Microstructural Aspects of the Hot-Pressed Hexagonal Boron Nitride Ceramics with Limited Content of Boron Oxide

Abstract

Hexagonal boron nitride (h-BN) based ceramics have been fabricated by hot-pressing in a nitrogen atmosphere at 1900°C for 60 min. under a pressure of 50 MPa with B2O3 as sintering additive. The phase composition was confirmed by XRD. The microstructural properties of h-BN powders and hot-pressed surfaces were evaluated by SEM-EDS analysis. The formation of the secondary phase was also investigated with particular interest on grain boundaries. The sintered densities were also determined by Archimedes’ method. The maximum relative density was obtained to be about 92 %. The effect of the existence of boron oxide on the boron nitride grain growth behaviour was discussed on the basis of microstructural features.

Powder Preparation, Properties and Industrial Applications of Hexagonal Boron Nitride

“…Apparent density values of 81-96 % of the theoretical density were obtained in related literature studies carried out on h-BN based ceramics formed by hot pressing depending on the process temperature and the composition [18].…”

“…Unlike the typical platelet shaped h-BN powder particles, near-spherical shaped particles can be observed in the SEM images in Fig.10. These spherical particles could be explained as molten B 2 O 3 particles that were not converted to BN during synthesis [18]. An average value of 92 % of the theoretical density was achieved by the hot pressing process.…”

“…In other words, a high degree of orientation of the hexagonal boron nitride grains leads to mechanical anisotropy. Densification via hot-pressing occurs by three consecutive mechanisms: re-arrangement of the powders, removal of open porosity by plastic deformation and elimination of closed porosity by bulk diffusion processes [18].…”

“…The low degreee of orientation prevents efficient packing and lowers density besides basal planes near parallel to the hot pressing direction act as flaws on the microstructural scale and decreases flexure strength. Thus orientation and densification characteristics are important from the mechanical point of view [18]. The hot-pressing operation, which involves the hexagonal boron nitride grains containing boron oxide, is not just a physical process.…”

“…Prior to SEM analysis, agglomerated BN particles were dispersed in acetone and adhered on a carbon tape and then coated with carbon. For the sintered bodies, the formation of the secondary phase was also investigated with particular interest on grain boundaries via SEM-EDS analysis [18].…”