Investigation Of CMAS Corrosion Mechanism And CMAS-Resistant Thermal Barrier Coatings

Thermal barrier coatings（TBCs）is applied on the critical components of gas turbine engine to insulate the thermal conduction and decrease surface temperature of metallic component.Calcium-magnesium-alumino-silicates（CMAS）corrosion of TBCs has become more critical with the increasing of engine-operating temperature.CMAS arises from the volcanic ash,dust and fine sand in the intake air,which will deposit on the surface of the TBCs during engine operation,finally form a CMAS melt when temperature exceeds the melting point.CMAS melt could severely corrode the state of the art 7-8 wt%yttria-stabilized zirconia（8YSZ）TBCs,thereby causing the premature failure of TBCs.Currently,the CMAS corrosion has significantly affected the reliability of TBCs,and needs to be addressed urgently.In this thesis,the CMAS corrosion behavior of classical 8YSZ TBCs was investigated firstly,especially,the corrosion method was focused.The CMAS corrosion mechanism of 8YSZ was elucidated,and the strategies for inhibiting the CMAS corrosion were proposed.And then,three kind of CMAS-resistant materials were developed.The CMAS corrosion behavior of those materials were systematically investigated,and the CMAS-resistant mechanisms as well as CMAS resistance were discussed.The main contents and conclusions of this thesis are summarized below:（1）The CMAS corrosion behavior of 8YSZ TBCs was investigated firstly.It is found that 8YSZ TBCs showed severe grain boundary（GB）CMAS corrosion,which caused the terrible CMAS resistance of 8YSZ.Furthermore,the CMAS corrosion behavior of YSZ bulk materials with different YO_1.5 content was investigated.The results indicated that all YSZ materials had the preferential corrosion through GB,the reason was mainly attributed to the Y element segregation at GB.The extent of preferential GB corrosion depended on the doping content of YO_1.5in Zr O₂.The YSZ with low YO_1.5content,like 8YSZ and 19YSZ,were mainly corroded through GB.In contrast,the samples with high YO_1.5 content,like Y₂Zr₂O₇ and Y₄Zr₃O₁₂,basically showed a uniform corrosion through grain and GB,and the preferential GB corrosion was very slight.It is elucidated that the preferential GB corrosion could be mitigated by decreasing the Y segregation extent in GB,increasing the volume of precipitated products,and/or increasing the precipitation kinetics of products.（2）For solving the preferential GB corrosion issue,an equimolar YO_1.5 and Ta O_2.5co-doped Zr O₂(Zr_0.66Y_0.17Ta_0.17O₂,ZYTO)was developed as the new CMAS-resistant materials.The CMAS corrosion behavior of ZYTO bulk material was investigated firstly.It was found that ZYTO bulk has a significantly enhanced CMAS resistance than 8YSZ bulk.The CMAS corrosion rate of ZYTO was fitted as 15.9μm·h^-0.37 in contrast to～15μm·h^-1 in 8YSZ.Compositional and morphological analyses on the CMAS reaction zone revealed that the excellent CMAS resistance of ZYTO bulk originated from the uniform corrosion through grain and GB,along with densification of the reaction layer.Furthermore,the ZYTO coating was prepared by air plasma spray（APS）technology.It was found that as-sprayed ZYTO coating showed a severe phase decomposition,which was attributed to the elements segregation during APS process.After short-time annealing,the coatings could basically revert to the tetragonal ZYTO,but some secondary phases still left in the grain boundary of ZYTO.Both as-sprayed and annealed ZYTO coatings showed terrible CMAS resistance,the reasons were attributed to the rapid corrosion through secondary phases and the lack of dense reaction products layer in coating samples.（3）In order to ensure a good CMAS resistance in both bulk and coating materials,the Zr O₂ doped YTa O₄(Zr_xY_0.5-x/2Ta_0.5-x/2O₂（x=0,0.1,0.2,0.28）)were developed as the CMAS-resistant TBCs material.The CMAS corrosion behavior of the Zr O₂ doped YTa O₄ was investigated firstly.The results indicates that all compositions had the much better CMAS resistance than YSZ materials.The CMAS resistance was attributed to the rapid formation and followed thickening of dense reaction product layer.Furthermore,the effects of Zr O₂ doping content on CMAS resistance of YTa O₄ was discussed.It was found that minor doping of Zr O₂ can ensure the excellent short-and long-term CMAS resistance,but heavy doping of Zr O₂ will degrade the long-term CMAS resistance.Finally,Zr_0.1Y_0.45Ta_0.45O₂ was proposed as an optimal composition,which had the best CMAS resistance with the corrosion rate of 12.9μm·h^-0.35,and the corresponding coating was prepared by APS technology.The as-sprayed Zr_0.1Y_0.45Ta_0.45O₂ coating also showed a phase decomposition,therefore could not resist the CMAS corrosion.However,after short-time annealing,the coating showed the significantly enhanced CMAS resistance than as-sprayed sample,which was attributed to the formation of dense reaction product layer.Note that CMAS resistance of annealed Zr_0.1Y_0.45Ta_0.45O₂ coating was still weaker than that of bulk samples.The reasons were attributed to rapid corrosion through the residual secondary phase in annealed coatings.（4）A new strategy based on thermodynamic equilibrium design was proposed to develop the better CMAS-resistant TBCs material.And apatite-type Gd₁₀（Si O₄）₆O₃ was developed as a novel CMAS-resistant TBCs materials based on the strategy.The CMAS corrosion behavior of Gd₁₀（Si O₄）₆O₃ bulk was investigated.It was found that Gd₁₀（Si O₄）₆O₃ had a significantly lower intrinsic CMAS corrosion rate(～1.09μm·h^-0.5).More importantly,the CMAS corrosion only altered the composition of Gd₁₀（Si O₄）₆O₃,but did not change the crystal structure or destroyed microstructural integrity.During reaction,the surface Gd₁₀（Si O₄）₆O₃ rapidly transformed into Ca₂Gd₈（Si O₄）₆O₂in suit by inter-diffusion with CMAS melt and then become thermodynamically stable with CMAS melt,thereby inhibiting the further CMAS corrosion effectively.However,the CMAS resistance of Gd₁₀（Si O₄）₆O₃ was susceptible to the structural defects,like pores and cracks,which would cause a deeper infiltration.Therefore,a dense structure was necessary for this CMAS mitigation strategy.