Download Applying Silicon Carbide to Optics

OPTICS | ENGINEERING
Engineers
prepare the
3.5-m SiC
primary mirror
of the Herschel
Space Telescope,
which is slated
to launch in
September 2008.
© EADS Astrium/Patrick Dumas
Applying Silicon Carbide to Optics
David A. Bath and Eric A. Ness
Silicon carbide (SiC) is a promising optical material that offers several performance advantages
over traditional optics. Due to its unique physical and thermal properties, it can be used for a
wide range of applications—from large space-based telescope systems to small galvo mirrors.
I
n general, silicon carbides have a low
density, high modulus (stiffness), low
thermal expansion and high thermal
conductivity. SiC is very hard, making
it polishable to fine surface finishes
(< 10 Å RMS).
When these properties are combined,
SiC materials exhibit a high specific
stiffness that enables them to perform
as lightweight, stiff structures. Silicon
carbides also have high thermal stability,
allowing structures to quickly dissipate
heat and maintain size during temperature changes. Silicon carbide optics combined with SiC support structures can be
integrated in an athermal optical system
that provides unparalleled performance in
a dynamic thermal environment.
Advantages over
traditional materials
Silicon carbide is stronger and stiffer than
typical optical glasses. A SiC optic that
has the same deformation characteristics
as glass can be made at one-third the
weight, resulting in a significant savings—especially for space-based applications. Silicon carbide also has a higher
[ Properties of common optical materials ]
Al
Material/ Property
units
Invar TA6V
ULE
6061
Density, r
Zero- Si
dur
SiSiC
(30%)
C- Sintered SiC
SiC
SiC
CVD
Be
1.8
g/cm3
8.1
4.4
2.7
2.2
2.5
2.3
2.9
2.6
3.1
3.2
GPa
148
114
71
67
93
130
330
220
410
465 300
Thermal Expansion,TCE
ppm/K
1.3
8.6
23.9
.03
.05
2.6
2.6
2.0
2.2
2.2 11.5
Thermal Conductivity,Tc
W/mK
10.7
7.1
167
1.3
1.6
155
155
125
175
200 216
Modulus, E
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PM_May_OPN.qxd
fracture toughness and better fatigue
resistance than glass. In addition, it is
chemically inert and can perform at temperatures from cryogenic up to 1500° C.
When compared to aluminum, SiC offers
greater stiffness and polishability, and it is
more temperature-insensitive due to the
low thermal expansion.
The optical material with the closest performance to SiC is beryllium,
primarily due to its high specific stiffness.
Beryllium has a 10-percent higher specific
stiffness compared to SiC, but less than a
quarter of the steady-state thermal stability. Aerospace engineers in the United
States typically rely on beryllium, while
the European space community is leading
in the implementation of silicon carbide
materials. Growth in the use of SiC is
anticipated as processing techniques
improve and engineers gain a better
understanding of how to design systems
with this ceramic. A significant drawback
of using beryllium is the toxicity of the
precursor materials; this is not the case
with SiC.
Types of silicon
carbide materials
A wide variety of silicon carbides are
commercially available as either singleor two-phase systems. Each type has
different properties:

Chemical vapor deposition (CVD)
SiC is the most pure, fully dense form;
it is fabricated from chemically pure
gases. Available in coatings or sheets,
it is also used as a cladding on other
types of SiC.



4/4/08
11:47 AM
OPTICAL
POWER METERS
Sintered silicon carbide is a 99 percent pure, single-phase form that is
produced from a powdered precursor.
The optic can be machined while in
the compacted, unfired state, but then
it experiences a shrinkage of approximately 20 percent.
SiSiC is reaction-bonded or Si-infiltrated SiC, typically with 30 percent
free silicon remaining in the body.
SiSiC experiences very little shrinkage during infiltration; this can be an
advantage in net-shape forming.
C-SiC or CeSiC is a SiC-vapor
infiltrated material with short carbon
fibers. The inclusion of fibers significantly increases the fracture toughness.
These materials are available as
monolithic bulk materials, CVD-coated
materials, lightweight foam, and as a
constituent in ceramic and carbon matrix
composites. For intricate shapes, some
of the materials can even be electrodischarge machined.
Single-phase materials develop no
internal stresses when cooled from
processing temperatures, ensuring
mechanical stability at the temperature
of application.
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[ Athermal optics performance ]
Thermal stability [Tc/CTE]
100
CVD SiC
90
Sintered
intered
SiC
80
70
60
50
Silicon
ULE
40
30
Better
performance
C-SiC
SiSiC
ZERODUR
20
INVAR
10
0
0
20
AI-6061
TA6V
40
Be
60
80
100
120
140
160
180
Specific stiffness [E modulus/density]
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Combining the thermal and physical properties of SiC translates into better performance.
OPN May 2008 | 11
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OPTICS | ENGINEERING
Processing of sintered
silicon carbide
Processing starts by mixing silicon carbide powder with sintering aids, which
enable densification during sintering, and
a binder that allows the powder to hold
a compacted shape. The blended and
spray-dried powder is then isostatically
pressed into a billet, ranging in weight
from fractions of a gram to more than
half a ton. This “green” unfired billet has
a chalk-like consistency, and can easily be
machined with diamond-tipped carbide
milling tools.
The green part is designed about
20 percent larger in all dimensions to
account for shrinkage during sintering.
As many features as possible are designed into the green part; this minimizes diamond machining after firing.
For instance, to make lightweight, stiff
optics, pocketing of the backside material
leaving ribs supporting the optic surface
are formed at this stage, as in the image
below.
Sintering is done in a graphite-lined
vacuum furnace at 2100° C. Due to
natural variations in shrinkage, the final
dimensions are accurate to ±0.5 percent.
Thus, critical features such as the optical
[ A production process used to make sintered SiC optical substrates ]
CAD design &
CNC programming
Spray-dried,
blended
SiC powder
Pressed green
billet
Multi-axis green
machining
Cold isostatic
pressing
Debindering & sintering
Optical surface
grinding & polish
facesheet and attachment points must
be machined after firing. Sintered SiC is
extremely hard (the fourth hardest known
material) and can only be machined using
diamond tools.
The basic optic geometry is defined by
grinding; final geometry is achieved by
diamond lapping and polishing. Sintered
silicon carbide has 2 to 3 percent porosity, which limits the surface finish that can
be achieved. For many optical applica-
Final quality inspection
tions, this porosity must be eliminated, so
cladding with CVD SiC is helpful. This
cladding’s thermal and physical properties are nearly identical to those of the
sintered substrate, so the bond between
them is uniformly excellent. The CVD
cladding can then be polished to extremely fine finishes. Standard reflectance
coatings can be deposited on a cladding
to complete the optic.
Design considerations
The 1-m parabolic
SiC mirror backface
has a lightweight
rib structure.
Fabricating very large optics is limited by
available equipment. For instance, one of
the largest cold isostatic presses available
is 1.5 m across, which limits the fired optic (after shrinkage) to about a 1-m diameter. An alternative to fabricating a single
monolithic part is to design the part
from smaller pieces and assemble them
via a brazing process (Proc. SPIE 6666,
66660L). A large furnace is still required
for the brazing process. However, due to
the relatively low temperatures involved,
obtaining such a furnace is more feasible
than building a large sintering furnace or
CVD chamber.
One braze alloy, based on silicon, was
developed to be a CTE (coefficient of thermal expansion) match with the sintered
SiC. The actual brazed joint width can be
less than 25 µm, yielding a joint strength
nearly equivalent to the bulk material.
This process has been used successfully
Courtesy CoorsTek Inc.
12 | OPN May 2008
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to develop the primary mirrors for the
1.5-m-diameter Aladin and the huge
3.5-m diameter Herschel telescope.
Perhaps the largest hurdle to adopting
SiC for optical applications is figuring
out how to design to the strength and
reliability specifications of brittle materials. This issue has been addressed when
designing glass optics. However, to implement a fully athermal design of SiC, both
the optics and their structural supports
need to be made from SiC.
Due to the low fracture toughness
of ceramics, new design methodologies
incorporating Weibull statistics should be
used. To be statistically valid, these design
approaches require access to extensive
test data of the specific SiC type from the
manufacturer providing the material. An
example of such a design approach is the
CARES/Life reliability code (NASA/TM2003-106316). This code incorporates
material strength test data, fracture mechanics parameters, and an FEA analysis
for calculating reliability estimates.
Up-to-date, Relevant Information
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Conclusion
Adopting new material systems can be a
long and arduous path, particularly for
ceramics, because of their brittle nature.
However, thanks to recent advances in
ceramic processing technologies and
growth in commercial competence, SiC
is now being used for mirrors and other
optics. The unique thermal and physical
properties of silicon carbide make it an
ideal material choice for athermal optical
designs. Moving forward, the engineering community should generate material
databases in order to use design methodologies that account for their low fracture
toughness. t
[ David A. Bath ([email protected]) and
Eric A. Ness ([email protected]) are with
CoorsTek, Inc., in Hillsboro, Ore., U.S.A. ]
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[ References and Resources ]
>> D.A. Bath et al. “Fabrication and optical
characterization of a segmented and
brazed mirror assembly,” Proc. SPIE 6666,
66660L (2007).
IEEE Information Driving Innovation
>> N.N. Nemeth et al. “CARES/Life Ceramics
Analysis and Reliability Evaluation of Structures Life Prediction Program,” NASA/TM2003-106316, February 2003.
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