Chopped C/C Vs Long Fibre C/C

Posted on September 25, 2014 · Posted in Uncategorized

Chopped C/C in today’s modern heat-treatment industry

 

When looking for a C/C Composite, there are there are two key areas that should be focused on; Inter-laminar sheer strength (ILSS) and density. (As a rule the higher the density the longer the life span)

There are other areas you may also need to consider when evaluating your individual specifications depending on how and where the C/C Composite would be used in an application. But overall the above two specifications are the priorities, especially if the material is going to be machined in to complex shapes and structures.

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Density is the first area most users focus on, higher the density the stronger the material, the longer the life span. The average density for long-fiber C/C is around 1.3-1.4cm/3, this can be increased by further stages of impregnation and baking and you can raise the density to around 1.5-1.6. To achieve a higher density you would on average need to do ten cycles of baking and impregnation. The uniformed structure of Long-fiber limits the amount of open pores for the matrix to penetrate, this is why we see several cycles during the bake and impregnation stages to make sure the matrix fully bonds to the fiber. Even so there is a limit to the maximum density of long-fiber.

Chopped fiber does not suffer from the same issues during production, if we look at PC70 as an example of chopped fiber C/C composite. The arrangement of the fibers is non-uniform; the random structure of the fibers allows it be more porous granting easier impregnation of the matrix. After one impregnation and bake cycle the density is 1.7g/cm3.

Inter-laminar sheer strength or ILS also plays a key role in determining the overall strength, and this is especially applicable when machining C/C.

ILS describes the force that is needed to cause cracking between layers of fibers that leads to delamination. Fibers arranged in a plain weave stacked fashion (2D) are prone to cracking due to the layers not being bridged by other fibers. A good example of when this happens is during machining, precision parts like bolts or threaded studs are prone to delamination because the cutting process effectively pulls the layers apart.

3D structures, like chopped fiber do not suffer from this issue. Chopped fiber effectively covers a 360 degree bonded area at the X and Y while there is also an increased bonds on the Z-axis. The stronger bonds found between each layer of Chopped fiber greatly increases the ILS.

 

Inter-laminar strength of 2D plain-woven long fiber C/C Composite ranges between: 5.2 and 13.2Mpa

 

The inter-laminar strength of chopped fiber, for example PC70 C/C composite is 17Mpa

 

The high density and ILS, coupled with the other benefits of C/C Composites gives chopped fiber a unique advantage over other commonly used materials in High-temp environments. Especially when it comes to machining, chopped fiber offers very similar machining characteristics to isotropic graphite. Another benefit is the durability when loading and unloading a furnace. Chopped fibre is very durable against shock and accidental knocks.