Sintering and strength of hot-pressed ceramics based on titanium diboride

The paper examines how the grain-size composition of starting titan diboride powders and calcium silicon and calcium hexaboride additives influence the consolidation of titanium diboride in hot pressing and the structure and properties of hot-pressed materials. It is shown that 3 and 5 wt.% of calcium silicon decrease the pressing temperature by 200 °C, refine the structure, and improve the density and mechanical properties of titan diboride materials.     

Analysis of the Effect of Nonhydrostatic Compression Conditions on Direct Phase Transformations in Carbon

We have analyzed the effect of deviations from hydrostatic compression conditions on the driving force for transformations of graphite to dense phases. We consider transformations of the hexagonal (2H) and rhombohedral (3R) modifications of graphite to lonsdaleite and diamond by martensitic mechanisms, and also a direct diffusional transformation. Comparative estimates of the driving forces were made for hydrostatic and uniaxial compression schemes, and also for shear loading. We show that a loading scheme combining hydrostatic compression with uniaxial compression is more favorable for the martensitic transformation of 2H-graphite to lonsdaleite, while shear loading and uniaxial loading are more favorable for the transformations of 3R-graphite; and lonsdaleite is formed in the first case, while diamond is formed in the second case.     

Formation of long-period and unidimensionally disordered structures in the AlN-SiC-O system

An investigation was made of the formation of long-period structures (LPS) in solid AlN-SiC solutions with added oxygen. It was found that the crystallographic parameters of these structures are similar to those of sialon. A structural model was developed which connects LPS with the formation of structural and chemical inhomogeneities in the basic 2H-structure. It was established that a considerable fraction of LPS exhibit packing defects which are unidimensionally disordered.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 5–6, pp. 97–101, May–June, 1994.     

Diffusionless Nucleation of Lonsdaleite and Diamond in Hexagonal Graphite under Static Compression

We have used transmission electron microscopy to study the initial stages of diffusionless formation of lonsdaleite and diamond crystals when pyrolytic graphite is subjected to static compression. We consider the effect of plastic deformation of the matrix phase on development of the transformation. We propose a dislocation model for nucleation of dense phases, making it possible to explain the reason for the formation of a metastable lonsdaleite phase, the nature of its structural disordering, and also the possibility of diffusionless nucleation of diamond directly from hexagonal graphite.     

Structure formation of the superhard carbon ceramics in sintering powders of the carbon amorphous phase in a mixture with nanocrystalline diamond at high static pressures

The results have been discussed of studying the structure and hardness of polycrystals produced from nanodispersed powders of mixtures of the amorphous carbon phase and nanocrystalline diamond. The initial powders have been synthesized by high-temperature shock compression and sintered at a pressure of 13 GPa and temperatures from 1100 to 2000°C. The sintering has been accompanied by the transformation of amorphous carbon into diamond. As the diamond content of the sample has been increased from 10 to 100%, the hardness increased from 45 to 80 GPa.     

Phase transformations of disordered structures of graphite-like Boron Nitride under high-temperature shock compression

The pattern of phase transformations in boron nitride under high-temperature shock compression has been studied using a previously proposed method for high-temperature shock-wave synthesis of high-pressure phases followed by rapid quenching. Fine powders of turbostratic and partially ordered graphite-like BN were used as initial structures. Shock compression was carried out in ring devices at a pressure of 30 GPa and a temperature above 2500 K. A mixture of dense phases (wurtzitic and sphaleritic) was found to form from the graphite-like structures under those conditions; the total yield of those phases and the relative amount of the sphaleritic modification are considerably higher when turbostratic BN is the starting material. Both of the dense phases formed have a nanocrystalline grainstructure. The wurtzitic phase does not transform into the sphaleritic phase under those conditions, which points to cubic BN forming directly from the graphite-like structures.     

Structure, mechanical and erosive properties of AlN-MoSi2 composite materials and their electrospark-deposited coatings

The paper examines the phase composition, structure, and properties of AlN-MoSi₂ alloys and associated electrospark-deposited coatings on titanium. It is shown that the most intensive erosion and mass transfer are characteristic of alloys containing 60–80 wt.% MoSi₂ and that the erosive characteristics essentially depend on the electrospark treatment parameters. A protective coating with a thickness of 30–40 µm and hardness to 1 GPa is formed on the substrate. There is a thermal impact area up to 300 µm deep and 1.4 GPa hard under the coating. The high-temperature holding of coated samples promotes the rapid formation of heat-resistance phases in the coating and the formation of a secondary structure in the thermal impact area. As a result, this area becomes thicker and the hardness of its material increases up to 1.9 GPa.     

Structure of polycrystals produced by sintering nanocrystalline powders of cubic and wurtzitic boron nitrides

The structure and some properties of polycrystals produced by sintering nanocrystalline powders of the dense modifications of shock—wave-synthesized BN have been studied. The sintering was conducted at a static pressure of 7.7 GPa and temperatures from 1100 to 1800° C. The highest density (3 g/cm³) and microhardness (up to 20 GPa) have been exhibited by polycrystals produced by sintering the powder containing wurtzitic and cubic modifications in amounts that are approximately equal. In the temperature range from 1100 to 1300° C the wurtzitic phase transformed into the cubic one. In this temperature range the average size of cBN grains changed from 20 to 50 nm. The structure of compacts is characterized by the presence of grain (grain-boundary) interlayers 2–5 nm in thickness.