An Experimental Study of Powder Melting During Low Pressure Plasma Deposition

ABSTRACTLow pressure plasma deposition (LPPD), a recent advancement in plasma spray metallizing, is currently being developed for high performance materials applications. An experimental study of particle melting within the plasma jet was pursued to identify the effect of the process variables and the material properties on the resultant deposit. In tandem the experimental results have been utilized in the development and verification of a mathematical model for the melting of powder particles during the process.Two binary iron based model alloys - Fe-20 wt% Mn and Fe-20 wt% Cu - were plasma sprayed using Ma 2.4 and Ma 3 guns. Two different methods were used to evaluate the degree of particle melting within the plasma jet. The first method intercepts the particle path in the plasma jet with a glass slide, whereas in the second method the powder particles are collected in free flight using a powder collector (resolidification of powder particles occurs before they impact the collector walls). The droplets which impacted on glass slides and the collected (using the powder collector) powder particles were studied for mode and degree of powder particle melting using scanning electron microscopy. “Sweet spot” deposits (with no relative motion between the plasma gun and the substrate) were also made for the two model alloys in different size ranges using the Ma 2.4 and Ma 3 guns. The resulting deposits were metallographically evaluated. Mode and degree of particle melting injected under different process variables as well as the resultant deposit structures are presented.