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TRANSPARENT POLYCRYSTALLINE
MATERIALS RESIST BULLETS
Capable of stopping 50-caliber, armor-piercing rounds, ALON
and Spinel are hard and durable transparent polycrystalline
materials manufactured using powder based processes.
Mohan Ramisetty, Suri Sastri, FASM*,
Surmet Corp., Burlington, Mass.
A
new class of advanced polycrys-
talline materials with a cubic
crystal structure—including alu-
minum oxynitride (ALON) and magne-
sium aluminate (Spinel)—is making its
way into many advanced applications.
Due to their unique combination of me-
chanical, optical, and chemical prop-
erties, along with recent advances in
manufacturing, these materials are
now being used for critical defense, in-
dustrial, and civilian applications.
ALON has attracted consider-
able media attention lately, based on
its ability to stop 50-caliber armor-
piercing rounds, coupled with night vi-
sion capability, and crystal clear trans-
parency. Sometimes referredtoas “trans-
parent aluminum,” the material appears
regularly in movies, documentaries, and
*Member of ASM International
TABLE 1 —KEY PROPERTIES OF ALON AND SPINEL
[1,2,3]
Property
ALON
Spinel
Chemical formula (nominal)
Al
23−1/3X
O
27+X
N
5−X
MgAl
2
O
4
Crystal structure
Cubic, spinel
Cubic, spinel
Lattice parameter/constant (nm)
0.794-0.796
0.8085
Density (g/cc)
3.69
3.58
Strength (MPa)
300-700
70-300
Hardness (kg/mm2, Knoop)
1800-1870
1550-1650
Young’s modulus (GPa)
323
277
Refractive index (at wavelength 0.5 µm)
1.80
1.723
Transmission wavelength range
(at 2 mm thick)*
0.25 to 6
0.25 to 6.5
Refractive index in-homogeneity
(~4" aperture, RMS)
~5
<10
Transmittance in the visible
(no AR coating)* (%)
84-85
80-86
Haze* (%)
<2
<10
Clarity* (%)
>98
>95
*Varies depending on processing conditions and thickness.
Fig. 1 —
Engineer inspects a 19 × 36 in.
ALON window.
popular fiction. For example, the movie
Jurassic World
used ALON for its gyro-
spheres, ALON Armor was featured in an
episode of Discovery Channel’s
How Stuff
Works,
and the material also appeared in
Lee Child’s best-selling novel
Personal.
CHEMISTRY AND PROPERTIES
Although both ALON and Spinel
exhibit a cubic spinel crystal struc-
ture, the lattice constants and chemi-
cal bonds vary and result in significant
property differences. Table 1 summariz-
es the key properties of these materials.
ALON is significantly harder and
stronger than Spinel, primarily due to
the slightly increased covalent nature
of its bonding versus a predominantly
ionic bond in Spinel. ALON is the hard-
est transparent polycrystalline ma-
terial available for commercial use
[1]
.
Compared to ALON, Spinel’s lattice
structure features a slightly expand-
ed unit cell and transmits further into
the mid-wave infrared (MWIR) region
(Fig. 2), making it desirable in many
midwave infrared sensor applications.
MANUFACTURING PROCESSES
AND CHALLENGES
Manufacturing both ALON and Spi-
nel involves many of the steps required
to make any technical ceramic mate-
rial. However, attaining crystal-clear
A D V A N C E D
M A T E R I A L S
&
P R O C E S S E S |
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