Page 32 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2. Characterization of interfaces 15

Table 2.2
Elemental composition of fibersa

Fiber Bulk Surface analysis Probable functional
group

E-glass Si, 0, AI, Ca, Mg, B, S, 0, AI -Si-OH, -SiOSi
F, Fe, Na
Carbon C, 0, N, H, metal C, 0, H JZOOH, C-OH, C=O
impurities
Boron (B/W core) W2B~, WB4 Bz03 as methyl B-OH, B-0-B
(inner core), borate
B (outer core)
Silicon carbide Si, W (inner core), Si, C Si-0-Si, Si-OH
(SiC,/W core) C (outer core), 0, N

"After Scolar (1974)

Special cases of reaction bonding include the exchange reaction bond and the
oxide bond. The exchange reaction bond occurs when a second element in the
constituents begins to exchange lattice sites with the elements in the reaction product
in thermodynamic equilibrium (Rudy, 1969). A good example of an exchange
reaction is one that takes place between a titanium-aluminum alloy with boron
fibers. The boride compound is initially formed at the interface region in an early
stage of the process composed of both elements. This is followed by an exchange
reaction between the titanium in the matrix and the aluminum in the boride. The
exchange reaction causes the composition of the matrix adjacent to the compound
to suffer a loss of titanium, which is now embedded in the compound. This
eventually slows down the overall reaction rate.
The oxide bond occurs between the oxide films present in the matching surfaces of
fiber and matrix. The reaction bond makes a major contribution to the final bond
strength of the interface for some MMCs, depending on the fiber-matrix
combination (which determines the diffusivity of elements from one constituent to
another) and the processing conditions (particularly temperature and exposure
time). A general scheme for the classification of interfaces in MMCs can be made
based on the chemical reaction occurring between fiber and matrix according to
Metcalfe (1974). Table 2.3 gives examples of each type. In class I, the fiber and
matrix are mutually non-reactive and insoluble with each other; in class 11, the fiber
and matrix are mutually non-reactive but soluble in each other; and in class 111, the
fiber and matrix react to form compound(s) at the interface. There are no clear-cut
definitions between the different classes, but the grouping provides a systematic
division to evaluate their characteristics. For pseudoclass 1 composites that include
B-AI, stainless steel-A1 and Sic-A1 systems, hardly any interaction occurs in solid
state diffusion bonding, but a reaction does occur when the A1 matrix is melted for
liquid infiltration.
In general, in most CMCs, chemical reaction hardly occurs between fiber (or
whisker) and matrix. However, an extremely thin amorphous film can be formed,