ABSTRACT:The surface of polyacrylonitrile-based carbon fiber was appropriately coated with dilute phosphoric acid to protect or retard its thermal oxidation occurring during an exposure to high temperatures above 600"C in air. Microscopic behavior on surface damage and size change in the uncoated and coated fibers was compared using thermogravimetric and scanning electron microscopic techniques. The result showed that the state of the carbon fiber surface was found to be remarkably improved by the coated phosphorous compound, minimizing the surface pitting. and size reduction of carbon fiber by high temperature oxidation. The microscopic evidence for the effective protection of carbon fiber from thermal oxidation also indicated that use of this technology may be positively considerable to other related materials. KEY WORDSCarbon Fiber / Thermal Oxidation Protection / Microscopic Behavior / Phosphorous Compound/ Carbon fibers are extremely important in the application of carbon fiber related materials, especially carbon fiber-reinforced polymer, ceramic or metal composites, and carbon-carbon composites. They have been widely used as a key component of advanced composite materials in aircraft, aerospace, and missile applications, experiencing high temperature aerial environments. However, because carbon fibers exposed to high temperatures for a long duration above 500°C are extremely sensitive to oxygen, use of carbon fiber-reinforced composite materials under very high temperature conditions in air has been often restricted in practical performances. 1 -3 It is very clear that if the strong interfacial bonding between the fiber and matrix in the composite and the fibrous reinforcement itself are damaged and weakened by severe thermal oxidation, resulting in debonding at the interfaces, then a serious deterioration of various properties in the composite would be unavoidable. Due to significant influence of the thermal response of carbon fiber on various properties of the related composite materials, oxidation is one of major limiting factors in determining the lifetime and performance of carbon fibers and their composites in many applications. Therefore, many researches to investigate the oxidative phenomenon of carbon materials have been driven over the last decade. 4 -7 Phosphorous compounds are well-known as one of the materials contributing to the antioxidation and flame resistance of various organic materials including carbon. 8 -10 Some researches to improve the oxidation protection of carbon-based materials using phosphorous compounds have been progressed. 11 -14 During the pyrolysis of a carbon-based composite material a significant quantity of oxygen can penetrate into the inner region of the composite through the resultant enlarged t Author to whom all correspondence should be addressed.pores. It can be proposed that at this step the phosphorous compound may preferentially react with oxygen atoms rather than carbon atoms in the carbon fibers (or in the carbon matrix in the case of carbon-carbon compo...

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