Chapter 5 - Magnetic, mechanical, and tribological properties of hexagonal boron nitride

Abstract

Hexagonal boron nitride (h-BN) emerges as an important ceramic engineering material. The nonuniform charge distribution caused by electronegativity difference (between B and N) made it different from graphite and is the reason for its unique properties like low dielectric coefficient, less tangent loss, high sublimation temperature, resistance from thermal shock, and good mechanical properties. Each layer of h-BN is made up of covalently bonded sp2 hybridized B and N atom and various layers are connected to each other through van der Waals interaction. Electronic structure of h-BN depends on number of layers as indicated by the change of band gap from direct to indirect. Semiconducting behavior and optical conductivity of h-BN nanosheets in parallel and perpendicular electric field made them a good candidate for their electronic and optoelectronic devices. Distribution of pentagonal and hexagonal edges with smaller number of homopolar bonds (B-B and N-N) increases the energetic stability of three-layered h-BN nanoribbons. h-BN nanoplates have great potential to act as biological nanovectors to carry proteins by cross-linking immobilization, biocompatible materials, and vaccine adjuvants. Pure h-BN ceramics showed excellent mechanical strength even at high temperature, whereas in the case of h-BN nanosheets, it gets affected by temperature, cracks and percentage defects, and its random distribution. Although h-BN is nonmagnetic, yet local magnetic moment generated through spin polarization by introducing vacancy or edge defects. A change in band gap and electronic structure of h-BN results in the formation of an unconventional magnetic semiconductor. Friction and wear value of h-BN particles decreases with decrease in size from semimicron and micron to nano. Improvement in specific wear rate and friction coefficient can be obtained in polyetherketone/h-BN and 316L stainless steel composites at elevated temperature.