Fundamental challenges in CMAS mitigation
June 24-29, 2018
CMAS degradation of thermal and environmental barrier coatings (T/EBCs) is recognized as a fundamental barrier to progress in gas turbine technology; melting of the precursor silicate deposits, typically at ~1200°C, limits the temperature capability of the coatings and by extension the achievable engine efficiency. Molten CMAS dissolves all coating materials of interest in the protection of superalloys and ceramic composites, sometimes with preferential grain boundary penetration, and often leads to the formation of new or modified crystalline phases. Notwithstanding the thermo-chemical attack, the more deleterious form of damage is arguably thermo-mechanical. In TBCs the CMAS melt flows into the network of pores and cracks/segmentations that enable strain tolerance during thermal cycling, stiffening the coating and elevating the strain energy available for delamination. The mitigation strategy is generally based on the reaction of CMAS with the coating material, consuming locally the melt and yielding the precipitation of crystalline phases in sufficient volume to fill in the flow channels and arrest penetration. The extent of stiffening scales with the penetration depth, which depends in turn on a complex interplay between the infiltration, dissolution and crystallization dynamics. Predictive models are hindered by uncertainty in the geometric features of the pore network as well as the paucity of information on the dissolution and crystallization rates, compounded with the evolving composition and viscosity of the melt under the thermal gradient within the coating. Ideally, the reaction should be sufficiently rapid to overwhelm the flow dynamics, so the stiffened layer thickness is minimized. However, most oxides that exhibit the desirable reactivity also lack significant toughening mechanisms, so the reduction in penetration and the increase in strain energy are counteracted by a lower toughness. In essence, a coating with no effective mitigation mechanism but reasonable toughness, like 7YSZ, could in some instances resist thermal cycling induced delamination better than an oxide more resistant to penetration but with a lower toughness. Balancing these attributes represents a grand challenge in TBC design.
Penetration is generally not the critical issue in environmental barrier coatings (EBCs), which must be dense to perform effectively as barriers to the permeation of water vapor and, ideally, of oxygen. Consequently, EBCs are selected to minimize thermal expansion mismatch with the CMC substrate. This constraint limits the choice of materials, with rare earth silicates being favored in current systems. These silicates, however, are rapidly attacked by CMAS, especially by melts with higher Ca:Si ratios. While the recession of the EBC material is problematic in itself, the reaction with CMAS results in a layer of reaction products that is poorly matched thermally with the substrate. A source of substantial strain energy arises from this reaction layer, leading to the evolution of cracks that may delaminate the modified layer and/or the underlying EBC, but also penetrate into the bond coat and the CMC. This may expose the latter to environmental degradation of the fibers and the fiber/matrix interfaces that enable damage tolerance. The robustness of the system thus depends not only on the chemical reactivity of the CMAS/coating system but also on the toughness of the different layers, which are generally low. The grand challenge in EBCs is to approach prime reliance because CMCs are arguably less environmentally robust than current superalloys. At a minimum, this demands low reactivity with a relevant spectrum of CMAS compositions, and sufficient toughness to mitigate impact and/or thermo-mechanical damage.
This presentation will discuss the state of understanding of these challenges and the tools available to assess the response of the system and effectiveness of the CMAS mitigation approach against a spectrum of melt compositions. The insight is complemented by the presentation of Prof. Poerschke at this conference.
Acknowledgments: Presentation based on research contributions by R.W. Jackson, C.S. Holgate, K.M. Wessels, E.M. Zaleski, N. Abdul-Jabbar, B. Lutz, D. Park, J.S. Van Sluytman, M.R. Begley, and F.W. Zok, as well as collaborations with QuesTek Innovations, Pratt & Whitney, Siemens and Honeywell Aerospace. Work sponsored by the Office of Naval Research under awards N00014-08-1-0625, -12-M-0340 and -16-1-2702, as well as by the Pratt & Whitney Center of Excellence in Composites and the Honeywell-UCSB Alliance for Thermal Barrier Coatings.
Carlos Levi and David L. Poerschke, "Fundamental challenges in CMAS mitigation" in "Thermal Barrier Coatings V", Prof. Dr. Robert Vaßen, Forschungszentrum Jülich GmbH, Germany Brian Hazel, Pratt & Whitney, USA Prof. Dr. Uwe Schulz, German Aerospace Center, Germany Dr. Michael J. Maloney, Pratt & Whitney, USA Dr. Ram Darolia, GE Aviation (Retired), USA Eds, ECI Symposium Series, (2018). http://dc.engconfintl.org/tbcv/68