Microstructural characterisation of titanium coatings deposited using cold gas spraying on Al2O3 substrates

Microstructure formation in cold gas sprayed titanium coatings was investigated using optical microscopy, scanning and transmission electron microscopy and X-ray diffraction. Optical microscopy was employed to obtain an overview about the microstructure of the Ti coatings and to determine their porosity. Scanning electron microscopy in combination with the electron back-scatter diffraction was used to quantify the grain and sub-grain size in titanium and to get information about the mean misorientations of sub-grains within the Ti grains. X-ray diffraction revealed microstrain in titanium before and after the cold gas spraying process, from which the changes of dislocation density were calculated. Transmission electron microscopy yielded information about the morphology of Ti/Al2O3 interfaces, about the size of Ti grains in different distances from the interface and about the mutual orientation of Ti and Al2O3 crystallites in vicinity of the interface. Particularly the information about the mutual orientation of Ti and Al2O3 crystallites supported the hypothesis that titanium can grow hetero-epitaxially on corundum. Furthermore, it was concluded from the experimental results that titanium particles are severely plastically deformed during the impact on the corundum substrate. This plastic deformation created microstructure defects in titanium that have a high stored energy. Together with a local increase of the temperature of the coating that is due to the impact of Ti on the substrate, the energy stored in the microstructure defects is considered to contribute to the local hetero-epitaxial growth of nanocrystalline Ti on Al2O3.