Stress evolution mechanism of NiCr thin films on titanium alloy

Abstract

The stress distribution associated with the film under tensile loading of the substrate was investigated by the analytical model proposed by F. Ahmed et al. The interfacial shear stress was evaluated by the modified analytical model by B. F. Chen et al. The validity of both the models has been tested on samples with NiCr thin film and titanium alloy substrate. These samples were progressively tensile tested and subjected to intermittent scanning electron microscopy for microstructural investigations. Insitu microscopy was adopted to identify the onset of cracks during tensile loading and the crack spacing is measured. Finite Element Analysis (FEA) is performed on the film-substrate system to study the stress pattern in the film and at the interface. The interfacial shear stress follows the modified shear lag model and is in good agreement between the stress profiles proposed in the model and the FEA results. The evolution of the tensile stresses and the cracking behavior are mapped as a function of the applied strain in the analytical model. The concept of critical intercrack spacing is introduced and the stress relaxation as a function of the inter crack spacing is discussed. The interfacial shear stress evaluated is found to be in agreement with the yield shear strength of the substrate.