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 which may be undesirable. 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, which can limit 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 systems (the newly installed state-of-the-art Impact Innovations 5/11 and CGT GmbH Kinetiks ® 4000 47kW). The newly installed 5/11 system offers significant increases in operating temperature and pressure (N2 gas at up to 1100°C and 60 bar) over previous systems, delivering improved coating quality. The cold spray guns can be operated with or without pre-heating the gas and with a variety of spray nozzles. This enables the systems 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.
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