BORIDES IN THIN-FILM TECHNOLOGY

The borides of transition and rare-earth metals are considered for app lication as wear- and corrosion-resistant, decorative or thermionic co atings. After a review of physical vapor deposition (PVD) techniques u sed for the deposition of these coatings, a survey of investigations t o apply these coatings is given, As a result of the strong directional ity of covalent boron-boron bonds, boride coatings show an increasing tendency to amorphous film growth with increasing B/Me atomic ratio an d, for rare-earth hexaborides, with decreasing metallic radius of the rare-earth metal. Mechanical and optical properties are strongly influ enced by the crystallographic structure of the boride phase, Because o f their high hardness combined with good adhesion, crystalline films b ased on the diborides of transition metals seem to be promising candid ates for wear resistant coatings on cutting tools. Alloying of these f ilms with nitrogen by reactive PVD processes results in the formation of extremely fine-grained multiphase hard coatings with excellent trib ological and corrosion behavior, thus offering new applications in the coating of engineering components. Because of their distinct colorati ons, some of the hexaborides of rare-earth elements may be used as dec orative coatings on consumer products like wristwatch casings or eyegl ass frames. Another promising field is the development of thermionic c oatings based on rare-earth hexaborides, which may offer the possibili ty of the production of inexpensive and simple high emission filaments . (C) 1997 Academic Press.