Figure 1: Schematic of Selective Laser Sintering Process. Scanner System. Laser Source. Sealed Chamber. Fabricated Parts.

Transcription

1 SUPPORT-FREE INFILTRATION OF SELETIVE LASER SINTERED (SLS) SILION ARBIDE PREFORMS B.Y. Sevis, D.L. Buell, J.J. Beama J., Mechaical Egieeig Depame, Labay f Feefm Fabicai, The Uivesiy f Texas a Ausi, 1 Uivesiy Sai, 22, Ausi TX USA Reviewed, acceped Sepembe 14, 26 Absac N-meallic bjecs ae fe difficul maufacue due high melig empeaues, p sieabiliy, limied duciliy, ad difficuly i machiig. Feefm fabicai echiques cupled wih liquid ifilai ffe a cs-effecive ad apid maufacuig mechaism f cmpsie pas wih cmplex gemey ad adequae ppeies. Selecive lase sieed (SLS) silic cabide () pefms ifilaed wih liquid silic develp lcalized ifila veexusis sufaces ad a suface iegulaiies. Seveal shikage mechaisms icludig desificai wee sudied as pssible causes f hese vefillig exusis, ad he esuls ae discussed belw. This eseach was supped by NSF Ga Numbe DMI Backgud Reaci bded silic cabide (RB) is a egieeig ceamic ha ca be fmed by desifyig pus silic cabide pefms wih silic cabide pduced i siu via chemical eaci bewee silic ad cab. These maeials ae f iees whee high empeaue segh ad sabiliy ae equied. Applicais iclude elecics pcessig wafe caies ad bas, kil cmpes, ad wea pas. Selecive lase sieig wih subseque liquid ifilai pvides a cs-effecive ad elaively easy maufacuig ue f RB, ad pemis a geeus feedm f desig. I supp-fee selecive lase sieig, phelic ad silic cabide pwdes ae mixed ad laded i a SLS machie. A cmpue-clled scaig lase guided by a.sl file mels a shape i a hi laye f he pwde. A ew laye f pwde is lled p f his ad he pcess epeas, ceaig a 3-dimesial pa. Oly he phelic is meled ude he lase, ad i bids he silic cabide paicles gehe ceae a gee pa. Figue 1 shws a schemaic f he SLS pcess. Sealed hambe Scae Sysem Lase Suce Rlle Mechaism Fabicaed Pas Build ylide Pwde Delivey Sysem Figue 1: Schemaic f Selecive Lase Sieig Pcess The gee pa is heaed i vacuum cabize he bide, hus ceaig a bw pa which is fuhe heaed i he pesece f silic pelles iduce liquid silic ifilai. Favable suface eegy cdiis, lw viscsiy, ad he paamees allw he mle silic quickly ad fully ifilae he pus bw pa. I he pcess, silic eacs wih he cab 359

2 fm ew silic cabide, i.e. eaci bded silic cabide. The e esul is a fully dese cmpsie pa f silic ad silic cabide. Pessueless ifilai f silic i he silic cabide pefm is dive by suface capillay fces, which equies amg he higs a weig agle < 9 ad favable suface fee eegies. Based Washbu s elaiship f he divig pessue P a liquid dawig up a ube, ie ad c-wkes develped a heshld pessue f spaeus ifilai P* i a pus medium (Mese ad ie, 1987; Oh, e al., 1989): P* 6 cs lv (1 )D (1) whee is he ai f he paicle acual suface aea he suface aea f a sphee f ideical vlume, lv is he liquid suface fee eegy, is he elaive desiy ad D is he paicle size. The equilibium heigh f liquid h max ifilaig i a pus medium ad wkig agais gaviy i pessueless ifilai has bee deived (Ge, 1995): h max 2 cs lv (2) g whee g is he acceleai due gaviy ad is he mle ifila desiy. Applyig Equai 1 silic ifilai i a silic cabide pefm, he fllwig paamees may be ake (Wag, 1999): = 1.3, lv = 846 mj/m 2, =.4 ad D = 4 μm. The heshld pessue P* is calculaed be -83kPa. This is effecively a vacuum dawig mle silic i he pe sucue. The maximum heigh f silic ifilai i a silic cabide pefm may be calculaed usig Equai 2 wih addiial paamees = 41, = 2 μm, g = 9.8 m/s 2 ad = 2.65 g/cm 3. The esul, 2.5 m, is cmpaable 2 m baied by Ge (1995). Based his ifmai, e migh expec silic eadily ifilae silic cabide pefms. 2. Expeimeal mmecial gade -28 gi silic cabide () pwde pvided by Idusial Supply, Ic., ad GP-5546 phelic esi fm Gegia Pacific Resis, Ic. ha was fuhe milled 11 m, wee mixed i a especive 1:1 weigh ai. Afe 5 hus f mixig i a ceme mixe, he pwde was selecive lase sieed i a 3D Sysems Siesai 2. The Siesai 2 peaig paamees wee as fllws: lase pwe, 1W; ulie lase pwe, 4W; laye hickess, 75 μm; sca spacig, 1 μm; pa bed empeaue, 75 ; sca speed, 1.3 m/s; beam diamee, 45 m; ad ige pcess gas was used. Afe sieig, he gee pas wee emved fm he Siesai ad ifilaed wih BJB T-1622 A/B epxy mixed wih xylee i a 1:1 ai educe he viscsiy. They wee he ps-cued accdig he maufacue s ecmmedais. Nex, he pas wee placed i a b iide caed gaphie cucible i idiec cac wih silic pelles, Figue 2. Usig a high empeaue gaphie vacuum fuace, he pas wee heaed wih he fllwig empeaue pfile: 18 /h fm m empeaue 2, 3 /h 5 duig which he phelic ad epxy decmpsed, 3 /h 14, 5 /h 165, ad he immediaely fuace cled. lig ime was appximaely 7 hus. The fuace was backfilled wih ag befe fuace pcessig ude vacuum, ad ag was pulsed hugh he fuace chambe duig pcessig sweep away bypducs f he phelic decmpsii ad he chemical eacis. 36

3 Figue 2: Fuace seup shwig pa i idiec cac wih silic pelles Pas f vaius gemeies wee ceaed chaaceize he vefillig. Bulk ad facue sufaces f he pas wee examied usig scaig elec micscpy (SEM). F desificai sudies, five 25mm cubes wee ceaed usig he abve pe-fuace pcesses. These cubes wee weighed ad measued wih a calibaed digial micmee (±2.5 m) a 15 lcais each afe five pcessig pis, f a al f 75 measuemes pe pcessig pi. Measuemes wee ake afe: SLS pcessig (as gee pas), epxy ifilai, cvesi bw pa up 8 (3 /h fm m empeaue 5 ad he 3 8 ), heaig 165 (25 /h fm m empeaue 14 ad he 5 /h 165 ), ad silic ifilai 165 (25 /h fm m empeaue 14 ad he 5 /h 165 ). Pas wee allwed cl cmpleely bewee seps elimiae he ifluece f hemal expasi he measuemes. I addii he physical expeimes, a Ashby desificai pl based ifmai fm Gema (1998) was csuced f pessueless silic cabide heaig. 3. Resuls ad Discussi SEM facue suface images ake f he gee pa, Figue 3a, ad afe bw pa cvesi, Figue 3b, cfim ha he phelic bidges bewee he silic cabide paicles cve cab ligames afe hea eame. The cab ligames ae visible i he cee f Figue 3b. Phelic Bidge Paicle Paicle ab Ligame (a) Figue 3: SEM image f gee pa (a) ad bw pa heaed 8 (b) (b) 361

4 SEM ad pical micscpe images ake f silic ifilaed pas veify ha he esulig pas ae fully dese silic-silic cabide cmpsies, Figues 4a ad 4b. Figue 4a is a facue suface image, ad Figue 4b is a diamd saw-cu css-secial view. Silic Silic (a) Figue 4: SEM image (a) ad pical micgaph (b) f silic ifilaed pa, heaed 165 Suface defecs he pas ccued as w fms f vefillig: macscpic vefillig a gemeical sess cceais (Figue 5), ad micscpic vefillig a bulk sufaces (Figue 6). The micscpic vefills ae pese as small hemispheical pusis f silic spead me less uifmly ve he suface f he pa. These pusis ae 1 μm 2 mm i diamee, ad hey geeally ccupy he suface a a fequecy he de f 6-1 pusis pe cm 2. While he macscpic vefillig ca be explaied as he esul f he mle silic flw beig dive i aeas f high sess cceais, he micscpic vefillig he bulk sufaces is as iuiive ad is he pimay fcus f his ivesigai. (b) 1 1 mm Figue 5: Macscpic Ovefillig a Gemeical Sess ceais Figue 6: Micscpic Ovefillig Bulk Sufaces 1 mm 362

5 Shikage f he silic cabide maix duig he liquid silic phase was hugh be he cause f he micscpic vefillig. Oce he pefm is cmpleely ifilaed, ay shikage f he maix culd pesumably fce mle silic u he suface. Fm calculais based he aveage amu f vefillig bseved i he pas, i was deemied ha ly a small amu f shikage i he silic cabide maix was eeded pduce he bseved amu f vefillig. This measued vlume chage i he silic cabide, V/V, is appximaely -(5 6)%, a equivale dimesial chage L/L f appximaely -1.5% -2%. Aleaively, his is effecively a chage i elaive desiy / h fm Thee causes f shikage wee expled. Fis, hemal caci duig silic slidificai was csideed. Silic has a lwe cefficie f hemal expasi (TE) ha silic cabide ad wuld heefe be ude cmpessi duig clig, bu he diffeece i vlumeic chages bewee he w maeials is vasly isufficie cause he expeimeally bseved amu f vefillig. The secd csideai was he fmai f ew silic cabide fm he eaci Si (3) ( l) ( s) ( s) Bu he silic cabide fmed i he eaci ccupies much geae vlume ha he pecus cab did, s he maeial expeieces a e expasi, caci as eeded explai he vefillig mechaism. This is see fm simple mla vlume calculais. Fially, ligh desificai f he silic cabide duig heaig was csideed. suci f a Ashby desificai map (Figue 7) pediced viually desificai wihi he empeaues, ime, ad pessues used i pa pcessig. Alhugh desificai is sigifica whe high pcess pessues ae used (Dua, 1989), ude vacuum desificai ccus h Relaive Desiy Vlume Diffusi uvaue Dive 5 1 h.5 1, h Tempeaue, Figue 7: Ashby Desificai Map f i vacuum Fuheme, daa fm he five 25mm cubes idicaed ha while shikage ccued duig he empeaue age 2-14, e ccued duig he empeaue age i which silic ifilai ad slidificai ccued (Figue 8). The shikage duig he lwe empeaues ca be aibued he expeced mass lss duig phelic ad epxy decmpsii, which is sigifica i he empeaue age 4-7 (K, 2). Fially, micscpic vefillig was bseved afe pas had bee pe-desified, i.e. hey wee heaed 165 wihu silic ad he eheaed 165 wih silic ifilai. Viually all desificai wuld have ake place duig he fis sep, hus elimiaig vefillig duig he secd sep. 363

6 Dilaai G e e E p x y E p x y B w 8 B w B w S i I f i l. Figue 8: Dilaai Bewee Pcessig Seps f SLS Si- Pas 4. clusis Usig selecive lase sieig ad liquid silic ifilai, fully dese eaci bded silic cabide pas have bee successfully ceaed. Hweve, he supeficial exusis ceaed i he pcess mus be elimiaed befe i is f seius cmmecial value f his maeial sysem. These exusis ae difficul emve usig bead blasig, ad he pcess is me cs-effecive wihu ps-pcess fiishig. Based he expeimeal ad heeical wk descibed abve, shikage ad/ ligh desificai f he silic cabide duig he liquid silic phase have bee excluded as causes f he bulk suface vefillig i he SLS silicsilic cabide pas. Fuhe ivesigai f he pblem ciues ad may equie a me kiemaical appach. 5. Refeeces Dua, S. (1989) High-Segh Silic abides by H Issaic Pessig, Pceedigs f he Thid Sympsium eamic Maeials ad mpes f Egies, Las Vegas, V.J. Teey, ed., The Ameica Sciey, Ic., Gema, R.M. (1998) slidai Piciples ad Pcess Mldig i ASM Hadbk Vlume 7: Pwde Meal Techlgies ad Applicais, ASM Ieaial, Maeials Pak OH USA, Ge, F.H. (1995) Ieaci Bewee apillay Flw ad Macscpic Silic ceai i Liquid Silicized ab/ab, eamic Tasacis, 58, 149. Heady, R.B. ad ah (197) J.W., A Aalysis f he apillay Fces i Liquid-Phase Sieig f Spheical Paicles, Meallugical Tasacis, 1(1),

7 K, T. (2) Micsucual hages f Phelic Resi duig Pylysis, J. Applied Plyme Sciece, 81, Mese, A. ad ie, J.S. (1987) O he Ifilai f Meal Maix mpsies, Meallugical Tasacis A, 18A, Wag, H. (1999) Advaced Pcessig Mehds f Micelecics Idusy Silic Wafe Hadlig mpes, Ph.D. Disseai, The Uivesiy f Texas a Ausi, Ausi TX USA,