Unmelted particles in plasma sprayed coatings

A technique is described for investigation of unmelted particles in plasma spraying and for porosity investigation in coatings. A study of unmelted particles is made. The technique is based on large area section (LAS) images, superimposed to obtain a densely packed “column” of images as a 3-D map of a certain volume of the plasma sprayed coating. Optical micrographs of a polished cross section of the coating were used. The LAS technique combines two parameters—magnification and field size. Such a combination is normally beyond the range of optical microscopy. The method makes it necessary to polish the sample again and again and after each polishing step align precisely the new LAS with the previous one. The unmelted particles are considered to have been spheroidized in the plasma plume during plasma spraying, but to have cooled below the melting point before reaching the substrate. Such particles are embedded in the coating as previously solidified, i.e. with globular shape. The 3-D map makes it possible to observe their distribution inside the coating. These globular particles represent defects in the deposited coating. Moreover they have a tendency to behave isotropically under the influence of stress or other physical factors and in this way serve in the anisotropic structure of the coating as centers of secondary defects deteriorating the functional properties of the coatings.