Recession of BN coatings in SiC/SiC composites through reaction with water vapor

Virginia E. Collier, Wenbo Xu, Robert M. McMeeking, Frank W. Zok, Matthew R. Begley*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

Abstract

The mechanical response of ceramic matrix composites depends critically on the slip along the matrix–fiber interface, which is usually achieved with thin coatings on the fibers. Environmental attack of such coatings (enabled by ingress of reactants through matrix cracks) often leads to significant degradation, through the removal of the coating via volatilization and oxidation of exposed SiC surfaces. The extent of the volatilization region extending from the matrix crack plane (i.e., recession length) is strongly coupled to the formation of oxide, which ultimately fills open gaps and arrests further reactions. This paper presents models to quantify these effects over a broad range of environmental conditions, coating thickness, and matrix crack opening. Analytical solutions are presented for the time to close recession gaps via oxidation, and the associated terminal recession lengths obtained near free surfaces. A broad parameter study illustrates that recession behaviors are controlled by a competition between volatilization and oxidation rates. As such, the extent of recession is highly sensitive to water vapor and temperature, providing an explanation for disparate observations of recession under seemingly similar conditions. The extent of recession in the interior of composites is also illustrated, using a straightforward reaction-transport model. Recession lengths decay rapidly away from the free surface, with the extent of recession penetration scaling with maximum recession at the free surface.

Original languageEnglish
Number of pages14
JournalJournal of the American Ceramic Society
Early online date24 Aug 2021
DOIs
Publication statusE-pub ahead of print - 24 Aug 2021

Keywords

  • boron nitride recession
  • ceramic matrix composites
  • oxidation resistance
  • silicon carbide
  • water vapor

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