High-temperature evaluation of solid lubricant coatings in a foil thrust bearing

Abstract

PS304 and Korolon™ 1350A foil bearing coatings were evaluated in startup/shutdown testing of compliant thrust pads against thrust runner disks. Tests were conducted at 540 °C, with pads loaded at a constant 3.44 kPa pressure while disk speed cycled between 0 and 19 m/s. As opposed to the case of uncoated Inconel X750 pads against uncoated Inconel 718 disks, PS304 disk coatings allowed hydrodynamic conditions to be more readily attained, as indicated by reduced speeds at which pad liftoff and touchdown occurred and lower coefficients of friction that existed at full speed. During low-speed portions of the cycle where direct sliding occurred, the PS304 disk coatings also lowered friction. PS304 disk coatings that were high-velocity oxyfuel (HVOF)-deposited attained these improved tribological performances following fewer required run-in cycles than those that were plasma-sprayed, as a result of lower attainable levels of as-finished roughness. HVOF deposition required reduced-size feed powder, with which PS304 disk coatings could be deposited using conventional propylene-fueled in addition to hydrogen-fueled HVOF systems. HVOF-deposited PS304 additionally possessed higher as-deposited strength than plasma-sprayed PS304. If instead the thrust pad was coated, with K1350A, even better hydrodynamic performance was attained with substantial further reductions in liftoff and touchdown speeds and coefficients of friction at full speed. When coupled with a PS304-coated disk the K1350A pad coating immediately provided this better hydrodynamic performance, with very few run-in cycles required before attainment. However, one drawback of K1350A pad coatings appeared to be an increased level of friction during low-speed sliding conditions where direct sliding contact with the disk occurred.