Brazing is a soldering process at higher temperatures. The grits are held mechanically and chemically in the bond. Therefore, the grit exposure can be higher than for electroplated wheels leading to bigger chip space (Fig. 3.19) [MARI04, p. 416 f.]. Chemically bonded grits allow for even thinner bonding layers than electroplated grits [CHAT90]. This has advantages of higher chip storage space and achievable material removal rate.
The wheel body material acts as substrate for the brazing process and has to be cleaned before [DING05]. The solder, also known as braze material, is deposited on the body (Fig. 3.20). This can be done by spreading the solder as powder. The abrasive grits are either deposited along with the solder or fixed temporarily before the brazing takes place. Brazing is done at temperatures of up to 1000 °C; the solder builds up around the grits and puts them under tensile stress when cooling [MARI04, p. 416 f.]. The mechanisms within the brazing process include complex
Fig. 3.19 Structure of a brazed layer after [MARI04, p. 416, DING05]
steps, such as dissolution of the substrate, diffusion of elements, formation of reaction products, and wetting phenomena [DING05].
In the case of diamond, conventional brazing often results only in mechanical retention. Therefore, vacuum soldering is applied [BENZ91]. Using metals such as titanium, zircon, niob, or tantal in the solder enable carbide forming and therefore chemical grit retention. Because of the high oxygen affinity of these metals, the soldering process must be conducted in inert gas atmosphere or in high vacuum [BENZ91].
For CBN, high temperature metal alloys, for example based on Ni/Cr, are used [MARI04, p. 416]. Density and distribution pattern of the CBN grits define the performance of a brazed grinding tool [CHAT94]. Research on Ag-Cu-Ti alloy solders for CBN grinding wheels shows that reaction layers of TiN and TiB2 can form [DING05]. These layers are key factors in achieving a strong bond between the CBN grits and the steel body of the grinding wheel.