Evolution Of Microstructure And Degradation Mechanism Of Performance In Thermal Barrier Coatings Corroded By Volcanic Ash Under High Temperature

It is well known that increasing the gas inlet temperatures improves gas turbine engine efficiency,thereby reducing operating costs.However,restricted by the melting temperatures of single crystal superalloys,this increased temperature leads to new challenges.To achieve this demand,thermal barrier coatings(TBCs)are applied to the hot sections of the engines to improve the life of the associated components and to satisfy the continuously increasing demand for efficiency.However,when a turbine operates in a harsh environment,for example a volcanic ash attack,sand and ash particles ingested by the engine could be deposited on the TBCs surfaces as molten calcium-magnesium-alumino-silicate(CMAS).CMAS melts and penetrates into TBCs at high temperatures,which causes a loss of strain tolerance and results in the premature failure of the top coat.It is recognized that the formation of CMAS is inevitable due to exposing the turbine to sand and ash;therefore,CMAS mitigation solutions are among the top challenges for materials scientists.To some extent,CMAS is the biggest weakness of the traditional 7-8 wt.%yttria-stabilized zirconia(YSZ)TBCs material.In recent years,the ingestion of volcanic ash(VA)by engines has been widely recognized as a potentially catastrophic risk for aircraft operation.Most of the current studies focus on the microstructure and phase changes of thermal barrier coatings after corrosion,but seldom involve the changes of thermal and mechanical properties of coatings.To contribute to a better understanding of these relationships,the present study combines the investigations of microstructure evolution and mechanical properties of the coating after VA corrosion.(1)The VA utilized in this study was obtained from the March 10th 2009 eruption of Sakurajima volcano,Kagoshima Prefecture,Japan.Particle size distributions of VA were obtained using a particle-size analyzer.In order to study the high temperature interactions between the VA and TBCs,the composition of the VA was measured by X-ray fluorescence(XRF)and its crystallinity was measured using X-ray diffraction(XRD).Differential scanning calorimetry(DSC)was used to study the onset of the phase transformation,melting and reactions between the VA and as-prepared ceramics.The differences in composition,phase structure and thermal properties between VA and artificial CMAS were compared.The results shown that using real VA can accurately simulate the real environment faced by TBCs.(2)Thermal conductivity,thermal expansion coefficient,hardness and Young's modulus of the coatings subjected to VA attack were investigated.The molten VA deposit was found to readily penetrate the APS YSZ top coat and filled the pores and interlamellar cracks.Sintering of VA-corroded coating subsequently took place,which was accompanied by the disappearance of interlamellar structure and nano-pores.Yttria is leached from YSZ and into the VA melt,which reacted with VA to form YIG,accelerating the detrimental phase transformation from t-ZrO2 to m-ZrO2.The thermal conductivity increases due to the presence of m-ZrO2 and ZrSiO4 which have higher thermal conductivity than t-ZrO2.The hardness and Young's modulus measured on the cross-section increased significantly after VA-corroded.The increases in the hardness and Young's modulus were primarily attributed to the sintering effect of the coating.(3)Due to the column structure of the EB-PVD coatings,VA penetrates almost the entire layer of YSZ along inter-column gaps,which severely damages the column morphology due to the sintering of the columns.The feather-like structure at the column edges is then filled with the VA.The thermal conductivity increases due to VA-corrosion which may reduce the insulating property of TBCs.The results of testing the indentation showed a significant increase in E and H due to the VA corrosion,even at a loading of 3 mg/cm2.The higher E and H values were due to the VA deposits cooling within the coating and the stiffening of the column structure.The stiffening of the microstructure limited the lateral movement of the columns,which thus reduced the strain tolerance of the coating.(4)The degradation mechanisms of phase transformation were investigated by in-situ high temperature X-ray diffraction.The experimental results confirmed the change in the phase structure from t-ZrO2 to m-ZrO2 due to depletion in Y and the formation of ZrSiO4 by the relevant chemical reaction.The phase transformation occurs when the temperature is reduced to 600? and the chemical reaction occurs after holding for approximately 2 h at 1150?.The melting point variation mechanisms of VA corrosion YSZ were characterized by DSC.The formation energy of VA corroded YSZ products were determined by first-principle calculations.The phase transformation and chemical reaction can change the thermochemical and thermomechanical properties of the YSZ coating.The results revealed the YSZ degradation mechanisms associated with the infiltration and chemical reaction of the VA deposition.(5)The key to affect coating performance is phase stability and chemical reaction.Based on the degradation mechanisms of YSZ exposure to VA,three samples(Al2O3-TiO2 doped YSZ,rare earth oxide doped YSZ and La2Ce2O7/YSZ double coating)were produced to study the effects of the amount of solute in coatings.It is believed that crystalline products with high melting temperatures stop further VA penetration.The Al2O3-TiO2 doped YSZ,rare earth oxide doped YSZ and La2Ce2O7/YSZ double coating have the potential to provide effective protection against molten volcanic ash.This study has provided new insights that can be used for the further development of VA-resistant TBCs.