CP620, Shock Compression of Condensed Matter — 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
THE FAILURE OF ALUMINIUM NITRIDE UNDER SHOCK
I.M. Pickup, N.K. Bourne*
*Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK.
Defence Science and Technology Laboratory, Chobham Lane, Chertsey, Surrey, KT16OEE, UK
Abstract. The shear strength of aluminium nitride has been measured over a range of impact stresses
by measuring lateral stresses in plate impact experiments. The range of impact stress spanned several
key shock thresholds for the material, pre and post Hugoniot elastic limit and up to values where the
hexagonal to cubic phase transition starts. The shear strength measurements indicate significant
inelastic damage at stress levels in excess of the HEL, but a significant recovery of strength at the
highest impact stress was observed. This stress equates to the phase transition stress. The shear strength
behaviour is compared to that of silicon carbide, which does not exhibit a phase change at these impact
velocities.
INTRODUCTION
this with ballistic performance. The shear strength
in the shocked ceramic was determined by
embedding manganin gauges in plate impact
specimens to monitor the lateral stresses.
Over the last 10 years aluminium nitride (A1N)
has been the subject of a significant number of
shock and ballistic studies. It has some interesting
features in both fields. The material has been the
subject of ballistic trials over a wide range of
kinetic energy projectile velocities [1-2] (up to
5000 ms"1). It has been noted that at lower velocities
(1300 ms"1) the ballistic penetration resistance of
A1N is significantly less than that of other nonoxide ceramics, e.g. B4C, SiC and TiB2. At higher
velocities (-2500 m s"1) it is apparently significantly
greater. In shock studies several investigations have
reported a wurtzite (hexagonal) to rock-salt (cubic)
phase transformation initiating at pressures from 16
to 24 GPa depending on material, particularly
grain-size, and measurement technique [3-6]. The
work presented here describes the initial
experiments in a programme which compares the
deviatoric strength of A1N and SiC from relatively
low impact stresses to stresses up to the phase
transition in A1N with a view to determining the
relative shear strength behaviour and correlating
EXPERIMENTAL
The impact experiments were conducted using a
50 mm diameter gas gun. Impact velocity was
measured to an accuracy of 0.5% using a sequential
pin-shorting method and tilt was fixed to be less
than 1 mrad by means of an adjustable specimen
mount. Impactor plates were made from lapped
copper and aluminium discs and were mounted
onto a polycarbonate sabot with a relieved front
surface in order that the rear of the flyer plate
remained unconfmed. Targets were flat to less than
2 |um across the surface. Lateral stresses were
measured using manganin stress gauges of type
J2M-SS-580SF-025 (resistance 25 ft). The gauges
were placed at varying distances from the impact
face as shown in Fig. 1. They had an active width of
240 jam and which contrasts with the 2 mm wide
gauges used by Rosenberg [7].
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5 mm tungsten flyer travelling at 975 m s"1. These
impacts induced stresses of magnitude 5.6, 14.0 and
19.5 GPa respectively. The magnitude of the HEL
for the material is around 9.5 GPa [6] whilst the
phase transformation has been reported to initiate at
-19.5 to 20 GPa for A1N with a similar grain size.
FIGURE 1. Experimental arrangement for lateral and
longitudinal stress experiments, a) Longitudinal and lateral stress
gauge mounting positions with rear PMMA plate, b). Sectioning
for multiple lateral gauge measurements.
The lateral stress, cry, was used along with the
longitudinal stress, crx, to calculate the shear
strength of the material, r, using the well-known
relation
(1)
1
This quantity has been shown to be a good
indicator of the ballistic performance of the material
[8]. This method of determining the shear strength
has the advantage over previous calculations of
being direct since no computation of the hydrostat
is required.
The materials used in this study were
manufactured by Cercom Inc. A linear intercept
method was used to measure grain size, yielding
values of 4±3 and 2±2 jim for A1N and SiC B
respectively. The density was measured by a water
immersion technique to be 3.19 and 3.22 g cm"3 for
A1N and SiC B respectively.
The traces below the HEL show features typical
of polycrystalline materials such as alumina. The
stress rises to a plateau at the first gauge and after
400 ns rises again to a second higher lateral stress.
This second rise is interpreted as the arrival of a
failure wave behind which the strength reduces [8].
The gauge at 6 mm does not rise above the first
level, implying that a failure wave does not
penetrate to this thickness during this loading time.
The trace at an intermediate stress between HEL
and phase transition does not show the stepped rise
described above. It may be that at this impact stress
the failure wave may travel with the shock as has
been seen in lead-filled glasses described elsewhere
[9, 10]. The 6 mm gauge shows a lower lateral
stress than that at 2 mm which may indicate that the
failure does not penetrate equally from the impact
face through the thickness of the tile.
The traces obtained above the phase
transformation show an entirely different
behaviour. Whereas the other sensors observe the
lateral stress rising in a step from a lower to a
A1N
This work
A1N
Rosenberg (7)
SiCB
v
mm ns~*
10.46
mm jus'1
6.14
0.24
3.23
10.72
6.27
0.24
3.22
12.26
7.78
0.16
2
FIGURE 2. Impacts of Al alloy, copper and tungsten alloy
flyers onto AIN. Lateral gauges at 2 mm (bold lines) and 6 mm
(fine lines) from the impact surface.
TABLE 1. PROPERTIES OF ALUMINIUM NITRIDE
gcm~3
3.19
Time/MS
RESULTS
Lateral stress histories are shown in Fig. 2 for
three impacts upon A1N with a 10 mm thick
aluminium flyer travelling at 556 m s"1, a 5 mm
thick copper flyer travelling at 833 m s"1 and a
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higher value, these traces show the opposite
behaviour. The lateral stress falls from a high to a
lower value indicating a strengthening of the
material. It is acknowledged that at the highest
stress the manganin gauges are approaching an
upper performance limit, prior to the electrical
breakdown of the polymer gauge encapsulation at
stresses greater than 24 GPa. Such a breakdown is
mediated by the use of thicker polymer films and
choice of polymers.
Lateral traces for SiC B [11], impacted at
similar stresses to the 19.5 GPa A1N are shown in
Fig. 3. SiC does not undergo a phase change in this
stress regime and in contrast to A1N behaves as
described above with the two stage trace indicating
strength loss behind a failure wave. The initial and
final value of the stress are used to calculate shear
strengths which are nominated ahead and behind
respectively in Fig. 4.
greater width and a different material. His work did
not exceed the phase transformation and the
thickness of the sensors precluded resolution of the
two components to the failure wave, but
nevertheless the strength up to the phase
transformation was mapped thoroughly for his
material.
For the SiC material the shear strength follows
the elastic line up to stresses approaching it's HEL
(~ 15 GPa). Beyond this, the shear strength deviates
from the elastic behaviour indicating both
instantaneous damage and a further small reduction
in strength behind the failure wave.
For AIN the shear strength deviates from the
elastic line at approximately the HEL value (9.5
GPa) indicating significant instantaneous damage,
with the initial stages of the lateral stress history
suggesting inelastic behaviour. The shear strength
of the AIN in the current study levels off at ~ 2
GPa, slightly lower than that of Rosenberg [7]. For
the AIN specimen impacted at 19.5 GPa
longitudinal stress, due to the strong reduction in
the lateral stress observed in the stress history in
Figs. 2 and 3, there is an apparently significant
strengthening effect. The shear strength of the AIN
increases to a value similar to the shear strength of
SiC B rising from 3.25 GPa to 7.25 GPa. It is
significant that the transition in behaviour occurs at
the phase transition onset stress.
AIN 19.5 GPa
0
10
9
0
0.5
1
1.5
8
Time (iiS)
7
FIGURE 3. Contrasting lateral stress histories of SiC B and AIN
impacted at -20 GPa longitudinal stress (ax is indicated by each
curve).
The calculated shear strength (using equation 1)
is shown in Fig. 4. The rising diagonal lines
represent the elastic values calculated using the
longitudinal stress and the Poisson's ratio (solid line
for SiC B and dotted for AIN). The Phase
transformation initiation stress for AIN is indicated
as a vertical dotted line. Two points are plotted for
each experiment. The first corresponds to the value
ahead of the failure front (filled symbol) and the
second to that behind it (open symbol). The crosses
show the data of Rosenberg [7] using gauges of
10
15
20
25
Longitudinal stress /GPa
FIGURE 4. The shear strength of AIN and SiC plotted against
longitudinal impact stress. The diagonal lines are the calculated
strength based on elastic properties (solid for SiC and dotted for
AIN).
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At present neither the reasons for the strength
increase nor the significance of the structure on the
lateral A1N trace for the 2 mm gauge (impact stress
19.5 GPa) is understood. The phase transition
which occurs at this level of stress has an associated
(non-recoverable) volumetric compression of -20%
[5]. It is therefore unlikely that the apparent
strength increase results from a bulking effect
increasing pressure and consequently effective
strength. Mashimo [6] has measured a significant
increase (nearly 50%) in the bulk modulus of the
high pressure, cubic phase. This may significantly
affect the measured lateral stresses.
3.
Vollstadt, H. Ito, E., Akaishi, S. and Fukunaga, O.,
Proc. Jpn. Acad., Ser. B: Phys. Biol. Sci., 66, p.7,
1990.
4. Ueno, M., Onodera, A., Shimomura, O and
Takemura, K., Phys. Rev. B 45,10123, 1992.
5. Kipp, M.E. and Grady, D.E., 'Shock phase
transformation and release properties of aluminium
nitride', DYMAT 94 Internat. Conf. Mechanical and
Physical Behaviour of Materials under Dynamic
Loading, Journal de Physique, p. 249, 1994.
6. T. Mashimo, T., Uchino, M., Nakamura, A.,
Kobayashi, T., Takasawa, E., Sekine, T., Nuguchi,
T., Hikosaka, H. and Fukuoka, K., J. Appl. Phys. 86,
pp.6710-6716, 1999.
7. Rosenberg, Z., Brar, N.S. and Bless, S.J., J. Appl.
Phys. 70,pp.l67-171,1991.
8. Bourne, N. K. and Millett, J. C. F., 'On impact upon
brittle solids', DYMAT 2000, Internat. Conf.
Mechanical and Physical Behaviour of Materials
under Dynamic Loading, Journal de Physique IV,
pp. 281-286,2000.
9. Bourne, N. K. and Millett, J. C. F., The Dynamic
response of Soda-lime glass', in Shock Compression
of Condensed Matter-1995, edited by S. C. Schmidt
et al., AIP Press, pp.567-572, 1996.
10. Bourne, N.K., Millett, J.C.F., Rosenberg, Z. and
Murray, N.H., J. Mech. Phys. Solids 46, pp. 18871908, 1998.
11. Pickup, I. M. and Barker, A. K., 'Deviatoric strength
of silicon carbide subject to shock,' in Shock
Compression of Condensed Matter-1999, edited by
M. D. Furnish et al., AIP Press, pp. 573-576, 1999.
CONCLUSIONS
The deviatoric strength of A1N has been
measured in shock studies from sub-HEL levels to
stresses where the hexagonal to cubic phase change
initiates. At the lowest stresses, failure wave
characteristics were observed. At impact stresses in
excess of the HEL inelastic behaviour, probably in
the form of instantaneous damage travelling with
the shock front was evident. Very significant shear
strength recovery was apparent for impact stress at
the level which initiates the phase transition. The
shear strength behaviour of silicon carbide was
compared over the same stress regime using exactly
the same measuring techniques as a standard
material which does not phase transform in the
stress range. The unusual response observed in the
A1N, at the highest impact stress, i.e. significant
reduction in lateral stress, was not apparent in SiC
at similar stresses. This tends to suggest a material
response was governing the reduction rather than
gauge breakdown.
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Orphal, D. L., Frantzen, R. R., Piekutowski, A. J.,
Cunningham, B.J., Hord, B.L and Kusubov, A.S.,
Int. J. Impact. Eng. 25, pp. 221-231, 2001.
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