Cold Spraying of Titanium: A Review of Bonding Mechanisms, Microstructure and Properties

Cold gas dynamic spraying (CGDS) is a relatively new branch of surface engineering that involves modification of the surface of substrates to provide specific engineering advantages, which the substrate alone cannot provide. Cold spraying, as a metal deposition technique, involves spraying of typically 10-40 μm particles which are accelerated by a propellant gas to 300-1200 m/s at a temperature well below the melting point of material, and upon impact deform and adhere to the substrate. The deposition process in cold spraying occurs in a solid state which results in reduced oxidation and absence of phase changes; whereas, in thermal spraying deposition occurs of molten or semi molten particles. Over the last decade the interest in cold spraying has increased substantially. Considerable effort has been invested in process developments and optimization of coatings like copper. However, bonding in cold spraying is still a matter of some debate. The most prevalent theory is that when a particle travels at a minimum required velocity the particle deforms at a very high strain rate upon impact and during this deformation thermal softening dominates over work hardening in impact zone and a material jet is produced. This material jet removes oxides from the surface of the materials and the metal-to-metal contact is established between the freshly exposed surfaces. However, precisely how this high strain rate deformation behaviour of material promotes bonding is still unclear and requires further investigations. This article provides a comprehensive review of the current theories of bonding in cold spraying based on numerical modelling of impact and experimental work. The numerical modelling of the impact section reviews adiabatic shear instability phenomena, critical velocity, critical particle diameter, window of deposition of particles, particle impact on various substrates and the role of adhesion and rebound energy. The review of the experimental section describes the shear lip formation, crater formation on the substrates, role of surface oxides, characterization of bond formation, role of substrate preparations, coating build up mechanisms and contributions of mechanical and metallurgical components in bonding. Cold spraying of copper and aluminium has been widely explored in the last decade, now it is of growing interest to the scientific and engineering communities to explore the potential of titanium and its alloys. Titanium and its alloys are widely util