Sputter deposition of decorative boride coatings

Abstract

Decorative hard coatings have to meet a variety of requirements such as attractive colour as well as high wear and corrosion resistance. After a review of PVD coatings used for decorative metal-finishing, an account is given of studies to extend the range of useful coating materials. Within the scope of the search for new colours several borides such as zirconium diboride ZrB 2, lanthanum hexaboride LaB 6 and zirconium dodecaboride ZrB 12 were evaluated using magnetron sputtering. The nitrogen flow in a partially reactive process was used as an additional deposition parameter to influence film composition and properties. The interrelationships between structure, chemical composition and mechanical as well as optical properties were investigated. By means of non-reactive sputtering, crystalline films containing the ZrB 2 or the LaB 6 phase were formed. In the case of sputtering from the ZrB 12 target, the ZrB 2 phase was accompanied by a highly disordered boron or boron-rich phase. The incorporation of nitrogen leads to increasing lattice distortion and amorphous film growth at high concentrations. Among the coating types with interesting properties are nitrogen containing films sputtered reactively from ZrB 2 targets that had an anthracite coloured surface and outstanding corrosion resistance. Violet coloured coatings deposited non-reactively from LaB 6 targets are also potential candidates for decorative applications.