High Entropy Thin Films by Magnetron Sputtering: Deposition, Properties and Applications

Abstract

Surface coating is of a great interest to increase the performances of the materials and extend its lifetime. High entropy films (HEFs) become the hot spot for developing surface engineering applications due to their good performances. They are reported to have superior properties such as good corrosion, wear resistance and excellent high temperature oxidation. Various deposition techniques have been exploited to fabricate HEFs such as laser cladding, spraying, sputter deposition and electrochemical deposition. These techniques are known to be an easy process to achieve a rapid quenching. Magnetron sputtering is seen as the most efficient methods to deposit the HEFs. Different gas can be used to prepare the ceramic materials. Besides, the deposition parameters reveal a strong influence on the physicochemical properties of HEFs. Working pressure, substrate temperature, bias voltage and gas mixture flow ratios have been reported to influence the morphology, microstructure, and functional properties of HEFs. The chapter overviews the development of the recent HEFs prepared by magnetron sputtering technique. First, it describes the principal of the technique. Then, it reports the classes of HEFs followed by the effect of the deposition parameters on their different properties. Applications have been developed using some HEFs for biomaterials and machining process.

1. Introduction

Conventional alloys are usually based on one main element. To improve the structural and functional properties of the alloy, other elements can be added. However, this strategy leads to the formation of several phases. Indeed, physical metallurgy and phase diagrams show that multifunctional alloys can develop dozens of structures with several phases. Structurally, they can be fragile, and scientifically their analysis will be difficult. On the other hand, high entropy alloys (HEAs) [1, 2] are characterized by high mixing entropy. Therefore, they have become primordial structures for developing potential applications tanks to their superior properties. HEAs materials are characterized by four effects: high entropy, severe lattice distortion, slow scattering and the cocktail effect. They are detailed in the reference [3]. Excellent properties of HEAs have been reported such as outstanding thermal stability [4], good wear resistance [5], good corrosion resistance [6] and best oxidation resistance [7].

On the other hand, the surface is seen as an important component of the material for developing industrial applications. It can be easily modified and adapted to improve the performance of the materials according to demanding conditions. Thus, the quality of the material surface has a significant impact on its lifetime. Thin films are found in many applications with improved surface properties for high-performance materials. For example, tools used in machining are often coated with protective and hard thin films to achieve high mechanical properties and better wear resistance [8, 9, 10]. Better physical properties such as a good oxidation resistance are also required for aerospace and automotive applications. Protective films can also be found in biomedical applications such as bio-implants [11, 12]. Review articles have been reported on the effect of process parameters on the phase structure of HEFs with also a discussion on the preparation process and the functional properties of the films [13]. Others have been focused on the development of HEAs/HEFs operating in extreme conditions and other characteristics [14, 15, 16].

Magnetron sputtering is a widely used technique to deposit thin films. It is used in several industrial applications. This technology has been continuously developed to improve the target utilization and increase the deposition rates by reducing operating costs. Preparing high entropy films (HEFs) by this technique is of considerable interest to provide coatings with superior properties. To this end, several research works report on the development of coatings with improved performances but with low-cost materials. Conventional hard coatings, such as traditional nitrides and carbides, have shown the potential to increase wear resistance. However, these traditional materials seem to not meet the current needs. For example, some traditional nitrides have a limited oxidation resistance. Recently, HEFs have shown much improved performances. A comparison of different alloys is presented in the reference [17] where the oxidation resistance of an HEF reaches 1300°C. (AlCrNbTaTi)N HEF shows an oxidation resistance at 850°C for 100 hours [18] which is better than various traditional nitrides. Many HEFs, prepared by magnetron sputtering technique, revealed other excellent performances [19, 20, 21, 22].

By using magnetron sputtering technique, various deposition parameters are exploited to control the properties of HEFs. Among these parameters, we find gas flow rate, substrate temperature, working pressure, and bias voltage. The chapter overviews magnetron sputtering process, the classes of different HEFs and it shed light on the effect of the different parameters on the properties of HEFs. Applications of some HEFs are also presented.

2. Magnetron sputtering deposition

There are several ways to prepare thin films. The most common methods used in research laboratories and in industry are PVD and CVD techniques. Among the PVD procedures, magnetron sputtering shows several advantages. It offers the possibility to obtain a stoichiometry like that of the used target. The quenching rate is high (10⁹ K/s) leading to the formation of sutured solid solutions. In the following paragraph, the process of the magnetron sputtering will be briefly introduced.

2.1 Direct current magnetron sputtering (DCMS) deposition

The synthesis of coatings by using magnetron sputtering technique can be done in three steps. In the first step, an atomic vapor is created by extracting the atoms from the target thanks to applied potential difference between the target and the reactor walls. Then this vapor is transferred to the substrate in a rarefied atmosphere of chemically neutral gas. In the last step, the atomic vapor condenses into the substrate surface allowing the germination process and consequently the growth of the film.

This basic process is limited by various effects like low deposition rate, low ionization coefficient in the plasma, and a substrate heat. To circumvent these limitations, a magnetron dispositive is integrated into the process. The magnets are placed behind the cathode. They generate a magnetic field parallel to the surface of the target, perpendicular to the electric field. The electrons emitted by the cathode and present in the gas and are trapped by the field lines (Figure 1). The probability that an electron meets argon is so high. The ion bombardment of the target results in a higher sputtering rate and the deposition rate increases.

Figure 1.

Magnetron sputtering process.

2.2 High power impulse magnetron sputtering (HiPIMS) deposition

The difference between the High Power Impulse Magnetron Sputtering (HiPIMS) and DCMS is the use of high power densities. HiPIMS is a recent advance in sputtering process using magnetron sputtering with a high voltage power source. High voltage with short duration is used to generate high-density plasma resulting in high degree of ionization of the coating material. HiPIMS, has various advantages. The highly energetic ions, produced by high voltage, result in denser film compared to that deposited by conventional techniques. HiPIMS bombards the sample with high-energy gas ions that can remove oxides and therefore clean the surface. This can improve the adhesion of the coating to the surface. A description of plasma process using HiPIMS can be found in Anders’s tutorial [23].

Few studies have been reported in the literature on using HiPIMS to prepare the HEFs. The papers revealed the formation of dense microstructures of the films compared to that obtained by DCMS process. The change of the microstructure strongly influences the mechanical and electrochemical performances of HEFS. For example, Bachani and co-workers [24] investigated (TiZrNbTaFe)N using HiPIMS process and found that the film containing 32 at.% of nitrogen exhibits a very dense microstructure compared to others. Its hardness is improved (36.2 GPa). The corrosion resistance is increased, according to the variation of the nitrogen content, due to the densification of the films. CuNiTiNbCr dual-phases were formed at different working pressures as reported by Li and co-workers [25]. AlCrTiVZr HEFs have been studied under the effect of nitrogen. Due to its densification, the nitride obtained at 12 sccm presents a hardness of 41.8 GPa which is the super-hard film compared to others. Figure 2 presents SEM images showing the difference in the morphology of (AlCrNbSiTiV)N films obtained by both processes DCMS (Figure 2a, b) and HiPIMS (Figure 2c, d) [26].

Figure 2.

SEM images showing the surfaces and cross-sectional microstructure of (AlCrNbSiTiV)N deposition using DCMS (a,b) and HiPIMS (c,d). The figure is reproduced with permission of [26].

3. HEFs classes

HEFs can be classified into three categories: metallics, ceramics and composites films as presented in Figure 3.

Figure 3.

Classes of HEFs.

Metallic HEFs: they consist basically of Cantor-based elements. They are mostly composed of transition elements such as Al, Cr, Fe, Ti, Mo, etc. Refractory elements are also used to develop coatings for high-temperature applications. The refractory elements, such as Hf, Ta, Nb, V, W, etc have much higher melting point. These materials are classified into HfNbTaZr, CrMoNbTa [27].

Ceramic HEFs: consist of nitrides, carbides, oxides and borides. These materials can be deposited on substrates by using reactive mode (introduction of gas). Solid solutions are reported and strong nitride-, carbide-, oxide-forming elements like Zr, Cr, Si and Ti are used. These ceramics exhibit superior properties such as high oxidation resistance, good corrosion resistance and high tribological performances.

Composite HEFs: These materials can be prepared by reinforcing the film matrix with ceramics. Various ceramics like WC, TiC, NbC, and others have been used as reinforcements to improve the properties of HEFs. Metallic reinforcements have been also used. For example, Tian and co-workers [28] prepared a compact ACoCrniFeTi/Ni coating with Ni splats uniformly distributed in the matrix (ACoCrniFeTi). Due to this reinforcement, the results reveal an improvement in its tensile test compared to that of the matrix alone.

4. Properties of HEFs

4.1 Morphology and structural of HEFs

DCMS was largely used to prepare the HEFs under different conditions. The most-reported films have three different morphologies, columnar, dendrite-like and fibrous-like. The deposition parameters are reported to have a strong influence on the HEFs morphology. The mobility of particles into the surface of the substrate is the main reason of resulted morphology of the films. Studies have reported the effect of gas mixture on the films properties. Nitrogen, carbon and oxygen are used to form high entropy ceramics. For example, AlCrNbYZr exhibits dendrite-like morphology. By adding the nitrogen (AlCrNbYZr)N films reveal V-shaped columnar morphology. Zhang et al. [29] studied the effect of nitrogen on CrNbTiAlV films. Smooth surface and dense cross-sectional morphology are observed in the metallic film (CrNbTiAlV). With addition of the nitrogen, (CrNbTiAl)N gradually changes into columnar morphology. Some carbides show a different trend. Jhong et al. [21] studied CrNbSiTaZr as a function of CH₄ flow rate. They show that all films exhibit smooth surface and featureless cross-sectional morphology. The mobility of the atoms on the surface can be the main reason for the formation of dense structures. Oxygen gas also influences the morphology of HEFs. In the case of AlCoCrCu0.5FeNi, the films exhibit agglomerated grains at a low oxygen flow rate. However, as the oxygen flow increases the grains become equiaxed with a size of 35 nm.

The morphology of HEFs can be also influenced by other parameters like the pressure and the bias voltage. AlCrTiWNbT shows a columnar morphology when the bias voltage changes from −50V to −100V but at −150V fine spaced striation lines are formed of the film [30]. Fine fibers morphology is formed for CrNbSiTiZr at −50V which is transformed to compact at −200V [31].

For the structure, the prepared HEFs by magnetron sputtering are reported to have amorphous or crystalline structure. The most crystalline structures exhibit fcc solid solution. The structure can change under the effect of various parameters such as the high entropy and the atomic mismatch. The high entropy promotes the formation of the solid solution instead of metallics compounds. However, if the difference in atomic mismatch is enough, the film remains amorphous. In the case of HEFs by magnetron sputtering, three different structures are reported: amorphous, fcc and bcc. AlCoCrNi [32], NbTiAlSiZr [33] and FeCoNiCuVZrAl [34] HEFs exhibit an amorphous structure which remains unchangeable even after adding the nitrogen. On the other hand, other studies revealed the phase transformation from amorphous to crystalline structure upon increasing the nitrogen contents in the films. Cheng et al. [22] examined the effect of nitrogen on AlCrMoTaTiZr by varying its flow rate from 0% to 50%. The percentage of the flow rate is calculated according to argon quantity by the followed formula R_N=N₂/(Ar+N₂). The results show that the film exhibits an amorphous structure at R_N=0%. However, when the nitrogen flow increases fcc-single phases structures are formed as is presented the Figure 4.

Figure 4.

X-ray diffractogramme of (AlCrMoTaTiZr)N as function of nitrogen flow rate (N₂). The figure is reproduced with permission of [22].

The nitrogen atoms are adsorbed in interstitial sites leading to the formation of nitrides. The interaction between the nitrogen and the elements varies along with the periodic table. Group 4–6 are strong nitride former while metals in group 7–11 are weakly nitride former. This interaction has a mix of ionic, metallic and covalent bonding. The reported HEFs nitrides have NaCl-type structure as mentioned above. Because of the similarities of the structure between different standard nitrides, extended homogeneity regions for solid solutions can be obtained.

Among other ceramics, carbides can be also prepared. Up to now, they have not been widely investigated like the nitrides. Most references focus on the use of strong carbide forming metals. Kuang et al. have investigated the tribological properties of CrNbTiMoZr carbide films [35]. Kao and co-workers studied the effect of carbon content on the mechanical and electrochemical properties of CrNbSiTaZr films [36]. There is also others few studied on the same subject [21, 37].Figure 5 shows XRD diffractograms of (CrNbSiTiZr)Cx films as a function of CH₄ rate flow. In the case of carbides, textured solid solutions are also formed. (111), (200), (220) and (311) peaks of the prepared films reveal the formation of fcc NaCl-type structure (Figure 5).

Figure 5.

X-ray diffractogramme of (CrNbSiTiZr)C_x as function of nitrogen flow rate (N₂). The figure is reproduced with permission of [21].

4.2 Surface chemistry of HEFs

X-ray photoelectron spectroscopy (XPS) is a powerful technique to provide information on the composition and the chemical binding between the elements. Up to date, few XPS analysis are reported on HEFs studies. More efforts are needed to provide more information on the binding nature and the compound of different elements constituting HEFs.

Khan et al. [38] examined AlCoCrCu_0.5FeNi nitride films by XPS. They showed that the porosity of films grows with a higher nitrogen flow fraction facilitating than the atmospheric oxidation. XPS analysis confirmed the formation of protective oxides AlO₃, CrO₃ and nitrides AlN and CrN on the films surfaces [38].

Khan and co-workers [38] used XPS to determine the oxidation states of AlCoCrCu_0.5FeNi HEFs. The films were deposited at various nitrogen flow rates. The results revealed the formation of both nitrides and oxides on the surface of the films. At higher nitrogen flow, binary oxides Al₂O₃ and Cr₂O₃ were formed together with nitrides AlN and CrN on the films surfaces. Feng and coworkers [39] studied (ZrNbTaTiW)N HEFs and reported by XPS the formation of a mixture of metallic (Nb, W, Ta), nitride (ZrN, TiN, TaN) and oxide ZO₂.

Our group studied the nitridation effect on AlTiTaZrHf, prepared by the magnetron sputtering technique [40]. All the elements are nitride after adding the nitrogen. As the nitrogen flow rate increases, the nitride content changes according to the affinity of each element. The atomic percentage is estimated according to XPS analysis and is presented for each individual element in Figure 6. Both metal Ta and Hf show a quick increase of nitridation followed by quasi-stable evolution when R_N2=N₂/(N₂+Ar) increases from 0 to 50%. Al and Zr elements show a weak increase at R_N2=5% and stabilization when R_N2 continues to grow. However, Ti reveals a quasi-stable formation of nitride even though R_N2 increases (Figure 6). The results demonstrate that all the elements are nitride during the deposition leading to the formation of high entropy nitride films.

Figure 6.

Atomic percentage of individual elements, Al, Ti, Ta, Zr, and Hf as a function of nitrogen flow rate R_N2=N₂/(N₂+Ar) during the preparation of AlTiTaZrHf(-N) HEFs. The curves are presented according to XPS analysis.

4.3 Mechanical properties

The mechanical properties have been investigated for large amounts of HEFs and the results revealed an improvement in the materials' performances. Indeed, good hardness and wear resistance make the HEFs as well as their nitrides promising candidates for cutting tools for example. HEFs prepared by the magnetron sputtering technique showed that the mechanical properties are influenced by various deposition parameters. For example, Yu and co-workers [31] studied CrNbSiTiZr by changing the substrate bias voltage. The results showed that the hardness increased at a maximum of 12.4 ± 0.3 GPa with −50 V bias followed by a decrease to 9.8 ± 0.2 GPa when the bias is −200 V. Young’s modulus shows the same trend by increasing at 187.7 ± 3.3 GPa for −100 V and a decreasing to 162.3 ± 3.7 GPa for −200 V. The working pressure is also another parameter influencing the mechanical properties of HFEs. Kim and co-workers [32] reported high mechanical properties of AlCoCrNi films obtained with a pressure of 1.33 × 10⁻¹ Pa. Indeed, at low pressure (1.33 Pa), the hardness is measured at 8.9 ± 0.9 GPa and the modulus at 142 ± 11 GPa. However, when the pressure reaches 1.33 × 10⁻¹ Pa, the hardness increase to 16.8 ± 0.5 GPa and the modulus to 243 ± 39 GPa.

The nitrides show an increasing of mechanical properties as the nitrogen flow rate increases followed by a decrease as the flow continues to increase. This trend is seen for various high entropy nitrides [29, 41, 42, 43, 44]. An example is presented in Figure 7a. (AlCrMoTaTiZr)N HEFs, obtained with 40% nitrogen flow ratio, is the hardest film compared to others with a hardness of 40 GPa with Young’s modulus higher than 370 GPa. Residual stresses have been also studied and their evolution is depending on the nitrogen content. Zhang and co-workers [29] investigated the residual stress of (CrNbTiAlV)N HEFs at different nitrogen flows. The result is presented in Figure 7b. The metallic film has the lowest value of −2.35 GPa while a maximum (−6.55 GPa) is obtained for the nitride at 38 sccm.

Figure 7.

Hardness (a) and Residual stress (b) of (CrNbTiAlV)Nx films deposited under different nitrogen flows. Figure reproduced with permission from [29].

In the case of oxides, as the oxygen flow rate increases the microstructure became dense and consequently the mechanical properties are improved. At a high flow rate, these properties degrade. The hardness of AlCoCrCu_0.5FeNi reaches the maximum (11.3 ± 0.9 GPa) at 25% of O₂ and decreases as the flow continues to increase.

Other results show that this trend is not always true. Khan and co-workers [45] reported no change in the mechanical properties by studying AlCoCrCu_0.5FeNi films. These laters are prepared as a function of the working pressure. Their hardness was measured at 13 GPa while Young's modulus was evaluated at 204 GPa and no change was revealed as the pressure increased.

4.4 Tribological performances

The change in microstructure could be the result of several factors such as preferential orientation and variation in crystallite size. These phenomena improve the tribological performance of HEFs. The presence of defects can also prevent plastic flow during deformation in the material that can change its hardness. The change in the hardness will lead to a change in its tribological properties.

The tribological performance of (AlCrNbSiTiMo)N has been studied by Lo and co-workers [46] at ambient temperature and after annealing at 700°C. The results showed a decrease in the friction coefficient, especially after annealing. This reduction difference in the coefficient of friction was measured at 0.2. At room temperature, the coefficient was 0.68 ± 0.09 while after annealing at 700°C it became 0.48 ± 0.08 as shown in Figure 8. The result revealed an improved wear resistance due to the formation of MoO₃ after annealing, which acts as a lubricating effect.

Figure 8.

Friction Coefficient of the AlCrNbSiTiMoN coatings. Figure reproduced with permission from [46].

By changing the content of the elements, the tribological properties can be improved. For example, the effect of Al was studied by Cui and co-workers [47] on FeCoCrNiMnAl_x alloy. As the Al content increases, the friction coefficient of these films decreases. On the other hand, the incorporation of carbon, producing a lubricating effect can also reduce the friction coefficient [35].

4.5 Corrosion

Corrosion is described as a physical-chemical interaction between a metal and its environment leading to changes in its properties and significant degradation of its function. Developing corrosion-resistant materials is a necessary need to resolve the issue and improve their performance. Due to the elevated entropy, HEFs form solid solutions rather than intermetallic compounds. This makes the materials with best functional properties. The corrosion of HEFs has been mostly studied in nitric acid, salt (NaCl) and in HCl. It has been reported that Cr, Ni, Co and Ti elements can improve the corrosion resistance in acid solution. Mo element could inhibit pitting corrosion in a solution containing Cl. Such phenomena have been carried out for HEFs prepared by magnetron sputtering. The results revealed that these properties are influenced by different deposition parameters. To carry out the electrochemical measurements of the films, potentiodynamic polarization tests are used. The parameters include corrosion potential (E_corr), pitting potential (E_pit) and corrosion-current density (I_corr). This later can be used to estimate the corrosion rate as described by the equation below [48]:

$$\mathit{Corroion}\mathit{rate}\left(\frac{\mathit{mm}}{\mathit{year}}\right)=3.27\times {10}^{-3}\times \frac{I_{\mathit{corr}}}{\rho }\times EW$$E1

Where ρ is the density of the alloys (g/cm³), I_corr (μA/cm²) and EW present the equivalent weight given by:

$$EW={\left(\sum \frac{n_i{f}_i}{W_i}\right)}^{-1}$$E2

n_i, f_i and W_i are the ith elements, the masse fraction and the atomic weight of ith element in the alloy respectively.

Anti-corrosion performance of (CrNbTaTiW)C has been studied by Malinovskis and co-workers [19] in HCl solution with a concentration of 1M. The results revealed that the carbides showed the best corrosion resistance compared to that Stainless Steel. Gao and co-workers [49] performed a deposition of (CoCrFeNiAl_0.3) on silicon by using magnetron sputtering. The films show better corrosion resistance compared to austenitic 304L stainless steel. Wang and a coworker [50] studied corrosion behavior of AlCoFeNiTiZr HEFs in NaCl solution. Three coatings, (Fe-Co-Ni)₂₅(Al-Ti-Zr)₇₅, (Fe-Co-Ni)₂₀(Al-Ti-Zr)₈₀, (Fe-Co-Ni)₁₅(Al-Ti-Zr)₈₅ have been tested. According to the reported results, (Fe-Co-Ni)₂₅(Al-Ti-Zr)₇₅ exhibits the lowest I_corr and the highest E_corr revealing its best corrosion resistance compared to other films. Wang and co-worker [37] investigated the electrochemical properties of (CrNbSiTiZr)C in a 3.5 wt% NaCl solution. Figure 9 presented the potentiodynamic polarization curves of the film. The result is compared to that of 304L stainless steel (SS). The study reported that (CrNbSiTiZr)C shows a E_corr of −189 mV and I_corr of 0.0026 μA/cm². E_corr is higher and I_corr is smaller than that of 304L SS (E_corr= −319 mV; Icorr= 0.13 μA/cm²). Th result then reveal that (CrNbSiTiZr)C exhibits a higher corrosion resistance compared to that of 304L SS.

Figure 9.

Potentiodynamic curve of (CrNbSiTiZr)C in 3.5 wt% NaCl solution. Figure reproduced with permission from [37].

The change in composition (variation in the elements contents) influences the microstructure. Therefore, the change of the microstructure will improve the functional properties such as corrosion resistance. Wang and co-workers [50] reported an improved corrosion resistance of (Fe-Co-Ni)_x(Al-Ti-Zr)_100-x as a result of the increase in Fe-Co-Ni content. As a result, (FeCoNi)₂₅(AlTiZr)₇₅ showed a better corrosion performance. The addition of Al in the refractory HEF films VNbMoTaW was beneficial in terms of increasing the corrosion resistance. With 2.37 at.% of Al present in the alloy, excellent corrosion resistance was measured compared to 304 stainless steels in 0.5M of H₂SO₄ [51]. However, increasing the Al content can have a negative effect on the quality of the film. Indeed, at high Al content porous oxides can be formed and the pores can easily facilitate the diffusion of acid. As a result, the corrosion resistance decreases.

Other deposition parameters strongly influence the anticorrosion performance of HEFs. Kao and co-workers [36] reported an improved corrosion resistance of CrNbSiTaZr films. The films were prepared in a C₂H₂ containing environment. Varying the bias voltage can also change the properties of the films. Von Fieandt and co-workers [52] showed a better corrosion resistance of (AlCrNbYZr)N_x films compared to stainless steel. The electrochemical measurements were done in HCl by changing the polarization voltage and the temperature.

4.6 High temperature oxidation

Various HEFs have been prepared to investigate their high-temperature oxidation behavior. (Al_0.34Cr_0.22Nb_0.11Si_0.11Ti_0.22)₅₀N₅₀ HEFs have been annealed at 900°C for 50 hours [17]. Two different oxides were formed: α-Al₂O₃ and rutile-TiO₂. The dense Al₂O₃ formed on the top layer of the films was a key reason to improve their oxidation resistance. Compared to traditional films TiN and TiAlN prepared under the same conditions, (Al_0.34Cr_0.22Nb_0.11Si_0.11Ti_0.22)₅₀N₅₀ HEFs reveal the best oxidation resistance and can be potential candidates for developing high-temperature applications. The element content is one of the influencing parameters that can improve the oxidation resistance of the films. Indeed, Al and Si can lead to the formation of dense α-Al₂O₃ and α-SiO₂ layers on HEFs at high temperature which improves their oxidation resistance. The oxidation behavior of (AlCrNbTaTi)N films, in air as a function of Si concentration, was reported by Kretschmer and co-workers [18]. The films were annealed at 850°C for 100 hours. Without Si, the oxide thickness of the film is important (2700 nm), however when Si was added the oxide thickness was measured at 280 nm. This means that Si forms a dense layer on the surface during the oxidation preventing then the diffusion of the oxygen in the film.

Tsai and co-workers [53] reported the same trend of Si effect. Figure 10 shows the variation of the oxide thickness formed according to Si content in the films. As Si content increases, the thickness of oxide layer decreases revealing a good oxidation resistance at high temperature.

Figure 10.

Cross-sectional SEM micrographs of the (AlCrMoTaTi)N HEFs with (a) 0 at.%, (b) 2.77 at.%, and (c) 7.51 at.% of Si coatings after annealing at different temperature in air. Figure reproduced with permission from [53].

In the case of HfNbTaTiZr film HEFs, it was shown that oxygen reacted with all elements forming oxide nanoclusters. XPS was used to analyze the oxidation behavior and the results revealed 66 at.% of oxygen content where no oxide was determined by other techniques like SEM, TEM and X-ray diffraction [54]. The oxygen was found to preferentially bind to Ti, Zr and Hf rather than other elements.

5. Applications

HEFs deposited by magnetron sputtering techniques have been exploited to develop some applications. They can serve as surface protective materials. Among the different applications, materials for biomedical and for machining will be presented in this section.

5.1 Biomedical application

Various scientific research has been focused on coatings to improve the performance of implants and prostheses. Compared to traditional coatings using this field, HEFs become the hot spot in surface engineering development. Two films, (HfNbTaTiZr)N and (HfNbTaTiZr)C, have been prepared by magnetron sputtering technique [55]. The corrosion property of these films was simulated in body fluid. The results have revealed a very small ratio of dead cells that were observed for both (HfNbTaTiZr)N and (HfNbTaTiZr)C HEFs. Si was used to improve the biocompatibility of the materials. Valdescu and co-workers [11] have replaced Ta with Si in the case of (TiZrNbTaHf)C. Considering the role of electrostatic interactions between the biomaterial surface and the cells, the authors examined the effects of surface charge (characterized by electrical potential and work function) on the biocompatibility property. A low electrical potential and high work function of (TiZrNbSiHf)C film was obtained revealing that this film exhibits best biocompatibility.

5.2 Machining application

The dry machining process is seen as the best alternative to replace the oils in the industry. Because the oils have a negative impact on both operator health and the ecology. An environmental transition is, therefore, necessary to develop clean processes. HEFs are now interesting materials where scientific efforts are underway to improve the performance of cutting tools. thermal and machining properties of (Al_0.34Cr_0.22Nb_0.11Si_0.11Ti_0.22)₅₀N₅₀ HEF, have been examined by Shen and co-workers [56]. The cutting performances of the films are better as the milling was operating at a high temperature. Due to its superior properties like high hardness, good thermal stability and outstanding oxidation resistance, the HEF shows great potential to be exploited in machining applications. The study reported that, after 900 m of cutting, the wear depth is 226, 202, 184, and 175 μm for uncoated, TiN, TiAlN and (Al_0.34Cr_0.22Nb_0.11Si_0.11Ti_0.22)50N₅₀ HEF respectively.

6. Conclusion

The chapter reports and discusses briefly various properties of HEFs fabricated by magnetron sputtering. Intensive scientific efforts have been paid to this area for improving the materials surfaces and developing innovant materials. The preparation of the films is performed according to various deposition parameters.

Two processes are reported, standard direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) that are used to prepare HEFs in different environments. HiPIMS process led to the formation of denser microstructure compared to that with DCMS. Substrate bias voltage, working pressure, gas flow rate as deposition parameters, all are discussed and revealed that they strongly influence the physico-chemical properties of HEFs. Amorphous to crystalline structure of the most prepared HEFs transition took place upon introduction of gas like N₂ or CH₄ or O₂.

Two functional properties, electrochemical (corrosion) and physical (oxidation) are reported and discussed. Th both properties have been reported to be influenced by different deposition parameters. The preparation of dense films prevents acid attack and improve corrosion resistance. The formation of some oxide layers such as α-Al₂O₃ and α-SiO₂ on the top film surface plays a great role in protecting the materials from oxidation at high temperature.

Some HEFs are exploited to develop application in the various materials field. Examples are reported on biomaterials and machining processes showing the best performances of the films compared to traditional coatings.

The prepared HEFs revealed enhanced surface protection ability. Even with the promising performances that possess the HEFs, more efforts are needed to develop a deep understanding of this class of materials. The complexity of the materials increases with the number of possible combinations of elements. Traditional metallurgy cannot meet the requirements of quick results, which requires a lot of time depending on the combinations. Advanced characterization techniques are needed which must be combined with theoretical simulation to solve this type of problem. Artificial intelligence can also be added to the knowledge of traditional metallurgy to define new approaches for studying innovative HEFs.

Acknowledgments

Authors ME, FS and FSC thank the Université de Technologie de Troyes (UTT) and Commissariat à l'Energie Atomique et aux énergies alternatives (CEA) Saclay.

Conflict of interest

The authors declare no conflict of interest.