Study of Thermal Barrier Coating: Processing, Residual Stress and CMAS (CaO-MgO-Al2O3-SiO2) Corrosion Resistance

熱障塗層研究:加工、殘餘應力與耐CMAS(鈣鎂鋁矽酸鹽)腐蝕性能

Student thesis: Doctoral Thesis

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Award date11 Jun 2021

Abstract

Thermal barrier coatings (TBCs) are extensively applied to the hot components of turbine engines, i.e., the turbine blades and the combustion chambers. TBCs are of great importance in increasing the operating temperature of hot sections in turbine engines, and improving the efficiency of turbine engines. Defects originating from 1) the ceramic topcoat and the bondcoat/substrate interfacial zone, and 2) the calcium-magnesium-aluminum-silicon (CMAS) attacked topcoats are broadly accepted as the driving force for the premature failure of the yttria-stabilized zirconia (YSZ) TBCs which are adopted in the aerospace industry. Although a lot of efforts have been done to extend the service life of YSZ TBCs, there are still some failure mechanisms which need to be clarified due to the complex microstructure fabricated by various techniques. E-beam physical vapor deposition (EB-PVD) and air plasma spray (APS) are regarded as the first-generation manufacturing methods for TBC, while plasma spray physical vapor deposition (PS-PVD) is considered to be the latest manufacturing methods. The microstructures manufactured by different methods have their own features, i.e., columnar and splat-like microstructures.

In this dissertation, YSZ TBCs manufactured by APS were used to investigate the effect of the major processing parameters on coating adhesion via evaluating the interfacial fracture toughness. It was suggested that the increasing coating thickness and the substrate temperature during manufacture both had a positive impact on improving interfacial fracture toughness in the TBC system. It indicated that a thicker coating and higher substrate temperature within the appropriate range can enhance coating adhesion in actual processing. Meanwhile, the depth-resolved residual stress was another important intrinsic failure reason. The focused ion beam (FIB) milling coupled with digital image correlation (DIC) method is a promising solution to highly localized residual strain measurement. Since the damage of surface morphology during ion milling process affected the conventional pixel-level image correlation, a DIC algorism based on the atomic strain calculation method was proposed. A series of miniature ring cores were processed by FIB at various depths of the coating at the cross-sectional ceramic topcoat, and the corresponding released strain of each core surface was evaluated by the DIC program. The non-monotonic increase tendency of the measured residual strain as a function of depth revealed that there was a complex local strain field at 250μm away from the coating surface towards the topcoat/bondcoat interface which was possibly related to the cracking in the topcoat. This FIB-DIC approach provided new insight into the micro residual stress analysis of TBCs. This method can also be applied to the local strain/stress measurement in bulk ceramics, and amorphous materials due to its working principle which is unrelated to crystal structures.

In addition, the CMAS resistance of PS-PVD TBCs was studied. Although PS-PVD TBCs have fine thermal insulation and good strain tolerance, their CMAS resistant behavior is no better than that of the APS and EB-PVD TBCs. At the initial corrosion stage, lattice distortion resulting from ion-exchange released via grain rotation at elevated temperatures. The YSZ coating exhibited anisotropy corrosion resistance. The (102) plane showed stronger anisotropy under CMAS attack compared to the low-index planes of tetragonal ZrO2. Along with the CMAS infiltration at 1200°C in an air atmosphere, the PS-PVD TBCs presented a unique microstructure evolution: quasi-columnar feather-like microstructure, fingerprint-like microstructure, and finally the nanocrystalline stacking. It revealed lower CMAS resistance than TBCs manufactured by classical methods such as EB-PVD. The changes in microstructure accelerated the phase transition from tetragonal to monoclinic, as a result of increasing the contact area.

These mechanisms can be applied to optimize the processing and operation conditions to increase the service time of plasma sprayed TBCs. Also, some of these studies are more related to the microstructure other than the chemical composition of the coatings. Thus, these results are accordingly meaningful to other coating systems which have similar microstructures or are manufactured by the same technique.

    Research areas

  • thermal barrier coating, residual stress, interfacial adhesion, focused ion beam, digital image correlation, corrosion mechanism