Oxidation Behavior of (Mo,Hf)Si₂-Al₂O₃ Coating on Mo-Based Alloy at Elevated Temperature

Abstract

To improve the oxidation resistance of Mo-based alloys, a novel (Mo,Hf)Si₂-Al₂O₃ composite coating was fabricated on a Mo-based alloy by the method of slurry sintering. The isothermal oxidation behavior of the coating was evaluated at 1400 °C. The microstructure evolution and phase composition of the coating before and after oxidation exposure were characterized. The anti-oxidant mechanism for the good performance of the composite coating during high-temperature oxidation was discussed. The coating had a double-layer structure consisting of a MoSi₂ inner layer and a (Mo,Hf)Si₂-Al₂O₃ outer composite layer. The composite coating could offer more than 40 h of oxidation-resistant protection at 1400 °C for the Mo-based alloy, and the final weight gain rate was only 6.03 mg/cm² after oxidation. A SiO₂-based oxide scale embedded with Al₂O₃, HfO₂, mullite, and HfSiO₄ was formed on the surface of the composite coating during oxidation. The composite oxide scale exhibited high thermal stability, low oxygen permeability, and enhanced thermal mismatch between oxide and coating layers, thus improving the oxidation resistance of the coating.

1. Introduction

Molybdenum (Mo) alloys are of great interest as high-temperature structural materials because of their attractive properties, such as high melting points, outstanding mechanical properties at elevated temperatures, low coefficient of thermal expansion (CTE), high thermal conductivity, and good thermal shock resistance [1,2,3,4,5]. In practical applications, high-temperature structural materials are not only required to have excellent high-temperature mechanical properties, but also withstand complex environments, such as high-temperature oxidation, corrosion, and ablation. Nevertheless, the catastrophic behavior of molybdenum alloys under high-temperature oxidizing environments has been a key bottleneck to their high-temperature applications [6,7]. An oxidation-resistant coating is necessary for the high-temperature applications of these Mo-based alloys in oxidizing environments.

Silicide is an intermetallic material that has excellent oxidation resistance at high temperatures resulting from the formation of a continuous and protective SiO₂ layer at temperatures above 1000 °C, and has been widely used as an oxidation-resistant coating for refractory metals [8,9,10,11,12]. Among them, MoSi₂ has attracted much attention due to its high melting point (2030 °C), moderate density of 6.24 g/cm³, and similar CTE (8.1 × 10⁻⁶/°C) to refractory metals (e.g., Mo 6.7 × 10⁻⁶/°C). However, MoSi₂ suffers from rapid pesting oxidation between 400 and 600 °Cm which can lead to the disintegration of the material [13]. In addition, it performs limited protective performance at higher temperatures due to the volatilization of SiO₂, which has hindered its use under high-temperature oxidizing environments [14,15]. Attempts have been made to introduce alloying elements such as aluminum (Al), titanium (Ti), zirconium (Zr), and hafnium (Hf), as well as their oxides (e.g., Al₂O₃, ZrO₂, HfO₂), into MoSi₂-based coatings to enhance their properties at elevated temperatures [16,17,18,19,20,21,22]. Hf has been widely adopted to improve the high-temperature performance of superalloys [23,24,25,26] (e.g., Ni-, Co-, Fe-, and Nb-based superalloys) and the high-temperature coatings on those superalloy substrates (e.g., β-NiAl coating with B2 structure [27], aluminide coating [28], AlCoCrFeNi high-entropy alloy coating [29]). These reports have shown that small additions of Hf can effectively increase the oxide scale adhesion and thermal stability, thus increasing the service life of the coating [30]. There have been discussions on the effect of Hf on MoSi₂-based composites and coatings. Refs. [31,32] declared that Mo-Si-B bulk material doping with Hf exhibited enhanced mechanical properties and oxidation resistance at 1650 °C due to the formation of dense HfO₂ and HfSiO₄ oxides. Refs. [18,33] reported that the Hf-doped Mo-Si-B coating exhibits excellent oxidation resistance due to the formation of a protective surface oxide layer based on SiO₂+HfO₂+HfSiO₄. In addition, Al₂O₃ has been a favorable additive for MoSi₂ due to its high melting point (2054 °C), high thermal stability, and moderate CTE (8.3 × 10⁻⁶/°C) similar to that of MoSi₂ (8.1 × 10⁻⁶/°C). It has been confirmed that the addition of Al₂O₃ to MoSi₂ coatings is beneficial to restrain pest oxidation at low temperatures (500 °C) and improve high-temperature oxidation resistance (above 1500 °C) by generating a SiO₂-mullite composite oxide layer with high thermal stability and low oxygen permeability [5,17,34]. In recent years, attempts to synthesize MoSi₂ composites with superior properties for high-temperature applications by combining the alloying element (e.g., Ti, Al) and composite (e.g., Al₂O₃) have been reported [16,35]. Zhang C. et al. [16] reported the preparation and corrosion behavior of a Al and Al₂O₃ co-doped MoSi₂-based coating on the surface of molybdenum. Tian et al. [36] investigated the effect of Y₂O₃/yttrium (Y) on the Mo-Si-B coating on pure Mo.

In this work, Hf and Al₂O₃ were co-doped to a MoSi₂ coating to form a new (Mo,Hf)Si₂-Al₂O₃ composite coating on a Mo-based alloy substrate. Among the various coating techniques to prepare MoSi₂-based silicide coatings on refractory metals, vacuum slurry sintering is probably the most competitive, which offers the advantages of being low-cost, effective, and easy to operate [37,38]. Moreover, a metallurgical bond between the coating and substrate can be expected. Accordingly, the Hf-Al₂O₃ co-doped MoSi₂ coating was fabricated on a Mo-based substrate by the method of slurry sintering. The isothermal oxidation behavior of the composite coating was evaluated at 1400 °C in static air. The microstructure evolution, element distribution, and phase composition of the coatings before and after oxidation were characterized, and the antioxidant mechanism of the coating at high temperatures was also discussed.

2. Materials and Methods

2.1. Sample Preparation

Long strip specimens (80 mm × 10 mm × 2 mm) used as substrates were cut from Mo-0.7 wt.% La₂O₃ alloy plate. All samples were hand-polished with SiC-grit papers of 400, 600, and 1000 mesh, cleaned in an ultrasonic ethanol bath, and then dried in vacuum at 80 °C for 2 h.

The (Mo,Hf)Si₂-Al₂O₃ coating was prepared on Mo-La₂O₃ alloy substrates through the slurry sintering process. Firstly, a Si-Mo-Hf-Al₂O₃ slurry was prepared from a mixture of pure Si powders (≥99%, 1~3 μm), Mo powders (≥99%, 1~2 μm), Hf powders (>98%, 1~2 μm), and Al₂O₃ powders (≥99%, 5~10 μm) with a weight ratio of 65:20:5:10 for Si:Mo:Hf: Al₂O₃. A small amount of halide (NH₄F) and organic binder (nitrocellulose) was added to the powder mixture, and then the powder mixture was attrition milled for 6 h using ethyl acetate as solvent. Subsequently, the slurry was sprayed evenly on the surface of the Mo-La₂O₃ alloy substrate through an air compressor and a manual spray gun until a suitable coating thickness was obtained. The spraying process parameters for the Si-Mo-based slurry have been reported in previous work [5]. Finally, the as-sprayed samples were dried using an infrared baking lamp, taken into a vacuum furnace, and then sintered at 1480 °C for 20 min in vacuum (<1 Pa). Subsequently, the sintered coating samples were furnace-cooled to room temperature.

2.2. Oxidation Test and Characterization

Isothermal oxidation tests were conducted at 1400 °C in static air using an electric furnace, equipped with real-time display and automatic data recording. The sample temperature was measured by infrared radiation thermometer. Mass change of the specimens after oxidation for different times was measured using an analytical balance with an accuracy of 10⁻⁴ g to study the oxidation characteristics of the composite coating. The weight gain P₀ can be calculated by the following equation:

P₀ = △m/S

(1)

where △m is the mass gain of the coating after oxidation (mg), and S refers to the superficial area of coatings (cm²).

The phase composition of the coating before and after oxidation was analyzed by X-ray diffraction (XRD, D/Max 2500, Cu-Ka radiation, Tokyo, Japan). The surface and cross-sectional morphology of the as-coated and oxidized coating were obtained using scanning electron microscopy (SEM, FEI Sirion 200, Waltham, MA, USA) coupled with an energy-dispersive spectrometer (EDS) device. An electron-probe micro-analyzer (EPMA, JEOL JXA-8230, Musashino, Japan) was utilized to characterize the elemental distribution of the coating before and after oxidation.

3. Results and Discussion

3.1. Microstructure and Phase Composition of the Coating

Figure 1a shows the back-scattered electronic (BSE) image of the surface of the as-coated sample. The coating surface was relatively loose and rough, and mainly consisted of two kinds of particles. Meanwhile, there were some pores and pits on the coating surface; however, no noticeable microcrack was observed. It can be seen that dark gray particles (Spot 1) and white particles (Spot 2) were embedded with each other on the coating surface, while a number of white particles became intermingled together to form island-like clusters, as indicated by the blue arrows. The particle size of dark gray particles was in the range of 5~10 μm. The maximum size of the white island-like clusters was up to 20 μm while the minimum size of small particles was less than 2 μm. EDS results in Table 1 reveal that the composition of Spot 1 contained Al and O elements, and the ratio of Al:O was approximately 2:3, while the composition of Spot 2 contained Mo, Hf, and Si elements, and the ratio of (Mo+Hf):Si was about 1:2. Figure 1b–f shows the element mapping of the surface of the coating, which further confirms the distribution and element composition of the white particles and dark gray particles. Al and O elements were mainly distributed in the dark gray particles (Spot 1 in Figure 1a, while Mo and Si elements were mainly distributed in the white particles (Spot 2 in Figure 1a). It is noteworthy that the distribution of Hf was similar to that of Mo and Si, which was also distributed mainly in the white particles.

Figure 2 displays the XRD pattern of the surface of the as-prepared coating. According to the XRD pattern, the coating was composed of MoSi₂ and Al₂O₃. No other phases were detected. Combined with the XRD, EDS, and element mapping results, dark gray particles (Spot 1) and white particles (Spot 2) could be identified to be MoSi₂ with Hf existing in the solid solution and Al₂O₃, respectively.

Figure 1. (a) Surface morphology, (b–f) element mapping of the surface of the as-prepared coating.

Figure 1. (a) Surface morphology, (b–f) element mapping of the surface of the as-prepared coating.

Table 1. EDS analysis of Spots 1–2 in Figure 1 and Spots 3–5 in Figure 3.

Spot	Composition (at.%)					Main Phase
	Mo	Hf	Si	Al	O
1	0.29	0.05	0.61	39.42	59.63	Al2O3
2	30.83	2.09	63.06	1.17	2.85	(Mo,Hf)Si2
3	31.55	1.62	64.43	0.83	1.57	(Mo,Hf)Si2
4	32.32	-	67.25	0.15	0.28	MoSi2
5	0.51	0.04	0.84	38.90	59.71	Al2O3

Table 1. EDS analysis of Spots 1–2 in Figure 1 and Spots 3–5 in Figure 3.

Spot	Composition (at.%)					Main Phase
	Mo	Hf	Si	Al	O
1	0.29	0.05	0.61	39.42	59.63	Al2O3
2	30.83	2.09	63.06	1.17	2.85	(Mo,Hf)Si2
3	31.55	1.62	64.43	0.83	1.57	(Mo,Hf)Si2
4	32.32	-	67.25	0.15	0.28	MoSi2
5	0.51	0.04	0.84	38.90	59.71	Al2O3

Figure 2. XRD pattern of the surface of the as-prepared coating.

Figure 2. XRD pattern of the surface of the as-prepared coating.

Figure 3. (a) Cross-sectional morphology; (b–f) element mapping of the as-coated coating.

Figure 3. (a) Cross-sectional morphology; (b–f) element mapping of the as-coated coating.

Figure 3a shows the cross-sectional microstructure of the as-prepared coating. A typical double-layer structure consisting of an outer layer (Layer I) and an inner layer (Layer II) can be observed in the coating. There was no obvious diffusion layer between the coating and the substrate. Apparently, the inner layer showing a dense structure was firmly adhered to the Mo-based alloy substrate and constituted the principal part of the coating, which had 48~50 μm of thickness, while the outer layer showed a loose but continuous structure, which had 30~38 μm of thickness. The outer layer also adhered well to the inner layer, which is an important requirement for the long anti-oxidation life of the coating. Gray and black gray phases were observed inlaid with each other in the outer layer. Figure 3b–f displays the element mapping of the cross-section of the coating in Figure 3a. As revealed by the element mapping, the inner layer was rich in Mo and Si elements, while the outer layer was rich in Mo, Si, Hf, Al, and O. It seems Hf mainly distributed uniformly in the Mo-Si rich phases in the outer layer, and barely any of the Hf element appeared in the inner layer. Element mapping combined with EDS analysis and XRD pattern reveals the phase distribution, wherein Spots 3 and 4 were (Mo,Hf)Si₂ and MoSi₂ phases, respectively, while the black particles (Spot 5) were Al₂O₃. It can be inferred that the inner layer was mainly composed of MoSi₂, while the outer part of the coating was identified to be a (Mo,Hf)Si₂-Al₂O₃ mixed layer.

Figure 4 shows a schematic diagram of the microstructure evolution of the composite coating during preparation. The slurry containing Si, Mo, Hf, and Al₂O₃ powders was sprayed onto the surface of the Mo-based alloy substrate and then sintered at 1480 °C. Since the sintering temperature was above its melting point (1410 °C), Si was in a molten state and diffused inward to the direction of the matrix and reacted with the Mo-based substrate to form a Mo-silicide inner layer by liquid phase reaction, resulting in a relatively dense structure. High melting point elements Mo and Hf had a much slower inward diffusion rate and mainly enriched in the outer layer to form (Mo,Hf)Si₂ phases, as shown in Figure 3a,d. At the same time, the solid Al₂O₃ particle would not participate in the chemical reaction and was distributed between the sintering reaction products on the coating surface, forming a relatively loose (Mo,Hf)Si₂-Al₂O₃ outer layer on the surface, as shown in Figure 3a,e,f. The formation of pores and pits on the coating surface was related to the gaseous volatilization resulting from the decomposition of the nitrocellulose during the sintering process.

3.2. Oxidation Behavior of the Coating

Isothermal oxidation tests of the coating were conducted at 1400 °C in static air. The results show that the (Mo,Hf)Si₂-Al₂O₃ composite coating exhibited excellent oxidation resistance and could provide effective oxidation protection for Mo-based alloy up to 40 h at 1400 °C. Figure 5 exhibits the weight gain of the coating as a function of duration time ranging from 0.25 to 40 h. It can be seen that the coating exhibited continuous weight gain with increasing oxidation time, containing a rapid weight gain stage (Stage I, ranging from 0 to 1 h) and a slow weight gain stage (Stage II, ranging from 1 to 40 h), as shown in Figure 5. At the early stage of oxidation (Stage I), the mass change within an approximately linear region 0–1 h was 2.34 mg/cm², while the final weight gain rate of the coating after 40 h oxidation was only 6.03 mg/cm² after 40 h of oxidation. Changes in weight obeying a parabolic law were observed for the (Mo,Hf)Si₂-Al₂O₃ coating in this work and MoSi₂-Al₂O₃ in our previous work [5].

The revealed regularities of oxidation of the coating were attributed to the oxidation behavior during oxidation. In the initial stage, the coating oxidized, and the oxide scale (containing, e.g., HfO₂, SiO₂) was formed on the coating surface, resulting in rapid weight gain of the coating. However, the oxide scale was insufficient to wrap the substrate; consequently, significant oxidative weight gain was observed in Stage I. In Stage II, a relatively dense oxide scale was formed on the coating surface with an oxidation time extension. Since a protective oxide scale formed on the surface of the coating, the oxidation rate was under the control of the permeation of oxygen through the dense oxide scale, thus the mass gain rate decreased.

3.3. Phase Composition of the Oxidized Coating

Figure 6 shows the XRD patterns of the surface of the (Mo,Hf)Si₂-Al₂O₃ coating after oxidation for different times, and the phase composition of the coating after oxidation is given in

Table 2. Phase composition of the composite coating after oxidation at different times.

Time (h)	Phase Composition
0.5, 1, 5	MoSi2, Mo5Si3, mullite, Al2O3, HfO2, SiO2, HfSiO4
10, 20, 40	Mo, Mo5Si3, Mo3Si, HfO2, Al2O3, SiO2, mullite, HfSiO4

. Markers show positions of the peaks corresponding to MoSi₂, Mo₅Si₃, mullite, Al₂O₃, HfO₂, SiO₂, and HfSiO₄ after oxidation for 0.5 h. The phase composition of the coating surface after oxidation for 1 h and 5 h was the same as that of 0.5 h, while the intensity of diffraction peaks differed. At an exposure duration of 0.5–5 h, the diffraction peak intensity of MoSi₂ in the oxidized coating gradually decreased, while the peak intensity of Mo₅Si₃, HfO₂, SiO₂, HfSiO₄, and mullite increased with the extension of oxidation time, indicating that the content of MoSi₂ phases gradually decreased, while the content of Mo₅Si₃, HfO₂, SiO₂, HfSiO₄, and mullite increased. It is worth noting that the diffraction peak of MoSi₂ was absent, and Mo₃Si appeared significantly after 10 h of oxidation. The appearance of the Mo diffraction peak in the XRD pattern after 20 h of oxidation is supposed to be the Mo-alloy substrate. Compared with the XRD result of the unoxidized coating, it can be inferred that the presence of Mo₅Si₃, HfO₂, and SiO₂ was attributed to the oxidation of MoSi₂ and (Mo,Hf)Si₂. Meanwhile, the production of mullite and HfSiO₄ is obviously resulting from the synthetic reactions between Al₂O₃, HfO₂, and SiO₂. The presence of Al₂O₃ and HfO₂ phases during the whole 40 h oxidation demonstrates that only part of them reacted with SiO₂ to form mullite and HfSiO₄.

3.4. Microstructure Evolution of the Coating during Oxidation

3.4.1. Surface Morphology

Figure 7a–f reveals the surface morphology of the composite coating after oxidation for the time ranging from 0.5 to 40 h. In comparison with the surface of the as-prepared coating, the composite coating after oxidation exhibited a relatively compact and smooth structure. Glassy phases were formed on the surface of the coating after oxidation for 0.5 h, and there were granular and flaky particles inlaying in the glassy phases. Meanwhile, a ceramic layer was observed near the glassy oxides, as shown in Figure 7a. Figure 8 shows the element mapping of the coating after 0.5 h of oxidation. It can be seen that the glassy phases were rich in Si and O (SiO₂-based oxide), while the ceramic layer was rich in Al and O. The Hf element was mainly distributed in the glassy oxides. It can be inferred that the white particles (Spot 2 in Figure 1a, identified to be MoSi₂ with Hf dissolved in it) were oxidized to form SiO₂-based glassy oxides with HfO₂ inclusions. The dark gray particles (Spot 1 in Figure 1a, identified to be Al₂O₃) were stable for oxygen. However, the small size of the white particles (silicides) was observed to be found in the cavity of Al₂O₃ particles (Figure 1a) and would oxidize to form SiO₂-based oxides. Moreover, the Al₂O₃ layer on the coating surface was relatively loose, and oxygen could pass through the Al₂O₃ layer and react with silicide beneath the Al₂O₃ layer to generate SiO₂-based oxides. The Al₂O₃ particles would further be likely to react with SiO₂ to form mullite, leading to the formation of mountain-like clusters, which were marked by a purple outer oval, as shown in Figure 7a,b. Distinguished from the MoSi₂-Al₂O₃ coating in our previous work [5], the SiO₂-based glassy oxides formed on the surface of the (Mo,Hf)Si₂-Al₂O₃ coating in this work were distributed uniformly and would not bond together to form a complete and dense glassy oxide film on the coating surface. Instead, ceramic phases (e.g., Al₂O₃, mullite) and glassy phases alternated on the coating surface, as shown in Figure 7a–c. The surface morphologies of the composite coating after oxidation for 1 h and 5 h are similar to that of 0.5 h. The amount and size of both granular and flaky particles increased with oxidation time. Figure 7d–f shows the surface morphology of the composite coating after exposure duration for a longer time (10, 20, and 40 h). Obviously, the coating surface had a tendency to ceramicize, and the SiO₂-based glassy oxides were almost placed by a large number of island-like ceramic clusters, which illustrates that the glassy oxide was consumed. Surface morphology evolution (Figure 7) well corresponded with the XRD analysis results (Figure 6). It is believed that SiO₂ reacted with pre-existed Al₂O₃ and newly generated HfO₂ particles to form mullite and HfSiO₄, respectively, according to the XRD result in Figure 6, which resulted in a rough surface of the oxide layer. It has been reported that the formation of mullite and HfSiO₄ was beneficial to the anti-oxidation property resulting from their excellent thermal stability, low oxygen permeability, and the improvement of thermal mismatch between oxide and coating layers.

3.4.2. Cross-Sectional Microstructure

Figure 9 shows the cross-sectional microstructure evolution of the composite coating after oxidation ranging from 0.5 to 40 h. It can be seen from these pictures that the composite coating exhibited a multi-phase structure. As shown in Figure 9a, a four-layer structure marked as Layer I, Layer II, Layer III, and Layer IV from inside to outside is observed in the cross-section of the coating after oxidation for 0.5 h, with a thickness of 14.3 μm, 30 μm, 35 μm, and 16.7 μm respectively. It is concluded that the thickness of the MoSi₂ layer reduced by about 20 μm (from 50 μm to 30 μm), while the thickness of the Mo₅Si₃ layer increased by about 14.3 μm (from 0 to 14.3 μm), compared to the as-prepared coating. Figure 10 shows the element mapping of the cross-section of the oxidized coating, where it is indicated that Layer I was an oxide scale mainly containing Si, Al, Hf, and O elements, while Layer II was a silicide–oxide composite layer mainly containing Si, Mo, Al, Hf, and O elements. It seems that Layers III and IV both mainly contained Si and Mo elements, of which Layer III was rich in Si while Layer IV was Si-poor Mo-silicide.

Table 3. EDS analysis of Spots 1–9 in Figure 9.

Spot	Composition (at.%)					Main Phase
	Mo	Hf	Si	Al	O
1	58.92	-	39.84	-	1.24	Mo5Si3
2	31.68	0.12	66.91	-	1.29	MoSi2
3	29.69	0.26	66.93	-	3.12	MoSi2 (Hf)
4	55.87	-	40.02	-	4.11	Mo5Si3
5	-	0.95	12.14	32.98	53.93	SiO2, Al2O3, HfO2
6	62.20		23.88	-	13.92	Mo3Si
7	53.65	-	36.46	-	09.89	Mo5Si3
8	65.81	-	25.54	-	08.65	Mo3Si
9	-	0.54	35.73	6.97	56.76	SiO2, Al2O3, HfO2

gives the average chemical composition of Spots 1–9 in Figure 9. According to the atomic ratios of the elements and the XRD pattern (Figure 6), Spots 1–5 were Mo₅Si₃, MoSi₂, (Mo,Hf)Si₂, (Mo,Hf)₅Si₃, and Si-Al-Hf-O oxide (a mixture of SiO₂, Al₂O₃, and HfO₂), respectively. Consequently, Layers I, II, III, and IV were identified to be Mo₅Si₃, MoSi₂, MoSi₂+Mo₅Si₃, and Si-Al-Hf-O oxide, respectively. A thick Mo₅Si₃ diffusion layer was formed between the MoSi₂ layer (Layer II) and the substrate, and a small amount of Mo₅Si₃ phase existed between the oxide scale and the MoSi₂ layer. The thin Mo₅Si₃ layer formed between the oxide scale and MoSi₂ layer was caused by the oxidation of MoSi₂ (Si diffusion outward to react with oxygen), while the Mo₅Si₃ layer (Layer I in Figure 9a) between the substrate and MoSi₂ layer was formed due to decomposition of MoSi₂ (Si diffusion from the coating to the substrate). The coating structures after oxidation at 1400 °C for 1 h (Figure 9b) and 5 h (Figure 9c) also consisted of four layers, which is similar to that of 0.5 h. Moreover, the phase composition of each layer was also the same, while the thickness differed. The thickness of the MoSi₂ layer (Layer II) reduced to 31.3 μm, while the Mo₅Si₃ layer (Layer III) and the oxide scale (Layer IV) thickened to 25 μm and 16.6 μm, respectively, after 1 h of oxidation. The value of the thickness of the MoSi₂ layer, Mo₅Si₃ layer, and oxide layer was 24.3 μm, 51.5 μm, and 21.4 μm, respectively, when the oxidation time reached 5 h. The results illustrate the continuous consumption and decomposition of MoSi₂ during oxidation, giving rise to the increase in the thickness of the Mo₅Si₃ and SiO₂-based scale. It can also be seen from Figure 9a–f that a large amount of HfO₂, HfSiO₄, Al₂O₃, and mullite was distributed in the outer layer of the oxide scale (Layer IV), while a small part of them was distributed in the silicide layer (Layer III), as shown in Figure 9a–c. Concurrently, the amount of white particles in the outer layer of the oxide scale (Layer IV) continued to increase with the extension of oxidation time, especially after 20 and 40 h of oxidation, as shown in Figure 9e,f. This is attributed to the persistent oxidation of Hf to form HfO₂, and HfO₂ would further generate HfSiO₄ by reacting with SiO₂, which could be confirmed by the XRD analysis. HfO₂, HfSiO₄, Al₂O_3, and mullite particles inlaid into the SiO₂-based oxide scale and could form a skeleton structure, in which HfO₂, HfSiO₄, Al₂O_3, and mullite particles play a pinning role on the SiO₂-based oxide scale and could enhance the structural stability of the oxide scale.

Figure 11 shows an enlarged view of the interface between the oxide scale and coating after 5 h of oxidation. A ceramic layer with a large amount of oxides (e.g., Al₂O₃, HfO₂) and ceramic particles (e.g., mullite, HfSiO₄) was observed outside the oxide scale. Mo₅Si₃ and MoSi₂ were closely bonded. SiO₂-based oxides containing Al₂O₃ and HfO₂ were embedded in the gap of the Mo₅Si₃ and MoSi₂ phases. Simultaneously, HfO₂ particles were partly dispersed in the MoSi₂ phases, which illustrates the internal oxidation of (Mo,Hf)Si₂. The growth and agglomeration of HfO₂ particles can also be observed. The HfO₂ particles had a maximum size of ~5.5 μm. The distribution of Al₂O₃ particles is similar to that of HfO₂.

It is worth noting that, as the oxidation time was further extended to 10 h, although the coating still displayed a four-layer structure, significant changes had taken place in the thickness and composition of each layer of the coating. To distinguish the structure of the coating before and after oxidation for 10 h, the four layers in Figure 9d are marked as Layer i, Layer ii II, Layer iii, and Layer iv from inside to outside. According to the EDS (Table 3) and XRD results (Figure 6), it can be inferred that the phase composition of Layers i, ii, and iv was Mo₃Si, Mo₅Si₃, and Si-Al-Hf-O oxide, with a thickness of 10.7 μm, 81.1 μm, and 24.9 μm, respectively, while Layer iii was a composite layer mixture of Mo₅Si₃ and Si-Al-Hf-O oxides. The MoSi₂ layer was absent after 10 h of oxidation, which is consistent with the XRD analysis in Figure 6. These above results indicate the complete consumption of the MoSi₂ layer. As compared to the microstructure of the coating after oxidation for 5 h, a Mo₃Si layer was newly generated between the Mo₅Si₃ layer (Layer ii) and the substrate after 10 h of oxidation. However, no Mo₃Si formation was observed between the Mo₅Si₃ layer (Layer ii) and the oxide scale (Layer iv). The coating structures after 20 h and 40 h of oxidation were similar to that of 10 h oxidation. The thickness of the Mo₃Si layer (Layer i) and oxide scale both increased with oxidation time.

Figure 12 displays the thickness variation of coating layers after oxidation. It is obvious that the oxide layer was growing constantly during the exposure duration of 0.5–40 h. During the whole oxidation process, the thickness of the Mo₅Si₃ layer increased gradually and reached the maximum (81.1 μm) after 10 h of oxidation, then began to decrease with the increase in oxidation time. When the oxidation time reached 40 h, the thickness of the Mo₅Si₃ layer decreased to 75.7 μm. The thickness of the Mo₃Si layer (Layer i) increased to 17.1 μm and 21.4 μm, and the thickness of the oxide scale increased to 25.5 μm and 29.7 μm, respectively, after 20 h and 40 h of oxidation.

According to the element mapping of the coating after oxidation in Figure 10, it is concluded that during oxidation, Hf and Al elements tended to concentrate on the silicide–oxide composite layer (Layer III and iii) and the oxide scale (Layer IV and iv), which reveals that HfO₂, HfSiO₄, Al₂O₃, and mullite particles were mainly distributed in the oxide scale and silide–oxide composite layer. This is consistent with the cross-sectional morphology observation in Figure 9.

Microcracks were found in the MoSi₂ layer after 0.5 h of oxidation and reached the Mo₅Si₃/MoSi₂ interface after 1 h of oxidation. When the oxidized samples were cooled down from the test temperature to room temperature, cracks could be produced induced by large thermally induced stresses. After long-time oxidation (20 h and 40 h), cracks passed through the Mo₃Si layer and propagated to the Mo₃Si/substrate interface. Simultaneously, there were some Kirkendall voids in the Mo₅Si₃ layer after oxidation for 1 h, which were marked with a blue circle, and the number of Kirkendall voids increased with the extension of oxidation time to form porous zones in the Mo₅Si₃ layer, as shown in Figure 9b–f. The occurrence of voids in the Mo₅Si₃ layer was attributed to the faster diffusion of Si than that of Mo in the Mo₅Si₃ phase [5]. The mismatch of the CTE between the oxide scale and the MoSi₂ layer easily leads to the cracking and shedding of the oxide scale. However, no visible cracks were revealed in the oxide scale after 10 h of oxidation in this work, indicating that the oxide scale containing Al₂O₃, HfO₂, mullite, HfSiO₄. and SiO₂ exhibited enhanced thermal matching with the MoSi₂ layer. Nonetheless, cracks were formed in the Mo₃Si layer after 20 h of oxidation, as shown in Figure 9e,f. It is confirmed that the Mo₅Si₃ layer (Layer I in Figure 9a–c) and the Mo₃Si layer (Layer i in Figure 9d–f) were beneficial to prevent cracks propagating toward the Mo-based alloy substrate since no cracks were found to propagate into the matrix even after 40 h of oxidation [5,39]. In addition, It can also be seen that even after oxidation at 1400 °C for 40 h, the (Mo,Hf)Si₂-Al₂O₃ composite coating remained intact, which indicates that the composite coating was still efficiently protective.

3.5. Antioxidation Mechanism of the Composite Coating

Figure 13 displays the schematic oxidation mechanism of the (Mo,Hf)Si₂-Al₂O₃ composite coating at 1400 °C in air. The composite coating exhibits a double-layer structure consisting of a MoSi₂ inner layer and an outer layer of a mixture of (Mo,Hf)Si₂ and Al₂O₃ (Figure 13a). Reactions (2)–(9) may occur when the coating samples are exposed to a high-temperature oxidizing environment at 1400 °C. The standard Gibbs free energy (based on 1 mol oxygen) of Reaction (2) is lower than that of Reaction (3), which is −420.27 kJ/mol and −559.84 kJ/mol, respectively [39,40]. Similarly, the standard Gibbs free energy of Reaction (4) is lower than that of Reaction (5). Thus, Reactions (2) and (4) are dominant. In the initial stage of oxidation, the oxidation of MoSi₂ and (Mo,Hf)Si₂ gives rise to form SiO₂, HfO₂, Mo₅Si₃, and MoO₃ on the coating surface according to Reactions (2)–(5). A thin Mo₅Si₃ layer could be observed between the oxide scale and the coating (Figure 9a and Figure 13b). In addition to Reactions (2)–(5), the pre-existing Al₂O₃ and newly generated HfO₂ were partly converted to mullite and HfSiO₄, respectively, according to Reactions (6) and (7). Since MoO₃ would volatilize rapidly at the temperature for the isothermal oxidation test in this work, a glassy SiO₂-based oxide scale with HfO₂, Al₂O₃, mullite, and HfSiO₄ inclusions was formed on the coating surface. Since the silicide–oxide composite layer (Layer II in Figure 3a) was relatively loose, oxygen could also diffuse inward through the pores and pits directly and react with MoSi₂ and (Mo,Hf)Si₂ in the sublayer (Layer II in Figure 3a). Thus, glassy SiO₂-based oxides were also formed inside the silicide–oxide composite layer. Al₂O₃ particles embedded in the MoSi₂ phases in the as-prepared coating (Figure 3a) were then embedded in SiO₂-based oxides (Figure 10) along with the MoSi₂ transformation to SiO₂ during oxidation. With the formation of the oxide scale, the pores and pits could be filled, and the inward diffusion of oxygen to the coating was hindered.

Compared with the MoSi₂-Al₂O₃ composite coating in our previous work [5], the composite coating in this work did not generate a complete and dense glassy SiO₂-based oxide scale to cover the whole coating surface after 0.5 h of oxidation. Instead, a large amount of white particles (e.g., HfO₂, HfSiO₄, Al₂O₃, mullite) were concentrated on the coating surface to form a ceramic outer layer. This phenomenon was partly related to the preferential oxidation of Hf. Hf shows a higher affinity with oxygen relative to Si and could be oxidized preferentially at high temperatures. Through that, the reaction of Si with oxygen to form SiO₂ could be relatively suppressed, and the driving force for its outward diffusion could be decreased as well. Hence, a large amount of white particles (e.g., HfO₂) were agglomerated and enriched in the outer layer of the oxide scale, which is confirmed by the surface morphology of the oxidized coating (Figure 6). Since Al₂O₃ particles were distributed mainly in the outer layer in the as-prepared coating, they would distribute mainly in the outer layer of the oxide scale in the oxidized coating. Furthermore, Reactions (6) and (7) concerning the formation of mullite and HfSiO₄ consumed a certain amount of SiO₂, which led to the coating failing to form a glassy oxide film properly at the place where HfO₂ and Al₂O₃ particles were enriched. However, a dense oxide film was formed below, acting as a diffusion barrier of oxygen, as shown in Figure 9a–f. Meanwhile, the formation of the composite oxide scale embedded with high-temperature stable mullite and HfSiO₄ is believed to improve the high-temperature resistance of the composite coating because of the following factors: (1) Al₂O₃, mullite, HfO₂, HfSiO₄, and SiO₂ could develop a special glass–ceramic skeleton structure on the coating surface and possessed the peculiarities of the stress tolerance of glass scale and the structural stability of the ceramic phase, in which Al₂O₃, HfO₂, mullite, and HfSiO₄ particles have a pinning effect on the SiO₂ glassy oxide [20]. (2) The permeability of oxygen through this composite oxide scale is supposed to be lower than that of oxygen in the SiO₂ scale [41]. (3) The CTE of Al₂O₃ (8.3 × 10⁻⁶/K), HfO₂ (5.8 × 10⁻⁶/K), mullite (5.6 × 10⁻⁶/K), and HfSiO₄ (4.6 × 10⁻⁶/K) is higher than that of SiO₂ (0.55 × 10⁻⁶/K). The CTE of the composite oxide scale is higher than that of the pure SiO₂ scale and closer to that of the coating, which is expected to improve the CTE mismatch between the oxide scale and the coating. For the above reasons, the composite oxide scale could be well stabilized on the coating surface and provide long-term oxidation protection for Mo-based substrates by retarding oxygen diffusion into the internal coating and suppressing crack propagation in the oxide scale.

In addition to the surface layer, the microstructure inside the coating also changed. A diffusion layer of Mo₅Si₃ was formed between the MoSi₂ layer and substrate according to Reaction (8), as shown in Figure 13b. It is believed that the formation of the thin Mo₅Si₃ layer between the oxide scale and MoSi₂ layer was attributed to the oxidation of MoSi₂, while the Mo₅Si₃ layer between the MoSi₂ layer and the substrate was derived from the inward diffusion of Si (MoSi₂) from coating to substrate according to Reaction (8) rather than oxidation [5]. During oxidation, Si atoms diffuse outward to generate Mo₅Si₃ and SiO₂, and inward to the substrate to form Mo₅Si₃ (Layer I); at the same time, Hf would diffuse outward and react with oxygen to form HfO₂.

With the extension of oxidation time, the continuous consumption of MoSi₂ led to a gradual decrease in thickness, accompanied by an increase in the thickness of the Mo₅Si₃ layer and the oxide scale layer. When the oxidation time reached 10 h, the MoSi₂ layer was absent (Figure 13d) and a diffusion layer of Mo₃Si was generated between the tMo₅Si₃ layer and the substrate. It is believed that the formation of Mo₃Si was attributed to the diffusion of Si from Mo₅Si₃ to the Mo-based substrate driven by the concentration gradient according to Reaction (9).

The outward diffusion of Si and Hf from the coating to the oxide scale and the interdiffusion between the coating and substrate during oxidation resulted in a multi-layer structure of the coating after oxidation. Since the thermal stress for the Mo₅Si₃/Mo interface is lower than that for the MoSi₂/Mo substrate [5], the formation of the multi-layer structure of the coating after oxidation is beneficial to release the residual stress induced by thermal mismatch between the composite coating and the substrate [42,43], which would restrain the crack formation in the coating and prevent crack propagation toward the substrate even after 40 h of oxidation (Figure 9f and Figure 13d).

5/7MoSi₂ + O₂ = 1/7Mo₅Si₃ + SiO₂

(2)

2/7MoSi₂ + O₂ = 2/7MoO₃ + 4/7SiO₂

(3)

(Mo,Hf)Si₂ + O₂ →Mo₅Si₃ + HfO₂ + SiO₂

(4)

(Mo,Hf)Si₂ + O₂ → MoO₃ + HfO₂ + SiO₂

(5)

3Al₂O₃ + 2SiO₂ = 3Al₂O₃·2SiO₂ (mullite)

(6)

HfO₂ + SiO₂ = HfSiO₄

(7)

MoSi₂ + 7/3Mo = 2/3Mo₅Si₃

(8)

Mo₅Si₃ + 4Mo = 3Mo₃Si

(9)

4. Conclusions

In this study, a highly oxidation-resistant MoSi₂-based composite coating with Hf and Al₂O₃ co-doping was prepared by the slurry sintering method. Additionally, its oxidation behavior was systematically investigated at 1400 °C. The coating was composed of MoSi₂ with Hf existing in solid solution and Al₂O₃ phases and had a double-layer structure consisting of a MoSi₂ inner layer and a (Mo,Hf)Si₂-Al₂O₃ outer composite layer. The composite coating exhibited excellent oxidation resistance at 1400 °C, and the mass gain was only 6.03 mg/cm² after 40 h of oxidation. During oxidation, Hf diffused outward and underwent preferential oxidation to generate HfO₂, while Si was susceptible to diffuse outward to form SiO₂, Mo₅Si₃ on the coating surface by oxidation of MoSi₂ and inward to form Mo₅Si₃ diffusion layer between coating and substrate by decomposition of MoSi₂. Pre-existing Al₂O₃ and newly generated HfO₂ particles partly reacted with SiO₂ to form thermal stable mullite and HfSiO₄. SiO₂, Al₂O₃, HfO₂, mullite, and HfSiO₄ developed a dense protective oxide scale on the surface of the composite coating during oxidation. The composite oxide scale developed a skeleton structure with enhanced thermal stability and improved adhesion between the oxide scale and coating, which enhanced the thermal stability of the oxide scale. Simultaneously, element (e.g., Si, Hf) diffusion led to the formation of a multi-layer structure in the coating, which is beneficial to release the residual stress induced by thermal mismatch between the coating and the substrate and inhibit the tendency of crack formation or prevent crack propagation directly on the substrate.