A Novel Approach for Fabrication of Reaction Bonded Silicon
Carbide (Rbsc) Composites
N. Frage, S. Aroati, M.Cafri, H.Dilman and M.P.Dariel
Department of Materials Engineering, Ben-Gurion University,
P.O.Box 653, Beer-Sheva 84105, Israel
ABSTRACT
RBSC composites are fully dense materials fabricated by pressure-less infiltration of
molten silicon into compacted mixtures of silcon carbide and carbon. Free carbon is
formed as a result of the pyrolysis of an organic resin, (the carbon source) and reacts
with molten silicon to form secondary SiC grains that precipitate on the original SiC
particles. Non-reacted residual silicon fills the free areas between the SiC grains. The
presence of the residual silicon decreases the mechanical properties of the composites;
moreover, the pyrolysis process is environmentally unfriendly.
In the present study, boron carbide powder was used as an alternative carbon source,
leading to reduced fraction of the residual silicon. In addition, the volume available for
the infiltrated silicon was reduced by using mixtures of multimodal silicon and boron
carbides powders and thereby obtaining a relative density of about 80% in the
compacted body. The final composites fabricated by infiltration with molten silicon,
that has optimal composition display a relatively low fraction of residual silicon and
improved mechanical properties (hardness 1960±300HV, elastic modulus 365±2GPa,
flexural strength 271±15 MPa). It is also noteworthy that the suggested approach is
environmentally friendly.
INTRODUCTION
RBSC is an attractive material for light armor applications. A conventional approach
of RBSC fabrication involves infiltrating a compacted SiC/carbon mixture with molten
Si. The reaction between molten silicon and free carbon leads to the formation of
silicon carbide grains, which precipitated on the initial SiC particles, interconnect
them, and provide continuous ceramic skeleton. The typical microstructure of RBSC
composites consists of the about 85% inter-connected silicon carbide phase and
residual silicon1.
The main advantage of the RBSC fabrication process is it’s relatively low temperature
(~1500°C) in comparison with pressure-less or hot pressing densification. On the other
hand, RBSC composites have two main problems. The first one is the presence of the
residual silicon that leads to lowering mechanical properties of the final composites
and the second one is related to using carbon containing organics (resins) as a source
of free carbon, which is formed as a result of a pyrolisis process. This process takes
place at elevated temperatures and accompanied with releasing toxic gases and is
strongly environmentally unfriendly. In order to reduce the amount of the residual
silicon in reaction bonded composites it was suggested to use powder mixture with
appropriate particles size distribution2. This approach allows to fabricate compacts
with green density of about 20vol. % and to reduce significantly the amount of the
residual silicon (<10vol. %). In our previous work was also established that boron
carbide directly reacts with liquid silicon and serves as carbon source in the reaction
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bonded boron carbide composites3. In the present work we have combined these two
features in order to fabricate RBSC composites with decreased amount of the residual
silicon and using boron carbide as an alternative source of free carbon instead carbon
containing organics.
EXPERIMENTAL PROCEDURE
Fabrication of the Composites
A commercial coarse (~120m) silicon carbide powder was grinded and sieved in
order to obtain requested particle size distribution in the powders mixtures. Some of
the fractions in the SiC powders mixtures were replaced by boron carbide powders
with different particle size. Boron carbide powder (~1m, grade HS) was supplied by
H.C. Starck, and B4C powders with average particle size of 13µm was supplied by
"Modan Jang" a Chinese Company. 5 mixtures of silicon and boron carbides powders
were prepared and each mixture corresponds to the desired particle size distribution
(Table 1).
Table 1. The powder compositions and particle size distribution that were used at this
work
Vol. % of different powders
RB-1 RB-2 RB-3 RB-4 RB-5
53
53
53
53
53
106
10
10
10
10
10
50
SiC
18
18
12
7
2
13
19
4
0
0
0
3
0
0
0
0
0
106
0
0
0
0
0
50
B4 C
0
0
6
11
16
13
0
15
19
19
19
3
24
24
23
23
21
Porosity of green compact , %Vol.
Particle size, D50 ,µm
The mixtures were dry mixed in a planetary mixer for 8 hours. 20mm diameter and
5mm high performs were uniaxially compacted under 180MPa. The compacts were
infiltrated with liquid silicon (98.4%, Alfa Aesar) at 14800C for 15 min under vacuum
10-4 torr.
Characterization
Microstructure and composition
The microstructure of the samples was studied using optical microscope (OM, Zeiss
Axiovert 25) and scanning electron microscopy (SEM, JEOL-35) accompanied with
an energy dispersive spectrometer (EDS). The samples for the microstructure
characterization were prepared using standard metallographic procedure that includes
a last stage of polishing by 2.5μm diamond paste. Image analysis using the Thixomet
software was applied in order to determine the amount of the residual silicon in the
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composites. The phase composition was determined qualitative by x-ray diffraction
(XRD).
Mechanical properties
Micro-hardness values were determined by Vickers hardness (Buehler-Micromet
2100) with 2000gr load. Flexural strength was measured with LRX plus LLOYD
machine (Lloyd Instruments, Fareham Hants, U.K.) using 1.5x2x20mm3 samples. The
elastic modulus of the composite was derived from ultrasonic sound velocity
measurements according to the "Pulse Echo" method. Density of the infiltrated
composites was measured using Archimedes principle.
RESULTS AND DISCUSSIONS
Microstructure
The microstructure of the composite (Fig.1) fabricated with addition of 35%vol. boron
carbide powders consists of B12(C, Si)3, SiC and the residual silicon. The presence of
the B12(B,C,Si)3 grains with a gray color slightly lighter than a color of SiC particles
are clearly observed. It is very important to point out that B12(B,C,Si)3 grains connect
ceramic particles and provide continuous ceramic skeleton, similar to that for the
composites fabricated with free carbon additions and its formation is in a good
agreement with the reported previously our results for reaction bonded boron carbide
composites3.
Fig.1. Optical microscope images of the composites microstructure with 35% Vol.
B4C particles (the whole B4C original particles converted to rim)
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1000
a - SiC - 35Vol.% B4C powders mixture before infiltration
b - SiC - 35Vol.% B4C
infiltrtaed composite
Counts.
b
SiC
B12(C,Si,B)3
500
a
B4C
0
35.0
35.5
36.0
36.5
37.0
37.5
38.0
2
Fig.2. XRD pattern of the composites.
According to XRD analysis (Error! Reference source not found.) the initial boron
carbide particles (curve a) were completely converted to the ternary B12(B,C,Si)3 phase
(curve b).
It was established in our previous work that in the reaction bonded boron carbide
composites fabricated without free carbon additions the secondary SiC phase displays
plate-like morphology4. In contrast, for the reaction bonded SiC, fabricated with boron
carbide as the carbon source, the presence of secondary SiC phase with plate-like
morphology was not observed. This difference may be attributed to the fact that
surface of the initial SiC particles, serves as a preferred site for heterogeneous SiC
nucleation. The presence of a thick secondary SiC layer indicates that boron carbide
particles serve as an effective source of carbon.
Fig.3. SEM images (SE) of the composites microstructure with 35%Vol. B4C particles
addition
Mechanical Properties
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The mechanical and the physical properties of the composites that were fabricated
with different fraction of B4C were summarized in
Table 2.
Table 2. Properties of the infiltrated composites
RBSC*
RB-1
RB-2
RB-3
Density ,
2.85-3.10
2.92
2.89
2.89
gr/cm3
Young’s
320-340
344
343
345
Modulus,
GPa ±3
Flexural
190-250
204
255
256
strength ,
MPa ±18
Hardness, Hv 1500-2200 1534±202 1673±262 1835±262
Residual Si,
15-35
23.4
19.4
15.6
vol. %
*Range of the composites properties reported in literature1,5,6 .
RB-4
RB-5
2.83
2.83
351
353
237
270
1928±307
1963±331
12.0
10.6
According to the results presented in
Table 2 the density of the composites decreasing with the added amount of boron
carbide. The enhanced amount of SiC and the reduced amount of the residual silicon
stand behind the increased hardness values. As was expected the values of Young
modulus do not depend on the particle size of added boron carbide and increases with
boron carbide volume fraction. In order to estimate the reliability of the composites,
Weibull modulus (m) for flexural strength was calculated according to equation:
1
*
ln ln(
)  m ln  max  m ln  0
1 P
i  0.5
Where, P 
, “i” is the specimen number from N samples,  max is the flexural
N
*
strength of specimen i and  0 is mean value of the flexural strength.
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b
1.5
1.0
ln(ln(1/1-P))
0.5
0.0
SiC-Hy-5-FP
Linear fitting
y = 16.10x - 90.65
2
R = 0.95
Weibull modulus : 16.10
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
5.40
5.45
5.50
5.55
5.60
5.65
5.70
ln()
Fig.4. Weibull plots for flexural strength of composites fabricated with 35% vol. B4C
The outcome of the experimental results treatment for the composites with 35vol. %
carbide particles is presented in Error! Reference source not found.. The higher
value of Weibull modulus for the composites fabricated by using B4C powder reflects
high level of the microstructure homogeneity.
CONCLUSIONS
RBSC composites were fabricated using various fractions of boron carbide as an
alternative source of carbon. The composites with B4C addition display high reliability
and slightly better mechanical properties than the composites fabricated by
conventional approaches. The suggested approach is environmentally friendly and
allows avoiding the formation of carbon containing organic materials.
REFERENCES
1
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2
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distribution on the microstructure and the mechanical properties of boron carbide
based reaction bonded composites”, International Journal of Applied Ceramic
Technology , 1 – 9 (2008)
3
S. Hayun , N. Frage and M.P. Dariel, "The morphology of ceramic phases in BxCSiC-Si infiltrated composites" , Journal of solid state chemistry , 179 (2006) p:28752879.
4
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5
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6
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content and temperature on the strength and toughness of reaction-bonded silicon
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