Oxidation-Protective Coatings for Carbon-Carbon Composites

Oxidation-Protective Coatings for Carbon-Carbon Composites

Omid Ashrafiyan (University of Tehran, Iran), Mohsen Saremi (University of Tehran, Iran), Amirhossein Pakseresht (University of Tehran, Iran & Materials and Energy Research Center, Iran) and Ehsan Ghasali (Materials and Energy Research Center, Iran)
DOI: 10.4018/978-1-5225-4194-3.ch016
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Carbon-carbon composites have attractive properties for high-temperature applications but unfortunately, oxidized at a temperature over 500 °C in the air. Various methods used for protection of C-C composites oxidation such as surface treatment, adding inhibitors in the matrix, and outer coating. High-temperature outer coatings are the most effective method to the protection of C-C composites oxidation. Protective coatings should have low oxygen permeability, compatibility with C-C composites, and adherent to the substrate. SiC is a suitable coating but limited in temperature over 1700 °C and below 1100 °C due to evaporation of silica layer and high viscosity of silica layer respectively and so, multilayer coating system used to solve these problems. SiC coating usually is as a bond layer in coating systems.
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Introduction And Background Of C-C Composites

Carbon fibers are as reinforcement agent in metals, polymers, and ceramics composites from past decades. Properties of the carbon fibers are a few micrometers in diameter, lightweight, high tensile strength, high stiffness, and high chemical resistance (Chand, 2000). Carbon matrixes reinforced with carbon fiber are called carbon-carbon composites and have a density of 1.6 to 2 g.cm-3 which is much lower than many of metals and ceramics density. The carbon fibers can be weaved from one to 3-dimensional as the reinforcement agent in the carbonaceous agent. (Savage, 2012).

The carbon can take continuous forms from amorphous and glassy carbon to crystalline graphite. Therefore, although C-C composite made from only one element, both constituents of the composite have different properties that will bring together to produce the carbon-carbon composite. Furthermore, the structure of graphite is consisted hexagonal planes which bonds together by weak van der Waals force, the anisotropy of graphite structure can create different structures based on the orientation and cavities (Buckley & Edie, 1993).

Production of C-C Composite

The carbon fibers are made from PAN, rayon, and pitch and woven in preforms for producing C-C composites. The carbon fibers should place in a carbon matrix. Carbon powders cannot be sintering. For the process of the matrix can use chemical vapor deposition (CVD) and pyrolysis of carbon materials such as pitch. In CVD method, carbon layer deposit on carbon fibers by the organic gas such as methane at high temperature. High densification is difficult and slow in CVD method (Chand, 2000; Deborah, 2010; Shirvanimoghaddam et al., 2017).

The producing of C-C composites by using of pitch or thermosetting resin such as phenolic includes following steps:

  • Carbon fibers pre-impregnated with pitch

  • Oxidation of Pre-impregnated

  • Carbonization of the sample, which production composite with porous structure

  • Densification

The final stage is the densification of carbon-carbon composites. This step can be carried out by using the chemical vapor diffusion method, in which carbonaceous gases are pressurized into voids inside the composite or crack formed, then heated up to 1400 °C. Gases tend to fill the cracks of the outer surface of the composite, in resulting the inner cavity can be close and densification incomplete. Another method for densification of carbon-carbon composites is using the liquid phase of pitch and phenolic resin (Savage, 2012).

The condensation process must be repeated to obtain a condensed matter. The pyrolysis process can be carried out both at ambient pressure and high pressure (HIP) in the process of the matrix. The number of saturation cycles much less at high pressure and can improve the composite properties. Therefore, HIP used more than other methods (Raunija & Sharma, 2015)

Properties of C-C Composite

C-C composites have low-density, chemically inert, and high hardness and toughness in temperature around 2000 °C. (Ferreira, Coppini, & Neto, 2001). At high temperatures, C-C composites show 10 to 20 percent increase in mechanical properties (Manocha, 2003). Carbon materials have a thermal shock resistance so that these can be heated from under zero degree of centigrade up to 1500 °C at less than a few seconds. Also, the C-C composite does not severely fracture (Raunija & Babu, 2013).

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