Rapid Fabrication of Mirrors with Nanolaminate Facesheets
and Composite Structures
Jack Massarello, Brett deBlonk, Arup Maji
Space Vehicles Directorate
Air Force Research Laboratory
3550 Aberdeen Ave SE
Kirtland AFB, NM 87117-5776
Abstract
The Air Force Research Laboratory Space Vehicles Directorate in less than two weeks total time has produced a lightweight
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mirror prototype with an area density of 13 kg/m and diameter of 15cm. This project seeks to use mirror replication
techniques to promptly fabricate an ultra-lightweight optical-quality mirror with an area density less than 15 kg/m2.
Improvements over conventional mirrors include the use of nontraditional mirror materials that eliminate the need for polishing
and kinematic support.
Summary
The purpose of this research is to fabricate an ultra-lightweight passive mirror composed of a deep support for bending rigidity,
composite replica for quality backing surface, and a nanolaminate face sheet for a high-quality reflecting surface. This
approach will enhance the structural and manufacturing performance of advanced mirrors by eliminating separate kinematic
supports, eliminating polishing requirements, and reducing area density.
With the two nanolaminates that the Air Force Research Laboratory (AFRL) received from Lawrence Livermore National
Laboratory (LLNL), one mirror has been fabricated, and one is planned. Table 1 summarizes the actual or measured
properties of Mirror Prototype #1 and the actual or projected properties of Prototype #2.
Table 1: Parameters summary of the two prototype mirrors at AFRL made with nanolaminate face sheets.
Prototype #1 has been fabricated, and #2 is planned.
Parameter
Diameter
Area Density
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Assembly/ Fabrication Time
Backing Structure
Surface Figure/ Accuracy
Mirror Prototype #1
Mirror Prototype #2
15 cm (6.0 in)
13 kg/m2
72 hours
Carbon Foam
4.5 waves P-V (HeNe)
15 cm (6.0 in)
5 kg/m2
72 hours
Isogrid Composite
(same as mandrel to first order)
Introduction
The fabrication of lightweight mirrors for space-based applications is an extremely challenging technical problem because the
(1) required quality in the surface shape is so high over a relatively large surface area, (2) the relative stiffness requirements
for aerospace applications make the use of traditional mirror materials difficult, and (3) the desired fabrication time and cost
are well under the capability of modern technology. Many different materials and fabrication techniques are being developed
to address these challenges, as described well in the current literature. Matson and Mollenhauer [2003] explain and compare
many different mirror technologies. MacEwen [2003] describes the advantages of separating the reflector function from the
structural function in a mirror, and this idea of separation is pursued in the current research effort.
The most significant concerns during mirror construction include mirror image quality and dimensional stability of the
supporting structure, the two concerns that separation of function address. To address this complex issue, an innovative
combination of graphite-fiber epoxy-resin (GrEp) composite backing structure and copper-zirconium nanolaminate film
[Barbee, 2003] is manufactured to characterize the fabrication parameters of an advanced lightweight mirror for space-based
optical systems. This paper describes the in-house research efforts at AFRL to build such a mirror prototype.
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Includes composite manufacturing time but excludes nanolaminate preparation.
Approach
Nanolaminate Fabrication
A critical technology of the current approach is the nanolaminate itself, of which relatively few are available for study.
Production of metallic nanolaminate mirror substrates at Lawrence Livermore National Laboratory (LLNL) results in thin
reflecting surfaces with 45 to 150micron total thicknesses. A direct replication process involves magnetron sputtering onto a
master tool or mandrel to manufacture each nanolaminate mirror over a period of approximately 7 days. During synthesis the
nanolaminate substrate reaches equilibrium at or near a temperature of ~ 50˚C (122°F). The nanolaminate used in this effort
has an initial reflective layer of corrosion-resistant gold, is followed by periods of Cu and amorphous CuxZr, and has a final
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layer of Zr/ZrO2; the overall thickness is approximately 45microns .
The mandrel has a 1.3 to 1.6nm RMS roughness with a figure that varies from part to part by approximately 3.5 waves peakto-valley (PV). Since the goal is to replicate an optical quality surface, the final nanolaminate/ composite mirror assembly
surface texture and figure will be characterized and compared to the as-received mandrel.
Assembly
The primary focus of this effort was to demonstrate the feasibility of incorporating a surface veil and gel coat to sufficiently
suppress micron level fiber print-through patterns on the supporting structure—creating an overall lightweight mirror assembly.
There are two main components that make up the complete lightweight mirror – the reflective nanolaminate and the composite
backing structure. Affixing one to the other presents fabrication challenges, the most prominent being the acquisition of an
adhesive material that will sufficiently adjoin the unabraded metallic nanolaminate to the reinforced polymer.
Appendix A details (1) the decision process for selecting the adhesive and (2) the prototype history of learning how to use the
adhesive. Appendix B describes the background of the GrEp backing structure that is replicated off a glass mandrel to ensure
surface quality and shape accuracy.
Fabrication of Mirror Prototype #1
Figure 1 represents the layers in the first mirror prototype. The GrEp replica is first made and bonded to the carbon foam
while being constrained on the front surface by a glass mandrel (see Appendix B). This GrEp replica mirror — with printthrough problems — is bonded to the nanolaminate according to the procedures learned through initial prototypes (see
Appendix A).
Nanolaminate
Adhesive
GrEp Replica
Adhesive
Carbon Foam
Figure 1: Diagram of layers for Mirror Prototype #1. Drawing is not to scale.
Figure 2 shows the mirror at several stages of this fabrication approach. More advanced processing techniques such as resin
encapsulation or assemblage under vacuum were not pursued with this prototype for simplicity, and the potential problem is air
voids behind the nanolaminate. Future assembly in vacuum will eliminate the issue of entrapped air.
The structure was allowed to cure for 24 hours and then placed in an oven at 50˚C (122˚F) to meet or exceed the as-sputtered
equilibrium temperature of the nanolaminate for ease of separation from the mandrel. Additionally, since a post cure
temperature of 149˚C (300˚F) is recommended for maximum exploitation of adhesive properties, care must be taken to avoid
the crystallization of the amorphous CuxZr layer; the risk is alteration of the nanolaminate morphology and properties at or
around 95˚C (203˚F).
After 30 minutes of oven time, the complete assembly was removed, followed by immediate delamination of the nanolaminate
from the float glass mandrel. Although the nanolaminate was fairly brittle, the component separated after considerable care
and effort and resulted in a rough-edged final mirror assembly as shown in picture 4 of Figure 2. The final mirror, with edges
trimmed, is shown in Figure 3.
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All dimensions and properties of the mandrel and laminate come from personal communication with Troy Barbee of the
Lawrence Livermore National Laboratory.
Figure 2: Assembly sequence for Mirror Prototype #1 (total processing time ~ 72 hours):
(1) Nanolaminate on mandrel as received from LLNL. Gray ring shores up adhesive.
(2) Support structure (graphitic foam & composite face sheet) adhered to mirror.
(3) Assembly is removed from mandrel.
(4) The nanolaminate successfully separated despite its brittleness.
Figure 3: Photograph of the completed 6” Mirror Prototype #1.
Surface Figure Results
In order to identify how closely an optical surface conforms to its intended shape, a measure of its surface figure, or accuracy,
is needed. Surface accuracy, as determined by an interferometer that provides interference data between a known flat and
test part, is typically specified in terms of the wavelength of light from a HeNe laser (λ = 632.8 nm) [Smith, 2000]. The final
mirror sample was submitted for interferometric analysis. The resulting data are a measure of the low frequency deviations of
the mirror’s surface from the reference wavelength. Figure 4 provides a comprehensive view of low frequency data, with an
overall surface figure of 4.5 waves PV. In comparison to the reference mandrel, replication of the mandrel surface appears
satisfactory.
Figure 4: Output of Zygo interferometer measurement on Mirror Prototype #1. Interferometry indicates 4.5 waves PV surface
figure, as compared to the replicated mandrel surface of ~ 3.5 waves PV (HeNe wavelength of 633nm).
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A surface figure of 4.5 waves PV was attained, of which 3.5 waves were attributable to the quality of the float glass mandrel.
Due to the presence of air voids in the underlying adhesive, a total surface figure of 4.5 waves PV overall is promising,
considering the contribution of the mandrels existing 3.5 waves PV surface figure. With improved procedures, such as higher
quality mandrels used as deposition substrates and minor alterations in fabrication techniques, it is highly feasible that a
quality precision reflector could be assembled.
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According to the supplier, the nanolaminate prototypes studied here are early versions with relatively low shape accuracy in
the mandrel.
Fabrication of Mirror Prototype #2
Figure 5 shows the mirror fabrication technique for Mirror Prototype #2, using the isogrid GrEp replica described in Appendix
B. The mirror will be fabricated under vacuum to eliminate any problems with pockets of air remaining under the
nanolaminate.
Nanolaminate
Adhesive
GrEp Replica
Figure 5: Diagram of layers for Mirror Prototype #2. Drawing is not to scale.
Conclusion
AFRL/VS produced the first in-house, lightweight carbon-foam mirror prototype with an area density of 13 kg/m2 within 72
hours. This project sought to use replication techniques for fabrication of an ultra lightweight optical quality mirror with an area
density less than 15 kg/m2. A significantly improved GrEp substrate structure, planned for the second prototype, has an area
density of only 5 kg/m2.
Surface figure data exhibits 4.5 waves PV, with 3.5 waves attributable to the as received float glass/ nanolaminate assembly.
This leaves an additional wave of error, likely due to the presence of air voids and cure shrinkage in the adhesive material.
Based on attainable improvements in mandrel surface figure and minor alterations in assembly techniques, the current
methods should result in an optical-quality, dimensionally stable, ultra-lightweight mirror prototype with low mirror fabrication
rates and cost relative to industry.
Future Work
Due to the limited availability of nanolaminates, a more detailed interferogram of the final reflector should be conducted, taking
the time to map out and differentiate between surface figure errors attributable to adhesive air voids and figure errors
dependent on the float glass mandrel. Efforts to enhance the nanolaminate application continue. Although the current epoxy
adhesive in use has many applicable properties, efforts to acquire better adhesives are ongoing. Further considerations
include elimination of the two additional degrees of freedom as presented in this paper: the surface veil and the adhesive/gel
coat. Elimination of these two structural features requires extensive efforts in improving GrEp surface features by eliminating
the effects of fiber print-through patterns.
Acknowledgements
This research was sponsored in part by the Air Force Office of Scientific Research, AFOSR, Dr. Les Lee, program manager.
The authors would also like to acknowledge the support of Touchstone Research Group and Lawrence Livermore National
Laboratory for supply of material and to thank Jack Hochhalter for his engineering support.
Notes
Reference herein to any specific commercial product, process or service by trade name, trademark manufacturer or otherwise,
does not constitute nor imply endorsement by the United States Government, the Department of Defense, or the Air Force
Research Laboratory.
References
1. Barbee, T.W., 2003, “Nanolaminate thin-shell mirror structures”, UV/Optical/IR Space Telescopes: Innovative
Technologies and Concepts, Proceedings of the SPIE, Volume 5166.
2.
MacEwen, H. A., 2003, “Separation of functions as an approach to development of large space telescope mirrors,”
UV/Optical/IR Space Telescopes: Innovative Technologies and Concepts, Proceedings of the SPIE, Volume
5166.
3.
Matson, LE, and D. Mollenhauer, 2003, “Advanced Materials and Processes for Large, Lightweight Space-Based
Mirrors,” presented at the 2003 IEEE Aerospace Conference, Big Sky, Montana.
4.
Smith, W.J., Modern Optical Engineering, 3rd Edition, McGraw-Hill, New York, 2000, p. 561
Appendix A: Development of Adhesive: Selection and Application Technique
Successful bonding of the different materials in the current fabrication approach will depend on the quality of the adhesive
material chosen. Bonding of the composite support structure will be to the backside of the nanolaminate substrate – the
exposed native ZrO2 surface. Hundreds of adhesives exist that are available to the aerospace industry; however, few if none
meet the stringent requirements imposed by a precision reflector assembly. An extensive materials search was conducted
based on the following material property requirements and characteristics for the adhesive:
a. Sufficient joining of unabraded metallic nanolaminate to the organic polymer
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b. Low viscosity requirements for ease of fabrication and out life
c. Low/assembly matching CTE to minimize thermal influence between backing structure and nanolaminate
d. Unfilled material (otherwise the original problem of print-through reemerges, and is transferred to the reflector)
e. Low cure shrinkage (to not influence nanolaminate surface quality)
f. Room-temperature cure to minimize residual stresses after cure
g. Heat resistance with a reasonable Tg to withstand variations in service temperatures
The adhesive of choice that satisfactorily fulfilled all of these standards was a low viscosity (1900 cP), low CTE (8.5 ppm/°F),
unfilled, low cure shrinkage (< 1%), RT curable, heat resistant epoxy with a max Tg of 338°F after post cure. Outgassing and
moisture uptake tests, peel strength tests, and thermal-mechanical analysis will need to be conducted to confirm the validity of
as provided adhesive specifications.
The basic steps of the fabrication process include shoring, adhesive pour, and manual insertion of the composite support
structure onto the adhesive pool. Prior to this first trial assembly, ‘hands-on’ practice was performed with transparent
Plexiglas, adhesive, and Mylar materials to simulate to scale the final prototype method of assembly (Figure A.1). An obstacle
to proper adhesion and stability is the scattered presence of millimeter to sub millimeter air bubbles within the epoxy adhesive.
Although a vacuum had been pulled on the adhesive prior to assembly, additional air bubbles were introduced as the
composite face sheet and adhesive joined to form an interface.
However, by slowly rotating the face sheet ±30˚ and spreading out the epoxy adhesive, a reasonably air pocket free interface
may be created. Figure A.1 vouches for the age-old adage ‘practice makes perfect’!
Figure A.1: Progressive steps to eliminating air entrapment between a simulated backing
structure and nanolaminate (transparent Mylar, adhesive, composite face sheet
support for Trial 1; transparent Plexiglas, adhesive, and Mylar for Trials 3 &5.
Appendix B: Preparation of GrEp Mirror Substrates
Composite mirrors are a new grade of ultra lightweight, high optical quality mirrors. They are fabricated via replication of a tool
surface using an elevated temperature cure, continuously reinforced pre-preg material. A quasi-isotropic fiber orientation on
the order of [0/(±60)2/0]S or [0/45/90/-45]S is typically utilized. Initial prototypes have been made in a flat shape (see Figure
B.1), to be followed eventually by doubly-curved surfaces. This method of fabrication does, however, have its problems. In
short, the large temperature difference ∆T of curing as well as coefficient of thermal expansion (CTE) mismatch between tool
and part (more specifically, between fiber reinforcement and matrix), adds to the high frequency surface errors due to the
presence of fiber print-through. Numerous research avenues are currently under way at AFRL/VS to address the issue of
fiber-print through. Consequently, this particular research effort includes the integration of an adhesive layer and a reflective
nanolaminate surface with an existing composite face sheet and support grid.
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Production technique potentially imposes a limit on part size and complexity and is therefore related to the adhesive system
used: the lower the viscosity of the adhesive, the larger and thicker the part can be (the probability/odds increase for ease of
processing).
Figure B.2: Photograph of GrEp replica (right) and the glass substrate on which the replica was made.
To date, numerous graphite polymeric composite reflectors have been successfully designed and manufactured to meet many
stringent requirements set forth by the aerospace community. Unfortunately, composite reflectors that meet stability and
density requirements are limited to the infrared or longer wavelengths of the electromagnetic spectrum due to the presence of
micron-size surface defects, otherwise attributable to the so-called “print-through” of the fibers on the surface. Some level of
progress has been made in eliminating the deleterious effects of print-through. The General Motors Polymers Laboratory and
the AFRL Materials Directorate are just a few of the commercial and government participants that have shown an interest in
contributing to this endeavor.
In the current effort two different substrate approaches were chosen (see Figures 1 and 5 for cross-section diagrams). Figure
B.2 shows the carbon foam used for a substrate in the first prototype as well as the replica integrated with an isogrid backing
structure for the second prototype. In each case, the composite surface bonded to the nanolaminate has been manufactured
as the replica of a glass surface.
Figure B.2: Mirror substrates for Mirror Prototypes #1 and #2, respectively. The carbon foam is the CFoam product from
Touchstone Research Laboratories. The isogrid composite was made in-house at AFRL/VS and includes offnodal innovations at the hex-cell joints that make the fiber placement (and article construction) process easier.