Non-ambient X-ray diffraction residual stress analysis of thin films: tracing nanosize-related effects on thermoelastic constants and identifying sources of residual stresses

The stress evolutions at non-ambient temperatures in Ni and Pd layers (the thickness of each metal layer was 50 nm) were investigated employing in-situ X-ray diffraction measurements in a temperature range between 173 and 523 K. Stress determinations were performed by analysing the X-ray diffraction data according to the sin²ψ method on the basis of the crystallite group variant. The stress evolution in the Ni layer was correlated with the change of its coherently diffracting domain size and its microstrain with temperature. The advantage of measurements at temperatures below ambient to distinguish the thermal stresses from the effects of stress relaxation and emerging secondary stresses, arising from thermally activated processes (such as grain growth), is highlighted in this work for the first time. The possibility of using the in-situ X-ray diffraction stress measurement to obtain coefficients of thermal expansion and thermoelastic slopes of thin films (with nanosized grains) is discussed in detail.