Diagnostics and modeling of an argon/helium plasma spray process

The natural instability of the are in direct current (DC) plasma torches us ed in spray processing is one of the most important causes for variations i n heating of sprayed particles, leading to inconsistencies in the final coa ting quality. A relatively simple diagnostic system has been set up to moni tor the plasma jet instability, as well as some important process character istics. Effects of the operating parameters and the anode condition on prop erties of plasma jets, particle properties, and coatings have been measured . These results show that the inconsistency caused by the jet instability i nfluences the plasma spray process in several ways. The coating porosity an d the deposition efficiency can be correlated to an average jet length obta ined from a series of high speed images, Selected frequency peaks in the po wer spectrum of the acoustic signal are correlated with the average jet len gth, and these results are used to derive a simple control scheme, which ad opts a fuzzy look-up model indicating the condition of the anode. Increasin g the are current is the most effective way to counteract the negative effe cts of anode erosion.