Page 232 - Engineered Interfaces in Fiber Reinforced Composites

214 Enxineered interfaces in fiber reinforced composites

on the decomposition parameters, which are responsible for the crystal structure, the
layer thickness and the chemical attack of hydrogen on the carbon fiber.

5.5.4. Boron fibers

5.5.4.1. Introduction
Boron fibers are normally made by CVD of boron on a substrate, such as a fine
tungsten wire or carbon core. Fig. 5.26 shows a schematic drawing of boron fiber
production. A fine tungsten wire of 10-12 pm in diameter is pulled into a reaction
chamber at one end through a mercury seal and out at the other end through
another mercury seal. There is a critical temperature (above about 1150°C) to obtain
a boron fiber with optimum properties and structure, depending on the substrate
wire speed (Krukonis, 1977). The structure and morphology of boron fibers are
controlled by the conditions of deposition, temperature, composition of glasses, gas
dynamics, etc. In general, boron fibers have a corn-cob structure, as shown in
Fig. 5.29, which consists of nodules separated by boundaries originating from the
nature of CVD process. However, the surface of boron fibers based on a carbon
substrate is relatively smooth, a reflection of the smooth surface of the carbon core.
Boron fibers produced on a tungsten substrate contain a series of compounds,
such as W2B, WB, W2B5 and WBS, which are formed at the interface region by
diffusion of B into W. On the contrary, boron fibers with a carbon core do not
produce boron carbides as the interface reaction compound. Due to the composite
nature of boron fibers, complex internal stresses and defects such as voids and
structural discontinuities result from the presence of a core and the deposition
process. The strength of a boron fiber is cquivalent to the intrinsic strength of boron,
with an average tensile strength of about 3.8 GPa and a Young's modulus between
380 and 400 GPa. Representative properties of boron fibers and other non-oxide
inorganic fibers are given in Table 5.15. Fibers are often post-treated chemically or
thermally to make them more compatible with metal and ceramic matrices for
composites fabrication. Chemical treatment also serves to remove the surface
defects, whereas thermal treatment is intended to remove the residual stresses.

5.5.4.2. Reaction barrier coatings on boron jibers
Coatings on boron fibers have been applied to prevent the formation of reaction
products at the interface region with common matrices, such as AI and Ti, and other
ferrous metals including Fe, Co and Ni. The oxide films preexisting on the fiber
surface and the AI matrix may act to delay the reaction if the composite is made in
the solid state. However, the oxides are not useful in the presence of molten AI.
Coating materials developed specifically for aluminum matrices include Sic, BC and
BN (Carlsson, 1986). A Sic coating of thickness in the range 1.5-2.0 pm is applied
by a CVD process at 120&1300"C using a mixture of hydrogen and chlorosilane,
while a BC coating of 2-8 pm in thickness is obtained by a similar CVD process at
1150-1300°C using a mixture of hydrogen, boron trichloride and methane. A BN
coating is also applied through oxidation of the fiber surface in air at 1000°C,
followed by heating in the presence of ammmia at 1100°C. It is also found that Sic