Essential Principles of the Deposition Process

The core idea of Low Pressure Cold Spray is that fine metal particles in solid state are accelerated by a supersonic air stream to the speed of several hundreds meters per second. Colliding with a substrate during high-velocity impact, particles adhere to it, forming a continuous coating. Although the air is preheated in the spraying gun to 300 - 400ºC (See the clicakble figure on the left) the particles working temperature is much below melting point. The temperature on the treated surface does not exceed 150 - 200ºC and depends on the distance between the nozzle and surface, temperature mode and physical properties of materials.

Although in comparison with the well-established thermal spraying processes such as HVOF, plasma or arc spraying, there is no melting of metal powder in Low Pressure Cold Spray before impact of the particles with the substrate, both their thermal and kinetic energies are essential for the coating formation. A successful bonding can be achieved only when the temperature and velocity of the particles exceed their critical values. Deliberate variations in supersonic nozzle design and careful consideration of sprayed metal powders' properties may put new materials into practice. Thus, special nozzles have been developed for spraying babbits, which can be used for sliding bearings restoration.

Mechanism of Adhesion

Coating formation occurs as a result of a sequence of high-speed impacts of fine metals particles on a substrate or on the previous layer of deposited particles. It was established that there is a combination of some basic processes playing a crucial role in adhesion of metal particles with a substrate and with each other (See the clicakble figure on the right):

• high-velocity impact brings crystal lattices of metal particles and substrates into close contact which in turn leads to formation of metal bonds. This mechanism is similar to the mechanism of adhesion in welding by explosion;

• the fusion and spot microwelding of sprayed particles with the substrate can occur on separate burrs and surface irregularities due to localised heating after impingement caused by high friction and shear;

• intermolecular interaction of different material may occur in close contact of juvenile surfaces of these materials. A typical example of such a mechanism can be observed during deposition of an aluminium coating on a glass mirror;

• mechanical bonding of particles with irregularities on the substrate surface can play a certain role, in particular, in the presence of deep penetration and intrusion of particles into a substrate. Rough surface developed by, for example, sand blasting, may improve this type of adhesion.

The concrete parity of a relative role of the above mechanisms of adhesion in various cases may significantly vary from each other.