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

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<ul><li><p>Synthesis, microstructure and properties of SiCNceramics prepared from tailored polymers</p><p>G. Ziegler, H.-J. Kleebe*, G. Motz, H. Muller, S. Tral, W. WeibelzahlInstitute for Materials Research (IMA), University of Bayreuth, D-95440 Bayreuth, Germany</p><p>Dedicated to Prof. Dr. S. Somiya on the occasion of his 70th birthday</p><p>Abstract</p><p>Different liquid polymers in the system SiCN with tailored structures were prepared by ammonolysis from functionalized chlorosilanes.</p><p>Crosslinking to an unmeltable polymer with initiators at low temperatures and subsequent ceramization were studied applying 29Si solid-</p><p>state nuclear magnetic resonance (NMR) spectroscopy in combination with Fourier transformed infrared (FTIR) spectroscopy and</p><p>thermoanalytical techniques.</p><p>Microstructure development, in particular, the devitrification of the corresponding bulk polymer-derived SiCN glasses was investigated</p><p>by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Preparation of monolithic samples was performed</p><p>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.</p><p>The material prepared from the HVNG precursor revealed a homogeneous amorphous micro structure with only a small fraction of</p><p>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</p><p>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</p><p>residual porosity can affect the crystallization behavior, which strongly differs depending on the polymer employed.</p><p>In addition to the crystallization phenomena observed, different oxidation response was monitored for the two SiCN ceramics discussed</p><p>here. Moreover, fracture strength and hardness data were recorded, which, however, did not substantially differ between the polymer-</p><p>derived ceramics investigated. # 1999 Elsevier Science S.A. All rights reserved.</p><p>Keywords: Synthesis; Microstructure; Properties; SiCN ceramics; Tailored polymers</p><p>1. Introduction</p><p>Organometallic compounds (precursors) have attracted</p><p>considerable interest in recent years, owing to their promis-</p><p>ing potential for the formation of high-purity non-oxide</p><p>ceramics, amorphous fibers and surface coatings [13].</p><p>Since the pioneering work of Verbeek and Winter [4] in</p><p>addition to Yajima [5] in the mid 1970s, a wide variety of</p><p>precursors have been developed for the preparation of</p><p>different non-oxide ceramics [68]. The major advantages</p><p>of such polymer-based materials is their intrinsic homoge-</p><p>neity on an atomic level, low processing temperatures, since</p><p>the precursors can be transformed into amorphous covalent</p><p>ceramics at temperatures between 80010008C, and the</p><p>applicability of established polymer processing techniques.</p><p>In general, processing of ceramic materials via organome-</p><p>tallic compounds involves the synthesis of the precursor</p><p>from monomer units followed by crosslinking into an</p><p>unmeltable, preceramic network and finally the pyrolysis</p><p>at elevated temperatures. The latter heat treatment initiates</p><p>the organicinorganic transition and results in an amor-</p><p>phous, non-oxide covalent glass. Post-annealing of such</p><p>amorphous non-oxide ceramics at temperatures exceeding</p><p>10008C yields a partially or completely crystallized ceramic.A number of studies reported in literature address the</p><p>pyrolysis behavior of the polymeric precursors at tempera-</p><p>tures around 10008C, whereas less work has been focused onthe crystallization behavior and the thermal stability of these</p><p>precursor-derived amorphous structures. TEM investiga-</p><p>tions by Monthioux and Delverdier [9,10] as well as Kleebe</p><p>et al. [11,12] focused on the crystallization phenomena</p><p>Materials Chemistry and Physics 61 (1999) 5563</p><p>*Corresponding author.</p><p>E-mail address: achim.kleebe@uni-bayreuth.de (H.-J. Kleebe)</p><p>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</p></li><li><p>observed in SiCN-based glasses, while the work reported by</p><p>Bill and Aldinger [13] described the microstructure devel-</p><p>opment of monolithic SiBCN and SiPCN ceramics. Up to</p><p>now, little understanding has been developed concerning the</p><p>problem of thermal degradation of the amorphous SiCN-</p><p>ceramic materials. Various aspects may be important, start-</p><p>ing from the polymer architecture, the chemical composi-</p><p>tion, the residual porosity within the amorphous structure</p><p>(open/closed system), local kinetics and thermodynamics as</p><p>well as the ambient atmosphere. It is also thought that the</p><p>aforementioned parameters affect the resulting material</p><p>properties such as fracture strength, fracture toughness</p><p>and oxidation resistance [1416].</p><p>Here we report on the study of two different SiCN</p><p>ceramics, derived from tailored precursors, starting from</p><p>polymer synthesis followed by detailed microstructure</p><p>characterization of bulk ceramics in addition to the acquisi-</p><p>tion of their corresponding properties such as oxidation</p><p>behavior and mechanical response. This general approach</p><p>synthesis-characterization-microstructure reflects the con-</p><p>cept followed at the Institute for Materials Research in</p><p>Bayreuth.</p><p>2. Experimental procedures</p><p>2.1. Polymer synthesis and characterization</p><p>All preparation steps were carried out in an inert gas</p><p>atmosphere due to air and moisture sensitivity of both educts</p><p>and products. Synthesis followed standard procedures [7],</p><p>i.e., dissolving of different di- and trifunctionalized chlor-</p><p>osilanes in toluene and passing ammonia through the solu-</p><p>tion. When the reaction has ended, it is necessary to purge</p><p>with argon to eliminate excess ammonia. Subsequent filtra-</p><p>tion of the ammonium chloride from the reaction mixture</p><p>and distillation of the solvent leads to colorless or pale</p><p>yellow silazane precursors. Rheological measurements</p><p>were performed on a cone-plate-viscosimeter (Rheolab</p><p>MG 10, Physica Metechnik, Germany). Molecular weights</p><p>were determined cryoscopically in cyclohexane or p-xylene.</p><p>The precursors were crosslinked by using dicumylper-</p><p>oxide as a radicalic initiator and subsequent thermal treat-</p><p>ment at 3008C for 5 h in N2-atmosphere. For all theexperiments, powder samples were used, which were</p><p>obtained from the as-received unmeltable solids by ball</p><p>milling with zirconia milling media. The powders were</p><p>sieved and the fraction </p></li><li><p>perforation and subsequent light carbon coating to minimize</p><p>electrostatic charging under the electron beam.</p><p>2.4. Properties</p><p>Oxidation stability was examined employing thermogra-</p><p>vimetry (STA 409, Netzsch) at temperatures ranging from</p><p>11008C to 14008C for 72 h in flowing air (150 ccm/min).Before testing, the polished specimens were tempered at</p><p>14508C in N2-atmosphere to exclude any possible mass lossdue to the escape of hydrogen. The hardness values of the</p><p>specimens were determined using a Vickers indenter (98 N),</p><p>with an average of 10 indentations for each sample. Fracture</p><p>strength was measured by four-point bending tests of five</p><p>specimens each using a 40/20 mm support span and a</p><p>crosshead speed of 0.5 mm/min. Youngs modulus was</p><p>determined from the load/displacement curves by following</p><p>Eq. (1)[25]:</p><p>E Fl20l1</p><p>16Jy0; (1)</p><p>where F represents the load applied, l0 is the distance</p><p>between the inner load points, l1 gives the distance between</p><p>the inner and outer supports, y0 the deflection of the center of</p><p>the specimen relative to the position of inner supports, and J</p><p>is the moment of inertia, J bh312</p><p>, where b is the width of the</p><p>specimen and h represents its height in the direction of the</p><p>deflection.</p><p>3. Results</p><p>3.1. Synthesis and characterization of polymers</p><p>The polymer synthesis is mainly based on two concepts.</p><p>First, various reactive functional groups at the silicon atoms</p><p>were introduced to control further branching reactions via</p><p>hydrosilylation (&gt;SiHH2C=CHSi&gt;) and/or polymeriza-tion of the vinyl substituents. These reactions can be</p><p>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</p><p>is achieved by either using di- and trifunctional chlorosi-</p><p>lanes or by bonding sterically different substituents to the</p><p>silicon atoms. As a result of both concepts, the polysilazanes</p><p>HVNG and HPS were synthesized and can be described by</p><p>the structural units given in Fig. 1. The polysilazane HVNG</p><p>consist of mixed di- and trifunctional units, i.e., every</p><p>second silicon atom is bridged by two and the other half</p><p>by three nitrogen atoms to other silicon atoms. In contrast,</p><p>the HPS precursor only consist of twofold bridged silazane</p><p>units.</p><p>With the additional possibility of crosslinking, the mole-</p><p>cular weight was raised from 440 g/mol (HPS) to 620 g/mol</p><p>(HVNG). The viscosity of the silazanes also strongly</p><p>depends on their intrinsic structure. Therefore, a viscosity</p><p>increase from a highly liquid (HPS, 0.05 Pas) to a honey like</p><p>precursor (HVNG, 29 Pas) was recorded (compare Table 1).</p><p>In all silazane systems, first a mass change was observed</p><p>during pyrolysis between 1508C and 3508C. In general, atlower degrees of branching (HPS) the mass loss is about 15</p><p>30 wt%, owing to the release of gaseous oligomers besides</p><p>ammonia, as identified by coupled TG-FTIR measurements</p><p>(Fig. 2). When, however, increasing the degree of branching</p><p>(HVNG), the mass loss is reduced to about 5 wt%. A second</p><p>stage of mass loss was observed between 3508C and 7508C,where methane is released which leads to the degradation of</p><p>the organic substituents (methylene groups, ethylene</p><p>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</p><p>lower (73 versus 82 wt%), due to the escape of more volatile</p><p>Fig. 1. Structural units of the precursors HVNG and HPS.</p><p>Table 1</p><p>Properties of the precursors and the resulting polymer-derived ceramics.</p><p>Educts Precursor Molecular</p><p>Weight (g/mol)</p><p>Synthesis</p><p>Yield (%)</p><p>Viscosity at</p><p>208C (Pas)Ceramic yield</p><p>at 10008CElementary composition at 10008C(mass %)</p><p>ViSiCl3 HVNG 620 92 29 82 Si50.3 O0.85Me(H)SiCl2 C20.6 H</p></li><li><p>oligomers and a higher amount of methyl groups within the</p><p>HPS. In addition, the elementary composition shows a</p><p>higher carbon concentration in the resulting amorphous</p><p>ceramic (Table 1). Changes in the structure of the solid</p><p>intermediates during thermal treatment were investigated by</p><p>29Si solid state NMR spectroscopy. Fig. 3(a) (HVNG) and</p><p>Fig. 3(b) (HPS) show the 29Si-spectra of both polysilazanes</p><p>as a function of pyrolysis temperature up to 15008C. Solidstate NMR characterization of silazanes is typically diffi-</p><p>cult, since rather broad signals appear in the NMR spectra of</p><p>the crosslinked polymers as well as the amorphous materi-</p><p>als. In the present study we used the peak assignment</p><p>reported in literature [1722]. The 29Si-spectrum of the</p><p>crosslinked HVNG polymer cured at 3008C shows threesharp peaks (Fig. 3(a)). In contrast to the corresponding</p><p>solution spectrum of this precursor, a new signal at</p><p>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</p><p>having a 29Si-chemical shift in the range of 2.5 ppmcorrespond to silicon atoms on (N)2Si(C</p><p>sp3)2 sites and</p><p>denote crosslinking via hydrosilylation [22]. An exact</p><p>assignment of the signals appearing after crosslinking can</p><p>only be made via comparison with other SiCN precursors</p><p>like HPS. This spectrum shows only two peaks (Fig. 3(b)).</p><p>The chemical shift with high intensity at 3.5 ppm isrelated to the hydrosilylation reaction, while the higher</p><p>intensity compared to the signal at 22 ppm (unreactedSiH groups) and the fact that a peak for Si-vinyl groups in</p><p>the range of about 15 ppm cannot be observed, indicatethat all vinyl groups reacted via hydrosilylation and/or</p><p>polymerization. In the 29Si spectrum of HVNG at 5008C,</p><p>Fig. 2. FTIR spectra (coupled with TG) of gaseous species which escaped</p><p>during pyrolysis (HVNG).</p><p>Fig. 3. 29Si NMR spectra of (a) HVNG-and (b) HPS-derived powder samples heat treated at various temperatures.</p><p>58 G. Ziegler et al. / Materials Chemistry and Physics 61 (1999) 5563</p></li><li><p>the peak corresponding to unreacted vinyl groups has dis-</p><p>appeared, indicating that crosslinking is completed. More-</p><p>over, these signals are broadened (higher degree of</p><p>crosslinking) and shifted to higher fields, owing to the</p><p>degradation of organic groups and the enrichment of SiN</p><p>surroundings. The latter is in agreement with TG-FTIR</p><p>measurements (Fig. 2), where the evolution of methane</p><p>was observed. The same effects were detected in the</p><p>5008C 29Si NMR spectrum of the HPS precursor.Based on TG analysis, a second mass loss of about 3 wt%</p><p>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-</p><p>cates rearrangements in the amorphous state, whereas at</p><p>10008C only one broad peak was monitored, which corre-sponds to a homogeneous amorphous SiCN matrix. Anneal-</p><p>ing at 15008C, however, leads to a heterogeneous SiCNmaterial. The broad peak finally separates into the three</p><p>signals for SiC4, SiN3C and SiN4 [18,2024]. Longer</p><p>annealing times (48 h) at 15008C cause the formation ofthe thermodynamically stable crystalline phases SiC</p><p>(16 ppm) and Si3N4 (48 ppm). At 16008C, no Si3N4but only a SiC signal is detected by 29Si NMR measure-</p><p>ments, which narrows at 17008C indicating crystal growthof SiC. Upon crystallization, the density increases from</p><p>about 2.6 to 3.25 g/cm3 (corresponding to SiC) with a</p><p>substantial mass loss of 26 wt% due to the decomposition</p><p>of amorphous SiCN and Si3N4 by nitrogen evaporation.</p><p>3.2. Microstructure</p><p>The materials investigated exhibited a residual open</p><p>porosity of about l5 vol% after high-temperature annealing</p><p>at 15408C for 6 h in N2-atmosphere and can, therefore, beconsidered as open systems that allow for the escape of</p><p>gaseous species formed during pyrolysis. This open porosity</p><p>in turn affects the high-temperature stability of these poly-</p><p>mer-derived glasses, as will be discussed in the following.</p><p>On the other hand, since the materials revealed a high degree</p><p>of coalescence between the powder particles and the binder</p><p>phase upon pyrolysis, as can be seen at the fracture surfaces</p><p>of the HVNG- (Fig. 4(a)) and the HPS-derived (Fig. 4(b))</p><p>bulk glasses after annealing at 15408C, the materials locallycontain regions without residual porosity which is consid-</p><p>ered here as...</p></li></ul>