Defect formation in thin polyelectrolyte films on polycrystalline NiTi substrates

This paper reports on the mechanical properties of ultrathin PAA/PAH (polyacrylic acid/polyallylamine hydrochloride) polyelectrolyte films deposited by a layer-by-layer technique on a polycrystalline Nickel–Titanium (NiTi) substrate. Since thin polyelectrolyte films are potentially suitable coatings to reduce the Ni release in biomedical applications, the mechanical properties of the thin films were determined by applying monotonic and cyclic tensile strains of 5% and 3%, respectively. While single tensile strains up to 5% revealed the amazing strain to failure of the applied coating, cyclic strains resulted in defect formation within the polyelectrolytes.To provide a better understanding of the mechanisms that are determining the defect formation, macroscopic and microscopic defect localizations were determined by digital image correlation–and EBSD (electron back-scattered diffraction)–techniques. Defects emerged particularly within areas of elevated local strain differences and were predominantly observed in the vicinity of grain boundaries. To relate these findings to the transformation behavior of polycrystalline NiTi considering strain localizations and intergranular constraints, crystallographic data obtained from the EBSD measurements were correlated with the defect distribution. EBSD data revealed a distinct dependence of defect formation on misorientation of neighboring grains.