Abstract
Residual stresses in thin films are usually evaluated either by measurement of elastic strains in the film or from the curvature or deflection of a film coated substrate material (the Stoney technique). Both methods are prone to serious measurement errors particularly for thinner films or smaller curvatures or deflections. The Stoney technique is popular because the analysis does not require the elastic modulus of the film. However, residual stress can be determined more accurately using the film modulus. A simple strain–energy analysis using this modulus has been developed to investigate residual stresses in boric acid–sulphuric acid (BSA) anodised films, based on bending measurements of thin coupons. The method is an improvement over Stoney's equation because the square root of the thickness of the film, rather than the thickness itself, is involved, therefore reducing one of the larger sources of error. The modulus of the film was determined from nanoindentation measurements. The strain–energy method allows analysis of residual stress development in the coupons as functions of film thickness and time. The results show that the residual stress in BSA anodised films formed on high strength Al–Cu alloy is highly tensile with stress levels up to 400 MPa. The effects of coupon geometry on stress development were also studied and were found to influence the final levels of residual stress developed. These results have implications for the measurement of residual stresses by beam deflection methods.
Highlights
THE MEASUREMENT OF FILM RESIDUAL STRESSExperimental techniques for measuring residual stresses in thin films on substrates fall into two classes: those based on the direct measurement of elastic strains in the film using X-ray diffraction and other similar techniques, and those based on the associated curvature or deflection of the substrate material.In general, X-ray diffraction techniques are the most informative, allowing determination of all the principal stress components in the film, provided the elastic constants of both the film and the substrate material are known
These results show that the residual stress developed in boric acid-sulphuric acid (BSA) films is tensile for both the uniaxial and the biaxial case
4 CONCLUSIONS The results of these experiments show that the strain-energy method is a simple and viable method for determining the residual stress developed in anodic oxide films
Summary
Experimental techniques for measuring residual stresses in thin films on substrates fall into two classes:. When the elastic constants of the thin-film coating are not known, or the cost implications too great, the residual stress in films is usually determined using a specimen curvature technique based on a simple bending analysis first proposed by Stoney [1] in 1909. Even for crystalline materials where the elastic constants are known, this substrate curvature technique is often preferred since it is more convenient to use and easier to apply under difficult conditions Such conditions include in situ heating or cooling or stress measurement during the course of film growth or subsequent processing. Equation (10) allows the residual stress in the film to be determined from the output of strain gauge(s) attached to the substrate material It applies to the uniaxial stress case, but can be extended to biaxial stresses, when it becomes ( )( ) " c =.
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