DEFLECTION MEASUREMENTS AS METHOD TO DETERMINE RESIDUAL STRESS IN THIN HARD COATINGS ON TOOL MATERIALS

Abstract

Current techniques for residual stress measurement can be divided into two general classes: those based on direct measurelnent of the elastic strains in the film using X -ray diffraction; and those based on the associated curvature or deflection of a thin substrate. The most widely used deflection technique is to coat a relatively thin silicon substrate and then measure the curvature of radius resulting from the coating residual stress. However, since this residual stress strongly depends on the substrate material, it is desirable to develop a coating stress measurement technique applicable to any coated material. Thus, the purpose of the present work was to study the feasibility of the deflection technique for residual stress measurements of thin hard coatings on tool materials. This means, in practice, obtaining a suitable technique for mechanical thinning of coated tool materials. Residual stress values obtained using X-ray diffraction data (sin² ψ method) and literature values of the coating elastic constants were used for comparison. The deflection technique gave compressive residual stress values of 4·] ± 0·4 and 2·] ± 0·3 GN m^-2 in titanium nitride coatings on high speed steel and diamond coatings on cemented carbide respectively. Corresponding values obtainedfrom X-ray measurements were 3·9 ± 0·8 and ]·9 ± 0·4 GN m^-2. In summary, the major advantages of the deflection method are that no information about the coating elastic properties is required to calculate the residual stress and that it can be applied to most coated substrates, including technically important materials such as tool steels and cemented carbides.