Figure 1 shows the microscopic images of the cross sections of the as-deposited TBC specimens eroded at selected impingement angles. It is evident that at all impingement angles the top coat was severely cracked by the attack of solid particles. Additionally, as the impingement angle was raised, the crater depth generated by the impact increased, thus resulting in thickness reduction of the top coat layer.However, it was noticed that although severe cracking and fracturing occurred in the top coat during the solid particle erosion process, no obvious damage was found on the bond coat and at the interface between the bond coat and topcoat. This implied that the failure of the APS-TBC under solid particle erosion would be a result of top coat removal by deformation wear, which differed from the failure of the APS-TBC under isothermally exposure process. Since the thickness reduction of the top coat significantly depended on particle impingement angle, the erosion behavior of the APS-TBC was governed by particle impact condition.Certainly, at the same particle impingement angle, higher particle impact velocity would expedite the removal of the top coat, leading to shorter service life of the TBC.
The SEM images in Fig.1 demonstrate that the solid particle erosion behavior of the APS-TBC top coat was predominantly controlled by wear with monotonically increasing erosion rate from acute to normal angle. The eroded surface mostly exhibited a particle-pounded morphology rather than ductile cutting. Cracks were generated around the splats to facilitate the removal of either individual or cluster of splats. Although these cracks can provide a certain degree of strain tolerance, they made great contribution to the erosion damage of the top coat, because they served as stress concentrators and can easily "link-up"to create large cracks or connect splat boundaries, resulting in increased erosion. The fragmentation of the splat tops and the microcrack network within the YSZ top coat were observed close to the area of particle impact with eroding alumina of the top coat. The propagation of microcracks caused the debonding at the splat boundaries and consequently successive removal of the splats during the erosion process.
Fig.1 Cross-sectional microscopic images of as-deposited APS-TBC specimen eroded with particle impact velocity 84 m/s at impact angle (a)30°,(b)45°,(c)60°and(d)90°
Figure 2 shows the SEM microscopic cross-sectional images of the eroded TBC specimens at particle impingement angle 30° and particle impact velocity 100 m/s which were heat-treated for 72 h at 1000 and 1150℃before erosion tests. It appeared that while the exposure duration reduced the erosion rate, the solid particle erosion mechanism remained unchanged for the top coat. Higher aging temperature(1150℃) was found to enhance the erosion resistance of the top coat than that at 1000C, which was attributed to the reduction of porosity due to sintering effect during the heat treatment. According to the three erosion mechanisms proposed by Eaton and Novak, the above surface morphology analyses reveal that the second mechanism of moderate erosion rate most likely occurred for the APS-TBC specimens of the present study.
Fig.2 Cross-sectional microscopic images of heat-treated APS-TBC specimen eroded at particle impact angle 30°with impact velocity 100 m/s:(a) aged at 1000℃ for 72 h and (b) aged at1150℃ for 72h
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