Fatigue of Thick Thermal Barrier Coatings

Abstract

Thick thermal barrier coatings (TTBCs) of plasma-sprayed 8% Y,O,-ZrO, were fatigued in compression as part of a reliability and durability study to evaluate their potential use in high-performance diesel engines. Test specimens were designed to test the bulk ceramic uniaxially, independent of the substrate. A test machine was designed to alleviate the mechanical gripping and alignment difficulties associated with cyclically stressing brittle ceramics in compression. Higher fatigue limits, 375 vs 200 MPa, were observed at 800°C than at room temperature. Specimens tested at room temperature after high-temperature compressive cycling also had higher fatigue limits, indicating that the strengthening was permanent. At temperatures of SOO'C, the coatings showed evidence of low-temperature, pressure-induced sintering. The extent to which sintering occurred was determined by studying the change in the elastic modulus as a result of the application of varying temperatures and static stresses.

Stiffness of Plasma Sprayed Thermal Barrier Coatings

“…Sharp changes in stiffness after short sintering times have also been observed by others [28][29][30]. While pronounced sintering of zirconia powders is not generally expected at temperatures below 1400 • C, detectable sintering effects have been reported in plasma-sprayed zirconia at temperatures as low as 800 • C [31].…”

In Situ Observation of Crack Behavior in Compressively Loaded Plasma‐Sprayed 7‐wt%‐Yttria‐Stabilized Zirconia

“…For example, there is evidence that the compressive modulus of some plasma-sprayed materials increases under increasing applied loads. This has been observed in plasma-sprayed 8 wt% yttriazirconia 1 and alumina. 2 It has been speculated that the increase in modulus is a result of microcrack closure caused by loading of the plasma-sprayed coating.…”

“…Although the bulk modulus of dense zirconia is ϳ200 GPa, 5 current values are within the same order of magnitude as other reported values for the compressive modulus of plasma-sprayed YSZ. 1,6 The region investigated in the ESEM during loading was the outer surface of the YSZ tube; this region was chosen because it 960 journal contained cracks of various widths oriented parallel, perpendicular, and off-axis (or angled) with respect to the applied load. Figure 2 shows the specific regions of longer cracks investigated in this study under no load; these included two cracks oriented vertically and three cracks oriented horizontally with respect to the applied stress.…”

Microstructures and Properties of Plasma‐Sprayed Segmented Thermal Barrier Coatings

“…During the development of thick TBCs, it turned out that the thermal shock resistance decreased with increasing coating thickness. [2][3][4][5] The major reason for this phenomenon is the strain energy stored in coatings during thermal cycling. This energy typically increases with increasing coating thickness.…”