Ballistic Evaluation of Nanocomposite Ceramic
M. Bolduc1, B. Anctil2, J. Lo3, R. Zhang3, S. Lin4, B. Simard4, K. Bosnick5, M.
Bielawski6, A. Merati6
1
Defence Research and Development Canada, Quebec, QC, Canada,
[email protected]
2
Biokinetics and Associated Ltd., Ottawa, Ontario, Canada,
3
NRCan-RNCan, Hamilton, ON, Canada,
4
NRC-Security and Disruptive Technologies Portfolio, Ottawa, ON, Canada,
5
NRC-Security and Disruptive Technologies Portfolio, Edmonton, AB, Canada,
6
NRC-Aerospace Portfolio, Ottawa, ON, Canada,
Abstract. To improve ballistic resistance and to reduce weight of ceramic composite armour, carbon nanotubes
and boron nitrite nanotubes were added into ceramic matrix of alumina and silicon carbide by Pressureless
Sintering (PS) or Hot Pressing (HP) techniques. Small samples were fabricated to evaluate mechanical properties
such as hardness, density and fracture toughness. Results showed little improvement in hardness but significant
improvement of fracture toughness encouraging fabrication of larger samples to conduct Depth of Penetration
(DOP) tests to assess ballistic performance. Commercial monolithic alumina and silicon carbide tiles were used
initially to evaluate the effect of ceramic-backing bonding (no adhesive, grease or glue) and sample confinement
(free, frame, lid). Test parameters providing the most repeatable DOP measurements were then used to test
different nanocomposite ceramic configurations. Test samples consisted of commercial monolithic alumina and
silicon carbide, single wall carbon nanotubes reinforced alumina, boron nitrite nanotubes reinforced alumina and
silicon carbide, alumina fibre mat reinforced alumina and alumina fibre mat with carbon nanotubes reinforced
alumina. The mass efficiency, ballistic efficiency factor, and critical thickness were calculated to assess the
ballistic performance of different configurations. Fracture patterns of reinforced composites were examined with
electron microscopy to better understand how the nanotubes influence fracture propagation. Ballistic impact
resistance did not improve for the various configurations tested but the results provided key insights to design and
manufacture nanocomposite ceramic. Future efforts will focus on applying the knowledge gained from these
experiments to improve ballistic efficiency of the next generation of ceramic armour.
1. NANOCOMPOSITE CERAMIC ARMOUR
DRDC Valcartier with a group of Canadian experts utilized nanotechnology to fabricate ceramic
composite with the goal of creating a lighter armour material with increased multi hit capabilities. The
ceramic development was presented previously [Ref. 1] before the availability of ballistic test results.
This paper compares the mechanical and ballistic resistant properties of monolithic and
nanocomposite ceramics. The ballistic method is described and the failure mechanisms observed are
briefly discussed
2. MECHANICAL PROPERTIES
The nanocomposite ceramic program has required significant efforts to develop formulations for
integrating nanotubes into ceramic [Ref 2, 5, 9], to evaluate the mechanical properties of the samples
produced [Ref 3, 7, 8, 10] and to link the results to the specific needs of armour ceramic [Ref. 1, 4, 6].
The mechanical properties generally associated with ballistic resistance are the hardness, for the
potential to fracture the projectile, and the fracture toughness, for its ability to resist fracture and
restraining spalling. Hardness and fracture toughness values for the configurations of nanocomposite
ceramic investigated are shown in Table 1. The 3D structure refers to a forest of nanotubes grown on
the fibre mat before being sintered in a ceramic matrix [Ref. 1]. All Al2O3 nanoceramic composites
were fabricated with particles having a grain size of 3 µm before sintering. For comparison purposes, a
few samples were fabricated with particles having a grain size of 1 µm before sintering. Interestingly,
the addition of nanotubes increased the hardness for Al2O3 while reducing it for SiC. A possible
explanation is a higher void content seen in SEM for SiC samples when nanotubes are integrated Ref.
[8]. The fracture toughness improved for all samples with nanotubes, especially the Al2O3 3D structure
which showed an increase of 72% in comparison with the monolithic sample.
DRDC-RDDC-2016-P064
1
Table 1. Mechanical properties
Vickers Hardness *4
GPa
Ceramic Configurations
Al2O3 Particulates grain size before sintering
(surface measurement only)
1 µm
3 µm*1
Monolithic Al2O3 matrix (3 µm) + 1% CNT*2
Monolithic Al2O3matrix (3 µm) + 1% BNNT*3
Al2O3 matrix (3 µm) + 18% Al2O3 fibre mat (2D)
Al2O3 matrix (3 µm) + 18% Al2O3 fibre mat + CNT (3D)
Monolithic SiC
Monolithic SiC matrix + 1% CNT*2
Monolithic SiC matrix + 1% BNNT*3
18.94
15.57
15.87 (+2%)
16.83 (+8%)
17.00 (+9%)
17.97 (+15%)
22.87
22.26 (-3%)
20.41 (-11%)
Fracture
toughness*5
MPa X m1/2
3.85
3.83
3.98 (+4%)
3.96 (+3.4%)
4.44 (+16%)
6.60 (+72%)
2.73
2.78 (+1.8%)
2.84 (+4%)
*1
the particulate grain size before sintering of 3 µm was used in all nanocomposite ceramics samples
CNT : Carbon Nanotubes
*3
BNNT : Boron Nitride Nanotubes
*4
the hardness was evaluated following ASTM C1327 method with the indentation measure of 1 kgf
*5
the fracture toughness was evaluated following ASTM C1421 for the chevron notch method
*2
With these promising results, Depth of Penetration (DOP) tests were conducted and followed by
a study of the ceramic microstructure to understand the failure mechanisms.
3. DEPTH OF PENETRATION METHOD (DOP)
In this study, the ballistic performance evaluation of ceramic materials was done using the simple test
method called Depth of Penetration (DoP). This test uses flat ceramic samples fixed on a thick backing
material. The DoP corresponds to the depth at which the projectile imbedded itself in the backing
material (Figure 1). The projectile and its impact velocity are selected to overmatch the ballistic
resistance of the armour material ensuring significant penetration into the backing material. The DoP is
used with other parameters (e.g. backing material density, ceramic density, ceramic thickness, and
unprotected backing material penetration depth) to calculate the mass efficiency [Ref. 14-15], the
ballistic efficiency [Ref. 16] and the critical thickness to defeat the projectile [Ref. 17]. Equations are
provided in Table 2. The results allow the relative performance ranking of the different ceramic
formulations.
Figure 1. Schematic illustration of a DOP test
Table 2. Performance evaluation equations
𝐸𝑚 =
𝜌𝑏𝑎𝑐 𝑃𝑎𝑐
𝜌𝑐𝑒𝑟 𝑡𝑐𝑒𝑟 + 𝜌𝑏𝑎𝑐 𝐷𝑂𝑃
1a) Mass Efficiency (Em)
𝜂=
𝜌𝑏𝑎𝑐 (𝑃𝑏𝑎𝑐 − 𝐷𝑂𝑃)
𝜌𝑐𝑒𝑟 𝑡𝑐𝑒𝑟
1b) Ballistic Efficiency (η)
where
ρbac is the density of the backing material
2
𝑡𝑐𝑟𝑖𝑡 =
𝑡𝑐𝑒𝑟 × 𝑃𝑏𝑎𝑐
𝑃𝑏𝑎𝑐 − 𝐷𝑂𝑃
1c) Critical Thickness (tcrit)
(1)
ρcer is the density of the ceramic
Pbac is the penetration depth into the unprotected backing material
tcer is the thickness of the ceramic
DOP is the depth of penetration into backing after striking the ceramic target
(Pbac - DOP) is the reduction in penetration depth due to ceramic
Typically, DoP testing uses steel or aluminium as backing material [Ref. 11 to 13]. When used
with small calibre rifle bullets, low DoP are generally recorded due to the relatively high backing
material densities.
To increase resolution of the DoP measurement for a better discrimination of the results, a lower
density backing material such as a polycarbonate was proposed. Parameters to ensure test repeatability
with polycarbonate backing material were defined. The parameters included in our study were: sample
thickness, ceramic-backing bonding method, and sample confinement. All tests were conducted with
7.62 mm APM2 bullet launched from a universal receiver located at 5m from target. The first test
series evaluated different sample thicknesses to find out which one will generate significant DOP
measurements for better comparison between ceramic configurations. No ceramic was used for the first
shot. The bullet impacted the polycarbonate rod directly to establish the DoP reference. Four different
thicknesses (5, 6, 7 and 8 mm) of SiC ceramic tiles were evaluated. Square (100mm x 100mm) sample
tiles were secured with grease (Dow Corning MOLYKOTE® G-4700) to the backing polycarbonate
rod to obtain a uniform contact. DoP results are shown in Figure 2. The 6mm sample thickness (DoP ≈
60mm) was considered a good compromise and selected for all remaining tests.
Figure 2. SiC tile thickness influence on DoP
For the second test series, seven different test configurations were tried: unbonded, two ceramicbacking bonding methods (grease Dow Corning MOLYKOTE® G-4700 and glue AC-Tech AC-240
Class B-1/2) and three different tiles confinement (free, frame, and lid, Figure 3). Polycarbonate rod
used for all tests had a diameter of 150 mm (6 in) and a length of 300 mm (12 in). The test matrix is
summarized in Table 3 and results are shown in Figure 4. The free/glue configuration show the smaller
dispersion and was adopted for the remaining tests.
a. Free (no confinement)
b. Frame (radial confinement)
Figure 3. Confinement configurations
3
c. Lid (full confinement)
Table 3. Second series tests matrix
Series
Ceramic
Thickness
(mm)
Confinement
Bonding
Number of
Repetition
1
None
N/A
N/A
N/A
1
2
Al2O3
6
Free
None
3
3
Al2O3
6
Free
Grease
3
4
Al2O3
6
Lid
Grease
3
5
Al2O3
6
Frame
Grease
3
6
Al2O3
6
Free
Glue
3
7
Al2O3
6
Lid
Glue
3
8
Al2O3
6
Frame
Glue
3
Figure 4. Depth of Penetration comparison for different bonding and confinement
4. BALLISTIC EVALUATION OF NANOCOMPOSITE CERAMIC
DoP tests were done on all Al2O3 ceramic configurations made using the same particulate sizes (3 µm)
and all SiC (Table 1). The sintering of SiC + CNT was unsuccessful and no samples were available for
the DoP tests.
The DoP measurement corresponds to the straight line from point of penetration to the deepest
distance in the backing material (red line in Figure 5b). However, few bullets did not follow a straight
path (green line in Figure 5b). To use a better representation of the total energy dissipated in the
backing material, the DoP measurement used in Equations 1 corresponded to DoP Total Length
identified in Figure 5b.
4
a
b
Figure 5. DoP measurement
The mean DoP total length and other mean performance criteria are shown in Figure 6. To rank
the ballistic performance of the different ceramic configurations, a score from 1 to 7 was given to each
material, 7 corresponding to the best performer and 1 to the worst performer. An overall ranking was
obtained by adding the 4 scores of each material. The highest possible score would be 28. Results are
shown in Table 4. The best performer was the monolithic SiC followed by the Al2O3. It was both
surprising and disappointing that all new ceramic configurations with the addition of nanotechnology
did not improve the ballistic performances. These results were investigated further through failure
analysis to better understand what happened and to identify potential solutions.
DoP
Critical Thickness
Mass Efficiency
Ballistic Efficiency
Figure 6. Ballistic evaluation of nanocomposite ceramic configurations
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Table 4. Ballistic performance ranking
Material
DoP
Em
η
tcrit
Total
Ranking
Al2O3
7
6
5
6
24
2
Al2O3 + 1% BNNT
4
4
3
5
16
4
Al2O3 + 1% CNT
5
5
4
5
19
3
Al2O3 matrix + 18% Al2O3 fibre mat (2D)
2
2
2
2
8
6
Al2O3 matrix + 18% Al2O3 fibre mat + CNT (3D)
1
1
1
1
4
7
SiC
6
7
7
7
27
1
SiC + 1% BNNT
3
3
6
3
15
5
5. FAILURE ANALYSIS
The failure analysis presented in this paper focuses on the Al2O3 3D sample considering the high
expectation from quasi static fracture toughness results.
Ceramic fragments were recovered and reassembled to get a general overview of failure
mechanisms. For the unreinforced Al2O3 (Figure 7a), large radial and linear failures are seen with
many small fragments in the area close to penetration. In comparison, the Al2O3 3D reinforced (Figure
7b) fragments are much bigger, with less radial and circumferential failure pattern.
A scanning electron microscope (SEM) was used to provide further details on the failures
observed. For the Al2O3 3D, failures happened within the fibre mat (Figure 8) and in some cases, a
complete delamination was observed.
a) Unreinforced Al2O3
b) Al2O3 + 18% Al2O3 fibre mat + CNT (3D)
Figure 7. Unreinforced and reinforced Al2O3 failures
6
Figure 8. SEM of Al2O3 + 18% Al2O3 fibre mat + CNT (3D) - failure in the fibre mat
Enlarging the SEM image (Figure 9) showed that the outside layer of the fibre mat was strongly
bonded to the matrix and the failure occured directly behind.
Figure 9. SEM of Al2O3 + 18% Al2O3 fibre mat + CNT (3D) - failure behind 1st layer of fibres
When a complete delamination occurred within fibre mat, we observed that the forest of
nanotubes had not fully developed and that the zone without nanotubes created weak areas where
failures could propagate (Figure 10). Furthermore, deeper in the fibre mat, nanotubes did not develop.
This indicates that the procedure used to create the forest of nanotubes was unable to penetrate a dense
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fibre mat uniformly. Even if nanotubes had developed uniformly, the ceramic matrix would not have
been able to penetrate the fibre mat making the nanotube free and useless.
Figure 10. SEM of Al2O3 + 18% Al2O3 fibre mat + CNT (3D) - zone with no nanotubes developped on
fibre mat
6. CONCLUSION
A small percentage of carbon nanotubes and boron nitride nanotubes (1%) were integrated to Al2O3
and SiC matrix to investigate the potential of nanocomposite ceramic for ballistic armour applications.
A new method was developed to grow nanotubes on fibre mat creating a 3D ceramic structure.
The addition of nanotubes improved the mechanical properties but better ballistic performance
was recorded for the monolithic ceramics (without reinforcement).
For the new 3D ceramic structure, SEM images showed a strong bond between nanotubes and the
matrix but with failures occurring where no nanotubes had developed. Failure analysis also showed
that the nanotubes forest developed on the outer surface of the fibre mat, resulting in a weak internal
structure, favouring delamination.
Ceramic samples with small content of nanotubes did not improve ballistic performance. In fact,
the small content of nanotubes created limited reinforced zones which redirected the failure
propagation to areas without nanotubes. It is equivalent to having less material to distribute the impact
energy, thus resulting in lower ballistic performance.
Therefore, to take full benefit of nanotube properties, a ceramic having a homogeneous
distribution and very high content (possibly > 50%) of nanotubes is required to recreate a structure
similar to a monolithic ceramic.
Future work will focus on two new approaches: 1) to grow the nanotubes directly on ceramic
particulate before sintering to ensure a very high content and uniform distribution of nanotubes, and 2)
to use a uniform structure of open cells, e.g. ceramic foam, where nanotubes can be developed on the
open cell structure and later be infiltrated with ceramic matrix powder.
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