Preceramic polymer composite: Fabrication process and mechanical performance

Abstract

Preceramic polymers, which are converted into ceramics upon heat treatment, offer an advantage of combining the shaping and synthesis of ceramics parts. In this work, the Polymer Infiltration and Pyrolysis (PIP) process is employed to fabricate ceramic composite prototypes using preceramic polymers with carbon/glass fabrics, followed by experimental investigations of the mechanical properties of the ceramic composite samples (e.g. hardness and compression strength). X-ray micro computed tomography and SEM analysis were conducted to examine the effectiveness of the PIP.

Full text

Article published in
Materials Letters, Volume 318, 1 July 2022, 132218
https://doi.org/10.1016/j.matlet.2022.132218
Preceramic polymer composite: Fabrication process and
mechanical performance
Keivan A. Nazari1, Phuong Tran1, Ping Tan2, Abdallah Ghazlan3, Tuan D. Ngo3, Yi Min Xie1
1) School of Engineering, RMIT University, GPO Box 2476, Melbourne, Australia
2) Defense Science and Technology Group, Australia
3) Department of Infrastructure Engineering, The University of Melbourne, Melbourne, VIC,
Australia
Abstract
Preceramic polymers, which are converted into ceramics upon heat treatment, offer an
advantage of combining the shaping and synthesis of ceramics parts. In this work, the Polymer
Infiltration and Pyrolysis (PIP) process is employed to fabricate ceramic composite prototypes
using preceramic polymers with carbon/glass fabrics, followed by experimental investigations
of the mechanical properties of the ceramic composite samples (e.g. hardness and compression
strength). X-ray micro computed tomography and SEM analysis were conducted to examine
the effectiveness of the PIP.
Keywords
Preceramic polymers; Ceramic Composite; Polymer Infiltration and Pyrolysis; Ceramic
Sintering
Introduction
Recently, ceramic polymer composites are attracting the attention of researchers and engineers
for armour applications including ballistic protection. For example, Colombo et al. [1]
conducted ballistic testing on ceramic–polymer composites, which are composed of an alumina
ceramic face and silicon carbide foams infiltrated with different polymeric materials. Chabera
et al. [2] carried out ballistic testing using an armour-piercing (AP) shell to assess the ballistic
performance of Al2O3/PU2.5 ceramic-elastomer composites. Preceramic polymers need lower
temperatures for pyrolysis 850–1100˚C compared to ceramic powders which need
temperatures up to 1500 ˚C for long time. In addition, sintered ceramic powders require

machining which is very expensive and time consuming. Preceramic polymers can also be used
to produce ceramic components with complex geometries such as bioinspired structures, which
have been fabricated using diverse processing techniques including conventional ceramic
processing and additive manufacturing techniques such as stereolithography [3]. Hence,
preceramic polymers have shown advantages over ceramic powders to fabricate bioinspired
armours.
Monolithic ceramics suffer from brittleness which could be improved by combining additives
e.g., fibre (either reactive or inert), or carbon nanotubes (CNTs) with preceramic polymers [4-
6]. Ma et al. [4] used additives such as particles (e.g. carbon nanotube) and short chopped fibre
in preceramic polymers to increase the rigidity and strength of a ceramic composite. However,
only slight improvements in mechanical properties were observed in these composites. The
advantages of preceramic polymers, including economic and easy processing, lead to a number
of applications such as low-loss binders in powder metallurgy [7], coatings and armour systems
[1]. Technological benefits like near-net-shape technology, low process temperatures, together
with advantageous material properties such as high-temperature resistance without loss of
strength, are convincing reasons for using these materials.
This study aims to develop novel ceramic composite components for bioinspired armour
applications. The polymer infiltration and pyrolysis (PIP) process is used to fabricate ceramic
composite armour samples using preceramic polymers (i.e., SL 680, SPR 688) with fibres (i.e.,
carbon or glass), which have been infiltrated with or without SPR 212. Subsequently, various
experimental tests are conducted to obtain the mechanical properties of the ceramic composite
armour samples (e.g. hardness and compression strength).
Materials and fabrication process
Three Polysiloxane resins including SL 680, SPR 688 and SPR 212, and a platinum catalyst
are used in this study, which have been developed by Starfire (USA). The platinum catalyst (a
ratio of 2%) is used to simplify casting and decrease the curing temperature, as well as to
decrease the amount of gas bubbles in the green body. After adding a catalyst to the determined
amount of resin, the resin is exposed to a vacuum chamber for 24 hours to remove all existing
gas. Carbon fibre and glass fibre (S2) were used to prepare a fibre reinforced ceramic
composite.
The fibre mass fraction for the composite samples fabricated using carbon fibre/SL 680, carbon
fibre/SPR 688, glass fibre/SL 680, and glass fibre/SPR 688, is 36%, 53%, 52%, and 61%,

respectively. In order to produce the fibre reinforced ceramic composite green bodies, the
composite samples are placed in the furnace, in which the temperature is increased to 200 ˚C
at the rate of 5 ˚C/min over 60 min. The samples are then cured under ambient temperature in
the furnace to reduce the occurrence of cracks in the structure. This stage results in a fully
crosslinked polymer matrix in preparation for the pyrolysis process. It is highly recommended
that this process is carried out in the furnace under vacuum when fabricating large samples for
impact/ballistic tests to remove gas bubbles [8]. Subsequently, the green bodies are placed in
the furnace under an Argon (Ar) atmosphere, and the temperature is increased to 900 ˚C at a
rate of 5 ˚C/min over 60 min, and then allowed to cool down to ambient temperature in the
furnace at a rate of 10 ˚C/min.
The quality of the ceramic composite samples is inspected using x-ray CT scan analysis. Also,
compression tests are conducted for the fibre reinforced ceramic composite both with and
without SPR 212 infiltration. The hardness of the fibre reinforced ceramic composite is
measured using nanoindentation method. Compression tests were performed on rectangular
samples (50 mm×35 mm×20) in triplicate using an Instron 50 kN machine at a constant
crosshead speed with an initial strain rate of 10-3s-1. The microstructure of the sintered
composite is examined using scanning electron microscopy (SEM) (FEI Nova NanoSEM).
Results and discussion
The PIP process is detailed in Figure 1 (a) for the ceramic composite samples made of carbon
fibre/SL 680, glass fibre/SL 680, carbon fibre/SPR 688, and glass fibre/SPR 688, respectively.
The SEM micrographs of the ceramic composite samples, which were infiltrated with SPR 212
resin, are shown in Figure 1 (b-d). There are, however, noticeable voids at the intersection of
fibres because SL 680/SPR 688 or SPR 212 resin cannot be fully infiltrated into these areas.
There are also cracks near the voids which were formed during sintering due to the evaporation
of volatile elements.

Figure 1. (a) Schematic illustration of fabricating process of fibre reinforced ceramic
composite samples, SEM images of the ceramic samples pyrolyzed at 900 ˚C and made of (b)
carbon fibre/SL 680, (c) glass fibre/SL 680, (d) carbon fibre/SPR 680, (e) glass fibre/SPR 688
Figures 2 shows typical X-ray micro-CT scan images of the rear surface of two ceramic
composite samples made of carbon fibre/SL 680 and pyrolyzed at 900 ˚C both with and without
SPR 212 infiltration. The regular existence of the small (blue) and large (red) voids among the
fibres are observed in the micro-CT images. The porosities in the sample without SPR 212
infiltration seem to be interconnected (Figure. 2(a)). After infiltration with SPR 212, the
number of the small and large voids reduces considerably (see Figure. 2(b) for the selected
area). Hence, it is expected that the mechanical properties of the samples could be improved
by infiltration with SPR 212 by several orders of magnitude (six times in this study). Baily [8]
and Yang [9] used both carbon fibre and carbon nano tube (MWCNT) to enhance the flexural
strength and stiffness of a ceramic composite. Therefore, using fibres and nano fillers to

fabricate preceramic samples are necessary to achieve ceramic samples with improved
mechanical properties after infiltration with SPR 212 resin.
(a)
(b)
Figure 2. The X-ray CT scan images of the ceramic samples pyrolyzed at 900 ˚C and made of
carbon fibre/SL 680 (a) without SPR 212 infiltration and (b) with SPR 212 infiltration, which
shows the volume percentage of the voids.
The corresponding compressive stress–strain (σ–ε) curves of the SL 680, SPR 688, and fibre
reinforced ceramic composite samples with and without SPR 212 infiltration are plotted in
figure 3. The mechanical properties and the volume percentage of the voids in the samples are
listed in table 1. The results depict that fibre reinforced composite samples, which have been
pyrolyzed (without further SPR 212 infiltration) possess a lower compression strength, elastic
modulus and density compared to those with SPR 212 infiltration due to their higher volume
percentage of voids. For the samples with SPR 212 infiltration, the density, compression
strength, elastic modulus and hardness for the samples made of SL 680 are higher than SPR
688, especially the compression strength and elastic modulus. This is because the SL 680
samples have a lower void volume fraction after the infiltration process due to having
additional SiC fillers. The highest compression strength belongs to carbon fibre/SL 680 sample
related to the lowest volume percentage of voids.

(a)
(b)
0
2
4
6
8
10
12
14
16
18
20
00.1 0.2 0.3 0.4 0.5 0.6
Compressive stress (MPa)
Compressive strain (mm/mm)
SL 680
SPR 688
0
1
2
3
4
5
6
00.5 11.5 22.5
Compressive stress (MPa)
Compressive strain (mm/mm)
Glass fibre+SL 680
Carbon fibre+SL 680
Glass fibre+SPR 688
Carbon fibre+SPR 688

(c)
Figure 3. Nominal compressive stress–strain curves of pyrolyzed samples made of carbon
fibre/SL 680, glass fibre/SL 680, carbon fibre/SPR 688, and glass fibre/SL 680 (a) sintered
preceramic samples at 900 ˚C (b) without being infiltrated and (c) infiltrated with SPR 212,
respectively.
Table1. Density, compression strength, elastic modulus and hardness (via indentation method
according to ASTM C1327-15) for the selected samples without and with being infiltrated by
SPR 212, * the measurement of the hardness was not possible due to porosity.
(a) Without SPR 212 infiltration
Materials
Density
(g/cm
3
)
Strength
(MPa)
Elastic modulus
(GPa)
Hardness
(GPa)
Void percentage
Carbon fibre/SL
680
1.66
3.2
5
N/A*
22.81
Glass fibre/SL
680
1.78
5.5
5
N/A*
11.25
Carbon fibre/SPR
688
1.31
3.2
3
N/A*
23.87
Glass fibre/SPR
688
1.53
2.5
3
N/A*
31.76
(b) With SPR 212 infiltration
Materials
Density
(g/cm
3
)
Strength
(MPa)
Elastic modulus
(GPa)
Hardness
(GPa)
Void percentage
Carbon fibre/SL
680
1.98
73
65
7
5.25
Glass fibre/SL
680
2.15
65
76
6.5
2.54
0
10
20
30
40
50
60
70
80
00.5 11.5 22.5 3
Compressive stress (MPa)
Compressive strain (mm/mm)
Glass fibre/SL 680
Carbon fibre+SL 680
Glass fibre+SPR 688
Carbon fibre+SPR 688

Carbon fibre/SPR
688
1.62
13
15
6.5
7.38
Glass fibre/SPR
688
1.92
17
6
6
8.65
(c) Properties of both glass fibre S2 and carbon fibre
Property
Value (S.I.)
Glass fibre S2
Carbon Fibre
Poisson's Ratio
0.21-0.23
0.23
Shear Modulus (GPa)
35-39
10–15
Tensile Strength (GPa)
4700-4800
3–7
Young's Modulus (GPa)
86-93
183
Conclusion
The SEM micrographs and X-ray CT scan analysis indicated that the cracks and voids were
presented in both samples made of SL 680 and SPR 688 after pyrolysis at 900 ˚C with or
without SPR 212 infiltration. SEM micrographs also demonstrated that the intersection of the
fibres could not be fully filled with SPR 212 resin, which leads to lower mechanical properties.
The existence of cracks and/or voids in the samples resulted in poor compression strength and
more brittle behaviour. Embedding fibres (e.g., glass and carbon fibre) in Polyramic® SL 680
and SPR 688 were required to improve the mechanical properties of ceramic composites.
It was noted from the X-ray micro-CT scan image analysis that for the fibre reinforced ceramic
composite considered, the volume fraction of voids ranges from 11.25% to 31.76% in the
samples without vacuum infiltration of SPR 212 resin, and conversely 2.54% to 8.65% in those
infiltrated with resin. Infiltration with SPR 212 results in a significant reduction of voids in the
samples and thereby increases density, compression strength, elastic modulus and hardness.
Hence, the infiltration with SPR 212 is the key process in decreasing the voids and
consequently increasing the mechanical properties. Also, it was noted from the present study
that the samples made of SL 680 had a higher compression strength due to the existence of SiC
filler in the SL 680 resin.
Acknowledgment
This project was funded by the Defence Science and Technology Group (MvIP: ID8419),
Australia.
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