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
SUB-MOLECULAR FRACTURE STEPS IN SHOCK-SHATTERED
RDX CRYSTALS AND FOLLOW-ON NANO-INDENTATION
EVALUATION OF EARLY STAGE PLASTICITY
J. Sharma1, C.S. Coffey2, R.W. Armstrong3, W.L. Elban4 and S.M. Hoover1
1
Carderock Division Naval Surface Warfare Center, Bethesda, Maryland 20817
Indian Head Division Naval Surface Warfare Center, Indian Head, Maryland 20640
3
ASFRL/MNME, 2306 Perimeter Road, EglinAFB, Florida 32542
4
Loyola College, Baltimore, Maryland 21210
2
Abstract Nano-crystallites of RDX produced by aquarium shock, were examined using atomic force
microscopy and found to contain sharply defined, apparent shear-type (Mode II) fracture steps having
heights less than the size of an RDX molecule. The sub-molecular steps run for substantial distances
along crystallite surfaces, thus indicating concerted disregistry in depth between juxtaposed molecules
across the crack surfaces. The observation of locally jumbled regions of displaced/misoriented
molecules suggests an obstacle barrier, not previously considered, to either shear crack propagation or
dislocation pile-up release with sufficient stress intensity to cause hot spot initiation. Recent follow-on
nanostructural results are reported for separate cleaved RDX crystals containing nano-indentations
showing only plastic deformation with evidence of cracking.
laboratory-grown crystals revealed [2,3] that submolecular fracture steps had formed, running long
distances along the surfaces. Step heights range
from 0.05 to 0.4 nm, which are smaller than the size
(0.5 nm) of an RDX molecule. The sub-molecular
steps are associated with an extensive network of
straight-line (planar) and sometimes non-linear
cracks in bulk crystal specimens that were
recovered largely intact.
The steps relate
geometrically to the familiar macro-scale Mode II
shear fracturing. Figure 1 shows a line plot of a
representative step.
The crystallograpliic character of the step is
obvious even in a 4 nm x 4 nm area image, of Fig.
2. Further, hardly any lateral gap is discernible at
the step. From the spacings of molecules in a line
going perpendicular across the step, it appears that
the gap is 0.1-0.2 nm wide at most. Of particular
interest now is the jumbled appearance of the
INTRODUCTION
Aquarium-shocked RDX (cyclotrimethylenetrinitramine) crystals have shown sub-molecular
high fracture steps which can potentially provide a
trigger decomposition mechanism. To assess the
slowly produced plastic deformation of the
nanometer regime, RDX crystals were indented
with ultra sharp spikes and followed with an atomic
force microscope (AFM).
SURFACE STRUCTURE OF SHOCKED
RDX
Application of the atomic force microscope to
view surfaces of nano-crystallites (20-500 nm) of
RDX resulting from aquarium shock [1] of
837
densities in the respective crystal lattices [4,5]. This
consideration was subsequently used to understand
[6] the formation of nitroso compounds as
decomposition products in production-grade RDX
that had been drop-weight impacted [7]. The
possibility exists that the side nitro groups in RDX
serve as obstacles needed in the dislocation pile-up
avalanche model [8] for hot spot formation in
energetic crystals. The foregoing discussion is
based on analysis of defect-free crystal lattices
containing molecules in regular positions and
orientations.
Given the current nano-scale
observations, locally jumbled regions of
displaced/misoriented molecules are proposed as
potential obstacles in dislocation pile-up formation,
thus providing a new nanostructural-based
explanation of hot spots in crystalline materials
undergoing plastic deformation.
surface molecules shown in Fig. 2. The relative
positions/orientations of molecules at and away
from the two crack surfaces have been effected.
This has important ramification for the chemical
reactivity/decomposition of RDX. It is envisioned
that conformational changes and the rubbing action
of molecules at fracture surfaces could provide an
important trigger mechanism. The creation of steps
may involve the breaking of intramolecular bonds.
The extent of such a reaction pathway would
depend on how far the fracture surfaces extend into
the nano-crystallites, but potentially a large number
of molecules could be involved.
Figure 1. Line plot image of a fracture step smaller in
height than an RDX molecule observed on nanocrystallites,
produced by aquarium-shock.
Figure 3. SEM image of the spike grating used to produce
indentations in RDX crystal. The spikes were 700 nm high with
tips less than 10 nm radius of curvature and 20 degree tip angle.
NANO-SCALE PLASTICITY IN INDENTED
RDX
In order to study the plasticity of RDX at the
nano-scale in a controlled way, a novel, inexpensive
technique was devised to deform freshly cleaved
surfaces at very small loads. RDX was pressed into
a
calibration
grating,
manufactured
by
MikroMasch*, using a microindentation hardness
tester, without affixing the usual diamond pyramid
(Vickers or Knoop) indenter. A force of 25 g was
applied for 10 s dwell. The grating consists of
approximately 106sharp spikes in square array over
a 2 mm x 2 mm area on a single crystal silicon
Figure 2. Line plot of a molecularly resolved AFM image,
showing that the 0.25 nm high fracture step can be distinctly
seen even on a 4nm x 4nm area image.
The relatively high hardness of RDX and related
energetic crystals has been explained at the
nanometer scale by the difficulty of dislocation
motion occurring because of hindered shear
displacements between the juxtaposed irregularlyshaped molecules at relatively high packing
838
for an obstacle character of the (boundary-type) line
structure is shown by the discontinuous elevation
changes associated with the small indentations
revealed in Fig. 5.
wafer, resulting in a 2 um spacing between adjacent
spikes. Individual spikes have circular crosssections, are 700 nm high and have a 20° apex
angle; the utmost tip radius of curvature is <10 nm.
The use of ultra sharp spikes, set in a patterned
way, as seen in Fig.3, has made it possible to
examine plastic deformation even when the
impressions are only nanometers deep.
1500
-8000
1000
8000
500
4000
nm
2000
500
1000
1500
Figure 5. The indentation on the top left shows how the
surface gets pressed down over a dimension of a few hundred
nanometers before any hole is produced. Firmer indentations
result in prominent holes.
nm
Figure 4. AFM image of the indented surface of RDX,
showing deep and shallow indentations produced in a square
pattern. The deeper ones show sharply defined 100-200 nm deep
holes.
At points of least force 3-10nm deep
impressions extended over 200-300 nm are
produced (See left top of Fig.5.). At higher force, a
sharply defined impression is produced. Figure 5
reveals, in addition, that the indentations have
polygonally-shaped edges.
The other two indentations of Fig. 5 show
prominent hole formation from deeper penetration
of the spike. They are approximately 100 nm deep
as measured by the AFM, but the scanning tip of
the AFM is too fat to probe deep and give full
depth. The larger indentations were observed to
have built-up regions of higher elevation distributed
immediately adjacent to the residual impression.
Figure 6 shows an example of these pile-up
mounds.
It is instructive to relate the nanoscale
indentations of various sizes to microindentation
results reported for RDX crystals. First, there is the
important observation that no cracking has occurred
at these small indentations, also made at small
effective load values, despite the fixed cone angle
of the indenter points, corresponding to an
Since the RDX cleavage surfaces are not
perfectly flat, the contact was not uniform causing
the resultant nano-indentations to have varying
sizes, which allowed successive stages of material
response prior to crack formation to be followed.
The distances between adjacent impressions were
sufficient to allow the corresponding deformations
to be independent of each other. Sometimes during
the indenting process a little lateral movement of
the crystal surface relative to the grating occurred,
which is apparent in the AFM images. This led to
streaking and very closely spaced, multiple
impressions at some sites.
The indented crystal surface, indicated to be
(001) as the preferred cleavage plane, also exhibited
a striated appearance of hills and valleys (See Figs.
4 and 5.). The undulated surface presumably
relates to an underlying internal growth sector
structure first reported for RDX crystals by van der
Steen and Duvalois [9]. Here, important evidence
839
ACKNOWLEDGEMENTS
otherwise large effective strain value.
Elban,
Armstrong and Russell [10] reported an absence of
cracking for effective small strains and load values
applied in a ball microindentation test. Next, there
is the apparent two-step process indicated for
increasing nano-indentation sizes of primary
dislocation flow occurring to form the indentations
at small loads, in the absence of secondary, volume
conserving slip surrounding the residual nanoindentations only at larger indentation sizes [11].
Such secondary slip is very evident in Fig. 6 where
the raised areas are readily identified. In Fig. 6
also, there is indication of the indentation strain
field being heavily pitted, perhaps, relating to the
speculative Frank suggestion of dislocations
exhibiting hollow cores for large Burgers vector
dislocations [12].
0
nm
500
This AFM based research has been solely
supported by the U.S. Office of Naval Research
(ONR). N0001401WX20909 (Program Officer Dr.
Judah M. Goldwasser).
REFERENCES
1.
Sandusky, H.W., Beard, B.C., Glancy, B.C., Elban,
W.L. and Armstrong, R.W., "Comparison of
Deformation and Shock Reactivity for Single
Crystals of RDX and AP," in Mater. Res. Soc. Symp.
Proc. 296,1993, pp. 93-98.
2. Sharma, J., Armstrong, R.W., Elban, W. L., Coffey,
C.S. and Sandusky, H. W., Appl Phys. Letters 78,
457-459(2001).
3. Coffey, C.S. and Sharma, J., J. Appl Phys, 89,
4797-4802(2001).
4. Armstrong, R.W. and Elban,W.L., "Microstructural
Origin of Hot Spots in RDX Crystals," in Energetic
Material Fundamentals Workshop, Los Alamos
National Laboratory, 1986, (Chemical Propulsion
Information Agency [CPIA] Publication 475, 1987)
pp. 177-182.
5. Dick, J.J., Mulford, R.N., Spencer, W.J., Pettit,
D.R., Garcia, E. and Shaw, D.C., J. Appl Phys. 70,
3572(1991).
6. Armstrong, R.W., J. Physique IV, Colloque C4,
Supplement au J. Physique III, 5, C4-89-102 (1995).
7. Hoffsommer, J.C., Glover, D.J. and Elban, W.L. J.
Energetic Mater., 3, 149-167(1985).
8. Armstrong, R.W., Coffey, C.S. and Elban, W.L,
ActaMetall, 30,2111-2116 (1982).
9. van der Steen, A.C. and Duvalois, W., "What Do
Explosive Particles Look Like?" in ONR/TNO
Workshop on Desensitization of Explosives and
Propellants, Preprints Volume 3, Prins Maurits
Laboratory, Rijswick, The Netherlands, p. 1.
10. Elban, W.L., Armstrong, R.W., and Russell, T.P.,
Phil Mag., 78, 907-912 (1998).
11. Elban, W.L. and Armstrong, R.W., Acta Mater., 46,
6041-6052(1998).
12. F.C. Frank, Acta Cryst, 4,497-501 (1951).
* Distributed by K-TEK International, Inc, Portland, OR.
97223.
1000
Figure 6. Surface mode image of a large indentation
(approximately 700 nm in size) from which pile-up material (510 nm high) has been pushed out on all sides. The pile-up
material has sharp borders and is full of 100-200 nm size welldefined, 3-10 nm deep, dislocation core holes.
SUMMARY
Sub-micron size indentations on RDX crystals,
produced with very sharp silicon spikes, having
radius of curvature smaller than 10 nm, have been
reported for the first time. It appears that before the
tip penetrates into the crystal, extended but shallow
deformation of the surface takes place. Under
higher force, sharply defined indentations are
produced and mounds arise around them. The
surface of the mounds shows very small craters,
having a surface density of 108 cm"2, that are
interpreted as dislocation core holes.
840