Shear strength determination using the nanoscratch technique and its application to thin solid films

Reliable measurement of mechanical properties of thin solid films has been challenging, despite widespread usage of thin films in applications such as semiconductor, magnetic storage, and microelectromechanical systems. Some of the challenges include instrument limitations and inadequacy of theoretical models to obtain quantitative prediction of thin film properties. In this article, we propose a technique to extract the shear strength of thin films from nanoscratch experiments using a contact mechanics analysis of a sliding sphere. Based on the stress field analysis by Hamilton, the stress status around the contact point is obtained at the initiation of yield, and is used to establish a direct correlation between contact pressure/surface traction and shear strength. Nanoscratch experiments were also performed on an extremely thin diamond carbon overcoat used in supersmooth magnetic storage disks, and the shear strength was successfully obtained using the proposed technique. These results were comparable with hardness values reported in the literature, assuming Tabor’s empirical relation (hardness ≈ 3*yield strength) and Tresca yield criterion. Finally, a finite element model was developed to simulate a rigid sphere sliding over a deformable solid to further verify the validity of the proposed model. The finite element analysis confirmed that the calculation results from the proposed relation are in good agreement with experimentally measured bulk property values of shear strength.