Synthesis, microstructure and properties of SiCN ceramics prepared from tailored polymers
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Synthesis, microstructure and properties of SiCNceramics prepared from tailored polymers
G. Ziegler, H.-J. Kleebe*, G. Motz, H. Muller, S. Tral, W. WeibelzahlInstitute for Materials Research (IMA), University of Bayreuth, D-95440 Bayreuth, Germany
Dedicated to Prof. Dr. S. Somiya on the occasion of his 70th birthday
Different liquid polymers in the system SiCN with tailored structures were prepared by ammonolysis from functionalized chlorosilanes.
Crosslinking to an unmeltable polymer with initiators at low temperatures and subsequent ceramization were studied applying 29Si solid-
state nuclear magnetic resonance (NMR) spectroscopy in combination with Fourier transformed infrared (FTIR) spectroscopy and
Microstructure development, in particular, the devitrification of the corresponding bulk polymer-derived SiCN glasses was investigated
by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Preparation of monolithic samples was performed
by mixing liquid polysilazane with SiCN-powder particles, derived from the same precursors by heat treatment at 3008C, and subsequentannealing at temperatures exceeding 10008C to initiate crystallization. Depending on the functionalities of the SiCN-precursor and theprocessing conditions, different microstructures were obtained.
The material prepared from the HVNG precursor revealed a homogeneous amorphous micro structure with only a small fraction of
crystallized spherical inclusions after exposure at 15408C for 6 h in nitrogen atmosphere. In contrast, investigating ceramic monolithsderived from another SiCN precursor, a different crystallization sequence was observed. The material derived from the HPS precursor
showed crystallization of large a-Si3N4 grains within the bulk. As will be discussed in detail, devitrification of these polymer-derivedglasses is promoted by local rearrangements and possible phase separations within the amorphous bulk. Moreover, local decomposition and
residual porosity can affect the crystallization behavior, which strongly differs depending on the polymer employed.
In addition to the crystallization phenomena observed, different oxidation response was monitored for the two SiCN ceramics discussed
here. Moreover, fracture strength and hardness data were recorded, which, however, did not substantially differ between the polymer-
derived ceramics investigated. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Synthesis; Microstructure; Properties; SiCN ceramics; Tailored polymers
Organometallic compounds (precursors) have attracted
considerable interest in recent years, owing to their promis-
ing potential for the formation of high-purity non-oxide
ceramics, amorphous fibers and surface coatings .
Since the pioneering work of Verbeek and Winter  in
addition to Yajima  in the mid 1970s, a wide variety of
precursors have been developed for the preparation of
different non-oxide ceramics . The major advantages
of such polymer-based materials is their intrinsic homoge-
neity on an atomic level, low processing temperatures, since
the precursors can be transformed into amorphous covalent
ceramics at temperatures between 80010008C, and the
applicability of established polymer processing techniques.
In general, processing of ceramic materials via organome-
tallic compounds involves the synthesis of the precursor
from monomer units followed by crosslinking into an
unmeltable, preceramic network and finally the pyrolysis
at elevated temperatures. The latter heat treatment initiates
the organicinorganic transition and results in an amor-
phous, non-oxide covalent glass. Post-annealing of such
amorphous non-oxide ceramics at temperatures exceeding
10008C yields a partially or completely crystallized ceramic.A number of studies reported in literature address the
pyrolysis behavior of the polymeric precursors at tempera-
tures around 10008C, whereas less work has been focused onthe crystallization behavior and the thermal stability of these
precursor-derived amorphous structures. TEM investiga-
tions by Monthioux and Delverdier [9,10] as well as Kleebe
et al. [11,12] focused on the crystallization phenomena
Materials Chemistry and Physics 61 (1999) 5563
E-mail address: email@example.com (H.-J. Kleebe)
0254-0584/99/$ see front matter # 1999 Elsevier Science S.A. All rights reserved.PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 1 4 - 5
observed in SiCN-based glasses, while the work reported by
Bill and Aldinger  described the microstructure devel-
opment of monolithic SiBCN and SiPCN ceramics. Up to
now, little understanding has been developed concerning the
problem of thermal degradation of the amorphous SiCN-
ceramic materials. Various aspects may be important, start-
ing from the polymer architecture, the chemical composi-
tion, the residual porosity within the amorphous structure
(open/closed system), local kinetics and thermodynamics as
well as the ambient atmosphere. It is also thought that the
aforementioned parameters affect the resulting material
properties such as fracture strength, fracture toughness
and oxidation resistance .
Here we report on the study of two different SiCN
ceramics, derived from tailored precursors, starting from
polymer synthesis followed by detailed microstructure
characterization of bulk ceramics in addition to the acquisi-
tion of their corresponding properties such as oxidation
behavior and mechanical response. This general approach
synthesis-characterization-microstructure reflects the con-
cept followed at the Institute for Materials Research in
2. Experimental procedures
2.1. Polymer synthesis and characterization
All preparation steps were carried out in an inert gas
atmosphere due to air and moisture sensitivity of both educts
and products. Synthesis followed standard procedures ,
i.e., dissolving of different di- and trifunctionalized chlor-
osilanes in toluene and passing ammonia through the solu-
tion. When the reaction has ended, it is necessary to purge
with argon to eliminate excess ammonia. Subsequent filtra-
tion of the ammonium chloride from the reaction mixture
and distillation of the solvent leads to colorless or pale
yellow silazane precursors. Rheological measurements
were performed on a cone-plate-viscosimeter (Rheolab
MG 10, Physica Metechnik, Germany). Molecular weights
were determined cryoscopically in cyclohexane or p-xylene.
The precursors were crosslinked by using dicumylper-
oxide as a radicalic initiator and subsequent thermal treat-
ment at 3008C for 5 h in N2-atmosphere. For all theexperiments, powder samples were used, which were
obtained from the as-received unmeltable solids by ball
milling with zirconia milling media. The powders were
sieved and the fraction
perforation and subsequent light carbon coating to minimize
electrostatic charging under the electron beam.
Oxidation stability was examined employing thermogra-
vimetry (STA 409, Netzsch) at temperatures ranging from
11008C to 14008C for 72 h in flowing air (150 ccm/min).Before testing, the polished specimens were tempered at
14508C in N2-atmosphere to exclude any possible mass lossdue to the escape of hydrogen. The hardness values of the
specimens were determined using a Vickers indenter (98 N),
with an average of 10 indentations for each sample. Fracture
strength was measured by four-point bending tests of five
specimens each using a 40/20 mm support span and a
crosshead speed of 0.5 mm/min. Youngs modulus was
determined from the load/displacement curves by following
where F represents the load applied, l0 is the distance
between the inner load points, l1 gives the distance between
the inner and outer supports, y0 the deflection of the center of
the specimen relative to the position of inner supports, and J
is the moment of inertia, J bh312
, where b is the width of the
specimen and h represents its height in the direction of the
3.1. Synthesis and characterization of polymers
The polymer synthesis is mainly based on two concepts.
First, various reactive functional groups at the silicon atoms
were introduced to control further branching reactions via
hydrosilylation (>SiHH2C=CHSi>) and/or polymeriza-tion of the vinyl substituents. These reactions can be
induced by heating to 3008C or preferably at a lowertemperature of about 1308C by adding a radicalic initiator.Second, modification of the molecular weight and viscosity
is achieved by either using di- and trifunctional chlorosi-
lanes or by bonding sterically different substituents to the
silicon atoms. As a result of both concepts, the polysilazanes
HVNG and HPS were synthesized and can be described by
the structural units given in Fig. 1. The polysilazane HVNG
consist of mixed di- and trifunctional units, i.e., every
second silicon atom is bridged by two and the other half
by three nitrogen atoms to other silicon atoms. In contrast,
the HPS precursor only consist of twofold bridged silazane
With the additional possibility of crosslinking, the mole-
cular weight was raised from 440 g/mol (HPS) to 620 g/mol
(HVNG). The viscosity of the silazanes also strongly
depends on their intrinsic structure. Therefore, a viscosity
increase from a highly liquid (HPS, 0.05 Pas) to a honey like
precursor (HVNG, 29 Pas) was recorded (compare Table 1).
In all silazane systems, first a mass change was observed
during pyrolysis between 1508C and 3508C. In general, atlower degrees of branching (HPS) the mass loss is about 15
30 wt%, owing to the release of gaseous oligomers besides
ammonia, as identified by coupled TG-FTIR measurements
(Fig. 2). When, however, increasing the degree of branching
(HVNG), the mass loss is reduced to about 5 wt%. A second
stage of mass loss was observed between 3508C and 7508C,where methane is released which leads to the degradation of
the organic substituents (methylene groups, ethylene
bridges). Above 8008C, no significant mass changes wereobserved. After heating to 10008C, the ceramic yield for theHPS in comparison to the HVNG precursor is markedly
lower (73 versus 82 wt%), due to the escape of more volatile
Fig. 1. Structural units of the precursors HVNG and HPS.
Properties of the precursors and the resulting polymer-derived ceramics.
Educts Precursor Molecular
208C (Pas)Ceramic yield
at 10008CElementary composition at 10008C(mass %)
ViSiCl3 HVNG 620 92 29 82 Si50.3 O0.85Me(H)SiCl2 C20.6 H
oligomers and a higher amount of methyl groups within the
HPS. In addition, the elementary composition shows a
higher carbon concentration in the resulting amorphous
ceramic (Table 1). Changes in the structure of the solid
intermediates during thermal treatment were investigated by
29Si solid state NMR spectroscopy. Fig. 3(a) (HVNG) and
Fig. 3(b) (HPS) show the 29Si-spectra of both polysilazanes
as a function of pyrolysis temperature up to 15008C. Solidstate NMR characterization of silazanes is typically diffi-
cult, since rather broad signals appear in the NMR spectra of
the crosslinked polymers as well as the amorphous materi-
als. In the present study we used the peak assignment
reported in literature . The 29Si-spectrum of the
crosslinked HVNG polymer cured at 3008C shows threesharp peaks (Fig. 3(a)). In contrast to the corresponding
solution spectrum of this precursor, a new signal at
2.5 ppm appears, whereas the peak at 33.5 ppm andthe high intensity of the signal at 20 ppm refer tounreacted vinyl and SiH groups, respectively. Resonances
having a 29Si-chemical shift in the range of 2.5 ppmcorrespond to silicon atoms on (N)2Si(C
sp3)2 sites and
denote crosslinking via hydrosilylation . An exact
assignment of the signals appearing after crosslinking can
only be made via comparison with other SiCN precursors
like HPS. This spectrum shows only two peaks (Fig. 3(b)).
The chemical shift with high intensity at 3.5 ppm isrelated to the hydrosilylation reaction, while the higher
intensity compared to the signal at 22 ppm (unreactedSiH groups) and the fact that a peak for Si-vinyl groups in
the range of about 15 ppm cannot be observed, indicatethat all vinyl groups reacted via hydrosilylation and/or
polymerization. In the 29Si spectrum of HVNG at 5008C,
Fig. 2. FTIR spectra (coupled with TG) of gaseous species which escaped
during pyrolysis (HVNG).
Fig. 3. 29Si NMR spectra of (a) HVNG-and (b) HPS-derived powder samples heat treated at various temperatures.
58 G. Ziegler et al. / Materials Chemistry and Physics 61 (1999) 5563
the peak corresponding to unreacted vinyl groups has dis-
appeared, indicating that crosslinking is completed. More-
over, these signals are broadened (higher degree of
crosslinking) and shifted to higher fields, owing to the
degradation of organic groups and the enrichment of SiN
surroundings. The latter is in agreement with TG-FTIR
measurements (Fig. 2), where the evolution of methane
was observed. The same effects were detected in the
5008C 29Si NMR spectrum of the HPS precursor.Based on TG analysis, a second mass loss of about 3 wt%
was detected in the temperature range between 8008C and14008C in conjunction with a density increase from 2.3 to2.6 g/cm3. The accompanying 29Si-NMR spectrum indi-
cates rearrangements in the amorphous state, whereas at
10008C only one broad peak was monitored, which corre-sponds to a homogeneous amorphous SiCN matrix. Anneal-
ing at 15008C, however, leads to a heterogeneous SiCNmaterial. The broad peak finally separates into the three
signals for SiC4, SiN3C and SiN4 [18,2024]. Longer
annealing times (48 h) at 15008C cause the formation ofthe thermodynamically stable crystalline phases SiC
(16 ppm) and Si3N4 (48 ppm). At 16008C, no Si3N4but only a SiC signal is detected by 29Si NMR measure-
ments, which narrows at 17008C indicating crystal growthof SiC. Upon crystallization, the density increases from
about 2.6 to 3.25 g/cm3 (corresponding to SiC) with a
substantial mass loss of 26 wt% due to the decomposition
of amorphous SiCN and Si3N4 by nitrogen evaporation.
The materials investigated exhibited a residual open
porosity of about l5 vol% after high-temperature annealing
at 15408C for 6 h in N2-atmosphere and can, therefore, beconsidered as open systems that allow for the escape of
gaseous species formed during pyrolysis. This open porosity
in turn affects the high-temperature stability of these poly-
mer-derived glasses, as will be discussed in the following.
On the other hand, since the materials revealed a high degree
of coalescence between the powder particles and the binder
phase upon pyrolysis, as can be seen at the fracture surfaces
of the HVNG- (Fig. 4(a)) and the HPS-derived (Fig. 4(b))
bulk glasses after annealing at 15408C, the materials locallycontain regions without residual porosity which is consid-
ered here as the corresponding closed systems. Apart
from the porosity present, the matrix of the polymer-derived
materials revealed a homogeneous glass-like fracture sur-
face, as shown in the SEM micrographs of Fig. 4. Distinc-
tion between former powder particles and binder phase is
not possible. This coalescence between powder particles,
pre-heat treated below 6008C, and the binder phase uponpyrolysis indicates the possibility of structural rearrange-
ments within these polymer-derived compounds, due to the
presence of various functionalities.
The pore sizes of the HPS-derived material are in the
range of 13 mm in diameter, whereas the pore diameters ofthe HVNG-derived material are much larger, up to 10 mm indiameter. A second major difference, besides the pore size,
was the occurrence of crystallized spherical inclusions
commonly observed in the HVNG-derived glass, as
depicted in the TEM micrograph of Fig. 5(a). These sphe-
rical inclusions, only observed in the HVNG material,
contained the thermodynamically stable crystalline phases
Si3N4, SiC and graphite (compare the HRTEM image of
Fig. 6(a)). It should be emphasized that in order to ratio-
nalize the observed phase assemblage, a nitrogen over-
pressure within these globules of about four atmospheres
is required. Except of these spherical inclusions, the bulk
material of the HVNG material remained completely amor-
In contrast, using the HPS-polymer for preparation of the
monolithic SiCN-glass sample, no spherical inclusions
could be found. The material appeared completely homo-
geneous and amorphous, when employing SEM as the
characterization tool (Fig. 4(b)). Additional TEM investi-
gations, however, revealed large a-Si3N4 crystallites within
Fig. 4. SEM micrographs of fracture surfaces of (a) HVNG- and (b) HPS-derived ceramic monoliths annealed at 15408C, 6 h, N2 atmosphere, containing3008C polymer powder particles. A distinction between former powder particles and void filling binder phase is not feasible. Note the different pore size ofthe two materials which, however, contain the same overall porosity.
G. Ziegler et al. / Materials Chemistry and Physics 61 (1999) 5563 59
the glass after exposure to 15408C (see Fig. 5(b)). More-over, employing HRTEM imaging, it could be revealed that
the bulk of the HPS-derived material was in fact not
completely amorphous as suggested by SEM, but showed
in some areas the formation of SiC nuclei, shown in the
HRTEM image of Fig. 6(b).
The characterization of the two different precursor mate-
rials clearly revealed different crystallization phenomena. In
the one case, spherical precipitates, filled with SiC, Si3N4and graphite, were observed while the HPS-derived sample
revealed large a-Si3N4 crystallites besides a small numberof globules which only contained SiC. However, the actual
reason for this marked difference in high-temperature
response, i.e., the respective crystallization behavior, is
not yet unequivocally known and a generalized discussion
proved to be rather complex, as will be shown in Section 4.
Density and open porosity of the infiltrated and pyrolyzed
HVNG bodies changed from 1.68 to 2.05 g/cm3 and from
25% to 8%, respectively, after four infiltration cycles. The
effort to decrease the residual porosity below 8%, using a
higher number of infiltration cycles was not successful,
because all the accessible pore channels were already closed
after four infiltration cycles. This leads to small porosity
gradient within the sample with a rather dense outer rim and
a porous inner core structure.
The Vickers hardness strongly depends on the annealing
temperature of the material (Fig. 7), whereas different pre-
cursors show only a small variance in hardness. The Vickers
hardness increases between 10008C and 12008C from 7.9 to12.8 GPa and from there on remains constant up to 15008C.Annealing the specimen at 15508C, however, leads to apronounced decrease of the hardness to about 5.5 GPa, since
crystallization occurs which creates additional porosity due
to the strong density change.
Fracture strength shown in Fig. 8 depends on the overall
microstructure and on the silazane used . Due to process
optimization by die pressing at 1408C, large structuraldefects could mainly be eliminated which resulted in higher
strength values. Therefore, the fracture strength improved
from an average value of about 104 MPa (pyrolysis tem-
perature of 14008C) to about 130 MPa. Monolithic samples
Fig. 5. TEM micrographs of (a) one spherical inclusion observed in the HVNG- derived SiCN-material, (b) a-Si3N4-crystallites within the matrix of theHPS-derived material after annealing at 15408C, 6 h, N2 atmosphere.
Fig. 6. HRTEM micrographs of (a) one spherical inclusion in HVNG revealing the crystalline phases a-Si3N4, SiC and C, and (b) the matrix of the HPS glassafter annealing at 15408C, 6 h, N2-atmosphere. Note that the formation of small SiC nuclei was observed in some regions within the glass structure.
60 G. Ziegler et al. / Materials Chemistry and Physics 61 (1999) 5563
prepared from HPS showed the highest strength values with
a maximum of 235 MPa. The Youngs modulus of the
HVNG material also increased from 109 to 118 GPa by
employing the warm die-pressing technique.
The oxidation resistance of the monolithic SiCN ceramic
was tested by isothermal oxidation in air. In general, the
SiCN materials are stable due to the formation of a SiO2-
protection layer. Commonly, porous non-oxide ceramics
oxidize by internal and external oxidation (e.g., RBSN),
whereas internal oxidation dominates at lower temperatures
and larger channel radii. The complete mass gain of the
pyrolyzed HVNG ceramic was not larger than 1% at 14008Cafter 72 h oxidation. Increasing the oxidation temperature
leads to an increase in mass gain, as given in Fig. 9. The
oxidation behavior of bulk material derived from the HPS-
precursor differs strongly from the HVNG material. In this
case, the total mass gain after isothermal treatment for 72 h
at 14008C was 0.07% and, hence, about two orders ofmagnitude lower as compared to the HVNG material.
The general idea of using polymer powders (crosslinked
at 3008C) derived from polymers with different basic struc-tural units was based on the assumption that the micro-
structure development and the respective thermal stability of
these polymer-derived materials is directly influenced by
the polymer architecture. It was suggested that the devi-
trification of polymer-derived SiCN glasses can be
described by a stepwise change of the microstructure,
initiated by a rearrangement of the polymer network upon
heat treatment, which yields phase separation within the
amorphous state. The phase separation and, consequently,
the thermal stability of the glass structure can therefore be
controlled by the architecture of the starting polymer.
However, the resulting pore structure (open/closed system)
can also strongly affect the stability of the amorphous
ceramic. Both precursors studied here yielded a homoge-
neous microstructure after pyrolysis, where coalescence
between powder particles and polymer binder had occurred.
However, employing the HPS-precursor for preparation of
the SiCN glass, a microstructure with a high amount of
small pores was observed, whereas the HVNG-precursor
resulted in a microstructure with only a small amount of
much larger pores. The crystallization phenomena, in par-
ticular, the occurrence of all the stable phases of the SiCN
system within one spherical inclusion in the HVNG-derived
material, supports the assumption that this reflects the
crystallization behavior of a closed system. In contrast,
the HPS system can be considered as an open system, since
the regions without any porosity between the pore network
are very small. It becomes evident, that no crystallization
areas, containing all the thermodynamically stable phases
SiC, Si3N4 and graphite, can be found within the HPS-
derived material since the open system allows for the escape
of nitrogen during rearrangement and decomposition of the
amorphous SiCN network prior to crystallization. This
results in SiC enriched areas and the formation of SiC
Fig. 7. Vickers hardness (HV10) of different precursors (HVNG, HPS)
annealed at various pyrolysis temperatures (6 h, N2).
Fig. 8. Four-point fracture strength of monolithic HVNG samples
prepared at different forming and pyrolysis temperatures. Note that one
fracture strength data point of the HPS material is also shown (triangle),
giving the highest strength value.
Fig. 9. Mass change of two SiCN ceramics (HVNG, HPS) due to the
formation of a protective silica layer by isothermal oxidation in air at
G. Ziegler et al. / Materials Chemistry and Physics 61 (1999) 5563 61
nuclei within the matrix. In addition, it is assumed that the
large a-Si3N4 crystallites, observed in the HPS material,were formed in proximity to closed pores, where the gen-
eration of a sufficiently high nitrogen partial pressure, which
allows for the formation and stabilization of Si3N4, was
enabled. It is important to note, that, apart from the polymer
architecture, the residual porosity plays a dominant role
with respect to crystallization of the bulk polymer-derived
The resistance of SiCN ceramics against oxidation is
affected by both, temperature and precursor type. Since
processing of the investigated silazanes resulted in different
microstructures of the pyrolyzed monoliths, giving different
pore structures, it is assumed that the detected higher
oxidation rates of the HVNG specimens are in fact a result
of the much wider pore channels. The isothermal oxidation
experiments imply that above 12008C and after an initialoxidation stage, the pore channels are mostly closed by a
protective silica scale which prevents further oxidation.
Oxidation of the HPS-precursor material, however, shows
nearly no detectable mass change due to the smaller pore
size of the material. The above-mentioned results again
emphasize the effect of the residual porosity and the diffi-
culty to distinguish between the influence of polymer
architecture itself and the given pore structure. The latter
is also thought to affect fracture strength obtained as well as
KIc and hardness.
One of the key topics when employing newly developed
polymer-derived glasses, is their stability at high tempera-
tures, in particular, the stability of the amorphous state.
Crystallization of bulk SiCN glasses is controlled by a
stepwise change in microstructure, which is thought to
yield phase separation within the amorphous phase,
structural rearrangement as well as chemical degradation.
The crystallization strongly depends on the polymer
architecture and on the residual porosity of the system.
One question that remains to be solved in this context is,
if the process of phase separation is required for crystal-
lization to occur and, therefore, would be responsible for
the observed degradation in thermal stability. Or, on the
other hand, if the residual porosity, i.e., the pore size and
the ratio between open and closed porosity, in fact over-
rules the influence of the polymer architecture and
The authors would like to thank the Volkswagenstiftung
Hannover and the Deutsche Forschungsgemeinschaft
(DFG) Bonn for financial support throughout the work.
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