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Unusual Oxidative Limitations for Al-MAX PhasesAlumina-forming MAX phases are well-known for their excellent oxidation resistance, rivaling many metallic NiAl, NiCrAl, and FeCrAl counterparts and with upper temperature capability possible to approximately1400C. However a number of limitations have been emerging that need to be acknowledged to permit robust performance in demanding applications. Ti2AlC and Ti3AlC2 possess excellent scale adhesion, cyclic oxidation/moisture/volatility resistance, and TBC compatibility. However they are very sensitive to Al content and flux in order to maintain an exclusive Al2O3 scale without runaway oxidation of ubiquitous TiO2 transient scales. Accelerated oxidation has been shown to occur for Al-depleted, damaged, or roughened surfaces at temperatures less than 1200C. Conversely, Cr2AlC is less sensitive to transients, but exhibits volatile losses at 1200C or above if common Cr7C3 impurity phases are present. Poor scale adhesion is exhibited after oxidation at 1150C or above, where spallation occurs at the Cr7C3 (depletion zone) interface. Delayed spallation is significant and suggests a moisture-induced phenomenon similar to non-adherent metallic systems. Re-oxidation of this surface does not reproduce the initial pure Al2O3 behavior, but initiates a less-protective scale. Cr2AlC has also been shown to have good long term bonding with superalloys at 800C, but exhibits significant Beta-NiAl + Cr7C3 diffusion zones at 1100C and above. This may set limits on Cr2AlC as a high temperature TBC bond coat on Ni-based superalloys, while improving corrosion resistance in lower temperature applications.
Document ID
20170001576
Acquisition Source
Glenn Research Center
Document Type
Technical Memorandum (TM)
Authors
Smialek, James L. (NASA Glenn Research Center Cleveland, OH, United States)
Date Acquired
February 15, 2017
Publication Date
February 1, 2017
Subject Category
Metals And Metallic Materials
Report/Patent Number
GRC-E-DAA-TN37630
E-19328
NASA/TM-2017-219444
Report Number: GRC-E-DAA-TN37630
Report Number: E-19328
Report Number: NASA/TM-2017-219444
Funding Number(s)
WBS: WBS 109492.02.03.02.02
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.
Keywords
oxidation resistance
ceramics

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