High-temperature compatibility of carbon fibres with nickel

A study has been made of the elevated temperature degradation of a number of carbon fibre types coated with nickel by a variety of methods (electroless, electrolytic, carbonyl and physical vapour deposition). At high temperatures, Ni-coated fibres undergo a transformation of structure to crystalline graphite with a consequent loss of strength and elastic modulus. Resistance to this recrystallization is related to the fibre type and structure and increases in the order HTS PAN-based, HM PAN-based, HM rayon-based. For PAN-based fibres the resistance increases with the degree of structural order and orientation. The recrystallization of HTS fibres is consistent with a simple model of dissolution and reprecipitation controlled by diffusion of carbon in nickel. To explain the higher stability of HM fibres an additional factor must be introduced. For example, their behaviour can be explained in terms of a highly stable surface layer between about 0.1 and 0.5m thick. Rapid recrystallization occurs when the nickel breaks through this layer e.g. by dissolution. The recrystallization was not greatly affected by the type of nickel coating but the recrystallization temperature of HM fibres was considerably reduced by a small proportion of air in the heat-treatment atmosphere. HTS fibres were not affected in this way but the fibres were severely weakened through surface attack by both air and hydrogen at temperatures well below the recrystallization temperature.