To gain more information about the solid particle erosion performance of the APS-TBC system, microstructural analysis was performed on the eroded surface of the APS-TBC specimens using SEM. The morphologies of the eroded surfaces of the as-deposited TBC specimen are shown in Fig.1 for the tests at particle impingement angles of 30°,45°,60° and 90 °with the particle impact velocity of 84 m/s.
As seen in Fig.1(a), most of the splats in the top coat were damaged visually. The scars in the coating surface were caused by the impacts of the sharp-edged alumina particles. From the erosion test, the erosion rate was observed the least at 30° impingement angle while the most at 90° impingement angle, which indicated that the top coat exhibited erosion behavior in brittle manner, as most of ceramics-based coatings do. At 45° impingement angle, the erosion damage became severe, characterized by deeper and wider craters that were formed because of particle impacts(Fig.1b). Moreover, in addition to the damage induced by continuous particle impact, the cracks propagating around and through the splats were pronounced visually, leaving the erosion process governed by the splat fracture process. With further increase in particle impingement angles to 60° and 90°, the surfaces were damaged more severely and featured by deep and large cracks formed, as shown in Fig.1(c) and(d). From the damage mechanism,a large particle impingement angle increased projection of impact force on the target surface, at the impact angle of 90°, the total impact force was normal to the target surface, providing the largest breaking force on the brittle ceramic coating surface, resulting in serious cracking of the coating.
Fig.1 SEM eroded surface morphology of the top coat in as-deposited APS-TBC specimen under solid particle erosion with particle impact velocity 84 m/s at impact angle(a)30°,(b)45°,(c)60°and(d)90°
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