Download - Synthesis, microstructure and properties of SiCN ceramics prepared from tailored polymers

Synthesis, microstructure and properties of SiCNceramics prepared from tailored polymers

G. Ziegler, H.-J. Kleebe*, G. Motz, H. MuÈller, S. Traûl, 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

Abstract

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

thermoanalytical techniques.

Microstructure development, in particular, the devitri®cation 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 subsequent

annealing at temperatures exceeding 10008C to initiate crystallization. Depending on the functionalities of the SiCN-precursor and the

processing 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 monoliths

derived 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, devitri®cation of these polymer-derived

glasses 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

1. Introduction

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 ®bers and surface coatings [1±3].

Since the pioneering work of Verbeek and Winter [4] in

addition to Yajima [5] in the mid 1970s, a wide variety of

precursors have been developed for the preparation of

different non-oxide ceramics [6±8]. 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 800±10008C, 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 ®nally the pyrolysis

at elevated temperatures. The latter heat treatment initiates

the organic±inorganic 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 on

the 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) 55±63

*Corresponding author.

E-mail address: [email protected] (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 [13] 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 [14±16].

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 re¯ects the con-

cept followed at the Institute for Materials Research in

Bayreuth.

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 [7],

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 ®ltra-

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 Meûtechnik, 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 the

experiments, 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 <125 mm was used for thermal

treatments up to 16008C (�T�100 K). 29Si NMR spectra

of the solid intermediates between 3008C and 15008C were

obtained on a Bruker AVANCE DSX 400 spectrometer

applying the magic-angle spinning technique (MAS) with

spinning rates of 5±7 kHz. From 3008C to 5008C, a signal-

to-noise enhancement using the cross polarization (CP)

method was possible, due to the presence of a suf®ciently

high proton content in these samples. At temperatures

exceeding 5008C, the CP method became inef®cient since

the proton concentration was too low. Therefore, single

pulse excitation was employed for recording 29Si spectra

in the latter case. The chemical shift data of the 29Si NMR

spectra are listed with respect to tetramethylsilane as an

external standard. The gaseous species evolving during

pyrolysis were monitored with a coupled TG-FTIR set-

up. Powder samples were annealed using a Netzsch STA

409 thermobalance and the volatile species were detected by

FTIR absorption spectra using a heated transmission line.

2.2. Processing of the monoliths

The preparation of compact monolithic SiCN ceramics

was performed under inert conditions. The oxygen content

of the ®nal pyrolysis product was therefore below 1 wt%.

During processing, the liquid precursor was ®rst converted

into an unmeltable crosslinked polymer by thermal treat-

ment at 3008C for 3 h. The thermal curing was catalytically

supported by adding 1 wt% of dicumylperoxide, which

initiates polymerization of the functional vinylgroups and

reduces the evolution of low molecular oligomeric sila-

zanes. After ball milling and sieving, the polymer powder

fraction <32 mm was blended with 30 vol% of the starting

liquid precursor. The added liquid precursor acts both as

®ller of the pores in the green compact and as a reactive

component which coalesces with the powder particles.

Uniaxial die pressing at 10 MPa, partially at elevated tem-

peratures (120±1408C), yielded a powder compact

(70�6�4) mm with green density of about 75% th.d. Heat

treatment at 650±800 8C for 6 h resulted in an intermediate

product, which was in®ltrated with liquid precursor at a low

partial pressure of <1 mbar. This procedure was repeated up

to four times, with the ®nal heat treatment at 10008C for 1 h.

Subsequent annealing at temperatures ranging from 11008Cto 15508C in N2 atmosphere for 6 h was applied to study the

effect of heat treatment on resulting mechanical properties.

The samples employed for TEM inspection were not

rein®ltrated but only annealed at 10008C for 1 h in nitrogen

environment and subsequently heat treated at elevated

temperatures, in order to eliminate possible artefacts intro-

duced by multiple in®ltration.

2.3. Microstructure characterization

The overall microstructure characterization of the poly-

mer-derived materials after pyrolysis and subsequent

annealing was performed by SEM, using a Jeol 6400

microscope, equipped with an ultra-thin window Ge energy

dispersive X-ray (EDX) detector, as well as by TEM

utilizing a Philips CM2FEG (®eld emission gun) micro-

scope also equipped with an EDX system and an electron

energy-loss spectrometer with parallel detection (PEELS,

Gatan 666). TEM-foil preparation followed standard tech-

niques, which involves diamond cutting, ultra-sound dril-

ling, mechanical grinding, dimpling, Ar-ion thinning to

56 G. Ziegler et al. / Materials Chemistry and Physics 61 (1999) 55±63

perforation and subsequent light carbon coating to minimize

electrostatic charging under the electron beam.

2.4. Properties

Oxidation stability was examined employing thermogra-

vimetry (STA 409, Netzsch) at temperatures ranging from

11008C to 14008C for 72 h in ¯owing air (150 ccm/min).

Before testing, the polished specimens were tempered at

14508C in N2-atmosphere to exclude any possible mass loss

due 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 ®ve

specimens each using a 40/20 mm support span and a

crosshead speed of 0.5 mm/min. Young's modulus was

determined from the load/displacement curves by following

Eq. (1)[25]:

E � Fl20l1

16Jy0

; (1)

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 de¯ection of the center of

the specimen relative to the position of inner supports, and J

is the moment of inertia, J � bh3

12, where b is the width of the

specimen and h represents its height in the direction of the

de¯ection.

3. Results

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 (>Si±H�H2C=CH±Si>) and/or polymeriza-

tion of the vinyl substituents. These reactions can be

induced by heating to 3008C or preferably at a lower

temperature of about 1308C by adding a radicalic initiator.

Second, modi®cation 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

units.

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, ®rst a mass change was observed

during pyrolysis between 1508C and 3508C. In general, at

lower degrees of branching (HPS) the mass loss is about 15±

30 wt%, owing to the release of gaseous oligomers besides

ammonia, as identi®ed 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 signi®cant mass changes were

observed. After heating to 10008C, the ceramic yield for the

HPS 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.

Table 1

Properties of the precursors and the resulting polymer-derived ceramics.

Educts Precursor Molecular

Weight (g/mol)

Synthesis

Yield (%)

Viscosity at

208C (Pas)

Ceramic yield

at 10008CElementary composition at 10008C(mass %)

ViSiCl3 HVNG 620 92 29 82 Si�50.3 O�0.85

Me(H)SiCl2 C�20.6 H<0.1

N�26.9

ViMeSiCl2 HPS 440 85 0.05 73 Si�51.4 O�0.39

Me(H)SiCl2 C�26.5 H<0.1

N�21.9

G. Ziegler et al. / Materials Chemistry and Physics 61 (1999) 55±63 57

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. Solid

state NMR characterization of silazanes is typically dif®-

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 [17±22]. The 29Si-spectrum of the

crosslinked HVNG polymer cured at 3008C shows three

sharp peaks (Fig. 3(a)). In contrast to the corresponding

solution spectrum of this precursor, a new signal at

ÿ2.5 ppm appears, whereas the peak at ÿ33.5 ppm and

the high intensity of the signal at ÿ20 ppm refer to

unreacted vinyl and Si±H groups, respectively. Resonances

having a 29Si-chemical shift in the range of ÿ2.5 ppm

correspond to silicon atoms on (N)2Si(Csp3)2 sites and

denote crosslinking via hydrosilylation [22]. 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 ÿ3.5 ppm is

related to the hydrosilylation reaction, while the higher

intensity compared to the signal at ÿ22 ppm (unreacted

Si±H groups) and the fact that a peak for Si-vinyl groups in

the range of about ÿ15 ppm cannot be observed, indicate

that 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.


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 ®elds, owing to the

degradation of organic groups and the enrichment of Si±N

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 and

14008C in conjunction with a density increase from 2.3 to

2.6 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 SiCN

material. The broad peak ®nally separates into the three

signals for SiC4, SiN3C and SiN4 [18,20±24]. Longer

annealing times (48 h) at 15008C cause the formation of

the thermodynamically stable crystalline phases SiC

(ÿ16 ppm) and Si3N4 (ÿ48 ppm). At 16008C, no Si3N4

but only a SiC signal is detected by 29Si NMR measure-

ments, which narrows at 17008C indicating crystal growth

of 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.

3.2. Microstructure

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, be

considered 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 locally

contain 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 upon

pyrolysis 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 1±3 mm in diameter, whereas the pore diameters of

the HVNG-derived material are much larger, up to 10 mm in

diameter. 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-

phous.

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, containing

3008C polymer powder particles. A distinction between former powder particles and void filling binder phase is not feasible. Note the different pore size of

the two materials which, however, contain the same overall porosity.


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, ®lled with SiC, Si3N4

and graphite, were observed while the HPS-derived sample

revealed large a-Si3N4 crystallites besides a small number

of 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.

3.3. Properties

Density and open porosity of the in®ltrated and pyrolyzed

HVNG bodies changed from 1.68 to 2.05 g/cm3 and from

25% to 8%, respectively, after four in®ltration cycles. The

effort to decrease the residual porosity below 8%, using a

higher number of in®ltration cycles was not successful,

because all the accessible pore channels were already closed

after four in®ltration 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 to

12.8 GPa and from there on remains constant up to 15008C.

Annealing the specimen at 15508C, however, leads to a

pronounced 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 [25]. Due to process

optimization by die pressing at 1408C, large structural

defects 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 the

HPS-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 glass

after annealing at 15408C, 6 h, N2-atmosphere. Note that the formation of small SiC nuclei was observed in some regions within the glass structure.


prepared from HPS showed the highest strength values with

a maximum of 235 MPa. The Young's 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 of

magnitude lower as compared to the HVNG material.

4. Discussion

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 in¯uenced by

the polymer architecture. It was suggested that the devi-

tri®cation 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 re¯ects 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

different temperatures.


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 suf®ciently 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

ceramics.

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 initial

oxidation 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 dif®-

culty to distinguish between the in¯uence 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.

5. Conclusions

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 in¯uence of the polymer architecture and

chemistry.

Acknowledgements

The authors would like to thank the Volkswagenstiftung

Hannover and the Deutsche Forschungsgemeinschaft

(DFG) Bonn for ®nancial support throughout the work.

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