Mechanical Properties, Cracking Mechanism And Cracking Detection Of The Top-coat In Thermal Barrier Coating System

The uses of thermal barrier coatings (TBCs) have decrease the surface temperature of high-temperature alloy, and thus increase the thrust-weight ratio of engine. However, the cracking in the top coatings of TBCs has been observed during they are in service, which can decrease the service life and influence the safe operation of the coating systems. Correspondingly, the top coatings’mechanical properties will be measured firstly in the present study through using nano-indentation method. The effect of oxidized time will be obtained. In the same time, the changing tendency of mechanical properties is also got when the oxidized time is maintained unchangeable. Fracture of coatings is observed in-situ using Scanning Electron Microscope (SEM) during the single-axis cycle tensile testing. The initiation, extension and fracture of the cracks in the top coatings are observed. Digital speckle correlation method (DSCM) is used to analyze the deformation and to study the effects of pores in the coating. Rare-earth doped coatings is deposited and their cracking is detected by Raman spectrum.It was found that the elastic modulus and hardness of the top-coat is independent with the applied load when the loading is larger than the limited value. The mechanical properties in the direction parallel to the spraying direction increase firstly and then decrease with the increment of coating thickness. With the thickness of TGO (Thermally Grown Oxide) increase, the elastic modulus and hardness show the decreasing tendency correspondingly.The cracks in the coatings propagate slowly firstly with the increase of cycles in the cycle tensile testing. And then the coating fracture instantly when the cycles increase further. The failure limit of different samples presents great dispersion. The strain in the vertical direction of Top Coating distributes in a pattern of strip. It shows there are weak parts in the vertical direction in the coating, which can lead the cracking easily in the top coating.The intensity ratio of YSZ:(Tm, Dy) coating varies with the different applied load. The intensity ratio of coatings shows an increment tendency with the increase in the load, while some downward fluctuations are found along the rising curve. New cracks appear in the coatings during the loading process, which cause the luminescent intensity of coatings decrease. The downward fluctuation of emission intensity may be used to monitor the appearance of new cracks in the coating.