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Abstract
The process-property-microstructure relationship is investigated in the in-house fabricated unidirectionally reinforced PAN-phenolic C/C composites comprising surface-treated and untreated carbon fibers. The composites are subjected to a final heat treatment of 1000° or 2000°C. The mechanical properties of the composites are found sensitive to the process parameters (particularly the final HTT), as well as fiber surface condition (surface-treated or untreated). For the composites comprising surface-treated fibers, the flexural strength of the composites heat treated at 2000C is higher than that of the composites treated at 1000°C. For the composites with untreated fibers, the results are the opposite. Microstructure, particularly fiber-matrix morphology, has been used to explain the strength variation among the various C/C samples, as discussed in the text.
The requirements for higher mechanical properties at high temperatures from Air Force and NASA research programs resulted in the reinforcement of carbon and the development of carbon/carbons. Three types of matrix systems are used for the industrial manufacturing processes for carbon/carbons: thermosetting resins, thermoplastic precursors, and gas-phase-derived carbons or ceramics. Recently, the mechanical properties of carbon/carbons have found to be profoundly affected by their fiber/matrix interfacial properties. The chapter deals with such recent progress regarding the strengths of carbon/carbons, including studies of their tensile, shear, compressive, and fatigue strengths. Thermal properties of C/C composites, such as the thermal conductivity and thermal coefficient expansion, are very important characteristics for the main applications of these materials, such as brakes, re-entry shields, rocket nozzles, first wall tokamaks, and heat exchangers. Oxidation protection of carbon/carbon has to be applied for all materials which are used in an oxidative atmosphere above 420°C.
The mechanical properties and microstructure of unidirectional carbon/carbon (UD C/C) were investigated. The strength of one type of UD C/C, produced with an intermediate modulus fibre treated to four different levels of an oxidative surface treatment, was determined after each step of the production cycle (of loose, impregnated and carbonized impregnated fibre bundles). The impregnated bundle had a strength 1.9-4.3 times the strength of the loose bundle, whereas after carbonization the strength of the bundle dropped below the strength of the loose bundle. It is suggested that this is mainly caused by the formation of defects in the fibres due to the shrinkage process during carbonization. These defects are larger if a good fibre/matrix bond strength in the green material exists. The possibility that the low strength of carbon/carbon could be caused by stress-concentration effects was excluded with the aid of TEM investigations. They showed that the carbon matrix mainly consisted of vitreous carbon, the modulus of which (E = 35 GPa) does not become effective due to micro- and macrocracks.
Carbon fiber/carbon composites were prepared with thermosetting resin-derived carbon matrix and two groups of carbon fibers (i.e., surface treated and non-surface treated) with and without sizing treatment. These composites were heat treated at 1000°C and 3000°C. Fracture behavior and flexural strength of the composites were studied at ambient temperature. Surface treatment of carbon fibers played an important role on fracture behavior and strength of the composites. The composite with surface-treated carbon fibers heat treated at 1000°C showed low strength and a catastrophic fracture pattern, whereas those heat treated at 3000°C showed a pseudo-plastic fracture pattern. However, the behavior was just the opposite in the composite with non-surface-treated carbon fibers.Graphitization of composites with two series of carbon fibers showed an altogether different matrix microstructure. Composites with surface treated fibers showed a columnarlike carbon matrix whereas those made with non-surface-treated fibers possessed a lamellar type carbon matrix well oriented around carbon fibers.
Surface oxides on various carbon fibres were determined after modified oxidation treatment with nitric acid. Improved wetting was investigated by contact angle measurements.The results of these systematic studies are correlated with the mechanical properties of UD-composites, prepared from the matrix precursors with which the wetting experiments have been performed.A possible mechanism of the bonding between epoxy matrix precursors and reinforcing fibres was determined by contact angle measurements, by surface tension measurements and by chemical analysis of the chemisorpted diamine hardener.
The influence of carbon fibre type and carbon fibre surface treatment on the mechanical properties of phenolic-based, unidirectionally reinforced carbon/carbon composites has been investigated.It was found that only the reinforcement of carbon/carbon composites with untreated type I fibres results in best mechanical properties. Surprisingly in that case also an optimised surface treatment of the fibres improve the yield of fibre strength. The experimental results have shown that the applicability of the rule of mixtures for the precalculation of the strength of carbon/carbon composites is limited and that the fracture behaviour is controlled by the amount of adhesion between carbon fibres and carbon matrix.
17th Biennial Conf. Carbon 314, Kentucky
- E. Fitzer
- A. Gkogkidis
- A. Rajamohan