Melting behavior of alumina particles in plasma spraying

The collection of flying particles in plasma spraying has been performed by use of a chemical filter and a carbon adhesive tape and subjected to scanning electron microscopy (SEM). The observation reveals that the fusion mechanism of large and thick ceramic particles is different from that of smaller ceramic or metal particles. After introducing alumina particles into plasma jet, the separation occurs at the 1-2 micron under the surface of particles by the thermal stress. This separated thin layer prevents the internal thermal conduction in the particle and is heated up to completely melted by plasma flame. The melted part moves to the end of solid part and remains at the side edges of solid. At the same time the next separation and melting sequence starts at the solid surface followed by the melting parts apart from the solid to be a single and connected sphere particles before impact the substrate. This observation also provides an excellent method visualizing the particle melting process in the plasma jet without a special equipment.