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AWITEK, B.PALOSZ, S.STEL'MAKH, S.GIERLOTKA, R.PIELASZEK, E. EKIMOV, *t * V.FILONENKO, A.GAVRILIUK & V.GRYAZNOV High Pressure Research Center UNIPRESS, ul.Sokolowska 29, 01 142 Warsaw, Poland * Institute for High Pressure Physics, Russian Academy of Sciences, 142092 Troick, Russia; ABSTRACT Compacts of polycrystalline diamond were made in toroid-type high-pressure camera under the pressure of 8 GPa using temperatures between 800 to 2150'C without the use of additive components. Nanocrystalline commercial DALAN, and microcrystalline ASM diamond powders were used. The compacts were characterized by helium pycnometry, Vickers hardness measurements, X-ray diffraction and SEM methods. The starting and sintered nanocrystalline grain compacts were found to have strongly one-dimensionally disordered cubic modification. The nanocrystalline powder had a bimodal grain size distribution function as determined from X-ray diffraction data and ab initio intensity calculations performed with the use of Debye functions. It was found that neither the grain size nor one-dimensional disordering change under high-pressure high-temperature conditions. There is a general tendency in a decrease of density of compacts with increase in the sintering temperature what resulting partly from graphitization above 1000-1200 0C. The main factor which determines the density of the diamond compacts is closed porosity. Typically, the nanocrystalline diamond compacts sintered from 30 sec. to 6 min. have densities around 90% of the theoretical value. Their Vickers microhardness is 24 GPa and less. INTRODUCTION

Conventional diamond tools like cutters, dies and drilling bits are prepared with using metal additives like Co, Ni, Si, carbonates etc. [1], which facilitate sintering but limit applications of dense compacts. Preliminary studies showed that significant improvement of operational properties of diamond compacts can be achieved by decreasing the size of the powder particles [2 to 6]. This follows from the fact that small grain size ceramics is more crack steady, and thus less prone to abrasive wear and more suitable for manufacturing of machining tools. Sintering without metal additives should allow for (i) a use of diamond compacts as substrates for electronic devices and windows for electromagnetic and neutron radiation and, what is even more attractive (ii) an increase in the working temperature of diamond tools up to 1000-1200 TC (due to a strong reduction of graphitization in the absence of metal additives). Chemical activity of nanocrystalline particles is higher than that of microcrystalline materials. Therefore mass-transfer proceeds more actively and under lower temperatures than that observed in conventional microcrystalline materials. This may facilitate a formation of dense, well-sintered polycrystals with low-energy interfacial structure under some conditions. However, a practical use of nanocrystalline materials for fabrication of ceramics requires 115

Mat. Res. Soc. Symp. Proc. Vol. 499 01998 Materials Research Society

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