Boron nitride coatings : Preparation, characteristics and applications

摘要

Boron nitride (BN) is in many respects similar to the elementary carbon. BN exists in 4 crystalline forms of which the hexagonal, thermodynamically stable at NTP, and cubic are the best known as those similar to graphite and diamond. The third crystalline form of BN is analogous to the rhombohedral form of carbon, the fourth, BN form with hexagonal wurtzite structure being the only one without its exact carbon counterpart. Crystal structures of h-BN, c-BN and wurtzitic BN are represented in Fig. 1.1. The pressure temperature diagram of BN is presented in Fig. 1.2. Analogy of hexagonal BN (h-BN) with graphite goes further in that, due to the similarity in bonding, h-BN has lattice parameters close to those of graphite, its main structural defects are stacking faults leading to a so called turbostratic structure, and its properties present a marked anisotropy. However, h-BN differs from graphite by its electronic structure: unlike graphite, in BN there are no quasi-free electrons responsible for the "semi -metallic" propert i es of graphite. The h-BN is therefore, unlike graphite, an excellent electrical insulator. Furthermore, h-BN does notmake intercallation compounds because its electronic structure does not allow the charge transfer enabling intercallation of many molecules between the hexagonal layers in graphite. Chemical properties of h-BN are rather similar to those of graphite, one difference being in a better oxidation resistance of h-BN. Thus, h-BN is electrically insulating, chemically inert, thermally stable, resistant to corrosion and having desirable mechanical properties and is considered to be one of the wide-gap semiconductors. Potential and already existing applications of h-BN involve high-temperature dielectrics, heat-dissipation coatings, passivation layers, diffusion sources of Band sodium barriers. Since BN is highly transparent to X-rays, it can also be used in the fabrication of masks for X-ray lithography. However, the very properties of BN are responsible for high prices of some BN parts produced by conventional methods, e.g. crucibles for growing single crystals of semiconductor materials. On the other hand, a number of the mentioned applications demand thin BN films. The cubic BN has different crystal structure and properties. Thus, it is avery hard and high-friction material, as compared to the soft and lubricant h-BN. In addition, its cubic structure is intrinsically isotropie. [...]