Abstract
Somewhat unconventional plasma sprayed TBC systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. Approximately 250 µm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, in order to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur contents of 1 ppmw, total coating separation took place at relatively short times (200 hr at 1100oC). Reductions in the sulfur content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen annealing in the lab (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5-10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. To subvert this mechanism, samples surfaces of melt desulfurized PWA 1484 were EDM’ed with a grid of grooves or ribs (~250 µm wide and high), resulting in a segmented TBC surface macrostructure. Now failure only occurred as independent single segments events. For grooved samples, 1100 C segment life was extended to ~1000 hr for 5 mm wide segments, with no failure observed out to 2000 hr for segments ≤ 2.5 mm wide. Ribbed samples were even more durable, and segments ≤ 6 mm remained intact for 2000 hr. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidative effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low sulfur substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.
