Gordon England Surface Engineering Forum

COLD GAS DYNAMIC SPRAY COATING PROCESS


Cold Spray Process Schematic

Schematic Diagram of Cold Spray Process



Dr Antolli Papyrin and colleagues at the Russian Academy of Sciences were the first to demonstrate the cold spray process in the mid-1980s.

The Cold Spray or cold gas-dynamic spraying process is the next progressive step in the development of high kinetic energy coating processes. Similar in principle to the other thermal spray methods, it follows the trend of increasing particle spray velocity and reducing particle temperature as with the HVOF/HVAF processes, but to a more extreme level that it could be asked whether the process fits under the description of thermal spray.

The process basically uses the energy stored in high pressure compressed gas to propel fine powder particles at very high velocities (500 - 1500 m/s). Compressed gas (usually helium) is fed via a heating unit to the gun where the gas exits through a specially designed nozzle (laval type mostly) at very high velocity. Compressed gas is also fed via a high pressure powder feeder to introduce powder material into the high velocity gas jet. The powder particles are accelerated  and moderately heated to a certain velocity and temperature where on impact with a substrate they deform  and bond to form a coating. As with the other processes a fine balance between particle size, density, temperature and velocity are important criteria to achieve the desired coating.
The particles remain in the solid state and are relatively cold, so the bulk reaction on impact is solid state only. The process imparts little to no oxidation to the spray material, so surfaces stay clean which aids bonding. No melting and relatively low temperatures result in very low shrinkage on cooling, plus with the high strain induced on impact, the coatings tend to be stressed in compression and not in tension like liquid/solid state reactions of most of the other thermal spray processes. Low temperatures also aid in retaining the original powder chemistry and phases in the coating, with only changes due deformation and cold working.

Bonding relies on sufficient energy to cause significant plastic deformation of the particle and substrate. Under the high impact stresses and stains, interaction of the particle and substrate surfaces probably cause disruption of oxide films promoting contact of chemically clean surfaces and high friction generating very high localised heating promoting bonding similar to friction or explosive welding.

Coatings at present are limited to ductile materials like aluminium, stainless steel, copper, titanium  and alloys. Hard and brittle materials like ceramics can not be sprayed in the pure form, but may be applied as composites with a ductile matrix phase. Substrate materials are also limited to those that can withstand the aggressive action of the spray particles. Soft or friable substrates will erode rather than be coated.

The cold spray process is still primarily in the research and development stage and only now becoming commercially available.

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