Development and characterisation of high thermal performance diamond-metal matrix composites

Abstract

Most published research on the production of diamond metal matrix composites make use of energy and technological intensive processes to prepare interfaces onto diamond powders. In this project, electroless deposited interlayers containing carbide forming elements were investigated as precursors for interfaces in copper and silver diamond metal matrix composites for materials having enhanced thermal properties. Electro less deposition techniques make use of chemical reactions to deposit metallic layers and can be carried out using simple apparatus found in most laboratories. NiWP, NiCrB were prepared but only NiWP was chosen for finiher experimentation . . Copper and silver matrix composites having diamonds pre-coated with various amounts of NiWP interlayers were produced following a two-step method, first compaction then sintering at high temperature. Characterisation of the interlayers shows diffusion of Ni and P into Cu matrix forming a W rich layer at the carbon matrix boundary, whilst a complex interlayer transformation was observed for Ag matrix composites. X-ray diffraction patterns show the formation of NhP upon NiWP coated diamond powders were heat treated at 800 °C under vacuum. The produced DMMCs had an average relative densification of 92 %, this is as a result of large voids present between the diamonds and the matrix due to incomplete sintering and poor wetting prope1iies. Reduced coefficients of thermal expansion were measured when %-wt interlayers exceed 5 %, reaching values as low as 14.1 ppm °C-1 • This value was obtained for Cu matrix composites. Thermal conductivity measurements show that the composites possess thermal conductivities lower than 200 W m-1 0 c-1 , which decrease with increasing %-wt interlayer. This indicates that although wetting increases the diamond character in the composites (reduced CTE), the interlayer I interfaces produces might still have inherent large thermal resistivity.