Fabrication and thermophysical properties of short graphite fiber/Cu composites with Mo2C coating by vacuum pressure infiltration

High-performance mesophase pitch-based graphite fiber reinforced metal matrix composites offer a good thermal conductivity and tailorable coefficient of thermal expansion. These composites are regarded as a promising heat dissipation materials for electronic packaging application. It is reported that short graphite fiber/Al composites were fabricated by vacuum pressure infiltration, which is a suitable procedure for the fabrication of complex shaped components at high production rates and low cost. The thermal conductivity of short fiber/Al composites is lower than 230 W m^-1 K^-1. It is expected that the composites using copper as the matrix, which has a higher thermal conductivity and lower coefficient of thermal expansion than aluminum, may have a better thermal properties. However, there is few investigation on fabrication of graphite fiber/Cu composites since copper does not wet graphite. This work focuses on the fabrication of short graphite fiber/Cu composites for thermal management application by vacuum pressure infiltration technique. A Mo₂C coating was synthesized on the surface of graphite fiber through molten salts method for improving the wetting between graphite fibers and copper. Certain Grfs reacted with molybdenum oxides powders in a molten salts mixture composed of NaCl-KCl at 1000 °C for 30 min under a flowing argon atmosphere. The obtained coating is compact and homogenous with the thickness of 0.3 μm. Copper was infiltrated into Mo₂C-coated graphite preform at 1150 °C with low pressure and Cu matrix composites with 20 vol%-40 vol% fibers were fabricated. Due to the unidirectional compression in the perform and infiltration process, the fibers were uniform distribution in the plane perpendicular to the press direction (X-Y plane) and showed a two dimensional orientation. The interface of composites was examined by EDS line-scan. Results show that there was copper infiltrated into - he Mo₂C interlayer. A good adhesion was observed in composites in which the Mo₂C interlayer tightly adhered to both fiber and Cu matrix. The obtained composites present high relative density above 99.5%. With increasing volume fraction of fiber from 20% to 40%, the thermal conductivity of composites in X-Y plane slightly decreased from 356 W m^-1 K^-1 to 311 W m^-1 K^-1. The coefficient of thermal expansion in X-Y plane decreased from 14× 10^-6 to 7 × 10^-6 K^-1. The experimental values of thermal conductivities of composites were compared with the theoretical calculation. With the introduction of Mo₂C interlayer, the interface thermal conductance of graphite fiber/Mo₂C interlayer/copper matrix had to be taken into consideration. So the effective thermal conductivity of coated fiber was firstly calculated by using the acoustic mismatch model (AMM). Then, a model of Maxwell-Garnett effective medium approach on the anisotropic short fiber reinforcement was used to estimate the thermal conductivity of composites. The result showed that the theoretical predictions are in good agreement with the experimental values. The thermal conductivity of composites could be further improved if perform with single axile orientation fiber was infiltrated.