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The second zone, of the same thickness, consists of Cr 7C 3 (89–90 % Cr). The presence of two intermediate zones can be clearly seen on microsections and the curves of distribution of chromium concentration in the thickness of the new phase (Fig. 21a). 5–1 µm thick can be seen on the microsection and in scanning of the probe is less distinctive than the other two phases. The authors and Ref. 78 assumed that this zone consists of the Cr 23C 6 (91–92 % Cr) carbide. 21 Dependence of the variation of the content of copper, chromium and titanium at copper–chromium alloy–diamond (a) and copper–titanium alloy–diamond (b) interfaces in transition from the alloy to diamond.

Thus, the processes of interaction of diamond (graphite) with carbideforming metals in the solid state and with melts containing additions of these metals are as a rule accompanied by carbide formation at the interface; the intensity and rate of growth of the carbide in the initial stage are determined by the kinetics of chemical reactions at the interface and then by the diffusion of components (carbon and metal) through the layer of the new phase. The second stage is usually controlling. However, in cases in which the temperature is high and the contact time is short, the ‘reaction’ stage of the process can play a significant and, possibly, controlling role.

Indexing of the x-ray diffraction patterns was carried out using the information presented in Ref. 141 and 142. Niobium and tantalum coatings were deposited on natural diamond powders with a grain size of 400/315 µm in vacuum annealing in a metallising agent consisting of a mixture of surface-oxidised powders of niobium and tantalum and also NaF activator (1–4%) by the method proposed in Ref. 143. In the metallising diamonds by these metals 41 without the activating agents the coating did not form even at relatively high temperatures (1100–1200 °C).