This fibre was first introduced by Talley in 1959 . In commercial production of boron fibres, the method of Chemical Vapour Deposition (CVD) is used. The CVD is a process in which one material is deposited onto a substrate to produce near theoretical density and small grain size for the deposited material. In CVD the material is deposited on a thin filament. The material grows on this substrate and produces a thicker filament. The size of the final filament is such that it could not be produced by drawing or other conventional methods of producing fibres. It is the fine and dense structure of the deposited material which determines the strength and modulus of the fibre.
In the fabrication of boron fibre by CVD, the boron trichloride is mixed with hydrogen and boron is deposited according to the reaction
In the process, the passage takes place for couple of minutes. During this process, the atoms diffuse into tungsten core to produce the complete boridization and the production of and . In the beginning the tungsten fibre of 12 diameter is used, which increases to 12 . This step induces significant residual stresses in the fibre. The core is subjected to compression and the neighbouring boron mantle is subjected to tension.
The CVD method for boron fibres is shown in Figure 1.7.
Figure 1.7: Schematic of reactors for silicon carbide fibres by Chemical Vapour Deposition
The key features of this fibre are listed below:
These are ceramic monofilament fiber.
Fiber itself is a composite.
Circular cross section.
Fiber diameter ranges between 33-400 and typical diameter is 140 .
Boron is brittle hence large diameter results in lower flexibility.
Thermal coefficient mismatch between boron and tungsten results in thermal residual stresses during fabrication cool down to room temperature.
Boron fibres are usually coated with SiC or so that it protects the surface during contact with molten metal when it is used to reinforce light alloys. Further, it avoids the chemical reaction between the molten metal and fibre.
Strong in both tension and compression.
Exhibits linear axial stress-strain relationship up to 650.
Since this fibre requires a specialized procedure for fabrication, the cost of production is relatively high.
The boron fibre structure and its composite is elucidated in Figure 1.8.
Figure 1.8: Boron fibre structure and its composite