Spray technology comes in from the cold

Thursday, 13 November, 2003

Cold spray technology, or cold gas-dynamic spray technology, is a revolutionary and rapidly emerging industrial coatings technology, which offers wide opportunity for Australian manufacturing. Cold spray, or more appropriately 'room-temperature spray' technology, unlike traditional thermal spraying, applies metal and alloy particles at temperatures much lower than the melting temperature of either the coating or substrate.

The distinguishing feature of cold spray is the ability to produce coatings with a gas jet temperature, which is lower than other processes such as powder flame, wire arc, plasma arc and high-velocity oxygen fuel. This eliminates the detrimental effects of high temperature on coatings and substrates.

It is a revolutionary system that avoids the raft of high-temperature effects such as oxidation, vaporisation, melting, crystallisation, residual stresses and gas release.

Not surprisingly, Dr Mahnaz Jahedi of CSIRO Elaborately Transformed Metals offers enthusiastic descriptions of cold spray technology.

"It is the next milestone for the thermal spray industry," say Jahedi.

The CSIRO, and Dr Jahedi in particular, have made it possible for easy access to the development of cold sprayed, exotic coatings for Australian industry, by importing the first cold spray system to Australia.

The CSIRO says that there are useful applications for cold spray technology in just about any industry utilising thermal spraying: from the biomedical industry where it can be used for prostheses with improved wear characteristics, to the aerospace industry where it offers coatings of greater fatigues resistance, to the chemical and mineral processing and die casting industries, to applications in the electronics, paper, oil, gas and glass industries.

Dr Jahedi says, "By virtue of its 'low temperature' nature, cold spray technology is also expected to be useful for the 70 per cent of materials which could be spray coated but are ruled out by the high temperatures required by current thermal technology."

What is cold spray technology?

Conventional 'thermal spray' processes require the sprayed materials to be preheated so the particles are in a semi-molten state when they reach the substrate, allowing them to splash across the surface. But as the 'splats' cool, they contract slightly, creating residual (stored) stresses or flaws at the interface and within the coating that can cause defects later.

Cold sprayed materials typically remain at or near room temperature until impact, slamming into the substrate so fast (500-1500 metres per second) that a tight bond is formed without the undesirable chemistry changes and stresses associated with conventional thermal processes.

Dr Jahedi says, "Researchers believe this 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."

Cold sprayed materials experience little to no defect-causing oxidation during flight and exhibit remarkably high densities and conductivities once fabricated.

Other possible uses of the technique include fabricating layer by layer, low-defect small piece parts, joining chemically dissimilar materials with bonds that gradually transition from one material composition to another, and as a low-temperature alternative to welding.

Cold spray as a fabrication process also has some significant advantages in developing industrial prototypes and advancing new design quickly and comparatively inexpensively compared to the usual prototyping processes.

"Direct fabrication for more cost-effective product development is a major plus," says Dr Jahedi.

However, using the process for specific applications requires tailoring by CSIRO.

Dr Jahedi says research is required to identify the best materials, particle sizes and impact velocities as well as an examination into gas dynamics, plastic deformation and spray nozzle configurations for each job.

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