Thermal spray coatings are extensively used in aerospace, pump and valve, printing, paper and a variety of industrial components for enhancement and modification of surface properties. One of the key quality control characteristics of thermal spray coatings processing is thickness. There are two basic points of thickness control. If the component is to be used as is, then the as sprayed thickness is the only determining factor. If subsequent operations such as grinding of the thermal sprayed coating and such are to be performed, then the finished coating thickness also comes into play. We will briefly address the measurement of thermal spray coating thickness in this article for the beginner.

Many newcomer engineers and inspectors in thermal spray shops get confused when it comes to the measurement and reporting of thicknesses of flame sprayed coatings. To keep things simple, thermal spray coating thickness measurement methods can be divided into four techniques. Measurement using the flat anvil micrometer is the first option. Generally, the as plasma sprayed or as arc wire sprayed coating depicts a significant peak valley topology. Reading of the peaks is what is denoted by the flat anvil micrometer. The second method is the use of the vernier caliper and this also gives the value of the peaks of the flame spray coating. The third method is the use of pin micrometer. The high readings using the pin micrometer denote the peaks and the low readings denote the valleys. The fourth technique is the use of electronic gages such as the Fisherscope and such wherein an electronic indication of the thickness is reported based upon values from a sensor. A fifth method of measuring coating thickness is the use of metallographic techniques where the coating thickness is determined from a metallurgically prepared sample. Regardless of the method used to determine the thermal spray coating thickness, it is important that the inspection report clearly describe the method used in addition to the thickness value obtained. This is critical, because otherwise erroneous interpretation of data is quite feasible. For example a ten thousandths of an inch report based upon flat anvil micrometer reading is not the same as the low value readings of a ten thousandths of an inch obtained by a pin micrometer. In addition to final inspection acceptance or rejection criteria, in process coating thickness measurements produce good insight into the process behavior of the system components and value for statistical process control, especially in high volume thermal spray processing. For example, when the thermal spray gun parameters are running accurately to settings, drops in coating thickness values gradually could potentially be traced to powder feeder malfunctions that could be fixed and repaired prior to parts deviating beyond levels of acceptability. Informative articles such as these can be found at the thermal spray coatings blog entitled http://thermalsprayedcoatings.blogspot.com

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