Cohesive and adhesive properties of nanocrystalline Ti thin films on polyimide substrates

In this work, cohesive and adhesive properties of Ti thin films on polyimide substrates were studied systematically as a function of grain size by in situ optical microscopy, resistance measurements and synchrotron X-ray diffraction under uniaxial tensile testing. The characteristic of cohesive and adhesive behavior can be well reflected in the stress-strain curves determined by synchrotron X-ray diffraction. Generally, the yield strength decreases while the cracking strain increases with increasing grain size, due to the constraint effect of grain size on dislocation activity. Different from the yield strength, the fracture strength is almost unchanged with grain size. The fracture toughness is found to increase as the grain size increases, which is attributed to the enhanced plasticity during fracture process. Additionally, the interfacial adhesion energy is determined to be 3.8 J m⁻². The buckling strain increases with increasing grain size as a result of enhanced plastic dissipation. The formation of patches, instead of buckles, in Ti film with smaller grain size is ascribed to the low deformation capability of the film.