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Additive Manufacturing of Ceramics from Preceramic Polymers: A Versatile Stereolithographic Approach Assisted by Thiol-Ene Click Chemistry

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Here we introduce a versatile stereolithographic route to produce three different kinds of Si-containing thermosets that yield high performance ceramics upon thermal treatment. Our approach is based on a fast and inexpensive thiol-ene free radical addition that can be applied for different classes of preceramic polymers with carbon-carbon double bonds. Due to the rapidity and efficiency of the thiol-ene click reactions, this additive manufacturing process can be effectively carried out using conventional light sources on benchtop printers. Through light initiated cross-linking, the liquid preceramic polymers transform into stable infusible thermosets that preserve their shape during the polymer-to-ceramic transformation. Through pyrolysis the thermosets transform into glassy ceramics with uniform shrinkage and high density. The obtained ceramic structures are nearly fully dense, have smooth surfaces, and are free from macroscopic voids and defects. A fabricated SiOC honeycomb was shown to exhibit a significantly higher compressive strength to weight ratio in comparison to other porous ceramics.
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Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
1
Additive Manufacturing of Ceramics from Preceramic Polymers: A Versatile
Stereolithographic Approach Assisted by Thiol-Ene Click Chemistry
Xifan Wang1*, Franziska Schmidt1, Dorian Hanaor1, Paul H. Kamm2, Shuang Li3, and Aleksander
Gurlo1
*correspondence to: xifan.wang@ceramics.tu-berlin.de
1 Fachgebiet Keramische Werkstoffe / Chair of Advanced Ceramic Materials, Technische Universität
Berlin, Hardenbergstr. 40 BA3, Berlin, 10623, Germany.
2 Helmholtz-Zentrum Berlin für Materialien und Energie, Institut für Angewandte Materialforschung /
Institute of Applied Materials, Hahn-Meitner-Platz 1, Berlin, 14109, Germany
3Functional Materials, Department of Chemistry, Technische Universität Berlin, Hardenbergstr. 40
BA2, Berlin, 10623, Germany
Abstract
Here we introduce a versatile stereolithographic route to produce three different kinds of Si-containing
thermosets that yield high performance ceramics upon thermal treatment. Our approach is based on a
fast and inexpensive thiol-ene free radical addition that can be applied for different classes of preceramic
polymers with carbon-carbon double bonds. Due to the rapidity and efficiency of the thiol-ene click
reactions, this additive manufacturing process can be effectively carried out using conventional light
sources on benchtop printers. Through light initiated cross-linking, the liquid preceramic polymers
transform into stable infusible thermosets that preserve their shape during the polymer-to-ceramic
transformation. Through pyrolysis the thermosets transform into glassy ceramics with uniform
shrinkage and high density. The obtained ceramic structures are nearly fully dense, have smooth
surfaces, and are free from macroscopic voids and defects. A fabricated SiOC honeycomb was shown
to exhibit a significantly higher compressive strength to weight ratio in comparison to other porous
ceramics.
Keywords
Additive manufacturing; stereolithography; polymer derived ceramics; silicon oxycarbide; compressive
strength
1. Introduction
In industrial applications, ceramics are most
commonly formed by slip casting, injection
molding and various pressure-assisted
techniques. As all of these methods rely on the
usage of molds, the shape of ceramic parts is
restricted to relatively simple geometries. The
machining of complex shapes is further
hindered by the high hardness and brittleness of
ceramic materials. Additive manufacturing
(AM) heralds a new era in the fabrication of
ceramics. As AM technologies are based on the
layer by layer consolidation of ceramic
powders, they neither require molds nor are they
limited by the hardness and brittleness of
ceramics. This in turn allows for the fabrication
of complex parts that utilize the exceptional
properties of ceramics such as high strength,
thermal stability and chemical resistance [1,2].
Traditional AM approaches towards ceramics
rely on powder- and slurry-based technologies
including stereolithography of ceramic slurries
containing UV-curable photopolymers [3,4],
laser sintering of ceramic powder beds [5,6]
and binder jetting, where a liquid organic binder
is injected into a ceramic powder bed [7,8].
Forming dense objects by consolidating
ceramic powders without pressure is a
challenging task due to the unavoidable
presence of pores and cracks that impair the
mechanical performance. For the fabrication of
dense ceramics, formulations with high solid
loadings are required, which consequently
restrict the suitability of slurries for the
manufacturing of fine structures due to their
decreased flow ability, increased viscosity and
inhomogeneity. When shaping ceramic slurries
with light, a significant mismatch between the
refractive index of the ceramic powder and that
of the photocurable resin strongly reduces the
curing depth and additionally causes a
coarsening of the curing resolution due to the
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
2
side scattering of incident light [9].
Consequently, it is far more challenging to 3D
print ‘dark’ ceramic materials (Si3N4, SiC)
relative to alumina, silica or zirconia.
Polymer-derived-ceramics (PDCs) define a
class of ceramic materials that are synthesized
by thermal treatment (usually pyrolysis) of
ceramic precursors (so-called preceramic
polymers) under an inert or reacting atmosphere
[10]. By using suitable preceramic polymers,
various ceramic compositions such as
amorphous silicon carbide (SiC), silicon
oxycarbide (SiOC), and silicon carbonitride
(SiCN), can be obtained after heat treatment at
1000-1100 °C in an inert atmosphere (argon or
nitrogen). Because there is no sintering step,
PDC parts can be formed without pressure at
lower temperatures relative to traditional
ceramic powder shaping technologies.
Preceramic polymers can be processed using
existing technologies suitable for polymers in
general. Due to their distinctive nanostructure
of carbon-rich and free carbon domains, PDCs
show exceptional stability against oxidation,
crystallization, phase separation and creep even
up to 1500 °C [10].
Currently, PDCs have been successfully
utilized for the fabrication of ceramic fibers,
ceramic matrix composites (CMCs) and
microstructures, e.g. microelectromechanical
systems (MEMS). Since some preceramic
polymers contain silyl and vinyl groups, they
can act as negative photoresists, exploiting a
UV activated hydrosilylation reaction. For this
reason, photolithography is frequently used to
achieve fine two-dimensional patterning in
PDC MEMS [1116]. However, UV activated
hydrosilylation requires adding large amounts
of photoinitiators (PI) and long exposures to
high energy UV light, which makes its
utilization unrealistic in additive manufacturing
processes. To overcome this limitation certain
polysiloxanes can be modified by the
attachment of curable functional groups to the
silicone backbone in order to render them UV
active and facilitate rapid photopolymerization.
The additive manufacturing of macro-sized
complex SiOC parts with excellent mechanical
strength and stability against high temperature
oxidation by stereolithography of preceramic
polymers was first reported in [17,18]. In the
first approach, two silicones terminated with
vinyl and mercapto functional groups were
blended together to trigger cross-linking
[18,19]. In the second approach [17], to obtain
a photocurable polymer suitable for
stereolithography, a commercial polysiloxane
(Silres ® MK Powder) was modified by
attaching functional acrylate groups to the
silicone backbone. Despite significant
developments in the field, the full potential of
PDC stereolithography has not yet been
realized. The range of readily photocurable
preceramic polymers remains quite confined
and has thus far been limited to polysiloxanes,
consequently only SiOC ceramics have been
additively manufactured in macro scale
[2,17,18,2022]. A recent study examined 3D
printing of SiC by blending acrylate based
photopolymers with SiC precursors [23].
However, oxygen contamination from the
photopolymers is unavoidable and is likely to
largely degrade the product material’s
thermomechanical properties. Another
disadvantage of this route is the extremely low
ceramic yield [20,23], as the mixed
photopolymers are completely eliminated
during pyrolysis.
Classical radical photopolymerization pathways
rely mainly on the addition reaction of carbon
double bonds in acrylate or methacrylate, and
they have been widely used in photolithography
and stereolithography processes. Acrylate or
methacrylate based photopolymerization routes
suffer from fundamental limitations, including
high levels of polymerization-induced volume
shrinkage and consequent stress development,
and oxygen inhibition of the cross-linking
reaction. An alternative pathway involves thiol-
ene free radical addition, where SH groups
undergo addition reactions with carbon-carbon
double bonds. This polymerization route is
considered as a “click reaction” and therefore
exhibits the following characteristics [24]: (a) a
high reaction yield (b) fast reaction rate, (c)
insensitivity to solvent parameters (d)
negligible volumetric shrinkage, and (e)
insensitivity towards oxygen and water. These
overall properties render thiol-ene click
chemistry a promising approach towards
photopolymerization in general and additive
manufacturing in particular. It has been shown
that the preceramic polymer polycarbosilazane,
can be photopolymerized with thiol-ene
reaction in 2D geometries and converted to
silicon carbonitride based material upon
pyrolysis [13].
In this work, we introduce a versatile
stereolithographic route, based on VAT
photopolymerization, using fast and
inexpensive thiol-ene click reactions to 3D print
Si-containing thermosets. These are
subsequently pyrolyzed to obtain high
performance Si-containing ceramics. Our
approach is applied for different classes of pr-
ceramic polymers containing carbon-carbon
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
3
double bonds, including polysiloxane,
polycarbosilane and polycarbosilazane with
side vinyl groups. Due to the rapidity and
efficiency of thiol-ene click chemistry reactions
used here, the AM process can be effectively
performed with conventional light sources
(such as projectors) on benchtop Digital Light
Processing (DLP) printers. Due to their high
optical transparency, which minimizes the
scattering effects, resin mixtures are further
applicable for more sophisticated techniques
such as two-photon polymerization [25,26] and
microstereolithography [27].
2. Materials and methods
2.1 Materials:
The preceramic polymers used in this work are
a liquid methylvinylhydrogen polysiloxane
(polyramic SPR-212) and
allylhydrydopolycarbosilane (StarPCSTM
SMP10), bought from Starfire Systems (USA),
as precursors for SiOC and SiC ceramics
respectively. For SiCN, a liquid
methylvinylhydrogen polycarbosilazane
(Durazane 1800) bought from Merck,
Germany) was selected. 1,6-hexanedithiol,
phenylbis(2,4,6-trimethylbenzoyl)phosphine
oxide (BAPOs), Sudan Orange G, and
hydroquinone were purchased from Sigma
Aldrich , Germany. All chemicals were used
without further purification.
Formulation of photopolymerizable preceramic
resins for stereolithography was conducted as
follows: The preceramic resin was prepared by
mixing preceramic polymers with phenylbis
(2,4,6-trimethylbenzoyl) phosphine oxide
(BAPOs) as photoinitiator (PI), Sudan Orange
G as photoabsorber and hydroquinone as free
radical scavenger. In order to homogenize the
resin, it was treated in an ultrasonic bath for 2
hours, followed by degassing under vacuum.
After that, 1,6-hexanedithiol (2T), which
provides the thiol groups to react with the
alkene functionalities of preceramic polymers,
was dissolved in the resin mixture via mild
stirring for 1 hour. The prepared resin was
subsequently stored in a brown glass bottle to
avoid exposure to light.
Upon exposure to light with wavelengths below
450 nm, which are included in the emitted white
light from the projector, the PI undergoes a
cleavage process generating free radicals (see
Fig. S1 in SI), which then initiate the thiol-ene
click reaction as seen in Fig. 1. Sudan Orange
G, which exhibits suitable optical absorbance in
the relevant working spectrum of the
photoinitiator, was utilized to modify the
sensitivity of prepared resin and therefore limit
the penetration depth of the emitted light. As a
result, the z-axis resolution can be controlled.
Detailed information of resin sensitivity and
critical exposure time can be found in Fig. S2 of
the provided supplementary information.
Hydroquinone acts as free radical scavenger to
avoid any unwanted photopolymerization
induced by background light or scattering, and
was found to significantly prolong the shelf life
of the resin.
Fig. 1. Schematics of the photoinduced thiol-ene click reactions (cross-linking) of vinyl-containing Si-
based preceramic polymers with 1,6-hexanedithiol via an anti-Markovnikov addition.
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
2.2 Stereolithography and thermal pyrolysis:
Stereolithography of preceramic polymer resins
was conducted in an Open 3D Resin Printer
LittleRP2 (LittleRP, USA) using an Acer
X152H projector as a light source. The lens
system of the projector was modified for short
distance focusing. The z-axis resolution,
namely the layer thickness of each print, was set
at 100 µm to enable rapid printing. As a result,
objects with 2 cm height could be fabricated in
approximately one hour. The printing
parameters including the exposure time are
reported in Table S1. After printing, the parts
fabricated from polysiloxane were washed with
isopropanol, while those fabricated from
polycarbosilane and polycarbosilazane were
washed with anhydrous cyclohexane.
Subsequently parts were post cured under
exposure to UV irradiation between 350 and
400 nm for 30 mins. To transform these parts
into ceramics they were pyrolyzed in a tubular
furnace at 1100 °C for 2 hours under flowing
nitrogen. The heating/cooling rate was set to 40
°C /h. A schematic illustration of the complete
printing process is shown in Fig. 2.
Fig. 2. Additive manufacturing of ceramics from preceramic polymers: schematic representation of
thiol-ene click chemistry based stereolithography. (A) The preceramic photopolymerizable resin was
prepared by mixing different preceramic polymers (polysiloxanes, polycarbosilane, polycarbosilazane)
with 1,6-hexanedithiol dithiol along with phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (BAPOs)
as photoinitiator (PI), Sudan Orange G as photoabsorber and hydroquinone as free radical scavenger
(for more details, see Materials and Methods). (B) Photopolymerization of preceramic resins in a
benchtop DLP printer utilizing a commercially available projector as a light source. (C) A
representative printed cross-linked thermoset and the resultant glassy ceramic formed upon pyrolysis
at 1100 °C under N2 atmosphere.
2.3 Characterization:
Fourier transform infrared spectroscopy (FTIR)
in Attenuated Total Reflection (ATR) mode
carried out in an Equinox 55 (Bruker, Germany)
in the range of 500 4000 cm-1 was used to
characterize the free radical initiated thiol-ene
polymerization as well as the polymer to
ceramic transformation. In-situ Differential
Scanning Calorimetry (DSC) under exposure to
UV light in the range between 350 and 600 nm
(100 mW/s) was utilized in a DSC Q2000 (TA
Instruments) to characterize the heat flow and
reaction kinetics during photopolymerization.
X-ray photoelectron spectroscopy (K-Alpha
TM, Thermo Scientific) was used to determine
the elemental composition of printed parts. The
polymer to ceramic conversion was investigated
in nitrogen with a heating rate of 10 °C min-1
with thermal gravimetry/differential thermal
analysis (TG/DTA) on STA 409 PC LUXX
(Netzsch, Germany) coupled with a mass
spectrometer OMNi Star GSD 320 (Pfeiffer
Vacuum, Germany). Solid-state 29Si NMR
spectra was recorded with a Avance 400 MHz
spectrometer (Bruker, Germany) operating at
79.44 MHz to characterize the chemical
environment of silicon in polysiloxane derived
SiOC ceramics. Transmission electron
microscopy (TEM) was used to prove the
amorphous nature of SiOC. X-ray imaging
(radiography) was performed using a
microfocus X-ray source and a flat panel
detector (Hamamatsu, Japan) with an area of ~
120 x 120 mm² and a pixel size of 50 µm as
reported in [28]. Sample magnification was set
between 6 and 10, depending on the size of the
sample, resulting in an acquired projection with
a pixel size between 5 and 10 µm. For the
tomographic acquisition 1000 projections were
taken over a sample rotation of 360°. The
printing quality and microstructure of 3D
printed SiOC ceramics was evaluated by
Scanning Electron Microscopy in Leo Gemini
1530 (Zeiss, Germany).
2.4 Mechanical properties:
Mechanical tests were conducted on SiOC
honeycombs additively manufactured from
polysiloxane SPR212 and pyrolyzed at 1100 °C
for 2 hours under flowing nitrogen.
Nanoindentation of SiOC honeycomb that had
been embedded in epoxy resin and polished to a
1 µm finish, was conducted using a TI 950
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
5
TriboIndenter (Hysitron Inc., USA) with a
tetrahedral Berkovich diamond tip at a load of
10 mN and a load rate of 50 µN/s. The elastic
modulus and hardness are calculated by
evaluating the slope of the curve dP/dh upon the
elastic unloading with 200 indents. The
compressive strength of SiOC honeycombs was
determined using a universal test machine
RetroLine (Zwick/Roell, Germany) with a
cross-head speed of 0.1 mm/min. For this test,
samples were first ground to achieve relatively
parallel surfaces and a thickness of 2.78 mm.
These were then sandwiched between two steel
face sheets with epoxy glue to prevent
horizontal movement. The solid cell wall
density s) of SiOC honeycombs was
determined via Archimedes’ method using
distilled water, while the cellular density of this
honeycomb (ρ*) was calculated according to
[29] by dividing the foam mass by its volume,
which is the product of its length (a), width (b)
and height (c).
3. Results and discussion
3.1 Thiol-ene click chemistry assisted
stereolithography
To demonstrate the versatility of our approach
we applied three commercially available Si-
containing polymers from three different
classes as precursors for three different ceramic
compositions, i.e. polysiloxane SPR212 as
precursor for SiOC, polycarbosilane SMP10 as
precursor for SiC and polysilazane
Durazane1800 as precursor for SiCN. The thiol-
ene click reaction assisted AM process is
schematically illustrated in Fig. 2. Preceramic
polymers mixed with 1,6-hexanedithiol and
BAPOs (PI) are cross-linked in less than 7
seconds using a commercial digital light
processing (DLP) based projector.
Upon exposure to the white light, the PI
undergoes a cleavage process generating free
radicals that initiate the thiol-ene
polymerization reactions shown in Fig. 1 in
which thiol groups react with the alkene groups
of preceramic polymers leading to a thioether
linkage. Because of this propagating cross-
linking, the liquid preceramic polymers
transform into robust infusible thermosets that
are essential for the preservation of 3D printed
geometries during subsequent processing
including washing and pyrolysis. Insufficient
cross-linking would lead to soft structures that
would deform prior to or during the
ceramization process.
Fig. 3. (A-C) ATR-FTIR spectra of preceramic resins before (black) and after photoinduced curing (red):
(A) with polysiloxane SPR212, (B) polycarbosilane SMP10 and (C) polycarbosilazane Durazane1800.
The thiol-ene click polymerization is confirmed by the strong reduction of the absorption bands of the
vinyl-groups. (D) In-situ differential scanning calorimetric characterization of preceramic resins
exposed to light demonstrating that the preceramic resins are effectively cured within 15 secs, which
makes them suitable for stereolithographic additive manufacturing: (black) the curing reaction heat and
(blue) the conversion rate (the integral curve of heat flow after normalization)
.
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
The evolution of the thiol-ene click reaction can
be observed using FTIR spectroscopy by
monitoring the intensity of bands associated
with vinyl groups at 1407 cm-1 (=CH2
scissoring) and 1597 cm-1 (C=C stretch) and
with thiol groups at 2570 cm-1. Fig. 3 A-C
display the FTIR spectra of SPR212, SMP10
and Durazane1800 polymers before and after
the thiol-ene assisted photopolymerization.
Because of the low concentration of thiols and
relatively weak absorption associated with the
S-H groups, the thiol peaks are not observed in
the FTIR spectra even before the UV
irradiation. Nonetheless, the significant
decrease of the intensity of the vinyl absorption
bands in the FTIR spectra as well as the
elimination of the strong thiol odor indicate a
successful thiol-ene click reaction in all
polymers exposed to light. The appearance of
the C=O absorption band in SMP10 based resin
is induced from the addition of BAPOs.
However, in-situ real-time calorimetric
characterization under UV irradiation reveal
significant differences between the polymers
studied in this work.
As shown by the black lines in Fig. 3D, the
polysiloxane SPR212 as well as polysilazane
Durazane1800 polymers exhibit strong and
sharp exothermic peaks immediately after UV
exposure, which indicate very high curing
efficiencies for these polymers. The normalized
integral of these exothermic peaks shows that
more than 50% of the resin mixture can be
cross-linked within 5 secs, which is an ideal
time frame for the AM process, where fast and
efficient reaction kinetics are required. In
contrast, the polymerization of the SMP10
polymer was significantly slower and was
accompanied by smaller heat evolution. This is
because SMP10 has far fewer vinyl groups
compared to SPR212 and Durazane1800, which
leads to a lower degree of photoinduced cross-
linking. Furthermore, the SMP10 polymer
absorbs light in the blue region of the visible
spectrum, as evident by its intrinsic orange
color, resulting in a slower photopolymerization
process. This difference has implications for the
ceramization process as discussed below.
Nonetheless, all preceramic polymers studied
here can be applied in the stereolithographic
fabrication of complex thermoset parts as
demonstrated in Fig. 4.
Fig. 4. Representative examples of thermoset parts fabricated from three different polymer classes: (A)
Kelvin cell structure printed with polysiloxane SPR212, (B) cog wheel printed with polycarbosilane
SMP10 and (C) turbine structure printed with polycarbosilazane Durazane1800.
3.2 Polymer-to-ceramic transformation and
the microstructure of additively
manufactured ceramic components
To transform the cross-linked thermosets into
ceramics, they were subjected to further
processing steps. First, they were rinsed with
organic solvents (isopropanol for polysiloxane
and anhydrous cyclohexane for polycarbosilane
and polycarbosilazane), to remove the
remaining non-polymerized resin, and post
cured under UVA light between 350 400 nm
to enhance their mechanical stability. In the next
step they were transferred to a tube furnace and
pyrolyzed in N2 at 1100 °C. During pyrolysis
the cross-linked preceramic polymers
decompose into amorphous Si-containing
ceramics; this transformation is accompanied
by the release of hydrogen, methane and other
carbon- and hydrogen-containing species. This
process entails mass loss, isotropic volume
shrinkage and densification of the material.
Upon the pyrolytic conversion of cross-linked
polymers to the corresponding ceramics, a
linear shrinkage of roughly 30 % is observed
(see Fig. 2C). Since the shrinkage is isotropic,
the thermoset is transformed into a ceramic
without any noticeable distortion. The
preservation of geometry demonstrates that
thiol-ene click reactions are an effective and
easily accessible pathway towards the AM
fabrication of ceramic parts with excellent
resolution and smooth surfaces from preceramic
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
7
polymers. To avoid cracking or explosion
during pyrolysis, the geometry of printed
preceramic polymers is designed with a feature
size smaller than 2 mm in at least one direction.
This reduces the diffusion path of evolving gas
and facilitates the shrinking process.
Using the procedures developed in the present
work, further ceramic compositions beyond the
SiOC, SiC and SiCN materials studied here can
be obtained by means of suitable preceramic
polymers. General guidelines for the selection
of appropriate ceramic precursors for use with
thiol-ene click reactions can be stated as
follows: (i) an inorganic-organic hybrid
polymer or monomer, where the inorganic core
comprises species (such as Al, B, Si, Ti or Zr)
that form stable oxides, carbides, nitrides or
oxycarbides upon pyrolysis; (ii) the presence of
unsaturated carbon double or triple bonds, such
as those in vinyl groups; (iii) the propensity to
form thiyl radicals through photoinitiation.
Furthermore, the methodology developed here
can be easily adapted to produce composite
ceramics containing fillers to impart catalytic,
photocatalytic or magnetic functionalities.
In this work we were able to fabricate complete
black glassy ceramics composed of silicon
oxycarbide and silicon carbonitride. Fig. 5 A-D
displays several examples of ceramic parts -
ranging from cellular lattices to bulk structures
- fabricated by our approach. Substantial
internal flaws were not observed in
corresponding radiography images (Fig. 5 E -
H), which validates our approach towards AM
of high performance ceramics. The SiOC and
SiCN(O) parts were successfully fabricated
with excellent resolution and very smooth, pore
free surfaces. Surprisingly, SiC parts fabricated
from the polycarbosilane SMP10 cracked and
exploded during pyrolysis when the heating
temperature reached 600 °C. At temperatures
between 550 and 800 °C, SMP 10 transforms
into an inorganic SiC based material
accompanied by release of hydrogen (H2) and
methane (CH4), as well as volumetric shrinkage,
which indicates the polymer to ceramic
conversion [3032]. As discussed before,
relative to SPR212 and Durazane 1800, the
SMP10 polymer was cross-linked at a lower
degree and more importantly its polymer
structure contains greater amounts of silyl (Si-
H) and carbosilane (Si-CH2-Si) groups, leading
to greater hydrogen gas evolution during
pyrolysis. Therefore, the significant hydrogen
and methane gas egression during the
ceramization process of SMP10, along with
shrinkage stresses, resulted in the formation and
rapid propagation of cracks manifesting in the
explosion of samples
.
Fig. 5. Examples of ceramic components fabricated by pyrolyzing the cross-linked thermosets showing
(A,B) Kelvin cell structure and turbine impeller of SiOC and (C,D) cog wheel and turbine structure of
SiCN(O). Corresponding radiography images (E-H) demonstrate the fabricated ceramic components
are free from macro sized voids and defects.
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
Fig. 6. STA characterization of the polymer-to-
ceramic transformation of the cross-linked
thermosets fabricated from SPR212, SMP10
and Durazane 1800: (A) selected MS data for
SPR212 indicating the release of SO2 (m/z= 64),
CH3SH (m/z= 48), C4H8 (m/z= 56) and H2S
(m/z= 34) due to the decomposition of thioether
linkages during the major mass loss region and
(B) TG curves indicating the overall weight loss.
The polymer to ceramic transformation was
examined by thermal gravimetry coupled with
mass-spectrometry, with results shown in Fig.
6. The overall weight loss of dithiol modified
SPR212 resins is significantly larger in
comparison to the weight loss of the unmodified
preceramic polymer, while the differences in
SMP10 and Durazane 1800 resins are less
significant [33,34]. The additional weight loss
is attributed to the evaporation and breakdown
of thioether linkages during pyrolysis. As seen
in Fig. 6 A, a substantial release of sulfur -
containing low molecular weight species is
observed between 400 and 600 °C. In contrast,
in SMP10 and Durazane 1800 resins, oxidation
in air leads to the formation of Si-O and Si-OH
groups, which transform into thermally stable
Si-O-Si bonds. The resultant ceramic yield is
thus not strongly affected by the presence of
dithiol compound.
Fig. 7. Characterization of additively manufactured polymer-derived ceramic materials: (A) ATR-FTIR
and (B) X-ray photoelectron spectra of SiOC (red), SiC(O) (green) and SiCN(O) (blue line) synthesized
at 1100°C from SPR212, SMP10 and Durazane 1800, respectively. Analysis of SPR212-derived SiOC
ceramic pyrolyzed at 1100 °C: (C) TEM image, with selected area electron diffraction (SAED) pattern
shown in inset indicating a homogeneous amorphous structure (D) solid state 29Si NMR spectra,
revealing mixed bonding between Si, O and C.
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
9
Table 1. Elemental composition of ceramics pyrolyzed at 1100 °C in flowing nitrogen
Preceramic
polymer
Fabricated
ceramic
Elemental composition, at. %
Si
O
N
S
Polysiloxane
SPR212
SiC1.39O1.53S0.01
25.48
38.88
0
0.17
Polycarbosilane
SMP10
SiC1.22(O0.78)S0.04
33.00
25.77
0
1.19
Polycarbosilazane
Durazane 1800
SiC1.02N0.59(O1.12)
S0.04
28.52
28.92
15.27
1.07
As verified by FTIR and X-ray photoelectron
spectroscopy and TEM characterization (Fig.
7), parts pyrolyzed at 1100 °C in flowing
nitrogen are amorphous silicon-containing
ceramics with a general formula of SiCxOyNz
with small amount of residual sulfur. Their
chemical composition depends on the
preceramic polymer applied for the
manufacturing (Table 1).
From XPS data, it is clear that ceramics with
targeted compositions of SiC and SiCN possess
significant oxygen content and these materials
further demonstrate broad absorption bands
around 1100 cm-1 in the FTIR spectra attributed
to Si-O-Si stretching vibrations. Since the
additive manufacturing process was performed
in ambient air, the silane (-Si-H) and silazane
(Si-N-Si) bonds contained in the preceramic
polymers reacted with atmospheric oxygen and
moisture rapidly leading to an increased oxygen
content. Consequently, “reacted” SMP10 and
Durazane 1800 polymers are thermally
converted into SiC(O) and SiCN(O) ceramics
with deviated properties and microstructures
compared to those in the intrinsic oxygen free
SiC and SiCN ceramics. To minimize oxidation
in stoichiometric SiC and SiCN ceramics, the
entire fabrication process would have to be
conducted under an inert atmosphere.
We thus focus on the representative SiOC
ceramics fabricated from polysiloxane SPR212.
As described above and as verified by TEM, the
SiOC formed by pyrolysis at 1100 °C in flowing
nitrogen is amorphous. Further insight into the
SPR212-derived SiOC microstructure is
provided by solid-state 29Si-NMR
characterization (Fig. 7 D). The characteristic
peaks at chemical shifts of -114 and -14 ppm
correspond to SiO4 and SiC4 units and the peaks
at -82 and -39 ppm can be assigned to mixed-
bond SiO3C and SiO2C2 units [35]. The
fabricated SiOC parts exhibited a measured
Archimedes density of 2.10 g/cm3, which
approaches 97 % of the calculated theoretical
density of. 2.17 g/cm3 assuming densities of 2.2,
3.1 and 1.45 g/cm3 for amorphous SiO2,
amorphous SiC and pyrolytic carbon,
respectively [36]. This confirms that the
fabricated SiOC parts are nearly fully dense
with negligible pore volume. This is essential
for ceramic materials, since cracks and pores act
as stress concentrators under load and initiate
failures. As can be seen in the radiography
images (Fig. 5), SiOC components are free from
observable internal cracks and closed pores. 3-
D printed cellular cubes of SiOC are completely
dense and free of macro sized defects as
confirmed by the homogeneous absorption of
X-rays in the tomographic images of the
diamond lattice (Fig. 8 A) and Kelvin cell (Fig.
8 E) structures. The corresponding SEM
images (Fig. 8 B-D and F-H) demonstrate again
the delicate microstructures and fine features
which are achievable only in additive
manufacturing. Along the printing direction,
step-like surfaces are clearly visible which is
related to the layer-by-layer printing process. In
this work the layer thickness during printing
was set to be 100 µm to yield rapid fabrication.
At higher magnification, glassy smooth
surfaces are observed, exhibiting no porosity.
This is consistent with the radiography images,
indicating the high quality fabrication of
ceramic components.
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
10
Fig. 8. Morphology and microstructure of additively manufactured SPR212-dervied SiOC: (A and E)
tomographic images of the diamond lattice (A) and Kelvin cell (E) structures. The delicate design is
perfectly replicated without any detectable flaws. (B-D, F-H) Corresponding SEM images at higher
magnification showing the step-like surfaces as well as the crack free struts.
3.3 Mechanical properties
The hardness and elastic modulus of the SiOC
ceramics were evaluated in nanoindentation
experiments (Fig. 9). Fig. 9A shows the 3D
printed honeycomb used for measurements
along with an AFM image of an indent. A
typical load-hold-unload curve is shown in Fig.
9B. 200 measurements were conducted to
evaluate the reduced elastic modulus and
hardness, as given in Fig. 9 C and D. Over a
range of penetration depths, the mean values of
the reduced Young’s modulus and hardness are
found to be 106±6 and 12±1 GPa, respectively.
An earlier study showed that SiOC ceramics
with lower O/C ratios behave similarly to
amorphous-SiC materials, while the mechanical
properties of SiOC with higher O/C ratio are
similar to those in amorphous SiO2[37]. In the
present work, the hardness and elastic modulus
of AM fabricated SiOC materials lie between
the predicted limit of amorphous SiO2 [37] and
amorphous SiC [38] and are further in good
agreement with reported values for silicon
oxycarbide glasses with variable C/O ratio
[37,39].
Fig. 9. Results of nanoindentation tests performed on 2D honeycombs of additively manufactured
SPR212-derived SiOC. (A) A polished SiOC honeycomb imbedded in epoxy resin. Inset shows the AFM
image of one indent. (B) Load-Hold-Unload curve of one indent. Elastic modulus is determined via the
elastic unloading process. (C, D) Reduced modulus and hardness as a function of the penetration depth.
200 measurement results are shown here.
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
It is worth emphasizing that our fabrication
method resulted in SiOC ceramics with
comparable or even slightly better mechanical
properties than those of SiOC glasses fabricated
either via hot-pressing or via radio frequency
(RF)-magnetron sputtering. Both of these
methods yield high surface quality SiOC
components (bar and thin films) with
minimized defects and pore volume.
Since our manufacturing methods allow for the
fabrication of 2D and 3D periodic cellular
structures, in the following step we evaluate the
out-of-plane compression strength of 2D SiOC
honeycombs as a representative example of a
material with high strength to density
characteristics, and apply Gibson and Ashby’s
model [29] to analyze these results. The
honeycomb geometry is particularly strong
when loaded along the main axis of the
hexagonal prism (out-of-plane, X3) compared to
the in-plane directions (X1 and X2), as thin cell
walls are much stiffer under extension or
compression than in bending. Therefore,
honeycombs are mostly used as cores in
sandwich panels to provide high strength to
weight ratios. Likewise, honeycombs made
from ceramic materials are frequently used as
catalyst supports, filtration membranes and
components in heat exchangers due to their
superior chemical and mechanical stability in
harsh environments.
Fig. S3 shows the corresponding stress-strain
curves. 2D SiOC honeycomb structures
fabricated in this work were found to exhibit a
foam density of 0.61 g/cm3 and a compressive
strength of 216 MPa as seen in Table S2 and
Fig. S3. The SiOC honeycomb demonstrates
linear-elastic behavior as well as catastrophic
failure, which is accompanied by a sudden drop
in the measured stress. The plateau presented in
the stress-strain curves before failure is resulted
from slight misalignments of load bearing faces
in the fabricated lattices, which in an ideal
structure should be perfectly parallel, as well as
an edge effect that, cell walls from internal to
external can’t fail simultaneously. Therefore, in
our experiments the compressive strength is
taken as the value at which this plateau occurs
rather than the maximum crushing stress.
Gibson and Ashby’s model relates the
mechanical properties of a cellular solid to its
relative density ρrelative (the density ρ* of the
foam divided by the density ρs of the solid
which the foam is made of) and its solid
strength/bulk modulus s ,Es). The strength (σ)
as well as the modulus (E) of the cellular
material can then be expressed as follows:
 

 

where C is a dimensional constant and the
exponent m depends on the cell morphology. In
the case of a 2D honeycomb carrying loads on
the faces normal to X3, equation (1) and (2) can
further be simplified to (3) [40]:


In this case the Young’s modulus of the 2D
honeycomb simply reflects the solid modulus
scaled by the load-bearing area. Using equation
(3) with calculated relative density of 0.29 and
the Young’s modulus of 106±6 GPa from the
nanoindentation experiments, we obtain 31±2
GPa for the theoretical honeycomb stiffness.
Because of the measurement error, which
occurred when evaluating the true strain of
SiOC honeycomb under compression, we are
unable to compare the measured Young’s
modulus with this calculated theoretical value
here. More detailed explanation can be found in
SI with Table S2 and Fig S4.
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
12
Fig. 10. Strength-density Ashby chart for designing light strong structures. In this chart our additively
manufactured SiOC honeycombs are compared to other ceramic (i.e. SiOC, Al2O3, SiC) honeycombs as
well as to other natural and technical materials. Figure and material properties are generated by
Nicoguaro using JavaScript and D3.js (https://commons.wikimedia.org/wiki/File:Material-
comparison--strength-vs-density_plain.svg) except the data points of different ceramic honeycombs,
which were inserted manually from the corresponding references.
In Fig. 10, SiOC honeycombs printed in this
work are compared, in terms of density and
compressive strength, to natural and engineered
cellular solids as well as other honeycombs
made from SiOC [18], SiOC nanocomposites
[41], Al2O3 [4244], and SiC [45]. The silicon
oxycarbide honeycombs fabricated in this work
display an outstanding strength to density ratio
that is significantly higher than other materials
of similar density. Its absolute strength is also
comparable with fiber-reinforced polymers and
metal alloys. The compressive strength of 216
MPa is comparable to that exhibited by α-Al2O3
honeycombs of approximately twice of the
density and is around one order of magnitude
higher than commercial SiC honeycombs. The
silicon oxycarbide honeycombs fabricated in
this work further demonstrate a higher
compressive strength compared to SiOC
honeycombs with the densities ranged from 0.5
to 0.8 g/cm3 reported in previous works (see e.g.
[18] [41]). The exceptional performance of the
additively manufactured SiOC honeycombs is
the result of their dense and defect-free
structure, which was observed by radiography
and SEM characterization. As shown in
equation (3), the strength of a cellular solid,
whose cells walls display a stretch-dominated
behavior, scales linearly with its relative density
[46]. This behavior is represented as a line with
a slope of 1 in the double-logarithmic strength-
density plot. Accordingly, the theoretical
compressive stress of cellular polymer derived
SiOC ceramics can be defined by these lines
passing through bulk silicon carbide, silicon
nitride and silica, which thus delineate the
maximum obtainable performance for PDCs
[47]. As can be seen, SiOC honeycombs in this
work with a density of 0.6 g/cm3 reach the
theoretical compressive strength of silica based
materials while approaching the predicted
theoretical value of silicon carbide and silicon
nitride. Shifting the ceramics compositions
towards higher C to O ratios by choosing
polymers of higher carbon content and avoiding
oxidation can further boost its specific strength
towards the theoretical limit of SiC. By utilizing
polysilazane in an oxygen free environment,
silicon carbonitride ceramics can be obtained,
offering further avenues towards high
mechanical performance of printed cellular
structures.
4. Conclusions
We have closely examined the utility of thiol-
ene click chemistry towards additive
manufacturing using preceramic polymers. We
demonstrated a versatile stereolithographic
Wang, X., Schmidt, F., Hanaor, D., Kamm, P.H., Li, S. and Gurlo, A., 2019. Additive manufacturing of ceramics from
preceramic polymers: A versatile stereolithographic approach assisted by thiol-ene click chemistry. Additive
Manufacturing, 27, pp.80-90.
13
manufacturing route whereby high performance
polymer-derived ceramics are obtained with
well controlled geometry. The developed
approach can be applied for the entire range of
known PDC systems. For precursors to be used
in thiol-ene click assisted stereolithography
they should exhibit an inorganic backbone,
unsaturated carbon bonds and the
photoinitiation of thiyl radicals. High levels of
hydrogenated groups in precursors are
detrimental as the evolution of a significant
volume of hydrogen gas during pyrolysis can
cause failure under certain firing conditions.
The photocurable preceramic polymers of this
method are easily prepared and can be applied
in any SLA/DLP printer, as well as techniques
like microstereolithography and two-photon
polymerization to produce microstructures
beyond the resolution limit of DLP techniques.
The additively manufactured polysiloxane-
derived SiOC components fabricated here are
nearly fully dense, achieving 97 % of theoretical
density. 2D SiOC honeycombs with a cellular
density of 0.61 g/cm3 exhibit a compressive
strength of 216 MPa, surpassing the
performance of comparable porous ceramics
and stretching the boundaries of material
property space in terms of strength to weight
ratio under compression. The presently
developed methods represent a flexible and
rapid route towards high-performance
additively manufactured polymer-derived
ceramics that can find broad application in
various engineering fields, particularly for load
bearing materials applied in harsh environments
and high temperatures. With appropriate AM
techniques, ceramic components can be
produced with the developed methods across a
range of length-scales with high precision,
representing a valuable new capability for
industries such as aerospace, automotive,
energy conversion and chemical engineering.
Acknowledgements:
The research was supported by the Deutsche
Forschungsgemeinschaft under the grant “GU
992/17-1”. We thank Jörg Bauer for helping
with UV-DSC, Anke Maerten for conducting
the nanoindentation measurements and Jan
Epping for conducting the solid-state NMR
experiments, respectively. We also thank
Gaofeng Shao and Wuqi Guo for technical
support in obtaining optical images.
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... Some of the PCPs contain silyl and vinyl groups and can be used as negative photoresists in SL, exploiting a UV activated hydrosilylation reaction [255]. The limitation of UV activated hydrosilylation is that it requires addition of a large amount of photoinitiator (PI) and long exposures to high energy UV light, which can be overcome by modifying the PCPs. ...
... The advantage of SL of PCPs is the better resolution (20 μm or less) compared to most other AM techniques, where the smallest features are typically in the range of 50-200 μm [34,255]. The high resolution is attributed to the fine control of incident photon density at the focal spot. ...
... The ones described above are the traditional free radical photopolymerization and flexible routes to high resolution, dense, crackfree samples possessing good mechanical properties. Different from radical photopolymerization Wang et al. [255] introduced a thiolene click chemistry-based versatile approach to DLP print high performance ceramics upon pyrolysis, as shown in Fig. 10 (k-m). The preceramic formulation involves the mixing of PCPs such as polysiloxane or allylhydrydopolycarbosilane with phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide as photoinitiator (PI), Sudan Orange G as photo-absorber and hydroquinone as free radical scavenger. ...
Article
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Artificially structured ceramic components with extraordinary properties are of immense demand in various industries. Additive manufacturing (AM) or 3D printing technologies are promising for the fabrication of ceramic components. However, printing of ceramics directly from their raw powders is a daunting task and requires multistep processing. Preceramic polymers (PCPs) offer an attractive pathway towards AM of preceramic structures featuring heterogeneous architectures and their direct conversion to polymer-derived ceramics (PDCs) – the corresponding ceramics. This review reports a detailed summary of recent research progress on the additive manufacturing of PCPs and the corresponding PDCs manufactured for different applications. The approaches towards the synthesis of various PCPs are discussed along with easily tunable chemical formulations that can be employed in AM processes. Further, the review discusses conventional PDC technology as well as AM technologies that can be employed with PCPs and the associated superiorities and drawbacks in comparison to powder-based ceramic 3D printing. Complex-shaped PDC structures and their properties and potential applications are also discussed. Overall, this review illustrates the AM capabilities of PCPs for cost-effective fabrication of advanced ceramics with high resolution, superior performance, lower environmental impact and new functionalities.
... In this context, the works of Eckel et al. [7,8] should be mentioned in particular, who created complex SiOC structures by using a thiol-ene system consisting of mercapto polysiloxane and vinyl polysiloxane. A similar approach was employed by Wang et al. [9] to produce SiOC ceramic articles using a vinyl polysiloxane-dithiol system. Other works reported on the use of non-active polysiloxanes mixed with photoactive polysiloxanes, thus facilitating a desired processability through a straightforward blending method [10]. ...
Article
In order to overcome challenges typically encountered during additive manufacturing of ceramics via the polymer precursor route, a novel polymer-derived SiOC/SiC composite system suitable for advanced geometric designs achievable by lithography-based ceramic manufacturing was established. The photoreactive resin system filled with 20 wt.% SiC exhibits suitable viscosity characteristics, adequate stability against sedimentation, and a fast photocuring behaviour. After printing and pyrolytic conversion, SiC particulates were well-dispersed within the polymer-derived SiOC matrix. A direct comparison with the unfilled polysiloxane-based resin system showed that the addition of particulate SiC increases handleability, reduces shrinkage, and significantly increases critical wall thicknesses up to 5 mm. The biaxial Ball-on-Three-Balls testing methodology yielded a characteristic strength of 325 MPa for SiOC/SiC composites. The results highlight the high potential of particle-filled preceramic polymer systems toward the fabrication of high-performance SiC-based materials by lithography-based additive manufacturing.
... The PVS-Ti(b)-300 sample shows no loss up to 300 °C and a continuous mass loss of 20.65% up to 750 °C. The cessation of mass loss after 750 °C indicated that the polymer to ceramic transformation was complete at this temperature, with the mass loss over the 300-750 °C temperature range attributed to the evaporation of H 2 , carbon in the form of CH 4 , and other hydrocarbons (C n H m ) during the decomposition of the polymer precursor [44]. ...
Article
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The current investigation describes the synthesis of SiTiCNO ceramics derived from the mixture of a preceramic polymer (polyvinylsilazane) and tetrabutyl orthotitanate precursors by crosslinking at 300 °C and pyrolysis in the temperature range of 900–1400 °C in flowing nitrogen atmosphere. Crosslinked precursor was studied by thermogravimetry to estimate ceramization temperature as well as ceramic yield. Further, the evolution of phase and nanostructure with temperature in the composite SiTiCNO ceramics was analyzed by the help of different characterization techniques, such as XRD, Raman, and electron microscopy. The Ti-doped SiCN ceramic system appeared as single-phase SiTiCNO amorphous up to 1100 °C. The phase separation of SiTiCNO ceramics started at 1200 °C and exhibited TiO2 nanocrystals distributed in the amorphous SiCN matrix. Ti-doping was found to accelerate the separation of the free carbon phase from the SiCN matrix, and the said carbon had better graphitic order in the Ti-doped SiCN samples as compared to the undoped SiCN of equivalent thermal history. At 1400 °C, high temperature stable phases such as TiC and TiC0.3N0.7 were formed along with predominant rutile-TiO2 phase within the Si–Ti–O–C–N composite. A uniform distribution of these nanocrystals in the SiCN matrix at 1400 °C was observed by high resolution transmission electron microscopy. The current work exhibits the formation of a unique multiphase composite with the co-existence of nanocrystalline phases uniformly distributed within a polymer derived ceramic matrix.
... 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. ...
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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.
Article
Previous research often failed to prepared polymer-derived SiOC ceramic components with dense and thick monoliths/skeletons based on additive manufacturing processes, due to the porosities and cracks generated during the pyrolysis process. Herein, we report a simple solution which combines the introduction of phenolic resin (PR) as an additive and the use of controlled strategy of pyrolysis, for the vat photopolymerization additively manufactured SiOC ceramics. As a result, crack-free dense monolith and lattice skeleton structural polymer-derived SiOC ceramics are successfully prepared despite the low ceramic yield of 19.3 wt% and large linear shrinkage of 50% generated after pyrolysis. The prepared dense monoliths possess a thickness of over 5 mm and the lattices show attainable maximum dense skeletons thickness of ~2 mm. The maximum dense thickness of the samples after pyrolysis is over 7 times that of other samples prepared based on additive manufacturing reported in the literature. As an indicator of dimensional retention, the specific dense thickness to ceramic yield ratio (T/Y) is deduced, resulting maximum values of 101.9 µm/(wt%) for lattice skeletons and 254.7 µm/(wt%) for monoliths. The former is even larger than that for dense monolith SiOC structures prepared by other conventional gel casting and isostatic pressing techniques with much higher ceramic yields of > 80 wt%, and is about 6.1 times the largest T/Y ratio (=16.7 µm/(wt%)) achievable in the AM-based works reported in the literature. This shows predominance of the current work over other types of polymer-derived ceramic structures prepared by additive manufacturing and even conventional methods. The lattice structures’ apparent density and specific strength reached 0.55 g/cm³ and 6.6 × 10⁴ N∙m/kg, respectively. The mechanical performance surpasses other structural polymer-derived ceramic structures with similar apparent density reported in the literature. Through the advanced characterization and analysis of material phase and morphology evolution during the pyrolysis process, the mechanism of defect-free densification is revealed. That is, the introduction of PR enables a highly smooth gas-releasing process due to the generation of ordered gas-releasing channels in the sample bodies during the pyrolysis process. As a result, the generation of cracks and defects was successfully prevented. Moreover, due to the high carbon residue of the PR, the samples containing 15 wt% PR in the preceramic state still possessed amorphous SiOC ceramics after pyrolysis at 1400 ℃, while 0 wt% PR samples showed SiC crystallization. Compared with similar materials reported, the ceramics prepared in this study exhibit enhanced structural and mechanical performance. The strategy investigated in the study can be a viable route for the additive manufacturing of high-performance crack-free and thick monolith and lattice skeleton SiOC ceramic structures based on vat photopolymerization.
Article
Green bodies with ceramic particles and polymer binders can be shaped with additive manufacturing (3D-printing) techniques without the need for a mold. The dimensions and properties of the final ceramic part produced from printed green bodies depend on the porosity created from polymer burn-off and the densification of the ceramic particles during the post-printing steps. Predictions of the part dimensions and the internal stress states of the final ceramic part can assist the designers in properly sizing and shaping the green body. In this paper, the SOVS model was implemented in a three-dimensional finite element software as a user-defined creep model. The parameters required for the SOVS model, namely viscosity and surface energy, were calibrated by minimizing the error between predicted and experimental relative densities. The parameters were obtained using an error measure definition and minimizing the error with the well-known Differential Evolution global optimization. The dimensional changes of selected additively manufactured green bodies through the polymer burn-off and sintering process are analyzed. Density changes are tracked through the time-temperature cycles typically used to create ceramic parts from their green bodies.
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Conventional crosslinked photopolymers possess permanent network structures, which cannot be remolded/recycled and generally discarded as waste after use. Research has begun to incorporate dynamic bonds into crosslinked photopolymers to make them reprocessable/recyclable, however, previous studies typically require tedious pre‐syntheses to obtain photoreactive building blocks that contain desired dynamic linkages/functionalities. In this study, we report a simple, one‐step, and scalable synthesis of chemically recyclable crosslinked thiol‐ene photopolymers that contain dynamic disulfide bonds from commercially available building blocks. Specifically, liquid polysulfides containing both reactive thiol end groups and internal disulfide bonds, together with multifunctional alkenes, are selected as the building blocks to simultaneously incorporate disulfide bonds during network formation via thiol‐ene photopolymerization. The incorporated dynamic disulfide bonds allow these thiol‐ene networks to be chemically recycled into photoreactive thiol oligomers through base‐catalyzed thiol‐disulfide exchange reactions. The resulting thiol oligomers can be effectively reused, together with the original multifunctional alkenes, for the next‐generation syntheses of crosslinked thiol‐ene photopolymers with chemical compositions and material properties that are nearly identical to those of the originally crosslinked sample. This chemical recycling process can be repeated many (infinite in theory) times to produce recycled thiol‐ene photopolymer networks with full property retention. Overall, the unique chemistry demonstrated in this study could potentially provide a route towards a circular economy of crosslinked photopolymers.
Thesis
Les céramiques techniques 3D à géométrie complexe font actuellement l’objet d'une demande croissante pour diverses applications dans des conditions sévères. Elles présentent des propriétés importantes telles qu’une excellente stabilité chimique, une grande résistance à l’oxydation et la corrosion ainsi que des propriétés mécaniques importantes les rendant des composants fiables dans les domaines électronique, aérospatial et biomédical. En particulier, les céramiques à base de Si sont caractérisées par d’excellentes propriétés thermochimiques et une résistance au fluage. La fabrication additive (AM) est la technologie de choix pour concevoir des objets de forme quasi nette avec un contrôle parfait de la géométrie et de la porosité à différentes échelles. Cependant, les poudres céramiques traditionnelles sont difficiles à imprimer à cause de leur dureté et friabilité. Par conséquent, les polymères précéramiques conviennent à l'impression 3D car ils sont facilement modifiables à l'état moléculaire. Dans ce travail, nous avons combiné l'impression 3D avec la chimie des précurseurs (PDCs) pour fabriquer des céramiques à base de Si à géométrie complexe. Deux technologies d’AM ont été investiguées : Fused Deposition Modeling (FDM) basée sur la fusion d'un filament thermoplastique et UV-LCD basée sur la photopolymérisation d’une résine sous rayonnements UVs. La caractérisation des polymères précéramiques et des céramiques qui en dérivent a été systématiquement étudiée. L’influence de la composition chimique et structure du polymère sur son imprimabilité a été présentée. Tout d'abord, nous avons élaboré des céramiques type carbonitrure de silicium (SiCN) et carbure de silicium (SiC) par impression indirecte en couplant la technologie FDM avec une approche de réplique. Nous avons imprimé par FDM des motifs acide polylactique (PLA) en nid d'abeille qui ont été revêtus par le polyvinylsilazane (PVZ) ou l’allylhydrydopolycarbosilane (AHPCS) précurseurs respectifs de SiCN et SiC. Ces polymères ont été chimiquement réticulés avec le dicumyl peroxide. Une pyrolyse ultérieure sous atmosphère contrôlée permet la réticulation thermique des polymères à 130°C, la décomposition du PLA à 320°C et la céramisation à 1000°C. Un rétrécissement volumique a été observé suite à la décomposition du PLA et de la céramisation. SiBC (dérivé de AHPCS modifié par le bore) a également été élaboré pour étudier l'effet du bore sur l’imprimabilité de Si-C. Deuxièmement, nous avons synthétisé des polymères précéramiques photosensibles précurseurs de SiOC, SiC ou SiCN selon deux approches : i) mélange du polymère avec une résine photosensible commerciale et ii) synthèse d'un polymère précéramique sensible aux UVs par fonctionnalisation avec des unités photosensibles. Nous avons procédé ensuite à l’impression 3D par UV-LCD de ces polymères en optimisant plusieurs paramètres : temps d'exposition, épaisseur et nombre de couches. Dans la première approche, deux types de précurseurs de SiOC ont été utilisés : méthyl silsesquioxane (Silres MK) et polyméthylhydrosiloxane (PMHS) qui ont été mélangés avec une résine commerciale photosensible. Un meilleur comportement mécanique de la céramique en forme de pastille est observé avec Silres MK dû à son meilleur rendement céramique. Dans la deuxième approche, Silres MK photosensible est préparée soit par un simple mélange avec des agents photoréticulables type triméthylolpropane triacrylate (TMPTA) et/ou 1,6 hexanediol diacrylate (HDDA), soit par greffage de fonctions triacrylates du 3-(triméthoxysilyl) propyle méthacrylate (TMSPM) sur les fonctions Si-OH du Silres MK. Les céramiques 3D SiOC obtenues après pyrolyse montrent une conservation de la forme de la pastille presque sans fissures. Notre approche a également été appliquée à la fabrication de céramiques non-oxydes SiC et SiCN, prouvant la versatilité de synthétiser des polymères précéramiques photosensibles oxyde et non-oxyde.
Article
Digital light processing of ceramic precursor was used to prepare SiC rich ceramic parts in this study. In order to achieve appropriate light curing rate, the ceramic precursor allylhydropolycarbosilane (LHBPCS) was mixed with acrylate monomers tripropylene glycol diacrylate and trimethylolpropane triacrylate. The content of acrylate monomers was optimized to increase the ceramic yield and reduce the shrinkage during pyrolysis. According to the results of thermogravimetric analysis and photolithography experiment, 15 wt% acrylate monomers was appropriate. 330 mJ/cm² UV irradiation dose was selected for every layer with a thickness of 25 microns, and green bodies with different shapes were successfully printed. During pyrolysis, these printed parts changed from transparent yellow to black accompanying uniform shrinkage. At 1000 °C, the shrinkage was 24.0–26.0%, and crack-free SiC rich ceramic parts with density of 2.11 g/cm³ and chemical formula of SiC1.31O0.26 were obtained.
Article
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Four-dimensional (4D) printing involves conventional 3D printing followed by a shape-morphing step. It enables more complex shapes to be created than is possible with conventional 3D printing. However, 3D-printed ceramic precursors are usually difficult to be deformed, hindering the development of 4D printing for ceramics. To overcome this limitation, we developed elastomeric poly(dimethylsiloxane) matrix nanocomposites (NCs) that can be printed, deformed, and then transformed into silicon oxycarbide matrix NCs, making the growth of complex ceramic origami and 4D-printed ceramic structures possible. In addition, the printed ceramic precursors are soft and can be stretched beyond three times their initial length. Hierarchical elastomer-derived ceramics (EDCs) could be achieved with programmable architectures spanning three orders of magnitude, from 200 μm to 10 cm. A compressive strength of 547 MPa is achieved on the microlattice at 1.6 g cm⁻³. This work starts a new chapter of printing high-resolution complex and mechanically robust ceramics, and this origami and 4D printing of ceramics is cost-efficient in terms of time due to geometrical flexibility of precursors. With the versatile shape-morphing capability of elastomers, this work on origami and 4D printing of EDCs could lead to structural applications of autonomous morphing structures, aerospace propulsion components, space exploration, electronic devices, and high-temperature microelectromechanical systems.
Article
Full-text available
Most existing methods for additive manufacturing (AM) of metals are inherently limited to ~20–50 μm resolution, which makes them untenable for generating complex 3D-printed metallic structures with smaller features. We developed a lithography-based process to create complex 3D nano-architected metals with ~100 nm resolution. We first synthesize hybrid organic–inorganic materials that contain Ni clusters to produce a metal-rich photoresist, then use two-photon lithography to sculpt 3D polymer scaffolds, and pyrolyze them to volatilize the organics, which produces a >90 wt% Ni-containing architecture. We demonstrate nanolattices with octet geometries, 2 μm unit cells and 300–400-nm diameter beams made of 20-nm grained nanocrystalline, nanoporous Ni. Nanomechanical experiments reveal their specific strength to be 2.1–7.2 MPa g^(−1) cm^3, which is comparable to lattice architectures fabricated using existing metal AM processes. This work demonstrates an efficient pathway to 3D-print micro-architected and nano-architected metals with sub-micron resolution.
Article
Polymer‐derived ceramics (PDCs) represent a class of ceramics that are preparatively accessible from inorganic polymers (also called preceramic polymers). The synthesis and processing of PDCs from polymeric precursors have been shown within the past four decades to be an excellent way to design ceramics, which provides unique tools to control and tune structural features and consequently properties in PDCs. Thus, the molecular architecture and the chemistry of the preceramic polymers strongly correlate to the polymer‐to‐ceramic transformation characteristics as well as the structural features of the resulting PDC materials. By carefully designing the preceramic polymer, fine‐tuning of the chemical and phase composition of the resulting ceramics is possible, which results in unique microstructures and behavior thereof. The present article aims to give a brief introduction to the field of PDCs. Typically, an introduction of preparative tools to access PDCs from tailored inorganic polymers will be done. In addition, a critical consideration of the main structural features of PDCs will be given, with emphasis on the strong correlation between the nano/microstructure of the PDCs and the molecular architecture of their polymeric precursors. Also, various technologies being used to process PDCs in the form of porous materials, coatings, fibers, complex‐shaped monolithic parts, and so on will be introduced and discussed. Finally, some selected structural and functional properties (e.g., high‐temperature behavior, electrical properties, optical properties, and bioactivity) of PDCs will be highlighted and some emerging application fields in which PDCs may be highly suitable material candidates will be introduced. The present article does not intend to provide an exhaustive review of the activities from the past three to four decades in the field of PDCs, but rather to give a short overview of the particularities and the potential of this technology. The article refers in the section “Related Papers” to some excellent reviews and book chapters from the past 20 years, which address and summarize various aspects related to PDCs.
Article
Multiple metals doped polymer-derived SiOC ceramics with octet truss structure were prepared by employing a photosensitive methyl-silsesquioxane as preceramic polymer through sol-gel method and Digital Light Processing 3D printing. The physical and chemical properties of the preceramic polymers and printed octet truss structure SiOC ceramics were investigated. Results show that the organosilicon preceramic polymers have outstanding photocuring properties and could transform into amorphous SiOC ceramics at 800–1200 °C. It is illustrated that the excellent mechanical properties of SiOC ceramics with octet truss structure (after 3D printing and pyrolysis) are attributed to the metal elements pinning in the amorphous matrix on the atomic level. Doping other metal elements such as Fe, Ni, Co, Pt, etc, is thought to bring promising properties for the lattice structure SiOC ceramics and potentially further expand its applications in the future.
Article
3D microarchitected metamaterials exhibit unique, desirable properties influenced by their small length scales and architected layout, unachievable by their solid counterparts and random cellular configurations. However, few of them can be used in high-temperature applications, which could benefit significantly from their ultra-lightweight, ultrastiff properties. Existing high-temperature ceramic materials are often heavy and difficult to process into complex, microscale features. Inspired by this limitation, we fabricated polymer-derived ceramic metamaterials with controlled solid strut size varying from 10-m scale to a few millimeters with relative densities ranging from as low as 1 to 22%. We found that these high-temperature architected ceramics of identical 3D topologies exhibit size-dependent strength influenced by both strut diameter and strut length. Weibull theory is utilized to map this dependency with varying single strut volumes. These observations demonstrate the structural benefits of increasing feature resolution in additive manufacturing of ceramic materials. Through capitalizing upon the reduction of unit strut volumes within the architecture, high-temperature ceramics could achieve high specific strength with only fraction of the weight of their solid counterparts.
Article
The high hardness, melting temperature and environmental resistance of most ceramic materials makes them well-suited for propulsion, tribilogical and protective applications. However, these same attributes pose difficulties for manufacturing and machining of ceramics and ultimately limit the achievable design space of these materials. Recently, a new class of preceramic photopolymers has been developed that enables additive manufacturing of ceramics using commercially available stereolithography systems. By consolidating preceramic monomers via layer-wise exposure to ultraviolet light and subsequently pyrolyzing under an inert atmosphere to form a ceramic, this method allows for complex geometry parts that cannot be produced with traditional sintering, pressing or vapor infiltration processes. In order to retain geometric fidelity and generate flaw-free microstructures, volumetric and gravimetric changes during the polymer-to-ceramic conversion must be quantified. To this end, we present x-ray micro-computed tomography (micro-CT) measurements of the dimensional stability and uniformity of additively manufactured silicon-based ceramics as a function of geometry and processing conditions.
Article
Additive manufacturing using photocurable polymers is one method to answer the increased demand of ceramic structures with complicated morphology by fabricating ceramic parts with high resolution and good surface quality. We introduce here a new method to fabricate SiOC ceramic structures by utilizing a simple physical blend between two different preceramic polysiloxanes, one providing photosensitive acrylate groups while the other one a high ceramic yield. Different blend ratios have been realized and respectively optimized concerning the printing additives and setting times to fabricate exact replications of highly complex polysiloxane structures by Digital Light Processing. After pyrolysis, a uniform, homogenous shrinkage was observed yielding dense, pore- as well as crack-free SiOC ceramics. By adjusting the ratio between the different polysiloxanes, parameters such as the ceramic yield, shrinkage, chemical composition and resolution after pyrolysis could be tailored in a wide range of values.
Article
3D printed ceramic articles are receiving increased interest recently. Stereolithography (STL) is the method of choice where surface quality, high resolution and high aspect ratio architectures are concerned. Recently, we have developed a UV curable system consisting of allylhydridopolycarbosilane (AHPCS) and multifunctional acrylates. In our present work we investigate the photo-crosslinking mechanism and use selected formulations for the 3D printing of SiC rich ceramic articles using a desktop STL device. High resolution and complex shape articles are demonstrated. The versatile curing method can be used for the STL of practically most other vinyl/allyl modified preceramic polymers. The nano-porosity as well as SiOxCy composition can be tailored in a wide range for specific applications by the ratio of acrylate to AHPCS and by the type of acrylate and AHPCS used.
Article
Hierarchical cellular structures are ubiquitous in nature because of their low-density, high-specific properties, and multifunctionality. Inspired by these systems, we created lightweight ceramic architectures composed of closed-cell porous struts patterned in the form of hexagonal and triangular honeycombs by direct foam writing. The foam ink contains bubbles stabilized by attractive colloidal particles suspended in an aqueous solution. The printed and sintered ceramic foam honeycombs possess low relative density (∼6%). By tailoring their microstructure and geometry, we created honeycombs with different modes of deformation, exceptional specific stiffness, and stiffness values that span over an order of magnitude. This capability represents an important step toward the scalable fabrication of hierarchical porous materials for applications, including lightweight structures, thermal insulation, tissue scaffolds, catalyst supports, and electrodes.
Article
A process for the laser pyrolysis of a ceramic composite coating system composed of an organosilazane (Durazane™ 1800) with ZrO2 and glass particles as fillers was developed. Firstly, the mild steel substrates were dip-coated with a perhydropolysilazane (PHPS) bond coat, onto which the composite coating slurry was applied by spraying. After drying, pyrolysis using a Nd:YAG laser led to the formation of a dense semi-crystalline ceramic coating system with a thickness up to 20 μm in a short time. The resulting coatings possess a significantly different morphology compared to the same coating system pyrolyzed in a furnace, due to different forming mechanisms. Laser irradiation led to the unexpected formation of oxygen vacancies in the crystalline lattice of ZrO2, which increased the absorption of the laser radiation, enabling the transformation into a ceramic coating. Simultaneously, reactions between the glasses and the monoclinic ZrO2 fillers were activated, resulting in the formation of dendritic tetragonal-stabilized ZrO2 crystals. The thermal stability of the coating components was analyzed by thermogravimetric analysis (TGA) and the coatings were investigated by attenuated total reflectance infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD).