17. Porosity of Ni-Based and Ti-Based Cold-Sprayed Coatings

Cold spray technology is aimed to produce coatings thanks to the high energy impact of metallic and ceramic particles on similar and dissimilar substrates. The high energy is provided to the impacting particles by high-pressure-carrying gases capable of accelerating the coating material to very high speeds in the order of 1000 m/s. Cold spray temperatures are in the order of 0.6T_m leading the sprayed particles to severely plastic deform at the contact with the substrate or with already deposited coating. In all the technological available coating processes, the intrinsic required properties are mainly thickness, porosity, adhesion strength, deposition efficiency and surface finishing. Among the latter, porosity is the coating property most influencing the mechanical behaviour of the substrate-coating system. The low processing temperatures minimize or eliminate phase transformations and reduce porosity and residual stresses that normally characterize high-temperature thermal spray processes. To take advantage of the material properties of the feedstock powder, cold spraying was developed because phase transformations, chemical changes, residual stresses and porosity are minimized, if not eliminated, in these coatings due to the low processing temperatures and high velocities. Because of its non-combustive nature, cold spray process is of great interest for spraying temperature-sensitive materials such as Ni and Ti. Ni-based coatings are used in applications where wear resistance, combined with corrosion resistance, is required. Ti and its alloys are extensively used for aerospace components that work till elevated temperatures like airframe and jet engine components. The aim of the present chapter is to describe the processing parameters, feedstock properties and materials coupling leading to the coating’s porosity variation and its effect on the mechanical behaviour of the deposited coatings.