The thermal expansion of carbon/carbon composites from room temperature to 1400 °C

Abstract

The coefficient of thermal expansion (CTE) is one of the most important properties for materials used under variable temperatures. Carbon/carbon composites must be capable of high geometric stability and low thermal expansion when they are used in engine turbine blades, rocket nozzles and seal rings in a liquid propellant rocket engine [1]. The thermal expansion of C/SiC composites and pyrocarbon films has been studied by Cheng et al. [2] and Taylor et al. [3], respectively. Carbon/carbon composites were developed using high-pressure impregnation/carbonization techniques and their thermophysical properties studied by Manocha et al. [4]. The thermophysical properties of five different carbon/carbon composites were discussed by Luo et al. [5]. Baxter et al. studied the effect of density on thermal expansion [6]. Piat and Schnack [7] studied pyrocarbon with different texture degrees and attempted to model the influence of microstructure on the coefficients of thermal expansion by collecting experimental data from the real physical structure using SAED and HRTEM. A material with a specific thermal expansion can be designed by adjusting the microstructure. Here we report the influence of heat treatment, preform architecture and the microstructure of the matrix carbon on the thermal expansion of carbon/carbon composites. A layer of carbon fiber cloth without a weft thread and a random short fiber layer were laminated alternately to prepare porous carbon fiber preform felt with an apparent density around 0.6 g/cm. The carbon cloth layer was stacked in such a way that each layer was oriented at 90 to the adjacent layer. Carbon/ carbon composites with a density of 1.75 g/cm were prepared by a thermal gradient CVI process using natural gas (98%CH4, 0.3%C3H8, 0.3%C4H10, 0.4% other hydrocarbons) as precursor. Specimens were subjected to heat treatment at different temperatures in an argon atmosphere. The detailed description of the preparation had been reported elsewhere [8]. The coefficient of thermal expansion of the densified composites was measured parallel and perpendicular to the carbon fiber cloth layer, or the random short fiber layer, from room temperature to 1400 C in an argon atmosphere by using a Netzsch DIL 402C dilatometer. The specimen size was 3.5 · 3.5 · 20 mm. The microstructures were analyzed using an X’Pert X-ray diffraction analyzer. Data for diffraction peaks (002) was obtained. The microstructures were examined by a Olympus PMG3 polarized light microscope. The thermal expansion of graphite materials is anisotropic and is higher perpendicular to the graphite layers than parallel. In carbon/carbon composites, the graphitic flat-layered structure crystallites of pyrocarbon are arranged on the surface of the carbon fiber, and grow around the carbon fiber in the form of a ringed multilayer structure, in which the graphitic layers are parallel to the fiber axis. Therefore, the thermal expansion is anisotropic as well if the preform J.-g. Zhao (&) K.-z. Li H.-j. Li C. Wang Y.-q. Zhai School of Materials Science, Northwestern Polytechnical University, Xi’an 710072, P.R. China e-mail: jgzhaoshi@163.com