Masanori Ueki

High-temperature behaviour of hexagonal boron nitride

Abstract Three boron nitride processes have been studied—synthesis of turbostratic powder, sintering of hexagonal ceramics and transformation into diamond-like phases. It is shown that the crystallization of boron nitride proceeds in three distinctive steps. The first step is the recombination of boron and nitrogen forming semi-organic oligomers; this is followed by a double coalescence on the basis of this mechanism, three fundamental sub-polytypes of layered boron nitride (turbostratic, mesographitic and graphitic) have been defined. Compared with most other inorganic materials, the chemical properties and the character of the interaction with the environment are strongly dependent on the crystallographic direction. The crystal structure of boron nitride is thus a result of a competition between the combination of boron nitride monolayers and their chemical interaction with the environment.

COPPER-BORON NITRIDE INTERACTION IN HOT-PRESSED CERAMICS

Ceramics were prepared by hot pressing of chemical mixtures of turbostratic boron nitride with copper. A positive effect of copper on boron nitride crystallization and densification has been found. Preferred orientation of boron nitride grains has been revealed in such ceramics, being opposite to general experience: the c-axis of boron nitride crystallites was arranged preferably within a plane perpendicular to the direction of uniaxial pressure. Such an arrangement is interpreted as a result of the uniaxial compression of the boron nitride grains extensively growing within the c crystallographic axis. {copyright} {ital 1997 Materials Research Society.}

Effect of Graphite Addition on the Microstructure and Mechanical Properties of Hot-pressed Titanium Carbide

Titanium carbide with a wide range of grain size was consolidated by controlling hot-pressing temperature and small amount of graphite addition. In the TiC without graphite addition, increase in hot-pressing temperature from 2073 to 2373K resulted in the significant grain growth in the sintered body from 7.0 to 27.7μm, which caused decrease in flexural strength from 670 to 430MPa. Graphite addition to TiC caused a distinct inhibition of grain growth resulting increase in flexural strength. Increase in grain size caused increase in fracture toughness of the hot-pressed TiC with and without graphite addition, which may be resulted from increase in traction of fracture surface behind the crack tip. Graphite addition increased in fracture toughness of the TiC due to certain toughening mechanisms such as crack deflection and bridging.

Tribology of Ceramic Matrix Composites against Metals

Abstract The sliding of silicon nitride matrix–silicon carbide whisker composites against bearing steel has been investigated, and it was observed that the wear of the ceramic specimens decreased with increasing amount of incorporated whiskers. In hot-pressed silicon nitride based ceramics, the appearance of anisotropy in the mechanical properties has generally been observed. Such an anisotropy was also found in the tribological properties of those ceramics. A series of investigations on ceramics sliding against pure iron was also carried out, with as result that composite ceramics did not always show better wear resistance than monolithic ones. The tribological compatibility must be considered with first priority when the sliding countermaterial is selected. In this chapter, the discussion is based on the viewpoint of the adhesive-wear mechanism with special emphasis on the tribological compatibility of the sliding materials