COMBUSTION SYNTHESIS OF SILICON NITRIDE / SILICON CARBIDE COMPOSITE MATERIALS

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1 Proceedings of the South Dkot Acdemy of Science, Vol. 82 (23) 89 COMBUSTION SYNTHESIS OF SILICON NITRIDE / SILICON CARBIDE COMPOSITE MATERIALS Bert Lieig, Griel Bosk, Dnielle Smith nd Jn A. Puszynski Deprtment of Chemistry nd Chemicl Engineering South Dkot School of Mines nd Technology Rpid City, SD ABSTRACT Mny nonoxide cermic mterils cn e synthesized economiclly nd with high purity y comustion technique. Even complex solid solution, sed on silicon nitride structure, could e synthesized y direct high pressure nitridtion of silicon in the presence of other oxides or nitrides. These importnt mterils find ppliction in mnufcturing of engine prts, high performnce erings, nd cutting tools. This pper focuses on in-situ synthesis of silicon nitride silicon cride composites. licon cride/silicon nitride composites exhiit good wer nd therml shock resistnce. In this reserch silicon cride/silicon nitride composites were synthesized using chemiclly-ssisted comustion technique. During tht process composites were formed from elementl silicon nd cron powders in the presence of nitrogen nd poly-fluorinted ethylene (teflon). licon nitride/silicon cride composites with the mount of silicon cride phse vrying from 5 to 25 wt% were synthesized using this method. By ltering comustion conditions it ws possile to synthesize composites with different content of lph nd et silicon nitride phses. Altertion of those conditions hs lso significnt effect on verge prticle size of synthesized composite powders. The effect of silicon cride composition in comustion synthesized composites on microstructure is presented s well. Keywords Chemiclly enhnced comustion synthesis, 3 N 4, C/ 3 N 4 teflon, mmonium ifluoride. composites, INTRODUCTION licon nitride/silicon cride composite mterils hve their mjor ppliction s refrctory mteril nd s mteril for structurl ppliction (Richerson 2, Weimer 1997, Weinstein 1991) s utomotive vlves, high performnce erings, nd cutting tools. These mterils exhiit gret corrosion resistnce nd stle mechnicl properties t elevted tempertures. Conventionlly, these mterils re produced in rection sintering process (Eckstroem1993). In the 9 s some reserchers (Agrfiotis et l. 1991, Puszyn-

2 9 Proceedings of the South Dkot Acdemy of Science, Vol. 82 (23) ski nd Mio 1998) did some pioneering work in the field of the formtion of silicon cride y comustion synthesis. Comustion synthesis cn e descried s clss of highly exothermic rection, which ecome self-sustining once they re loclly ignited y n externl force (Lieig nd Puszynski 1997, Vrm et l.1998). In the cse of non oxide cermic such s TiC, TiN, AlN, 3 N 4, BN, this synthesis process hs proven to e highly effective due to its energy costs, nd high purity of the product. The comustion synthesis of silicon cride is two-step process. First 3 N 4 is formed y comustion of silicon t elevted nitrogen pressure. Then, in the presence of oth 3 N 4 nd cron, the formtion of C took plce. According to the C/ 3 N 4 equilirium digrm, depicted in Figure 1, the C- 3 N 4 equilirium depends on the temperture nd the gs pressure. The morphology nd the phse content o Temperture, C C 3N Nitrogen pressure, Figure 1. C/ 3 N 4 equilirium phse digrm. EXPERIMENTAL PROCEDURE of oth C nd 3N4 cn lso e controlled y the mount of hlide dditives such s NH 4 HF 2, NH 4 F, NH 4 Cl, nd teflon. This delierte use of dditives during the formtion process is clled chemicl enhnced comustion synthesis. This reserch pper focuses on recent developments in the field of comustion synthesis of C, com- 3 N 4, nd C/ 3 N 4 posites. The comustion synthesis experiments were crried out in high pressure rector (see Figure 2). The rection mixture ws plced in refrctory ot nd ignited with resistively heted molydenum wire. rector ignition nd thermocouples wires grphite ot ignition wires nd wires for thermocouples grphite ot wter cooling gs thermocouples rectnt powder + diluent + dditves nitrogen or pressure: up to 3 MP Figure 2. Experimentl set-up to crry out comustion synthesis. The detiled setup cn e found elsewhere (Puszynski nd Mio 1999). The rw mterils used were supplied form the following sources. licon powder ( vg. prticle size 7 µm) ws provided y Elkem Corp. Ammonium ifluoride nd teflon were purchsed from Alph Aesr. licon nitride ws o-

3 Proceedings of the South Dkot Acdemy of Science, Vol. 82 (23) 91 tined from Strck (M11). The cron powder ws Cron Blck from Rven Corp. A silicon silicon nitride mixture with weight rtio 1:1 ws prepred. The mount of mmonium ifluoride dded to the rectnt mixture vried etween 1 wt% nd 5 wt%. In order to synthesize the 3 N 4 /C composite powders, rectnt mixtures consisting of silicon nd silicon nitride, in the weight rtio 1:1, were mixed with 5 wt% cron nd vrile mount of teflon (2.5, 5, 1, 2 wt%). RESULTS AND DISCUSSION The equilirium digrm (equilirium composition (mol %) versus temperture) of the silicon nitrogen system without dditives ws generted y using HSC-thermodynmic dtse softwre from Outokumpo Inc. (see Figure 3). Results of similr clcultion with the presence of 5 wt% mmonium ifluoride is shown in Figure 4. It ws lso possile to determine the ditic comustion tempertures with respect to mount of dditives. The result of those clcultions is liner decrese of the ditic temperture with the incresing mount of mmonium ifluoride, which is shown in Figure 5. The comustion experiments of silicon nitride with 1 wt% to 5 wt% of mmonium ifluoride s dditives were crried out. The x-ry diffrctogrms of the comustion synthesized smples with the ddition of 1 wt% nd 5 wt% of the mmonium ifluoride s dditive re shown elow (Figs. 6 nd 7). Compring oth diffrctogrms, it is ovious tht the mount of lph silicon nitride incresed with the mount of hlide dditive dded. A reson for tht might e the incresed mount of gseous silicon mol-% N4 N2(g) Tempertur C Figure 3. Equilirium digrm (equilirium composition (mol %) versus temperture) for the siliconnitrogen system without ny dditives t 4 psi mol-% F4(g) H2(g) 3N4 N2(g) HF(g) HF3(g) F3(g) F2(g) Figure 4. Equilirium digrm equilirium composition (mol %) versus temperture for the siliconnitrogen system with 5 wt% of NH 4 HF 2 s dditive. Tempertur C

4 92 Proceedings of the South Dkot Acdemy of Science, Vol. 82 (23) o Aditic Temperture C) Additives (wt%) Figure 5. Clculted ditic temperture with respect to the mount of mmonium ifluoride dded. Intensity / Two Thet / degree Figure 6. X-ry diffrctogrm of comustion synthesized 3N4 with 1 wt % NH 4 HF 2. Intensity / Figure 7. X-ry diffrctogrm of comustion synthesized 3N4 with 5 wt % NH 4 HF 2. - lph silicon ni - et silison nit / / Two Thet / degree / contining species, which form lph silicon nitride when recting with nitrogen. When no dditives were used, like in the simultion depicted in Figure 3, it is more likely tht the formtion of silicon nitride tkes plce vi liquid phse mechnism. This process fvors the formtion of the high temperture et phse. Figure 8 depicts the reltion etween the quntity of hlide dditives used nd the phse content of comustion synthesized silicon nitride. The lrger the mount of dditives, the more lph- 3 N 4 cn e found in the products (see Figure 8). However, if more thn 5 wt% mmonium ifluoride were dded to the rectnt mixture, the comustion rection ws not self-propgting nymore. In further experiments, the comustion synthesis rection ws modified to synthesize 3 N 4 /C composites mterils. This ojective ws chieved y dding smll mount of cron nd poly -fluorinted ethylene (teflon) to the silicon nitrogen system. It should e noted tht the ddition of 2 wt% of cron lredy hd n impct on the phse content of the product. The x-ry diffrctogrm depicted in Figure 9 indictes tht the mount of lph silicon nitride incresed to 28 wt%. When teflon ws dded, the et silicon nitride phse further decreses. This cn e oserved in the x-ry diffrctogrm of the smple when 5 wt% teflon ws dded to the rectnt mix- - lph silicon ni - et silicon nit /

5 Proceedings of the South Dkot Acdemy of Science, Vol. 82 (23) 93 ture (see Figure 1). Furthermore, when compring the SEM imges of the smples without telfon (Fig. 11), nd with telfon (Fig. 11), one cn oserve reduction in prticle size. Without teflon, n verge prticle size ws 7 µm (Fig. 11). The fluorine introduced to the system through teflon, rects with the silicon nd led to the formtion of smller crystls (Fig. 11 less thn 2 µm, needle-like). Figure 12 shows comprison of the phse content in 3 N 4 /C composite powders when different mounts of teflon were used in the rectnt mixture. The incresed mount of fluoride introduced y teflon decreses the formtion et silicon nitride y (i) enhncing gs phse rection nd (ii) y decresing the ditic temperture Percentge o α nd β phse4 in the product % free hlide dditive lph et Figure 8. Phse content of lph nd et 3 N 4 with respect to the mount of mmonium ifluoride. Intensity/ C/ /C 5 / / //C / / Two Thet/degree C - lph silicon nitr - et silicon nitrid C- silicon cride Figure 9. X-ry diffrctogrm of C/ 3 N 4, when only 2 wt% of cron ws used s dditive. CONCLUSIONS Chemicl enhnced comustion synthesis cn e pplied to form 3 N 4 nd C/ 3 N 4 with n incresed mount of lph 3 N 4 phse. Equilirium clcultions support the mechnism of the formtion of silicon nitride in the presence of mmonium ifluoride. The clcultions lso hve shown tht the ditic comustion temperture decresed with in- Intensity/ C C/ Two Thet/degree Figure 1. X-ry diffrctogrm of C/ 3 N 4, when 5 wt % teflon ws used in the rectnt mixture next to the 2 wt% of cron. - lph silicon nit - et silicon nitri C- silicon cride / / // /C / /

6 94 Proceedings of the South Dkot Acdemy of Science, Vol. 82 (23) Figure 11 nd 11. SEM imges (5X) of C/ 3 N 4, () without nd () with teflon (5 wt %) dded to the rectnt mixture dditionl to the 2 wt% of cron. Figure 12. Phse content of comustion synthesized C/ 3 N 4 composite powder s function of concentrtion of teflon concentrtion in the rectnt mixture. cresing mounts of tht dditive. It ws found experimentlly tht the use of mmonium ifluoride significntly effects phse content, morphology, nd prticle size of comustion synthesized silicon nitride sed mterils. The ddition of teflon to silicon-nitrogen-cron system results in the formtion of the composite powder with smller prticle sizes. Also, the phse content of 3 N 4 - C-composite is effected y the ddition of teflon. Further sintering experiments to form C/ 3 N 4 composite mterils for structurl ppliction re underwy. ACKNOWLEDGEMENT The uthors grtefully cknowledge the support Form The NSF-sponsored Reserch Experience for Undergrdutes (REU) Progrm conducted t the Deprtment of Chemistry nd Chemicl Engineering NSF-grnt No. EEC

7 Proceedings of the South Dkot Acdemy of Science, Vol. 82 (23) 95 REFERENCES Agrfiotis C., Lis J., Puszynski, J.A., Hlvcek, V Comustion Synthesis of licon nitride -licon Cride Composite J. Am. Cerm. Soc. 73 (11), P Ekstroem,T Reserch on nd Engineering ppliction of silicon nitride sed cermic. Mterils Forum 17, P Lieig, B., Puszynski, J.A High Pressure Synthesis of licon Nitride-sed Mterils with Controlled Morphology nd Phse Composition. Int. J. of Self-Propg. High-Temp. Synth., 7[1],, P Richerson D. 2. The Mgic of Cermics. Am. Cerm. Soc., Westerville Ohio, P Puszynski J.A. Mio S.1999 Kinetic Studies of synthesis of C powders nd whiskers in the presence of KClO3 nd Teflon. J. Int. SHS Vol 8. (3). P Puszynski J.A. Mio S Chemiclly-ssisted Comustion synthesis of silicon cride from elementl powders in Innovtive Processing nd Synthesis of Cermics, Glsses, nd Composites IV, y J.P. ngh (eds.) Cermic Trnsctions, Vol. 95, Americn Cermic Society, Westerville, Ohio. P Weimer, A.W Cride, Boride, nd nitride mterils Synthesis nd Processing Chpmn& Hll, New York. Weinstein J Minerl Processing in Cermics nd Glsses Volume 4 -- Engineered Mterils Hndook, ASM Interntionl. P Vrm, A., Rogchev, A. Muksyn, A. Hwng, S Comustion Synthesis of Advnced Mterils: Principles nd Applictions. Advnces in Chemicl Engineering Vol. 24, Acdemic Press.P