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Unformatted text preview: along the ﬁber-matrix
interface while an airplane is on the ground, the moisture may freeze upon ﬂight, causing
further degradation of the interface and cracking. Thus, the nature of bonding at the
interfaces is important.
The ﬁber-matrix interfacial bond strength can be characterized by the shear stress
required to cause sliding between the ﬁber and the matrix. The interface bonding can be
mechanical, chemical, or both.
Under certain conditions the differential thermal expansion described above can actually aid in the formation of a mechanical bond between the matrix and the ﬁber. Since
most matrix materials have a higher coefﬁcient of thermal expansion than the ﬁbers,
cooling from elevated processing temperatures results in compression, or frictional,
bonding across the ﬁber-matrix interface. Curing temperatures for thermosetting resins
are in the 100–200 C range, and the expansion coefﬁcients of these materials are high
compared with glass and carbon (see Tables 14.3–1 and 14.3–2), which are the most
common reinforcing materials. Thus, this type of bonding can be easily accomplished in
these composites. Mechanical bond strength can be increased by making the ﬁber surface
rougher, which must be done carefully to avoid compromising ﬁber strength. However, in
most instances, mechanical bonding alone is not sufﬁcient; it is used to supplement
strength derived from chemical interactions.
Chemical bonding can be in the form of a wettability or secondary bond, which results
from electron interactions between the ﬁber and matrix. This type of bonding occurs
when, during fabrication, the molten matrix “wets” the ﬁber. As discussed in Section 8.2.4, the important variables are the surface energies of the matrix and ﬁbers, which
in turn determine the contact angle, the area of contact, and, ultimately, the bond strength.
Because the electronic interactions associated with this type of bond occur over a distance
of only 2–4 angstroms, it is important that the two bonding surfaces come into intimate
contact with each other. Cleanliness of the ﬁber surface is necessary for such contact to
occur. Layers of dust or oil can prevent these interactions, thereby reducing the bond
A second type of chemical bonding, referred to as reaction or primary bonding, can
occur as a result of mass transfer through solid-state diffusion between the ﬁber surface
and the surrounding matrix. This can result in the formation of a compound at the interface. In such cases, the interface becomes less sharply deﬁned and is more appropriately | e-Text Main Menu | Textbook Table of Contents 12.01.98 plm QC2 rps MP 589 pg590 [V] G2 7-27060 / IRWIN / Schaffer Part III Properties ;;;;;
;; Interface Solid solutions M F Intermetallics
Metal F Concentration M Fiber Fiber Metal FIGURE 14.3–5 Schematic of the interface region in a metal-matrix composite showing the metal (M) and ﬁber (F)
concentrations and the metal-ﬁber reaction compounds that may form in the interface region. (Source: K....
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