Page 224 - Engineered Interfaces in Fiber Reinforced Composites

206 Engineered interfaces in fiber reinforced composites

polymeric matrices. For polymer matrix composites (PMCs), although the chem-
istry and thermodynamic compatibilities required are as complicated as in other
composites, the prime objectives of the interface characteristics are mostly a strong
bond for efficient stress transfer and a good resistance to prolonged environmental
attack in service conditions. Similarly, a strong interface bond with high strength
properties are often desirable in metal matrix composites (MMCs). The reaction
products formed at the interface region at high processing temperatures generally
increase the chemical bonding, but degrades the gross mechanical properties of the
composite. Therefore, a compromise is necessary between the desired properties,
and proper control of chemical reaction is a primary concern in the design and
manufacture of MMCs. Because the service temperatures for most MMCs are
significantly lower than those for ceramic matrix composites (CMCs), the avoidance
of environmental attack is of secondary importance, as is the issue of fracture
toughness due to their inherently high ductility and inelastic fracture behavior.
The high Young’s modulus and good thermal stability of CMCs coupled with
their low density and corrosion resistance, compared with MMCs, make them very
attractive for high temperature applications. In particular, those reinforced with Sic
and A1203 fibers are increasingly applied to manufacture engineering components
requiring high performance. As for other composite materials, the structure and
mechanical characteristics of the fiber-matrix interface provide a key to effective
control of damage in designing CMCs. The aim is usually to make the interface
rather weak in order to improve the fracture toughness through interface-related
failure mechanisms, because the weakest aspect of most CMCs is their inherent
brittleness and extremely low damage tolerance. On the other hand, an adequate
interfacial strength is sometimes required to provide good strength properties, in
particular in the transverse direction. Indeed, proper control of the interface
properties is essential to achieving often conflicting requirements of high strength
and high fracture toughness.
In this section, surface treatments and coatings of fibers now popularly used for
MMCs and CMCs, particularly carbon, boron (B/W), silicon carbide (Sic) and
alumina (A1203) fibers are discussed. Several approaches have been used to achieve
the desired compatibility between fiber and matrix, that include the applications of
diffusion barrier coating and fiber size, and the control of matrix composition (i.e.
alloying elements), impurity content and composite fabrication procedure. Among
these techniques, fiber coating and alloying element control methods are used
extensively, that are the subject of the present section. Recent advances on the
development, availability and applications of a wide range of new and existing
inorganic fibers have been reviewed in a special volume of the journal, Composites
Science and Technology (Mai, 1994).

5.5.2. Selection of coating materials and coating techniques

5.5.2.1. Coating materials
The interfacial characteristics can be optimized by the application of appropriate
coatings to the fiber, allowing desired mechanical properties of the composites to be