SAMPE Journal - March/April 2024 - 10

FEATURE / HIGH-TEMPERATURE APPLICATIONS
Typical Fiber Properties
Density
Weight/Length (24K)
Approximate Yield (24K)
Tow Cross-Sectional Area (24K)
Carbon Content
its on-and-off search for a PAN-based solution for
ablative carbon-carbon.
Hexcel commercialized a PAN-based carbon
fiber, HexTow® 85 in 2018 and this is the only USdomestic
supplier for carbon fibers that can be
used in ablative carbon-carbon applications.[4]
An
image of a spool of HexTow® 85 is shown in Figure
1. It was developed as a drop-in replacement of
rayon-based carbon fiber and can be processed
through stretch-breaking, spinning, weaving,
prepregging and part
fabrication with a new
sizing formulation developed for optimizing these
manufacturing processes. Several companies have
successfully used HexTow® 85 for carbon-carbon
production and it is qualified for use for defense
and commercial applications. The properties of
HexTow® 85 are listed in Table 1.
HexTow® 85 delivers
equivalent or better
strength and ablative performance to traditional
rayon-based carbon fiber in carbon-carbon
composite structures. Being PAN-based, the
supply chain is greatly simplified and provides a
robust and predictable supply of fibers for ablative
carbon-carbon structures.
IV. CERAMIC-CONVERTED CARBON FIBERS
A new carbon fiber processing method being
developed involves additional processing of the
carbon fiber to achieve additional performance
utility in the carbon fiber. Using a low-cost nonChemical
Vapor Infiltration (CVI) process, PAN,
pitch, or carbon nanotube (CNT) continuous
carbon fibers and yarns, either in tow or pre-woven
form, are exposed to a continuous controlled
gaseous atmosphere at elevated temperature
Table 1. HexTow® 85 physical properties.
US Units
0.0643 lb/in3
100 x 10-6
836 ft/lb
15.5 x 10-4 in2
85%
lb/in
SI Units
1.78 g/cm3
1.78 g/m
0.56 m/g
1.0 mm2
85%
which then results in a nano-thin metal carbide
layer formed into each individual carbon
filament. The nano-layer material can consist of
silicon carbide, tantalum carbide or other metal
carbides. The nano layer adds additional benefits
to carbon fibers. Benefits include suppression
of carbon vaporization above 2000°C which may
extend carbon -carbon service life at extremely
high temperatures. The nano-metal carbide layer
has been seen to function as a novel fiber-matrix
debonding interface in composite matrices which
can reduce or eliminate this costly manufacturing
step. Being a nano-thick layer, the carbon fiber
density is slightly higher being in the range of 1.82.2
g/cc. Other features include the nano-layered
fibers maintain original fiber drapability, modulus
and a high percentage of original fiber strength.
Active research and development are on-going to
include process and high temperature fiber and
composite characterization. Processing capability
scaleup is planned supporting transition of the
ceramic-carbon converted fibers into products,
such as 3-dimensional fiber architecture reinforced
composite fasteners. Figure 2 shows the nano-SiC
layer on a carbon fiber and a resulting 2D weave of
SiC/C converted fiber.
V. CARBON-CARBON MATRIX SYSTEMS
A. Phenolics
Phenolic based resin systems have been the most
widely used system to produce carbon-carbon
materials. Phenolics exhibit excellent char yields
which is the percentage of the resin that is converted
to carbon in the carbonization process. Barraday
has commercialized a product with HexTow® 85
Figure 2. Nano-Silicon carbide layer on carbon filaments (Provided by Advanced Ceramic Fibers, Inc.).
10 | SAMPE JOURNAL | MARCH APRIL 2024
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