Preparation of Graphite Ceramic Coatings

Graphite and its composite materials which exhibited excellent physical and chemical properties in the metallurgical, mechanical, chemical, aerospace and other fields are widely used. With the continuous development of science and technology, we put forward higher requirements on the performance of graphite materials. The graphite materials are easily oxidized in the high temperature oxygen atmosphere so that affected the using of graphite materials at high temperatures seriously. Therefore, the preparation of anti-oxidation coating which prevented the diffusion of oxygen into the interior of the graphite substrate at high temperatures and extended the time of graphite materials at the high temperature of oxygen atmosphere has become the main research direction.
In the process of graphite production and preparation, there are impurities and residual stresses in the graphite material, which provide the active point for the graphite material to be oxidized in the high-temperature oxygen-containing environment, make the structure of the graphite material destroyed, mechanical properties decline, and limit its performance in the high-temperature oxygen-containing conditions. Therefore, solving the problem that the graphite material is oxidized in a high-temperature oxygen-containing environment has become a main starting point.
According to the oxidation mechanism of graphite material, the essence of graphite material oxidation is that oxygen diffuses to the surface of the material. Oxygen molecules adsorb on the active point of graphite material due to the diffusion movement of molecules, and oxygen and carbon react to form carbon oxides.
Graphite industry has broad prospects for development, but graphite materials begin to oxidize under oxidation atmospheres over 450℃, the reaction rate increases rapidly at 750 ℃, which greatly limits the use of graphite materials in high temperature oxygen-containing conditions. It is very important to study the surface coating technology of graphite material, improve the surface properties of graphite material and make it meet the application conditions at high temperature. The key to improving the anti-oxidation pathway of graphite is to use an inhibitor that can prevent oxidation of the graphite material or to isolate the graphite material from oxygen in a high temperature oxygen-containing atmosphere. At present, there are two main methods to improve the oxidation resistance of graphite materials: one is matrix modification, the other is coating technology.
The principle of the matrix modification technology is to improve the oxidation resistance of the carbon material by modifying the material components. The matrix modification method generally adds an oxidation inhibitor to the matrix to improve the overall oxidation resistance of the carbon material. The oxidation inhibitor acts in the high temperature oxidation atmosphere, and the oxidation inhibitor preferentially oxidizes to the matrix of the graphite material to form a glassy substance. For example, the boron-based oxidation inhibitor B is contacted with oxygen in a high-temperature oxidizing atmosphere and oxidized to form B2O3 having a lower viscosity and having good affinity with the graphite material, coated on the surface of the graphite material and filled with the pores of the graphite material substrate. The rate of oxygen entering the matrix of the graphite material is suppressed, and the oxidation resistance of the graphite material is improved.
Coating technology is the most important and effective means to improve the oxidation resistance of graphite materials. The preparation of an anti-oxidation coating on the surface of the graphite material can effectively improve the oxidation resistance of the graphite matrix. In addition, the preparation of coatings with different chemical compositions on the surface of the graphite substrate can improve the high temperature ablation, high temperature oxidation resistance, corrosion resistance and radiation resistance of the matrix material. The basic principle of the coating method is to prepare an anti-oxidation coating on the surface of the material. The anti-oxidation coating prevents the direct contact between the air and the substrate to a certain extent, so that the material has a certain high temperature thermal stability. For the oxidation protection of graphite materials at higher temperatures, the coating technology can meet the requirements of effective oxidation resistance.
With the development and growth of the graphite industry, the oxidation of graphite materials is particularly important. Many researchers have carried out research on the preparation methods of anti-oxidation coatings for graphite materials, and have achieved many research results. At present, the preparation methods of the anti-oxidation coating for the graphite material mainly include an embedding method, a slurry method, a sol-gel method, a chemical vapor deposition method and the like.
The process principle of the embedding method is to embed the graphite material in the already mixed embedding powder, and then heat the graphite crucible in a high temperature sintering furnace under a vacuum or an inert gas atmosphere. As the sintering temperature increases, the embedded powder in direct contact with the graphite begins to react with the graphite material in a liquid or gas phase to form a coating of a certain thickness on the surface of the substrate. The advantage of the embedding method is that the preparation process is simple, and the product size change before and after embedding is small. The disadvantage of the embedding method is that some ceramics have a higher melting point and require higher temperatures for preparation.
The process principle of the slurry method is to mix the raw materials for preparing the coating with the adhesive and the solvent into a slurry, and adjust the parameters of the slurry until the experimental requirements are met, and the prepared slurry is applied to the graphite material. The surface or the substrate is immersed in the slurry, and after high temperature sintering, a coating is formed on the surface of the substrate. The advantage of the slurry method is that the process is simple and the operation is convenient. The disadvantage is that the coating is less dense and the bonding strength between the coating and the substrate is poor.
The sol-gel method is a method in which a metal organic salt or a metal inorganic salt is formulated into a homogeneous solution, which is converted into a gel from a sol by a chemical reaction such as hydrolysis or polycondensation at a low temperature, and then heat-treated at a relatively low temperature to synthesize the glass, ceramic and other coatings.
The principle of chemical vapor deposition is to first decompose the coating material into steam, and the steam enters the interior of the reactor and adheres to the substrate due to the pressure difference. The advantage of the chemical vapor deposition method is that the resulting coating has a high density and a uniform coating thickness. The disadvantage is that the process is complicated and the experimental period is long. When depositing the inner wall of the irregular hole, the vapor of the prepared coating is first deposited on the outer layer of the hole and closed due to the temperature difference between the inner and outer sides of the hole, thereby The coating of the inner layer is deposited to a poor density, uneven coating, and poor integrity.
Author: Tian Shuai