Effect of chemical vapor infiltration on erosion and thermal properties of porous carbon/carbon composite thermal insulation

• Published in: Carbon (New York) 2000, Vol.38 (3), p.441-449
• Main Authors: Baxter, R.I, Rawlings, R.D, Iwashita, N, Sawada, Y
• Format: Article
• Language: English
• Subjects: A. Carbon/carbon composites; Applied sciences; B. Chemical vapor infiltration; Building materials. Ceramics. Glasses; Ceramic industries; Chemical industry and chemicals; D. Thermal conductivity; Exact sciences and technology; Miscellaneous; Refractory products; Special refractories; Technical ceramics; Thermal expansion
• ISSN: 0008-6223; 1873-3891
• DOI: 10.1016/S0008-6223(99)00125-6

A highly porous carbon/carbon composite, known as carbon bonded carbon fiber (CBCF) and used as thermal insulation, was densified by chemical vapor infiltration (CVI). The erosion resistance, thermal conductivity and thermal expansion coefficient were measured with interest to utilization of the CVI densified composite as erosion protection in furnaces that employ inert gas quenching. It was found that the erosion rate decreased exponentially as bulk density increased from 0.18 to 1.00 Mg·m −3 and this was related to the Ryshkewitch and Duckworth relationship which relates mechanical properties to the porosity of ceramic materials. The thermal conductivity of the CVI densified composite both parallel and perpendicular to the fiber array was greater than that of CBCF in the measured range between 1000 and 2000°C. The measured values of thermal conductivity showed reasonable agreement with values calculated from a model structure. For CBCF and CVI densified composite the contribution of solid conduction was calculated to be dominant compared to the contribution of radiative heat transfer and heat transfer taking a path through the gas present in the pores of the composite. As a function of temperature thermal conductivity decreased for the CVI densified composite whereas an increase was found for CBCF. This opposite behavior is a result of the intrinsic properties of the dominant component in the composites: fibers in CBCF and pyrolytic carbon deposit in the case of CVI densified composite. Both materials exhibited anisotropy in the thermal conductivity; the thermal conductivity parallel to the fiber array being about 2.8 times that in the perpendicular direction in the case of CBCF. The thermal expansion coefficient in both directions increased as CBCF was densified by CVI; the increase was greater in the direction perpendicular to the fiber array which increased from 1.8×10 −6 K −1 for CBCF to 5.3×10 −6 K −1 for the CVI densified composite (1.10 Mg·m −3). The thermal expansion anisotropy was consistent with a c-axis radial orientation of the pyrolytic carbon around the fibers in the CVI densified composite.