Microstructure and Antioxidative Performance of Y₂O₃-CeO₂ Co-Modified Molybdenum Silicide Coatings

Abstract

To enhance the high-temperature oxidation resistance of molybdenum-based materials, Y₂O₃-CeO₂ co-modified silicide coatings were produced on molybdenum substrates using two-step pack cementation. The microstructure and phase composition of Y₂O₃-CeO₂ co-modified composite coatings were examined both before and after oxidation. A detailed analysis of the antioxidant properties of the co-modified coatings and the mechanisms behind the modifications was also conducted. The incorporation of 1.0 wt.% CeO₂ and 1.5 wt.% Y₂O₃ into the composite coatings resulted in a dense, non-porous, maximum-thickness microstructure. This microstructure is characterized by the uniform distribution of parallel MoSi₂ and MoB layers on the substrate. In particular, the coating containing 1.5 wt.% Y₂O₃ exhibited superior oxidation resistance, with a weight gain of 0.29 mg/cm² and an oxidation rate constant of 6.68 × 10⁻⁴ mg²/(cm⁴·h) after oxidation at 1150 °C for 255 h. During oxidation, a dense SiO₂ oxide film is formed through the cooperation of Y₂O₃ and CeO₂, inhibiting further Si diffusion into the substrate and reducing the formation of the Mo₅Si₃ layer.

1. Introduction

Refractory metal molybdenum and its alloys exhibit exceptional properties, including high melting points and superior electrical and thermal conductivity [1,2]. These materials are commonly utilized as structural components in high-temperature settings within the aerospace and nuclear energy sectors [3,4]. However, molybdenum and its alloys are highly susceptible to rapid oxidation, resulting in the formation of volatile molybdenum trioxide (MoO₃) at temperatures above 600 °C [5]. This process causes material degradation and a significant reduction in performance. The limited oxidation resistance significantly hinders the advancement of molybdenum and its alloys across various industries [4,6,7]. Therefore, the critical issue of poor oxidation resistance requires prompt resolution.

Currently, the enhancement of oxidation resistance in molybdenum and its alloys primarily relies on protective coatings [8,9]. Molybdenum disilicide (MoSi₂) is the main silicide of molybdenum and is commonly utilized in various coating systems due to its impressive characteristics, including a high melting point (2030 °C), low density (6.24 g/cm³) and low coefficient of linear expansion (7.8 × 10⁻⁶/K) [10,11,12]. When temperatures exceed 1000 °C, molybdenum disulfide (MoSi₂) forms a dense protective layer of glass silica (SiO₂) on the coating’s surface, effectively preventing oxygen penetration and inhibiting oxidation on both the coating and substrate [13,14,15,16]. Currently, coating methods for preparing MoSi₂ coatings on molybdenum surfaces include vapor deposition, plasma spraying, slurry process, and pack cementation (HAPC) [17,18,19,20]. Among these, HAPC is preferred due to its cost-effectiveness, versatility in coating complex geometries, and ability to produce dense and adherent coatings with superior oxidation [8,21]. Researchers like Zhai et al. [22] have successfully prepared MoSi₂ coatings on Mo substrates through pack cementation, observing the formation of a glass-like SiO₂ layer post-oxidation at 1300 °C. This layer aids in repairing coating cracks to some extent and prevents oxygen diffusion through them. However, the MoSi₂ coating is subject to microcrack initiation during oxidation, leading to “pest oxidation” and coating failure, which severely limits its application [23,24].

Studies have shown that the incorporation of additives, such as aluminum, boron, and rare earth elements, can significantly improve the performance of MoSi₂ coatings [25,26]. Al-doped MoSi₂ forms the Mo(Si, Al)₂ phase, which generates a protective Al₂O₃ film with a low coefficient of volume expansion during high-temperature oxidation, minimizing stress mismatch and preventing “pest oxidation” [27]. B-modified silicate coatings were able to fill defects such as pores and cracks promptly by producing lower viscosity B₂O₃, but B-modified silicide coatings had low density and lacked significant structural gradients, producing weak bonding between coatings [28]. Xiao et al. [29] demonstrated that CeO₂ effectively refines the grain and increases the thickness of the MoSi₂ coating. Similarly, Chen et al. [30] discovered that Y₂O₃ can improve the grain size and density of MoSi₂ coatings. Some disilicide coatings modified with rare earth elements have shown promising antioxidant properties. Zhang et al. [31] investigated the oxidation stability of rare earth co-modified coatings applied to Nb-Ti-Si-based high-temperature alloys. It was demonstrated that these coatings exhibited enhanced antioxidant properties during cyclic oxidation tests at 1250 °C, resulting in a 31% reduction in oxidation rate. However, the simultaneous introduction of CeO₂ and Y₂O₃ to enhance the functionality of the coating on molybdenum surface variations had rarely been reported, and the precise mechanism underlying the co-modification of multiple rare earth elements within composite coatings had yet to be unraveled in full.

In a previous study [32], it was established that a Ce content of 1.00 wt.% in the pack cementation process resulted in the formation of a MoSi₂ layer with a uniform density. The objective of this paper is to examine the impact of Y₂O₃-CeO₂ co-modification on the coating formation process and the antioxidant process by preparing Y₂O₃-CeO₂ co-modified silicide coatings using two-step cementation. In view of the established optimal Ce content of 1.00 wt.%, the present study specifically investigated the influence of Y content on the coating’s phase composition, microstructure evolution, and high-temperature oxidation behavior. Furthermore, the mechanisms through which Yttrium (Y) and Cerium (Ce) facilitate the antioxidant processes were examined in depth.

2. Materials and Methods

2.1. Specimen Preparation

The Mo substrate (chemical composition listed in

Table 1. Chemical composition of molybdenum substrate/%.

Mo	Fe	Ni	O	C	Si	N	Ca	Mg
Balance	0.0031	0.001	0.003	0.003	0.003	0.001	0.0005	0.0003

) was processed into a sample with 10 mm × 10 mm × 10 mm cut by wire-cut electrical discharge machining (WEDM). The surfaces were gradually ground using sandpaper (from 400 to 2000 grit) to achieve a smooth finish, followed by polishing with a diamond paste (particle size: 1.5 µm) to ensure a mirror-like surface. Subsequently, an ultrasonic cleaning apparatus was employed to clean the surface, which was then subjected to a drying process using a vacuum oven.

The experimental process is shown in Figure 1. A two-step pack cementation method obtained the coated samples of Y₂O₃-CeO₂ co-modified silicide coatings. The powder mixtures consisted of pure boron (B), sodium fluoride (NaF), alumina (Al₂O₃), silicon (Si), aluminum (Al), silica (SiO₂), cerium oxide (CeO₂), and yttrium oxide (Y₂O₃), serving as boronizing agent and siliconizing agent. Detailed compositions of the pack mixtures are summarized in

Table 2. Composition and content of embedding agent.

Process Steps	Sample	Pack Mixtures (wt.%)
		B	NaF	Al2O3	Si	SiO2	CeO2	Y2O3
First boronizing	Boronizing agent	4	5	91	-	-	-	-
Second Si-Ce-Y depositing	Siliconizing agent-1	0.5	5	-	20	73.5	1	0
	Siliconizing agent-2	0.5	5	-	20	72.75	1	0.75
	Siliconizing agent-3	0.5	5	-	20	72	1	1.5
	Siliconizing agent-4	0.5	5	-	20	70.5	1	3

. Both powders were milled in a planetary ball mill at 600 r/min for 4 h. During the first boronizing process, the molybdenum substrate was fully encapsulated in porcelain boat. The porcelain boat was sealed using a mixture of silane coupling agent and Al₂O₃ powder, then transferred into a corundum-lined tubular furnace (OTF-1200X) under an argon atmosphere (flow rate: 10 mL/min). The temperature was precisely controlled by a K-type thermocouple positioned near the sample, with a heating rate of 5 °C/min to the target temperature of 1000 °C. The sample was isothermally sintered for 4 h to form a boride coating. For the second siliconizing process, the boride coating sample was re-encapsulated in siliconizing agent (with varying Y₂O₃/SiO₂ ratios) and sintered at 1100 °C for 6 h under identical argon protection.

2.2. Characterization of Microstructure

The microstructure of the coating was observed before and after oxidation using a cold field emission scanning electron microscope (Hitachi S-4800 Cold Field Emission Scanning Electron Microscope (SEM), Tokyo, Japan). The cross-sectional morphologies of the coatings were analyzed using ImageJ 2 software and the thickness of each coating was measured. Additionally, the chemical composition of the samples was examined using X-ray spectroscopy. The phase composition of the coating was analyzed by an X-ray diffractometer (Bruker AXS D8 ADVANCE X-Ray Diffractometer, Karlsruhe, Germany). The target is copper, the diffraction angle is in the range of 20°~90°, the scanning speed is 10°/min, the electron acceleration voltage is 40 kV, and the current is 40 mA.

2.3. Oxidation Test

The cyclic oxidation experiments were conducted within a high-temperature tubular furnace. The specimens were individually held in alumina crucibles during the course of the tests, and the oxidation cycle was 12 h, 24 h, and 36 h until the predetermined oxidation time was reached. The mass gain of the coated samples was then determined using an analytical balance (PTX-FA210, ±0.1 mg, OTF-1200X Tube Furnace, Hefei Kejing Material Technology Co., Ltd., Hefei, China). Three parallel samples were set in each group, and the final result was the average value of the measured data. The mass change per unit area of the sample was calculated by the Formula (1) and characterized the high oxidation resistance of the coating.

$${\displaystyle \frac{\Delta m}{S}}={\displaystyle \frac{m-m_0}{S}}$$

(1)

In which Δm (mg) is the change in the specimen weight, and S (cm²) is the surface area.

3. Results and Discussion

3.1. Microstructure of the Y₂O₃-CeO₂ Co-Modified Silicide Coating

Figure 2 illustrates the XRD pattern of a Y₂O₃-CeO₂ co-modified coating with varying Y₂O₃ amounts. The prominent phases detected in the coating are MoSi₂ and MoB. However, due to the limited Y₂O₃ and CeO₂ doping, the XRD patterns do not show diffraction peaks of Y and Ce. Post-co-modification, the intensity of MoSi₂ and MoB diffraction peaks was notably higher compared to solely CeO₂-modified samples. With increasing Y₂O₃ content, the MoSi₂ diffraction peaks initially rose around 40 degrees and then gradually declined. The peak intensity peaks at 1.5 wt.% Y₂O₃, indicating that an optimal Y₂O₃ content had a more pronounced effect on promoting MoSi₂ crystallization. This enhancement was attributed to Y₂O₃ acting as nucleating agents, fostering MoSi₂ and MoB crystal growth [33]. Additionally, as Y₂O₃ content increases within a specific range, the half-width of the MoSi₂ diffraction peak gradually widens. According to the Debye–Scherrer Formula (2):

$$D={\displaystyle \frac{K\gamma }{B\mathit{cos}\theta }}$$

(2)

In which D is the grain size, K is the constant, γ is the X-ray wavelength, B is the half peak width of the diffraction peak of the specimen, θ is the diffraction angle. This indicates a reduction in MoSi₂ grain size, meaning that Y₂O₃ and CeO₂ accumulation at grain boundaries can impede the growth and refinement of MoSi₂ grains [34]. The leftward shift (toward lower angles) of the MoSi₂ diffraction peaks with increasing Y₂O₃ content, particularly noticeable at 40° (as shown in the insert of Figure 2), can be attributed to lattice expansion caused by the incorporation of Y³⁺ ions into the MoSi₂ crystal structure. When Y³⁺ substitutes for Mo⁴⁺ or occupies interstitial sites in the MoSi₂ lattice, it introduces tensile stress, leading to lattice expansion [35]. According to Bragg’s Formula (3), the increase in interplanar spacing (d) due to lattice expansion directly leads to a reduction in the diffraction angle (θ), which accounts for the observed leftward shift of the peak.

$$2d\sin \theta =n\lambda$$

(3)

Figure 3 depicts the cross-sectional morphology of the Y₂O₃-CeO₂ co-modified composite coating, demonstrating a uniform double-layer structure across all samples. The distribution of Mo, Si, and B in the modified coatings was analyzed by EDS line scanning along lines A, B, C, and D, respectively. A detailed summary of the EDS results obtained for the phases identified in the coatings is presented in

Table 3. The elemental composition at different points in Figure 3.

Spot	Composition/at.%			Main Phase
	Mo	Si	B	-
1	54.4	0.1	45.4	MoB
2	34.5	65.2	-	MoSi2
3	32.3	67.6	-	MoSi2
4	53.6	0.2	46.2	MoB
5	54.4	0.1	45.4	MoB
6	54.2	0.1	45.67	MoB

. The combined results of the X-ray diffraction (XRD) analysis and the aforementioned tests confirm the presence of MoB phases at points 1 and 4. Points 2 and 3 exhibit an atomic ratio of Mo to Si close to 1:2, indicating a bilayer structure in the composite coating with an outer layer of MoSi₂ and an inner layer of light grey MoB. These findings align with the outcomes documented in Wang’s research [25]. As shown in Figure 4, the thickness of the MoSi₂ layer and the MoB layer at 0 wt.%, 0.75 wt.%, and 1.5 wt.% Y₂O₃ doping amounts are 53.515 μm and 9.584 μm, 56.372 μm and 11.849 μm, and 58.691 μm and 15.834 μm, respectively. The thickness of each layer was observed to increase in proportion to the quantity of Y₂O₃ present, demonstrating a progressive enhancement of the catalytic effect. Previous studies [31] have demonstrated a significant difference in catalytic effects between CeO₂ and Y₂O₃. The reactivity of Si and B atoms near the substrate is enhanced by CeO₂, leading to an increase in the number of Si and B atoms participating in the reaction, and in the thickness of the thickness of MoSi₂ and MoB layers [36]. On the other hand, Y₂O₃ tends to accumulate at grain boundaries, vacancies, and defects, causing lattice distortion and creating a “gas group” structure that facilitates diffusion channels for activated Si and B atoms, promoting their diffusion to the substrate. The growth of MoSi₂ and MoB grains is further stimulated by the combined use of Y₂O₃ and CeO₂, ultimately resulting in an increase in the coating thickness. When the proportion of Y₂O₃ exceeds 3.00 wt.%, the thickness of the MoSi₂ and MoB layers decreased to 48.813 μm and 11.014 μm, respectively. This finding suggested that an excess of Y₂O₃ may facilitate the accumulation of numerous active atoms at the grain boundary, which could potentially impede the growth of the composite coating [31]. Furthermore, Figure 3e illustrates the elemental distribution of Y and Ce, respectively. It can be observed that both Ce and Y are distributed uniformly across all layers of the coating.

Upon augmenting the Y₂O₃ proportion to 1.50 wt.%, a homogenous and compact structure was exhibited by the Y₂O₃-CeO₂ co-modified composite coating. This occurred because large, rare earth (RE) atoms located at grain boundaries or defects could function as sinks for holes, leading to the elimination of a significant number of holes and consequently inhibiting the formation of holes [37]. However, a further increase in Y₂O₃ content to 3.00 wt.% led to the appearance of holes within the MoSi₂ layer of the composite coating. These holes were attributed to the inward diffusion of Si atoms in the MoSi₂ layer facilitated, causing vacancies at grain boundaries and phase boundaries. The accumulation of vacancies eventually leads to the formation of holes during the outward diffusion process [38]. Excessive Y₂O₃ doping resulted in a significant mismatch in diffusion rates between Y³⁺ and Mo⁴⁺, which led to the development of compositional segregation zones within the coating that accumulated over time and ultimately evolved into holes [37]. Additionally, excess Y³⁺ ions induced pronounced segregation at grain boundaries, accompanied by severe embrittlement [37]. The weakened grain boundaries acted as preferential sites for crack initiation, further promoting hole formation. It can be inferred that with an addition of 1.0 wt.% CeO₂ and 1.5 wt.% Y₂O₃, the modified composite coating exhibits a compact, hole-free structure with the greatest thickness, consisting of parallel MoSi₂ and MoB layers evenly distributed on the substrate.

The incorporation of doped Y₂O₃ had no impact on the growth mechanism of Mo. The MoB was observed to be distributed in parallel stripes (Figure 3), confirming that its production was facilitated by the inward diffusion of B atoms and their interaction with Mo, rather than by the epitaxial growth of Mo [39]. A strong bond between the metallurgical layer and the substrate resulted from this formation process, effectively reducing the risk of cracking under low-stress conditions. Additionally, the MoB layer acted as a barrier, preventing the diffusion of Si atoms from the MoSi₂ layer to the substrate during oxidation. The degradation of the MoSi₂ coating was minimized and its lifespan was prolonged by this barrier function [22]. The presence of diffuse light grey areas within the dark grey MoSi₂ layer is evident in Figure 3a–d. This is further supported by the quantitative analysis results at points 5 and 6, where the Mo: B ratio is approximately 1:1, thereby confirming that the light grey phase corresponds to the MoB phase. Zang et al. [28] showed that the reaction of B with MoSi₂ can produce the MoB phase. The formation of MoB precipitates within the MoSi₂ layer is a consequence of the kinetic limitations imposed by the high concentration of B in MoSi₂, which drives the reaction (4).

$$B+MoS{i}_2=MoB+2Si$$

(4)

3.2. High-Temperature Oxidation of the Y₂O₃-CeO₂ Co-Modified Coatings

The XRD patterns of the Y₂O₃-CeO₂ co-modified composite coatings after oxidation at 1150 °C for 255 h are displayed in Figure 5. The main phases identified in the coating after oxidation included SiO₂, Mo₅Si₃, MoSi₂, and B₂O₃. The formation of SiO₂ was attributed to the oxidation of MoSi₂, and the formation of B₂O₃ was attributed to the outward diffusion and oxidation of B within the Mo-B compound phase within the coating structure [40]. Upon increasing the Y₂O₃ content by 1.5 wt.%, the peak intensities of SiO₂ and B₂O₃ diffraction peaks also increase, suggesting that the co-modification of Y₂O₃ and CeO₂ facilitated the formation of a SiO₂ protective film. However, with a further increase in Y₂O₃ content to 3.00 wt.%, the peak intensities of SiO₂ and MoSi₂ diffraction peaks decreased, indicating a reduction in antioxidant capacity.

The cross-sectional morphology of various modified composite coatings following oxidation at 1150 °C for 255 h was displayed in Figure 6. Quantitative analysis is conducted on different regions, and the elemental composition is presented in

Table 4. The elemental composition at different points in Figure 6.

Spot	Composition/at.%				Main Phase
	Mo	Si	O	B	-
a	1.2	32.7	60.4	0.7	SiO2
b	62.0	36.7	-	0.3	Mo5Si3
c	34.5	65.3	-	0.2	MoSi2
d	63.4	36.5	-	0.1	Mo5Si3
e	53.4	1.3	-	44.3	MoB
f	60.4	38.8	0.8	-	Mo5Si3

. Based on the elemental composition at points a, b, c, d, e, and f, the main phases in the respective regions can be determined through atomic ratio calculations as SiO₂, MoSi₂, Mo₅Si₃, MoB, Mo₅Si₃, and Mo₅Si₃. During high-temperature oxidation, the Mo₅Si₃ layer was formed due to the diffusion of the Si element in the MoSi₂ layer of the coating into the Mo matrix. Meanwhile, the silicon present on the coating surface undergoes a chemical reaction with oxygen, resulting in the formation of silicon dioxide (SiO₂). The thickness of the SiO₂ layer increases to 13.42 μm with 1.50 wt.% Y₂O₃ and the thickness of the transition Mo₅Si₃ layer was thinner due to the inhibitory effect of Y₂O₃ and CeO₂ on the inward diffusion of B and Si, resulting in conducive to the formation of SiO₂. In particular, Y³⁺ can fill the gaps in the Si-O bond, creating a new Si-O-Y bond that enhances the density of the SiO₂ layer and prevents the inward diffusion of oxygen [41,42]. The thickness of the SiO₂ layer decreases as the amount of Y₂O₃ increased to 3 wt.%. Longitudinal cracks form in the MoSi₂ layer, leading to rapid inward diffusion of oxygen from the cracks and consumption of the MoSi₂ layer. This was due to the excess of Y³⁺ ions that could fill the gaps in the Si-O bond at high temperatures, which damaged the bond and created numerous pores that exacerbated the oxidation loss of the Y₂O₃-CeO₂ co-modified coating [41]. In conclusion, the high-temperature oxidation resistance of the coating can be improved by the addition of Y₂O₃. However, excessive amounts of Y₂O₃ can ultimately have a detrimental effect on the coating layer.

The mass change curves of the 255 h oxidized composite coatings at 1150 °C are displayed in Figure 7a. The oxidation process of the Y₂O₃-CeO₂ co-modified coating at 1150 °C can be divided into the early rapid oxidation stage and the slow oxidation stage. Notably, with the addition of 0.75 wt.% and 3 wt.% of Y₂O₃, the coating underwent rapid oxidation in the initial 50 h, resulting in a rapid weight increase. Only linear oxidation was undergone by the modified composite coating, with a minor weight increase of 0.13 mg/cm² observed in the first 15 h prior to oxidation at a Y₂O₃ content of 1.50 wt.%. In contrast, at Y₂O₃ concentrations of 0.75 wt.% and 3.00 wt.%, the modified coating underwent linear oxidation for 35 h prior to oxidation, with the coating weight increasing by 0.42 mg/cm² and 0.40 mg/cm², respectively. This was attributed to the synergistic effect of appropriate amounts of Y₂O₃, which accelerated the formation of a SiO₂ protective layer, thereby impeding oxygen diffusion. As shown in Figure 6d, the Mo₅Si₃ phases present in cracks undergo further oxidation, leading to the generation of MoO₃ volatile products. The rapid decay of the coating weight resulted from the interaction of the oxygen released from MoO₃ volatilization with Mo₅Si₃ in the transition layer.

Figure 7b shows the square of the weight gain per unit area (Δm/S)² as a function of oxidation time for composite coating samples at 1150 °C for 105 h. During the experiment, the kinetic curve of the specimen follows a parabolic law after the fitting process, and the parabolic rate constant k_P is calculated according to the following equation:

$${\left({\displaystyle \frac{\Delta m}{S}}\right)}^2=k_{p}t$$

(5)

In which Δm (mg) is the change in the specimen weight, S (cm²) is the surface area, and t (h) is the oxidation time. After oxidation at 1150 °C, the mass gain of the co-modified composite coating with 1.50 wt.% Y₂O₃ was 0.29 mg/cm², and it exhibited an oxidation rate constant of 6.68 × 10⁻⁴ mg²/(cm⁴·h) for 105 h. This rate constant was lower than that of other modified composite coatings with varying Y₂O₃ content and significantly lower than the 1.00 wt.% CeO₂-doped coating, which had a weight gain of 0.41 mg/cm² and an oxidation rate constant of 1.70 × 10⁻³ mg²/(cm⁴·h). These results indicate that the co-modified composite coatings with 1.50 wt.% Y₂O₃ and 1.00 wt.% CeO₂ demonstrates superior high-temperature oxidation resistance. This finding was supported by the analysis of the post-oxidation morphology of Y₂O₃ and CeO₂ co-modified composite coatings in Figure 6.

3.3. Modification Mechanism of Ce and Y in the Coatings

Figure 8 shows the formation process of the Y₂O₃-CeO₂ co-modified coating. During the high-temperature embedding, Si and B particles were activated under the action of the NaF activator. The reaction Equations (6)–(11) occurred in the diffusion of atomic Si and B to Mo substrate to form a composite coating [22].

$$\mathrm{B}\left(\mathrm{s}\right)+2\mathrm{N}\mathrm{a}\mathrm{F}\left(\mathrm{s}\right)\to \mathrm{B}{F}_2\left(\mathrm{g}\right)+2\mathrm{N}\mathrm{a}\left(\mathrm{g}\right)$$

(6)

$$2\mathrm{B}{F}_2\left(\mathrm{g}\right)\to \mathrm{B}{F}_4\left(\mathrm{g}\right)+\mathrm{B}\left(\mathrm{g}\right)$$

(7)

$$\mathrm{B}\left(\mathrm{g}\right)+\mathrm{M}\mathrm{o}\left(\mathrm{s}\right)\to \mathrm{M}\mathrm{o}\mathrm{B}\left(\mathrm{s}\right)$$

(8)

$$2\mathrm{S}\mathrm{i}\left(\mathrm{s}\right)+4\mathrm{N}\mathrm{a}\mathrm{F}\left(\mathrm{s}\right)\to \mathrm{S}\mathrm{i}{F}_4\left(\mathrm{g}\right)+\mathrm{S}\mathrm{i}\left(\mathrm{g}\right)+4\mathrm{N}\mathrm{a}\left(\mathrm{g}\right)$$

(9)

$$2\mathrm{S}\mathrm{i}\left(\mathrm{g}\right)+\mathrm{M}\mathrm{o}\left(\mathrm{s}\right)\to \mathrm{M}\mathrm{o}\mathrm{S}{i}_2\left(\mathrm{s}\right)$$

(10)

$$2\mathrm{S}\mathrm{i}\left(\mathrm{g}\right)+\mathrm{M}\mathrm{o}\mathrm{B}\left(\mathrm{s}\right)\to \mathrm{M}\mathrm{o}\mathrm{S}{i}_2\left(\mathrm{s}\right)+\mathrm{B}\left(\mathrm{g}\right)$$

(11)

The rare earth oxides doped in the penetrant can promote the infiltration of Si and B to a certain extent, but their action mechanisms were significantly different. CeO₂ was mainly distributed on the surface of the Mo substrate and promoted the diffusion of Si and B to Mo substrate to form a protective coating (shown in Figure 8 (step 2)) [31]. However, the Y atom (179.7pm) has smaller size than that of a Ce atom (185pm). The radius of Y³⁺ had a small radius (179.7pm) and significant diffusion rate and tended to aggregate at grain boundaries, vacancies, and other defects, resulting in lattice distortion and “gas group” structure (shown in Figure 8 (gas group structure)). Meanwhile, it can absorb active atoms and promote the diffusion of atoms from the “gas group” into the interior of the coating [31]. Under the synergistic effect of CeO₂ and Y₂O₃, the number of active Si and B atoms increased, and their diffusion rate was also enhanced. During the diffusion of active Si and B atoms, CeO₂ was driven to migrate to the interior of the coating so that the coating–substrate interface gradually moved towards the substrate, and the thickness of the coatings gradually increased (shown in Figure 8 (step 2)). The Ce²⁺ and Y³⁺ distributed at grain boundaries, vacancies, and other defects act as heterogeneous nucleating agents, reducing the activation energy and promoting the crystal growth of MoSi₂. Furthermore, the solute drag effect produced by these agents refines and hinders the growth of MoSi₂ grains, thereby enhancing the compactness of the coating microstructure and reducing the incidence of cracks. Therefore, as a consequence of the combined action of CeO₂ and Y₂O₃, a composite coating with a substantial thickness and a compact structure was formed.

Figure 9 illustrates the oxidation process of the Y₂O₃-CeO₂ co-modified coating. During oxidation, MoSi₂ reacted with O₂ to produce a small amount of SiO₂, Mo₅Si₃, and volatile MoO₃. The MoSi₂ layer was further diffused into the substrate by the Si atoms to form the Mo₅Si₃ layer (shown in Figure 9 (1150 °C, step 1)). A complete and dense SiO₂ layer was formed on the coating surface, and the MoB layer can inhibit the mutual diffusion of Si and Mo atoms in the MoSi₂ layer. The B₂O₃ produced in the oxidation process can improve the high-temperature fluidity of SiO₂ and enhance the self-healing of the SiO₂ layer to quickly cover the coating surface and fill in cracks [28]. Meanwhile, the B₂O₃-SiO₂ improved the compactness of the coating [17,43]. The doped Y₂O₃ can provide a large number of Y³⁺ to occupy the Si-O bond gap and form a Si-O-Y bond, thus improving the compactness of SiO₂ (shown in Figure 9 (1150 °C, step 2)) [41]. The strong absorption can stabilize the distribution of O atoms in SiO₂ and inhibit the cracking of the Si-O bond at high temperatures to inhibit the diffusion of O atoms and the consumption of SiO₂ [44]. In addition, a pinning effect can be produced by the segregation of CeO₂ and Y₂O₃ at the grain boundary, which hinders the internal diffusion of Si and promotes the external diffusion of Si atoms. On the other hand, the binding of rare earth elements to oxygen vacancies facilitates the formation of defect clusters, enhancing both the binding energy and phase stability of the material [45]. The lattice distortion results from the simultaneous introduction of multiple rare earth elements due to variations in atomic radius. Furthermore, the fluctuations caused by differences in elemental characteristics contribute to the emergence of defects, which subsequently impede the diffusion processes of the components [46,47]. Consequently, the introduction of supplementary doping elements serves to augment the solution entropy within the coating, thereby impairing the spontaneity of chemical reactions as a consequence of the augmented mixing entropy of a solid solution. The synergistic interaction between Y₂O₃ and CeO₂ during the antioxidant process significantly prolongs the service life of antioxidant coatings by promoting lattice distortion and facilitating defect clustering.

4. Conclusions

In this study, Y₂O₃-CeO₂ co-doped composite coatings were prepared on molybdenum substrates using a two-step pack cementation method. The co-doping of Y₂O₃ and CeO₂ in the composite coatings has been shown to enhance the thermal stability and oxidation resistance of the coatings under high-temperature environments. The results indicate that the co-doped composite coating exhibits superior high-temperature oxidation resistance compared to coatings modified with a single dopant. Additionally, the impact of varying Y₂O₃ content on the microstructure of the composite coating and its high-temperature oxidation behavior has been analyzed. The conclusions are as follows:

(1)

CeO₂ and Y₂O₃ have a synergistic modification effect, which can promote the diffusion of Si and B atoms to the substrate, refine and promote MoSi₂ crystal growth, and make the coating more compact. The thickness of the coating is the largest as the additional amount of CeO₂ and Y₂O₃ are 1 wt.% and 1.5 wt.%, respectively. The coating is composed of MoSi₂ and MoB layers, which are both distributed parallel to the substrate, and the thickness is 58.691 μm and 15.834 μm, respectively.

(2)

After oxidation, the coating structure from the inside out is the SiO₂ layer, Mo₅Si₃ layer, MoSi₂ layer, Mo₅Si₃, and MoB layer. The thickness of the SiO₂ layer increases to 13.42 μm with 1.50 wt.% Y₂O₃ and the thickness of the transition Mo₅Si₃ layer was 20.08 μm. The co-modified composite coating with 1.50 wt.% Y₂O₃ exhibits a mass gain of 0.29 mg/cm² after 105 h of oxidation at 1150 °C, with an oxidation rate constant of 6.68 × 10⁻⁴ mg²/(cm⁴·h).

(3)

The doped Y₂O₃ can provide a large number of Y³⁺ to occupy the Si-O bond gap and form a Si-O-Y bond, thus improving the compactness of SiO₂. CeO₂ and Y₂O₃ segregated at the grain boundary can also produce a pinning effect, hinder the internal diffusion of Si, and promote the external diffusion of Si atoms.