Coating thickness can be controlled to a large extent on plasma spray coating jobs but needs to be considered before the work is started and specified on drawings.
Typical Thickness Ranges
Typical thickness for wear coatings are between 0.2 mm and 0.5 mm + – 0.1 mm (as sprayed). Typically plasma spray thermal barrier coatings are applied to increase high temperature part surface temperature resistance and therefore are applied to a greater thickness.
How Thickness Affects Coating Properties
Increasing the thickness of the deposit also increases residual stress in the coating. While this stress can be released by cracking, it can also lead to poor adhesion and increased porosity. As shown in Fig. 5, porosity and density of thermal spray coatings are affected by pass count and standoff distance.
Post-Spray Machining for Tight Tolerances
As-sprayed coatings are typically of a rough surface nature and therefore require subsequent grinding or turning in order to achieve the required tight tolerances. As an example, tolerances of plus or minus 0.025 mm can be achieved by careful grinding of the coating, although this will depend on the type of coating that has been applied and the geometry of the substrate.
Masking and Dimensional Allowances
For cylindrical features such as bores, threaded holes, and shoulders that require to be left uncoated, masking must be applied prior to application. The edges of the mask create a step of approx. thickness of coating, which must be allowed for by creating a chamfer or relief in these areas.
If you are looking for Thermal Plasma Spray, see //www.poeton.co.uk/surface-treatments/thermal-metal-sprays/plasma-coatings.
Specifying Thickness on Drawings
Indicate on your drawing the minimum and maximum coating thicknesses required, i.e. indicate if the thickness is to be measured as-sprayed or post finish machined. Indicate also any areas to be left uncoated and clearly mark any coating boundaries, especially where there is to be no coating.
Inspection Methods
The thickness of the coating can be measured with eddy current or magnetic induction gauges. Metallography of a cross-section can also be used to measure coating thickness with very tight tolerances if required.
A simple drawing, with sensible tolerances, and some early discussions with your coating supplier will save a lot of time and money on your plasma spraying job.
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