Heterogeneous interfacial chemical nature and bonds in an Al matrix composite reinforced with W ‐coated diamond particles

Over the last decade, a diamond particle reinforced Al matrix, namely diamond/Al, composite has been developed for thermal management applications, mainly in the microelectronic industry. This composite has demonstrated an excellent combination of a Thermal Conductivity (TC) as high as 600 W/mK and a Coefficient of Thermal Expansion (CTE) lower than 10 ppm/K, being compatible with that of electronic components. However, considering the TC (~ 1800 W/mK) of the diamond particles incorporated, it is clear that the overall TC enhancement has not been completely exploited in the composite. One then points at the diamond/Al interface, which should provide good adhesion as well as maximal Interfacial Thermal Conductance (ITC) in order to facilitate thermal exchange across the diamond/Al interface. Our analytical modelling has recently predicted that introduction of a W interface nanolayer is one of the most efficient ways to achieve high ITC, which provides a practical guide for interface engineering. Accordingly, a cost‐effective sol‐gel process has been tentatively used to deposit the W coating for diamond surface metallization. Compared with the diamond/Al counterpart, TC of the composite with such a W nanolayer improved more than 20 %. In this work, Scanning Transmission Electron Microscopy (STEM)/Energy‐Dispersive X‐ray spectroscopy (EDX) and Precession Electron Diffraction (PED) have been performed in order to investigate interface configurations of a W‐coated diamond/Al composite. The aim is to study the effect of interface formation, reaction and diffusion on the ITC. The results indicate that the deposited W coating is discontinuous, and consists of nanoparticles with a size in the range 30‐400 nm and homogenously covering the surface of the diamond particle (Figs. 1a and 1b). The average coating thickness is estimated to be around 200 nm (Fig. 1c). A STEM/ADF image in Fig. 1d shows that the formed diamond/Al interface has a heterogeneous configuration at the nanoscale where Al grain contrasts and a particle with high Z contrast are revealed. STEM/EDX mapping in Fig. 2 displays a W and Al rich interfacial particle. Visible O traces can be related to fine microstructural features. Alternatively, a 'clean' diamond/Al interface is tightly‐adhered and is not rich in O (Fig. 3). As shown in Fig. 4, PED zone‐axis patterns recorded from the W and Al rich particles are indexed to be the Al 12 W phase (Cubic, I2/m‐3, No. 204). Such different chemical nature of the bonds at the interface can have a pronounced effect on the local ITC [1,2].