Defense Intelligence Reference Document Materials For Advanced Aerospace Platforms

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will eventually fracture at their weakest points, which, statistically, is often somewhere
in the matrix rather that at the mouth of the crack. These fractured fibers then must be
pulled out of the matrix during crack extension under continued loading, an action that
dissipates additional energy. Therefore, the total energy expended during fracture of a
CMC is much greater than it would be in the case of a monolithic specimen of the
ceramic matrix. The crack bridging and pullout mechanisms of toughening, with the
attendant increase in energy dissipation during fracture, are the conceptual foundation
for CMCs. In SiC fiber-reinforced SiC matrix (called SiC-SiC) CMCs, the toughness can
be as high as 10 times that of the unreinforced matrix. Clearly, the nature of the fiber-
matrix interface determines the resistance to fiber pullout after fracture and controls
the toughness. In high-temperature CMCs, the fibers are coated to achieve an
intermediate fiber-matrix interfacial bond strength that optimizes pullout toughening. If
the interface is too weak, the fibers pull out too easily. If it is too strong, the fibers
break without any pullout. Neither of these situations maximizes the toughness.
Therefore, the challenge in creating a tough CMC is not only creating this interface but
also finding a coating that will remain stable over time during exposure to elevated
temperatures. As service temperature increases, this latter requirement becomes more
challenging. For extended service, SiC-SiC CMCs currently are limited to about 1,400
°Celsius, but this is higher than the capability of metallic materials.
The matrix microcrack stress is also significant because once the matrix develops
microcracks, it allows the environment to gain access to the fiber-matrix interfaces. The
role of environment can be to alter the nature of the interface and reduce the extent of
pullout toughening. The obvious difficulty with this is that the properties are better
initially than after a period of exposure in service. This leads to a nonconservative
design that is dangerous. Absent an arbitrary knockdown, there currently exists no
means of estimating the reduced toughness as a function of service life.
Nevertheless, CMCs are the material class that holds the greatest promise of defeating
the temperature limits of current metals. An important issue at present is the limited
industrial base for producing CMCs and the even more restrictive range of suitable
ceramic fibers available for use as the reinforcement in CMCs. The cost of CMCs is
currently very high but should come down with increased demand, as happened with
TMCs. Even so, CMCs will always be expensive, making development of efficient designs
that make optimal use of this class of material important.
Carbon-Carbon Composites
Carbon-carbon composites (C-CCs) consist of carbon fibers in a typically amorphous
carbon matrix. In principle, this class of materials is very attractive because it has very
low density and high strength at elevated temperatures. C-CCs also have good
oxidation resistance at very high temperatures (1,200 °Celsius) because they use an
Si-rich coating that forms a stable, protective SiO2 film on the surface. This coating
system is "self healing" if breached because, among other things, it is glassy (viscous)
and reforms over cracks. The principal vulnerability occurs at intermediate
temperatures where the viscosity of the glass is high enough that it may not flow
rapidly enough to heal a breach, exposing the reactive C substrate to the damaging
oxidizing environment. As with CMCs, there also are some questions concerning long-
term changes in the fiber-matrix interfaces that are at least partially responsible for the
loss of impact toughness over time, as experienced in the Columbia accident.
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