Films for protecting graphite materials from oxidation (review)

Abstract

Graphite has a special place amongst nonmetallic materials because of its unique physicomechanical and chemical properties, of which the main ones are the high resistance to thermal shock, low elastic modulus, high thermal conductivity and specific heat, fairly high strength, low thermal expansion coefficient, small neutron capture cross section, and inertness in corrosive media (acids, and solutions of salts and organic compounds [1-3]). The main disadvantages indicated by the general use of graphite in high-temperature processes are the low heat resistance and the erosion and burning in gas flows [4, 5]. There are various techniques for avoiding such undesirable characteristics.One way of effectively protecting graphite from oxidation is to produce a protective layer on the surface. Coatings of various types have been tested: diffusion ones (deposited from vacuum or from gases), plasma ones, ones formed from the gas phase by the deposition of refractory-metal halides, metal condensation from vacuum, and the deposition of various coatings followed of the National Academy of Sciences of the Ukraine has made a considerable contribution to research on protecting graphite with rate metals, the trends in coating graphite, and the development of suitable technologies [6][7][8][9][10][11][12][13][14][15][16]. At high temperatures, the protection by rare metals is governed by reactions between the metal and the solid surface at the interface [7]. Transition metals (chromium, zirconium, and titanium) form stable chemical bonds with carbon and wet graphite well, while copper, lead, tin, antimony, bismuth, and other nontransition metals do not wet it properly. The most active of the transition metals are those with high defectiveness in the d or f electron shells. A small amount of titanium added to other transition elements such as copper and tin markedly increases the tendency to wetting, on account of reduction in the wetting angle and increase in the work of adhesion. The main shortcomings that reduce the protective power of the coatings made by immersion or deposition from the gas phase are the brittleness and the tendency to peel from the substrate.To overcome these drawbacks, a new method has been proposed [8-13] for making protective coatings on graphite that in,olves two successive operations: depositing the metal layer from the liquid phase under vacuum and diffusion treatment with carbon, boron, nitrogen, and silicon. Metal layer deposition from the liquid involves interaction of the graphite with the metal, which increases the adhesion strength, which rises during the subsequent diffusion saturation. The metal layers have fairly high strength. Good thermal resistance is provided by the transition diffusion layers, which reduce the stress gradients between the coating and substrate. Reliability and good adhesion are provided at high temperatures by the formation of the coating consequent on the interaction with the substrate.Under working conditions (at lower temperatures), the interaction of the coating with t...

Complex temperature-resistant protective coatings for carbon-carbon materials

“…There are many publications in which the authors have discussed various problems regarding the fabrication of stable high-temperature protective coatings on the surface of graphite and carbon-carbon composite materials [1][2][3][4][5][6][7]. It has been noted that the porosity inherent in carbon materials, the high pressure of forming oxidation products (CO, CO 2 ), and the substantial difference between the thermal expansion coefficients of the coating and substrate materials lead to some difficulties in the development of protective coatings.…”

“…Such coatings provided the stability of carbon materials in air at 1700 ° C for several hours. Similar compositions of coatings were also used to protect carbon materials in other works [1,2,4,5]; however, their formation was most often performed through fused deposition. As a rule, the stability of these coatings at high temperatures in air due to the presence of structural defects is insufficient with respect to requirements currently specified for materials used in developing branches of high-temperature engineering.…”

Carboborosilicide and oxide composite coatings on carbon materials

“…Among them in particular there are SiC and B 4 C that are compatible with carbon and have thermal expansion Table 2). [14][15][16]. As a rule these compounds in the coatings on carbon materials are formed at high temperature (1700-2000°C) that requires use of special high-temperature equipment.…”