DE4129102A1 - Uniformly depositing finely crystalline stoichiometric silicon carbide - using flowing gas mixt. comprising silicon source, carbon source and diluent gas at moderate temp. and pressure useful for fibre bundles - Google Patents

Abstract

Finely crystalline stoichiometric SiC is uniformly deposited from the gas phase onto the filaments of fibre bundles or into open porous substrates and/or onto substrates from a flowing gas mixt. at 0.1-1 bar total pressure and at 750-1100 deg.C. The gas mixt. mutually consists of a silicon source, a carbon source and a diluent gas and having a C to Si atomic ratio of 5-40. ADVANTAGE - The prcocess allows chemical vapour infiltration of uniform deposits of stoichiometric, very finely crystalline SiC, under moderate process conditions even into the interior of the complex components

Description

It is known that ceramic layers can be deposited from the gas phase by means of "chemical vapor deposition (CVD)". The starting materials are introduced in gaseous form to the substrate located at a higher temperature and form the desired product, which separates out as a solid layer, by chemical reaction. A mixture of methyltrichlorosilane CH ₃ SiCl ₃ (MTS) and hydrogen H ₂ is predominantly used for the deposition of silicon carbide SiC.

CVD processes are generally used at near total pressures 1 bar. The speed of the deposition is mostly determined by determines the diffusion in the gas boundary layer. This makes it a success a good all-round coating, but the uniform coating with complicated workpiece geometry we Difficulties due to local depletion of the gas phase in reactive species. In particular, the individual filaments of a company have so far been unsuccessful cover the bundle evenly with a CVD layer or the open to fill the pores of a component using CVD. In the case of SiC rejection There is another problem. Because of homogeneous gas freak tion and diffusion processes in the gas phase occur  shifts in the gas, which have the consequence that contrary to thermodynami Sheer predictions do not separate pure SiC, but SiC + Si or, under other conditions, SiC + C. Because the product properties here are strongly influenced, is generally a targeted stoichiometry separation required. It also has a very fine grain up X-ray amorphous form of deposition of the SiC preferred, since then its me Chan properties are particularly favorable.

For the technical objective of uniform coating of the individual fi laments of fiber bundles and the filling of open pores in components using CVD (also known as Chemical Vapor Infiltration CVl) with stoichio metric very fine-grained silicon carbide SiC is invented below deres, economically advantageous process described.

The object of the invention is to be economically justifiable and feasible specify conditions under which Chemical Vapor Infiltration at the same time moderate deposition of stoichiometric, very fine crystalline SiC too inside a complex component.

For the coating to be carried out inexpensively, it is advantageous to let the process run at or near atmospheric pressure, because expensive vacuum equipment can be dispensed with. It is known that at total pressures above 0.1 bar, the deposition speed depends essentially on the gas diffusion when 1100 ° C. is exceeded. However, CVl can only be realized if the gas diffusion is rapid compared to the deposition reaction on the substrate, i.e. at temperatures below half 1100 ° C. Lower temperatures also have the advantage that SiC is increasingly deposited in a fine crystalline to X-ray amorphous form. However, it is also known that at these temperatures MTS, which is generally used as an MTS-H ₂ mixture, is not deposited as pure SiC but as a mixture of Si and SiC. In contrast, such a gas mixture at higher temperatures, z. B. 1500 ° C, to stoichiometric SiC. If the MTS + H₂ mixture still contains a hydrocarbon at such high temperatures, free carbon is deposited in addition to SiC. The object is achieved in that the separation from a total pressure of 0.1 to 1 bar in the temperature range between 750 ° C and 1100 ° C, starting from a gas mixture of silicon carrier, carbon carrier and diluent gas, the atomic ratio C: Si in the gas mixture is between 5 and 40 and the linear flow velocity, based on 20 ° C. and 1 bar, is 0.02 and 0.1 m / s. For example, methyltrichlorosilane, methane and hydrogen are used as starting materials.

Embodiment: At a total pressure of 1 bar at 845 ° C, a mixture of (volume fractions) 3.3% methyltrichlorosilane, 46.7% H₂ and 50.0% CH ₄ with 0.058 m / s flow rate (based on 20 ° C) passed over a graphite substrate, where stoichiometric SiC in a very fine crystalline form as β-SiC separated uniformly at a speed of 5.7 µm / h.

Claims (7)

1. A method for the uniform deposition of fine crystalline, stoichiometric SiC from the gas phase on the filaments of fiber bundle or in open-pore substrates and / or on substrates, characterized in that the deposition from a stream of gas mixture at a total pressure of 0.1 up to 1 bar, at temperatures between 750 and 1100 ° C, the gas mixture in the initial state consisting of a silicon carrier, a carbon carrier and a diluent gas and the atomic ratio of carbon to silicon in this gas between 5 and 40 is set . 2. The method according to claim 1, characterized in that as Total pressure atmospheric pressure in the reactor is set. 3. The method according to claim 1, characterized in that a Reaction temperature between 800 and 900 ° C is set. 4. The method according to claim 1, characterized in that methyltrichlorosilane CH ₃ SiCl _{3 is} used as the silicon carrier. 5. The method according to claim 1, characterized in that methane CH _{4 is} used as a carbon carrier in addition to the Koh contained in the carbon lenstoff. 6. The method according to claim 1, characterized in that hydrogen H _{2 is} used as the diluent gas. 7. The method according to claim 1, characterized in that in Reactor a flow rate is set, which at the Sub linear gas velocity (based on 1 bar and 20 ° C) between 0.02 and 0.1 m / s.