June 1999
Materials Letters 39 Ž1999. 350–353 www.elsevier.comrlocatermatlet
Enhanced mechanical properties of oriented mica glass-ceramics Kangguo Cheng a
a,)
, Julin Wan b, Kaiming Liang
b
Department of Materials Science and Engineering, UniÕersity of Illinois at Urbana-Champaign, 1304 West Green Street, Urbana, IL 61801, USA b Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, China Received 6 July 1998; received in revised form 8 November 1998; accepted 17 November 1998
Abstract Oriented mica-containing glass-ceramics have been fabricated by hot-pressing the glasses after they were crystallized. X-ray diffraction analysis and scanning electron microscopy show that mica plates were aligned in the hot-pressed glass-ceramics, forming a Ž001. preferential texture. The formation of the oriented structure was attributed to the combination of viscous flow of residual glass and rolling of mica plates when hot-pressure was applied. By hot-pressing technique, the flexural strength and fracture toughness of the glass-ceramics were improved up to 380 MPa and 2.8 MPa m1r2 , respectively, two to three times higher than traditional mica glass-ceramics. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Mica glass-ceramics; Hot-pressing; Texture; Mechanical properties; Composites
1. Introduction The mechanical properties of glass-ceramics are not only dependent on the crystalline phase and their volume fraction, but also highly related to the spatial arrangement of the crystals w1,2x. For most conventional glass-ceramics, crystals are randomly dispersed in the glass matrices, and thus, they possess poor strength and toughness. In contrast, several oriented bulk glass-ceramics w3,4x and glass-ceramic fibers w5,6x have been prepared by processing such as hot-extrusion, gradient crystallization, etc., by which much higher strength and toughness have been achieved. Mica glass-ceramics are widely used as mechanical, electrical, and biomedical materials due
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Corresponding author. E-mail:
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to their unique machinability and good electrical properties w1,2,7–10x. Unfortunately, since flat particles of mica are randomly dispersed in traditional mica glass-ceramics, such materials show poor mechanical performances. Consequently, further applications of such materials are strictly limited. In order to improve their mechanical properties and consequently broaden their applications, we developed a processing technique by which oriented mica glassceramics have been successfully fabricated. The original results are reported in this letter.
2. Experimental The composition of the base glass in weight percent, from the phlogopite family, was SiO 2 50%,
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 0 3 3 - 6
K. Cheng et al.r Materials Letters 39 (1999) 350–353
Fig. 1. Schematic illustration of hot-pressing apparatus.
Al 2 O 3 19%, MgO 16%, K 2 O 8%, F 7% ŽMgF2 form.. An excess amount of TiO 2 Ž5 wt.%. was added to the glass frits for the nucleating agent. The
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raw batch materials were well-mixed by ball-milling for 6 h and then melted in a platinum crucible at 15508C for 2 h. Cylindrical glass ingots were prepared by casting the molten glass into a copper mold with an inner cavity of 20 mm in diameter and 50 mm in height. Crystallization, i.e., decomposition to form mica particles in the glasses, were achieved by holding them at 6508C for 0.5 h, and then heating up to 9008C at a rate of 58Crmin, and holding for 4 h in an electrical furnace. Phase analysis and preferential orientation were investigated by X-ray diffraction ŽXRD.. Microstructures were analyzed by scanning electron microscopy ŽSEM.. The well-crystallized glass ingots were quickly withdrawn from the furnace and put into a pre-heated copper mold with an inner cavity of 80 mm in diameter and 80 mm in height, and hot-pressed. Various deformations were achieved at 9508C within a hot-pressing load range of 50–200 kN. The appara-
Fig. 2. XRD patterns of mica glass-ceramics before and after hot-press with varied deformations.
K. Cheng et al.r Materials Letters 39 (1999) 350–353
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Fig. 3. Microstructures of mica glass-ceramics before and after hot-pressing.
tus for hot-pressing is schematically illustrated in Fig. 1. The amount of hot-pressing deformation is described by H0 y H H0
= 100%
Ž 1.
where H0 and H are the heights of the crystallized specimens before and after hot-pressing, respectively.
3. Results and discussion After crystallization, all the glass-ceramics consisted of fluorophlogopite as the major phase, a minor amount of magnesium titanate and residual glass phase, confirmed by XRD technique. Fig. 2 shows the XRD patterns of mica glass-ceramics before and after hot-press with varied deformations. It is obvious that Ž001., Ž003. and Ž005. peaks of mica increase with increasing hot-pressing deformation, while peaks referring to Žy131., Ž020., Ž200. and Žy331., which are perpendicular or nearly perpen-
Fig. 4. Flexural strength and fracture toughness of mica glass-ceramics before and after hot-pressing.
K. Cheng et al.r Materials Letters 39 (1999) 350–353
dicular to Ž001. plane, decrease with increasing hotpressing deformation. SEM images ŽFig. 3. reveal the microstructures of mica glass-ceramics before and after hot-pressing. It is obviously seen that mica plates are randomly dispersed in glass matrix before hot-press, while they show a high preferential orientation after hot-press. Such preferential orientation is achieved due to the fact that as the hot-pressure is applied, the residual glass in mica glass-ceramics can readily flow along the plane perpendicular to the hot-press direction and mica plates rotate simultaneously. Consequently, oriented mica glass-ceramics with Ž001. texture of the crystalline mica phase are formed. The measured flexural strength and fracture toughness of mica glass-ceramic with 75% deformation are 380 MPa and 2.8 MPa m1r2 , respectively, compared with 91 MPa and 0.83 MPa m1r2 before hot-pressing ŽFig. 4.. The significant improvement of the mechanical properties of oriented mica glassceramics results from the highly aligned mica plates, which can efficiently deflect the crack propagating perpendicular to mica plates w1,2x. In addition, more transcrystalline fracture may occur in oriented specimens than that which occur in grain-randomly-dispersed specimens. This may also make a contribution to the improvement of the strength and toughness of mica glass-ceramics w2x.
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4. Conclusions By using a novel hot-pressing technique, we have successfully prepared a new type of mica glassceramics where microstructure consists of preferentially aligned mica particles. The flexural strength and fracture toughness of the oriented materials can be improved up to 380 MPa and 2.8 MPa m1r2 , respectively, two to three higher than traditional materials. References w1x P.W. McMillan, Glass Ceramics, 2nd edn., Academic Press, London, 1979. w2x D.S. Baik, K.S. No, J.S. Chun, J. Am. Ceram. Soc. 78 Ž5. Ž1995. 1271. w3x H. Atkinson, P.W. McMillan, Mater. Sci. 12 Ž2. Ž1977. 443. w4x Y. Abe, T. Kasuga, H. Hosono, J. Am. Ceram. Soc. 67 Ž7. Ž1984. 142. w5x Maries, P.S. Rogerers, Nature 256 Ž31. Ž1975. 401. w6x K.H.G. Ashbee, Nature 252 Ž6. Ž1974. 469. w7x J.F. Bednarik, P.W. Richter, Glass Technol. 27 Ž2. Ž1986. 60. w8x K.G. Cheng, J.L. Wan, K.M. Liang, J. Non-Cryst. Solids 215 Ž2–3. Ž1997. 134. w9x H.S. Liu, L.S. Lai, T.S. Chin, Biomedical Eng. Appl. Basis Commun. 8 Ž1. Ž1996. 30. w10x T. Hamasaki, K. Eguchi, Y. Koyanagi, A. Matsumoto, T. Utsunomiya, K. Koba, J. Am. Ceram. Soc. 71 Ž12. Ž1988. 1120.
