Atomization of a molten mixture of BaF2 and CaF2 has been found to be superior to crushing of bulk solid BaF2- CaF2 as a means of producing eutectic BaF2-CaF2 powder for use as an ingredient of the powder feedstock of a hightemperature solid lubricant material known as PS304. Developed to reduce friction and wear in turbomachines that incorporate foil air bearings, PS304 is applied to metal substrates by plasma spraying. The constituents of PS304 are:
- An alloy of 80 weight percent Ni and 20 weight percent Cr,
- Ag, and
- The BaF2-CaF2 eutectic — specifically, 62 weight percent BaF2 and 38 weight percent CaF2.
The superiority of atomization as a means of producing the eutectic BaF2- CaF2 powder lies in (1) the shapes of the BaF2-CaF2 particles produced and (2) the resulting flow properties of the PS304 feedstock powder: The particles produced through crushing are angular, whereas those produced through atomization are more rounded. PS304 feedstock powder containing the more rounded BaF2-CaF2 particles flows more freely and more predictably, as is preferable for plasma spraying.
Two well-known atomization processes (gas atomization and water atomization) have been investigated, in comparison with crushing, as means of producing eutectic BaF2-CaF2 powders (see figure). The particles produced by gas atomization are the most nearly spherical, but each batch contains only a small proportion of particles in the size range (20 to 100 μm) suitable for plasma spraying and a much larger proportion of undesired finer particles. Water atomization yields particles that are less spherical in character but still more rounded than those produced by crushing, and yields a greater proportion of usable particles.
As one might expect from the intermediate nature of the shapes of water-atomized BaF2-CaF2 particle shapes, the flow properties of PS304 powders containing water-atomized BaF2- CaF2 are intermediate to those of PS304 powders containing equal proportions of gas-atomized BaF2-CaF2 and those containing equal proportions of crushed BaF2-CaF2. Inasmuch as water atomization tends to be less expensive and better suited to high-volume production than is gas atomization, water atomization could be preferable for applications in which the shapes of the eutectic BaF2-CaF2 particles are not required to closely approximate spheres and the intermediate flow properties are acceptable.
This work was done by Christopher DellaCorte of Glenn Research Center and Malcolm K. Stanford of the University of Dayton.
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