Theoretical Analysis About Interface Oxidation Of Thermal Barrier Coatings With Thermodynamics

Thermal barrier coatings (TBCs) are consisted of ZrO2+7wt.%Y2O3 ceramic coat, MCrAlY (M stands for Ni, Co, Ni and Co) bond coat and super-alloy substrate. TBCs have excellent high temperature oxidation resistance and thermal corrosion resistance. At the same time, TBCs have very low thermal conductivity. TBCs can effectively reduce the service temperature of the protected substrate. Therefore, TBCs have an extensive application in spaceflight, chemical industry, metallurgy and energy sources fields.During high temperature oxidation of TBCs, the total change of free energy of the system is the sum of the change of percent unit volume free energy of formation, surface free energy and strain energy. Surface free energy and strain energy are resistance to nucleation. The driving force of oxide formation is the change of percent unit volume free energy of formation. The change of percent unit volume free energy of formation is more precise to estimate selective oxidation of alloys than the change of standard free energy of formation.The interfacial diffusion of alloy elements between ceramic coat and bond coat has been studied by electron probe microanalysis and least square method. With increasing thermal cycles, Al is depleted, and Ni, Cr will oxidize near the interface between thermally grown oxide(TGO) and bond coat. Stress concentration due to the existence of thermally grown oxide will weaken the combination between top ceramic coat and bond coat, and consequently, the stresses will result in the spallation of thermal barrier ceramic coating.A diffusion-kinetic model of thermal barrier coating oxidation under isothermal condition is proposed. The thickness of oxide layer formed at bond coat/ceramic coat interface is regarded as a function of oxidation time and several parameters describing longitudinal and lateral oxygen transfer. The model is used to calculate the thickness of oxide layer. The oxidation laws for TBC are obtained, where is the thickness of TGO and is time. Various oxidation laws of such as linear, parabolic and cubic can be described by the universal kinetic equation.In the study of the effect of stress on the oxidation of TBCs, the mechanochemistry activities of elements in bond coat are calculated. The effect of stress on the mechanochemistry activities of elements is analyzed. Stress increases the activity ratio of Cr/Al and Ni/Al in bond coat. The relative activities of Cr and Ni are increased. The increase of activity of one element is useful to the formation of its oxide. In a word, the thesis gives a detailed study of high temperature oxidation of thermal barrier coatings from thermodynamics, diffusion, oxidation kinetics and mechanochemistry activity.