Boron nitride ceramics have various functional characteristics. Especially, hexagonal boron nitride (h-BN) has a good thermal resistibility and solid-lubrication. Besides, as the WC-Co alloy has low thermal expansion coefficient, it is frequently applied for a counter structural material to ceramics. Meanwhile, the wettability of h-BN to the brazing metals is relatively low compared to the other ceramics [1], therefore, it is difficult to braze h-BN to another material. Thus only a fewstudies have focused on the joining of boron nitride and the tungsten carbide alloy or other metals [2].
Laser brazing has good characteristics for dissimilar joining because of possibility of a short heating time and small heating area, and suppression of damage to the base materials compared to conventional long time heating in a vacuum furnace, but the behavior of the molten alloy braze during laser brazing has not been described in detail.
In this study, the time dependence of the wettability of h-BN by Ag-Cu-Ti alloy braze was determined using a high-speed video camera, and elemental analyses of the interface were performed to describe the dissimilar laser brazing of the h-BN and WC-Co alloy, and in order to investigate the characteristics of the dissimilar joint, cross-section observations, elemental and structural analyses, and shear strength measurements were performed.
The h-BN block was set on WC-Co alloy plate. The Ag-Cu-Ti alloy braze sheet, which was based on the eutectic Ag-Cu alloy with additional Ti as an activator, was placed on the h-BN block in a chamber filled with Ar. The laser heating was carried out by selectively irradiating the WC-Co alloy plate. Just after melting of the alloy braze including 1.68mass%Ti, the droplet was not wet to the h-BN, but 2 seconds later, it started to spread, and just before the end of the heating, its contact angle was acute. Most of the Ti in the braze was consumed to form a reaction layer at the interface.
Using the same laser heating process, we made a dissimilar joint of h-BN and WC-Co. In all the shear strength tests, every crack was on the h-BN side of the interface of the brazing joint which suggested that the brazing joint interface has a sufficient strength to connect the h-BN and WC-Co.
[1] M.G. Nicholas, D.A. Mortimer, L.M. Jones, R.M. Crispin. Some observations on the wetting and bonding of nitride ceramics. J Mat Sci 1990; 25(6): 2679-89.
[2] J. Felba, K. P. Friedel, P. Krull, I.L. Pobol, H. Wohlfahrt. Electron beam activated brazing of cubic boron nitride to tungsten carbide cutting tools. Vac 2001; 62: 171-80.