Influences of the deposition rate on the microstructure and hardness of composite films prepared by reactive ion-assisted coevaporation

Abstract

The influence of the deposition rate on the microstructure and hardness of TiO2-SiO2 and TiO2-MgF2 composites and their single component films prepared by reactive ion-assisted coevaporation in the deposition rate range 0.14 to 2 nm/s are investigated using an x-ray diffractometer, a transmission electron microscope, and a microhardness tester. It is found that the hardness of the composite films and their single components increase with a decreasing deposition rate. However, the hardnesses of TiO2-SiO2 and TiO2-MgF2 composite films were generally lower than that of pure SiO2 and MgF2, respectively, at the same component deposition rate. This indicates that the addition of TiO2 reduces the hardness of Si02 and MgF2 films. The ultrahigh hardness of pure MgF2 films deposited by ion assistance at the deposition rate of 0.14 nm/s probably results from the high packing density, low lattice defects, and the small grain size (<20 nm) with the preferred orientation of [110]. Furthermore, the homogeneous dispersion of these hard nanoscale MgF2 grains within the amorphous TiO2 matrix causes the hardening of the composite film.