Using Stoney's equation [18] and a Tencor surface profiler, the intrinsic stress in each film was determined from the radii of curvature of the substrates (measured before and after deposition) and the film thickness. Surface roughness was measured using a Veeco Dimension 3100 atomic force microscope (AFM) operated in tapping mode and calculated from 10 × 10 μm square areas. The depth dependent compositions of the thin films were determined using a Thermo Scientific K-Alpha X-ray photo-electron Spectrometer (XPS) equipped with an Al anode X-ray source and with an Ar ion beam operating at 1–2 keV. The C1s peak (284.8 eV) was used as a reference for the XPS peak positions.
X-ray diffraction (XRD) data was collected using a Bruker D8 DISCOVER microdiffractometer fitted with a GADDS (General Area Detector Diffraction System), using CuKα radiation. Cross-section TEM specimens were prepared using a combination of mechanical polishing and Ar ion beam etching. The specimens were imaged using a JEOL2010 TEM operating at 200 kV.
The mechanical properties of the thin films were studied using a Hysitron 950 Triboindenter fitted with a Berkovich indentation tip. To determine the hardness, H and effective elastic modulus, E*, twenty-five indents were made up to a maximum force of 1000 μN and hardness values were calculated using the Oliver and Pharr method [19]. The maximum penetration depth was ~ 55 nm. The elastic recoveries (wp) of the coatings were calculated from load–unload curves [20].