Characteristics of a tungsten film electrodeposited in a KF–B2O3–WO3 melt and preparation of W–Cu–W three-layered films for heat sink application

A tungsten film of 13 μm in thickness was obtained on a copper substrate by galvanostatic electrolysis at 30 mA cm−2 for 40 min in a KF–B2O3–WO3 (67:26:7 mol%) melt at 850 °C. By cross-sectional scanning electron microscopy observation and energy dispersive X-ray analysis, the tungsten layer was found to be compact and free from cracks, voids and melt inclusion. The X-ray diffractometry analysis revealed that the phase was α-tungsten, and that (222) plane was significantly oriented parallel to the substrate. By nanoindentation, its hardness was found to be 8.4 GPa, which was larger than that of single crystal tungsten. Its Young’s modulus was measured to be 410 GPa, which was similar with the reported value of single crystal tungsten. Its coefficient of linear thermal expansion and thermal conductivity were 4.5 × 10−6 K−1 and 178 W m−1 K−1, respectively, which were similar values for the tungsten produced by a conventional powder metallurgy method. Finally, W–Cu–W three-layered films were prepared for a heat sink application. It was confirmed that a three-layered film having a desired coefficient of linear thermal expansion can be prepared easily by this new molten salt method.