Shear strength determination using the nanoscratch technique and its application to thin solid films
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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. I. INTRODUCTION
The measurement of thin solid film mechanical properties has drawn significant attention in the literature as such films are used routinely in semiconductor, magnetic storage, and microelectromechanical system (MEMS) applications. Thin solid films of thicknesses less than several tens of nanometers have been used and reliable measurement of their mechanical properties is necessary. The mechanical properties of interest include elastic modulus, yield strength, ductility, and shear strength, which in the case of bulk materials can be readily measured using conventional uniaxial tensile and torsion tests. However, traditional testing methods are not readily applicable for extremely thin films. For example, Haque and Saif1 proposed an in situ MEMS-based force sensor to perform tensile testing inside a transmission electron microscope for sub-micron thick freestanding films. This method enables the direct tensile testing for such films, however, it requires the fabrication of MEMS structures, and cannot be used for existing films on engineering surfaces. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0277 2304
J. Mater. Res., Vol. 21, No. 9, Sep 2006
The scratch technique was originally developed by Mohs2 as a hardness measurement method. He developed a hardness scale ranging from 1–10, where 10 represents the hardness of diamond. Later it was found that a step increase in Mohs’ scale corresponded to an increase in Vickers hardness by a factor of 1.6.3 However, the scratch metho
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