Abstract
The paper presents the results of studies on mechanical and structural properties of detonation nanodiamonds obtained by HPHT (high-pressure high-temperature) annealing within a wide temperature range. The experiments were carried out using a high-pressure “split-sphere” type apparatus (BARS) under 5 GPa and at 1100 –1500°С. It is established that the thermobaric treatment allows the production of strong composites with the local hardness up to 15 GPa. It is shown that the average value of microhardness increases with the sintering temperature. The temperature increase from 1100 to 1500°С results in an enhancement of the average value of microhardness from 8.8 to 12.2 GPa. The obtained materials are structurally inhomogeneous, regions of higher hardness are located in the central part of the samples. However, when the sintering temperature increases, dispersion of the microhardness decreases from 6.4 to 1.4 GPa which is caused by an improvement of the structural homogeneity of the composite with an increase in temperature. The thermobaric effect results in the growth of cores of diamond nanocrystals from 4.2 to 6.9 nm in samples obtained at 1500°С. It is highly probable that this growth occurs as a result of embedding of non-diamond phase carbon into the diamond core lattice during thermobaric sintering. This conclusion is made on the basis of X-ray data analysis. The specific feature of transformation of detonation nanodiamond shells consists in a simultaneous occurrence of two processes — desorption of volatile impure compounds and formation of a newly-formed diamond phase or ordered graphite around diamond cores. This effect results in the formation of open fragments of diamond cores capable of contacting with neighboring nanodiamond crystals thus binding diamond grains into a composite. Sintering temperature is an essential factor that affects the nanocrystal size as well as the mechanical properties and homogeneity of the composite.
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