Impact Conditions for Cold Spraying of Hard Metallic Glasses

As compared to thermal spray techniques, cold spraying allows to retain metastable phases of the feedstock material like amorphous structures, due to lower process gas temperatures. Compared to crystalline metals, metallic glasses are brittle at ambient temperature but viscous at higher temperatures. Therefore, cold spray parameters must be optimized for conditions that allow softening of the amorphous spray material for successfully producing coatings. For this study, a FeCoCrMoBC metallic glass was used that in comparison to others offers advantages with respect to higher hardness, less costly feedstock powder, and minimum reactivity with the environment. Necessary impact conditions were investigated to meet the window of deposition in cold gas spraying. According to calculations and cold spray experiments, neither the glass transition temperature T g nor the melting temperature T m can describe required conditions for bonding. Thus, a so called softening temperature between the glass temperature and the melting temperature had to be defined to calculate the critical velocity of metallic glasses. With respect to the bonding mechanism, impact morphologies could prove that a transition to viscous flow gets more prominent for harsher spray conditions. By sufficiently exceeding the critical condition for bonding, coatings with rather dense microstructures can be processed at deposition efficiencies of about 70%. The coatings have a hardness of 1100 HV 0.3, but the results also demonstrate that further work is still needed to explore the full potential for bulk metallic glasses.