The depth distribution of residual stresses in (Ti,Al)N films: Measurement and analysis
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A method is introduced to determine the depth distribution of the residual stresses in (Ti,Al)N films. The films were gradually stripped by chemical corrosion, an optical system was designed to test the curvature change of the specimens, and the depth distribution of the residual stresses was calculated. The results show that the residual stresses increase gradually from the interface of film/substrate and reach a maximum value at the middle region, then decrease until the surface.
Hard thin films, such as TiN-system films, are widely used to protect metallic materials due to their excellent resistance to wear and corrosion. There is a rapidly growing trend to prepare these hard films by physical vapor deposition (PVD), such as arc ion plating (AIP) and magnetron sputtering, to satisfy the demand for ever-lower process temperatures. Previous research revealed considerable residual compressive stresses, on the order of GPa, as well as their nonuniform distribution along the thickness of deposited hard films.1,2 The status of stresses significantly influences the properties of a film–substrate system, such as adhesion and hardness. Therefore, accurate measurement of the depth distribution of residual stresses in hard films and study of the effect of deposition parameters on residual stresses are beneficial to know how to improve the properties of the film–substrate system by tuning the residual stress distribution. The x-ray diffraction (XRD) technique3 and the substrate curvature technique (SCT)4 are conventional methods for measuring residual stresses in thin films. Nevertheless, some defects and limitations exist in these methods, especially for the stress measurement of hard films. For example, the XRD technique has a rather narrow application for thin films due to thickness limitations, serious texture in the films, and the difficulty in determining the x-ray elastic moduli of films.5 Some researchers stated that the average stresses of (Ti,Al)N or TiN films found with XRD were intensively influenced by ion bombardment and increased with the film thickness,6–9 but the distribution of the residual stresses along the depth are still unknown. Although the conventional SCT was used to measure the stresses in various films, it is unsuitable because the stresses from hard thin films a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0363 J. Mater. Res., Vol. 22, No. 10, Oct 2007
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deposited on single side of substrate might be large enough to cause plastic deformation of the substrate, in which case the assumption of elastic deformation can not be met any longer. Recently, many researchers have also proposed other residual stress testing methods10–17 that were, however, not specifically designed for the study of the residual stresses of hard thin films. Therefore, in this work, the stripping layer substrate curvature technique (SLSCT), combining a chemical corrosion strip and conventional SCT, was specifically developed to
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