Thermal Spray Metallographic Sample Mount Variables

Metallurgical quality control of thermal spray coatings is an important step for any applicator of this family of coatings because unacceptable structure can lead to premature failure. More importantly, spelled hard coatings can lead to secondary damage in critical aerospace applications and as a limiting case in the situation where there is a single engine aircraft as in most military vehicles, failure of the engine can lead to mission failure and even fatalities.

Metallurgical laboratory evaluation of thermal sprayed coatings includes but is not limited to metallographic examination of the sample. Additional tests such as erosion testing, hardness testing, tensile testing, etc may form part of the overall quality control process. While this could be the subject of an entire treatise, we will limit ourselves in this brief article to metallographic examination and in particular to the variables involved in the preparation of the mount itself.

For those that are not familiar in the art, successful metallographic evaluation begins with the preparation of a good quality mount. There are several variables involved in this step as discussed in what follows. The first and foremost variable is in the choice of the mount material itself. Bakelite is a commonly used material. Two-part epoxy compounds are also utilized by many metallurgists. To provide added strength, fine aluminum oxide powder is sometimes added to two-part epoxy compounds. For quick evaluations, hard mounts made of metal are also utilized in the industry.

When utilizing Bakelite as the material, one needs to control the variables of pressure, temperature and time of pressing to reproduce high quality mounts time and again. When two-part epoxy compounds are used, the mixing proportion of the two parts need to be controlled as well as the amount of hardening additives such as aluminum oxide need to be controlled. Mixing time and subsequent hardening time are also variables in the process.

The choice of metal used in the quick mount method is very critical. It is important that the metal be sufficiently hard so smearing of it does not occur during the subsequent grinding and polishing steps. Sometimes hard metal strips can be used juxtaposed to the coating surface during the mounting process for proper edge retention. In such cases, alteration of the edge retention material could producing varying effects during subsequent sample preparation processes.

As one can see, there are several variables even in the sample mount preparation process which if not controlled can lead to erroneous interpretation of coating quality. The best practice to avoid pitfalls and ensure consistent repeatable quality is to document the controlling practice of all these as part of the laboratory manual.

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