Amorphous steel coatings deposited by HVOF and Cold Gas Spray processes

In this paper it is considered the deposition of a commercial steel powder with a chemical composition that allow to obtain amorphous coatings using thermal spray techniques. In fact, these processes are characterized by high cooling speeds (108+107K/s) of the particles after the impact upon the substrate. The powders were sprayed with two different processes: Cold Gas Spray (CGS) and High Velocity Oxygen Fuel (HVOF). A comparison between the samples obtained reveals that only the CGS coatings are completely amorphous while the HVOF ones present nanocrystalline phases, detected with XRD analysis and SEM micrographs. Furthermore, CGS coatings are more compact and show lower hardness, with a comparable Young modulus. An hypothesis is that amorphisation phenomenon observed is related to the plastic deformation at the impact (due to the high particles' energy), rather than to the temperature, with a mechanism that could resemble the severe plastic deformation process. Additional thermal treatments and mechanical tests are in progress to investigate the toughness and other mechanical properties.