Role of microstructure geometry and CMAS viscosity in CMAS Infiltration

Conference Dates

June 24-29, 2018


Abstract-The most successful strategy to date to reduce the harmful effect of CMAS on TBC is by dissolution of elements from the TBC that form a blocking phase such as apatite in the case of gadolinium zirconate. Clearly there is a kinetic race of infiltration rate vs. rate of formation of the blocking phase. This race is influenced by CMAS viscosity, crack widths, availability lateral flow paths in addition to through thickness paths of and other factors influencing the dissolution rate of the TBC elements needed to produce the blocking phase. This behavior has been studied experimentally using interrupted tests and deliberate alterations of the microstructural geometry via both powder and solution precursor plasmas spray in two systems, gadolinium zirconate and yttrium aluminum garnet. In addition by changes in CMAS composition, viscosity was also strongly altered. Viscosity of several CMAS compositions was also measured in a high temperature viscometer. Results begin to provide a semi quantitative understanding of the factors involved. From a practical point of view it becomes clear that there is a critical size of through thickness crack that completely defeats this arrest mechanism and that this crack size is in the range of single digit microns or smaller. It is also clear that CMAS viscosity that is not an engineered quantity has important effects that will be difficult to predict in practice given the uncontrolled nature of CMAS composition.

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