Understand where the thermal spray coating thickness will vary and specify a part accordingly.
Typical Thickness by Process
The process will also set up a sensible thickness range for wear resistant coatings. As an example HVOF would be expected to produce a coating in the region of 100-400microns thick. For dimensional restoration work using thermal spray the process of choice would be arc wire spraying. The coating thickness can be built up to anywhere in the range of 500microns to several mm. Flame spraying also produces a coating of similar thickness to arc wire but with a more porous structure hence the finish thickness needs to be increased to allow for material removed during subsequent machining.
Why Deposits Build Up Unevenly
A point to note is that the coating can vary in thickness across a part. As previously stated the thickness can vary by plus or minus 50 to 100 microns on simple shapes such as round shafts, whilst complex sections containing shoulders, flanges or recesses can vary by a lot more plus or minus 100 to 200 microns.
Allowance for Grinding After Spraying
Typical allowance for grinding or other post-spray machining: 150-250 microns (depending on the coating process and target dimension). This would allow a 200-250 micron grinding stock for HVOF coatings such as those applied to a shaft (for a 50-60 micron coating and 200-250 micron stock the ground dimension would be around 50.00 mm ± 0.005 mm). For Flame Sprayed coatings a slightly greater allowance may be required due to the generally higher surface roughness.
How to Call Up Thickness on a Drawing
Finished dimensions must clearly indicate whether the dimension is sprayed or finished. g. 025mm). g. 15mm) and this should be clearly indicated on the drawing. A useful reference for Thermal Sprays is https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays/.
Bond Coat Thickness Adds to the Stack
In addition to the top coat many coatings are applied with a thin bond coat. This typically consists of a nickel-aluminium alloy of around 50-75microns in thickness and is usually applied before the top coat. Therefore this must also be included in the overall thickness of the coating and included when calculating the finished dimension from the drawing. It is not uncommon for the actual diameter after spraying to be greater than the dimension shown on the drawing if only the top coat has been specified.
Getting it right from the start of a project will avoid costly rework.
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