What is the suitable segmentation crack density for atmospheric plasma sprayed thick thermal barrier coatings with the improved thermal shock resistance?

The optimization and control of the segmentation crack density (D-s) for the thick thermal barrier coatings (TTBCs) with the improved thermal shock resistance has been performed via finite element modeling. The simulation results based on the current property parameters of each layer of the TTBCs fabricated by atmospheric plasma spraying (APS) are well consistent with the experimental results of thermal shock test. The investigation results indicate that too large or too low D-s will be not beneficial to the improvement of the thermal shock resistance of the TTBCs. The D-s must be located at a suitable range, and this paper has revealed the objective law quantitatively. Based on our simulation and experimental results, the appropriate segmentation crack density is in the range of 2.38-4.76 cracks/mm which will be beneficial to improve the thermal shock resistance ability. It has been found that the as-sprayed TTBCs exhibited superior thermal shock resistance when the segmentation crack density is about 4 mm(-1). The stress intensity factor (K-I) and energy release rate (J integration) will increase with the increasing of segmentation crack length. The existence of segmentation crack will improve the strain tolerance of TTBCs. The strain tolerance has been characterized by D-s and segmentation crack length quantitatively. The failure mechanism of APS-TTBCs can be attributed to the propagation of segmentation crack at the top-coat of the TTBCs, formation and propagation of the horizontal crack at the top-coat/TGO (thermally grown oxide) interface. The propagation rate of the main segmentation crack has been calculated and the life prediction model of the TTBCs during thermal cycle has been established. The possible methods which can prolong the service life of the TTBCs have also been proposed. (C) 2017 Elsevier B.V. All rights reserved.