Study On Microstructure, Phase Composition And The Capacitance Characteristics Of YSZ Thermal Barrier Coatings Prepared By APS

A series of changes in the serving process of YSZ thermal barrier coating such as changes in the micro structure, ZrO2phase transformation,the growing and distribution of TGO between ceramics coating and adhesive layer and the extension of defects will cause negative effect to the coating performance.These changes can influence to the life of coating directly. In this paper, two different kinds of ZrO2-8%Y2O3powder is used to fabricate YSZ thermal barrier coating whose substrate is superalloy and whose adhesive layer is CoNiCrAlY by Atmospheric Plasma Spraying(APS) process. In the experiment, coating samples have been aging annealing for5hours in the condition of three different temperatures as1100℃,1150℃and1200℃and thermal shock resistance of it is tested in the condition of1100℃respectively. The microstructure and morphology of coating is observed by the Olympus Microscope and Philips SEM; The microhardness of coating is measured by MH-6Microhardness tester; The XRD diffraction of samples are analyzed by Rigaku X-ray diffractometer; The microanalysis of the coating and powder is analyzed by EDX technology; The capacitance value of coating is measured and the permittivity of coating is calculated by means of the Bridge method. The result illustrates:(1) There exists Non-stable tetragonal T’ in original YSZ coating which is diffent from the original YSZ powder. The phase transformation happen in the process of the deposition and condensation of YSZ powders on the substrate. And M phase content of Beijing YSZ coating is obviously less than the one in Yiyang YSZ coating. During the thermal shock cycles, obvious Tâ†'M transformation process doesn’t occur in both of the two YSZ coatings.After the10hours’s aging treatment with the temperature1200℃, M phase content increases of the coating while T phase content decreases, which proofs that Tâ†'M transformation mainly occur in the aging process.(2) The density and microhardness of both two coatings increase after high-temperature aging treatment, mainly because of the YSZ gains grow, the small-angle grain orientation rotation occurs and a small number of Tâ†'M transformation exists in the aging procedure.(3) During the aging treatment, the stability performance of Beijing YSZ coating is preferable to Yiyang YSZ coating, which is closely related with molten state and deposition condensing of two kinds of YSZ powder in the spraying process. The loosen and micro cracks construct of Beijing YSZ coating to some extent releases various stresses of the coating so that the performance of coating is more stable.(4) The capacitance value of coating gradually decreases with the increment of aging temperature. However it presents a wave, a relatively stable trend during the thermal shock cycles, and different test frequency has limited effect on the measurement of capacitor value of coating. The Tâ†'M transformation of the coating is the secondary factor which affects the trend. However, The TGO whose main component is Al2O3generated in the interface of adhesive layer and ceramics coating is the main factor which cause the change of the capacity value.(5) The thermal shock resistance of Beijing YSZ coating is markedly superior to the one of Yiyang with the temperature1100℃. There exists multiple failure modes in Yiyang YSZ coating. Failure mechanism is that the stress generated by the mismatching Thermal expansion coefficient of ceramic coating and metal substrate, along with the oxidation of adhesive layer and the occurance of intertwine cracks in YSZ coating under the repeatedly thermal stress which cause the failure of the coating rapidly.