Investigation on the interface structure and thermal conductivity of hot-forged Cu/(Ti/W-coated)-diamond composites

Abstract

Effectively dissipating heat is required for the high-power electronic equipment to ensure the equipment has a safe working environment. Cu/diamond composite is considered as a new generation of heat sink materials because of its potential of achieving high thermal conductivity and tailorable coefficient of thermal expansion. The poor chemical affinity between the copper and the diamond is the main barrier for obtaining Cu/diamond composites that have high thermal conductivity. The interface layer between the copper and the diamond plays an important role in facilitating heat transfer in the Cu/diamond composites. However, it lacks deep understanding on the influence of interface characteristics on the Cu/diamond composite’s thermal conductivity.

We aim to tailor the thermal conductivity of Cu/diamond composites by modifying the interphase layer’s phase composition and microstructure, and there are three primary research parts in this thesis. The first part is the study of the Cu/Ti-diamond composite prepared by induction heating and hot forging from pre-annealed Ti-coated diamond particles. Part two is the study of the Cu/W-diamond composite prepared with the identical process that used in the part one from the pre-annealed W-coated diamond particles. The third part presents Cu/Ti-diamond composites that are fabricated by the combination of vacuum sintering and hot forging from Ti-coated diamond particles. The fabricated composites’ microstructure and the interface morphology of diamond particles that are extracted from the fabricated composites are examined by scanning electronic microscopy (SEM) and atomic force microscopy (AFM), respectively, and the phase composition of the fabricated composites is analyzed by X-ray diffraction (XRD).

Results show that both TiC and W2C particles are easier to nucleate on the diamond-{111} facet than on diamond-{100} facet during pre-annealing, and the reaction temperature for forming TiC is 800℃ and W2C is 1050 ℃, respectively, and the higher the annealing temperature is, the more carbides nucleation and growing locations present on the diamond surface. It finds that the un-reacted diamond coating (titanium or tungsten) further reacts with the diamond to form metal carbides during the process of hot forging at 800 ℃. Ti-800-Cu/Dia composite has the highest thermal conductivity among the hot-forged Cu/Ti-diamond composites, with a value of 350W/ (mK), attributed to the formation of a continuous interface, almost no interfacial layer spallation, and a rough interface formed on the diamond particle surface. Due to a large amount of WC interface is formed after hot forging, W-1050-Cu/Dia composite shows the highest thermal conductivity (223 W/ (mK)) among the hot-forged Cu/W-diamond composites. After vacuum sintering, the Ti coating is fully reacted with the diamond to form TiC, however, the preformed TiC particles are easy to spall from the diamond surface during hot forging. This leads to weak interfacial bonding strength between the diamond and the copper. The thermal conductivity of the Cu/Ti-Dia-VS is only 46W/ (mK) because of its low relative density. After hot forging, the thermal conductivity of Cu/Ti-Dia-VSHF composite is improved to 241W/ (mK), due to the significant increase of the hot-forged composite’s relative density. It is possible to design the interface characteristics of Cu/diamond composites by changing the pre-annealing temperature of the coated-diamond particles prior processing.