Hexaborides of the rare-earth elements are considered for applications as wear- and corrosion-resistant hard coatings for decoration of consumer products such as eye-glass frames and wristwatch casings. After a review of physical vapour deposition (PVD) methods used for the deposition of rare-earth hexaborides, a survey of studies is given that extends the range of suitable decorative coating materials. Within the scope of the search for new colours, the interdependence between deposition conditions, microstructure and mechanical and optical properties of boride coatings based on LaB6, CeB6, SmB6 and YB6 is presented and discussed. As a result of the strong directionally of covalent boron-boron bonds, hexaboride coatings show an increasing tendency to amorphous 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 pronounced visual colour impression is observed for LaB6-based coatings corresponding to a dark violet colour. Proper selection of process parameters for these coatings may widen the conventional spectrum of decorative coatings.
