Influence of Repeated Bending Loading on the Residual Stresses in Multilayer Coating Materials

Abstract

The technology of multilayer thin films to improve performance is widely used in the fields of semiconductors and functional composite materials. However, each layer of the multilayer film generates a residual stress during deposition due to differences in Young's modulus and coefficient of thermal expansion. In addition, the residual stress of the coating material is significantly affected by the mechanical load during use. In this study, the coated materials with deposited multilayer thin films were subjected to repeated bending loads, and the effects on the residual stresses in the thin films and substrates were investigated. Carbon steel (JIS: S45C) was used as the substrate material for the specimens. The thin films were deposited by sputtering. Three kinds of specimens were used, in which Cu, TiN and SiO2 films were deposited as a single Cu layer, a Cu/TiN double layer and a SiO2/Cu/TiN multilayer. The residual stress values of the Cu layer within the thin films were evaluated by X-ray stress measurement method using 311 and 222 diffractions of Cu. As a result, the tensile residual stress values in the Cu layers within each thin film were approximately 60~70 MPa for the single layer and 110~120 MPa for the double and multi-layer films. The residual stress values in the substrate, carbon steel, were evaluated using 211 diffractions of α-Fe. As a result, a tensile residual stress of about 30 MPa is existed. When all the coated materials were subjected to repeated bending loads, the change in the residual stress values of Cu layer showed that the residual stresses relaxed rapidly after several cycles of repeated loading and saturated to a value of about 30 MPa after 100 cycles. On the other hand, the residual stress evaluation by 222 diffractions of Cu showed that for all coated materials the residual stress values increased and decreased repeatedly with increasing number of repeated loadings. For the carbon steel substrate, the residual stress values hardly changed due to repeated loading.