Effect Of CMAS With Ca/Si Change On Microstructure Of EB-PVD Thermal Barrier Coatings

Thermal barrier coatings?TBCs?mainly made of yttria-stabilized-zirconia?YSZ?can protect metal components of aero-engine away from the thermal attack.Thus,they are widely used in the aero-engine industry to improve efficiency of the aerodynamic turbines.Reliability and service life time of the TBCs are critical for the aerodynamic turbines.Previous studies have shown that failure of TBCs has close relation with melting CaO-MgO-Al2O3-SiO₂?abbreviated as CMAS?particles which can penetrate YSZ coating along the void space in YSZ and chemically attack YSZ structure.Depending on the geographical environments,CMAS can change a lot in the composition,leading to the complex microstructures of YSZ coatings after the CMAS corrosion.CaO and SiO₂ are believed to play an important role in the CMAS corrosion of YSZ with respect to the Al2O3 and MgO.However,knowledge about effect of Ca/Si atomic ratio in CMAS on the microstructures of the CMAS-corroded YSZ is very limited at present.Consequently,in this study,the microstructures of YSZ coatings that is prepared by electron beam physical vapor deposition?EB-PVD?on Al2O3 substrate and corroded by CMAS with the typical Ca/Si atomic ratios of 0.33,0.73,1.78 respectively are characterized by X-ray diffraction,raman spectroscopy,scanning electron microscopy?SEM?,and transmission electron microscopy?TEM?in order to understand the effect of Ca/Si atomic ratios on the CMAS-corroded microstructure of YSZ coatings.Main results are drawn below.When the Ca/Si atomic ratio of CMAS is 0.33,it is found that at top region of YSZ,the original feather-like columnar YSZ transforms into particle structure and the gaps between the YSZ particles are fully filled by the amorphous CMAS solid.At bottom layer of YSZ,the YSZ columns are still present,but there are some voids due to CMAS corrosion at the surface of the columns.Except the amorphous CMAS solid fills between the YSZ columns and degradation of the feather-like columnar structure,ZrSiO₄ phase has also been detected at the bottom region of YSZ.As the Ca/Si atomic ratio increases to 0.73,except the amorphous CMAS solid fills the gaps between the YSZ columns of whole coating,degradation of the columnar structure happens at the top region of YSZ coating,where the diameter of the columns reduces and some columns are broken.At the bottom region of YSZ,the number of the corrosion voids at the surface of the columns increases obviously.CaAl₂Si₂O₈ and MgAl₂O₄ phases are detected both inside YSZ layer and at the YSZ/Al2O3 interface.Cracks can also be found near these newly formed phases.Presence of the CaAl₂Si₂O₈ and MgAl₂O₄ phases has close relation with the cracks,although the cracks may be created during preparation of TEM sample.When the Ca/Si atomic ratio increases to 1.78,the columnar structure disappeared and tetragonal YSZ particles with square-like configuration appeares due to YSZ dissolution in the CMAS melt and re-precipitation from the CMAS melt.The materials filled between the YSZ particles have been detected to be a mixture of the amorphous CMAS and MgAl₂O₄.Based on the microstructure characteristics of YSZ coatings corroded by the CMAS with different Ca/Si atomic ratios,it is clear that corrosion is enhanced and more serious structural degradation of the YSZ columns happens as increase of the Ca/Si atomic ratios.In this case,the tetragonal to monoclinic phase transition can happen more easily.