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BORON FIBER

 


Introduction

Normally boron is a brittle material. Weintraub was the one who obtained

the first Boron fiber in 1911 by the reduction of boron halide with hydrogen  on a hot wire (substrate).

Boron fibers were developed after 1960s to increase the strength and stiffness of composite by reinforcement.

Fabrication

Ø Boron fibers are produced by CVD on a substrate by following two methods.

                      1.    Thermal reduction of boron halide

o  Low temperature.

o  Carbon coated glass fiber used as a substrate.

o  Weak and less dense boron fiber  is produced by this method because of gas entrapment.

2.    Reduction of Boron halide by hydrogen

o  High temperature.

o  Tungsten wire (12micron) used as a substrate which has a high density and melting point.

o  Uniform quality of Boron fiber is the result of this method.

Reaction :

2BX3 + 2H2 ------> 2B + 6HX

Where X =  Cl, Br, I, mostly Cl.

In the above reaction only 10% of BX3 is transformed to B.

Fig:- schematic of Boron fiber production by halide decomposition on W substrate.


Structure & Morphology

Boron fibers are worthwhile in the Amorphous form, whereas the crystalline form is not desirable as it is mechanically impair. They have normally nanocrystalline B-Rhombohedral structure which is obtained by CVD at 1300 C.

 At higher temperatures boron diffuses into Tungsten substrate resulting in the formation of borides. Generally WB4 and W2B5 are formed.

 Silicon carbide barrier coating by vapour deposition on Boron fiber  is used to restrict the reaction of Boron with a Matrix , especially Al.

 

Fig:- Structure of boron fiber

 

CORN-COB structure at the surface of Boron fiber in which nodules are discrete by grain boundaries as manifest below.

 

 

Fig:- Corn-cob structure at surface

 

Properties

 

Tensile Strength

3 – 4 GPa

Young’s Modulus

350 – 400 GPa

Density

2.6 g/cc for 100 micron dia

Melting Point

2040 C

 





Apart from this it has low thermal expansion coefficient up to 350 C. and it have relatively better properties in compression than other fibers.   

Boron fiber having a strength greater than 4 GPa , Tungsten boride (core) controls the fracture ,whereas for the strength less than 4 GPa surface flaws controls the fracture.

Typical example is a Boron fiber of 142 micron diameter have a tensile strength about 3.8 GPa  with a relatively low fracture energy.

        

Applications

Ø Military aircraft & Space shuttle

Ø Repair patches for PMCs

Ø Stiffening golf shafts & tennis rackets

Ø HY-BOR

 

 

Prepared by,

Parth Tailor,

Metallurgical and Materials Engineering,

FOTE,

The M.S University, Baroda

 

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