Size effect of thermal expansion and thermal/intrinsic stresses in nanostructured thin films: Experiment and model

The thermal expansion coefficient of nanocrystalline materials and its dependence on grain size was investigated in two different model systems, soft metallic Cr and hard ceramic CrN thin nanocrystalline films, both composed of grains having a cubic structure and sizes ranging between 10 and 30 nm. The dominant contribution to the enhancement of thermal expansion in nanocrystalline materials with respect to their coarse-grained counterparts was identified in the interfacial area containing weakly bonded atoms. Based on the experimental results a model is proposed which aids in understanding the thermal expansion of nanocrystalline solids and of the origin and development of thermal stress in thin nanocrystalline films. This model was experimentally validated by the correlation between the thermal expansion coefficient and the material grain size, which was controllably varied by the film deposition process, coating architecture and thermal treatment. The number of atoms in grain boundaries and their bonding character were, in addition, found to be crucial for the development of intrinsic stress. Its increase with increasing volume fraction of grain boundaries is attributed to the enhanced diffusional flux of weakly bonded surface adatoms into this area and enhanced defect generation due to the higher sensitivity of grain boundary atoms to displacement by incident particles.