Effects of HfO2 Interlayer on Microstructure and Mechanical Property of Al2O3 Thin Film on MgF2 Substrate

Al2O3 thin films with or without HfO2 interlayer between Al2O3 and substrate were deposited on MgF2 substrates by electron-beam evaporation technique. The as-deposited thin films were then annealed at 600 degrees C for 1 h to promote crystallization. The microstructure, IR transmittance and mechanical properties of the as-deposited and annealed samples were investigated by field emission scanning electron microscopy (FE-SEM), grazing incidence X-ray diffraction (GIXRD), Fourier Transform Infrared (FTIR) spectrometer, nanoindentation, and scratch tests, respectively. FE-SEM results show that a new branch-like layer is generated in annealed HfO2/Al2O3 double-layer thin films. The hardness of the newly formed layer is considered to be larger than 17.5 GPa. Because of the high hardness of the new layer, the MgF2 substrate can be protected from being drown out during scratch test. From GIXRD patterns, Al2O3 still remains amorphous while HfO2 interlayer transforms from amorphous to monoclinic phase after annealing process. It could be deduced that it is the phase transformation of HfO2 interlayer that promotes the formation of the harder branch-like layer.