| Thermal barrier coatings(TBCs) are refractory-oxide ceramic coatings applied to the surface of metallic parts in the hottest part of gas-turbine engines, enabling modern engines to operate at significantly higher gas temperatures and increasing in the energy efficiency of gasturbine engines. However,the complexity or diversity in the TBCs structures along with the harsh operating conditions make it difficult to predict their spallation failure, which severely shortens the service life and restricts the wide application. Among them, the molten CaO-MgO-Al2O3-SiO2(CMAS) results from the ingestion of siliceous minerals(dust, sand, and volcanic ash) within the intake air during flight. At the high temperature CMAS yields glassy melts and penetrates in TBCs with porous microstructures, thus changing the near-surface mechanical properties and enhancing the spallation tendency. But the failure modes and the damage evolution are not clear. Hence, a real-time nondestructive evaluation is necessary for understanding the failure mechanism of the TBC. In this paper,the process of high temperature CMAS corrosion in TBCs is real-time monitored by the acoustic emission(AE). To discrimination the failure mode of TBCs under high temperature corrosion by using the signal process theories, such as the clustering analysis, the frequency spectral analysis and wavelet analysis. The main contents in the paper are listed as follows:Firstly, the laboratory CMAS powder was prepared by the simulating the real ash composition, the average particle size of CMAS powder was detected 15.601 μm. The experiment for TBCs attacked by high temperature CMAS with gradient temperature was realized by the self-designed gradient temperature furnace. Based on high temperature waveguide technology, the failure process of TBCs under high temperature CMAS corrosion was monitored by acoustic emission.Secondly, based on the amplitude and the number of AE event, AE characteristics of the failure process in TBCs attacked by molten CMAS are analyzed. And compared to the substrate and the TBCs without CMAS, more serious damage is found to occur in the TBCs attacked by the molten CMAS. The result indicated that the frequency is the best characteristic parameters to discriminate the damage mode of TBCs under high temperature CMAS corrosion by the k-means clustering analysis. After the frequency spectral analysis, the result demonstrated that the characteristic frequency bands of 0.09~0.13 MHz, 0.20~0.25 MHz, 0.25~0.32 MHz and 0.38~0.45 MHz are corresponding to substrate deformations, surface vertical cracks, sliding interface cracks and opening interface cracks. The reliabilities of identified failure modes are verified by scanning electronic microscopy(SEM) observations. The frequency band of sliding interface crack is associated with two crack types, including the crack at the TC/BC interface and the parallel crack in the ceramic coating induced by the compressive stress. These two cracks dominate the whole failure process, and ultimately result in the spallation of coatings.Thirdly, the failure modes of TBCs attacked by molten CMAS are discriminated by wavelet packet analysis. According to the frequency distribution of different types of AE signals, the wavelet and decomposition scales are selected. Then four kinds of signals are analyzed by the wavelet packet time-frequency analysis, and attained the time-frequency distribution characteristics of signals. Then, the failure modes of TBCs are discriminated by the energy coefficient of wavelet packet transform. There are four damage modes in TBCs respectively, surface vertical cracks, sliding interface cracks, opening interface cracks and substrate deformations. The failure of the TBCs mainly occurs upon the cooling process. During the failure process, interface cracks and parallel cracks first generate in the coating. When the temperature approximately drops to 400°C, surface vertical cracks appear in the TBCs. |
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