QSTI GELCASTING OF SILICON PREFORMS FOR THE PRODUCTION

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1 QSTI GELCASTING OF SILICON PREFORMS FOR THE PRODUCTION OF SINTERED REACTION-BONDED SILICON NITRIDE J.. Kiggns, jr., S. D. Nunn, T. N. Tiegs, C. C. Dvisson, nd D. W. Coffey Ok Ridge Ntionl Lbortory, P.O. Box 28, Ok Ridge, TN J-P. Mri Pennsylvni Stte University, Stte College, P Abstrct Gelcsting of silicon metl for the production of sintered rection-bonded silicon nitride (SRBSN) ws investigted in order to identify ssocited dvntges over conventionl forming techniques, Le., die nd isosttic pressing. Compcts were formed from identicl powder mixtures by both gelcsting nd pressing, nd.were nitrided nd sintered to produce SRBSN cermics using both conventionl nd microwve heting. Chrcteriztion of the smples included mesurement of green density, green nd nitrided pore structure, weight gin during nitridtion, finl density, microstructure, toughness, nd flexurl strength. It ws found tht more uniform pore structure existed in the green gelcst smples. It is believed tht this pore configurtion ided in nitridtion, nd mnifested itself in more uniform finl microstructure. In ddition, improved mechnicl properties were chieved in the gelcst smples. This improvement cn be ttributed to green microstructure homogeneity. An dditionl finding of this study ws tht microwve heting combined with gelcst forming resulted in SRBSN mterils with improved mechnicl properties. Jntroduco 'on Silicon nitride-bsed mterils re the leding cndidtes for use s high-temperture structurl cermics due to their excellent overll combintion of mechnicl nd physicl properties [ 11. These mterils re of interest in numerous pplictions for such diverse items s cutting tools, rotors for turbine engines, nd vlves nd cm followers for gsoline nd diesel engines. Significnt progress hs been mde in recent yers in producing silicon nitride mterils with superior strength, frcture toughness, nd creep resistnce. However, these mterils tend to be prohibitively expensive due to the high cost of the silicon nitride powders used to produce them. Reduction of cost hs been recognized s mjor need for the successful introduction of silicon nitride cermics into the mrketplce [2,3,4]. SRBSN is n ttrctive lterntive to sintered silicon nitride for number of resons [5,6l. Silicon is economicl compred to high purity silicon nitride powders nd is more esily formed into shpes thn silicon nitride powders. Also, silicon preforms undergo less sintering shrinkge thn preforms mde of silicon nitride powders. However presently, silicon preforms of complex shpes must be mde by expensive cumbersome forming processes, such s injection molding. The door is open for inexpensive forming processes for mking these silicon preforms.

2 Gelcsting is simple, inexpensive forming process which hs been developed s method for forming cermic greenwre. Gelcsting involves the preprtion of cermic slip in mixed monomer solution. After preprtion of the slip, combintion of inititor nd ctlyst is dded to polymerize nd cross-link the monomers upon heting. Previous to this heting, the slip is poured into molds of potentilly complex geometry, whose shpe the finl gel will ssume. Numerous combintions of monomers, ctlysts, nd inititors hve been developed for use with both queous nd orgnic solvent.systems s dictted by the cermic powder being gelcst [7,8,9]. Also, it hs been shown tht gelcst preforms of lumin hve up to five times the mechnicl strength of die pressed green bodies of comprble volumes loding [lo]. The present work ws undertken to develop method for the fbriction of gelcst silicon preforms. It is felt tht the excellent uniformity nd dispersion tht is chievble in liquid suspension cn be frozen into the gelcst compct. It is lso believed tht this method will enble the formtion of complex green bodies of high green strength. In ddition, the present study ws conducted to investigte the use of microwve heting for the nitridtion nd sintering of the gelcst silicon preforms. Previous studies hve shown tht microwve heting results in improved mechnicl properties in the finl SRBSN product [11,121. ExDerimen A Si metl bsed powder mixture designted TM-145 ws used for this study. This composition consists of the following chemicls: 67 wt % Si-metl, Elkem metllurgicl grde 13 wt % Y2O3, Molycorp-56 4 wt % AI3O3, RCHP-DBM 14 wt % Si3N4, Strk LCION 2 wt % Si2, U.S. Silic-5 micron The powder mixture ws turbomilled for -2 hours with 4 mm Si3N4 medi in isopropnol (IPA) contining.5 wt % Drvn-C nd.5 wt % PW-K15 s dispersnts, nd using 4 mm Si3N4 medi. After milling, the powder ws dried nd sieved through 1 mesh screen. 1 g portions of the pre-milled TM-145 powder mixture were initilly die pressed in stinless steel cylindricl dies using steric cid in cetone s mold relese gent. After unixil pressing t 5 MP, the smples were then bgged nd isopressed t 117 MP. The pressures for pressing smples were chosen such tht the green densities were comprble to those obtined by gelcsting. The chemicl constituents of the gelcsting system hve been described in previous publiction A 5 g portion of the pre-milled TM-145 powder m i x t m ws dded to (PA) solution contining the gelcst chemicls nd milled with 4 mm Si3N4 medi in smll ttritor mill for 3 min. A mixture of ctlyst nd inititor ws then dded to induce polymeriztion nd crosslinking. Glss pltes seprted by 1 cm thick rubber gskets were used to cst flt pltes from 1 g portions of the cermic slurry. The slips were gelled in oven t 65OC for pproximtely one hour. After gelling nd cooling to room temperture, the gelcst pieces were dried overnight in closed dessictor box. Some of the gelcst pieces were then isopressed t 13 MP. All smples underwent binder removl in n ir furnce to finl condition of 5ZoC for 1 h. The smples were then nitrided in either grphite element or custom-built 2.45 GHz multimode microwve furnce. Smple crucibles nd conditions for the nitridtion nd sintering hve been discussed in previous publictions [11,12]. The weight gin vlues for nitridtion of the smples processed by microwve heting were mesured fter the single-step nitridtion nd sintering tretment. Weight gins for nitridtion of the smples processed by heting in grphite furnce were mde fter the nitridtion step. Densities of sintered pieces were determined using the Archimedes method. The pore structure of

3 the green nd nihded smples ws mesured using Micromeritics AutoPore II 922 mercury porosimeter. The finl microstructure of both frcture surfces nd polished plsm etched surfces of selected sintered smples were viewed with Hitchi S-8 SEM. 3 mm by 4 mm by 45 mm size specimens were mchined from sintered smples for mechnicl properties mesurements. Flexurl strength testing ws done by four point bending with inner nd outer spns of 2 mm nd 4 mm, respectively. Frcture toughness vlues were mesured using Chntikul's method of four point bending of specimens indented with Vickers indenter t 1 kg lod [14]. Results nd Discussion Tble 1 lists the smple bbrevitions, including description of the forming method nd furnce type used for processing, which re used in the grphs nd tbles. DI-MW DI-GE G-GE GI-MW GI-GE Tble 1. Smple Description Forming Process Furnce Type Designtion 1 Die nd Isopressed Die nd Isopressed Gelcst Gelcst nd Isopressed Gelcst nd Isopressed Grphite element Grphite element Microwve DI-GE GI-I" GI-GE Mesured vlues of green density, weight gin during nitridtion, nd sintered density re given in Tble 2. The weight gin dt indictes tht the level of nitridtion ppers to be independent of the forming process. The lower vlues weight gins observed in the microwve processed smples re due to weight losses during sintering. The theoreticl mximum weight gin is 66 wt %, however vlues pproching 6 wt % re tken to signify ner complete nitridtion, since weight losses occur during the sintering step [15]. Similrly, there ppers to be no reltionship between the finl sintered densities nd the green densities of the mterils. A density of 3.3 g/cm3 ws clculted to be the theoreticl density. Tble 2. Generl Properties Smple Type Green Density Sintered Density % T.D. % T.D. Nitrickion _ Three green body smples, one formed by die pressing nd isopressing, one by gelcsting nd isopressing, nd one by gelcsting lone, were nlyzed by mercury porosimehy. Figures 1 nd 2 demonstrte the incrementl nd cumultive pore res s function of pore dimeter. These plots show two significnt popultions of pores in the green smples. The popultions of the smllest dimeter pores re thought to be result of smple compression t very lrge mercury pressures, hence will be ignored. The pore popultions t higher pore dimeters demonstrte tht the gelcst smple hd the lrgest pores nd the nrrowest pore size distribution, furthermore; the die nd isopressed smple hd the smllest pores nd the widest size distribution. Uniform pcking of the fine prticles in the gelcst suspension would be expected to produce green compct with pores of nrrow distribution. Conversely, the presence of gglomertes in the dried nd sieved powder mixture would ccount for broder pore size distribution.

4 No distinct differences were seen in the mercury porosimetry dt from the nlyses performed on smples following nitridtion (dt not shown). The filling of pores which occus during the nitridtion process ppers to msk the differences tht re present in the green compcts. Figures 3 nd 4 give the vlues of strength nd toughness, respectively, for SRBSN smples formed nd processed s indicted in Tble 1. These results show tht the flexure strengths of ll die pressed smples were lower thn the two types of gelcst smples, irrespective of heting method. Also, these results show tht the gelcst-isopressed smple processed by microwve heting hd higher strength thn gelcst-isopressed mteril processed by conventionl heting. However, the die pressed-isopressed smple processed by microwve heting showed the reverse result. The lower strength of the die pressed, microwve processed mteril my be due to generl observtion mde in severl experiments tht microwve rdition cn preferentilly het flwed res within smples, with resulting negtive effects. Microwve heting of both die pressed-isopressed nd gelcst-isopressed smples gve slight improvements in the toughness vlues, s compred to conventionl heting of both of these smple types. SEM photomicrogrphs of polished nd plsm etched surfces of selected sintered SRBSN mterils re shown in Fig. 5. The grin structure of both die pressed-isopressed nd gelcstisopressed mterils which were processed in the microwve furnce contin numerous lrge ciculr grins. The gelcst-isopressed nd the die pressed-isopressed mterils which were processed in the grphite furnce pper to hve fewer ciculr grins, nd these grins hve smller spect rtio thn microwve processed mterils. The slightly lrger toughness vlues of the microwve processed mterils (Fig. 4) re probbly due to these microstructure differences. Conclusions A method hs been developed for gelcsting silicon metl nd sintering ids into green preforms for the production of SRBSN. This gelcsting method ws shown to be superior to trditionl die press forming process. The dispersion nd uniformity tht is regulrly chievble in liquid suspensions of cermic powders cn be preserved in solid compct through the gelcsting route. This preservtion leds to uniform pore structure, which is especilly importnt in the rection bonding of Si metl, where infiltrtion of nitrogen gs is crucil. The uniformity of the green nd nitrided pieces is then found in the sintered SRBSN cermics. In ddition, this study reinforces similr studies which hve shown tht microwve heting cn be used to fbricte SRBSN mterils with superior strength nd toughness, compred to those produced using conventionl heting techniques. This superior strength nd toughness of the SRBSN mterils is due to the incresed numbers of ciculr silicon nitride grins. Picknowledme n& Reserch sponsored by the U.S. Deprtment of Energy, Assistnt Secretry for Energy Efficiency nd Renewble Energy, Office of Trnsporttion Technologies, s prt of the Cermic Technology Project of the Propulsion System Mterils Progrm, under contrct DE-AC5-84R 214 with Mrtin Mriett Energy Systems, Inc. DISCLAIMER This report ws prepred s n ccount of work sponsored by n gency of the United Sttes Government. Neither the United Sttes Government nor ny gency thereof, nor ny of their employees, mkes ny wrrnty, express or implied, or ssumes ny legl libility or responsibility for the ccurcy, completeness, or usefulness of ny informtion, pprtus, product, or process disclosed, or represents tht its use would not infringe privtely owned rights. Reference herein to ny specific commercil product, process. or service by trde nme, trdemrk, mnufcturer, or otherwise does not necessrily constitute or imply its endorsement, recommendtion, or fvoring by the United Sttes Government or ny gency thereof. The views nd opinions of uthors expressed herein do not necessrily stte or reflect those of the United Sttes Government or ny gency thereof.

5 n w \ cu c E v $ c, C I 2! C I Die Press-lsopress.1.1 Dimeter (pm) Figure 1. Incrementl pore re s function of pore dimeter for green body compcts formed by die press-isopress, gelcst only, nd gelcst-isopress methods. + w cu c E v 2 L n m > Y c, E I f3- Die Press-lsopress _4 Gelcst On1 ( I Dimeter (pm) Figure 2. Cumultive pore re s function of pore dimeter for green body compcts formed by die press-isopress, gelcst only, nd gelcst-isopress methods.

6 n p" E v I I I I DI-GE DI-MW G-GE GI-GE GI-MW Smple ID Figure 3. Strength vlues for sintered SRBSN mterils processed in grphite element nd microwve furnces for smples defined in Tble 1. / DI-GE DI-MW G-GE GI-GE GI-MW Smple ID Figure 4. Toughness vlues for sintered SRBSN mterils processed in grphite nd microwve furnces for smples defined in Tble 1.

7 ) Figure 5. SEM photomicrogrphs showing the microstructure of SRBSN mterils produced from preforms which were ) die pressed-isopressed nd processed in microwve furnce, b) die pressed-isopressed nd processed in grphite furnce, c) gelcst-isopressed nd processed in microwve furnce, nd d) gelcst-isopressed nd processed in grphite furnce.

8 References 1. R. N. Ktz, Nitrogen Cermics , pp. 3-2 in Progress in Nitrogen Cermics, ed. F. L. Riley, Mrtinus Nijhoff Pub., The Hgue, Netherlnds (1983). 2. L. M. Shepprd, "Cost-Effective Mnufcturing of Advnced Cermics," Am. Cerm. Soc. Bull., 7[4]692-77( 1991). 3. T. Qudir, R. W. Rice, J. C. Chkrverty, J. A. Breindel, nd C. C. Wu, "Development of Lower Cost Si3N4,"Cerm. Eng. Sci. Proc., 12[ 9-1], (1991). 4. J. M. Schoenung, "Anlysis of the Economics of Silicon Nitride Powder Production," Am. Cerm. Soc. Bull.? 7 [l] (1991). 5. A. J. Moulson, "Recon-Bonded Silicon Nitride: Its Formtion nd Properties," J. Mter. Sci., Vol. 14, pp (1979). 6. F. L. Riley, "Nitridtion nd Rection Bonding," pp in Nitrogen Cermics, F. L. Riley (ed.), Noordhoff, Netherlnds (1977). 7...Omtete, M. A. Jnney, nd R. A. Strehlow, "Gelcsting-A New Cermic Forming Process," Cermic Bulletin, ul [lo], Oct M. A. Jnney nd.. Omtete, "Method for Forming Cermic Powders into Complex Shpes," U. S. Ptent No.4,894,194, Jn. 16, M. A. Jnney nd..omtete, "Method for Molding Cermic Powders using Wter-Bsed Gelcsting," U. S. Ptent No. 5,28,362, July 2, S. D. Nunn,.. Omtete, C. A. Wlls, nd D. L. Brker, "Tensile Strength of Dried Gelcst Green Bodies," Cerm. Sci. & Engr. Proceed., 15, [4] , J.. Kiggns, C. R. Hubbrd, R. R. Steele, H. D. Kimrey, C. E. Holcombe, nd T. N. Tiegs, "Chrcteriztion of Silicon Nitride Synthesized By Microwve Heting," in Cermic Trnsctions, Microwves, Theory,nd Applictions in Mterils Processing, Am. Cer. Soc.,Westerville, Ohio, (1991). 12. J,. Kiggns nd T. N. Tiegs, "Chrcteriztion of Sintered Rection Bonded Silicon Nitride Processed By Microwve Heting," pp in Microwve Processing of Mterils-III, Vol. 269, Mterils Reserch Soc., Pittsburgh, PA Omtete, J. P. Pollinger, nd K. OYoung, "Optimiztion of the Gelcsting of Silicon Nitride Formultion," to be published in the Proc.of the 19th Annul Coco Bech Conf., Jn. 12, P. Chntikul, G. R. Anstis, B. R. Lwn, nd D. B. Mrshll, "A Criticl Evlution of Indenttion Techniques for Mesuring Frcture Toughness: 11, Strength Method," in Jour. Amer. Cerm. Soc., (1981). 15. B. F. Jones nd M. W. Lindley, "Strength, Density, Nitrogen Weight Gin Reltionships for Rection Sintered Silicon Nitride, J. Mter. Sci., Vol. 1, pp , 1975.