Fabrication and thermal properties of copper matrix reinforced silver-coated carbon fibre composites

A solid solution composed of copper solvent and silver solute (Cu-Ag) was introduced as a thermal interface for carbon fibres reinforced copper matrix (Cu-CF) composites. The sources of copper and silver to form the metal matrix and the Cu-Ag solid solution were 100% produced by electroless coating processes. High volume of XN-100 carbon fibres was incorporated into the copper matrix to reduce the CTE and density of Cu-CF composites. In the fabrication of silver coated carbon fibres reinforced copper matrix (Cu-Ag/CF) composites, the carbon fibres were coated consecutively with silver and copper layers via an electroless coating technique. The thickness of the silver coating layer was kept at low level to minimize the effect of silver on the overall density of the Cu-Ag/CF composites. Based on the results of preliminary process capability studies, the content of silver and carbon fibres was controlled at 4 – 12 wt.% and 15 – 55 vol.%, respectively. The carbon fibres were individually coated at the same coating thickness and coverage. Since the coated carbon fibres were about the same size and shape, complications related to inefficient mixing could be avoided. Thus, the carbon fibres could be homogeneously distributed in the copper matrix. Then the coated carbon fibres were uniaxially compressed at 500 MPa into green compacts and sintered at 850oC under mixed atmosphere of 98% argon and 2% hydrogen gases. Copper-rich solid solution was formed by diffusion of silver atoms into the copper lattice sites during sintering. Large substitutional silver atoms imposed compressive strains on the surrounding copper crystal lattice and induced structural distortion which strengthened the matrix against thermal expansion. The diffusion of silver atoms into copper lattice sites increases the lattice parameter of the copper-rich solid solution from 0.36168 nm to 0.36327 nm when the silver concentration was increased from 4 wt.% to 12 wt.%. At the same time, the crystallite size decreased from 282.3 nm to 63.6 nm and the lattice strain increased from 0.3 x10-3 nm to 4.0 x 10-3 nm due to structural distortion. The effect and correlation between the wt.% Ag and vol.% CF on the thermal performance of Cu-Ag/CF composites were analysed by means of a full factorial design of experiment (DOE). JMP Pro software was used in designing the experiments. A 2 by 2 full factorial design with a standard least squares method was used to construct linear models in order to estimate the effect of each factor and their interactions. Based on the JMP analysis reports, the formation of Cu-Ag solid solution had positive impact on the thermal conductivity of the composites when the content of the carbon fibres was at high level (55 vol.%). As compared to the noncoated Cu-CF composites, the thermal conductivity of Cu-Ag/CF composites increased steadily by 56%, from 25.2 Wm-1K-1 to 39.4 Wm-1K-1, when Ag wt.% was increased from 4 wt.% to 12 wt.%. The resistance of the composites to thermal expansion also improved. The CTEs of the composites decreased more than 50% from 14.8 to 6.8 x 10-3 K-1. As compared to the Cu-X/CF (X = carbide compounds) composites reported in the literatures, the CTE values of Cu-Ag/CF composites were 30% better than Cu-X/CF composites. However, the thermal conductivity of Cu-Ag/CF composites is inferior to those of Cu-X/CF composites. High porosity was suspected to be the main reason behind the poor thermal conductivity of Cu-Ag/CF composites. The high aspect ratio of carbon fibres led to the formation of skeletal structures which impeded densification during sintering. As a result, cracks and pores were formed in the matrix. Heat transfer across pores is inefficient because air in the pores has an extremely low thermal conductivity. Porosity also acts as electron scattering centre and causes a reduction in the thermal conductivity of the composites.