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Degradation of Nextel™ 610‐based oxide‐oxide ceramic composites by aluminum oxychloride decomposition products
Author(s) -
Hay Randall S.,
Keller Kristin A.,
Zawada Larry P.,
Jacobson Nathan S.,
Fair Geoff E.
Publication year - 2018
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.15573
Subject(s) - materials science , composite material , ultimate tensile strength , oxide , microstructure , fiber , degradation (telecommunications) , ceramic , yttrium , decomposition , scanning electron microscope , metallurgy , telecommunications , ecology , computer science , biology
Nextel™ 610 alumina fibers and alumina‐ YAG (yttrium‐aluminum garnet) matrices were used to make oxide‐oxide ceramic matrix composites ( CMC s) with and without monazite (La PO 4 ) fiber‐matrix interfaces. Twelve sequential aluminum oxychloride (Al OC l) infiltrations with 1 hour heat treatments at 1100°C and a final 1 hour heat treatment at 1200°C were used for matrix densification. This matrix processing sequence severely degraded CMC mechanical properties. CMC tensile strengths and interlaminar tensile ( ILT ) strengths were less than 10 MP a and 1 MP a, respectively. Axial fracture of Nextel™ 610 fibers was observed after ILT testing, highlighting the extreme degradation of fiber strength. Extensive characterization was done to attempt to determine the responsible degradation mechanisms. Changes in Nextel™ 610 fiber microstructure after CMC processing were characterized by optical microscopy, SEM , and extensively by TEM . In Al OC l degraded fibers, grain boundaries near the fiber surface were wetted with a glass that contained Y 2 O 3 /SiO 2 or Y 2 O 3 /La 2 O 3 /P 2 O 5 /SiO 2 , and near‐surface pores were partially filled with Al 2 O 3 . This glass must also contain some Al 2 O 3 and initially some chlorine. Al OC l decomposition products were predicted using the FactSage ® Thermochemical code, and were characterized by mass spectrometry. Effects of Al OC l precursors on monazite coated and uncoated Nextel™ 610 fibers tow and filament strength were evaluated. A mechanism for the severe degradation of the oxide‐oxide CMC s and Nextel™ 610 fibers that involves subcritical crack growth promoted by release of chlorine containing species during breakdown of intergranular glasses in an anhydrous environment is proposed.