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A picture of silicon carbide chemical vapor deposition technology and equipment

May 19,2023

Carbon fiber reinforced nonoxide ceramic matrix composites are the most promising high-temperature structural materials at present. However, due to the mismatch of thermal expansion between fiber and matrix and improper control of preparation process, the density of materials is often low and there are a lot of cracks in matrix. In the use of materials, the reinforced phase carbon fiber is easy to react with oxygen, water and corrosive salts in the environment, resulting in a sharp deterioration of material properties. At present, coating protection is the most effective measure to solve the application of this kind of material in high temperature and harsh environment, and mainly chemical vapor deposition (CVD) silicon carbide coating system as the main component.

Trichloromethyl silane (MTS) is the most commonly used raw material for CVD preparation of SiC. H2 is commonly used as carrier gas and reaction gas, and Ar is used as dilution gas. The reaction is as follows:


The advantage of MTS as raw material is that the Si∶C in its molecule is 1∶1, which can be decomposed into stoichiological SiC, so high purity SiC can be prepared. Moreover, the temperature zone of MTS deposition is particularly wide, and deposition can occur at 900℃ ~ 1600℃.

Theoretical basis of chemical vapor deposition

Chemical vapor deposition reaction process

Chemical vapor deposition (CVD) is a very complex chemical reaction process, which mainly uses a kind of vapor precursor of coating raw material to be pyrolysis and reduced and deposited on the substrate surface at a certain temperature. It involves the temperature field distribution in the reaction chamber, the concentration field distribution of reactants and the multi-phase reaction of intermediate products, as shown in the figure.

In general, the CVD process consists of the following steps: the reaction gas mixture is transported through the boundary layer to the deposition region; The reactant molecules are adsorbed on the surface of the matrix; Reactions occur between adsorbents or between adsorbents and gaseous species on or near the surface of the matrix to generate solid products and by-products; The by-product molecules desorbed from the substrate surface; The by-product gas molecules diffuse from the surface area to the main air flow space.

Unreacted reactant molecules are expelled from the system.


Influence factors of chemical vapor deposition reaction

There are many factors that affect the quality of sic CVD coating, including deposition temperature, reactant concentration, ratio of reaction gas and carrier gas, and deposition pressure.

(1) Deposition temperature

For the same reaction system, the deposition temperature mainly affects or changes the control mechanism in the deposition process by changing the chemical reaction rate in the deposition process. When the temperature is low, the coating growth rate is lower than the diffusion rate of the reaction gas, and the deposition control process is the surface reaction rate control. When the temperature is high, the coating growth rate is accelerated, which is greater than the gas diffusion rate, and the deposition control process is the mass transfer control. Therefore, the deposition temperature is one of the most important factors affecting the morphology and structure of the coating.

(2) Reactant concentration

In the CVD process, the concentration of the reaction gas is closely related to the deposition rate. The higher the concentration of reaction gas is, the faster the deposition rate is, and the particle agglomeration phenomenon is easy to appear on the coating surface, and the thickness and surface roughness of the coating will also be affected. On the contrary, with the decrease of reactant concentration, the deposition rate is slower, the nucleation rate of the coating surface increases, the coating thickness is thinner, and the surface roughness decreases.

(3) The ratio of reaction gas and carrier gas

Gaseous precursors are usually brought in by H2, Ar and other gases as carrier gases when they enter the reaction chamber. When the ratio of carrier gas to reaction gas changes, the viscosity of the mixed gas, the diffusion coefficient of the precursor gas in the mixed gas and the density of the mixed gas will be changed, which will lead to the change of the growth rate of the coating, and even the grain orientation of the coating may change.

Chemical vapor deposition equipment

To prepare sic by chemical vapor deposition, MTS is used as the precursor to provide silicon and carbon sources for the deposition reaction. MTS were brought into the reaction chamber by bubbling with H2 as the carrier and reaction gas, and Ar as the dilution gas.

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