Structure and mechanical properties of nitrogen-containing Zr–Cu based thin films deposited by pulsed magnetron sputtering

Abstract

Pulsed-dc magnetron sputtered zirconium–copper films were deposited with a range of different compositions (of varying Zr/Cu ratio and nitrogen content). Adding nitrogen to the low-miscibility binary Zr–Cu system as a solution hardening and/or nitride-forming element permits the deposition of two- (or multi-) phase nanostructured coatings. Structure and morphology of the coatings was studied by means of x-ray diffraction and scanning electron microscopy. Elemental compositions and Zr/Cu atomic ratios were obtained by quantitative energy-dispersive x-ray analysis. Nanoindentation measurements were made to evaluate coating hardness and elastic modulus. Coating structure was found to depend on the chemical composition; at low nitrogen contents coatings exhibited a columnar morphology, while the maximum N2 flow rate used resulted in a compact and fully dense coating structure. ZrCu(N) films produced with little or no nitrogen (N2 gas flow rates of 0 and 1 sccm) showed a partially amorphous structure with broad, low intensity Zr and Cu x-ray diffraction peaks. An increase in N2 flow rate (3 sccm) developed coatings with nanocrystalline Zr and ZrN phases for the Zr-rich coatings, while increased amorphization, followed by Cu segregation, was observed for Cu-rich coatings deposited at the same N2 flow rate. At the highest N2 flow rate of 5 sccm crystalline ZrN-based coatings were produced. Zr-rich coatings deposited at 0, 1 and 3 sccm N2 flow rates (with Zr/Cu ratios of ∼2.2–6.2) demonstrated slightly higher hardness values than coatings exhibiting lower Zr/Cu ratios, while the elastic modulus in the majority of cases showed an opposite trend. This behaviour is shown to correlate well with film chemical composition and the expected mechanical properties of the resulting constituent phases—the exception being the nitrogen-free coatings which (surprisingly) appeared to develop a lower elastic modulus with increasing copper content.