How does SiC coating improve the oxidation resistance of carbon felt？

Carbon felt has excellent properties such as low thermal conductivity, small specific heat, and good high temperature thermal stability. It is often used as a thermal insulation material in a vacuum or protective atmosphere and has been widely used in the semiconductor field. However, in an environment with a temperature higher than 450℃, carbon felt will be rapidly oxidized, resulting in rapid destruction of the material. The processing environment of semiconductors is often higher than 450°C, so it is particularly important to improve the oxidation resistance of carbon felt.

Why choose SiC coating?

Surface coating is an ideal anti-oxidation method for carbon fiber products. Anti-oxidation coatings include metal coatings, ceramic coatings, glass coatings, etc. Among ceramic coatings, SiC has excellent high-temperature oxidation resistance and good physical and chemical compatibility with carbon fiber products. When SiC is oxidized at high temperature, the SiO2 generated on its surface can fill cracks and other defects in the coating and block the penetration of O2, making it the most commonly used coating material in carbon fiber product coatings.

How to perform SiC coating on carbon felt?

SiC coating was prepared on the surface of carbon felt carbon fiber by chemical vapor deposition. After ultrasonic cleaning, the prepared carbon felt was dried at 100℃ for a period of time. The carbon felt was heated to 1100℃ in a vacuum tube furnace, with Ar as the dilution gas and H2 as the carrier gas, and the heated trichloromethyl siloxane was led into the reaction chamber by bubbler method. The deposition principle is as follows:

CH₃SiCl (g)→SiC (s) +3HCl (g)

What does the SiC coating carbon felt surface look like?

We used D8 ADVANCE X-ray diffractometer (XRD) to analyze the phase composition of SiC coating carbon felt. From the XRD spectrum of SiC coating carbon felt, as shown in Figure 1, there are three obvious diffraction peaks at 2θ=35.8°, 60.2°, and 72°, which correspond to the (111), (220), and (311) crystal planes of β-SiC, respectively. It can be seen that the coating formed on the surface of the carbon felt is β-SiC.

Figure 1 XRD spectrum of SiC coating carbon felt

We used a MAGELLAN 400 scanning electron microscope (SEM) to observe the microscopic morphology of carbon felt before and after coating. As can be seen from Figure 2, the carbon fibers inside the original carbon felt are uneven in thickness, chaotically distributed, with a large number of voids, and a low overall density (about 0.14 g/cm3). The presence of a large number of voids and low density are the main reasons why carbon felt can be used as a thermal insulation material. There are a large number of grooves on the surface of the carbon fibers inside the original carbon felt along the fiber axis, which helps to improve the bonding strength between the coating and the matrix. 

From the comparison of Figures 2 and 3, it can be seen that the carbon fibers inside the coating carbon felt are covered with SiC coatings. The SiC coatings are formed by small particles tightly stacked, and the coatings are uniform and dense. They are tightly bonded to the carbon fiber matrix, without obvious peeling, cracks and holes, and there is no obvious cracking at the bonding with the matrix.

Figure 2 The morphology of carbon felt and single carbon fiber end before coating

Figure 3 The morphology of carbon felt and single carbon fiber end after coating

How is the oxidation resistance of SiC coating carbon felt manifested?

We conducted thermogravimetric analysis (TG) on ordinary carbon felt and SiC coating carbon felt, respectively. The heating rate was 10 ℃/min and the air flow rate was 20 mL/min. Figure 4 is the TG curve of carbon felt, where Figure 4a is the TG curve of uncoating carbon felt and Figure 4b is the TG curve of SiC coating carbon felt.It can be seen from Figure 4a that the uncoating carbon felt sample oxidizes slowly below about 600 ℃, and the oxidation rate is significantly accelerated after exceeding 600 ℃. At about 790 ℃, the residual mass fraction of the sample is 0, which means that it has been completely oxidized. 

As shown in Figure 4b, the coating carbon felt sample has no mass loss when the temperature rises from room temperature to 280℃. At 280-345℃, the sample begins to oxidize gradually, and the oxidation rate is relatively fast. At 345-520℃, the oxidation progress slows down. At about 760℃, the mass loss of the sample reaches the maximum, which is about 4%. At 760-1200℃, as the temperature rises, the mass of the sample begins to increase. That is, weight gain occurs. This is because the SiC on the surface of the carbon fiber is oxidized to form SiO2 at high temperature. This reaction is a weight gain reaction, which increases the mass of the sample.

Comparing Figure 4a and Figure 4b, it can be found that at 790℃, the ordinary carbon felt has been completely oxidized, while the oxidation weight loss rate of the SiC coating carbon felt sample is about 4%. When the temperature rises to 1200℃, the mass of the SiC coating carbon felt even increases slightly due to the generation of SiO2, indicating that the SiC coating can significantly improve the high temperature oxidation resistance of the carbon felt.

Fig. 4 TG curve of carbon felt

The SiC coating successfully prepared on carbon felt by chemical vapor deposition is evenly distributed, continuous, densely stacked, and has no obvious holes or cracks. The SiC coating is tightly bonded to the substrate without obvious gaps. It has very strong anti-oxidation ability.