Role of particle injection velocity on coating microstructure of plasma sprayed alumina — validation of process chart

The role of particle injection velocity and size on the microstructure of plasma sprayed alumina coating was studied with reference to the process chart formulated by the computational fluid dynamics (CFD) study reported earlier. For this, three grades of alumina powder with mean sizes of 25, 40 and 76 μm were utilized and sprayed under different injection velocities. The coating was characterized using scanning electron microscopy (SEM) for surface structure, X-ray diffractometry (XRD) for phase changes and Micro-Vickers hardness test measurements. The particle states were found to change depending on the injection velocity and size in correlation with the process chart, as revealed in the form of unmelted particles or well spread splats in the coating surface microstructure. This was further confirmed by the XRD results which showed a steady decrease in α-alumina content with increase in the injection velocity indicating a rise in the extent of complete melting of alumina particles in the case of 25 μm mean size powder. The micro-hardness also varied considerably depending on the injection velocity, due to the changes in the extent of the melting of alumina particles. A close correlation between the particle states depicted in the process chart and the coating microstructure was observed indicating the validity of the process chart and its suitability to put into actual coating applications.