Alumina/Glass Composites Fabricated by Melt-infiltration of Glass into. Porous Alumina Yung-Jen Lin 1,a, Li-Bin Chang 2,b

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1 Alumina/Glass Cmpsites Fabricated by Melt-infiltratin f Glass int Prus Alumina Yung-Jen Lin 1,a, Li-Bin Chang 2,b 1 Department f Materials Engineering, Tatung University, Taipei, Taiwan Prduct Develpment Center, Yung Optics Inc. Hsinchu, Taiwan a yjlin@ttu.edu.tw; b angusc@yungptics.cm Keywrds: Alumina, Cmpsite, Melt-infiltratin, Alumina-glass cmpsite Abstract. Alumina/glass cmpsites were successfully fabricated by melt-infiltratin f glass int prus alumina pellets. Alumina pwder was first pressed uniaxially at 100MPa t frm disc-shaped pellets, then, heated up t 1200 C fr 2 h t frm prus pellets with mderate strength fr subsequent infiltratin. A mixture f calcium aluminsilicate and magnesium brsilicate glass pwders were melt-infiltrated int prus alumina at 1200 C ~1250 C by capillary pressure t frm cmpsites. The infiltratin depths varied with the square rt f infiltratin time. And the activatin energy f the infiltratin prcess was estimated t be 621 KJ/mle. After cmplete infiltratin, the cmpsite had bulk density appraching 3.3 g/cm 3 (~ 96% f theretical density) and pen prsity reaching zer, with slight expansin f 0.5% in diameter. Its flexural strength was 150MPa and its Vickers micrhardness was abut 1000 Kg/mm 2. Intrductin Infiltratin methds are useful fr prcessing ceramic cmpsites. Prus ceramic prefrms with final shapes and suitable strength are fabricated by cnventinal prcessing, such as dry-pressing, slip casting and partial sintering. Then the liquid infiltrants were infiltrated t fill up the pres by capillary pressure r external applied pressure. Pst-infiltratin heat treatments may be needed t btain the desired micrstructures and/r crystalline phases by pyrlyzing the infiltrants r prmting the reactin between infiltrants and perfrms [1-8]. A variety f ceramic cmpsites have been fabricated using infiltratin methds. These include ceramic-ceramic cmpsites [1-8], ceramic-metal cmpsites [9-13] and ceramic-glass cmpsites [14-16]. Using infiltratin methds, unique micrstructure and/r surface mdificatin f the cmpsites can be achieved. Nevertheless, the mst appealing advantage f the infiltratin methds is that they culd be tailred t be near net-shape prcessing rutes. The dimensinal shrinkage culd be reduced by cyclic infiltratin f infiltrants which need pyrlyzatin t frm ceramics [3,4]. When the infiltrants are metals r glasses, the shrinkage after infiltratin is usually minimum, appraching near net-shape prcessing [9,14]. A well-knwn applicatin f infiltratin f mlten glass int ceramic prefrms t fabricate ceramic-glass cmpsites is VITA In-Ceram fr all-ceramic crwns [17]. It is a prcess f infiltrating lanthanum aluminsilicate glass int slip-cast, calcined alumina prefrms [14,17]. The

2 prcess is near net-shape and the prducts have excellent mechanical prperties: 508 MPa and 3.1 MPa m 1/2 fr strength and fracture tughness, respectively [14]. In view f the success with the dental applicatin, the glass infiltratin prcess is suppsed t be als applicable in the fabricatin f ceramic-glass cmpsites fr ther applicatins, which require high mechanical prperties as well as near net-shape prcessing. Fr example, by infiltrating glass int tape-cast alumina sheets, the difficulties assciated with the sintering shrinkage can be eliminated in prcessing f multi-layer ceramic (alumina) substrates. In this research, we chse cmmercially available glass pwders, which are intended t be used in electrnic applicatins, t fabricate alumina-glass cmpsites by melt infiltratin. The prcess and the prperties f the cmpsites were reprted. Experiments Prus alumina pellets. Alumina pwder (Baiklx CR15, Baikwski Internatial Crp., Charltte, NC) was ball-milled in ethanl with 2 wt% PVA fr 6 h. The slurry was dried with IR lamp and the dried pwder cake was sftly grund t pass thrugh 80-mesh sieve. Cmpacts f 15 mm in diameter and ~ 3 mm in thickness were btained by uniaxially dry pressing f 1 g f milled alumina pwder at 100 MPa. The cmpacts were then heated up t 1200 C and saked fr 2 h t btain prus alumina cmpacts with suitable strength fr subsequent infiltratin. On heating t 1200 C, the cmpacts were held at 700 C fr 2 h t burn ut PVA. The heating rate was 10 C/min. Glass pellets. A mixture f tw cmmercially-available glass pwders was used as the infiltrating glass. One was calcium aluminsilicate glass pwder and the ther was magnesium brsilicate glass pwder. The manufacturer s data f these tw glass pwders were listed in Table 1. Preliminary experiments f infiltratin f alumina with each glass alne turned ut t be fruitless. Each glass devitrified n heating t the desired temperature and culd nt infiltrate. Cnsequently, we mixed them in varius prprtins and fund ut a suitable prprtin f calcium aluminsilicate : magnesium brsilicate = 85:15 by weight. With this mixture, the infiltratin culd be perfrmed in the range f 1200 C ~1250 C. The mixing f these tw glass pwders was by ball-milling. The prcedures were similar t thse used fr alumina pwder. The glass pwder mixtures were als dry-pressed at 100 MPa t frm pellets f 15 mm in diameter and ~ 1.5 mm in thickness. Infiltratin and characterizatins. The glass pellets were placed n the alumina pellets in a bx furnace. Infiltratin was perfrmed at 1150 C t 1250 C fr varius times in air with 10 C/min f heating rate. After the infiltratin treatments were dne, the samples were characterized fr the infiltratin depth, dimensinal changes, crystalline phases and mechanical prperties. The infiltratin depth was measured in crss-sectins f the samples. By staining the crss-sectin with a dye slutin, the uninfiltrated part absrbs dye slutin (because f prsity) and became dark while the infiltrated part remains white. If the infiltratin frnt line is nt straight (especially at lwer temperature), the infiltratin depth is calculated by measuring the infiltrated area f the crss-sectin and divided by the width (i.e., t calculate the average depth.) The dimensinal change was assessed by the change f the diameter f the pellets after infiltratin. The density and

3 prsity f the samples were measured using water displacement methd (Archimede s principle). Phase identificatin f the samples was perfrmed using x-ray diffractmetry (D-5000, Siemens, Karuhlse, Germany.) Micrstructure was bserved in a scanning electrn micrscpe (JSM 5600, Hitachi, Tky, Japan.) The hardness was measured by a Vickers micr-hardness tester (FM-7, Future-tech, Japan.) The flexural strength was the average value f 5 measurements using 4-pint bending bars f 3x4x36 mm 3. Results and Discussins After the alumina cmpacts were sintered at 1200 C fr 2 h, its bulk density reached 1.99 g/cm 3 with an pen prsity f 50.5%. A linear shrinkage f 2.3% ccurred during this sintering prcess. The pre size distributin (measured with a mercury prsimeter) is shwn in Fig.1. It reveals that mst pres are f abut 0.13μm in diameter. Figure 2 shws the infiltratin depth f the samples at varius temperatures fr 1 h. Heat treatment at 1150 C shwed n evidence f infiltratin. Heat treatment at temperature greater than 1250 C resulted in significant shrinkage f the samples. Cnsequently, infiltratin temperatures were limited between 1200 C C fr this study. Figure 3 is a picture shwing the crss-sectin f the infiltrated pellets after heat treatment at different temperatures fr 1 h. It is quite bvius that the sample were fully infiltrated at 1250 C fr 1 h. It was als nted that there was ~0.5% f linear expansin after the pellets were infiltrated with glass. Figures 4 (a) and 4 (b) are the bulk densities and pen prsities f the infiltrated samples. It shws that infiltratin at higher temperatures culd reach higher density with negligible pen prsity than thse infiltrated at lwer temperatures. At lwer temperatures, the glass has higher viscsity, which slws dwn the infiltratin rate and leaves sme pres unfilled. Fr examples, samples infiltrated at 1212 C reached a final bulk density f 3.2 g/cm 3 with pen prsity f 3 %. Meanwhile, the samples infiltrated at 1225 C r 1250 C reached a final density f abut 3.3 g/cm 3 with negligible pen prsity. Nevertheless, the samples with negligible pen prsity d nt imply that they are withut prsity. Rugh calculatin f the theretical density f the alumina-glass cmpsite gives 3.44 g/cm 3. Hence, the sample fully infiltrated at 1250 C cntained ~4% clsed prsity. The kinetic f the infiltratin prcess can be studied by pltting the infiltratin depth against infiltratin time at different temperatures. Frm Darcy s law, the infiltratin depth f a liquid int prus media by capillary frce will be prprtinal t the square rt f time, i.e., [1,18] h = (2KP/η) 1/2 t 1/2.. Eq. 1 Where, h is the infiltratin depth, K is the permeability f the prus bdy, P is the pressure fr infiltratin (capillary pressure + external applied pressure internal pressure) ηis the viscsity f the liquid. The abve equatin is related t the temperature mainly by the change f viscsity η with

4 temperature. The variatin f the viscsity f glass with temperature is described by the empirical Vgel-Fulcher relatin:[19] η= A exp(b/(t-t )).. Eq. 2 where A and B are cnstants. Cnsequently, the infiltratin depth culd be related t the temperature in Arrhenius relatin. Fr simplicity, we let rate cnstant k=(2kp/η) 1/2 and k=k exp (-Q/(RT)), then h = k t 1/2 = k exp (-Q/(RT)) t 1/2.Eq. 3 This equatin can be cnveniently applied in the study f the kinetics f the infiltratin prcess. Hence, we pltted the infiltratin depth against the square rt f infiltratin time at different temperature and btained the slpe (i.e., rate cnstant k) at each temperature, as shwn in Fig. 5. Then, we pltted the lg k against 1/T, as shwn in Fig. 6, and btained the slpe (-Q/(RT)). The activatin energy f this infiltratin prcess was calculated frm the slpe f the fitting line in Fig.6 t be 621 KJ/ml, which is quite large. The large activatin energy is cnsistent with narrw temperature range (~ 50 C) f infiltratin. Figure 7 is a typical XRD pattern f the cmpletely infiltrated sample. After infiltratin the glass partly crystallized int spinel (MgAl 2 O 4 ) and anrthite (CaAl 2 Si 2 O 8 ). The crystallizatin was expected t increase the strength f the samples. Table 2 lists the hardness and the flexural strengths f the samples. The hardness and strength increased dramatically after infiltratin with glass. This is nt surprised because the pres f alumina pellets have been filled with glass. Nevertheless, the flexural strength f the sample is much lwer than thse f alumina-glass cmpsites as reprted in the literature [14-16,20]. It culd be due t the difference in the alumina cmpsitin (e.g., additin f MgO r ZrO 2 ), prsity befre infiltratin and infiltrating glass. Figure 8 is a micrgraph f the fracture surface f the samples. The clsed prsity is clearly shwn in this figure. In additin, the fracture appeared t be brittle, the characteristic fracture mde f ceramics and glass. Cnclusins Prus alumina with ~50% pen prsity culd be melt-infiltrated by capillary pressure with a mixture f calcium aluminsilicate glass and magnesium brsilicate glass. Suitable infiltratin temperature was between 1200 C and 1250 C. The infiltratin depths varied with the square rt f infiltratin time. And the activatin energy f the infiltratin prcess was calculated t be 621 KJ/mle. After cmplete infiltratin, the cmpsite had bulk density appraching 3.3 g/cm 3 (~ 96% f theretical density) and pen prsity reaching zer, with slight expansin f 0.5% in diameter. Its flexural strength was 150MPa and its Vickers micrhardness was abut 1000 Kg/mm 2. Acknwledgement This wrk was financially supprted by the Natinal Science Cuncil, Taiwan, ROC under grant number: NSC E

5 References: 1.W.-C Tu and F. F. Lange: J. Am. Ceram. Sc., 78 [12] (1995), p P. Hneyman-Clvin and F. F. Lange: J. Am. Ceram. Sc., 79 [7] (1996), p Y.-J. Lin and Y.-C. Chen: J. Am. Ceram. Sc., 84 [1] (2001), p B. R. Marple and D. J. Green: J. Am. Ceram. Sc.,72 [11] (1989), p B. R. Marple and D. J. Green: J. Am. Ceram. Sc.,73 [12] (1990), p B. R. Marple and D. J. Green: J. Am. Ceram. Sc.,74 [10] (1991), p B. R. Marple and D. J. Green: J. Am. Ceram. Sc.,75 [11] (1992), p S. J. Glass and D. J. Green: J. Am. Ceram. Sc.,79 [9] (1996), p Y.-J. Lin and T.-S. Shen: Key Eng. Materials, 249 (2003), p N.A. Travitzky, E.Y. Gutmanas and N. Claussen: Mater. Lett. 33 (1997), p J. Hj, O. Sagawa and H. Kimura: J. f the Japan Sc. f Pwder and Pwder Metall. 41 (1994) p W.-P. Tau, T. Watari and T. Trikai: Am. Ceram. Sc. Bull. 76 (1997), p C.A. Len and R.A.L. Drew: J. f Mater. Sci. 35 (2000), p D,J. Kim, M.H. Lee and C.E. Kim: J. Am. Ceram. Sc., 82 [11] (1999), p D.Y. Lee: J. f Mater. Sci. 39 [9] (2004), p X.P. Lu, J. M. Tian, Y.L. Zhang and L. Wang: Dental Mater., 18 [3] (2002), p F.A. Dullien: Prus Media-Fluid Transprt and Pre Structure. Academic Press, New Yrk, W.D. Kingery, H.K. Bwen and D. R. Uhlmann: Intrductin t Ceramics, 2 nd ed., Jhn-Wiley& Sns, New Yrk,

6 Table 1. Prperties f Glass Pwders Prperties Ca-Al-Si-O glass Mg-B-Si-O glass & Density (g/cm 3 ) Transfrmatin pint ( C) Wrking pint ( C) Thermal expansin cefficient ( x 10-7 / C) Ga 33, Nippn Electric Glass C, Ltd, Osaka, Japan. & Ga 44, Nippn Electric Glass C, Ltd, Osaka, Japan. Table 2. Mechanical Prperties f the Fully-infiltrated Samples Infiltratin temperature ( C) Micrhardness (H v ) (Kg/mm 2 ) Flexural strength (MPa) N infiltratin 50 ± 3 39 ± ± ± ± ± ± 18

7 INCREMENTAL INTRUSION VS DIAMETER INCREMENTAL INTRUSION, ( ml/g ) DIAMETER, ( MICROMETERS ) Figure 1 Pre size distributin f alumina pellets after sintered at 1200 C fr 2 h. (befre infiltratin) Infiltratin depth (%) Infiltratin temperature, deg C Figure 2 Infiltratin depth f the samples after infiltratin at varius temperatures fr 1 h.

8 Figure 3 Crss-sectin f the infiltrated pellets after infiltratin at different temperatures fr 1 h. Dark area: uninfiltrated regin, bright area: infiltrated regin. Distance between hrizntal lines = 1.5 mm.

9 Bulk density (g/c.c.) (a) (b) (c) (d) Figure 4(a) Bulk densities f the samples infiltrated at different temperatures fr different time. (a)1250 C, (b)1225 C, (c)1212 C, (d)1200 C Infiltratin time (min) 40 (b) (d) Open prsity (%) (a) (c) Figure 4(b) Open prsities f the samples infiltrated at different temperatures fr different time. (a)1250 C, (b)1225 C, (c)1212 C, (d)1200 C. Infiltratin time (min)

10 Infiltratin depth (mm) C C C C (Infiltratin time) 1/2 Figure 5 Infiltratin depth vs. square rt f infiltratin time at different temperatures. The slpe f each line is the rate cnstant (k) at each temperature ln k activatin energy = 621 kj/mle /(infiltratin temperature)x10 4 (1/K) Figure 6 Lg k vs. 1/T. The activatin energy f the infiltratin prcess is estimated t be 621 KJ/mle.

11 alumina spinel anrthite Intensity theta Figure 7 A typical XRD pattern f fully-infiltrated samples, shwing peaks f alumina, spinel (MgAl 2 O 4 ) and anrthite (CaAl 2 Si 2 O 8 ). This pattern was btained frm the sample infiltrated at 1250 C fr 1 h. Figure 8 Fracture surface f a bending bars, which was infiltrated at 1250 C fr 1 h. Nte the pres in the sample.