Thermal Spray Coating Inspection and Quality Checks
Thermal spray coatings are very demanding so a thorough check of them prior to issue is required.
Check Coating Thickness First
The first thing to check is the thickness of the coating. This is usually the easiest parameter to check and must be compared to the required thickness as detailed on the part drawing. There are a variety of gauges on the market including eddy-current gauges for non-metallic coatings on steel and magnetic gauges for ferrous coatings. It is advisable to take a number of readings from across the coated part to allow for variations in spray angle and standoff distance.
Test Adhesion Properly
A pull-off adhesion test is used to check the adhesion of a coating to a substrate. In the test a small dolly is bonded to the surface of the coating. After hardening the dolly is pulled off from the coating. The force required for the dolly to be pulled off from the coating indicates if the coating has adhesion to the substrate. Accepted test methods and the minimum required adhesion for thermal spray coatings are described in publications of ASM International.
Assess Porosity Under a Microscope
The amount of porosity in a thermally sprayed coating is typically around 5-10% of the coating area. The amount of porosity is a critical factor in the coating’s ability to resist corrosion. A sample for porosity testing would consist of a cross-section sample, mounted and polished for viewing under an optical microscope. The porosity can be estimated and reported as a percentage of the coating area. It is recommended that the user agree a maximum amount of porosity with the applicator prior to application. See also Thermal Sprays.
Measure Surface Hardness
Verify that the coating has achieved the required hardness for the specified material. A reading below the expected reading could indicate incorrect spray parameters or even contamination of the feedstock.
Verify Surface Finish
For sealed or mating surfaces, the surface roughness of the coating is critical. For other applications, a methodical assessment of surface roughness is generally required against the specified value as detailed on the drawing against a recorded value using a profilometer (surface roughness tester) expressed in Ra.
Confirm Documentation and Traceability
Also check for documentation of the complete process. This includes information about the used feedstock, the spray parameters, the operator, possible pre- and postcoating treatments etc. Without it, the root cause of any failure in service can’t be found anymore.
A simple check at the point of receipt of the part, coupled with a proper documentation of the coating application process, will ensure that any part issued into service will perform as intended.
