People typically associate thermal spray coatings with wear or corrosion protection. However, by using non-conductive materials such as ceramics within a thermal spray coating it is possible to prevent the flow of electricity. This article will go through the process of creating an insulating coating using thermal sprays and how it can be inspected.
Why Some Coatings Insulate Rather Than Conduct
For electrical insulation purposes thermal sprays use ceramic particles which when solidified form a non-conductive solid which can be used to insulate conductive surfaces. Insulation is the primary function of the coating, surface protection being a secondary function.
Which Ceramics Get Used and Why
Most commonly alumina is used for electrical insulation as it possesses excellent dielectric strength and cost effectiveness. The addition of titania to alumina produces alumina/titania coatings that have improved toughness and thermal shock resistance. Where wear resistance is also required for example for components which experience mechanical contact then chromium oxide coatings can also be produced.
Where This Turns Up in Real Parts
Busbars pass close to earthed surfaces or other conductors in a machine such as an alternator. Induction coil formers, heater elements and internal components of rotating electrical machines which require insulation to operate at high temperatures. Thermal Sprays is covered in more detail at https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays/.
Getting the Thickness Right
thickness. Work backwards from the required dielectric strength (measured in kV/mm) and set the coating thickness to achieve this. Ensure subsequent personnel measure out the coating to the same thickness to avoid adverse effects.
Why Porosity Matters So Much More
When an insulating coating is being applied then the main concern is to keep the porosity down to a level that will not allow the electrical breakdown to propagate throughout the coating. On a wear resistant coating a few small pores would not cause any problems, but on an electrical insulating coating a single pinhole or void can create a path of least resistance for the electrical charge to follow.
Checking the Finished Coating
Holiday testing using high voltage to detect pinholes or thin areas and subsequent breakdown voltage testing to verify coating voltage to be used in service are the only sensible methods to verify a sprayed coating. In order to set up realistic standards for testing dielectric strength of sprayed coatings, the same methods as described by national measurement standards bodies have to be adopted for verifying parts that have been coated with thermal spray coatings.
The thermal spray coating can be a surface protection coating and also an electrical insulation coating. In order to be an effective electrical insulation coating, the material must be of the correct type and the thickness of the coating must be within specified limits. Also, sufficient testing must be performed in order to verify the coating.
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