revealed a film composition similar to the one in as-deposited state. There are only minor reductions in the V and Ta contents in this case.   To further identify the reasons for the reduction of V during vacuum annealing, the vapor pressure for all constituting elements was calculated using the vapor pressure equations for elementary metallic elements according to [22,23] and is shown in Figure 3. Sublimation of an element or compound occurs when its vapor pressure is higher than the surrounding pressure at a given temperature. According to the calculated vapor pressures of single elements in dependence on annealing temperature in Figure 3, the vapor pressure of V is in the range of the chamber pressure at 1400 • C and above. Hence, the observed reduction in V concentration can be understood by the sublimation of V out of the films during vacuum annealing. It is further comprehensible that the sublimation of V causes the formation of defects in the films, which weakens their adhesion and causes the observed delamination. The latter is confirmed by the SEM images taken from the film annealed at 1500 • C in Ar atmosphere, i.e., without loss in V, which showed a roughening of the surface but no delamination (see Figure 1). The crystal structure of the films was analyzed by XRD and the recorded diffractograms are shown in Figure 4. The peaks for the as-deposited MoNbTaVW thin film suggest the formation of a single-phase bcc solid solution. The lattice constant a = 0.324 nm was calculated from the corresponding (110) peak and is in the range of the thermodynamically stable bcc structures of the different elements present in the film, i.e., a Mo = 0.315 nm, a Nb = 0.330 nm, a Ta = 0.330 nm, a V = 0.303 nm and a W = 0.316 nm [24]. However, the calculated lattice constants for the thin films are larger when compared to bulk MoNbTaVW HEAs (a = 0.318 nm) reported by Senkov et al. [8,25]. This is most likely due to the formation of defects during film growth and the presence of in-plane compressive stresses of about 0.6 GPa in the film as it is frequently observed for films synthesized by CAD [26]. With increasing annealing temperature first a peak shift towards higher diffraction angles relative to the as-deposited peak position was noticed at 1100 and 1200 • C. At temperatures above, a gradual shift to lower angles can be observed. These peak shifts are most likely related to defect recovery and grain coarsening as frequently observed for films deposited by physical vapor deposition when annealed at temperatures above deposition temperature as it was the case here [27]. The grain coarsening due to annealing is also visible by the increase in intensity and decrease in width of the peaks, in particular the (110) peak. After annealing at 1500 • C in vacuum and Ar atmosphere, peaks become visible that could be assigned to elementary bcc phases of Nb and Ta as well as Mo and W. This would indicate the onset of the decomposition of the solid solution, but higher annealing temperatures are necessary to clarify this effect. Alternatively, the small peaks could also be caused by superlattice reflections indicating an order-disorder transition. However, such a transition was not reported for vacuum-annealed bulk MoNbTaVW HEAs [8] and also in present case, the bcc-MoNbTaVW solid solution phase is still the dominating phase after the conducted annealing treatment.
The electrical resistivity of the deposited MoNbTaVW films stays rather constant regardless of the annealing temperature as shown in Figure 5a. The minor variations noticeable with increasing temperature are most likely related to defect recovery and grain coarsening [28,29] as well as to changes in the film morphology due to the sublimation of V. However, the resistivity of the MoNbTaVW HEA thin films is generally by a factor of 2-3 larger than the resistivity of the constituting bulk elements as reported in literature. It is known that defect-rich thin films show a higher resistivity than the respective bulk [30][31][32][33]. In the current case, the formation of the solid solution phase comprising all elements might also contribute to the increase in resistivity.  Hardness and Young's modulus of the as-deposited and annealed thin films are depicted in Figure 5b). While the Young's modulus remained between 290-350 GPa independent of the annealing temperature in vacuum, the hardness value decreased with temperature. The value of about 19 GPa for the as-deposited film is in agreement with the hardness of MoNbTaVW films measured in previous works [16,20,21]. Vickers microhardness values of bulk MoNbTaW and MoNbTaVW alloys, measured by Senkov et al. at room temperature, were around 4.6 GPa and 5.4 GPa, respectively [25]. In contrast, sputter-deposited MoNbTaW by Kim et al. showed hardness values of 12 GPa [17]. Higher hardness values of thin films over bulk materials are usually associated to the smaller grain size and higher stress levels in thin films [34]. However, with increasing annealing temperature the hardness of the MoNbTaVW films continuously decreases to around 9 GPa after annealing up to 1500 • C and, hence, approaches the hardness values of the bulk HEAs. This reduction in hardness is most probably attributed to recrystallization and thermally activated defect annihilation, i.e., decreasing of defect density and stress relaxation [27,35]. Furthermore, the hardness decrease is also influenced by the sublimation and the resulting loss of V in the thin film. The hardness of the film annealed at 1500 • C in Ar atmosphere remained at a value similar to the one after annealing at 1100 • C, i.e., a temperature below the sublimation of V.

Conclusions
The conducted studies demonstrate that MoNbTaVW HEA thin films synthesized by CAD exhibit promising mechanical properties and thermal properties. The bcc structure of MoNbTaVW HEA thin films is thermally stable up to 1500 • C.While the electrical resistivity of the films remains constant, a reduction in hardness from ∼19 to ∼9 GPa was observed after annealing in vacuum. This reduction is caused by defect annihilation and grain growth as it is typical for physical vapor deposited thin films. In addition, the loss of V due to its sublimation in vacuum enhances the hardness reduction. Only a slight loss in hardness was noticed after annealing in inert atmosphere at atmospheric pressure. The obtained results generally show the high potential of MoNbTaVW HEA thin films for high temperature applications where a high thermal stability of the thin film material is required. The present findings can aid the synthesis of such HEA thin films with optimized structure and properties for the desired application.