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 p lasma sprayed alumina coating was studied with reference to the process cha rt formulated by the computational fluid dynamics (CFD) study reported earl ier. For this, three grades of alumina powder with mean sizes of 25, 40- an d 76 mum were utilized and sprayed under different injection velocities. Th e coating was characterized using scanning electron microscopy (SEM) for su rface structure: X-ray diffractometry (XRD) for phase changes and Micro-Vic kers hardness test measurements. The particle states were found to change d epending 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 X RD results which showed a steady decrease in alpha -alumina content with in crease in the injection velocity indicating a rise in the extent of complet e melting of alumina particles in the case of 25 mum 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 c lose 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. (C) 2001 Elsevier Science B.V. All rights reserved.