Experimental measurement of the residual stress field within thermally sprayed rolling elements

Abstract

A non-destructive experimental approach using an X-ray diffraction technique was used to investigate the generation of residual stresses in thermally sprayed rolling elements. The rolling elements were detonation gun coated balls and high velocity oxy-fuel coated cones. A modified four ball machine was used to perform rolling contact fatigue (RCF) tests on tungsten carbide cobalt (WC-Co) coated samples on steel substrate. RCF tests were conducted in conventional steel ball bearing and hybrid ceramic configurations. Residual stress measurements were performed at different sample orientations on different coating thicknesses and various substrate geometries. Residual stress measurements on as-sprayed samples, pre-tested samples, and after the RCF tests were performed during this study. This enabled the measurement of residual stresses generated during the thermal spraying process and due to the RCF tests. Residual stress measurements are also made on the failed areas of the coatings. Residual stress measurement results are presented in the form of principal stress values, using complex stress/strain relationships. These results indicate that residual stresses are critical to the performance of coatings. The generation of residual stresses is not only dependent upon the coating process but also on the coatings thickness and substrate geometry. RCF tests induce tensile residual stresses within the contact area and coating microstructure. Residual stress magnitude depends upon the test configuration and time of failure. Compressive residual stresses caused by the coating process are helpful as they improve the RCF life of the coatings. A multiple-cause diagram relating to the generation of residual stresses within the coatings has been presented and stress measurements have been explained with the aid of figures and scanning electron microscopy observations.