In Cold Spray, powder particles (typically 10 to 50 µm) are accelerated to very high velocities (200 to 1000 m.s-1) by a supersonic compressed gas jet at temperatures well below their melting point. Upon impact with the substrate, the particles experience extreme and rapid plastic deformation which disrupts the thin surface oxide films that are present on all metals and alloys. This allows intimate conformal contact between the exposed metal surfaces under high local pressure, permitting bonding to occur and thick layers of deposited material to be built up rapidly. The deposition efficiency is very high, above 90% in some cases.
Whilst thermal spray is widely used in many applications, it uses thermal energy to melt or soften the feedstock. This can cause thermal degradation and partial oxidation of the coating material. Oxide formation is particularly undesirable if the coating is to be used for corrosion protection. For metallic materials that are very prone to oxidation, thermal spray needs to be conducted under protected atmosphere or a vacuum, introducing extra cost.
The heat input of thermal spray processes introduces residual stress into the coatings, limiting the thicknesses that can be attained. Furthermore, the thermal balance has to be carefully managed through part cooling and gun manipulation to avoid excessive internal stresses and, in the case of thermally sensitive substrates, potential substrate degradation.
With cold spray however, materials can be deposited without high thermal loads, producing coatings with low porosity and oxygen content.
TWI conducts research to explore the potential of cold spray and optimise the process for specific applications, for example:
- Repair techniques for lightweight aerospace alloys;
- Improved biocompatible coatings for medical implants;
- Thick metallic coatings for thermally sensitive substrates;
- Bond coatings for oxide ceramics such as thermal barrier coatings (TBCs)
TWI uses a high specification commercial system (CGT GmbH Kinetiks ® 4000 47kW) that can achieve process gas pressures of up to 40 bar and process gas temperatures of up to 800°C. The Active Jet Cold Spray Gun can be operated with or without pre-heating the gas and with a variety of spray nozzles. This enables the system to be configured depending on the requirements.
A number of materials have already been proven to be suitable for cold spraying. These include:
- Metals (Al, Cu, Ni, Ti, Ag, Zn, Ta, Nb)
- Refractory metals (Zr, W, Ta)
- Alloys (steels, Ni alloys, MCrAlYs, Al-alloys)
- Composites (Cu-W, Al-SiC, Al-Al2O3)
With these materials in mind, a wide range of applications can be explored, with possible end uses in sectors such as aerospace, automotive, oil and gas, power generation, motor sport, medical, petrochemical and electronics.
For more information, please contact us.
