On The Measurement Of Residual Stress And Fracture Toughness Of Thermal Barrier Coatings Attack By CMAS Glassy Deposits

Thermal barrier coatings (TBCs) have been widely applied in aerospace and otherfields, due to their excellent performances of heat protection, wear resistance, corrosionresistance, and allow for higher engine operating temperatures and therefore increasegas-turbine engines efficiency as the coated materials. However, the TBCs will failureduring engine in service, the spalling performance of TBCs depends on various factors,such as the complex structure and the extreme operating conditions. In this paper, theresidual stress distribution along the ceramic coating thickness direction and fracturetoughness change rules have been studied under the corrosion conditions at1200°C. Themain contents of this paper are stated as follows:Firstly, the residual stress distribution through the coating thickness directions andvariation with thermal cycles after corrosion in the top coat of APS8YSZ as well asEB-PVD8YSZ TBCs have been measured by Raman spectroscopy. Then using Ramanspectroscopy test the APS TBCs samples cross-section residual stress distribution andanalysis of the influence of different heating time on stress in coatings. The results showthat the as-deposited APS top coating has a low tensile residual stress near the surface andpresents compressive stress away from the surface and the compressive stress valuemaximum at the interface due to the influence of factors such as the preparation ofceramic thermal barrier coatings, the stress uneven distribution in coatings. The TBCssurface stress increased after high temperature corrosion, and with the increase of cyclingtimes, the compressive stress of near surface progressive increase, the stress changes smallnear the interface. The variation of residual stress within APS TBCs increased graduallywith heating time extension. The compressive residual stress within the EB-PVD TBCs islarger than APS TBCs as well as the similar changes process after thermal cycling.Secondly, the surface fracture toughness variation of APS8YSZ TBCs before andafter attack by molten glass deposits are estimated with modified Vickers indentation(LEM) method. The Vickers indentation technique results show that the fracture toughnessof the top coating changed from2.15MPa·m1/2to3.45MPa·m1/2under the suppression of117N, and the surface residual stress varied from-79.6MPa to-180.7MPa. The residualstress fracture toughness had increased after corrosion. Therefore, the thermal barriercoatings surface of mechanical properties affected by the corrosion.Thirdly, by using modified Vickers indentation (LEM) method which considering the influence of residual stress, studied three types APS8YSZ specimens interface fracturetoughness and residual stress. The molten glassy deposits are incorporated into TBCsspecimens deeply by long time high-temperature heating. The results show that thefracture toughness are range of0.12MPa·m1/2~0.72MPa·m1/2, the residual stress rangedfrom-44.8MPa to-268MPa. The Young’s modulus and hardness which considering thesize effect in the superalloy substrate materials were measured by nano-indentation theory.In general, the total performance of coated TBCs materials depends on variousfactors during service. The residual stress effect is one of important factor to determineoverall performance of coating system. In this paper, we obtain the evolution of fracturetoughness and residual stress of8YSZ TBCs after corrosion and thermal cycles. The studywill be significant in evaluating the durability of the8YSZ TBCs.