
* Porosity in Thermal Spray Coatings


Thermal spray coatings are susceptible to the formation of porosity due to a lack of fusion between sprayed particles or the expansion of gases generated during the spray process. The determination of area percent porosity is important to monitor the effect of variable spray parameters and the suitability of a coating for its intended purpose.


ASTM E 2109 Test Methods for Determining Area Percentage Porosity in
Thermal Sprayed Coatings
These test methods cover the determination of the area percentage porosity of thermal sprayed coatings. Method A is a manual, direct comparison method using seven standard images shown on figures in the standard. These figures depict typical distributions of porosity in thermal spray coatings. Method B is an automated technique requiring the use of a computerized image analyzer. The methods quantify area percentage porosity only on the basis of light reflectivity from a metallo-





* Coating Thickness
ASTM B 487 - Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of a Cross Section

This test method covers measurement of the local thickness of metal and oxide coatings by the microscopical examination of cross sections using an optical microscope.
Under good conditions, when using an optical microscope, the method is capable of giving an absolute measuring accuracy of 0.8 um.

The measuring device may be a screw (Filar) micrometre ocular or a micrometre eyepiece. An image splitting eyepiece is advantageous for thin coatings on rough substrate layers. The measuring device shall be calibrated at least once before and once after the measurement using a stage micrometre. The magnification should be chosen
so that the field of view is between 1.5 and 3 the coating thickness.
For the use of automatic image analysis see Section 18.5.5.
