Wettability studies of reactive brazing alloys on CVD diamond plates

Wettability of both the diamond and the insert surfaces by the filler metal in CVD diamond brazed-on cutting tools is a key condition for good brazing strength. The brazing process of CVD diamond thick plates still has to be improved, namely on the influence of the brazing alloy composition and of t he substrate surface finishing quality in wettability. In this study, conta ct angle measurements were performed in a dedicated high vacuum furnace cou pled with a video recording system. Diamond films with different thickness (75 < t < 300 mum), and thus having distinct grain sizes and roughness, wer e grown with fixed conditions by the MPCVD technique on Si substrates and c hemically detached for wettability experiments. Roughness parameters were e valuated by profilometry and AFM, which was used to observe the grown diamo nd surfaces of the self-standing films. The reactive Ag-Cu-Ti brazing syste m was investigated. Results showed a very good wettability in the temperatu re range 800-850 degreesC, namely for the diamond surface where a minimal c ontact angle of 10 degrees was reached. A Ti-rich thin reaction layer (0.5- 0.8 mum) was detected at the drop side of the substrate/brazing alloy inter face in both substrate materials, proving the affinity of Ti to carbon. The influence of the diamond roughness on the contact angle theta (R) is notab le, obeying a linear dependence of the type cos theta (R) = cos theta (0) k cos theta (0). (R-a/G)(2), where R-a and G, the grain size, are related to asperity height and width, respectively. This relationship is based on t he well-known Wenzel equation that correlates the real contact angle to the surface area increasing with roughness. (C) 2001 Elsevier Science B.V. All rights reserved.