Analysis of thermal properties by scanning thermal microscopy in nanocrystallized iron surface induced by ultrasonic shot peening

Nanocrystallized iron surface layer obtained by ultrasonic shot peening was studied by scanning thermal microscopy (SThM) that allows thermal conductivity to be mapped down to submicrometer scale. It is found that the microstructures obtained by ultrasonic shot peening show different thermal conductivities that strongly depend on the grain size: the thermal conductivity of the nanostructured surface layer decreases clearly if compared with that of the coarse-grained matrix of the sample. Preliminary analysis shows that the decrease of thermal conductivity is mainly due to the decrease of the electron and phonon mean free path and to electron and phonon scattering at the grain boundaries. The results demonstrate that SThM can be used as a tool for microstructural analysis of ultrafine-grained surface layer.