| Russian inventors first recognised its potential significance of cold spray technology while conducting high-velocity wind tunnel tests at the Institute of Theoretical and Applied Mechanics of the Russian Academy of Sciences in Siberia in the early 1990’s. Using experimental and computer-modelling, a consortium of eight US companies are collaborating at Sandia National Laboratories in Alberquerque (USA), to improve the fundamental scientific understanding of this emerging manufacturing technique. The Process Cold spraying involves injecting microscopic powdered particles of metal or other solids into a supersonic jet of rapidly expanding gas and shooting them at a target surface. When these particles hit the substrate, they ‘splat’ so hard they stick and form a coating. Differences between Cold Spraying and Thermal Spraying Cold spraying more appropriately might be called ‘room-temperature spraying’. Conventional thermal spray processes require preheating of the sprayed materials so the particles are in a semi-molten state when they reach the substrate. This allows them to splash across the surface. But as the ‘splats’ cool, they contract slightly, creating residual stresses or flaws at the interface that can cause defects later. By contrast, cold sprayed materials typically remain at, or near, room temperature until impact, slamming into the substrate so fast (500-1,500 m/s) that a tight bond is formed without the undesirable chemical changes and stresses associated with conventional processes. Areas not Fully Understood Although the science behind this bonding process is not yet well understood, the researchers think the high-velocity impact disrupts thin metal-oxide films on the particle and substrate surfaces, pressing their atomic structures into intimate contact with one another under momentarily high interfacial pressures and temperatures. Characteristics of Cold Sprayed Coatings Unlike thermal-sprayed materials, cold-sprayed materials experience little to no defect-causing oxidation during flight and exhibit remarkably high densities and conductivities once fabricated. In addition, deposition rates comparable to traditional thermal spray processes can be achieved. Carrier Gases Light gases such as nitrogen and helium are preferred due to their low molecular weight. This means their sonic velocity is as high as possible. While this rationale favours helium, economics favour nitrogen, unless the helium can be recycled. Pressure Effects Cold spraying has the advantage that it can be carried out at atmospheric pressure. Other processes require lower pressures such as vacuum to achieve similar quality coatings. While this rule holds true for materials such as copper, there are exceptions such as titanium which is more sensitive. Feed Materials It is believed that the process works by plastically deforming the materials as the particles hit, which disrupts surface oxides and forces the metal into intimate contact with the underlying material. Thus, it is suited to ductile materials. Non-ductile materials such as ceramics can't be sprayed as a pure material, but can be applied using ceramic/metal composite powders. In this case the metal acts as a ductile matrix. Spray Distance Unlike thermal spray techniques, heat input into the substrate from a flame or plasma is not a problem, so shorter distances can be used. Typical stand-off distances are about 1cm, while it is thought distances of 4 to 5cm will also work as the process is fairly forgiving. Nozzles While the design of the nozzle will determine how the particles are accelerated, it has not yet been determined what configuration is better. Howvere, it is thought that a longer (10‑20cm) nozzle is better. The tradeoff is that if when nozzle length gets it too long wall friction effects start to play a role. Spray materials and feed gases also play a role in nozzle design. Researchers have also been experimenting with round and rectangular nozzle holes. Rectangular nozzles have the advantage that you can put down a coating that is maybe 1cm wide in a single pass with reasonably sharp edges and without the need to do much masking. Larger openings can be used provided that enough gas can be supplied. In terms of acceleration, similar values can be achieved regardless of the orifice geometry. Feed Particle Size Feed particle size is extremely important. Ideally, powders should have as fine a material as possible, with the low end being defined by the fact that when this supersonic gas stream hits the target surface, you get a shockwave on that surface. When you get down below five microns (depending on the mass and momentum of the particles, they’ll follow the gas flow and decelerate near the surface. So typically, particles in the 5‑15 micron range are optimal, although, some materials up to as high as 30 microns and still get decent results. Substrate Substrates choice is limited by their ability to absorb the impact energy from the projected particles. Ceramics and metals are generally fine, but plastics and composites tend to be cut or the particles become embedded in the substrate rather than forming a coating. In short as ‘hard’ substrates can be coated by cold spraying. Bond strength and coating thickness buildup are also influenced by the substrate. Researchers have found that when spraying onto ceramics, it is often helpful to deposit a thin layer of aluminium down first (aluminium will stick to just about anything) and then deposit other materials on top of that. When copper, for example, is sprayed directly onto aluminium oxide, a thin layer can be deposited but it is difficult to get the coating to build very well. The reason for this is nit yet understood. Summary Although the process is still clearly in the experimental stage, the consortium behind cold spray research plan to use the technology to eventually create tough coatings on aircraft engine components and to deposit layers of conductive metals onto substrates for use as heat‑tolerant under‑bonnet automobile electronics. Other possible uses of the technique include fabricating layer‑by‑layer low‑defect small piece parts, joining chemically dissimilar materials with bonds that gradually make the transition from one material composition to another, and as a low‑temperature alternative to welding. |