JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014 - 54

CMC
Long vs. short fibre
reinforcement
CMCs may be reinforced by continuous (long) fibres or discontinuous
(short) fibres. Long-fibre CMCs
are typically comprised of continuous SiC fibres dispersed in a SiC
matrix. They offer superior structural
properties because the fibres carry
the majority of applied loads. The
matrix material serves primarily as a
binder, holding the fibres in place and
providing mechanical and chemical
protection. However, the fabrication
of long-fibre CMCs is difficult and
requires expensive raw materials.
Long-fibre CMCs are typically
fabricated using infiltration methods,
where a porous fibre preform of the
desired shape is "infiltrated" with a
pre-ceramic fluid (liquid or gas) to
form the matrix. The fibre preform
is first prepared by incorporating
continuous SiC fibres into flexible
sheets by resin impregnation, laying
up these sheets into a mould, and
then applying pressure or vacuum to
the mould at elevated temperatures.
Fabricating long-fibre CMCs is time
consuming, expensive, and requires
specialized equipment. Due to their
extremely high cost, the commercial
applications for long-fibre CMCs are
limited.
In contrast, short-fibre CMCs can be
produced by traditional ceramic fabrication techniques including pressing,
extrusion, and casting. In these processes, the dispersed phases (fibres)
are mixed with the matrix material
prior to processing. A wide range of
established techniques exist for the
production of parts of practically
any size and complexity. As a result,
short-fibre CMCs can be produced
using existing equipment, in high
volumes, and at relatively low costs.

ness, durability, and temperature
stability than conventional ceramics,
but do not require the structural
properties of long-fibre CMCs, which
are far too costly to offer competitive solutions in this cost-sensitive
market.
Short-fibre CMCs are an obvious
choice for these applications because
of their cost and manufacturing
advantages. Research has concluded
that short-fibre CMCs made with discontinuous SiC fibres have favourable
properties; yet, commercial acceptance does not exist. Why, then, have
short-fibre CMCs not been developed and commercialized? Why has
this market demand not been met?
The answer is that a suitable (costeffective) discontinuous SiC fibre has
not existed in the marketplace. Until
now, the only source of discontinuous
SiC fibre has been "chopped" continuous tow. The high cost of this fibre
prices short-fibre CMCs out of the
market, and has prevented broadbased commercialization
As a result, the commercialization of
CMCs has been limited to ultra-high
end applications that require ultimate
structural properties regardless of
cost. Thus, the development of CMC
technology has focused on continuous reinforcement.

SI-TUFF™ 7-Series:
Enablement for CMCs
Ever since CMCs came into the
industry spotlight, bold claims have
been made on their game-changing
potential. SI-TUFF™ 7-Series may
finally enable these claims to become
reality.

Fig. 3: SI-TUFF™ 7-Series is high-purity β-SiC
fibre and is a fraction of the cost of other SiC fibres

The commercial launch of SI-TUFF™
7-Series is planned for Spring 2014.
Typical properties are shown in Table
1. n
More information: www.acm-usa.com
david.mammarella@acm-usa.com
Tab. 1: Physical properties of GRE pipes

Composition

β-silicon carbide (β-SiC)

Crystal structure

Diamond cubic

Geometry

Discontinuous fibre

Mean diameter

7 µm

Mean length

65-70 µm (D50)

Modulus

350 GPa (estimated)

True density

> 3.0 g/cm3

Advantage: short fibre
There are large untapped markets for
CMCs which provide better tough-

Fig. 2: Market segmentation for CMCs based on reinforcement type. The demand for short-fibre
CMCs has not been met because no cost-effective fibre was available

54 jec composites magazine / No90 June - July 2014

http://www.acm-usa.com

JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #90 - June/July 2014