  Thermal stability of nanocrystalline multilayer thin film is of paramount
importance as the applications often involve high temperature. Here we report
on the layer instability phenomenon in binary polycrystalline thin film
initiating from the grain boundary migrations at higher temperatures using
phase-field simulations. Effect of layer thickness, bilayer spacing and the
absence of grain boundary are also investigated along with the grain boundary
mobility of individual phases on the layer stability. Layer instability in the
polycrystalline film is shown to arise from the grain boundary grooving which
originates spontaneously from the presence of grain boundaries. Our results
show that the growth of the perturbation generated from the differential
curvature follows Plateau-Rayleigh instability criterion. Increase in layer
thickness, lower bilayer thickness as well as lower grain boundary mobility
improve layer stability. Phase-field simulations show similar microstructural
evolution as has been observed in our Zirconium (Zr)/Zirconium Nitride (ZrN)
system experimentally. Detail analysis performed in this work to understand the
mechanisms of layer instability leads us to predict measures which will improve
the thermal stability of multilayer nanocrystalline thin film.
