ICMCTF2016 Session B1-1: PVD Coatings and Technologies

The design and development of new coatings is enabled through the understanding of the structure-property relationship of materials and the control of the film’s residual stress, inherent characteristic of films deposited on a substrate. Over the last decades, a lot of effort in thin film coatings research has been directed towards understanding and controlling these stresses, and also on investigating the relationship between the deposition processes and the material’s properties. However, most of the work done have focused on “thick” films deposited on hard substrates, and no clear correlation between surface morphology and stress has yet been reported. With the growing interest generated by lighter and flexible polymeric materials as substrates for the development of next-generation products, there is also a fundamental need to understand the seeding and growth of physical vapor deposition films on such substrates. To get further insight of the structure/property relationship of very thin films (< 50 nm), we have deposited, by dynamic magnetron sputtering, Cr films exhibiting various residual stresses on glass and polycarbonate substrates, characterized their electrical conductivity and photopic reflection and imaged their surfaces with an atomic force microscope (AFM). This work presents the relationship between quantitative aspects of the surface morphology and both the properties and the residual stress of such Cr films.

We observed an increase in electrical conductivity and photopic reflection of the films with the increase of the deposition power. Our analyses further revealed that the surface morphology is directly responsible for the enhancement of these properties, as surfaces presenting the least air fraction ratio were the most conductive and most reflective. We also demonstrated that calculating the residual film stress is an incomplete way of assessing the structure of the film, as films having similar residual stress exhibited different properties. In conclusion, the surface morphology and its roughness distribution represent a convenient way to estimate the bulk density of sputtered thin films.

The purpose of this study was to measure the residual stresses on both ZrN coating and Zr interlayer of ZrN/Zr/Si bilayer specimens, with different Zr thickness ranging from 50 nm to 200 nm, and understood the role of interlayer in stress relief. It is commonly acknowledged that using metal interlayer could relieve the compression residual stress and enhanced adhesion of transition metal nitride thin film on metallic or Si substrates. The thickness of interlayer was usually ranged from 50 nm up to about 300 nm, which has been widely applied in industry. However, there are few studies on the accurate stress measurement of both top nitride layer and interlayer. The reason is partly due to the difficulties of stress measurement by X-ray diffraction (XRD) methods, especially for thin films with thickness less than 500 nm. Recently we developed a method combining XRD and nanoindentation (NI) to carry out the stress measurement on TiN hard coatings [1,2], where the stress can be accurately measured down to a thickness of 350 nm using lab X-ray source. The technique used XRD to measure the average X-ray strain (AXS) and nanoindentation to measure the elastic constant of the coating and thereby obtaining the accurate stress that is comparable to the stress determined by laser curvature method within an accuracy of 10%. In this study, the AXS plus NI technique was further applied on a bilayer system of ZrN/Zr/Si. The ZrN/Zr thin film was prepared using unbalanced magnetron sputtering deposited on Si substrate, where the thickness of ZrN was fixed at 1 mm with Ti interlayer thickness ranging from 50 to 200 nm.

XRD cos²αsin²ψ method was used to measure the residual stresses in both Zr interlayer and ZrN film at several azimuthal angles. Since grazing incidence XRD was used in the stress measurement, the influence of anisotropy of the ZrN thin film can be avoided. The extent of stress relief with different interlayer thickness was discussed based on both elastic stored energy and energy dissipation of plastic deformation.