Control of interparticle cohesion in PS304 plasma spray deposited solid lubricant coating powder feedstock

The effects of eutectic barium fluoride - calcium fluoride particle morphology, particle size, size distribution and relative humidity level on PS304 powder feedstock flowability have been investigated in an effort to optimize the plasma spray deposition process. The eutectic fluorides were fabricated by comminution (angular particle morphology) and by gas atomization (spherical particle morphology). The angular fluorides were classified by screening to obtain 38–45μm, 45–106μm, 63–106μm, 45–53μm, 63–75μm and 90–106μm particle size distributions and the spherical fluorides were screened to obtain 45–106μm particles. The fluorides were added incrementally to the other powder constituents of the PS304 feedstock: nichrome, chromia, and silver powders. A linear relationship between feedstock flow rate and concentration of the fluorides was found from 0–10wt% using a Hall flowmeter. For the angular fluorides, the flow rate of the feedstock decreased linearly with increasing fluoride concentration. Flow of feedstock containing spherical fluorides was independent of fluoride concentration. Flow was degraded with decreasing fluoride particle size and with increasing particle size distribution due to interparticle friction. The angle of repose was distinct with respect to physical properties of the fluorides. The Hausner Ratio was less sensitive, though these data behaved predictably. Feedstock containing 10wt% 45–53μm and 90–106μm angular fluorides and 45–106μm angular and spherical fluorides were dried in a vacuum oven and cooled to room temperature under dry nitrogen. The flow of these powders was studied from 2–100% relative humidity (RH). The flow rate was only slightly degraded with increasing humidity below 66%RH, and a greater effect was apparent above 66%RH. No flow was observed above 88%RH for feedstock containing 45–106μm fluorides. The feedstock with narrower fluoride particle size distributions allowed flow up to 95%RH. These results offer guidance that enhances the efficiency of the plasma spray deposition process and the commercial potential for this material system and may have applicability to other powders that do not flow easily, such as cohesive ceramics.