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Research on Phase Transformation of Isothermal Oxidation of Thermal Barrier Coatings and Failure Analysis

Author: HanYuJun
Tutor: DongYun
School: Hebei University of Technology
Course: Materials Science
Keywords: aero-engine bond coat residual stress Laman fluorescence spectra SPFB technology spinel diffusion channel
CLC: TG174.453
Type: Master's thesis
Year: 2010
Downloads: 109
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Abstract


Thermal barrier coating is such a sophisticated and advanced surface spray technology that was employed to the field of aero-engine. The research of thermal barrier coatings involve with development of new types of top ceramic coats, optimization of ingredient and surface structure of bond coats, assessment of residual stress of TGO or interface of TGO/BC after isothermal oxidation, development of new coating preparation technologies and so on. The present study is part of research project that aims to enhance adhesion between bond coats and top ceramic layer by means of Supersonic Fine Particles Bombarding （SFPB） for the manufacturing of thermal barrier coatings （TBC）. Thermal barrier coating was prepared in the process of HVOF（CoNiCrAlY）+ SFPB+APS（8YSZ）and the sprayed samples were subjected to isothermal oxide at the temperature of 1000℃for 2h, 26h, 310h respectively. We eventually obtained high oxidation resistance with the main phase ofα-Al2O3 in TGO to avoid the failure of TBC system. XRD、SEM、EDS and Laman fluorescence spectra were employed to analyze the element distribution, phase transformation and microstructure of TGO. After HVOF（CoNiCrAlY）+ SFPB process, the bond coat is composed of phaseγ′-Ni3Al, phaseγ-Ni and phaseγ-Co. Phase NiO, phase Co3O4, spinel phase, phaseγ-Al2O3 and phaseα-Al2O3 were detected in TGO after isothermal oxidation for 2 hours. Phaseγ-Al2O3 forms initially, NiO and Co3O4 grains grow on the interface of Al2O3 grains subsequently. Due to chemical reaction there also exists small quantity of spinel phase. Metastable phaseγ-Al2O3 transformed into stable phaseα-Al2O3 after isothermal oxidation for 26 hours. After the pre-treatment of SFPB process, the density of dislocation increased evidently, which may generate a large number of diffusion channels in the region of the surface of bond coat. Numerous cations whose radii are larger than aluminum ion can transmit via diffusion channels during the transient state of isothermal oxidation. It may indirectly effect on the structure of metastableγ-Al2O3 and inhibit the phase transformation ofγ→θ→α. Laman fluorescence spectra and theoretical calculation indicated that the value of residual stress of micro-thermally growth increase initially and afterwards show the decrease trend in TGO after the process of isothermal oxidation treatment and slow cooling, and the value of isothermal oxide for 310 hours is lower than 26 hours by 0.476GPa. TGO layer was deduced stress on thick function relation. The residual stress of thermal mismatch in TGO can reach the value of 4.637 4.640 GPa, and after heat shock on the interface of TBC/TGO the total of residual stress can eventually reach the value of -3.44 GPa. The spallation of thermal barrier coatings may be ascribed to macro-residual stress of thermal mismatch.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Corrosion control and protection > Metal surface protection technology > Inorganic complex layer of protection > Ceramic stratified
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