Evolution of surface topography in dependence on the grain orientation during surface thermal fatigue of polycrystalline copper

Surface degradation due to cyclic thermal loading plays a major role in the Accelerating Structures (AS) of the future Compact Linear Collider (CLIC). In this article results on surface degradation of thermally cycled polycrystalline copper as a function of the orientation of surface grains are presented. Samples with different grain sizes were subjected to thermal fatigue using two different methods and were then characterized using roughness measurements and Orientation Imaging Scanning-Electron-Microscopy (OIM-SEM). Samples fatigued by a pulsed laser show the same trend in the orientation-fatigue damage accumulation as the sample fatigued by pulsed Radio-Frequency-heating (RF). It is clearly shown that [1 1 1] surface grains develop significantly more damage than the surface grains oriented in [1 0 0] and three reasons for this behaviour are pointed out. Based on observations performed near grain boundaries, their role in the crack initiation process is discussed. The results are in good agreement with previous findings and can be interpreted on the basis of existing theories. Finally the fatigue effects induced by the two different applied types of thermal cycling are compared.