Densification Studies of Silicon Carbide-Based Ceramics with Yttria , Silica and Alumina as Sintering Additives

Si li con car bi de has been ex ten si vely used in struc tu ral ap pli ca ti ons, es pe ci ally at high tem pe ra tu res. In this work, Y2O3, Al2O3 and SiO2 were ad ded to β-SiC in or der to ob ta in highly den se ce ra mics. Sin te ring was con duc ted in a di la to me ter and in a grap hi te re sis tan ce fur na ce and the den si fi ca ti on be ha vi our was stu di ed. Sin te red sam ples were cha rac te ri sed by den sity me a su re ments, the crystal li ne pha ses were iden ti fi ed by X-ray dif frac ti on. Mi cros truc tu ral ob ser va ti on of po lis hed and po lis hed/et ched sam ples was car ri ed out with help of scan ning elec tron mi cros copy. Si li con car bi de ce ra mics with more than 90% of the the o re ti cal den sity were ob ta i ned by pres su re less sin te ring if a su i ta ble pro por ti on of the ad di ti ves is used.


Introduction
Si li con car bi de is an im por tant ce ra mic used in struc tural ap pli ca ti ons, such as au to mo ti ve heat en gi nes, cut ting to ols, heat ex chan ge and me cha ni cal se als.The se ap pli ca tions are pos si ble be ca u se of the uni que pro per ti es of si li con car bi de ba sed ma te ri als, in clu ding high tem pe ra tu re strength and low den sity, as com pa red to con ven ti o nal steel, and ex cel lent ther mal shock and wear re sis tan ce 1,2 .
The che mi cal bonds bet we en si li con and car bon atoms in SiC are al most 88% co va lent, de cre a sing the ato mic diffu si vity and ma king it dif fi cult to achi e ve high den si fi ca tion 3 .Sin te ring ad di ti ves are usu ally re qui red to ob ta in high den sity, such as bo ron, car bon, alu mi ni um -or the ir compounds 4 .In the se ca ses, sin te ring oc curs through so lid sta te me cha nisms 5,6 .
Li quid pha se sin te ring, through so lu ti on re-precipitation pro cess, can also be em plo yed to den sify SiC ba sed ce ra mics [7][8][9][10][11] .The main ad van ta ge of this ap proach is the lo wer tem pe ra tu res in vol ved.Li quid pha se is gene rally pro du ced by re ac ti on of the ad di ti ves with SiO 2 pre sent on the SiC sur fa ce [12][13][14] .
Many pa ra me ters ac count for the pro duc ti on of highly den se ce ra mics, in clu ding pow der pro per ti es such as high spe ci fic sur fa ce area, the pro ces sing tech ni que used to conso li da te the pow ders, and the sin te ring con di ti ons, i.e. atmosp he re, tem pe ra tu re and time.The con trol of sin te ring con di ti ons is, the re fo re, of fun da men tal im por tan ce 15,16 .The amount and com po si ti on of the li quid pha se can be formu la ted to pro vi de su i ta ble vis co sity to make ma te ri al trans port more ef fi ci ent and, con se quently, to im pro ve den si fi ca ti on [17][18][19] .
Sin te ring can be con duc ted in a di la to me ter or in a graphi te re sis tan ce fur na ce.Di la to me tric me a su re ments ena ble a study of sin te ring pro vi ding the tem pe ra tu res of ma ximum shrin ka ge.Sin te ring in a di la to me ter pre sents the disad van ta ge of gre a ter we ight loss and lon ger pe ri ods of time to re ach equi li bri um con di ti ons.The lat ter is due to the con ti nu ous gas flow em plo yed du ring ex pe ri ments, which le ads to va ri a ble pres su re wit hin the system.Sin te ring in grap hi te re sis tan ce fur na ce can be more ef fec ti ve due to the pre sen ce of a pow der bed co ve ring the sam ples, which helps ma in ta in the va pour pres su re cons tant and de cre a se the sam ple we ight loss.
Sin te ring ad di ti ves ba sed on the SiO 2 -Al 2 O 3 -Y 2 O 3 system (anot her rare earth oxi de may re pla ce Y 2O3) have been found to be su i ta ble to pro mo te SiC den si fi ca ti on through li quid pha se sin te ring [20][21][22] .The se ad di ti ves have also shown to be ef fec ti ve to ob ta in high den sity si li con nitri de ce ra mics [23][24][25] .The for ma ti on of a eu tec tic tem pe ra tu re at about 1350 °C 26 is res pon si ble for li quid for ma ti on that can dis sol ve the si li con car bi de par ti cles at re la ti vely low tem pe ra tu res.Pres su re less sin te ring of si li con car bi de is, the re fo re, pos si ble using the se ad di ti ves, pro bably through a si mi lar me cha nism to that des cri bed for si li con ni tri de sin te ring.SiC gra in morp ho logy can be con trol led by se veral pro ces sing con di ti ons du ring sin te ring.In the pre sent work, the sin te ring be ha vi our of si li con car bi de using va ria ble pro por ti ons of SiO 2 -Al 2 O 3 -Y 2 O 3 was in ves ti ga ted.The sin te red sam ples were cha rac te ri sed in terms of den siti es, crystal li ne pha ses and mi cros truc tu ral ob ser va ti on.The in flu en ce of the oxi des pro por ti on on the se pa ra me ters can be es ta blis hed.

Experimental
Pow ders of Y 2O3 (H.C. Stark), SiO 2 (Sig ma Aldrich) and Al 2 O 3 (A16, Alcoa) were stir red for 4 h in se ve ral propor ti ons, ac cor ding to Ta ble 1.The 60-20Y com po si ti on was also mi xed using at tri tor mill (Sze gua ri) for 2 h.Both pro ces ses, stir ring and at tri ti on mil ling, were tested in or der to es ti ma te the ef fects of the par ti cle size dis tribu ti on and mean par ti cle size of the ad di ti ves on the sin te ring be ha vi our of si li con car bi de.For small amounts of ad di ti ves, at tri ti on mill is not usu ally re com men ded due to the high me dia wear du ring mil ling 27 , alt hough it pro vides bet ter pow der ho mo ge ne ity.
All com po si ti ons of Ta ble 1 were cal cu la ted wit hout con si de ring the low amount of SiO 2 (1.45%) which is present in SiC raw ma te ri al.They are lo ca ted sche ma ti cally in the SiO 2 -Al 2 O 3 -Y 2 O 3 com po si ti o nal di a gram (Fig. 1).These com po si ti ons were cho sen ba sed on Shelby data 17 , and are si tu a ted in the glass for ming re gi on.The 60-20Y mix ture is lo ca ted in the cen tre of this re gi on.Thus, 90 vol % of β-SiC (BF 17, H. C. Stark) and 10 vol-% of each ad di ti ve mix tu re were mi xed and at tri tor mil led for 4 h.
The the o re ti cal den si ti es of the ad di ti ve mix tu res, as well as of the fi nal mix tu res, were cal cu la ted using the mixtu re rule (Ta ble 1).The par ti cle size dis tri bu ti on of the raw ma te ri al and mix tu res was de ter mi ned by the se di men ta tion tech ni que (Se di graph, 5100, 3.1).Par ti cle morp ho logy was ob ser ved using scan ning elec tron mi cros copy (SEM).
The fi nal mix tu res were dri ed in a ro to e va po ra tor (He idolph WB 2000) and the pow ders were uni a xi ally pres sed at 20 MPa.The sam ples were cold isos ta ti cally pres sed at 200 MPa and the gre en den si ti es de ter mi ned.
Sin te ring stu di es were per for med in a di la to me ter (Netzsch, DIL, 402 E/7), in eit her ni tro gen or ar gon flow (1 atm), ap plying a he a ting rate of 15 °C/min up to 1950 °C.Sam ples were kept at this tem pe ra tu re for 1 h.The sin te red sam ples were cha rac te ri sed for den sity using the ge o me trical and Archi me des met hods.
In or der to fully cha rac te ri se the ma te ri als, the same com po si ti ons stu di ed un der di la to metry were pres su re less sin te red in a grap hi te re sis tan ce fur na ce, he re in using protec ti on pow der bed of 60-20Y com po si ti on.The same sche du le used in di la to metry ex pe ri ments was ap pli ed, using only ar gon flow.Sin te red sam ples were cha rac te rised for den sity and the sam ples were cut lon gi tu di nally.The crystal li ne pha ses of bulk sam ples were de ter mi ned by X-ray dif frac ti on (Phil lips, PW, 1710).Mi cros truc tu ral obser va ti on was car ri ed out using scan ning elec tron mi crosco pe (Phi lips, XL, 30) on the lon gi tu di nal sec ti on of po lis hed sam ples.The se were pre pa red by tra di ti o nal methods.The gra in morp ho logy was re ve a led af ter Mu ra kami's et ching.The et ching so lu ti on was pre pa red by mi xing KOH, Fe 3K(CN) 6 and dis til led wa ter in a ra tio of 1:1:2.

Results and discussion
Ta ble 1. Mix tu res com po si ti ons, the o re ti cal den si ti es (% dt) and mean par ti cle size (d50) of fi nal mix tu res.

Powder characterisation
Fi gu re 2 de picts the par ti cle size dis tri bu ti on of β-SiC and oxi des used in this study, as well as of the 60-20Y compo si ti on af ter mil ling.The 60-20Y was cho sen as a re fe rence in the Fig. 2 to re pre sent the typi cal be ha vi our of all ot her pow der mix tu res, which sho wed si mi lar par ti cle size dis tribu ti on.The par ti cle size dis tri bu ti on data pro vi ded the equi va lent sphe ri cal di a me ter at a cu mu la ti ve per cen ta ge of 50% (d50), as shown in Ta ble 1. Fi gu re 3 shows typi cal SEM ob ser va ti ons of the ad di ti ve mix tu re (60-20Y) and final mix tu re (ad di ti ve and 90% vo lu me of β-SiC).The powders are ho mo ge ne ously mi xed and do not con ta in lar ge ag glo me ra tes.It is not pos si ble to iden tify the dif fe rent oxides through this tech ni que.The mean par ti cle size of the final pow der mix tu re ob ser ved un der SEM is in good agre e ment with the par ti cle size dis tri bu ti on pre vi ously pre sen ted.Both tech ni ques re ve al the pre sen ce of a small frac ti on of par ti cles lar ger than 1 µm.

Density
The den si fi ca ti on re sults of 60-20Y sam ples, sin te red in the di la to me ter using eit her ar gon or ni tro gen are shown in Fig. 4.This com po si ti on was pre pa red by mi xing the ad diti ves in an at tri tor be fo re mil ling with si li con car bi de.Compa ring the den sity af ter sin te ring (Ta ble 2) and di la to me tric cur ves (Fig. 4), one can see that the sam ple sin te red un der ni tro gen does not un der go sig ni fi cant shrin ka ge.In this case, the ni tro gen atoms are in cor po ra ted into the li quid, the re fo re in cre a sing the li quid vis co sity.This im pli es less wet ting and the vis cous li quid is not able to dis sol ve the si licon car bi de par ti cles.For this re a son, furt her sin te ring expe ri ments were con duc ted un der ar gon at mosp he re.The se re sults agree with pre vi ously re por ted data 28 that com pa red both at mosp he res and sug ges ted ar gon flow as a more su ita ble at mosp he re to ob ta in hig her den sity.
<%-2>The sin te red den sity data lis ted in Ta ble 3 shows that sam ples sin te red in the di la to me ter ex hi bit lo wer density and hig her we ight loss when com pa red to sam ples sinte red in the grap hi te re sis tan ce fur na ce.As pre vi ously men ti o ned, the sam ples sin te red in the fur na ce were co vered with a pro tec ti ve pow der bed.This pro ce du re has a mar ked in flu en ce on the re sults.Se ve ral sam ples of each com po si ti on were sin te red un der the same con di ti ons, thus the re sults can be more re pre sen ta ti ve than di la to me tric sam ples.
The re sults ob ta i ned in the grap hi te fur na ce show that for hig her we ight loss the den sity de cre a ses.We igh loss occurs ma inly due to ad di ti ve vo la ti li sa ti on.Thus, we ight loss could be mi ni mi sed to achi e ve bet ter den si fi ca ti on.It is no ted that ke e ping a cons tant Y 2O3/Al 2O3 ra tio (1:1), whi le SiO 2 con tent is hig her ca u ses the den sity to decre a se.When the SiO 2 amount is kept cons tant (60 mol%), the den sity in cre a ses as the Y 2O3 amount is in cre a sed.This be ha vi our is pro bably re la ted to the vis co sity of the li quid for med, and de pends on the so lu bi lity of si li con car bi de.The li te ra tu re po ints out the pa ra me ters that in flu en ce this me cha nism 17,26 , the most im por tant be ing the vis co sity and tem pe ra tu re of the li quid.The fi nal den sity is re la ted to the amount of li quid pha se that could recrystal li ze.The fi nal den sity of SiC com pounds in cre a ses as the mix tu re den sity is hig her.The best den si ti es were ob ta i ned for the 50-25Y sam ple.

Dilatometric analysis
The den sity re sults ob ta i ned af ter sin te ring are in good agre e ment with the di la to me tric cur ves pre sen ted in Fig. 5.The 50-25Y sam ple, which ex hi bits hig her fi nal den sity, shows hig her shrin ka ge.It can be seen that at the tem pe ratu re of 1950 °C the re is no shrin ka ge, thus the den si fi ca ti on pro cess is hin de red for all sam ples at such tem pe ra tu re.
The sin te ring be ha vi our of si li con car bi de con ta i ning oxi de ad di ti ves is si mi lar to that of si li con ni tri de ba sed ce -ra mics [23][24][25] , im plying that si li con car bi de sin te ring oc curs through li quid pha se pro cess.
The li ne ar shrin ka ge rate (Fig. 6) re ve als two shrin ka ge pe aks.The first one, at lo wer tem pe ra tu re (at about 1300 °C) and bro a der, in di ca tes a slight re ar ran ge ment of si li con car bi de par ti cles as the li quid pha se starts to form, at a tem pe ra tu re near the eu tec tic.From the data of par ti cle rear ran ge ment shown in Ta ble 4, one can see that the 60-20Y com po si ti on shows lo wer ma xi mum shrin ka ge tem pe ra ture.This tem pe ra tu re is re la ted to the glass-forming re gi on, and con se quently, to the li quid com po si ti on for med du ring   the first sin te ring sta ge.The ma xi mum re ar ran ge ment tempe ra tu re se ems to be lo wer as the cen tre of the glass-forming re gi on is ap pro a ched, even though it does not lead to im pro ve den si fi ca ti on.
The shar per peak ob ser ved at hig her tem pe ra tu re in Fig. 6 re pre sents the for ma ti on of lar ger quan ti ti es of li quid and si mul ta ne ous dis so lu ti on of si li con car bi de par ti cles (so luti on-reprecipitation pro cess).Excep ti on to this is no ted for 60-15Y com po si ti on.
The ma xi mum shrin ka ge tem pe ra tu re data for li quid for ma ti on and so lu ti on-reprecipitation pro cess is gi ven in Ta ble 4. The 50-25Y sam ple has hig her den sity and shows hig her ma xi mum shrin ka ge tem pe ra tu re.The ot her samples show si mi lar ma xi mum shrin ka ge tem pe ra tu re, but dif fe rent fi nal den sity, sug ges ting that ot her pa ra me ters are in vol ved in the den si fi ca ti on pro cess.
Si mul ta ne ous li quid for ma ti on and so lu tion-reprecipitation pro cess is not ob ser ved for 60-15Y sample.This sam ple shows two dis tinct ma xi mum shrin ka ge at hig her tem pe ra tu res, in di ca ting that li quid for ma ti on and so lu ti on re pre ci pi ta ti on pro ces ses do not oc cur si mul ta neously.This ob ser va ti on should be in ves ti ga ted using compo si ti o nal data; furt her stu di es are re qui red to ex pla in this be ha vi our.The den si fi ca ti on pro cess can be re la ted, the refo re, to the lar ge amount of li quid that is de ter mi ned by the li qui dus tem pe ra tu re in the pha se di a grams of SiO 2-Al 2O3-Y 2O3 system, which de ter mi nes the li quid visco sity and the dis so lu ti on rate of si li con car bi de par ti cles.

Microstructural observation
The X-ray dif frac ti on pat terns of bulk sam ples are shown in Fig. 7. Three si li con car bi de polytypes are present: 3C, 4H and 6H.The pre sen ce of the 3C polytype in dica tes that the β-α trans for ma ti on is not com ple te.The pha se Y 2 Si 2 O 7 was iden ti fi ed by X-ray dif frac ti on as a crystal li ne se con dary pha se pre sent in all sam ples.YAG (Al 5Y3O12 ) was also iden ti fi ed in the 60-15Y com po si ti on, ke e ping cons tant the ra tio 3 Y 2 O 3 : 5 Al 2 O 3 as well as the ini ti al ad di ti ve con tent.The pre sen ce of this pha se could be re la ted to the two ma xi mum shrin ka ge pe aks ob ser ved in di la to metry for the same sam ple.It is pos si ble that YAG is also for med in the ot her com po si ti ons, but only in small quan ti ti es, which are not de tec ta ble by X-ray dif frac ti on analy sis.
Fi gu re 8a shows a typi cal mi cro graph of a po lis hed surfa ce ob ser ved un der scan ning elec tron mi cros copy.The secon dary pha se ap pe ars to be well dis tri bu ted in all sam ples, in di ca ting good mix tu re ho mo ge ne ity.The mi cro graphs of et ched sam ples re ve al the morp ho logy of si li con car bi de gra ins (Fig. 8b).All sin te red sam ples have si mi lar mi crostruc tu re wit hout se con dary gra in growth.

Conclusions
The use of 90% β-SiC, with se ve ral pro por ti ons of SiO 2 , Y 2 O 3 and Al 2 O 3 as ad di ti ves, was su i ta ble to ob ta in si li con car bi de with about 90% den sity by pres su re less sinte ring.
Di la to me tric ex pe ri ments re ve a led the pre sen ce of two shrin ka ge pe aks du ring sin te ring of si li con car bi de.The first is re la ted to a slight par ti cle re ar ran ge ment and a second peak, at hig her tem pe ra tu res, in di ca tes li quid for ma tion and so lu ti on re-precipitation.
The sam ples sin te red in the pre sen ce of pow der bed in a grap hi te re sis tan ce fur na ce show a lo wer we ight loss and, the re fo re, hig her den si fi ca ti on.X-ray dif frac ti on re ve a led the pre sen ce of β-SiC (3C polytype) and α-SiC (4H and 6H polytype) as well as small amounts of Y 2Si 2O7 and YAG.

Fi gu re 2 .
Par ti cle/ag glo me ra te size dis tri bu ti on of pow der and a typical mix tu re (60-20Y).

Fi gu re 3 .Ta ble 2 .
Pow der scan ning elec tron mi cro graphs.a) typi cal ad di ti ve mix tu re b) typi cal si li con car bi de mix tu re (60-20Y).Sam ple 60-20Y den si ti es (% the o re ti cal) and we ight loss (%) com pa ring the at morp he re.Atmosp he re Gre en Hi dros ta tic Ge o me tri cal We ight loss ni tro gen 54

234 3 .
Mar chi et al.Ma te ri als Re se archTa ble Sam ples den si ti es (% the o re ti cal) and we ight loss (%) du ring sin te ring.

Fi gu re 4 .Fi gu re 5 .Fi gu re 6 .
60-20Y Di la to me tric re sults: li ne ar shrin ka ge ver sus tem -Di la to me tric re sults in ar gon: li ne ar shrin ka ge ver sus tempe ra tu re for se ve ral com po si ti ons.Di la to me tric re sults: li ne ar shrin ka ge rate ver sus time for se ve ral com po si ti ons.