Why is carbon-graphite commonly used as the friction pair in mechanical seals?

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Carbon-graphite (hereinafter referred to as graphite) is a material frequently chosen for friction pairs due to its many excellent properties. 1) High thermal conductivity. The thermal conductivity of carbon is 20–40 W/(m·K), while that of graphite is 40–128 W/(m·K). This makes graphite second only to silver, copper, and aluminum in terms of thermal conductivity. It is the only non-metallic material with such a high thermal conductivity; its value is significantly higher than that of certain metals (such as stellite and Hastelloy). Therefore, it can dissipate the generated frictional heat in a timely manner, thereby reducing the temperature of the sealing surfaces. This is essential for improving their reliability and service life. 2) Low coefficient of linear expansion. The linear expansion coefficient of graphite is 2x 10-6 to 6x 10-6/°C, which is approximately 1/2 to 1/4 that of metals. At elevated temperatures, its thermal deformation is minimal, which is beneficial for maintaining the parallelism of the sealed end faces. Its low coefficient of expansion combined with high thermal conductivity gives it good thermal stability, as well as excellent resistance to heat, cold, and thermal shock; it does not develop cracks when exposed to sudden temperature changes. 3) Good corrosion resistance. Graphite has excellent chemical stability; it remains stable in air at temperatures below 400°C. Except for strongly oxidizing media such as aqua regia, chromic acid, concentrated sulfuric acid, and halogens, it can resist corrosion by other acids, bases, salts, and all organic compounds. It should be noted that its corrosion resistance varies depending on the type of graphite and the impregnant used. 4) Excellent self-lubricating properties. This is because when graphite rubs against a metal or other material, graphite transfers to the surface of the metal, resulting in friction between graphite particles. In addition, graphite has a low coefficient of friction. When graphite is in contact with metal, the coefficient of friction is 0.04–0.05; under fully fluid-lubricated conditions, it is merely 0.008–0.01. 5) Graphite has low tensile strength and high compressive strength; it is a brittle material. Due to its low hardness, it is commonly used as the soft ring in friction pairs. It can be easily processed in various ways. Its major drawback is a relatively high porosity, generally ranging from 18% to 22%. To overcome this drawback, graphite used in practical applications is impregnated to fill the pores and improve sealing performance.