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CHAPTER 4 VOLCANIC ASH AND ITS PENETRATION MECHNISMS IN TBCS 4.2.1 Delamination Type of Degradation Mechanism on TBCs Unlike traditional CMAS penetration, delamination happens when the thickness of the deposited CMAS/volcanic ash reaches a certain thickness, delamination occurs after only one thermal cycle [44]. This phenomenon has also been noticed in previous experiments for volcanic ash degradation tests. Figure 4.8 shows the cross-sectional morphology of the as-received TBC sample. By covering the thermal barrier coatings by a thick layer of volcanic ash (30mg/cm2), the YSZ top coat spalled off after one thermal cycle at 1250 °C as it shown in Figure 4.9. Xi Chen [44] has explained the relationship of the delamination crack and the thickness of CMAS deposit using finite element calculation. According to the finite element simulation, bending stress is formed due to the thermal expansion mismatch between CMAS, YSZ top coat, and bond coat layers and concentrate more on the upper-left and lower-right side of YSZ columns shown in Figure 4.10. Due to the high bending stresses, columns adjacent to the sintering edge cracks from the right side to the left side near the interior edge, as it shown in Figure 4.11A. By calculating individual YSZ columns, energy release rate G is increased along with the crack length (length of crack/diameter of YSZ column) with a fast speed, once the length of crack has been propagated for almost an entire column, G goes to infinity. By simulating the long delamination cracks for the whole TBC model shown in Figure 4.11B, once the crack starts propagating, energy release Page 55PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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