Page 28 - Mechanics Analysis Composite Materials

Chapter 1. Introduction 13

approximately the same way as for glass fibers. They do not absorb water and have
high chemical resistance, but demonstrate relatively low temperature and creep
resistance (see Fig. 1.8).
Boron fibers were developed to increase the stiffness of composite materials while
glass fibers were mainly used to reinforce composites of the day. Being followed by
high-modulus carbon fibers with higher stiffness and lower cost, boron fibers have
now rather limited application. Boron fibers are manufactured by chemical vapor
deposition of boron onto about 12 pm diameter tungsten or carbon fiber (core).
Because of this technology, boron fibers have relatively large diameter, 10C~200pm.
They are extremely brittle and sensitiveto surface damage. Mechanical properties of
boron fibers are presented in Table 1.1 and Figs. 1.7 and 1.8. Being mainly used in
metal matrix composites, boron fibers degrade under the action of aluminum or
titanium matrices at the temperature that is necessary for processing (above 5OOOC).
To prevent this degradation, chemical vapor deposition is used to cover the fiber
surface with about 5 pm thick layer of silicon carbide, Sic, (such fibers are called
Borsic) or boron carbide, B4C.
There exists a special class of ceramic fibers for high-temperature applications
composed of various combinations of silicon, carbon, nitrogen, aluminum, boron,
and titanium. The most commonly encountered are silicon carbide (Sic) and
alumina (A1203)fibers.
Silicon carbide is deposited on a tungsten or carbon core-fibre by the reaction of a
gas mixture of silanes and hydrogen. Thin (8-15 pm in diameter) Sic fibers can
be made by pyrolysis of polymeric (polycarbosilane) fibers under temperature of
about 140OOC in an inert atmosphere. Silicon carbide fibers have high strength
and stiffness, moderate density (see Table 1.1) and very high melting temperature
(2600°C).
Alumina (A1203)fibers are fabricated by sintering of fibers extruded from the
viscous alumina slurry with rather complicated composition. Alumina fibers,
possessing approximately the same mechanical properties as of Sic fibers have
relatively large diameter and high density. The melting temperature is about
2000"c.
Silicon carbide and alumina fibers are characterized with relatively low reduction
of strength under high temperature (see Fig. 1.9).
Promising ceramic fibers for high-temperature applications are boron carbide
(B4C)fibers that can be obtained either as a result of reaction of a carbon fiber with
a mixture of hydrogen and boron chloride under high temperature (around 18OOOC)
or by pyrolysis of cellulosic fibers soaked with boric acid solution. Possessing high
stiffness and strength and moderate density (see Table 1.1) boron carbide fibers
have very high thermal resistance (up to 2300°C).
Metal fibers (thin wires) made of steel, beryllium, titanium, tungsten, and
molybdenum are used for special, e.g., low-temperature and high-temperature
applications. Characteristics of metal fibers are presented in Table 1.1 and Figs. 1.7
and 1.9.
In advanced composites, fibers provide not only high strength and stiffness
but also a possibility to tailor the material so that directional dependence of its