Protection of carbon materials from high-temperature gas corrosion

Abstract

Articles made of carbon materials which exhibit unique physicochemical and mechanical properties are used in various branches of science and technology, i.e., from structural elements for space equipment to everyday articles. With the aim of expanding the range of application for carbon materials, particularly in the field of high-temperature technology, it is necessary to improve their heat resistance. As a rule this problem is resolved either by creating heat-resistant composites (for this purpose corrosion-resistant additions are made to the composition of the carbon matrix) or by applying protective coatings. The second path is assumed to be more promising.Information about multilayer coatings on carbon materials is presented in [1]. A substrate of titanium carbide with a thickness of the order of ten microns was precipitated from a mixture of titanium chloride and methane gases, and a protective coating of molybdenum disilicide with a thickness of 100 #m was applied by plasma deposition followed by annealing. The heat resistance of these coatings in air at 1600°C was of the order of tens of hours. The most promising are multilayer coatings of silicon carbides and nitndes, molybdenum and tungsten silicides, and also zirconium and hafnium oxides [2]. Thesecoatings arecapable ofprotecting carbon-carbonmaterials in oxygen-containing atmospheres at temperaturesup to 1800°C.The aim of the present study is to develop high-temperature and anti-corrosion coatings for carbon materials based on transition metal borocarbides and silicides.Considering the comparatively low linear expansion coefficients for carbon materials, the nonuniformity of their structure, high porosity, and high vapor pressures of carbon oxides with their greater chemical activity, in order to obtain high-temperature and heat-resistant coatings the methods of diffusion impregnation, deposition from the gas phase, and impregnation through the liquid phase and fusion were used. Experience in applying high-temperature (up to 2000°C) protective coatings on refractory metals and their alloys [3-5] was used.The original specimens made of carbon material, in particular graphite grade ARV and graphite bonded with pyrocarbon (GBP), were cylindrical in shape 8 mm in diameter 8 mm and 70 mm high. The carbon-borosilicides of refractory metals were selected as protective coatings. Titanium, niobium, and zirconium for subsequent preparation of a carbide substrate were applied by thermal decomposition from chlorides and iodides in the gas phase. A high-frequency generator or a furnace with an electric heater was used in order to heat the specimens. The temperature was measured by an optical pyrometer of the 'Promin' type or a tungsten-rhenium thermocouple. Titanium (zirconium) iodide was prepared directly in the device for applying coatings by passing iodine vapor through titanium (zirconium) turnings. The layout of the device and the procedure for applying coatings from the gas phase are described in [6]. In order to provide good coating adhesion with the base c...

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.…”

“…In some cases, such coatings satisfy the conditions of their operation; however, the presence of high internal stresses in surface layers of coatings can lead to their possible degradation these materials. In this respect, the above coatings require the appropriate modification in order to smooth the stresses and to Multilayer coatings on graphite of various grades and carbon-carbon composite materials were investigated in our earlier works [3,6], where molybdenum and tungsten silicides were used to protect carbon materials against high-temperature gas corrosion. In this case, molybdenum and tungsten were deposited onto the preliminarily prepared surface consisting of the corresponding carbonyls and the process of silicification was performed under vacuum.…”

Carboborosilicide and oxide composite coatings on carbon materials

“…Solution of this problem has been the subject of much work and questionable results have been obtained although currently it remains important. This is explained by the fact that protection of carbon composites from oxidation by applying anticorrosion coatings to them is connected with overcoming difficulties caused by the considerable porosity of carbon materials, the high volatility of their oxides, the nonuniform structure, low linear thermal expansion coefficient, and the anisotropy of physicomechanical properties [4,5]. We note that features of the porous structure of carbon materials, in particular pore size distribution, their permeability for oxidizing agent molecules, open and closed porosity determine to a considerable extent material behavior in oxidizing gas atmospheres.…”