is the preferred major metallic ingredient in the joint compound because the reaction product tungsten carbide has the lowest coefficient of thermal expansion among metallic carbides for this reason tungsten matches well with the low coefficient of thermal expansion coefficient of a carbon carbon smaller thermal expansion mismatch produces less thermal stresses in the joint and hence improves the joint strength the joint shear strength obtained using the reactive bonding method approaches 20 megapascals which is much higher than the interlaminar shear strength typically 7 9 megapascals of the carbon carbon composite material fine powders smaller than 100 micrometers in particle size both for metallic ingredients and carbon are used in the joint layers this is also essential to achieve a relatively low temperature joining operation because fine particle size due to its large surface area enhances the reaction rate and considerably reduces the joining temperature required to complete the carbides or suicides forming reaction in some cases short carbon fibers having an average length up to 5 mm in length are incorporated into the reactive bonding layer to further reinforce the joint and to achieve an even lower in plane coefficient of thermal expansion for the joint the advantages produced by the reinforcement effect of the fibers to ceramics will be readily apparent to those skilled in the art the lower coefficient of thermal expansion of the joint comes from the fact that carbon fibers usually have a much lower coefficient of thermal expansion in the longitudinal along the fiber length direction as compared to the transversal across the fiber section direction and the application of pressure during the joining process naturally aligns the fibers in the plane of the joint layer the in plane coefficient of.