Residue from the propellant combustion products penetrated both sides of the soft metal seal
to near the region of the fractures. It is not uncommon for gas penetration into very small cracks
to be observed in objects tested under these conditions. (This is the source of attack on the

adhesive when that method of mounting is used.)

Measurement of the angles of the window (exterior) and case (interior) confirmed that they
were within the limits that would normally be considered satisfactory with sapphire. However,
the soft metal sleeve showed clear deformation and loading only at the region of the fracture. An
examination of the interior shape and finish of the case seating area has revealed no major

condition that would have made this case unacceptable for mounting a sapphire window.

Since sapphire windows have not been failing in this mounting configuration, insufficient
evidence is available to compare these postmortem components with those from standard
sapphire components. However, the fabrication of this window was not by the usual sapphire
fabricator. Thus, it is not totally conclusive that this mounting technique will not work well with
ALON. It is interesting to note that the postmortem observations show that the hot end of the
window was not well-seated into the soft metal, as indicated by the residue deposits. Thus, the
amount of force causing the “chipping” of that surface is thought to have been smaller as
mounted here than if the window were seated into the metal in a manner sufficient to provide a
high-pressure seal. It appears that even with the forces present, the window experienced tensile

forces at the hot face that caused the observed cracking and spalling.

6. Conclusions

The tests in this report strongly indicate that properly mounted ALON will survive the
cannon environment. Further tests are required to study the long-term surface response to the
gun environment and to compare any degradation to that of crystalline sapphire. The
performance of ALON also suggests that other related materials, such as spinel (MgAl,Ox),
might be useful in this environment as well. These materials might be explored for potential

advantages characteristic of their chemical composition.

10


1400.

1200
400
© 1000 =
rs 300 &
5 800 =
oS
<4 o
o 60 200 5
i=
S 400 3
a 100 &
200
0 0
140 145 150 155 760

Time (msec)

Figure 3. Pressure and Temperature Measurement on Common Time Scale.

The pressure experienced by a window does not present a great design challenge for sapphire
(or ALON). Also, the peak temperature measured of near 1200 °C is within the temperature
range where sapphire might readily be used, especially for a short time. The failures of test
windows suggest that the combination of these two conditions simultaneously provides a unique

challenge to survival.

3. Window Fabrication

The ALON samples used in these experiments were provided by Raytheon Electronic
Systems, Lexington, MA. Three windows were fabricated by Kigre, Inc., Hilton Head, SC, into
the conical design that has been used successfully with single crystal sapphire. The basic shape
is shown in Figure 4. These windows are usually fabricated with all sides fully polished. In
order to minimize cost, these ALON windows were polished on the optical surfaces only; the
sides were ground. This general configuration is not unlike those found in earlier high-pressure

window designs, such as those of Stromberg and Schock [4].


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AW MIZIOLEK
JB MORRIS
R A PESCE-RODRIGUEZ
BMRICE
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MA SCHROEDER
J A VANDERHOFF

14


Acknowledgments

The materials for these windows were provided by Randall Tustison of Raytheon
Electronics. He also provided much encouragement and advice during the course of these
studies. The ALON-E window was assembled by Steven McKnight of the U.S. Army Research
Laboratory (ARL). Gary Gilde and Parimal Patel of ARL provided important advice and
assistance with the analysis. Photomicrographs were obtained with the assistance of

James Kleinmeyer of ARL.



7. References

Barrows, A. W., B. E. Forch, R. A. Beyer, A. Cohen, and J. E. Newberry. “Laser Ignition in
Guns, Howitzers, and Tanks: The LIGHT Program.” ARL-TR-62, U.S. Army Research
Laboratory, Aberdeen Proving Ground, MD, 1993.

Swab, J. J., J. C. LaSalvia, G. A. Gilde, P. J. Patel, and M. J. Motyka. “Transparent Armor
Ceramics: ALON and Spinel.” 23rd Annual Conference on Composites, Advanced
Ceramics, Materials and Structures, unpublished data.

Bundy, M. “Thermal Response of Sapphire to Propellant Combustion.” ARL-TR-1654,
U.S. Army Research Laboratory, Aberdeen Proving Ground, MD, 1998.

Stromberg, H. D., and R. N. Schock. “A Window Configuration for High Pressure Optical
Cells.” Rev. Sci. Instrum., vol. 41, pp. 1880-1881, 1970.

11



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Window ALON-M showed damage in two regions. The hot-face damage increased in area
by more than a factor of two. As shown in Figure 6, a sufficient center aperture remains clear;
this window would still be usable if this were the only damage. However, since this damage is
from a limited number of cycles, it is not likely that this method of seating is appropriate for
ALON. Thus, this damage alone would probably be sufficient to rule out ALON seated in metal

as a useful configuration for our purposes.

large face
damaged regions

interior damage ’
near small face ©,

Figure 6. Photograph (Left) and Diagram (Right) Showing Damage to High-Pressure
Face of ALON-M After Tests.

In addition to the surface damage, the window lost transparency in a region close to the small
(cold) face. Two images of this damage are shown here, as observed from the large (hot) face.
These images are the same as Figure 6, with the focus moved down to the damage plane and the
field of view decreased. Figure 7a shows the full clear aperture of about 7 mm. Figure 7b is a
magnified portion of the center of that same region. In addition, images were recorded using
crossed polarizers to identify regions of high strain. These images are not included here.
Evidence of internal strain was observed near the smaller face. It covered less than 25% of the
window cross section at that end. This residual strain was consistent with the damage to the

window.


INTENTIONALLY LEFT BLANK.


Table of Contents

Page
Acknowledgments ..............cssccsessssssscsssesssccsecseecsesssessucesseceseecsucssseeseesceneeseecseeees iii
List Of Figures ..............ccccssessssssesssssscsssscseessececssccecescsecsucesessassacesessessesessesecseessesses vii
List Of Tables............cscssscsssssssssecsssesssecsssecssscssecsucessssuceessessuessuecsseecseesecsessssseessess Vii
WMtroduction .........ccscsescssssssssessssecsssseesssseeessssessssecsssecssseccsaneesenseessueceasecneeeseessanes 1
Characterization of Cannon Environment ............:.csssccssssssssseeseseecseessecsesecessees 2
Window Fabrication ............ccccscssessssssesseccssessesssecnscsnesssscsssesssccsseeeseesessesseesseeseses 4
TOStS....secssesssscsnessssscssecsscesecssscesecssnessssessuscesscesssensecnscsuccauecsusessucssucsseeeneeseeseessessseess 6
Observations and Analysis ..........1..ssscsssccsesssscsseeseecneesseecseecssesssesesecseesecsecssecssecss 7
CONCHUSIONS ...........ssssesssesssecsscssssssseesssessssecesecsueessecnseeusessecsuessaeessesssesessesssesneesneess 10
References ..........sssssccsseeesseessssscssssssssseccssssecsssscessuecenseesnsessnusssstaccesseceaeessecesseesseess 11
Distribution List...............scssssssssssessssssssesssessssessessseensesssecsaeeeneeeaesseesnseessaceneesseees 13
Report Documentation Page ............s.sssscssesssssssesssssssessseecsecnseceseessceneeseeaessneesnees 15



INTENTIONALLY LEFT BLANK.


(a) ~ (b)

Figure 7. Images of Internal Damage to ALON-M Through Large Face: (a) Full Aperture
and (b) Closer View of Center Portion.

The window was pushed out of the case so that the window, case, and metal interface could
be examined in detail. It fractured cleanly into three parts, as shown schematically in Figure 8.
Two small slices were broken from the smaller end. Both breaks were nearly normal to the
window’s axis of symmetry. The lower break was quite flat. The other was curved slightly
toward the large face. The thickness of these pieces was measured at approximately 1.7 and
2.2 mm, respectively.

1.7mm residue

Figure 8. ALON-M After Removal From Case.



List of Figures

Figure Page
1. Simulation of Pressure-Time Behavior in a 155-mm Cannon ..........c.sssscssssessssseseeee 2
2. Temperature Measurement on Breech Face During Firing 3
3. Pressure and Temperature Measurement on Common Time Scale...............sssse0 4
4. Configuration of Windows and Approximate Dimensions...............s.ssssesessseeseeeeee 5
5. Pressure Record From Test No. 6 Of ALON-M ......cscssssssssssssssssssssessssssessesssceesseeseee 6
6. Photograph and Diagram Showing Damage to High-Pressure Face

Of ALON-M After Tests..........ccsccssssessessssssessessecnsseesessessssssesessssesssssesceesesensssenessess 8
7. Images of Internal Damage to ALON-M Through Large Face............scsssesssesesseees 9
8. ALON-M After Removal From Case .........scssssssssssessessesscescsssscssssssscssssessssessesenseaees 9

List of Tables

Table . Page

1. Peak Pressures Recorded for Windows in Test Fixture



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4. TITLE AND SUBTITLE
Evaluation of ALON for Cannon Window Application

‘5. FUNDING NUMBERS

622618.H80

6. AUTHOR(S)
Richard A. Beyer and Henry Kerwien*

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
U.S. Army Research Laboratory

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Aberdeen Proving Ground, MD 21005-5066

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13. ABSTRACT (Maximum 200 words)

In applying laser ignition to large-caliber cannons, a critical element is the window into the cannon chamber to admit
the laser energy. This window must repeatedly withstand a particularly harsh environment of highly reactive, high
temperature combustion products from the gun propellant at pressures up to 440 MPa (65,000 psi). Failure of the
window can be caused by either thermal gradients in the window, mechanical force, or a combination. Previous
successes with single-crystal sapphire have sometimes been limited by window deterioration modes suggestive of
crystalline behavior. Samples of aluminum oxynitride spinel (ALON) have been fabricated in the same design as the
standard sapphire windows and were qualified for gun testing. This process involves a series of experiments in a closed
chamber, where gun propellant is bumed to generate an environment similar to that inside the gun. Windows mounted
in two methods have been tested. One of these windows has survived the full pregun test series with no visible damage.

14. SUBJECT TERMS
ALON, window, laser ignition, cannon

15. NUMBER OF PAGES

19
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Army RESEARCH LABORATORY i)

Evaluation of ALON
for Cannon Window Application

by Richard A. Beyer and Henry Kerwien

ARL-TR-2358 October 2000

Approved for public release; distribution is unlimited.

20001212 O19



were fabricated; tests to simulate the gun environment were performed on two of them. The

details and conclusions of these tests are described in this report.

2. Characterization of Cannon Environment

During the interior ballistic cycle of a modern 155-mm cannon, propellant is burned at flame
temperatures near 2300 °C and pressures of 350 MPa (50 Kpsi) or more. The pressure and
highly turbulent combustion environment result in thin boundary layers and highly effective heat
transfer to the breech wall. The pressure in a cannon is a well-characterized parameter through
both measurement and highly sophisticated computer simulation. The pressure-time profile near

the breech from a simulation of a generic 155-mm cannon is shown in Figure 1.

400
350 | +50
300 |
= +40 =
& 250 4 a
2 <
& 200 4 +30 ©
1] >
Q n
8 150 8
g 420 &
100 |
+40
50 |
0 0
0 5 10 15 20

Time (msec)

Figure 1. Simulation of Pressure-Time Behavior in a 155-mm Cannon.

Heat transfer measurements to cannon tube walls are typically concerned with metal erosion
at regions in the forward parts of the breech. Because of the differences in flow velocity at the
breech wall (where the window is mounted), these are of little value in determining the

environment in which the window must survive. As a part of the window development program,


1. Introduction

Laser ignition of large-caliber cannons offers many potential advantages over conventional
ignition systems [1]. Chief among these are increased firing rate and reduced complexity of
design. Most proposed designs use an external laser, which requires a window to pass the light
energy through the cannon wall. Typical 155-mm cannons currently used or under development
use projectiles and propellant which are loaded separately into the gun breech. The propellant is
contained in a bag or case, which is consumed during the gun function. Thus, the normal
approach is to have a window which is incorporated into the breech of the gun and must

therefore survive multiple firings of the weapon.

While developing the window which is currently in use for research and developmental
cannons in the Army, many designs were attempted with mixed success. A recurring
phenomenon with the early failures that occurred was that they were often associated with crystal
behavior such as twining or crystal plane slippage. These modes of degradation are
noncatastrophic in the sense that the pressure integrity of the cannon breech remains in place.
However, they do result in reduced transmission of light into the cannon and loss in ignition
system efficiency. Because of the association of single crystal behavior with the most common
degradation modes, there has been an effort to identify potential candidate replacements among

other classes of materials.

In addition to degradation due to the loss of transmission through the crystal, the sapphire can
also suffer slower, long-term damage to the hot face which suggests chemical attack. This effect
results in a translucent layer developing after a large number of cycles. This degradation
mechanism is not addressed in the limited number of tests of the present work.

Aluminum oxynitride spinel (ALON) is currently under evaluation at the U.S. Army
Research Laboratory (ARL) as a candidate for transparent armor [2]. A small sample of ALON
was made available after determining that it possessed sufficient strength and other

characteristics required to perform as a window material in the cannon systems. Three windows



INTENTIONALLY LEFT BLANK.

vi


INTENTIONALLY LEFT BLANK.

12


eee]

Abstract

In applying laser ignition to large-caliber cannons, a critical element is the window into the
cannon chamber to admit the laser energy. This window must repeatedly withstand a particularly
harsh environment of highly reactive, high temperature combustion products from the gun
propellant at pressures up to 440 MPa (65,000 psi). Failure of the window can be caused by
either thermal gradients in the window, mechanical force, or acombination. Previous successes
with single-crystal sapphire have sometimes been limited by window deterioration modes
suggestive of crystalline behavior. Samples of aluminum oxynitride spinel (ALON) have been
fabricated in the same design as the standard sapphire windows and were qualified for gun
testing. This process involves a series of experiments in a closed chamber, where gun propellant
is burned to generate an environment similar to that inside the gun. Windows mounted in two
methods have been tested. One of these windows has survived the full pregun test series with
no visible damage.



15mm

Figure 4. Configuration of Windows and Approximate Dimensions.

Two ALON windows were mounted into threaded steel cases that allowed testing in standard
fixtures and cannons. The first window was assembled using a ductile metal sleeve interface
between it and the case. This technique works well with the single crystal sapphire windows. In
this process, after the clean components are put in place, the window is pressed into the case with
approximately 9 KN (1 ton) of force. This force serves to seat the window into the soft metal and
forms a low-pressure seal between the components. During this pressing portion of the process,
the ALON windows showed significant damage around the edge of the large end. The remaining
clear aperture was larger than the small aperture, so tests on one of these windows proceeded.

The window seated in metal is referred to as ALON-M.

Because of the damage to the first window assembled, adhesive was used to fix the second
window which was tested. This window was degreased and fastened into the case using a
high-performance aerospace grade primer and epoxy. The bond length of the epoxy was
minimized and thought to be approximately 0.025 mm (0.001 in). The assembly was cured at
elevated temperature. Although epoxy is an attractive assembly method, in some earlier tests the
sapphire windows showed limited durability; the epoxy eroded after the hot combustion gases
penetrated into the window-case interface. However, as a test of material suitability, this
mounting technique should prove satisfactory. This window is referred to as ALON-E.



The peak pressures recorded for these two windows are shown in Table 1. Tests were
terminated on ALON-M after the sixth test when the subsurface damage was noted. Because this
damage was internal and it was surface damage that was most closely monitored, it may have
begun to fail at a lower pressure. The target pressure for test no. 6 of window ALON-E was
between 55 and 60 Kpsi.

Table 1. Peak Pressures Recorded for Windows in Test Fixture

ALON-M ALON-E
Test No. MPa psi MPa psi

1 63 9,184 64 9,312

2 76 11,008 93 13,440
3 122 17,664 122 17,632
4 235 34,112 240 34,848
5 312 45,280 314 45,568
6 396 57,440 No data No data
7 _— — 376 54,592
8 _ = 402 58,240

5. Observations and Analysis

The response of the two windows was considerably different. Window ALON-E survived
the full battery of qualifying tests with no evidence of degradation. Because of sapphire’s
history of damage at the hot-face edge due to loss of support as the epoxy erodes, careful
inspection was made of this region. It does appear possible that small fragments of the ALON
may have been lost. The magnitude of the possible damage is small and inconclusive with this
small number of tests. Overall, the performance of this window is satisfactory within the limits
tested. The evidence suggests that this window and single-crystal sapphire would survive in a

cannon.



Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5066

ARL-TR-2358 October 2000

Evaluation of ALON
for Cannon Window Application

Richard A. Beyer
Weapons and Materials Research Directorate, ARL

Henry Kerwien
U.S. Army Armament Research, Development, and Engineering Center

Approved for public release; distribution is unlimited.

EE Eee



INTENTIONALLY LEFT BLANK.

16


The findings in this report are not to be construed as an official
Department of the Army position unless so designated by other
authorized documents.

Citation of manufacturer’s or trade names does not constitute an
official endorsement or approval of the use thereof.

Destroy this report when it is no longer needed. Do not return
it to the originator.


Bundy [3] made one series of measurements of the temperature history on the breech wall.
Although this study was subject to the many problems that can affect instrumentation in this
harsh environment, a limited number of temperature-time records were recorded that better
define the window environment. One of these is shown as Figure 2. Note that the time scale of
the temperature pulse is much longer than that of pressure. Probably, residual heat from the
adjacent cannon wall, as well as hot gases in the breech, slow the cooling rate of the temperature
probe. However, there is no claim that this profile exactly mimics the window surface
environment during a cannon firing. Bundy’s report suggests that the peak temperature may be
significantly higher than measured.

1200 4

Temperature (Cc),
8 8 8

:

°

0 100 200 300 400
Time (msec)

Figure 2. Temperature Measurement on Breech Face During Firing [3].

In order to compare the pressure and temperature profiles on a similar time scale, data from
these two figures are shown in Figure 3 with a time shift to place the nearly coincident rises. As
can be seen in the figure, the pressure event and the initial portion of the temperature record are
similar in duration.



4. Tests

Windows that will be used in gun or other highly demanding environments are typically
exposed to a series of pressures of increasing magnitude. This approach allows the window to
become fully seated in the ductile metal liner while confidence is established in the particular
assembly. The pressure for this phase of testing is generated by burning a charge of propellant in
a closed vessel with a capacity of approximately 1 liter. By varying the amount of propellant
charge, the pressure history of the window assembly is varied. The maximum pressure for this
test phase is usually around 400 MPa (60 Kpsi), which is comparable to the pressure a window
would normally experience in a cannon. (Higher pressures can be obtained for safety factors, as
required.) Although the flow of the hot gases is not the same as in a cannon, due to the fixed
volume, the pressure and temperature provide an excellent simulation of the cannon breech
environment. The pressure-time record for the final test of window ALON-M is shown in
Figure 5. The total decay time of the pressure pulse is slightly more than in a cannon, but the
rise time and peak pressure are well-simulated. During these tests, the window is directly

exposed to the burning propellant.

450
400 - + 60.0
350 4 + 50.0
& 300 + a.
S +400 ¢
= 250 | rd
2 200 4 + 30.0 FI
on ao
3 4 o
a 150 + 20.0 &
100 |
+ 10.0
50 4 10
0 - : : 0.0
) 10 20 30 40 50 60

Time (msec)

Figure 5. Pressure Record From Test No. 6 of ALON-M.