SAMPE Journal - May/June 2023 - 22

FEATURE / NEXT GENERATION CMC STRUCTURES
distributed fiber loading. However, the fibers were
clearly damaged during the composite fabrication
process as they were smaller in length than their
formed state during fabrication, and the fracture
surface was quite irregular. Thus, while the strength
of the composites is comparative to reported values
for other SiC/SiC composites which typically range
in the 40 to 60 ksi range
14-16
, there are still issues
that needed to be addressed. For instance, it was
unclear whether the carbon coating deposited
enough material to act effectively as an interphase.
Microscopy of the composites did show a smooth
fiber surface, as opposed to a rough crystalline
surface of as-prepared silicon carbide fibers. One
problem that arose was the fragility of the fiber
veils after the carbon treatment. An assumption
was that the carbon coating, if present, would
sufficiently hold the veil together. Whether the
carbon coating did not do that, or there was not
enough carbon coating to do so is unclear, but
the composites certainly suffered high weight
losses and other difficulties in the infiltration
process. These difficulties led to issues when
these composites were pressed and re-infiltrated
at Technology Assessment & Transfer and North
Country Composites, leading to smaller, less dense,
and thinner final composites, as shown in Figure 8.
In
subsequent experimental development,
three changes were made to the composite process
to both improve upon the previous coupon
results and to allow for ease of comparison to
industrial standards. The first change was the
improvement of the fiber coating rig; alterations
were made to increase the temperature from the
laser, with the goal of increasing the deposition
of carbon from the gaseous precursors. 1 " x1 "
sections of veil were sectioned from scrap veil
(silicon rich) to use for confirmation experiments
of carbon coating, and to get a better sense of the
relations between experimental parameters and
the resultant material. The second change was
changing the matrix material from Si3
N4 to SiC to
more adequately compare to industry materials.
The third change was the material and method of
matrix infiltration. Polysilazane-based polymeric
materials were secured from Extreme Environment
Materials Solutions (EEMS, Saratoga Springs, New
York) which has prompted a change in the prepreg
and green body fabrication process. Along with
resurrection of some processing equipment at the
Saratoga facility, such as a high temperature press,
these adjustments to a polymer infiltration and
pyrolysis (PIP) approach now allows the matrixforming
step to be conducted in-house. Figure
9 shows in-process composite panels that are
22 | SAMPE JOURNAL | MAY JUNE 2023
Figure 8. Fired composite coupons from Technology Assessment & Transfer.
undergoing the PIP polymer infiltration step. Work
is on-going to optimize the processing parameters
in forming the PIP-based matrix, with the next
round of composite sample panels planned for
thermomechanical testing at CTL.
Homogenous Joining of Ceramics and
Composites
As part of efforts supported by the Department of
Energy in commercial nuclear power generation
technology, an innovative approach to ceramic and
composite welding using a CMC material system
based on CVD processing has been developed.
Embedded-wire CVD (EWCVD) is a technology that
utilizes high performance fibers made by LCVD,
formed into a non-woven veil, and then attached
to underlying substrates using a remarkably quick
and efficient composite matrix phase fabrication
step, finally resulting in a unified welded composite
structure. A wire wrap secures the non-woven and
also delivers, through resistive heating of the wire,
the energy input to drive the gas-phase reaction.
This matrix formation process, akin to chemical
vapor infiltration (CVI), delivers the highest purity
material as expected from a CVD foundation. The
welded structure is referred to as a homogeneous
joint in which two similar pieces of material are
fused together into a single piece, such that the
joint material is then indistinguishable from the
original material. Welding is a form of homogenous
Figure 9. Infiltration cells for soaking PIP resin into non-woven veil.
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SAMPE Journal - May/June 2023

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