Effect of MoSi₂-Si₃N₄/SiC Multi-Layer Coating on the Oxidation Resistance of Carbon/Carbon Composites above 1770 K

Abstract

To improve the oxidation resistance of carbon/carbon composites at high temperatures (>1770 K), they were coated with MoSi₂-Si₃N₄/SiC. The slurry and pack cementation methods were adopted to deposit the inner SiC layer and outer MoSi₂-Si₃N₄ layer. The phase composition, microstructure, and elemental distributions in the coating were analyzed using SEM, XRD, EDS, and Raman spectroscopy. Oxidation tests show that the deposited multi-layer coating can protect the carbon/carbon matrix from oxidation at high temperatures (>1770 K) for 150h and that the coating can withstand 40 thermal cycles between 1773 and 300 K. It is observed that Si₃N₄ assists in the formation of a dense SiO₂ layer at a high temperature, which plays a vital role in increasing the thermal cyclic and oxidation resistance of the coating itself. The weight loss of coated carbon/carbon composite is attributed to the formation of micro-cracks and diffusion of SiO₂, MoO₃, and N₂ out of the material at high temperatures.

1. Introduction

For aggressive service environments in the nuclear and aerospace industry, carbon/carbon composites are candidate structural materials due to their attractive properties such as a high strength-to-weight ratio, thermal cyclic oxidation resistance, strength retention, and a low thermal expansion coefficient [1,2,3,4,5]. However, these composites exhibit limitations such as susceptibility to oxidation beyond 773 K and strength loss upon exposure to high temperatures. This makes them highly unfavorable for specific high-temperature applications [6,7,8]. It was shown by previous researchers that carbon/carbon composites could be protected against oxidation at high temperatures by the application of anti-oxidizing coatings [9].

In multi-layered coatings, MoSi₂ is used as an outer layer, while SiC forms the inner buffer layer. The peripheral multi-layer coating containing MoSi₂ exhibits a superior oxidation-resistant capability for carbon/carbon composites at 1500–1600 °C [10]. However, due to the mismatch of the thermal expansion coefficient between the SiC bonding layer and the MoSi₂ outer coating, micro-cracks in the MoSi₂ appear at extended high-temperature (i.e., 800–1000 °C) exposure [11,12,13,14]. Therefore, the protective temperature range of coatings containing MoSi₂ is constricted; this limits the structural and high-temperature applications of such materials. The coefficient of thermal expansion (CTE) of MoSi₂ (~8.1 × 10⁻⁶/K) is higher than that of carbon/carbon composites (~1.0 × 10⁻⁶/K). Therefore, the mismatch between the CTEs of the outer MoSi₂ layer and the inner SiC-coated carbon/carbon matrix causes the degradation of the coating under thermal cycling and ultimately lowers the coating durability [15,16].

MoSi₂ coating at a temperature range of 400–600 °C efficiently reacts with oxygen to form MoO₃ and SiO₂. These oxides result in the considerable volume expansion of the MoSi₂ matrix, which results in the disintegration of bulk MoSi₂ into powders and causes catastrophic failure of the coating [17]. This is called the “pest phenomenon” and is another primary reason for coating failure. In addition, MoSi₂ is a low toughness material which limits its industrial application even at ambient temperature [18].

Earlier reports have suggested that the addition of Si₃N₄ to MoSi₂ can minimize the coefficient of thermal expansion (CTE) mismatch between the outer MoSi₂ and inner SiC coatings. Si₃N₄ exhibits high flexural strength and excellent compatibility with MoSi₂ and SiC due to better mixing with these compounds. It also exhibits a reasonable resistance to creep, isothermal oxidation, and cyclic thermal oxidation [19]. The MoSi₂-Si₃N₄ coating exhibits excellent oxidation resistance with minute weight loss. At high temperature, MoSi₂-Si₃N₄ forms a dense glassy SiO₂ film on the coating surface. Owing to its low oxygen-diffusion coefficient, the SiO₂ film serves as an oxygen diffusion barrier and efficiently protects carbon/carbon composites from oxidation at 1773 K [20,21]. Due to good fluidity at high temperatures, SiO₂ can seal all the micro-cracks formed during the volatilization of MoO₃, CO₂, and N₂ [22]. Furthermore, the addition of Si₃N₄ provides better refractory and some oxidation-resistant properties, which decrease the possibility of pest disintegration of MoSi₂. Huang et al. [23] prepared a MoSi₂/Si₃N₄ coating on Mo substrate and described the effect of the Si₃N₄ content on the microstructure and antioxidant properties of multi-layer coatings.

In this study, coatings composed of a SiC bonding layer and a MoSi₂-Si₃N₄ outer layer were prepared by a simple and low-cost slurry method. The phase composition, microstructure, and oxidation resistance of the above multi-layer coatings at 1773 K in the air were investigated in detail.

2. Experimental Procedure

2.1. Coating Preparation

Small specimens (10 × 10 × 10 mm³) of carbon/carbon composite (Shandong Weiji Carbon Technology Co. Ltd., Jinan, China) with a density of 1.79g/cm³ used as substrates were cut from thin bulk sheet. The carbon/carbon composite used in this study was carbon fiber-strengthened graphite. The surface area of the samples was increased by abrading them with commercially available rough SiC papers. To remove any remaining debris and moisture, the samples were cleaned ultrasonically with distilled water and dried in an oven at 100 °C for 2 h.

The powder compositions used were Si 75 wt%, graphite 20 wt%, and Al₂O₃ 5 wt% respectively. All the powders used were lab-grade and had a purity ≥ 99% with particle size ranging between 1–10 µm. SiC inner coatings were applied to the specimens by the pack-cementation method. The carbon/carbon composites and pack mixtures were put in a graphite crucible and heated at 1873 K for 2 h in an argon atmosphere to form the SiC coating. After the completion of inner coating, the SiC-coated specimens were cleaned ultrasonically and dried in an oven at 100 °C for 2 h.

The liquid slurry mixture method was adopted for applying the outer coating of MoSi₂-Si₃N₄ over the already applied inner SiC coating. High-purity MoSi₂ and Si₃N₄ powders supplied by Turnnano Ltd., Henan, China) were used. The MoSi₂ and Si₃N₄ powders were mixed in 10mL aqueous solution of H-PSO, and a homogenous slurry solution was prepared. The SiC-coated carbon/carbon specimens were put in a slurry mixture of MoSi₂ and Si₃N₄. The as-coated specimens were put in a graphite crucible and heated up to 2073 K for two hours in the nitrogen atmosphere to apply the outer MoSi₂-Si₃N₄ coating on the samples.

The slurry method was preferred over others due to its compositional flexibility, ease of processing, and low cost. Also, it is known that the coating deposited via the slurry method exhibits reasonable resistance to oxidation and thermal cyclic oxidation [11]. The MoSi₂-SiC and MoSi₂-Si₃N₄/SiC coatings were prepared using a similar approach to elucidate the effect of Si₃N₄ addition on the oxidation resistance of the multi-layer coatings.

2.2. Oxidation Test

Isothermal oxidation tests on the MoSi₂-Si₃N₄/SiC multi-layer coated carbon/carbon specimens were performed in a corundum tube furnace at 1773 K for 150 h. After every 10 h to a maximum time of 150 h, the samples were taken out of the furnace directly and cooled to room temperature in air. The weight loss of each specimen at room temperature was measured with an electronic balance with a sensitivity of ±0.1 mg. The percentage weight loss (wt%) was calculated using Equation (1), where $m_0$ and $m_1$ are the weights of specimens before and after the oxidation test, respectively.

$$\mathrm{wt}\%=\frac{m_0-m_1}{m_0}\times 100\%$$

(1)

The thermal cyclic oxidation resistance of the MoSi₂-Si₃N₄/SiC multi-layer coating was investigated by conducting a test between 1773 K and room temperature. In this test, the coated carbon/carbon specimens were heated at 1773 K for 10 min, removed from the furnace, and air-cooled to room temperature for each cycle. The testing was carried out in 40 cycles.

2.3. Characterization of the Coatings

The morphology of the MoSi₂-Si₃N₄/SiC multi-layer coating before and after the oxidation test was analyzed using a scanning electron microscope (SEM), equipped with an energy dispersive spectroscope (EDS). Raman spectroscopy (LabRam-1B, 633nm line of the He-Ne laser) was used to identify the chemical composition and phase transformation. An X-ray diffraction (XRD) technique was used for the crystallographic characterization of the coating before and after the oxidation test.

To analyze the elemental distribution along with the depth of the coated samples, they were sectioned using a power cutter and were then ground and polished to prepare the surface perpendicular to the top coating surface. These samples were then analyzed using the same characterization techniques stated earlier in this section.

3. Results and Discussion

3.1. Microstructure of Coating before the Oxidation Test

The XRD pattern of the as-coated inner SiC layer is shown in Figure 1a. Three significant peaks at 2θ ≈ 35.6°, 60.0°, and 71.7°correspond to the (111), (220), and (311) crystalline planes of β-SiC, indicating the formation of the β-SiC crystalline plane during the pack cementation process. The low-intensity peak at 2θ ≈ 33.6° indicates the existence of stacking faults (SF) in the β-SiC coating. The stacking faults might be induced due to the thermal stresses produced in the coating process [24].

Figure 1b shows the surface microstructure of the as-coated inner SiC layer. Figure 2 shows the XRD analysis of the as-coated specimen, i.e., the MoSi₂-Si₃N₄/SiC-coated carbon/carbon specimen before the oxidation test. MoSi₂, Si₃N₄, and SiC phases are apparent. The more vigorous peak intensity of the MoSi₂ phase in Figure 2 indicates that it is relatively more abundant in the slurry.

Figure 3a shows the surface microstructure of the as-coated MoSi₂-Si₃N₄ outer layer before the oxidation test. The surface of the coating has no micro-cracks, indicating that the as-prepared coating is dense and homogeneous. In Figure 3b, the EDS pattern of the area marked with the red square in Figure 3a clearly shows that the smooth area of the as-coated MoSi₂-Si₃N₄ is composed of the elements Si, N, and Mo.

Figure 4 shows an overlay of element line scanning over the SEM image of the cross-section of MoSi₂-Si₃N₄/SiC coated C/C composite before the oxidation test. The multi-layer coating has an average thickness of 195 ± 3 µm. It can be observed from the line scan that the as-prepared coating is composed of a MoSi₂-Si₃N₄outer layer, SiC inner layer, and SiC transition layer, of thicknesses 40 ± 5, 61 ± 2, and 93 ± 2 µm, respectively. The SiC transition layer, formed possibly due to the infiltration of Si into the carbon/carbon substrate at high temperature, increases the oxidation and thermal cyclic oxidation resistance of the coating. The lack of voids between the outer and inner coating layers in Figure 4 indicate two things:

• An increase in the interfacial bonding strength of the coating [25];
• A reduction in the thermal mismatch of the expansion between the coatings and the C/C substrate.

3.2. Microstructure of Coating Post-Oxidation Test: Oxidation Resistance Evaluation

Figure 5 shows the XRD spectrum of the MoSi₂-Si₃N₄/SiC coating after the oxidation test at 1773 K. It is clear that the oxidation test resulted in the formation of several new phases. The MoSi₂ phase of the MoSi₂-Si₃N₄/SiC coating has undergone complete transformation, producing some new phases, SiO₂, Mo₅Si₃, and MoO₃. The appearance of large fractions of Si-based new phases indicates that MoSi₂ is completely oxidized. The partial oxidation of Si₃N₄ at oxidation temperature has resulted in the formation of the Si₂N₂O phase that is in accordance with the XRD pattern [26]. The possible reactions are as follows.

SiC (s) + 2O₂(g) → SiO₂ + CO₂ (g)

(2)

5MoSi₂ (s) + 7O₂ (g) → Mo₅Si₃ (s) + 7SiO₂ (s)

(3)

2Mo₅Si₃ (s) + 21O₂ (g) → 10MoO₃ (g) +6SiO₂ (s)

(4)

2MoSi₂ (s) + 7O₂ (g) → 4SiO₂ (s) + 2MoO₃ (g)

(5)

Si₃N₄ (s) + 3O₂ (g) → 3SiO₂ (s) + 2N₂ (g)

(6)

Si₃N₄ (s) + SiO₂ (s) → 2Si₂N₂O (s)

(7)

Mo₅Si₃ and SiO₂ play an important role in increasing the oxidation resistance of the coating. The Mo₅Si₃ phase increases the coating flexural strength and the coating-substrate compatibility and enhances the creep strength [27]. The SiO₂ phase, owing to its inherent low viscosity, good fluidity and low oxygen permeability, serves as a barrier against oxygen attack. In Figure 6, the Raman spectroscopy result also confirms the formation of SiO₂ in the MoSi₂-Si₃N₄ coating after oxidation at 1773 K. It can be found that the peak with wave number of 228 and 414 cm⁻¹ is the crystallite while the wave number of 1034 cm⁻¹ is the amorphous silica, which is accordance with the XRD result.

The effect of Si₃N₄ on the oxidation resistance of the multi-layer coating was investigated from the microstructure analysis and weight loss of the coatings constituting the MoSi₂ and MoSi₂-Si₃N₄ outer layers. Figure 7a shows the microstructure of the MoSi₂ surface of the MoSi₂/SiC multi-layer coating after the oxidation test. The incomplete formation of silica (SiO₂) was due to the non-uniform agglomeration of MoSi₂ particles in the brushing process [28]. These particles degraded the surface of the coating and gradually reduced its oxidation resistance. As a result, some deep cracks occurred and propagated through the randomly deposited MoSi₂.

Figure 7b shows the SEM micrograph of the cross-section of MoSi₂/SiC multi-layer-coated carbon/carbon composites. The cross-section is rough, porous, and oxidized. This is because of the large CTE difference between SiC and MoSi₂, leading to the incomplete formation of SiO₂ and pore/cavity formation [25,29]. These pores provide the diffusion channel for oxygen that penetrates the substrate and causes the failure of the coating. It is also noteworthy that the infiltration of oxygen via the cavities would reduce the thickness of the coating. Therefore, the porous morphology of the MoSi₂/SiC coating is detrimental for its oxidation resistance.

Figure 7c shows the microstructure of the outer surface of the MoSi₂-Si₃N₄/SiC coating after the oxidation test at 1773 K. A dense glassy SiO₂ layer on the surface is evident, which prevents the carbon/carbon matrix from oxygen penetration and protects the coating from oxidation. This makes the MoSi₂-Si₃N₄/SiC coating oxidation-resistant and an ideal choice for high-temperature applications.

In Figure 7d, the cross-sectional morphology of the post-oxidation tested MoSi₂-Si₃N₄/SiC coating exhibits excellent compatibility between the coating and the substrate, which is responsible for the high bonding and flexural strengths of the coating in high-temperature environments [30,31]. Furthermore, the absence of voids/holes at the coating–substrate interface indicates that the coating is resistant to high-temperature rupture. In addition, Si₃N₄ forms a suitable combination with MoSi₂ in the slurry, which helps to minimize the CTE difference between the outer MoSi₂-Si₃N₄ layer and the inner SiC-coated carbon/carbon substrate. Therefore, Si₃N₄ plays a vital role in increasing the oxidation protective ability of outer coatings at high temperatures [32]. The formation of microdefects (i.e., microcracks and micropores) is due to the volatilization of MoO₃. From experimental observations, these pores are far from detrimental to the coating and can be cured by the glassy SiO₂ layer at high temperatures.

Figure 8 shows a comparison of weight losses for the two coatings, with and without Si₃N₄, after the oxidation test at 1773 K. The MoSi₂-Si₃N₄/SiC coating experienced a weight loss of 0.9% after 150 h of oxidation treatment while the MoSi₂/SiC coating suffered a relatively high weight loss of 4.0% even after 90 h of oxidation.

The absence of Si₃N₄ in the MoSi₂/SiC coatings causes the pest disintegration of MoSi₂ which results in the coating’s degradation at high temperatures. The increasing weight loss with oxidation time is due to the insufficient amount of SiO₂ that results in the formation of large cracks on the surface of the coating. This severely affects the coating surface and allows oxygen access to the C/C composites, causing gaseous byproducts. At high temperatures, these gaseous byproducts quickly evaporate to cause the sudden weight loss of the coating. This de-gasification produces some micro-cracks and deep cavities in the coating surface, which are the principal contributors to weight reduction.

The lower weight loss in the MoSi₂-Si₃N₄/SiC coating indicates that the addition of Si₃N₄ is beneficial for the coating’s integrity at high temperatures. Additionally, the absence of debonding or spallation at 1773 K indicates that the Si₃N₄ in the coating yields better coating–substrate compatibility at high temperatures. This suggests that the MoSi₂-Si₃N₄ coating is more resistant to high-temperature oxidation.

3.3. Thermal Cyclic Oxidation Resistance of the Coating at 1773 K

Figure 9a shows the SEM image of the MoSi₂-Si₃N₄/SiC coating surface after the thermal cyclic oxidation test. It can be observed that whiskers are formed throughout the surface. The exposure of Si₃N₄ coatings to high temperatures can foster the formation of Si₂N₂O whiskers [33]. These whiskers play an essential role in increasing the oxidation resistance of the coating [34]. The cross-sectional view of the multi-layer coating in Figure 9b shows small pores, mainly caused by the rapid cooling from 1773 K to room temperature. These pores can be sealed by the formation of Si₃N₄ whiskers in the subsequent thermal cyclic oxidation process, which further protects the coating from high-temperature oxidation [35].

Figure 10 shows the weight loss curve of the MoSi₂-Si₃N₄/SiC-coated C/C specimen during its repeated thermal cycling between 1773 K and room temperature. It can be observed that the weight loss of the coated specimen was only 0.05% after 40 thermal cycles. During the thermal cycling, the coating remained intact, and no oxidation or spallation was found. This indicates that the coating exhibits excellent oxidation and thermal cyclic oxidation resistance, which can be attributed to the filling of micro cracks under oxidizing environments by abundant glassy oxides.

4. Conclusions

The effect of Si₃N₄ addition in MoSi₂/SiC coating on the oxidation and thermal cyclic oxidation resistance at 1773 K was investigated. The 0.5% weight loss in the MoSi₂-Si₃N₄/SiC coating after its oxidation for 150 h in comparison to the 4.0% weight loss in the MoSi₂/SiC coating after 90 h proves that the coating with Si₃N₄ has a better high-temperature oxidation resistance. The negligible weight loss after 40 thermal cycles between 1773 K and room temperature proves that the MoSi₂-Si₃N₄/SiC coating has an excellent thermal cyclic oxidation resistance. The presence of Si₃N₄ results in a complete transformation of MoSi₂ to SiO₂, which results in a dense glassy SiO₂ film. The SiO₂ film improves the coating’s resistance to high-temperature oxidation by preventing gas diffusion into the coating, thus shielding the C/C substrate from byproduct-forming gases and delaying the generation of micro cracks. The addition of Si₃N₄ to the multi-layer coating is found to be beneficial for both coating integrity and coating–substrate compatibility.