Corrosion Behavior and Mechanisms of Thermal Barrier Coatings in the Presence of CMAS,CMAS+NaVO3,and CMAS+Sea Salt
As a key thermal protection technology for aeroengine turbine blades,thermal barrier coatings(TBCs)can significantly improve working temperatures and prolong service lifetimes.The corrosion of environmental sediments(CaO-MgO-Al2O3-SiO2,CMAS)is a significant reason for the premature failure of engine TBCs.The aircraft service environment is complex and often experiences desert,inland,ocean,and other regional environments;some molten salts and sea salts are often coupled with CMAS,which together cause multiple and complex corrosions of TBCs.However,there are few reports on the multivariate coupled corrosion behavior of TBCs in the presence of CMAS and salt.The corrosion behavior of Y2O3 partially stabilized ZrO2(YSZ)TBCs under the attack of CMAS,CMAS+NaVO3,CMAS+sea salt is studied.After corrosion under different conditions,the coatings are characterized using X-ray diffraction,SEM and other methods,and the effects of temperature and corrosion species on the corrosion behavior are analyzed.The results showed that temperature was an important factor affecting the corrosiveness of CMAS,CMAS+NaVO3,and CMAS+sea salt.Only at high temperatures did the coating undergo obvious damage when the corrosion agent was completely melted.As the temperature increased,the penetration ability of the corrosive agents was further enhanced.The type of corrosive substance is another factor that affects corrosion behavior.Compared with CMAS,CMAS+NaVO3 and CMAS+sea salt damaged the coating at lower temperatures.After holding for 4 h at 1 200 ℃,CMAS cannot completely melt,thus its damage to the coating is limited.It did not interact with or penetrate the coating.However,the CMAS+NaVO3 and CMAS+sea salt melted completely under these conditions.The molten CMAS+NaVO3 and CMAS+sea salt dissolved and penetrated the coating surface,leading to the phase transformation of the YSZ.The penetration of CMAS+sea salt was the most severe.At 1 250 ℃,all the three corrosive substances completely melted and corroded the coatings.However,CMAS and CMAS+NaVO3 exhibited small penetration depths owing to their relatively low fluidity.They mainly remained near the surface of the coating,causing delamination and cracking.In contrast,the molten CMAS+sea salt penetrated the coating in large quantities owing to its greater fluidity.After heat treatment at 1 250 ℃ for 4 h,CMAS+sea salt completely penetrated into the coating,and the penetration depth exceeded 400 μm.Consequently,although the surface stratification of the coatings after CMAS+sea salt corrosion was less evident than that after CMAS corrosion,more phase transitions and cracks appeared in the coating interior.The coexistence of salts changes the properties of CMAS and enhances the permeability of the melt,which enables the mixed melt to corrode the YSZ coating at a lower temperature and has stronger permeability.This poses a serious threat to the interior and bottom of the coating.Multivariate coupled corrosion in complex environments poses a serious threat to TBCs.Among these,the influence of sea salt on the CMAS melt is particularly obvious.The damage mechanism and potential threat of the mixed melt to TBCs when the salt is coupled with CMAS are analyzed and expounded,providing a theoretical basis for developing TBC materials with high resistance to the coupling corrosion of CMAS+salt.
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