Mössbauer and electron microprobe studies of the metallic inclusions in HTHP synthesized diamond grits

Abstract Synthetic single crystal diamonds and diamond grits for industrial applications are generally produced in the high temperature high pressure (HTHP) process in which Fe–Co and Fe–Ni alloys are used as solvent–catalysts. During crystallization, some of the metal gets trapped as inclusions in the grits, and hence their size plays a crucial role in the durability of the grits. In the present work, Mossbauer spectroscopy and electron probe microanalysis (EPMA) are combined to infer information on the size and composition of the Fe-metallic inclusions in De Beers SDA ® 1 and SDB ® diamond grits, which were synthesized with Fe–Co and Fe–Ni solvent–catalysts, respectively. Four sets of similar size grits of each type, with sizes ranging from 210 to 650 μm, were studied. The Mossbauer spectra of the inclusions in the SDA grits were dominated (>90%) by ferromagnetic sextets with hyperfine fields above that of α-Fe. Temperature-dependent studies showed the Curie temperature to be above 700 K. The inclusions in the SDB grits, in contrast, showed no magnetic order in their Mossbauer spectra, which were fitted with a singlet and a doublet component. The elemental Fe, Ni and Co concentrations in the inclusions were investigated in EPMA, with 15 and 25 keV electrons. Only Fe and Ni were present in the inclusions in the SDB grits, with more than 80% of the sampled inclusions having a Fe/(Fe+Ni) ratio>0.8. The inclusions in the SDA grits showed the expected presence of Fe and Co, as well as that of Ni. The relative Fe/[Fe+Co+Ni] concentrations in the inclusions spanned a range 40 at.% to 100% Fe. The normalized, total Fe+Ni+Co concentrations in the inclusions in both suites of grits spanned at least two orders of magnitude, suggesting a similar distribution in inclusion size. Our results also show that α-Fe and several different Fe/Ni and Fe/Co phases are present in the inclusions. These results contradict earlier studies, which concluded that the inclusions were ≤10 nm in size and contained only one Fe/Ni phase.