Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.6 685- -691685F abrication of high performance 3D SiO2/Si3N4composite via perhydropolysilazane infiltrationand pyrolysis .QI Gongjin, ZHANG Changrui, HU Haifeng, CAO Feng, WANG Siqing& JIANG YonggangState Key Laboratory of Advanced Ceramic Fibers and Composites, College of Aerospace and MaterialsEngineering, National University of Defense Technology, Changsha 410073, ChinaCorrespondence should be addressed to Qi Gongjin (email: qgjin@tom.com)Received April 5, 2005Abstract Perhydropolysilazane， a low viscosity preceramic polymer with goodwettability and high char yield, was used to fabricate three-dimensional silica fiberreinforced silicon nitride matrix composites through the repeated infiltration-curing-pyrolysis cycles. With the increase of the pyrolysis temperature from T，T2 to Ts, thedensity of the composites increased all through, but the flexural strength showed amaximum value at T2 followed by a sharp decrease. The composite prepared at T2exhibited a good ceramization of the preceramic polymer, a high flexural strength of 144.9MPa and excellent dielectric property. The high performance of the composite resultedfrom the good state of the silica fibers, controlled fiber/matrix interfacial microstructuresand high-purity dense silicon nitride matrix.Keywords: silica fiber, preceramic polymer, silicon nitride, composite, high performance.DOI: 10.1360/102005-92The continuous fiber reinforced ceramic matrix composites have received consider-able attention for structural applications because of their excellent thermal stability, lightweight, and damage tolerance imparted by the reinforcing fibers. Silica fibers, with ex-cellent ablative resistance, thermal shock damage resistance, dielectric properties,chemical stability and flexibility, are suitable for fabricating high temperature antennawindow materials to meet the requirements of communication, control and thermal pro-tection of spacecrafts. However, the disadvantage of the silica fiber is that the high tem-perature processing of conventional inorganic ceramic systems will lead to serious fiberdegradation，hence it is arduous to pack ceramics as matrix densely intothree-dimensional silica fiber preforms. Through the low temperature processes such assol-gel and slurry infiltration, electrophoretic infil中國(guó)煤化rsilica fiberreinforced oxide ceramic matrix composites have bromagneticTYHCNMH GCopyright by Science in China Press 2005686Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.6 685- -691window materials during the last decades, including the short silica fiber reinforced sil-ica composite, the three-dimensional silica fiber reinforced silica or phosphate compos-ites, but these composites all had relatively low flexural strength of about 75 or 84MPal-4. Although the strength can be improved using organic resin (such as silicone)infiltration method, the carbon-containing resin will decrease the microwave transpar-ence under high temperatures. Therefore, it is a difficult problem to fabricate thethree-dimensional silica fiber reinforced ceramic matrix composites with high perform-ance. .To our knowledge, the silicon nitridelsl is also a candidate material for microwavetransparence due to its high mechanical strength, good thermal shock resistance and anacceptable dielectric property, but to date no paper has appeared on multidirectional sil-ica fiber reinforced silicon nitride composites, which could be prepared by the process ofpreceramic polymer infiltration and pyrolysis due to its advantages such as effectivenessin low temperatureslo. However, since materials containing carbon will decrease micro-wave transmission efficiency, it is not easy to find an adequate precursor for silicon ni-tride ceramic without carbon contamination. In this paper, perhydropolysilazane, a lowviscosity preceramic polymer without carbonous radicals, was used to fabricate highperformance composite of the three- dimensional silica fiber reinforced silicon nitridematrix (3D SiO2/SizN4).1 Experimental1.1 Raw materials and preparation of the compositesThe silica fibers, produced by Jingzhou Feilihua Quartz Glass Corporation, werewoven into the three-dimensional four-directional preform (see Fig. 1) with a fiber vol-ume fraction of 44% by Nanjing Fiberglass Research and Design Institute. The proper-ties of the silica fibers are shown in Table 1. Perhydropolysilazane (PHPS), the prece-ramic polymer for SizN4 ceramic matrix, was synthesized by the ammonolysis ofdichlorosilane-pyridine adduct . The precursor was a transparent liquid with a densityof 1.1一1.2 g/cm', a viscosity of 30- 50 mPa: s (25C) and a high ceramic yield (≥80wt%). 3D SiO2/SigN4 composites were prepared according to the following stages (seeFig.2). Before the vacuum infiltration of the polymer, a pretreatment process wasadopted not only to remove the organic impurities on the silica fiber surface, but also toprevent the possible reaction of the silica fiber and polymer-derived ceramic matrix dur-ing high temperature pyrolysis. The preforms filled with precursor were cured at 100一200°C for 1-3 h in an inert atmosphere, and then pyrolyzed in the ammonia atmos-phere. The infiltration-curing pyrolysis cycles were repeated for five times to densify theTable 1 The properties of silica fibersDensityTensile strengthElastiPurity (%)中國(guó)煤化m Lo/g . cm-/MPant tangent≥99952.21700YHCNMHG_0.0001Copyright by Science in China Press 2005High performance 3D SiO2/SisN4 composite687Fig. 1. Planform view of the silica fiber perform.Silica fiber preform↓PretreatmentPolymer infiltrationCuringOptionalcycles↑PyrolysisCompositeFig. 2. Process flow for preparing 3D SiO/SisNs composites.composites. To study the effect of the pyrolysis temperature on the composite properties,three different temperatures (T1 < T 2< T3) were adopted, and the corresponding com-posites were denoted as“3D-SS1",“3D SS2" and“3D-SS3”.1.2 Characterization中國(guó)煤化工The infrared spectra of PHPS and its pyrolyticMYHCN MH Gby Nicoletwww.scichina.com688Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.6 685- -691Avatar 360 FTIR spectrometer. The carbon content of the pyrolytic products was exam-ined by a chemical analysis method (GB/T16555.2-1996). The bulk densities of thecomposite samples were measured according to Archimedes' principle with deionizedwater as the immersion medium. The flexural strength of the composites was determinedon a WDW-100 computer controlled universal testing machine with a span of 30 mmand crosshead speed of 0.5 mm/min carried out at a test piece 4 mm wide and 3 mmthick. The dielectric property of the composite was measured at room temperature by theshort-circuited waveguide technique with the sample size 15.8X7.9X8 mm'. At leastthree samples were used for the average values of above data. Fractography of the com-posites was examined on JSM 5600LV scanning electron microscope.2 Results and discussion2.1 Pyrolysis of the preceramic polymer at different temperaturesThe FTIR spectra of perhydropolysilazane and its pyrolytic products at different tem-peratures are ilustrated in Fig. 3. Compared with ref. [7], absorption peaks in Fig. 3(a)located at 3372/1182 cm-' (N-H), 2161 cm^ 1 (Si-H), 1050- - 800 cm^ 1 (Si-N-Si), showeda formula of [H2SiNH].. Fig. 3(b) shows the FTIR spectrum of PHPS- derived ceramicpyrolyzed at T, the N-H absorbance peaks have disappeared and the intensity of Si-Habsorbance peak decreased remarkably. However, the absorption in the vicinity of 922cm-' was still a broad band, suggesting an incomplete organic-inorganic transformation.When pyrolyzed at T2 (see Fig. 3(c)), the Si-H vibration peak decreased further, and therelatively narrow absorption peak centered at about 922 cm-' attributed to Si-N-Si vibra-tion showed a good transition from PHPS to silicon nitride ceramic. For the pyrolyticproduct at T3 (see Fig. 3(d), the Si-H peak was very weak and the Si-N-Si vibration()(C)6)4000300020001000Wavenumber/cmFig. 3. FTIR spectra of PHPS and its pyrolytic products. (a) PHP中國(guó)煤化I工yrolytic prod-uct at T2; (d) pyrolytic product at T.YHCNM HGCopyright by Science in China Press 2005High performance 3D SiO/SisN4 composite689peak at about 922 cm- became narrower, indicating a better polymer-ceramic transfor-mation. Although no carbon- containing radicals exist in the molecular formula of PHPS,some carbon contamination may be introduced by the residual solvent. However, thesolvent will volatilize during pyrolysis of the preceramic polymer, and the ammonia at-mosphere could remove carbon due to its chemical reactivity. As a result, the aforemen-tioned pyrolytic products from PHPS at different temperatures all showed high purity,with residual carbon contents of less than 0.5 wt% by elemental analysis.2.2 Effect of pyrolysis temperatures on the mechanical properties of the compositesFig. 4 illustrates the flexural strength and bulk density of the composites pyrolyzed atdifferent temperatures. With the increase of pyrolysis temperature, the density of thecomposites increased all through. This is because that the precursor has a better poly-mer-ceramic transformation and denser ceramic matrix at the higher temperatures, andthe shrinkage of the matrix leaves behind more porosity for subsequent infiltration.Composite 3D-SS1 had a good flexural strength due to the lttle degradation of silicafibers and relatively dense matrix. With the increase of the pyrolysis temperature, thefiber strength decreases and the fiber/matrix interface bonding may be stronger, thusaffecting the strength of the composite. However, on the other hand, the ceramic matrixbecomes denser as a result of further ceramization, which plays an important role in theload transferring during deformation of the composite. Therefore, the final mechanicalproperty of the composite is determined by the above contrary factors. In comparisonwith 3D-SS 1, the little increase of flexural strength of 3D-SS2 may be resulted from thefact that the positive effect of denser silicon nitride matrix slightly exceeds the side ef-fect of silica fiber degradation on mechanical property. Although the ceramic matrix of3D- SS3 became denser than that of 3D SS2, the silica fibers suffered serious degrada-tion, which played a dominant role in controlling the flexural strength of the composite,thus leading to a sharp decrease of the strength.2502.5202.0 r00 t.0馬500.5TT;Temperature/CFig. 4. Flexural strength and bulk density of the compo中國(guó)煤化工，Jres.YHCNMH Gwww. .scichina.com690Science in China Ser. E Engineering & Materials Science 2005 Vol.48 No.6 685- -6912.3 Properties and microstructures of composite 3D-SS2Considering the above discussion, composite 3D SS2 exhibited the best comprehen-sive properties combining good ceramization of the matrix and high flexural strength.The overall properties of the composite are summarized in Table 2. After five pyrolysiscycles, the relative density of the composite was 86.8%, suggesting a good densificationthrough the present process of preceramic polymer infiltration and pyroysis. The com-posite did not fall apart after the flexural strength test and showed non-brittle fracturebehavior. The flexural strength 144.9 MPa was much higher than that of the other multi-directional silica fiber reinforced ceramic matrix composites'"- 4. What is more, the di-electric property of the composite was also very good, with a dielectric constant of 3.3,and a loss angle tangent value of 0.005, which resulted from the excellent dielectricproperty of silica fibers and the high purity of PHPS derived silicon nitride matrix.Table 2 The properties of composite 3D SS2PyrolysisPyrolysis Bulk densityRelativeFlexuralElasticDielectricLosstemperature(C) cycles/g.cm-density (%) strength/MPa modulus/GPa constant tangentI21.9186.8144.924.83.30.005Fig. 5 shows the SEM micrographs of the fracture surface of composite 3D SS2. As isseen, the precursor has been well infiltrated among the silica fibers due to its good wet-tability, thus improving the impregnation efficiency and ensuring a relatively high den-sity. There was fiber pull-out and fracture in the fracture surface, and the cross- section ofsilica fibers exhibited good state without apparent microcracks. The dense silicon nitridematrix improved the ability of absorbing fracture energy by transferring load during de-formation of the composite. The results showed that the appropriate process parameters(including preform pretreatment and pyrolysis temperature, etc.) have been adopted inthe present study, not only avoiding the strong adhesion of fiber/matrix interfaces, butalso improving the reinforcement ability of silica fibers and load transferring ability ofthe dense Si;N4 matrix, hence leading to high performance composite.20um5 pumFig. 5. SEM micrographs of the fracture suI中國(guó)煤化工YHCNMHGCopyright by Science in China Press 2005High performance 3D SiO2/SisN4 composite691Perhydropolysilazane was used to fabricate the high performance composite of thethree-dimensional silica fiber reinforced silicon nitride matrix by the preceramic poly-mer infiltration and pyrolysis process. This was a fast route for the fabrication of thedense composite due to the low viscosity, good wettability and high char yield of thepolymer. The composite prepared at T2 showed a high flexural strength and excellentdielectric properties. It was the good state of silica fibers, controlled interfacial micro-structures, and high purity of the PHPS- derived dense silicon nitride matrix that contrib-uted to the high performance of the composite.References1. Meyer, F. P, Fitzpatrick, R., Whither, R. E, Effects of CW high intensity laser iradiation on ceramic com-posite radome materials, AD-A096775, 1981.2. Chen, H, Zhang, L. M.. Jia, G. Y. et al, The preparation and characterization of 3D-silica fiber reinforced sil-ica composites, Key. Eng. Mat, 2003, 249:159- 162.3. Manocha, L. M.. Panchal, C. N., Manocha, S., Silica/silica composites through electrophretic infiltration, Ce-4. Tian, H. F, Preparation, microstructures and mechanical properties of silica reinforced phosphate composites,Master Thesis, Haerbin Institue of Technology, Haerbin, 2003, 68.5. Barta, J., Manela, M.. SisNs and SizN2O for high preformance radomes, Mat. Sci. Eng., 1985, 71: 265- -272.6. Sato, K, Suzuki, T, Funayama, O. et al, Fabrication of silicon nitride based composites by impregnationwith perhydropolysilazane, J. Cera. Soc. Japan. Int. Edit, 1992, 100: 450- -453.7. Isoda, T, Kaya, H, Nishi, H. et al, Perhydropolysilazane precursors to silicon nitride ceramics, J. Inorgan.Organomet. Poly, 1992, 2: 151- 160.中國(guó)煤化工MHCNMHGwww. .scichina.com