Microstructural characteristics and corrosion properties of cold-sprayed coatings

Abstract

Cold spraying is a thermal spray process which enables production of metallic and metallic-ceramic coatings with dense (very low porosity level) and pure (low oxygen content) structures. Several coating properties such as corrosion resistance and electrical conductivity rely on these properties. Cold spraying consists of two processes: high-pressure cold spraying (HPCS) and low-pressure cold spraying (LPCS) divided by the pressure level used in the processes (40 bar versus 10 bar). Generally, cold spraying is based on higher particle velocities and lower process temperatures than in other thermal spray processes. The coating is formed in a solid state when powder particles impact on a sprayed surface with high kinetic energy, deform and adhere to the substrate or to other particles. Therefore, a high level of plastic deformation and adiabatic shear instability are required for a tight bonding between powder particles and for the formation of dense microstructures. Moreover, in cold spraying, many factors, e.g., powder characteristics and compositions, spraying parameters, and post-treatments affect the formation and properties of the coating.

This work focuses on the characterization of the microstructural properties (microstructure, grain structure, particle deformation, and fracture behavior), corrosion resistance (denseness, impermeability, and corrosion properties), and mechanical properties (hardness and bond strength) of the cold-sprayed metallic and metallic-ceramic coatings. Furthermore, the aim of this work is to find relationship between microscopic details and macroscopic properties and affecting factors for these in order to produce fully dense coatings using cold spray processes. The coating materials are pure metals: Cu, Ta, and Ni, metal alloys: Ni-20Cr, Ni-20Cu, and Ni-30Cu, and metallic-ceramic composites: Cu+Al2O3, Ni-20Cr+Al2O3, and Ni-20Cr+WC-10Co-4Cr. Structural details are characterized using electron microscopy techniques (SEM, FESEM, and TEM) whereas denseness and corrosion properties are evaluated with corrosion tests (open-cell potential measurements, salt spray tests, and polarization measurements).

Typically, cold-sprayed coatings appeared to be dense without porosity or other defects in their structures according to visual examinations. However, corrosion tests revealed through-porosity in the coating structures, indicating an existence of weak points inside the coatings. Therefore, denseness and impermeability play an important role in the corrosion resistance of the coatings. Good corrosion resistance is based on the formation of a protective oxide layer in case of passivating metals and metal alloys. Fully dense coating structures are required for the capability of the cold-sprayed coatings to act as real corrosion barrier coatings and to perform well in all corrosion tests. Three ways to eliminate or decrease the number of weak points and defects in the coating structures were found. Firstly, the denseness improvement was done with an optimal combination of powder and spraying parameters (HPCS Cu, Ta, and Ni coatings). These combinations are strongly material-dependent. HPCS Ta coatings also have similar corrosion properties with corresponding bulk material, indicating excellent corrosion protection. Secondly, the denseness of metallic coating can be increased by adding ceramic particles into the metallic powder as a powder mixture. These hard particles keep the nozzle clean; activate the sprayed surface; and reinforce the coating by hammering the structure. The significant denseness improvement was observed with HPCS Ni-20Cr+Al2O3, HPCS Ni-20Cr+WC-10Co-4Cr and LPCS Cu+Al2O3 coatings. In addition to these, thirdly, the denseness improvement of HPCS Ni-20Cu coatings was done with heat treatments due to the void reduction and recrystallization.