Strong, Tough Ceramics Containing Microscopic Reinforcements: Tailoring In-Situ Reinforced Silicon Nitride Ceramics Page: 1 of 6
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Strong, Tough Ceramics Containing Microscopic Reinforcements:
Tailoring In-Situ Reinforced Silicon Nitride Ceramics
P. F. Becher
Metals and Ceramics Division
Oak Ridge National Laboratory
Oak Ridge, TN
Ceramics with their hardness, chemical stability, and refractoriness could be used to design
more efficient energy generation and conversion systems as well as numerous other
applications. However, we have needed to develop a fundamental understanding of how to
tailor ceramics to improve their performance, especially to overcome their brittle nature.
One of the advances in this respect was the incorporation of very strong microscopic rod-
like reinforcements in the form of whiskers that serve to hold the ceramic together making it
tougher and resistant to fracture.1 This microscopic reinforcement approach has a number
of features that are similar to continuous fiber-reinforced ceramics; however, some of the
details are modified. For instance, the strengths of the microscopic reinforcements must be
higher as they typically have much stronger interfaces. For instance, single crystal silicon
carbide whiskers can have tensile strengths in excess of > 7 GPal or > 2 times that of
continuous fibers. Furthermore, reinforcement pullout is limited to lengths of a few
microns in the case of microscopic reinforcement due as much to the higher interfacial shear
resistance as to the limit of the reinforcement lengths. On the other hand, the microscopic
reinforcement approach can be generated in-situ during the processing of ceramics. A
remarkable example of this is found in silicon nitride ceramics where elongated rod-like
shape grains can be formed when the ceramic is fired at elevated temperatures to form a
Figure 1. A larger elongated grain in a silicon nitride ceramic bridges across a crack behind
its tip. Note the the interface between the grain and the matrix has separated/debonded
along a portion of the grain's length (arrows). A glass network (white) surrounds all the
silicon nitride grains as well.
As seen in Figure 1, these large elongated silicon nitride grains can act as microscopic
reinforcements when the interface between the reinforcement and the surrounding
integranular glass separates (debonds) well before the reinforcement fractures. This
interfacial debonding process allows the reinforcement to form a bridge across the main
crack raising the fracture resistance of the silicon nitride. Thus, introduction of these larger
elongated grains combined with the interfacial debonding process can improve the fracture
resistance. However, it has now been shown that one must contol the generation of such
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Becher, P.F. Strong, Tough Ceramics Containing Microscopic Reinforcements: Tailoring In-Situ Reinforced Silicon Nitride Ceramics, article, June 27, 1999; Tennessee. (digital.library.unt.edu/ark:/67531/metadc678626/m1/1/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.