Ion-Exchange Strengthening a Mica-Based Machnable Glassceramic
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ION-EXCMNGE BTRENGTHEZXZG A XICA-BASED XACHXNABLE GLASSCERANIC T.S. CHIN, H.S. *Department of Hsinchu, 300, **Materials R & Lungtang, 325,
LIU , S.Y. CHIU , C.I. CHEN and S.E. HSU Materials Science & Engineering, Teing Hua University, Taiwan, Rep. of China D Center, Chun Shan Institute of Science and Technology, Taiwan, Rep. of China
ABSTRACT Ion exchange was carried out on a lithium-doped fluorophlogopite machinable glassceramic with composition, in wt%, 52.2 SiO2 , 14.0 A1 2 0 3 , 14.2 MgO, 12.5 MgF 2 , 1.4 Li 2 0, and 5.7 Na 2 0, to study the strengthening effect. and betaThe crystalline phases after annealings were Na-fluorophlogopite Ion exchange of Li by K in spodumene, with a volume fraction of about 2:1. 0 the glassceramic at 770 C for 4 h improved 2.1 to 2.3 times the three point bending strength (TPBS). Hence strong machinable ceramic with TPBS of greater than 300 MPa can be made. Ion exchange increases 100% electrical resistivity and improves 21% chemical stability, due to a surface layer of about 100 micrometer enriched with K.
INTRODUCTION of being able Mica-based machinable ceramics have the characteristics It is therefore applicable to be lathed, drilled, milled and threaded (1]. notably vacuum sealing, feedthroughs, mito a great deal of applications, crowave windows, etc. (2,3,4] Stookey firstly precipitated fluoro-mica phase from a K2 0-(SrO,BaO)MgO-A1 2 0 3 -B 2 0 3 -F glass (5]. Grossman precipitated the KMg2 . 5 Si 4 0 1 0F 2 phase which is isostructural with fluorophlogopite(KMg 3 XSi 3 0 10 F 2 , X=Al or B) from and discussed the effect of aspect a composition of K2 0-MgF 2 -MgO-SiO 2 [6], ratio of the platelet crystallite on the machinability. Beall et al studied the composition based on BaO-(SrO,CaO)MgF 2 -MgO-SiO 2 -F and the effect of doThereafter, several additive or replacement oxides such as ping ZrO2 (7]. Rb203, Cs 2 0 3 , As20 5 , Cr 2 0 3 , Fe 2 0 3 , Sb 2 0 3 , As 2 0 3 , K2 Cr 2 O7 ,KMnO 4 , CuO, Au and Sn were studied during the 70's [8-11]. The precipitation of beta-spodumene was found to strengthen the ceramic and lower thermal expansivity. In 1982, Beall precipitated (Ba,Sr) 0 .5Mg 3 A1Si 3 0 10 F2 phase to enhance the dielectric properties and mechanical strength while lowering density (12]. lower mechanical strength as However, machinable ceramics possesses compared to other structural ceramics, this is why lots of work is spent on It is our purpose to study the strengthening of these ceramics (10-12]. fluorophlogopite glasecethe possibility of strengthening the Li-modified ramic by ion exchange. Ion exchange is an old technique though (13,14), the application to machinable glassceramic has been scant in the literature.
EXPERIMENTAL The raw oxide and carbonate powders were weighed according to the composition (in wt%) of 52.2 Si02 , 14.0 A12 0 3 , 14.2 MgO, 12.5 MgF 2 , 1.40 Li 2 0 After and 5.7 Na 2 0. They were mixed with ethanol and ball milled for 24 h.
Mat. Res. Soc. Symp. Proc. Vol. 249. @1992 Materials Research Society
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being dried the mixture was melted in
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