From Molecules to Materials: Hard and Soft Boron Nitride Phases of Technological Importance3

Boron Nitride (BN) displays several phases of industrial importance, e.g. phases with threefold coordinated atoms (h-BN, r-BN, o-BN) display high thermal resitance, low friction and good insulating properties, whereas phases with tetracoordinated atoms (c-BN, w-BN) exhibit extreme hardness and can be used as wear resistant coatings. Due to these interesting thermal, electrical and tribological features, films and layers of these materials are of specific interest. In order to enhance a growth of hard phases (c-BN, w-BN) from the gas phase or at ambient pressure, it is important to understand the nucleation and growth mechanisms and avoid the formation of sp2- type BN. Due to the outstanding material properities of the individual BN phases, there is a huge interest in the specific synthesis of the individual modifications, and in general mainly infrared spectroscopy is used to determine the formation of c-BN films and nanoscale powders. A critical analysis of the characterization of the individual BN phases with IR- and Raman-spectroscopy is presented and their use and limitations are discussed. In the case of c-BN, polytypism was oberved for the first time with raman spectroscopy in analogy to silicon carbide (SiC), and the formation of boron nitride phases by chemical vapor deposition and by thermal decomposition of suitable precursors is presented. Amorphous boron nitride was obtained at temperatures below 400 °C, which does not show recrystallisation by heating up to 1500 °C. SEM, XRD- and IR- spectra indicate the formation of sp2- type boron nitride and display a new source for the selective formation of BN based nanoscale materials. By CVD of borazines, a selective formation of monoatomic layers of h-BN was achieved. Furthermore, the syntheses of boron nitride nanotubes and boron nitride based ceramic composites are presented. These investigations and studies are of principle interest for the understanding of the nucleation and growth of the distinct crystalline phases of BN.