When a part is subject to mechanical loads and at the same time needs to be electrically insulated, a ceramic plasma spray coating is the most practical solution.
Choose the Right Ceramic Material
Aluminium oxide (Al2O3) is the standard material for the electrical insulation of mechanical components. It has a good dielectric strength and wear resistance. Yttria-stabilised zirconia is used in situations where there is a lot of thermal cycling, as it has a large amount of expansion and contraction, without failing. The material chosen for a particular application will be a combination of the electrical, thermal and mechanical requirements of that part.
Understand Dielectric Strength and Thickness
A typical coating thickness of 250-500µm of aluminium oxide can achieve a dielectric strength of 15-40kV/mm, depending on the coating’s density and porosity. As coating thickness is increased, the breakdown voltage increases; however, there is a point at which the coating becomes too thick and excessive residual stress can occur. This should be taken into account by engineers specifying coatings for electrical insulation requirements.
Why Thermal Plasma Spray Is the Preferred Process
Spray coatings are applied using a process known as Thermal Plasma Spray. This process generates extremely high temperatures, greater than 8,000 °C, which are necessary to fully melt the feedstock and form a dense coating with low porosity. This is critical to the dielectric performance of the coating. Coatings applied using lower temperature processes, such as HVOF or Cold Spray, have high porosity due to the partial melting of the particles.
Prepare the Surface and Seal Afterwards
As grit blasting produces a clean, angular surface finish prior to coating this is superior for coating adhesion than a surface that has been contaminated during manufacturing or previously been coated. In addition, after coating surface connected pores can be filled with an epoxy or silicone-based sealant to prevent moisture absorption that can cause insulation to deteriorate over time.
Typical Applications to Bear in Mind
Motor shafts, bearing housings, and bus-bar supports are among the most common uses. For Thermal Plasma Spray, visit https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays/plasma-coatings/.
Specifying coatings correctly at the design stage to achieve correct thickness, porosity, and seal is critical to avoid unnecessary rework.