Page 335 - Engineered Interfaces in Fiber Reinforced Composites

316 Engineered interfaces in fiber reinforced composites

(2) For a given fiber system, the optimum CTE of the interlayer increases with
increasing modulus and CTE of the matrix.
(3) For a low matrix modulus, the optimum CTE of interlayer can be approximated
by the average CTE of the fiber and matrix, whereas for high matrix modulus,
the optimum CTE of interlayer approaches the matrix value.
Caruso et al. (1990) further defined the required properties of the compensating
interlayer for SCS-6 (Sic) fiber/Ti3Al + Nb matrix and SCS-6 (Sic) fiber/Ti-15-3
matrix composite systems: the interlayer should have a modulus 15% that of the
matrix and CTE approximately equal to that of the composite system without the
compensating layer. Although the addition of a recommended interlayer can
mitigate the matrix cracking problem, it causes a slight reduction in the composite
modulus. Plastic deformation of the matrix is taken into account later (Arnold et al.,
1990, 1992; Arnold and Wilt, 1992), proposing the yield point and hardening slope
also play a significant role in reducing the stress concentrations within the interlayer.
In addition to the above criteria, Arnold et al. (1992) proposed that:
(1) The interlayer CTE should be greater than the matrix CTE.
(2) The interlayer thickness to fiber diameter ratio should be as large as other
thermo-mechanical considerations would allow.
(3) The yield point and hardening slope of the interlayer should be low compared to
those of the matrix material.
A candidate interlayer consisting of dual coatings of Cu and Nb has been
identified successfully for the SiC-Ti3A1 + Nb composite system. The predicted
residual thermal stresses resulting from a stress free temperature to room
temperature (with AT = -774°C) for the composites with and without the
interlayers are illustrated in Fig. 7.23. The thermo-mechanical properties of the
composite constituents used for the calculation are given in Table 7.5. A number of
observations can be made about the benefits gained due to the presence of the
interlayer. Reductions in both the radial, or, and circumferential, oo, stress
components within the fiber and matrix are significant, whereas a moderate increase
in the axial stress component, cz, is noted. The chemical compatibility of Cu with
the fiber and matrix materials has been closely examined by Misra (1991).
Similar studies have been reported for CMCs, but with different perspectives
regarding the effects of residual stresses (Hsueh et al., 1988; Kuntz et al., 1993). The
tensile residual stresses in the hoop direction may cause cracking in the ceramic
matrix, especially when combined with external loading. More importantly, the
compressive clamping stresses normal to the fiber surface in the radial direction
increases the shear stress required for fiber pull-out, and tends to inhibit extensive
debonding along the interface. Interfacial debonding, fiber pull-out and associated
fiber bridging of cracked surfaces are known to be the major toughening
mechanisms for brittle matrix composites containing ceramic matrices. (There is
another source of normal stresses at the interface during fiber pull-out, namely the
radial strains arising from the fiber roughness, see Chapter 4 (Keran and
Parthasarathy, 1991; Jero and Kerans, 1991).) As such, the major purpose of
interlayers for CMCs is to minimize the residual thermal stresses at the interface in
an effort to improve the fracture toughness, which is considered to bc onc of the