Suspension Plasma Spray
What is Suspension Plasma Spray?
Suspension plasma spray (SPS), is a relatively new technology that produces coatings with micro- and nano-scale features by using ultrafine particle feedstock (less than 5 micron). To facilitate the use of these ultrafine powders, the particles are suspended in solution providing them with enough momentum to carry them into the plasma plume and make a coating. Suspension feedstocks can cover a range of material compositions including ceramics and metal alloy blends. Because the powder feedstocks used are ultrafine, the resulting coatings have a wider range of structures that can be produced including columnar, segmented, and fully dense.
The Suspension Plasma Spray Process
Suspension plasma spray is a process that enables the use of thermal spray feedstocks too small for conventional plasma spray processes. These feedstocks come in the form of a slurry, with micron and submicron sized particles suspended in water, alcohol, or other solvents. Generally, the concentration of the fine particles in the slurry can be controlled and ranges from 5-80% by weight. This allows for the best combination of coating microstructure and deposition rate to be achieved.
During the SPS process, the thermal spray slurry is pumped to the outlet of the thermal spray torch and injected into the thermal spray jet. Once entrained in the plasma, the droplets fragment and the liquid phase evaporates leaving ultrafine particles accelerating towards the substrate. By using these ultrafine particles sizes it is possible to generate uniform coatings as thin as 25 microns thick.
Suspension Plasma Spray Microstructures
Suspension plasma spray coatings typically have finer feature sizes than conventional air plasma spray coatings whose feedstock is an order of magnitude larger. As a result of the use of ultrafine particle feedstocks, the coating microstructure can be controlled to generate microstructures similar to Electron Beam Physical Vapor Deposition (EB-PVD): fully dense or columnar.
By controlling the particle size, velocity, particle temperature, and application conditions, the following different microstructures can be formed depending on the need. There are four basic types of suspension plasma spray microstructures:
- Columnar: Columnar coatings are produced when fine particles with moderate momentum flow across the surface of the substrate at a shallow angle forming individual porous vertical columns. These high strain tolerant coatings often have columns described as “feathery” and can provide performance similar to EB-PVD columnar structures.
- Segmented: Segmented coatings are formed when higher momentum particles are applied on the substrate surface at or near a 90-degree impact angle. The segmentation of the coating is formed by the development of vertical cracks generated by the high coating surface temperature and deposition rate. This type of erosion resistant microstructure resembles a dense vertically cracked (DVC)thermal barrier coating.
- Dense: Similar to segmented coatings, dense coatings are produced by high momentum particles impacting the substrate at near 90-degree impact angle. However, the substrate surface temperature and the deposition rate are controlled to prevent vertical cracks from forming.
- Porous: Porous coatings are similar to columnar coatings without the formation of columns. This type of microstructure is highly porous and thus yields very low thermal conductivity and hence the greatest thermal protection.
In addition to the coatings listed above, it is also possible to generate layered coatings of different chemistries and structures to develop coatings with a combination of properties. For example, a columnar 7 weight% yttria-stabilized zirconia (7YSZ) coating can be combined with segmented gaolinium zirconate coating to generate a high toughness, strain compliant coating that is also Calcium-Magnesium-Aluminosilicate (CMAS) resistant.
Benefits of Suspension Spray Plasma Coatings
The suspension plasma spray process is a coating process that can be used in many industries including:
- Power generation
- Mechanical Engineering
Some of the most common uses of plasma spray are found in the aerospace, power generation and maritime industries. In these cases, thermal barrier coatings generated by plasma spray are applied to hot section turbine components to protect them from high-temperature exposure. Other industries use the plasma spray process to provide protection from corrosion, abrasive wear, erosion, or cavitation.
At Praxair Surface Technologies, it’s our goal to help you get more protection, customization, and performance from your parts. To do that, we work with you to select or develop coatings based on your operating environment and production requirements. Not only does Praxair offer suspension plasma spray coating services, but we also develop the materials and power feeder used in the process. Contact us for additional information on how we can help you meet your sprayed thermal barrier coatings requirements.