Preparation of High-Entropy Silicide Coating on Tantalum Substrate by Silicon Infiltration Method and Its Antioxidant Performance

Abstract

High-entropy silicide (MeSi₂) coating was prepared by the slurry method and silicon infiltration method using Mo, Cr, Ta, Nb, W, and Si elemental powders as raw materials. The coating consisted of four layers, including a porous MeSi₂ layer, a (CrTa)Si layer, a TaSi₂ layer, and a Ta₅Si₃ layer from outside to inside. At 600 °C, Si was preferentially oxidized to form SiO₂ oxide film. The mass gain rate of the coating was 0.2 mg/cm² over a period of 100 h oxidation, eliminating the phenomenon of low-temperature pulverization. At 1200 °C, MeSi₂ coating had a protection time of 20 h. During the oxidation process, the coating generated metal oxides, forming a thin SiO₂ oxide film. TaSi₂ and Ta₅Si₃ gradually transformed into Ta₂O₅, and the coating eventually failed.

1. Introduction

Tantalum and its alloys have excellent high-temperature mechanical properties (high-temperature strength and creep resistance, etc.) and good room temperature performance, making them potential application materials for ultra-high-temperature component structural materials, and are especially applied in the aerospace industry [1,2]. However, the poor oxidation resistance of Ta and Ta alloys in high-temperature service environments limits the service range and lifespan of related components. To improve oxidation resistance, researchers have carried out a lot of work. Surface antioxidation coating has become one of the important means in the current thermal protection technology field of tantalum materials, with broad application prospects and good development potential. Song et al. [3] prepared Si-Ti coatings on the surface of a Ta alloy using the melt slurry method. Dong et al. [4] prepared ZrB₂ antioxidant ceramic coating on a Ta-W alloy surface using a two-step method. Majumda et al. [5] prepared a 40 μm multi-layer Al-Si composite coating on the surface of tantalum using embedding infiltration technology. Cai et al. [6] prepared MoSi₂-TaSi₂ ceramic coatings on pure Ta substrates using a three-step method. Yoon et al. [7] prepared nano Si₃N₄ particle reinforced TaSi₂ coating on the surface of a Ta substrate through in situ displacement reaction between Ta nitrides (TaN and Ta₂N) and Si.

Silicide coatings are currently the most used coating systems in the field of high-temperature oxidation resistance [6,7,8,9,10]. Their oxidation resistance mechanism mainly relies on the formation of a dense SiO₂ oxide film at high temperatures. The Gibbs free energy of the reaction between Si and oxygen at high temperatures is low, and the fluidity of the SiO₂ formed by the reaction has the ability to “selfheal” at high temperatures. Molten SiO₂ can flow into cracks and pores in the coating, improving the density of the SiO₂ oxide film, preventing oxygen diffusion to the substrate, and continuously protecting the tantalum substrate.

MoSi₂ coating has excellent antioxidant properties and self-healing ability at high temperature, making it the most used antioxidant coating system for tantalum and tantalum alloys. However, MoSi₂ coating shows a ‘pesting’ phenomenon at low temperature [10]. When the temperature is between 400 and 600 °C, the diffusion coefficient of Si in MoSi₂ is low, and complete SiO₂ film cannot be formed, while molybdenum is oxidized, forming powder products mainly with MoO₃ whiskers (or sheets) and SiO₂ clusters. Recently, high-entropy ceramic silicide has attracted a lot of attention [11]. As reported, high-entropy ceramic silicide can eliminate the ‘pesting’ phenomenon that usually appears for the single-silicide coating [12] and has advantages as a high-temperature oxidation resistance material [13]. The high-entropy ceramic silicide has great potential for application as a high-temperature protective coating [14]. At present, the main preparation methods for surface antioxidation coatings on tantalum and tantalum alloys include the halide embedding infiltration method, slurry sintering method, spray coating method, surface cladding method, etc. The halide embedding infiltration method uses gaseous halides as carriers to infiltrate protective elements (Si, B, etc.) into the surface of tantalum and tantalum alloys, forming protective coatings. The coating and substrate interface prepared by the embedding infiltration method have high bonding strength, a dense structure, a low oxygen permeation rate, and good oxidation resistance. Therefore, in this study, a (Mo_0.2Cr_0.2Ta_0.2Nb_0.2W_0.2)Si₂ high-entropy silicide ceramic coating on a tantalum substrate was prepared using the slurry sintering method and halide embedding infiltration method, and the oxidation behaviors of the coating at 600 °C and 1200 °C were evaluated.

2. Experimental Details

2.1. Preparation of Precursor Powders

Equal atom contents of Mo, Cr, Ta, Nb, and W powders with an initial particle size of no more than 325 mesh were weighed. First, the powders were mixed thoroughly in a mixer at 120 r/min for 24 h. After mixing, the powders were placed in a ball milling jar under argon protection environment. The ball-to-material ratio was 10:1. The milling time was 60 h with a speed of 400 rpm.

2.2. Preparation of Coating

Firstly, a high-entropy alloy coating was prepared using slurry method. The slurry was composed of elemental metal precursor powders, anhydrous ethanol, PVB, and silicon powders with a mass ratio of 42:52:1:5. The mixture was subjected to wet grinding, with a ball mill rotation speed of 200 rpm, a ball powder mass ratio of 5:1, and a ball milling time of 2 h. After wet grinding, the slurry was evenly brushed onto the surface of the tantalum substrate, and the sample was dried, and a coating with thickness of 100 μm could be obtained by repeating 4 times. Then the samples were dried and sintered in a vacuum sintering furnace. The furnace was heated from room temperature to 1420 °C for 1.5 h with a pressure of 1 × 10⁻³ Pa. Then the samples with sintered high-entropy alloy coating were placed in a sealed corundum crucible filled with a filler mass ratio (Si:NaF:Al₂O₃ = 30:5:65) and sintered at 1200 °C under an argon atmosphere for 2 h to obtain a MeSi₂ coating.

2.3. Oxidation Test

The antioxidant experiments were conduct on a muffle furnace at temperatures of 600 °C and 1200 °C. Before testing, the surface area of each coating sample was measured using a vernier caliper, then the samples were placed in an alumina crucible and measured for initial mass. Subsequently, the samples were transferred to a muffle furnace (SG-G03143, Zhonghuan Electric Furnace Co., Ltd., Tianjin, China) preheated to 600 and 1200 °C. At regular intervals, the samples were taken out and weighed. Then the mass change was calculated by dividing the change of weight by its surface area. To eliminate errors, 5 samples were tested to obtain the average value. The oxidation weight gain per unit area of the sample was calculated according to Formula (1):

$$\Delta W={\displaystyle \frac{\left(m_1-m_0\right)\times 100\%}{S}}$$

(1)

In the formula, ΔW is the oxidation weight gain per unit area of the coating sample (mg/cm²); m₀ is the initial mass of the sample (mg); m₁ is the mass of the oxidized sample (mg); S is the surface area of the sample (cm²).

3. Results and Discussion

3.1. Microstructure and Composition of Coatings

Figure 1 shows the XRD diffraction patterns of the elemental powders after different ball milling times. At the beginning, diffraction peaks of each element could be clearly detected in the mixed powder. After ball milling for 10 h, the width of the diffraction peaks slightly increased while the intensity of some peaks changed. The intensity of Si and Cr elements rapidly decreased, indicating the gradual formation of a solid solution. The changes in width and diffraction angle of the peaks represented the lattice distortion of the powder. In the solid-state reaction process, the alloying rate between various elements is mainly affected by the melting point of the elements, mechanical fragmentation between particles, and solid-state diffusion between elements. The lower the melting point of the elements is, the faster the alloying rate will be. Elements with higher melting points have smaller intrinsic diffusion coefficients, resulting in slower alloying rates. In addition, the interactions between different elements also have a certain impact on the diffusion rate, and negative mixing enthalpy is conducive to increasing the diffusion rate and promoting the solid solution process. It was found in the literature that Mo, Nb, Ta, Cr, W, and Si all had negative mixing enthalpies [15], resulting in decreases in the diffraction peaks of Si, Nb, Ta, Cr, and Mo. When the ball milling time reached 20 h, as the ball milling time continued to extend, the grains were further refined and the enthalpy stored at the grain boundaries increased, thereby accelerating the diffusion and solid solution processes. The peaks of Si and Cr elements completely disappeared, and the peaks of the mixed powders were mainly corresponding to a solid solution with the BCC structure as reported [16], with only the Ta and W elements being clearly detectable. When the ball milling time reached 40 h, the main diffraction peaks shifted right to 40.301°, and the intensity of the peak increased. After milling for 60 h, the peak continued to shift right to 40.367°. Then the effect of prolonging ball milling time was not significant, and the changes in the XRD pattern were no longer obvious.

Figure 2 shows the SEM image of the powers after milling for 60 h and the EDS map images of the elemental powders, in which Ta, Nb, Mo, Ta, and Cr elements are uniformly distributed. However, a small amount of the W element was enriched, which may have been related to the high melting point of W.

The sintering procedure of the slurry with elemental metal powder on the Ta substrate was 1420 °C and kept for 1.5 h. Then the sintered sample was placed in a corundum crucible, filled with filler (Si:NaF:Al₂O₃ = 30:5:65 wt%), and sealed with a high-temperature repair adhesive. According to reference [17], the silicon infiltration temperature and time were set to be 1200 °C for 2 h. The XRD patterns for the samples before and after silicon infiltration are present in Figure 3. The ball-milling metal powers after sintering formed a high-entropy alloy with a BCC structure, with only a small amount of the Ta₂O phase present. After silicon infiltration, a MeSi₂ phase formed on the surface. In addition, some WSi₂ phase had been observed due to the insoluble W element during the ball milling process.

Figure 4 shows the SEM images and the cross-sectional EDS spectrum of MeSi₂ coating. The coating consisted of four layers. Metal elements were mainly distributed in the outermost layer while tantalum and silicon elements were distributed in each layer, indicating the formation of other silicides. It is worth mentioning that the Cr element was enriched in the second layer from the outside to the inside. The rapid diffusion of Si led to the formation of many vacancies in the interface between the outer and inner layers. Except for Si, the melting point of Cr element was the lowest among the metal elements, so the Cr element had faster diffusion rate than other metals. In the interface, Cr reacted with Ta and Si to form a small number of intermetallic compounds [18,19].

The EDS energy spectrum analysis results of the MeSi₂ coating cross-section are shown in

Table 1. EDS composition of the different regions in Figure 5a.

Points	Composition (At%)							Possible Phase
	Si	Cr	Nb	Mo	Ta	W	O
1	69.4	3.5	6.5	4.4	6.5	5.7	4.0	MeSi2
2	40.8	14.8	0.4	0.4	37.8	0.1	5.7	low silicides of Cr and Ta
3	65.4	0.3	0.0	0.3	30.6	0.0	3.4	TaSi2
4	41.8	0.5	0.0	0.4	51.8	0.2	5.3	Ta5Si3

. The elements in region 1 are Si and metallic elements, with an atomic ratio of approximately 2:1, and the proportions of the Si, Cr, Nb, Mo, Ta, and W elements conform to the nominal molar ratio of MeSi₂. Based on the XRD pattern in Figure 3, it can be concluded that the outer layer had formed high-entropy silicide. Below the outermost layer, the element contents of Cr and Ta were relatively high. This was because when sintering, Si diffused and reacted quickly with the metal elements of the coating, leading to numerous vacancies in the coating. Cr, with the lowest melting point among the metal elements, has the fastest diffusion rate. So, Cr diffused along the vacancies. Therefore, a distribution of Cr elements in the Ta₅Si₃ layer and TaSi₂ were also observed. The atomic ratio of Si and Ta in the layer where point 3 is located in Figure 5 was about 2:1, indicating the formation of a TaSi₂ phase. For point 4, the atomic ratio of Si to Ta in the inner layer was about 3:5, forming the low silicide of Ta₅Si₃. The reason for its formation was that during the sintering process, Si element diffused into the tantalum substrate, forming a transition layer Ta₅Si₃ phase at the interface of the tantalum substrate. As the temperature increased and the holding time was prolonged, Si concentration in the tantalum substrate continued to increase, and Ta₅Si₃ continued to react with Si element to form a TaSi₂ layer.

During the silicon infiltration process, Si diffused from the embedded mixed powder to the surface of the tantalum substrate and was deposited on the surface. The possible reactions that may have occurred in the silicon infiltration process were as follows:

$$\mathrm{Si}\left(\mathrm{s}\right)+\mathrm{xNaF}\left(\mathrm{l}\right)\to \mathrm{Si}{\mathrm{F}}_{\mathrm{x}}\left(\mathrm{g}\right)+\mathrm{xNa}\left(\mathrm{g}\right)$$

(2)

$$2\mathrm{Si}{\mathrm{F}}_{\mathrm{x}}\left(\mathrm{g}\right)\to 2\mathrm{Si}\left(\mathrm{s}\right)+\mathrm{x}{\mathrm{F}}_2\left(\mathrm{g}\right)$$

(3)

$$\mathrm{Si}{\mathrm{F}}_{\mathrm{x}} \left(\mathrm{g}\right)+\mathrm{xNa}\left(\mathrm{g}\right)\to \mathrm{Si}\left(\mathrm{g}\right)+\mathrm{xNaF}\left(\mathrm{l}\right)$$

(4)

$$(\mathrm{x}+1)\mathrm{Si}{\mathrm{F}}_{\mathrm{x}}\left(\mathrm{g}\right)\to \mathrm{Si}\left(\mathrm{s}\right)+\mathrm{xSi}{\mathrm{F}}_{\mathrm{x}+1}\left(\mathrm{g}\right)$$

(5)

$$2\mathrm{Si}\left(\mathrm{s}\right)+\mathrm{Me}\left(\mathrm{s}\right)\to \mathrm{MeS}{\mathrm{i}}_2\left(\mathrm{s}\right)$$

(6)

$$3\mathrm{Si}\left(\mathrm{s}\right)+5\mathrm{Me}\left(\mathrm{s}\right)\to \mathrm{M}{\mathrm{e}}_5\mathrm{S}{\mathrm{i}}_3\left(\mathrm{s}\right)$$

(7)

$$7\mathrm{Si}\left(\mathrm{s}\right)+\mathrm{M}{\mathrm{e}}_5\mathrm{S}{\mathrm{i}}_3\left(\mathrm{s}\right)\to 5\mathrm{MeS}{\mathrm{i}}_2\left(\mathrm{s}\right)$$

(8)

During the silicon infiltration process, high-entropy alloys (Me) tended to form low silicides with active Si first, then reacted with Si to form MeSi₂.

3.2. Oxidation Behavior of Coatings

The oxidation behaviors of the samples coated with MeSi₂ coating at 600 and 1200 °C were tested. Figure 6 shows the cross-sectional and surface SEM morphology of the sample after 100 h of oxidation at 600 °C. The MeSi₂ coating showed no significant change compared to the coating before oxidation, and the macroscopic sample remained intact. No needle-shaped MoO₃ formed on the surface. The high-entropy effect in the MeSi₂ coating effectively avoided material pulverization caused by the ‘pesting’ phenomenon.

Figure 7 shows the mass gain curves and surface XRD pattern of the MeSi₂ coating after 600 °C oxidation for 100 h. The weight gain of the coating was 0.2 mg/cm². In the early stage of oxidation, weight gain maintained a linear increase, and the mass growth rate began to decrease around 10 h. After several tens of hours, the mass growth rate became extremely slow. The reason may have been that a complete SiO₂ oxide film gradually formed on the surface of the sample and the slow diffusion effect of high-entropy reduced the oxidation rate of the coating. In addition, the peaks in the patterns before and after coating oxidation were basically consistent, with only a small number of peak height changes. The reason for this may have been the distortion of lattice constants caused by the precipitation and oxidation of Si elements from the solid solution structure.

The XRD diffraction pattern mainly shows the phase structure of MeSi₂, with only a small number of SiO₂ diffraction peaks. To further determine the elemental distribution, EDS analysis was performed on the surface. Figure 8 shows the EDS elemental distribution of MeSi₂ coating after low-temperature oxidation for 100 h. The distribution of elements such as W, Mo, Ta, Nb, Si, and O was relatively uniform, and only a thin SiO₂ adhesion layer was formed on the surface. However, the distribution intensity of the Cr element was slightly weaker, which may have been related to the enrichment of the Cr element in the TaSi₂ phase as illustrated in Figure 4.

The different diffusion coefficients of metal elements M (M = Mo, W, Cr, Ta, and Nb), Si, and O in the SiO₂ layer became one of the controlling factors in the oxidation process. In the (MoWCrTaNb)Si₂, five cationic elements were randomly distributed on the cationic sites, and the difference in cationic atomic radii led to severe lattice distortion, introducing many stable vacancies with high migration energy barriers and slow diffusion effects, hindering cation diffusion [20]. The reaction between Si and O always had low Gibbs free energy between 0 K and 2000 K, and the reaction could occur spontaneously; The Gibbs free energy is much lower than that of other metal elements reacting with oxygen [19]. No other metal oxides were observed in the XRD pattern. The low-temperature oxidation process of MeSi₂ was not only influenced by Gibbs free energy but also controlled by the diffusion rates of elements. It has been proven that the high-entropy effect in high-entropy silicides under low-temperature oxidation environments promotes the selective oxidation of the O element to the Si element and inhibits the oxidation of metal elements. Under the combined action of Gibbs free energy and the high-entropy effect, the formation rate of SiO₂ is higher than that of metal oxides, and a complete SiO₂ protective film gradually forms on the surface of MeSi₂, which hinders the occurrence of the “pesting” phenomenon.

Figure 9 shows the cross-sectional morphology of the MeSi₂ coating oxidized at 1200 °C for different times. In the initial stage, the Si element from MeSi₂ in the coating and oxygen element reacted, generating SiO₂. When SiO₂ was molten, it had a certain fluidity, filling the pores inside the coating and improving the density of the coating to a certain extent. MeSi₂ gradually transformed into Me₅Si₃, and some pores appeared in the second layer (Figure 9b).

During the late stage of oxidation, oxygen diffused through the pores, causing oxidation reactions inside the coating. Metal elements were oxidized and metal oxides sublimated at high temperatures. As the oxidation time was further extended to 10 h, the original diffusion layer had basically disappeared (Figure 9c), and a large amount of SiO₂ had replaced the TaSi₂ layer. At the same time, MeSi₂ and Me₅Si₃ in the main layer of the coating were significantly decreased. Due to the high proportion of low silicides in the coating, the oxidation rate of the coating was faster. The high-entropy silicides were almost completely consumed, leaving only a small amount of residue. The low silicides reacted with oxygen and may have generated many oxides such as Ta₂O₅ and Nb₂O₅. The volatilization of these oxides caused a sharp decrease in the thickness of the outer oxide layer, weakening the protective performance of the coating. Figure 10 shows the SEM morphology and XRD pattern of the MeSi₂ coating surface after oxidation at 1200 °C for 20 h. A large number of needle-shaped oxides could be seen on the surface, and Ta₂O₅, Na₂O₅, MoO₃, and a small amount of MeSi₂ were detected in the XRD spectrum. After 20 h of oxidation, the high-entropy silicide inside the coating (Figure 9d) had basically been consumed, indicating the failure of the high-entropy silicide. Currently, the middle layer TaSi₂ played an antioxidant role, and the surface was covered with a SiO₂ oxide film with a thickness of about 10 μm.

The EDS energy spectrum analysis of the cross-section of the coating after 20 h of oxidation is shown in Figure 11. The EDS point scan data are shown in

Table 2. EDS compositions of the different regions in Figure 11.

	Composition (At%)							Possible Phase
	Si	Cr	Nb	Mo	Ta	W	O
1	56	0.1	5.2	4.4	16.6	5.7	12	MeSi2 + Me5Si3
2	31.5	0	0.7	0.3	1.5	0.1	65.9	SiO2 + Ta2O5
3	27.7	0	1.3	0.3	3.8	0.3	66.6	SiO2 + Nb2O5
4	34.7	0.1	0.2	0.6	47.4	0.3	16.7	TaSi2

. The element distribution at points 2 and 3 in the table indicates that the main phase in the oxide film was SiO₂, with a small amount of metal oxide. The element distribution at point 1 in the table shows that the proportion of Si atoms in the coating decreased and transformed into low silicides. The element distribution at point 4 shows that a small amount of the Ta element inside the oxide layer was oxidized to form Ta₂O₅. In the EDS element distribution map of the coating cross-section (Figure 11), the O element was enriched in the outermost region and did not diffuse extensively into the TaSi₂ layer and substrate. Due to the oxidation reaction at the coating interface during the previous oxidation process, a small number of pores appeared in the upper edge area of the TaSi₂ layer, and some pore areas were enriched with O. The reason for their formation was that in the later stage of oxidation, Si diffused up and down to form a small number of vacancies, and oxygen diffused along the vacancies, forming metal oxides in these areas. The volume expansion caused by metal oxides destroyed the structure of the TaSi₂ layer.

Figure 12 shows the weight gain curve and macroscopic morphology of the sample, indicating that the coating maintained weight gain before 10 h. But after 10 h, the weight gain began to decrease, which may have been due to the volatilization of metal oxidation products in the coating. In addition, the concentration of stress in the angular areas made the oxidation more pronounced (inset), and the outermost layer of the coating exhibited delamination. The volume expansion of Ta₂O₅ resulted in a penetrating crack, which extended from the TaSi₂ layer to the Ta₅Si₃ transition layer. The formation of a white phase was clearly visible at the corners of the macroscopic sample, indicating the failure of the MeSi₂ coating. Therefore, the overall oxidation failure time of the coating at 1200 °C was determined based on the failure of the MeSi₂ coating.

4. Conclusions

A MeSi₂ coating was prepared by the slurry method and halide silicon infiltration method. After vacuum sintering at 1420 °C for 90 min, the coating was composed of a BCC phase and a small amount of oxide phase. The silicon infiltration process involved holding at 1200 °C for 2 h and filling with fillers. The resulting MeSi₂ coating consisted of a four-layer structure, with an outer layer of MeSi₂ and a (TaCr) Si intermetallic compound layer, an intermediate layer of TaSi₂, and an inner layer of Ta₅Si₃. The outer layer of the coating was partially porous while the inner layer was dense, with a total thickness of approximately 120 μm. After 100 h of oxidation at a low temperature of 600 °C, the surface was covered with a thin SiO₂ oxide film. The XRD pattern showed no change before and after oxidation, and no metal oxidation products, and the weight gain rate of the coating was extremely low. There was no “pesting” phenomenon at this temperature. At an oxidation temperature of 1200 °C, the MeSi₂ coating could endure 20 h. During the oxidation process, the coating generated metal oxides, forming a thin SiO₂ oxide film. TaSi₂ and Ta₅Si₃ gradually transformed into Ta₂O₅, and the coating eventually failed.