Damage of Thermal Barrier Coated Superalloy Under Thermal Gradient Mechanical Fatigue

Thermal gradient mechanical fatigueThermal gradient mechanical fatigue (TGMF) tests were conducted on thermal barrier-coated (TBC) tubular specimens to investigate the damage behavior of the TBC under close-to-service conditions. Special attention was paid to the cracking behaviorCracking behavior of TBC under TGMF tests with different temperature ranges, mechanical strain ranges, and phase angles including in-phase (IP) and out-of-phase (OP) loading. Crack initiationCrack initiation, propagation, and coalescenceCoalescence within the ceramic top coat (TC) caused TBC failure during IP-TGMF tests with a 300–1000 ℃ temperature range and a mechanical strain range of 0.45–0.65%. Few cracks extend to the bond coat (BC) but did not further propagate into the substrate. Increasing the maximum temperature and mechanical strain range significantly changed the cracking behaviorCracking behavior of these TBCs. Cracks run straight through the TC and BC followed by partial penetration into the substrate. The OP-TGMF loading shortened the TBC cycles to failure and accentuated the TBC damage, with separation occurring between TC and BC as well as between BC and substrate. Severe delamination at the thermally grown oxide (TGO)/BC interface results in premature TBC failure when the TBC system reaches a certain critical thickness of TGO.