| Thermal barrier coating not only can bear high thermal temperature,but also can protect metal against corrosion, so it is widely used in aerospace field. But when aircraft fly in the sky, many small solid particles floating in the air will be involved, which will develop into CMAS deposition and then accelerate coating’s degradation. In order to get more information about corrosion failure mechanism caused by CMAS,we specially designed a tubular resistance furnace which has a thermostatic control system.The CMAS that prepared in laboratory are seriously modeled natural ash composition. This paper systematically discusses infiltration path and regularity of CMAS corrosion to the metal base of EB- PVD thermal barrier coating. Much attention were paied to the ways of crack initiation and propagation due to the CMAS deposition and corrosion,by means of SEM, XRD, EDS and other microscopic characterization methods such as nano indentation.Corrosion temperature and time, as well as the unit mase of CMAS play an important role. The main research results include:Firstly, on the basis of preparing CMAS powders,we mainly studied what environment will induce CMAS penetration.A lot of CMAS powders whose initial melting temperature was about 1170 °C were clustered together in 18 microns after manual grinding.Once CMAS arriving at its melting point,they can rapidly flowded into the gap of the columnar crystal within 15 minutes drived by capillary.As long as the contact time was long enough, a small amount of CMAS also can infiltrate into the ceramic layer in spite of the temperature below its melting point.Secondly, the evolution of cracks in the ceramic coating who was exposed to the melt CMAS under 1250 °C was analyzed step by step.Where CMAS powders were richly deposited,where Cracks mainly developed.So based on experimental studies, we found that the unit mass of the CMAS powders was one of the main factors influencing the quickly extension of cracks.There were a numble of microcracks growing in the ceramic coating’s surface when the infiltration time was limited.For comparison, the greater amount of interface cracks will be prevalent following by the increasing errosion time.More often,it can be observed that after1 h, 2 h, and 4 h, only several microcracks among the bottom of columnar crystal were generated. However, a bulky crack was extended over the interface because of lasting corrosion. As the time arrived at 24 h,the YSZ totally removed from the interface layer of the sample. The diffusion of Y element destroyed the stability of the structure of ZrO2, accelerating the phase change, and promoting the microcrack initiation. At the same time, the growth of Al2O3 and the generation of new material ZrSiO4 further exacerbated the microcrack extension, eventually led to the spalling of the coating.Thirdly, residual stress of TBC had been changed after interaction of molten CMAS under high temperature corrosion during different time. Nanoindentation technique was be used to examine elastic modulus of ceramic layer, and a flat plate model was established. It was found that the,residual stress of ceramic layer increased accompany with the increase of corrosion time. And there are a little stress drop within 24 h CMAS corrosion in some areas. |
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