Optical properties and corrosion behaviour of sputtered Zr-B and Zr-B-N coatings

Abstract

In this paper, we present results on the structure, optical properties and corrosion behaviour of Zr-B and Zr-B-N coatings employing non-reactive and reactive d.c. magnetron sputter deposition. The addition of nitrogen reduced the grain size—coatings deposited at nitrogen flow rates f(N 2) of less than approximately 14 sccm (standard cm 3 min -1) were extremely fine grained, while nitrogen flow rates of more than 14sccm led to fracture-amorphous coatings with a very smooth surface. The hardness and abrasion resistance decreased with the increase in nitrogen content. Pure Zr-B coatings are silver-greyish with metallic brilliance but nitrogen changes the colour from dark grey or black at low contents to interference colours in the amorphous state. Ellipsometric measurements of the refractive index n and adsorption coefficient k supported the results derived from electron microscopy, colour measurements and mechanical testing. The transition point between the fine-grained and fracture-amorphous structure lies in the range of f(N 2) around 10 sccm. The black coatings deposited with such nitrogen levels showed good corrosion and abrasion resistance combined with a satisfying hardness. In some cases, increasing the nitrogencontent improved the corrosion resistance. In potentio- dynamic experiments, more positive free corrosion and pitting potentials were demonstrated. However, in salt spray testing and immersion testing using an artificial sweat solution, no beneficial effect of high nitrogen additions was noticed. Zr-B-N coatings deposited with nitrogen flow values in the range 10–20 sccm offer an excellent choice for decorative purposes, owing to their dark grey or intense black colour and good corrosion resistance.