Effects of HfO$lt;inf$gt;2$lt;/inf$gt; Interlayer on Microstructure and Mechanical Property of Al$lt;inf$gt;2$lt;/inf$gt;O$lt;inf$gt;3$lt;/inf$gt; Thin Film on MgF$lt;inf$gt;2$lt;/inf$gt; Substrate

Al 2 O 3 thin films with or without HfO 2 interlayer between Al 2 O 3 and substrate were deposited on MgF 2 substrates by electron-beam evaporation technique. The as-deposited thin films were then annealed at 600 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 HfO 2 /Al 2 O 3 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 MgF 2 substrate can be protected from being drown out during scratch test. From GIXRD patterns, Al 2 O 3 still remains amorphous while HfO 2 interlayer transforms from amorphous to monoclinic phase after annealing process. It could be deduced that it is the phase transformation of HfO 2 interlayer that promotes the formation of the harder branch-like layer.