In principle, the severity of attack should be comparable at the TBC/substrate interface and within the bulk coating if the relative volume fraction of CMAS remained similarly small. However, two important differences arise because of the attack of the substrate by the melt. The first one is a result of the ensuing local separation of the TBC from the substrate, driven by the combination of substrate dissolution and thermal mismatch stresses,Jwhich allows further buildup of molten CMAS to form an interfacial layer several times thicker than the typical intercolumnar gap. This would explain the reactivation of the dissolution–reprecipitation process when the CMAS reaches the bottom of the coating, but not the changes in the phase constitution and chemical composition of the precipitated zirconia.These are arguably linked to the injection of excess alumina into the melt and the subsequent precipitation of an aluminate crystalline phase.
Insight can be gained by examining first the reaction scenario for CMAS with the substrate absent YSZ. In this case, there is reasonable knowledge of the quaternary diagram but visualization is greatly enhanced by focusing on the CAS metastable ternary section in Fig. 1. It is assumed again that crystallization of the primary pseudowollastonite phase is kinetically suppressed, and hence the liquid region at 13001C is metastably extended up to the C–S binary, as illustrated in Fig. 10. The projection of the quaternary CMAS composition onto the ternary is also marked in this figure, and a tie line representing the interaction with the alumina substrate is drawn to the corresponding corner of the diagram. The driver for dissolution–reprecipitation is immediately evident. Al2O3dissolves into the melt and shifts its composition toward the boundary of the anorthite (CA2S2)1liquid field. After the requisite supersaturation is achieved to nucleate CA2S2, the system will tend to establish local equilibrium between anorthite and the melt, continuing to dissolve Al2O3 and precipitate CA2S2 as the overall composition shifts toward the Al2O3 corner. It is further noted that the Al2O3 content of the residual CMAS in the near-substrate region is significantly lower than that throughout the rest of the coating. The inference is that the anorthite crystals continued to grow upon cooling after the reaction hold, gradually depleting the liquid composition beyond the boundary indicated in Fig. 1, as expected from the slope of the liquidus surface in that region.
Fig. 1.Metastable isothermal cross section for the AlO1.5–CaO–SiO2system, based on fig. 630 Levinet al.19The pseudowollastonite liquidus has been suppressed whereupon the equilibrium liquid field has been extended metastably to the CaO–SiO2binary. The projection of the quaternary calcium–magnesium alumino silicate (CMAS) composition onto this ternary is shown by the diamond symbol. The tie line between this composition and the alumina corner reflects the process that occurs when CMAS dissolves the alumina substrate and, upon saturation, precipitates anorthite (CA2S2) at the lower interaction zone
The physical characteristics of the anorthite andc-YSZ crystals suggest that both precipitate at temperature, although their sequence of nucleation is not completely clear. Nevertheless, the change in the local chemistry induced by the dissolution of alumina and reprecipitation of anorthite is arguably responsible for the shift in the redistribution pattern of Y and Zr upon crystallization of YSZ. The area EDS indicates that there is no significant change in the overall Y:Zr ratio relative to the original coating, so the Y enrichment inc-YSZ is presumably compensated by the incorporation of Zr into the anorthite. It is also evident from the volume of cubic YSZ particles that, while significant, the extent of YSZ dissolution in this region is much smaller than that in the upper reaction layer, consistent with the lower volume of CMAS available. The survival of much of the column root structure and small t0 grains detached from this region into the CMAS melt and further supports the view that the dissolution rate is moderated by the incorporation of alumina. This suggests possible mitigation strategies that will be explored in subsequent publications.
本文由桑尧热喷涂网收集整理。本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://www.sunspraying.com/
|
