Module 1: Introduction to Composites
  Lecture 3: Reinforcement: Materials
 

CVD on Tungsten or Carbon Core:

  • This fiber is similar in size and microstructure to boron.
  • The fibres are produced on both tungsten and carbon cores.
  • These fibres are relativity stiff due to thicker diameter of the fibres. The diameter of the fibres is about 140 .
  • The fibres have strength in the range of 3.4 – 4.0 GPa.
  • Failure strain is in the range of 0.8 - 1%.
  • The Young’s modulus is about 430 GPa.
  • The fibres show high structural stability and strength retention even at temperatures above 1000  .

The CVD with  as the reactant, SiC is deposited on the core as follows:

  • The SiC fibres produced on a tungsten core with a diameter about 12  . It shows a thin interfacial layer between the SiC mantle and the tungsten core. In case, when carbon fibre is used the fibre diameter of the carbon fibres is about 33 .
  • Both type of SiC fibre have smoother surfaces than a boron fibre. This is because there is a deposition of small columnar grains as compared to conical nodules in boron fibres.
  • The SiC fibres produced with carbon core are used in light reinforced alloys. These fibres are produced with a surface coating. The composition of this coating varies from carbon rich from inner surface to silicon carbide at the outer surface.
  • The fibres that are used to reinforce the titanium have a protective layer which varies from a carbon rich to silicon rich and again to a composition which is rich in carbon at the surface. The outer surface acts as a protective surface and when it comes in contact with molten and highly reactive titanium. The fibres are made by Specialty Materials Inc. under the trade name SCS-6. The coating increases the fibres diameter by 6 .
  • The fibre has low failure stresses due to surface flaws.
  • The higher strength of fibre is due to the controlled defects at the core-mantle interface.
  • The strength of SiC fibres produced using CVD is seen to be anisotropic. The radial strength is significantly lower than longitudinal tensile strength.
  • When the fibres are heated to above 800 ºC in air for a long period, they lose their strength due to oxidation of the carbon rich outer layers.

The CVD for SiC fibres is shown in Figure 1.9.

Figure 1.9: Schematic of reactors for silicon carbide fibres by Chemical Vapour Deposition