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CHAPTER 4 VOLCANIC ASH AND ITS PENETRATION MECHNISMS IN TBCS rate is increased and reaches its steady state (GSS) when the crack propagates to a long distance. πΊπ‘hππππ¦ = (βπ β3π=1 πΈπhπβπΌπ)2 π π 2 β3π=1 πΈπ hπ In the formula, ΞT is the temperature change, i refers to the various layers (e.g. CMAS, TBC, BC), ΞΞ±i is the mismatch of CTE between each layer and the substrate, Ei is the Young's modulus of corresponding layers, and hi refers to the heights of each layer. In Figure 4.12, it can be seen that when the crack length reaches the length of 30 - 50 times of the YSZ columns diameter, their G would be close to its GSS and once GSS is over the fracture toughness of YSZ columns (60 - 80 J/m2), the crack will propagate throughout the TBC YSZ top coat and lead to the spallation of entire YSZ top coat. By observing the diagram in Figure 4.12, once the height of CMAS reaches 20 - 30 percent of YSZ top coat height, the steady state energy release rate would reach the fracture toughness of YSZ and result in spallation. But in real situations, delamination cracks are induced by thinner layer of CMAS deposit [48]. Owing to the similarity of volcanic ash and CMAS in their composition and the degradation mechanisms on TBCs, the relationship between CMAS thickness and delamination crack propagation reported by Xi Chen [44] can still be used in this study. According to literature [70], the density of crystalized volcanic ash is in the range between 2700 - 3300 kg/m3, which means the amount of volcanic ash (30mg/cm2) using in this experiment provides a thickness about 91 - 110 ΞΌm, and Page 56PDF Image | Volcanic Ash Degradation on Thermal Barrier Coatings
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