Papers by Author: Guerold Sergueevitch Bobrovinitchii

Abstract: The high pressure and temperature, synthesis of diamond from carbonaceous materials, is a complex process highly dependent on variables such as the catalyst/solvent, the crystalline structure of the precursor material, the processing conditions and the type of compressive chamber. The optimum susceptible precursors to be transformed into diamond are those possessing the perfect hexagonal graphite structure, which is the thermodynamically most stable form of carbon at atmospheric pressure and ambient temperature. However, the majority of both industrial and natural graphites, presents a mixture of different atomic structural arrangements that greatly influence the process of diamond synthesis. In this works the influence of rhombohedral and hexagonal phases existing in the graphite was performed by means of a software refinement of the crystal structures using the Rietveld method. The thermobaric treatment, which determine the structural parameters, was conducted in a high pressure anvil type device with a central concavity. All experiments were carried out at 1200°C and pressures varying from 4.3 to 5.0 GPa. It was determined that the degree of graphite to diamond transformation is directly associated with the content of rhombohedral phase.

Abstract: t is well recognized that a catalyst agent is essential to synthesize diamond at high pressure and high temperature. The catalyst agent also acts as a carbon solvent and affects the properties of the final product. This is a consequence of its direct influence on the growth of diamonds crystals. The control of the growth, which is required to achieve a high quality level, is still a question open to discussion. In this work, an industrial diamond powder was synthesized from graphite using Ni-Mn as catalyst alloy added with up to 5% of iron. The synthesis was carried out at 4.5GPa and 1300°C for 10 minutes in high pressure device with central concavity. The analysis of the cubic-shaped diamond surface topography suggests that the formation of defects on the {111} faces is most likely a consequence of a complex growth process that influences the crystal productivity and morphology.

Abstract: The lifetime of drilling tools for petroleum well perforation is strongly affected by the type of insert at the crown of the tool. Usually, this insert is made a of hardmetal (WC-Co) cermet with embedded diamond particles. In the present work an investigation on the effect of Cu and Ti as doping agents on a WC-6%Co cermet was conducted. I was found a change in coercive force in the interval 12 to 34 kJ/m and harness from 73 to 89 HRA with improving adhesion. It was also shown that this could significantly increase the operational capacity of a drilling tool.

Abstract: Non-oxide covalent ceramics such as TiC and TiN display hardness and thermal stability that favor their use as cermet insert for high speed turning operations. The objective of this work was to assess the properties of the TiC combined with W-Mo as binding matrix. The cermet sintering was carried out inside a toroidal anvil type of high pressure device up to 1.0 GPa of pressure and temperatures up to 1800°C. It was found that at 1400°C the Mo reacts with TiC forming Mo₂C while at 1800°C the reaction with W forms WC. Moreover, the cermets density reaches 98% of the theoretically calculated value with strength of 942 MPa and Rockwell A hardness of 94. Efficient turning parameters are reported for machining operation using these cermets as tools insert.

Abstract: Polycristalline diamond composites (PDC) obtained by sintering of micro sized diamond particles together with silicon as binder is a conventional superhard material for high speed cutting tools. nanosized dispersed diamond particles have been recently used as precursor for a new generation of superhard composites. In the present work, PDCs were produced by sintering at high pressure and high temperature using a mixture of micro and nanosized diamond particles for optimized compactation. The values of density and hardness as well the results of X-ray diffraction suggest a plastic deformation of the larger diamond particles. The compression strength of these composites was found to be close to that of natural diamonds.

Abstract: Compositions of cutting tools based on cubic boron nitride phase added with different ceramics as binders have been the subject of several works. A major problem of these tools during machining of steel is the loss of cutting efficiency due the "chemical weathering" generated by the reaction between iron and cBN at the high temperatures reached by the machining operations. Aiming to eliminate these problems, this paper presents results on the development of composites obtained by sintering process at high pressures and high temperatures in association with a cyclic operation. Si₃N₄ was used as the binder agent. The compacts were analysed by x-ray diffraction as well as optical and scanning electron microscopy. The mechanical properties were evaluated by means of microhardness and fracture toughness tests. The degree of phase transformation of the Si₃N₄, the chemical reaction during the cyclic sintering process, and the sintering mechanism of the additive were discussed.

Abstract: The sintering of nanodiamond powders is of interest for both applied engineering of tool materials and fundamental materials science of nanodisperse covalent-type ceramic materials. It is a accept as a general notion that the driving force for sintering of monophase particles is determined by the level of the surface energy. In the case of diamond nanopowder, this level must be significantly higher which makes sintering a difficult process. This difficulty of sintering is connected with the low diffusive mobility of carbon causing the formation of a graphite structure onto surface of the diamond crystals. From this point of view the use of niobium oxide as a binder could be a solution. In an attempt to inhibit the diamonds graphitization process, Nb2O5 and small amounts of amorphous carbon were introduced in the reaction zone. Sintering process was conducted at 6.0 GPa of pressure and 1100-1400oC for a processing time of 30 seconds. At the end of the process, the samples were cleaned, and prepared to be characterized by X-ray diffraction, scanning electron microscopy, density and porosity. From these results it was proposed a densification mechanism based on the consolidation of the particle by diffusion and coalescence of clusters.

Abstract: Diamond-Si nanostructured composites were obtained by cyclic high pressure and high temperature sintering with different processing conditions to examine the dominant microstructural factors and the abrasive wear resistance. The microstructure of the composites was characterized by scanning electron microscopy. The abrasive wear behavior of the composites was evaluated by weight loss in abrasion tests. It was found that improved nanostructured composite properties and denser structures were obtained for sintering performed with more than one cycle of pressure and temperature.

Abstract: Ceramic based compacts used in tool bits are of fundamental importance for high speed machining operations. Pure ceramic, however, is too brittle. Therefore, conventional hardmetal cermets such as WC-Co are commonly used, in spite of thermal limitations due to the metallic matrix. The present work investigates the properties of sintered ZrO₂ nanopowder, stabilized with Yb₂O₃, as a tough enough pure ceramic with high thermal resistance for tooling compacts. Sintering of ZrO₂ nanopowder with particle size below 0.1µm in presence of 2-3% of Yb₂O₃ was conducted under pressure of 180 to 300 MPa and temperatures from 1500 to 1600°C, during 40 to 60 minutes. Both the density and strength pass through maximum values with temperature and processing time. Strength of 9.500 MPa and hardness of 17.5GPa as well as fracture toughness of 12.5MPa.m^1/2 were obtained

Abstract: In the present work, by selecting Si₃N₄, TiB₂ and Al₂O₃ as binding agents as well as La₃O₂ as an additive, sintered wBN composites were studied. By modifying the number of sintering cycles, the composites processed at 4.5GPa and 1800^°C showed improved mechanical properties. The degree of transformation of the wBN, as well as the chemical reactions during the sintering process were discussed. This new composite material was found to present polycrystalline structure that provides superior cutting properties. Moreover, owing to superior properties, the wBN composite sharpens itself during cutting.