Interface modification for increased fracture toughness in reaction-formed yttrium aluminum garnet/alumina eutectic comp
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Interface modification for increased fracture toughness in reaction-formed yttrium aluminum garnet/alumina eutectic composites Luke N. Brewer,a) Derrick P. Endler,b) Shani Austin,b) and Vinayak P. Dravid Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208
Joseph M. Collins Saphikon Inc., Milford, New Hampshire 03055 (Received 10 May 1999; accepted 13 July 1999)
The validity of controlling interfacial toughness in reaction-formed composites was explored using solid-state reaction processing and microanalysis techniques. A variety of rare-earth oxides was added to a yttrium aluminum garnet (YAG)/alumina powder mixture and then melted in air. Some melts retained the crystallography and microstructure of the pure, binary YAG–alumina eutectic. Using scanning transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy, rare-earth ions were observed both to segregate to the YAG/alumina interface and to form a third phase. Some evidence of increased crack deflection at these interfaces was observed via indentation fracture.
TABLE I. Starting melt compositions.
I. INTRODUCTION
The production of tough, reliable ceramic composites is a primary thrust of structural ceramics research. In brittle–brittle composites, a common approach for increasing fracture toughness has been to weaken the interface between fiber and matrix or between layers by coating the fiber with weakly bonding oxide species,1,2,3 refractory metals,4 easy-cleaving oxides,5,6 or porous structures.1,7 In each case, the strategy is to promote crack deflection and interface delamination at the fiber– matrix interface or at the coating–fiber interface. This ex situ toughened approach has had some success, especially in the case of monazite-coated fibers1 and other ABO43 coatings on fibers. However, for both the weakly bonding coatings and easy-cleaving oxide coatings, thermodynamic stability and phase compatibility at high temperature are of major concern in the usefulness of the resultant composite.2,8 Reaction-formed or in situ toughened oxide composites, such as oxide–oxide eutectics, may be able to circumvent the chemical compatibility problems, but must still be engineered to increase the fracture toughness. Oxide– oxide eutectics have demonstrated thermody-
a)
Address all correspondence to this author. e-mail: [email protected] b) As a part of the Research Experience for Undergraduates Program at the NSF-MRC at Northwestern University. J. Mater. Res., Vol. 14, No. 10, Oct 1999
Starting composition (wt%)
Target interphase
80.0% YAE 1.7% CaO 18.3% WO3
CaWO4
85.0% YAE 3.0% CeO2 12.0% Al2O3
CeAl11O18
56% YAE 20% CaO 24% Al2O3
CaAl12O19
86% YAE 3.0% Pr2O3 11% Al2O3
PrxAlyOz
86% YAE 3.0% Nd2O3 11% Al2O3
NdxAlyOz
86% YAE 8.0% La2O3 6.0% Nb2O3
LaNbO4
85% YAE 3.0% La2O3 12% Al2O3
LaxAlyOz
86% YAE 6.0% La2O3 8.0% Ta2O3
LaTaO4
86% YAE 6.0% Y2O3 8.0% Nb2O3
YNbO4
87% YAE 5.0% Y2O3 8.0% Ta2O3
YTaO4
© 1999 Materials Researc
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