Deposition efficiency, mechanical properties and coating roughness in cold-sprayed titanium

In the cold-spray process, metal powder particles develop into a coating as a result of ballistic impingment on a substrate. In cold-spray, compressed gas (air, nitrogen or helium), at pressures ranging between 1.4– 3.4 MPa (200–500 psi), but typically around 1.7 MPa (250 psi), flows through a manifold system containing a gas heater and a powder feeder. The pressurized gas is heated electrically to around 100–600 ◦C then passed through a Laval-type converging/diverging nozzle until the gas velocities reach supersonic speeds. The powder particles are introduced into the gas stream just in front of the converging section of the nozzle and are accelerated by the expanding gas. The powder feedstock is delivered on the high-pressure side of the nozzle by the metering device, which is heated and maintained at the elevated pressure of the manifold. During the supersonic expansion through the Laval nozzle, there is a temperature reduction. Thus, the temperature of the gas stream is always below the melting point of the particulate material, providing coatings developed primarily from particles in the solid state with very little oxidation [1–5]. As cold-spray is a 100% solid-state process, the deposition “in air” of titanium coatings without significant oxidation represent an important technical achievement. Titanium and its alloys are employed in corrosive environments, aerospace and bio-implants [6]. Beyond the solid-state characteristic, a fundamental feature of the cold-spray method is the concept of critical velocity (V ∗). For each coating and substrate combination there is a V ∗. Above the V ∗ the particles will have enough kinetic energy to be incorporated into a coating. Below the V ∗, the particles will be either reflected from the surface (bounced-off) or cause erosion of the substrate and any coating buildup which had begun. For particle velocities V > V ∗, the coating process occurs and the deposition efficiency is seen to increase with increasing V [1, 4, 5]. The actual mechanisms by which the solid-state particles deform and bond has not been well characterized. It seems plausible, though it has not yet been demonstrated, that plastic deformation may disrupt thin surface films, such as oxides, and provide intimate conformal contact under high local pressure, thus per-