*Proceedings of the 8th DoD Electromag. Windows Symposium*, pgs. 287-295
(2000)

# GELCASTING OF GD-1 CERAMIC RADOMES

# Kevin W. Kirby^1^, HRL Laboratories, Malibu, CA 

Anthony Jankiewicz, (Retired) Raytheon Missile Systems, Tucson, AZ

Mark Janney^2^, Oak Ridge National Labs, Oak Ridge, TN

Claudia Walls, Oak Ridge National Labs, Oak Ridge, TN

# Don Kupp, Fraunhofer USA, Newark DE

> A ceramic radome material originally introduced in the early 1980's
> has resurfaced due to innovations in material processing. GD-1, with a
> composition represented by the formula Si~6-z~Al~z~O~z~N~8-z~, where z
> ≈ 2, has long been considered an improved replacement for Pyroceram.
> However, difficulties in processing this material into a commercially
> viable radome has prevented its acceptance and application into
> missile programs such as AMRAAM and Standard Missile. Recently, it has
> been demonstrated that GD-1 can be inexpensively formed into full-size
> radomes using a near net-shape ceramic forming process known as
> gelcasting. This process, developed at Oak Ridge National Labs,
> together with a novel firing method, offer a path for producing GD-1
> ceramic radomes requiring a minimum of post-synthesis machining.

## INTRODUCTION

In the early 1980's, General Dynamics, Raytheon, and Hughes Aircraft
were separate companies competing for business sales on missile programs
such as AMRAAM and Standard Missile. This competition fostered
innovations in technology such that internal costs could be lowered on
each generation of missile, thereby providing an advantage in bidding
price for potential sales to the government. One technology area that
all three companies eventually targeted for cost reduction was the
ceramic radome on the front of the missile.

For the case of AMRAAM and most of Standard Missile, the radome material
of choice has been Pyroceram 9606, a material that is only produced in
radome form by Corning. Although all three companies were clearly
satisfied with the performance of Pyroceram, rising costs and an
unstable delivery schedule put them somewhat at the mercy of Corning in
terms of obtaining radomes. Part of the reason that costs and deliveries
became unstable had to do with the general synthesis method for
Pyroceram radomes \[1\]. The material itself is a glass-ceramic with a
composition similar to the

mineral cordierite (2MgO·2Al~2~O~3~·5SiO~2~). To form the radome, the
raw material is first melted and then cast as a glass to a shape having
about twice the thickness of the final

product. The glass body is then thermally treated below its melting
temperature in a "ceramming" process designed to create a crystalline
ceramic phase within the original glassy matrix. Once the desired
microstructure has been achieved (\~ 98 vol. % crystalline phase), the
radome is machined to the final shape. The radome surface then undergoes
a chemical fortification process to help prevent the spreading of
microcracks. According to reports from Corning, it is the machining, not
the material or processing, that is responsible for the majority of the
cost. Since the process also lends itself to making radomes in batches,
a supply stock has not always been available off the shelf, sometimes
affecting on-demand delivery.

## RADOME DEVELOPMENT

### Historical

To Corning's tribute, Pyroceram 9606 radomes have clearly been an
enabler for missile defense throughout the years. However, now that the
missile divisions of General Dynamics, Raytheon, and Hughes Aircraft
have merged, it is possible to openly compare the path that each company
chose in terms of technology development in order to drive radome costs
down, and to sever dependency on a single radome supplier.

During the early part of the 1980's the legacy Raytheon Company pursued
a material very similar to Pyroceram in terms of composition
(2MgO·2Al~2~O~3~·5SiO~2~), and named it Rayceram 8. Recognizing that
machining costs were a key driver, they developed an isostatic pressing
process for their material, thereby obviating the inefficient casting
method adopted by Corning. By carefully controlling the particle size,
composition, and final microstructure of the material, they were able to
engineer a radome that had superior thermal and dielectric properties to
Pyroceram. Demonstrating their intent to use Rayceram 8, they fabricated
and qualified radomes of this material on both Sparrow and AMRAAM. This
material, however, never became part of the Raytheon product line,
partially due to subsequent negotiations with Corning.

Recognizing the need for higher temperature radome materials, Raytheon
later enlisted the help of MIT in an effort to produce radomes from
reaction-bonded silicon nitride (RBSN). The RBSN process attempts to
overcome the inherently difficult diffusion bonding approach of hot
pressed silicon nitride. While the resulting material held much promise
in terms of its properties, the laser-induced formation method for the
silicon nitride powder was not cost effective. Efforts resulted in the
fabrication of prototype radomes, but full scale development was never
pursued.

During the same period (mid-1980's), General Dynamics attempted to
leap-frog the competition through the development of a radome material
they felt was superior to both silicon nitride and Pyroceram. Known
internally as GD-1 (i.e. General Dynamics-1), the material consisted of
a ß\'-SiAlON phase in the Si~3~N~4~-Al~2~O~3~-AlN-SiO~2~ quaternary
system (see fig. 1), with the nominal composition represented by the
formula Si~6-z~Al~z~O~z~N~8-z~, where z ≈ 2. This material,
independently discovered in Japan \[2,3\] and

![](media/image1.pct){width="4.55625in" height="3.5541666666666667in"}

**Fig. 1 -- Quaternary Phase Diagram Encompassing GD-1**

England \[4\] in the early 1970's, is isostructural with ß\'-Si~3~N~4~,
and tends towards some of its properties (see Table 1). However, in
order to reduce the dielectric and loss tangent values, as well increase
the ease of fabrication, the silicon nitride component is essentially
alloyed with aluminum nitride and their counterpart oxides. Although the
tetrahedral building blocks of the structure are preserved, the strongly
covalent nature of the SiN~4~ unit becomes more ionic as the silicon
atom is replaced with aluminum, and the nitrogen atom is replaced with
oxygen. By varying the amount of substitution, the properties of the
resulting compound become intrinsically tailored.

To produce GD-1 into radomes, an aqueous slip-casting process was
developed at General Dynamics in Pomona. While full scale monolithic
radomes were produced by this method \[5\], cracking and warping during
the drying process consistently kept yields low. In slip-casting,
cracking is caused by different rates of shrinkage between the interior
and exterior of the casting. The differential shrinkage comes as a
result of a moisture gradient throughout the thickness of the casting, a
problem enhanced in thin wall casts such as radomes. To overcome this
problem, thicker wall castings were made, although the additional
thickness added to the post-synthesis machining required to achieve the
desired final dimensions. For the case of GD-1, the hardness of the
ceramic made extensive machining economically unfeasible. Consequently,
despite the desirable material properties, the material processing
difficulties forced General Dynamics to abandon radomes based on the
GD-1 formulation.

Table 1

COMAPRISON OF RADOME MATERIAL PROPERTIES

[Property]{.underline} [GD-1]{.underline} [Pyroceram]{.underline}
[Silicon Nitride]{.underline}

![](media/image2.pct){width="5.99375in" height="3.5722222222222224in"}

It wasn't until well after the merger with General Dynamics missile
division that Hughes Aircraft Co. began to seriously address the issue
of lowering the cost of their ceramic radomes. In one such project,
efforts were initially focused on machining the Pyroceram radomes
in-house. To reduce cost, high speed machining methods were
investigated, including creep-feed grinding and laser machining \[6\].
Alternative synthesis methods and materials were also explored,
including arc-fused SiO~2~, MIT's RBSN method, and polysilazane based
ceramic composites. However, not until learning of Oak Ridge National
Lab's gelcasting process was the GD-1 material revisited as a possible
candidate for AMRAAM and Standard Missile radomes.

### Gelcasting of Radomes

The gelcasting process can best be described as a rapid, inexpensive,
net-shape or near net-shape process for producing ceramic bodies.
Developed at Oak Ridge National Labs, this process has been applied to
numerous complex ceramic parts, including turbine blades, rotors, and
now radomes \[7\]. In its most generic form, gelcasting represents a
ceramic forming process in which a slurry of ceramic powder in a
solution of organic monomers is cast in a mold. The monomer mixture is
polymerized in situ with the aid of a catalyst to form gelled parts.
Unlike slip casting, the formed green parts are strong and machinable.
In addition, since the fluid flow operation required to fill the mold is
separated from the gelation or setting process, many of the obstacles
associated with competing injection molding methods are avoided.

![](media/image3.pct){width="5.125in" height="3.797222222222222in"}

# Fig. 2 -- Gelcasting and Thermal Processes for Producing Radomes

Figure 2 depicts the essence of the gelcasting process for GD-1 radomes.
The process begins with the formation of the slip, a solution consisting
of the appropriate ceramic powders that compose GD-1, and a dissolved
monomer. Typical monomers used in gelcasting are methylacrylamide,
methylenebisacrylamide, or a combination of the two. The slip is then
ball milled for up to 18 hours to ensure proper mixing of the ceramic
powder and the monomer. During different stages of the mixing, various
wetting and defoaming agents are also added. The final mixing step
includes the addition of a catalyst to the solution to promote
cross-polymerization of the monomer. Once mixed, the slip is cast into a
solid metal mold. As shown in fig. 2, the mold imparts the shape and
dimensions of the radome exterior. Not shown in the figure is a male
mandrel that imparts the interior shape and thickness to the cast
radome. This mandrel is lowered slowly into the slip immediately after
being poured into the mold, and remains in place until after the slip
has significantly gelled. Once the mandrel is removed, the cast material
is allowed to dry for a period of 1-2 hours in the mold. After this
drying step, the material is rigid and is able to be lifted out of the
mold. Drying is then continued for a period of 14-24 hours, usually in a
humidity controlled oven. At this point, the cast radome has a very high
green strength, and can be easily handled or even lightly machined. A
photo of a gelcast AMRAAM radome at this stage of processing is shown in
fig. 3.

![](media/image4.pct){width="1.4097222222222223in"
height="2.783333333333333in"}

# Fig. 3 -- Photo of Gelcast Green-Body Radome

Particularly for the case of large gelcast parts, such as radomes, the
secret to success lies in the ability of this process to produce slips
or slurries with a very high solids loading. For GD-1, the typical
ceramic solids loading in the slip was on the order of 50-55 vol. %. The
high loading significantly reduced the amount of shrinkage, and
subsequent tendency for cracking, associated with the drying step. To
illustrate, when gelcasting a full-size AMRAAM radome, the amount of
shrinkage measured from the as-cast to the fully dried green-body part
was only 2.9 %.

##### Thermal Processing of Gelcast Radomes

To remove the polymer binder, the green body radome must first be fired
in air at an intermediate temperature (500-600ºC) before going to higher
temperatures for densification. This intermediate firing step involves
no measurable shrinkage, indicating that the binder is removed with no
densification or reaction of the gelcast ceramic components.

The final step in the radome fabrication is a high temperature firing
procedure that allows the individual ceramic components to react and
densify. In the case of GD-1, temperatures exceeding 1600ºC are required
to liquid phase sinter the material. Due to the presence of a small
amount of liquid at the densification temperature, the material has a
tendency to flow. While this property adds complexity to maintaining the
proper dimensions and shape of the radome, it also offers the
opportunity for conformally firing the radome to net-shape. Figure 4
illustrates how the GD-1 radome was conformally fired with the aid of a
graphite mandrel.

![](media/image4.pct){width="5.992361111111111in"
height="3.463888888888889in"}

# Fig. 4 -- Conformal Firing Process for a Gelcast GD-1 AMRAAM Radome

After firing at 600ºC in air, the gelcast radome consists of a slightly
porous mixture of unreacted starting reagents that together comprise the
composition for GD-1. To give the radome significant strength and begin
formation of the SiAlON phase, a thermal treatment at ≥ 1500ºC in
nitrogen is performed. The final step in the conformal firing process
consists of inserting a graphite mandrel into the base of the radome and
heating to \~ 1650ºC. The dimensions and positioning of the mandrel are
such that the thermal expansion of the graphite and shrinkage of the
dome produce the desired contour and size upon completing the thermal
treatment. Densification of the radome in the high temperature step
typically involves a uniform dimensional change of \~ 19 %, such that
the overall amount of shrinkage from the as-cast radome to the final
product is only \~ 22 % (i.e. the as-cast dimension is 1.22 times larger
than the final dimension). Once the high temperature firing has been
completed, the radome is removed from the mandrel and examined for
dimensional tolerance. While some machining has been necessary to obtain
the required dimensions, the results have indicated that this step may
be virtually eliminated with further development.

#### RADOME PROPERTIES

##### Thermal and Structural Analysis

Although no testing of a full-size AMRAAM radome produced by the
gelcasting process was performed, data from high temperature tensile,
thermal expansion, specific heat, thermal diffusivity, and 3-point
flexure tests were obtained from gelcast samples at the Southern
Research Institute, providing a basis for thermal and structural
modeling. To test the limits of the material, an analysis of the worst
case shoot-down thermal environment for AMRAAM (circa 1998) was
performed at Raytheon Missile Systems, Tucson. The analysis included the
AMRAAM + 5 inch motor and a total flight time of 40 secs. Standard
PATRAN finite element thermal and ANSYS structural models were used in
the analysis. From the modeling analysis, the temperature and maximum
hoop stress along the radome at any time during the simulated flight may
be determined. Table 2 shows the results from the analysis, and
indicates that for the worst case thermal environment the radome has a
positive margin throughout the flight.

Table 2

THERMAL AND STRUCTURAL ANALYSIS RESULTS FOR GD-1 RADOME

![](media/image4.pct){width="5.993055555555555in"
height="2.654861111111111in"}

##### Boresight and Transmission Loss Measurement

Measurements of the transmission loss and boresight error were performed
at Raytheon Missile Systems, Tucson on the first GD-1 prototype radome.
An AMRAAM seeker antenna was used, with data taken over the operational
range of the slotted array antenna. The results are presented in Table
3, and are compared to the values for a Pyroceram AMRAAM radome. As can
be seen from the data, the GD-1 radome had a significantly higher
insertion loss through the nose, and a greater boresight error for both
the in plane and cross plane measurements. The high insertion loss and
in-plane boresight error were reflective of a small amount of added
material present in the tip of the radome. This additional material
resulted from the inability to properly machine the interior tip region
to the proper half-wave thickness. The high cross-plane boresight error
suggested that there was some possible asymmetry in the radome. Despite
the poorer performance compared to Pyroceram, the results from the GD-1
prototype radome suggested that with slightly better dimensional
control, the material should be adequate from a seeker standpoint.

Table 3

SEEKER ANTENNA RESULTS FOR GD-1 AMRAAM RADOME

![](media/image4.pct){width="3.80625in" height="1.8701388888888888in"}

## CONCLUSIONS

The merger of General Dynamics, Raytheon, and Hughes Aircraft Co. has
brought together much of the talent in the area of missile radomes. As a
consequence, material processing problems that limited the use of GD-1
at one company have for the most part been solved through a synergistic
effort. The gelcasting technology from Oak Ridge National Labs has made
GD-1 a candidate radome material with many superior properties to
Pyroceram. However, the generic nature of the gelcasting process with
respect to casting ceramic materials, coupled with its rapid and
inexpensive approach, make it a viable fabrication process for a variety
of candidate radome materials.

## ACKNOWLEDGEMENTS

The authors would like to recognize the outstanding work of Bhagwan
Samyal (thermal and structural modeling) and Bill Salmond (seeker
antenna measurements), both from Raytheon Missile Systems, Tucson.
Special thanks to Bob Lauf (ORNL) and Joe Wysocki (HRL Labs, Malibu) for
coordinating the gelcast radome effort.

**REFERENCES**

1.  Noll, D. A., *Ceramic Radomes- Materials and Design*,
    Electro-Technology Magazine, June 1962, pg. 24

2.  Oyama, Y. and Kamigato, O., Jap. J. Appl. Phys. 10, 1971, pg. 1637

3.  Oyama, O., Jap. J. Appl. Phys. 11, 1972, pg. 760

4.  Jack, K. H., and Wilson, W. I., Nature Phys. Sci., London, 238,
    1972, pg. 28

5.  Lowell, R. F., *Radome Materials for Hypervelocity Missiles*,
    General Dynamics Engineering Research Report, 1984

6.  Kirby, et al., U.S. Patent #5,665,134

7.  Omatete, O., Janney, M., and Strehlow, R., Gelcasting -- A New
    Ceramic Forming Process, Ceramic Bulletin, Vol. 70, 10, 1991