STRUCTURE AND PROPERTIES OF DECORATIVE RARE-EARTH HEXABORIDE COATINGS

Hexaborides of the rare-earth elements are considered for applications as wear- and corrosion-resistant hard coatings for decoration of cons umer products such as eye-glass frames and wristwatch casings. After a review of physical vapour deposition (PVD) methods used for the depos ition of rare-earth hexaborides, a survey of studies is given that ext ends the range of suitable decorative coating materials. Within the sc ope of the search for new colours, the interdependence between deposit ion conditions, microstructure and mechanical and optical properties o f boride coatings based on LaB6, CeB6, SmB6 and YB6 is presented and d iscussed. As a result of the strong directionality of covalent boron-b oron bonds, hexaboride coatings show an increasing tendency to amorpho us film growth with decreasing metallic radius of the rare-earth metal . Mechanical as well as optical properties are strongly influenced by the crystallographic structure of the hexaboride phase. The most prono unced visual colour impression is observed for LaB6-based coatings cor responding to a dark violet colour. Proper selection of process parame ters for these coatings may widen the conventional spectrum of decorat ive coatings.