Thermal Spray Coatings thickness testing

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Measurement of thermal spray coatings thickness is as you may know, a critical quality control aspect. However, while this seems to be extremely simple, there sometimes seems to be confusing factors in techniques for thermal spray coatings thickness measurements especially in small production shops. I decided to concentrate on this aspect for this post. There are basically four techniques for performing thermal spray coating thickness measurements. The first method is the use of a flat anvil micrometer. In this case, the flat anvil of the micrometer will give the reading of the peaks of the thermal spray coating. If the component that has been thermalsprayed is to be used as is without subsequent grinding or other finishing operations, then this is the final coating thickness. The same is the case when using a vernier caliper; namely you get the reading of the peaks. The second method is the use of a pin micrometer. In this case, the lowest reading that you obtain is the thickness of the valleys and the highest reading you get is that of the peaks. If the difference between the lowest reading and highest reading is quite significant, then it simply means that the profile of the as coated thermal spray material is quite prominent. This could lead to some problems if the grinding allowance is lower than the difference between the valley readings and the peak readings. Quite simply upon grinding, the thermal spray coating will not clean up sufficiently and this may be cause for rejections. On the other hand, if the difference between the peak values and the valley values is not significant, then it means that the profile is fairly uniform. The third method of measuring thermal spray coatings thickness is by metallographic means whereby you attach a sample tab that is coated alongside the component and preferably bent to match the profile of the part being coated and then a metallurgical mount made of the section and then measured in an optical micrometer. Sometimes, depending upon customer requirements and parts sizes, actual parts themselves are mounted and metallographically evaluated and then valley thicknesses, peak thicknesses and average thicknesses can be reported. A fourth method is to measure coatings thicknesses by electronic gages such as Fisherscope and the like.

Obviously, other methods such as using height gages, etc are not addressed here. An important aspect of quality control is that along with the reporting of the thickness, the method of thermal spray coating thickness measurement MUST be stated in the inspection reports. Many times this aspect is overlooked and can lead to confusions on the part of the customer’s auditor since he is not then privy to this very important piece of information. Another case in point is that when electronic measurement techniques such as Fisherscope, etc are employed, there must be a hard correlation established for every batch between the electronic gage and a mechanical gage. In numerous instances, significant statistical process control charts and analyses are performed on coating thickness variations and control especially in high volume thermal spray; in such cases all of the data is suspect if the measurement techniques are not clearly understood and spelled out.