Molten Silicates-induced Buckling Failure Mechanism And Pore Filling Behavior Of Thermal Barrier Coatings

Thermal barrier coatings(TBCs)can protect metallic components in hot sections in gas turbine engines from hot gases.However,gas turbine engines can sometimes suck silicate debris based on calcium-magnesium-alumino-silicates(CMAS),and then part of these debris will deposit on the surface of TBCs.Then the silicate deposits will melt at elevated temperatures,infiltrate into TBCs and corrode the coatings.Although substantial effort has been invested to understand the degradation mechanisms,an understanding of some degradation mechanisms is still absent.For example,the buckling of air plasma spray(APS)TBCs caused by CMAS corrosion.Besides,many strategies have been proposed to solve the CMAS problem,but so far no proposed strategy has been proved to be effective in mitigating CMAS while meeting all the other important requirements for TBCs materials.This means that developing new strategies to migigate CMAS damage is still needed.Therefore,this thesis investigated the buckling mechanism of APS TBCs under CMAS attack.Besides,pore filling behaviors of YSZ pellets and typical APS TBCs during CMAS corrosion processs were investigated.Finally,this thesis investigated whether introducing big globular pores into an APS TBC can impove its CMAS corrosion resistance.The main contents and conclusions of this thesis are as follows.(1)The buckling phenomenon of yttria-stabilized zirconia(YSZ)APS TBCs induced by CMAS attack was investigated.The results showed that the amount of infiltrated CMAS was not limited by the porosity of the APS TBC.Excessive CMAS infiltration can cause significant volume expansion(up to 32% in this experiment)of the top-coat.When constrained by the substrate,it can cause severe buckling of the coating at elevated temperatures.Such kind of buckling could cause severe damage and should be considered as an important failure mechanism of APS TBCs under CMAS attack.(2)The pores(D50 ranging from 6 to 77 ?m)were introduced to YSZ pellets using poly methyl methacrylate(PMMA)as pore forming agents.Then the pellets were sintered to remove the pore forming agents and to achieve a similar volume fraction of porosity with TBCs.After CMAS infiltration,only some small pores in the CMASinfiltrated zones were filled by CMAS,whereas all large pores(larger than 13 ?m)remain unfilled;besides,the results also showed that even open pores can resist filling by CMAS.This study indicates that introducing “CMAS-proof” pores into TBCs may be a potential way to mitigate CMAS damage.(3)The pore filling behavior in a typical YSZ APS TBC during CMAS infiltration process was investigated using scanning electron microscopy and image analysis method.After fully infiltration,the total porosity of the coating decreased from 11.4% to 4.8%.However,the porosities and microstructures of the top and bottom regions were different.For the top region,the microstructures changed significantly,and its total porosity increased to 14.3%;almost all the unfilled pores were globular pores,and nearly all the crack network disappeared.For the bottom region,the microstructures did not change significantly;its total porosity dropped to 2.7%;in terms of pore types,almost all crack network disappeared,while 48 vol.% of globular pores still existed;and larger globular pores were more likely to remain.These results indicate that introducing relatively large globular pores into APS TBCs may mitigate CMAS damage.(4)To investigate whether introducing big globular pores into a TBC can impove its CMAS resistance,the pore filling behaviors of two different APS TBCs(termed TBC1 and TBC2,respectively)were investigated.Compared to TBC1,TBC2 has more bigger globular pores,and it has less crack network pores.The results showed that after corrosion for 3 h,the filling percentage of globular pores in TBC2 was higher than that in the slight interaction zone of TBC1;this may relate to the shape of globular pores.Despite of the worse CMAS infiltration resistance,the resistance of chemical corrosion of TBC2 was stronger than that of TBC1;besides,after CMAS corrosion without substrates,TBC1 gradually bent,while the shape of TBC2 did not change;this indicates that introducing CMAS into big globular pores in TBCs may lower their corrosion degree and the likelihood of failure via buckling at high temperatures.