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 scann ing electron microscopy (SEM). The observation reveals that the fusion mech anism of large and thick ceramic particles is different from that of smalle r ceramic or metal particles. After introducing alumina particles into plas ma jet, the separation occurs at the 1-2 micron under the surface of partic les 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 a t 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 f rom the solid to be a single and connected sphere particles before impact t he substrate. This observation also provides an excellent method visualizin g the particle melting process in the plasma jet without a special equipmen t.