High-precision determination of residual stress of polycrystalline coatings using optimised XRD-sin2ψ technique

The aim of the research is to optimise the XRD-sin2ψ technique in order to perform high precision measurement of surface residual stress. Residual stresses existing in most hard coatings have significant influence on the adhesion, mechanical properties and tribological performance. In the XRD-sin2ψ stress measurement, the residual stress value is determined through a linear regression between two parameters derived from experimentally measured diffraction angle (2θ). Thus, the precision coefficient (R2) of the linear regression reflects the accuracy of the stress measurement, which depends strongly on how precise the 2θ values are measured out of a group of very broad diffraction peaks. In this research, XRD experiments were conducted on a number of samples, including an electron beam evaporated ZrO2 based thermal barrier coating, several magnetron sputtered transition metal nitride coatings, and shot-peened superalloy components. In each case, the diffraction peak position was determined using different methods, namely, the maximum intensity (Imax) method, the middle point of half maximum (MPHM) intensity method, the gravity centre method, and the parabolic approaching method. The results reveal that the R2 values varied between 0.25 and 0.99, depending on both the tested materials and the method of the 2θ value determination. The parabolic approaching method showed the best linear regression with R2 = 0.93 ± 0.07, leading to high precision of the determined residual stress value in all cases; both the MPHM (R2 = 0.86 ± 0.16) and gravity centre (R2 = 0.91 ± 0.11) methods also gave good results in most cases; and the Imax method (R2 = 0.71 ± 0.27) exhibited substantial uncertainty depending on the nature of individual XRD scans.