CA1211476A - Glass-ceramic articles containing osumilite - Google Patents

Abstract

Abstract of the Disclosure This invention is directed to the production of glass-ceramic bodies exhibiting high strength and capable of being used at temperatures of 1300°C and higher. Barium osumilite constitutes the predominant crystal phase and the inventive bodies have overall compositions consisting essentially, in weight percent, of:
SiO2 51-68 Al2O3 17.5-30 MgO 5-12 BaO 3.5-15 Si 0-1 Nb2O5 0-20 Ta2O5 0-10 TiO2 0-12 ZrO2 0-6 The bodies can be utilized in monolith form and are very useful as matrices for fiber reinforced structures suitable for high temperature applications.

Description

~eall, Chirino, ~hyung, Mar~ih ~ ~aylor ~2-4-13-25-4 4'7~
GLASS-CERAMIC ARTICLES CONT~INING OSUMILITE

The produc~ion o glass-ceramic articles had i~s genesis in United States Patent No. 2,9~0,971. As is explained in that paten~, a gla~s-ceramic article is prepared through the controlled c~ystallization i~ situ of a pr~cur~or gla~s body. Th~ preparation lnvol~es ~hree basic ste~
first, a glas~ forming batch commonly containing a nucleating or crystallization-promoting agent is mel~.ed, second, the melt is simultaneously ~ooled to a ~emperature below the transformation range thereo~ and a ylas body of a desir~d g~ome~ry shaped theref~om; and, third, the glass body i5 exposed to temperatures above the nnealing point and often above the softening point of the gla~s to generate crystals in situ. To achieve greater uniformLty in crystal ~i2et ~he parent glass may fre~uently be initially exposed to a tempera-ture somewhat above ~he transorma~ion range ~o develop a myriad of nu~lei in ~he gla~s, following which the temperature is x~aised ~o ~au~e..~he.gxo~th.of cr.~stals on khose nuclei.
Glass-ceramic products ha~e also been prepared ~y firing ~lass rits, i.e., glasses in the form o inely-divided powder~, which frequently will no~ include a nucleating agent in ~heir compositions. That is, ~urface crystallization resulting from ~e high surface area presented by t~e ~ery finely-divided glass powders is r~lied upon to promote unifor~ly fine-~rained crystallization.
I~ general, glass-ceramic article~ are de~irably highly crystalline; Patent No. 2,920,971 speciies at least 50%
crystalline. Because of this high crystallinity, ~lass-ceramic articles ~ake on phy~ical proper~ies more closely akin to those of the crystal phase than those of the parent glass. Moreover, the composition of any residual glassy matrix will be quite dissimilar from that of the precursor glass inasmuch as the components of the crystal phase will have been removed therefrom.
Because of the wide variety of physical properties that can be enjoyed in glass-ceramic products through the many different types of crystal phases which can be developed therein, glass-ceramics have found utility in such diverse applications as radomes, dental constructs, culinary ware, printed circuit boards, dinnerware, and matrices for storage of radioactive materials.
A combination of thermal stability, thermal shock resistance, and mechanical strength is vital when a material is to be subjected to severe thermo-mechanical environmentsO Dielectric requirements may also dictate that the material be essentially free from alkali metals, especially sodium.
Serial No. 380,464, filed May 20, 1982 in the names of J.J. Brennan, C.K. Chyung, and M.P. Taylor under the title GLASS-CERAMIC COMPOSITIONS OF HIGH
REFRACTORINESS, now U.S. Patent 4,415,672 issued 15 November 1983, describes glass-ceramic compositions in the Li20-MgO-~1203-Si02 s~stem which are capable of long term use at temperatures up to 1100C, and short term exposure to 1200C. Those glass-ceramics contained beta-spodumene and/or beta-quartz solid solution as the predominant crystal phase and had, as their principal application, service as matrices for SiC fiber reinforced composite bodies. It was observed in that disclosure that, in an oxidizing atmosphere, SiC fib~rs react with the matrix to deleteriously affect the strength and fracture toughness of the composite articles, primarily due ~o the oxidation of the SiC fibers with the concomitant generation of gaseous species resulting , !~

7~
in fiber strength degradation~ The matrix viscosity at a desired use temperature should be at least on the order of 1013 poises (the annealing point);
otherwise, the load transfer through the shear strength of the matrix is too low to maintain efficient reinforcement.
As was noted above, U.S. Patent 4,415,672 discloses glass-ceramic compositions suitable for extended use at temperatures up to 1100C. and brief exposures to temperatures up to 1200C. For certain applications, e.g., jet engine components, glass-ceramics sufficiently refractory to withstand long term exposures to temperatures up to 1300'sC.
would be highly desirable. Also, the capability of acting as a matrix for SiC fibers, i.e., there being essentially no reaction between the matrix and the SiC
fibers, would be an added plus. However, to satisfy that high temperature requirement, the glass-ceramic matrix must demonstrate such refractoriness subsequent to the crystallization in situ process that the viscosity of the body is at least 1013 poises at 1300C.
In addition, where Sic fiber-containing composites are envisioned, it is much to be preferred that the glass-ceramic exhibit a relatively low coefficient of thermal expansion and good sinterability so that the composite articles can be fabricated at relatively low temperatures and pressures (~ 1000C and ~ 1000 psi). ~ot only is good sinterability at relatively low temperatures desirable from the practical points of view of ease and cost of producing composites, but also higher temperatures hazard reactions taking place between the matrix and the SiC fibers.
~s,~
The glass-ceramics of U.S. Patent 4,415,672 consist essentially, expressed in terms of weight percent on the oxide basis, of:

hi20 1.5-5 A1~03 15 ~5 sio2 ~a-7s Zr2 1-5 N~205 0`10 Ta205 0-10 Nb205 ~ ~a25 1~10 MgO 0-10 Where ~hsse compositions are to be used to ~abrica~e composite articles with SiC fiber~, TiO2 will be essentially absent therefrom and 0.5-3% AS203 will be incorporated into the compo.sition. Tio2 behav~s as a flux and, hence, adversely affects the refractorine~s of the product. Furthermore, TiO2 appears to form titanium silicid2 intermetallic compounds at the interface of the S C fiber-matrix interface during formation of the composi~e body~ thereby leading to reduced fracture toughness in the composite.
Arsenic, added as As205 to the pa~ent glas~ batch, substantially improves the re~i~tance of the glass-ceramics

2~9 t~o o~,d~a~,t.~on. I~ was :hypo~h~ized that, since arsenic can exist in ~wo oxidation s~ates, viz., A~+3 and As+5, i~ acts as an oxygen bufer to trap oxygen as it migrates inwardly from the surface of the composite.
Nb205 and Ta205 enhance the re~ractory chaxacter of ~he glass-ceramics and were ~heorized to perhaps perform a~
secondary nuclean~s (ZrO~ ~eing the primary nucleant~. More importantly, ~oweJer, .Nb205 and Ta2o5 were discovered to provide in situ protection from SiC-glass interaction through the formation of NbC and~or TaC at the SiC~glass int~rface and~or the development of a very thin protective layer around ~he SiC fi~er. W~a~ever mechanism i~ involved, '~Z~ 7/f;
the NbC and/or TaC reaction product acts to restrict active oxidation of the SiC fibers at elevated temperatures and to in~ibit SiC-glass interfacial reactivity. As a result, the Nb205 and/or Ta205 content in the glass~ceramic matrix will be reduced to the extent of the carbide layer.
To secure highly crystalline bodies wherein the crystals are quite uniformly fine-grained, the compositions will contain 2-3.5% Li20, 1.5-6% MgO, and 1-3% ZrO2.

Objectives of the Invention The primary objective of the instant invention is to provide glass-ceramic products capable of extended use at temperatures up to 1300C, exhibiting relatively low coefficients of thermal expansion (<30xlO 7/oC), high mechanical strength and fracture toughness, and excellent dielectric properties (a dielectric constant~at room temperature of about 5 and a loss tangent at microwave frequencies 0.001).
Another objective is to provide such glass-ceramic bodies which display excellent mechanical thermochemical stability at temperatures up to 1300C and higher and which do not chemically react with SiC fibers, thereby permitting the bodies to serve as matrices to be reinforced through the incorporation of SiC fibers therewithin.

Summary of the Invention Those objectives can be achieved via the production of highly crystalline glass-ceramic bodies wherein a bariumosumilite constitutes the predominant crystal phase.
The natural mineral osumilite has been studied by a number of researchers; reports of two such groups are W.C. Forbes et al., Amerlcan_Mineralogist G3, page 304 (1972) and P. C~rny e~. al., Canaaian Mineralo~ist 18, page 41 (1980). The general formula for minerals of ~he osumilite (milarite) group has been stated as A2~C12Dl~T(2)3T(1)12030 where the superscrip~s refer to coordination, T(l) and T(2) denote the inter-ring and ring tetrahedral positions, and A, ~, ~, and D axe various cation sites of higher coordination located betwe~n and within double-six-membered rings, the characteristic unit of the 3tructure~
For osumili~e the ~ and D positions are empty; A is commonly illed by Mg+2~ C is partially or completely filled by Ba+2 with, optionally, X , Ca~2 and/or Sr+2; T(2) is occupied by hl+3; and T(l~ is divided into Si+4 and Al Consequently, t~e normal ~onmula for a totally-stu~fed (C-filled) Ba-osumilita is Bao.2Mgo.3~l2o3-9si~2 or BaMg2A16 9 30 and the half-stuffed equivalent is BaOg 5M~2A15silo30 Ba-osumilite contains no alkali metal and is especially well-suited to accomplish the above objectives of the instant invention since it has a low coefficient of thermal expansion, i5 highly re~ractory, has excellent dielectric properties, and can be ~ormed and ~haped as a stable glass. This latter feature is rendered possible because, a~ can be seen from t~e mineral formula, the glass-former SiO2 is the pre~ominant oxide. Moreover, unliXe cordierite (2~gO.2A1203.5SiO2), a .related ring ~ilicate, osumilite glasses do not demonstrate a large volume increase when crystallized in situ; on the contrary, a slight s~rinkage occurs.
Highly crystalline, Ba-osumili~e articles can be prepared by two general procedures~ utilizing a devitriEyin~
rrit; and C2L utilizing an internal nuclea~ing agent. In --6~

11f,?~
the first practice, a finely~divided glass powder ~fri-t) is formed, the frit ~haped i~to a desired con~igurat~on, and then fired ~o ~ssentially simult neously sin~er the powder to a solid body and de~elop crystallization. The large surface area presented by the fine particles promotes nucleation of the crystals. In the second procedure, a ~ucleating agent selected from the group of silicon me~al, TiO2 and ~b205 i dissolved in the glass. Silicon metal is the most efficient nucleant i~ promoting osumilite crystalli-zation. N~05 is less effective but can be useful in improving the overall refractoriness of the products. TiO2 is the least e~fective nucleant and re~uires careful heat treatment to achieve the desired fine~grained, highly crystalline body. ZrO2 has proven ~o be relatively inefective in nucleating the inventive compositions. The addition of a small amount of Nb205 with TiO2 and/or 2rO2 appears to significantly improve the nucleation efficiency of Tio2 and/or ZrO2. TiO2 behaves as a flux at high temperatures so large amounts thereof will be avoided. Large amounts of ~rO2 will also b~ avoided since glass melting problems can be experienced and fhe undesirable formation of 2ircon crystals in the glass-ceramic product is hazarded~ The above-described combination of good glass forming and low ~olume change during cry~allization makes highly crystalli-ne bodies containing Ba-osumilite prepared via de~i~ri~ying frita or internal nucleation comparatively easy to manu-facture.
The following compositions suitable for achieving the above objectives are based upon the discovery that M~+~ ions can su~stitute into inter-ring tetrahedral sites in t~e osumilite struc~ure customarily occupied by A1~3 ions. To secure charge balance, some of the tetrahedral ring sites normally filled by Al 3 will be replaced with Si+4. Ca+2 and Sr+2 can be substituted in part for Ba and it is believed that Ca and Zn can also replace part of the Mg in the osumilite structure. Presumably, Ca and Zn enter the A
lattice position but it is possible that Zn , which is known to occur in tetrahedral coordination, can enter the T(l) position. In addition, it is conjectured that Fe and Mn are capable of replacing some of the Mg and Fe may enter tetrahedral positions, replacing Al 3 and/or Si . Ca ions help to stabilize osumilite relative to cordierite, because it will not enter the crystal structure of cordierite.

Laboratory experimentation has demonstrated that, when glasses are crystallized having the stoichiometry of half-stuffed Ba-osumilite, viz., BaO 5Mg2A15Si~o30 cordierite, rather than osumilite, is a major phase developed. The cordierite crystallization creates two serious problems: first, because it is low in SiO2 relative to the osumilite composition and the composition of the original glass, it results in a larger proportion of residual Siliceous glass in the matrix of the final glass-ceramic which allows the body to creep at relatively low temperatures; and, second, since crystallization of cordierite produces a significant volume increase as it crystallizes from the parent glass, cracking may be hazarded, particularly in silicon-nùcleated glass ceramics where crystallization occurs when the glass is at a high viscosity.

It has been discovered that the following substitution, viz., 0.5Mg +0.5SI ~ Al , ,~

in the half~tufed osumili~e ~ruc~ure stabilizes tha~
phase relative to the undesixed cordierite. Accordingly, in glasses having the appropria~e osumilite stoichiometry, the resulting crystallization contains very little, if any, cordierite.
The completely-stuffed and half-stuffed osumilites with ~hat substitution have the following respective stoich-iometries:
BaMg~ 5AlsSi9 5030 and BaO sMg2 5A14SilO 530 It is not possible to conduct this substitution beyond the points BaMg3A14Si9030 and Ba~ 5Mg3A13Sill30, because at these stoichiometries it is dif~icult to produce highly ~rystalline bodies. It appears that only about one-half of the T(2) tetrahedral inter-ring sites can be occupied by Mg~2.
~ighly crystalline glass-ceramic bodies satisfying the above objective5 can be produced from base gla~s compositions consisting e~sentially, expressed in terms o~ weight percent on the oxide basis, of about Si0251-68 A12317.5-30 MgO5-12 BaO3.5-15 Si 0-1 Nb2050-20 Ta250-10 TiO20-12 Zr2 0-6 2~ 4 7~

In like mann2r to the de~cription of Serial No. 380,464, ~he oxidation resista~ce of the inventive materials appears to be somewhat improved through ~he inclusion of about 0.5-

3% As2O3. However, contrary to that disclosure, the presence of Nb2Os and~or Ta2o5 is not necessary in the present inventive compositions to provide in situ protection from SiC-glass interactionj i.e., their înclusion is not required to restrict active oxidation o~ the SiC fibers at elevated temperatures and to i~hibit SiC-glass interacial reactivity. Up to about 15% total of the following divalent metal oxiaes may be included in the indicated propor~ions of 0-4% CaO, 0 12%
SrQ, and 3-10% ZnO. Minor amounts of other compatible metal oxides may be added, e.g~, re~ractory metal oxides such as WO3 and MoO3 or conventional transition metal oxide or rare earth oxide colorants in customary amounts. Never~heles~, to insure the desired development of highly crystalline, Ba-osumilite bodies exhibiting exceptional use temperatures, the sum of all additions to the base quaternary system, exclusive of any nucleating agent present, will not exceed ~o about l~%. ~he alkali metal ~xide-~ will desirably b~
totally absent from the compositions.
Where a glass~cexamic article is to be crystallized in situ through internal nucleation and crystàllization of a precursor glass body, i.e.~ not formed from glass frit, the nucleants, when utilized individually, will be employed in the minimum amount~ of lQ% Nb205, 5% Tio2, and O.1% silicon me~ The~preerr~ed levels of silicon me~al vary between about Q.2-0.5%.
In those instances w~ere the crystalline body is to be prepared via firiny glass fri~ the compositions o~ the base glass will be so designed that the mole ratio Al2o3:(Mgo~Bao+sro!

~10--will most desirably be less than 1. ~owever, if CaO should be present in amounts greater than 1% by weight, then the above mole ratio may be higher; i.e., it may be equal to or even greater than unity.
The complete absence of ZnO is preferred where silicon nucleation i~ to be utilized in order to preclude a reaction between those two inyredient~ resulting in the production of SiO2 nd Zn. BaO will most desira~ly be included in amounts in excess of 5% by weight where silicon nucleation is to be used and the glass batch is melted in air or under other atmospheres which react with ~ilicon, because BaO appears to retard gaseous di~usion in the molten glass and hence, inhibits oxidation of or other reac~ion wi~h th~ silicon nuclei except at the very surface.
In general, a temperature of at least 800C has been ~ound necessary to develop the desired cry~tallization. The maximum temperature suitable for cry~tallization will be dependent upon the composition of the precursor glass.
~owever, 1325C has been deemed to constitute a practical op t~e.mpera~ure.

Prior Art United States Pa~ent No. 3,573,939 describes the production of glass-ceramic ~rticles consisting essentially of crystals selected from the group of LiTaO3, beta-spodumene solid solution, and B~Ta2o5 nucleated with Ta2O5 and, optionally, Nb2O5. The base glasses ~lerefor consisted essentially, expressed in weight percent on the oxide basis, o~ about 2-7~ L~2O, 0-2S% A12O3~ 10-60~ SiO2, and 20-80~
~a25 ~ Nb205, wherein Nb2O5 i5 pxesent in an amount up ~o 3C 20~. The base compositions and th~ final products axe far :~Z~B~G~
removed from the instant invention~ There is no mention of osumilite.
United States Patent No. 3,713,854 discloses the production of glass-ceramic articles employing 0.15-2%
by weight silicon metal as the nucleating agent. A
wide variety of crystal phases is described but nowhere is Ba-osumilite mentioned and none of the 26 working examples provided has a composition which could be crystallized in situ to yield Ba-osumilite.
United States Patent ~o. 3,839,053 is concerned with the production of glass-ceramic articles containing crystals selected from the group of zinc petalite solid solution and beta-quartz solid solution nucleated With Zr02 and, optionally, Ti02. The base compositions therefor consisted essentially, expressed in terms of weight percent on the oxide basis, of 10-20% ZnO, 12-20% A1203, 1-10%
Ta205, 50-65% Si02, and 2-8~ Zr02. ~here is no mention of Ba-osumilite and the compositions are quite remote from the instant invention.

Description of Preferred Embodiments _ Table I records glass compositions, expressed in terms of parts by weight on the oxide basis, which, when crystallized in situ, will contain Ba-osumilite with or without other crystal phases. Inasmuch as the sum of the components totals or closely approximates 100, for all practicai purposes the levels reported may be deemed to reflect weight percent. The actual batch ingredients may comprise any materials, either the oxides or other compounds, which, when melted together, will be converted into the desired oxides in the proper proportions.

Th~ batch ingredients were compounded, ballmilled together to aid in obtaining a homogenesus mel~, and depo~ited in~o silica or alumina cru~ible~. The silicon metal waR
added as a powder finer than a No. 100 United States Standard Sieve (149 microns). The crucibles were covered, introduced into a rurnace operating at abou~ 1600C, and the batches melted or about 6 hours. Glass bodies were prepared from the melts in two ways. Ex~mples 1-9, containi~g no silison metal, were poured as a relatively fi~ stream in~o a container of water ~o produce finely-divided par~icles o glass which, after drying, were comminuted to form a frit passi~g a No. 325 United States Standard Sieve (44 microns).
ExampLes 10-14 and 16-18, containing silicon metal as a nucleant, were poured into steel molds to yield slabs having dimensions of about 8"xlO"xl" and those slabs were immediately trans~erred to an annealer operating at about 820C. Example~ 19-27, containing TiO2, ZrO2, and/or Nb205 as nucleants, were poured into s~eel molds to form sla~s about 6"x6"x0.5", and those slabs were transferred to an annealer operating a~ about 800 C. Cylinders and cones of glass about 0.25" in ~hicknes~ were centrifugally cast and pressed from Example 15. It will be appreciated that the above-described melting and forming procedures are illus trative of lahoratory practice and that the compo~itions of Table I can be melted and ~orme~ utili~ing commercial glass melting and forming tec~niques.

Tab:Le I
~Parts by Weight) SiO253 . 6 51. 8 64 . 452 . 9 55 . 6 67 . 554 . 5 51. B 52 q 0 A12O318 . 223 . 223 ~ 523 . 6 18 . 9 18 . 326 . 8 27 . 5 27 . 5 MgO 7.2 7.3 8.2 ~.3 11.2 10.3 5.7 5.6 6.6 BaO13 . 5 14 . 0 3 . 914 . 2 14 . 2 3 . 910 . 412 . 5 . 12 . 3 CaO ~ 2.7 2.6 1.3 ZnO 7 . 3 3 . 7 ~0 ll l2 ~ ~14 15. 16 l7 18 SiO259 . 354 . 8 52 . 26~ . 2 65 . 3 Zl. 255 . 9 56 . 7 52 . 2 A120325. 226 . 8 27 . 620 . 2 19 0 0 20 . 622 . 6 24 . :1 27 . 6 MgO 8.0 7.7 7.6 lû.0 10.0 9.9 9.5 8.8 7.6 BaO 7.6 11.0 12.6 7.6 5c7 8.3 12.0 10.4 12.6 Si 0.4 0.4 0.4 0.2 0.~ 0~ 0.2 0.2 0.2 19 20 21 22 23 24 25 2~ 27 . . _ _ _ SiO250.0 50.0 S0.050.0 57.4 57.4 59.3 59.3 62.3 A12O328 . 328 . 3~8 . 328 . 3 22 . 0 22 . û25 . 2 25 . 2 20 . 2 MgO 7.5 7.5 7.5 7.5 9.4 9.4 8.0 8.0 10.0 BaO 14.2 14.2 14.214.2 11.0 11.0 7.6 7.6 7.6 TiO2 5.0 600 - 5.0 - 6.0 6.0 6.0 8.û
Zr2 ~ 4.0 3.0 - _ 2.0 6.0 Nb2O5 - lO.û 5.0 20.0 5.0 200 2.0 A52O30.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.B

Bars of the proper ~ize for conducting measurements of coeficient of thermal expansion were cold pressed from Examples 1-9 and bars of like size were cut from the annealed slabs of Examples 10-14 and 16-lB. Those test specimens, plus the cylinders and cones of Example 15 were moved to an electrically-fired furnace and subjected to the heat treatment schedules recorded in Ta~le II. The temperature of each sample was raised at a rate of about 5C/minute to the levels cited. Upon complet~on of ~he heat ~reatment, the electric current to the furnace was cut off and the crystallized specimens left inside ~he furnace and allowed to cool to room témperature thereon. That pr~ctice, termed "cooling at furnace rate", has been estimated to average about 3-sC/minute.
Table II also report~ a visual description of the original glacs and ~he crys~alline body, an identification of ~he crystal phases present in each as determined via X-ray diffraction analyses (the phase listed first being predominant), and a measurement of the coefficient of ther~alj~ex~ansi~n (xlO 7/C) of each over a particular range of temperatures.

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~r ~n ~d ,~ 1 8 -~2~ 76 The proportion o~ cordierite is high in Example 17 and is predominant in ~xamples 18, 26 and 27. The cracking observed in Examples 17 a~d 18 illustrates the phenomenon that cordierite produoes expansion as it crystallizes from glass, especially in silicon-nucleated glass-ceramic bodies.
A comparison of Example 1~ with Example la, however, is o i~terest. The base composition of each is identical ~ut the heat treatment of Example 12 qs at a considerably lower temperature and the amount of nucleant therein is twice that of Example 18. Those two factors pxo~ided a crack-fxee, crystallized artic~e wherein Ba-osumilite, rather than cordierite, ccnstituted the p~e~ominant phase. A comparison of ~xample 25 with Example 26 demonstrates that the effect of high nucleant content is to develop cordierite in favor of Ba-osumilite. The developmen~ of cordierite in Example 27 is ~elieved to indica~e ~he ~fficiency of TiO2 as a nucleant~
Overall, the final products were highly crystalline, i.e., greater than 50~ by volume and fre~uently in excess of 90%. The dielectriç constant and loss tangent measured a~
Z 25C a~na ~t 8.6 GHz were 5.3 and 0.0003, respectively. Those electrical properties strongly recommend ~heir utility in the fabrication of radomes.
Table ITI lists another group of precursor glass compositions, expressed in terms of parts by weight on the oxide basis, illustrating further features of the inventive materials. Again, the sum of the individual components totals or closely approximates la0 so the values presented can reasonably be considered to reflect weight percent.
Examples 27-3Q and 36-38 are based upon fully-stuffed Ba-osumilite; Examples 31-35 represent half-stuffed Ba-osumilite;
Example 3g reflects 3/4-s~uffed Ba-osumilite; and Example 40 is a barium-containing cordierite.

~19--The batch ingredients were compounded~ ~allmilled tog~her, melted, and the melts rit~2~ in like manner to Examples 1-9 above. Cylinders having a diameter o~ 0.5"
were prepaxed by cold pressing the frit at 1000 psi to yield an unfixed density of about 50%.

~able III
, ~iQ249.7 48.2 47.3 5~.9 57.2 56.1 62.360.8 A12O328.127~3 26.8 25o0 ~4.3 23.8 20.22~.7 -10 MgO 7.4 7.2 7.1 7.9 7.7 7.5 10.0 9.0 BaO 14.1 13.7 13.4 7.5 7.3 7.2 7.6 7.6 AS2O30.7 0.7 0.7 0.7 0.7 0.7 - -Nb2O5 - 3. ~ - 3-Ta~O5 - - 5.0 ~ ~ 5-0 ~ -SiO2 51.4 50.0 52.8 51.4 41.3 A~2O3 26.0 28.3 23.6 25.3 35.1 MgO 8.4 7.5 9.3 7.3 12.3 BaO 1~.2 14.2 14.2 10.4 5~7 523 - - _ 0 9 0 7 l~b25 ' - - -Ta25 - - _ Si _ 0 4 ~2~ 6 Table IV reports heat treatments ~o which ~he above-de~cribed cylinders were subjected. Good sinterability is of interes~ because i~ renders fabrication of articles at relatively low temperatures possible (~loooC at a pressure ~1000 psi). Also, exposures to high temperature hazard reactions occurring between the glass-ceramic matrix and reinforcing elements embedded therein. Table IV records shrinkages (% linear) obsen7ed in the cylinders of Examples 28~33, 39, and 40 af~er ~iring a~ 900C and 1000C fQr 0.5 hour. Each specimen was then ~ired at 1200C for one hour to cause crystallization in situ. Table XVA report ~hrinkages (% linear) notad in the cylinders of Examples 34-38 after firing at 1025C for 0.25 hour. Thereaft~r, each specimen was fired at 1260C ~or 0.25 hour to effect crystallization in situ.
The precursor glasses were essentially clear and color-less and the glass-ceramics were white. The crystal phases present in ach sample, as identifie through X-ray diffrac-tion analysis, are also reported in Table IV, the phases being recorded in the order of amount present. Some cracking was observed i~ the crystalline product of Examples 31-35 and rather extensive cracking in Example 40. The composi-tions again illus~rate ~hat cordierite prodùces expansion as it crystallizes from a glass.
No cracklng was discerned in Examples ~8-30, 37 a~d 38 (fully-stuffed Ba~csumilite gla~-ceramics) and Example 39 (3~ u fed Ba-osumilite glass-ceramic~. No distortion was noted when ~odies of those stoichiometr1es were exposed to temperatures of 1300~C.

~LZ~ 76 Table IV
% Shrinka~e% Shrinkage Example900C o 0. 5 hour1000C - 0. S hotlrCrystal Phases 28 8.3 15.8 Ba-osumilite ' Cordierite Celsian 29 14.2 15.9 Ba-osumilite 5:~ordierite ` . Celsian 11.7 16.5 Ba-osumilite Cordierlte Celsian 31 6 . 7 16 . 5 t~ordierite Ba-osumilite Glass 3~ 9 . 5 16 . 7 Cordierite Ba-osumil ite Glass 33 5.1 16t 9 Cordierite Ba-osumilite Glass 39 10 .1 16 . 0 Ba-osumilite Cordierite 16.1 16.1 Cordierite Glass Table IVA
_ Example ~ Crystal Phases 34 15.6Ba-osumilit~, Cordierite 15.4Cordierite, Baosumilite 36 14.0Ba-osumilite, Cordierite 37 14 ~ O Ba osumilite 38 14.0 Ba-osumilite :*
~2'~
As can be discerned from Tables IV and IVA, the linear shrinkage ranges from 14.0-16.9~, thus indicating substantially complete densification. 'rhis study of the shrinkage character of the inventive materials signifies that 900-1000C would comprise an operable hot pressing temperature range.

It will be appreciated that the inventive materials readily lend themselves to conventional hot pressing techniques. Hence, a range of temperatures and pressures can be contemplated, the critical criterion being that the forming is undertaken at a temperature at which the glass exhibits a viscosity between about lO -10l2 poises. As is well-recognized, the forming pressure required to shape a body will be greater where the viscosity of the glass is greater. Stated in another manner, as the temp~rature of the glass is raised, the load applied for pressing can be reduced.

To illustrate the utility of the inventive materials as matrices for reinforcing elements, about 33.3% by weight glass powders of Examples 30, 33, and 40 were mixed into a solution consisting of about 60-67% by weight H20, 29-36~ triethylene glycol, and

4% by weight polyvinyl alcohol to form a slurry. A
yarn of SiC fibers was immersed into the solution to achieve impregnation and then wrapped on a drum to form a mat having unidirectional fiber orientation.
The mat was dried at room temperature, three-inch diameter discs cut therefrom, the discs stacked into a graphite mold, and the organic binder burned out in a furnace operating at about 350C and having a reducing atmosphere of forming gas. (It will be appreciated that other organic binders and burnout procedures can be satisfactorily utilized.) / ~

~LZ~ 7~
The mold was then transferred to a resistance-heated press ancl consolidated at the sintering temperature reported in Table V utilizing a 1500 psi uniaxial load and a vacuum environment. The heating schedule involved: rapid heat up ( ~, 43C/minute) to 650C, hold for 15 minutes, heat up to the sintering temperature at about 5C/,minute, the load being applied when a temperature of about 800C was reached, hold at the sintering temperature for 30 minutes; fill the press chamber with argon and cut off the electric current to permit cooling at furnace rate ( ~, 3-5C/minute), the load being raleased when the temperature reached about 800C.

Similar SiC fiber reinforced discs were prepared from Examples 34-38 in like manner to the above description. However, a different SiC yarn was employed and no load was applied to the press until after the mold had reached the maximum temperature for about five minutes.

In both practices, the discs were extricated from the molds, ground flat and parallel, and then cut into about 0.2" wide strips parallel to the direction of the fibers. Strips of several discs were exposed to a second heat treatment in air to investigate the effect thereof upon the mechanical strength of the composite. Modulus of rupture measurements were undertaken on each sample using a three-point bend test apparatus. Failure modes were classified as brittle (planar fracture surface, no fiber pull-out), woody (splintery fracture surface, ~Oi5 mm fiber pull-out), or brushy (individual fibers exposed, ' 0.5 mm fiber pull-out). The brushy failure mode correlated with the highest fracture energy.

Compositions 30, 33, and 40, represent composite articles containing 20-25% by volume of SiC fibers, whereas the . ,, ~

~ 6 composite bodies prepared from Examples 34-38 contained abou~ 40% by volume SiC ibers. Increasing the volume percentage of fibers can be expected to increase the mechanical strength of the composite.

- Table V

Pre~sing Additional Modulus of Example ~ Heat Tre~tment ~ Failure loonC - 41,000Brushy 1000C5C/min. to 1200C 51,000Brushy Hold for l hour 1000C5C/min. to 1320C 66,0008rushy H~l~d ~r ~-hours 33 960C - 24,000Brittle 33 960CImmediate exposure 26~000Brittle to 1200C
Hold ~ox 1 hour - 40 960C - 25,000Brittle, irregular 969CImmediate gxposure 15,000Brittle, to lO00 C irregular ~old for l hour 960C5C~min~ to 1000C 30,000Wood~
- Hsld or 1 hour 34 135GC - 52,100Brushy 1350C 40,900Woody 36 1350C - 39,000Brittle 37 1400C 36,000Brittle 38 1350C ~ 36,400Brittle As can be observe~ from Table V, both the half~stuffed barium osumilite matrix CExample 33) and the cordierite matrix CExample 40) produced relatively waak, hrittle com-posites in the aR-pressed state.. There was no visible evide~ce of fiher degradation and it is believed that the brittleness is due to very tight bonding between the ~iber -25~

4~

and matrix. The mechanism underlyi~g that tight bondi~g has not been fully explained, but has been th~orized to be either the resul~ of a re~ction between ~he fiber and the matrix or the thermal expansion mismatch existing between the fiber and the matrix. Each of the composites of Examples 33 and 40 exhi~ited some bloating and distortion after the suhsequent heat treatmen~. It was no~ed, however, that the programmed heating appeared to result in a less brittle fracture than when the specimen was plunged into a preheated furnace.
In contrast, the fully-stuffed Ba-osumilite composite article (Example 30) displayed moderate mechanical strength in the as-pressed state with a very tough failure and a bru~hy fracture surface. No bloating or distortion was noted, although a minor amount of bubbly glaze formed on thP
~urface of the article during the 1320~C heat ~reatment.
The str~ngth of the composite improved significantly after each ~upplemental hea~ ~rea~ment and the tough failure mode and ~rushy fracture surface were not altered by the subse-~uent heat treatments. In sum, Example 30 illustrate~ thatfully-stuffed Ba-osumilite glass-ceramic bodies can be employed as matrices for SiC fibers for use in applications ~ubject to temperatures of 1300C and higher.
Another advantage which the Ba-osumilite compo~itions di~play over cordierite compositions is the broader temperature range wherein good working (melting and fonminy) viscosities can b ob'ained.
The SiC yarn employed with Examples 34-38 contained a high proportion of oxygen~ viz., up to a~out 17 mole percent.
That factor is ~elieved to have contributed to the genexal brittleness and weakness of the composites.

'7~i Composite articles may be prepared from carbon (graphite) fiber mats and frits capable of being converted to fully-stuffed Ba osumilite glass-ceramics in like manner to that described above With SiC
fibers. Thus, similarly to the situation with respect to SiC fibers, it is believed that essentially no reaction occurs between the graphite fibers and the inventive glass-ceramic materials. TiO2 will preferably be essentially absent from the g~ass-ceramic compositions when SiC fibers constitute the reinforcing elements. TiO2 is believed to form titanium silicide intermetallic compounds at the interface of the SiC fiber-matrix interface, thereby resulting in reduced fracture toughness in the composite. No such reaction occurs with TiO2 and carbon fibers. ~bout 5-8% TiO2 will perform ~ery satisfactorily to nucleate a matrix containing carbon fibers.

Claims (17)

WE CLAIM:

1. A glass-ceramic body exhibiting high strength and a use temperature up to 1300°C and higher, wherein Ba-osumulite constitutes the predominant crystal phase, consisting essentially, expressed in terms of weight-percent on the oxide basis, of SiO2 51-68 Al2O3 17.5-30 MgO 5-12 BaO 3.5-15 Si 0-1 Nb2O5 0-20 Ta2O5 0-10 TiO2 0-12 ZrO2 0-6

2. A glass-ceramic body according to claim 1 also containing up to 15% total of the following materials in the indicated proportions of 0-3% As2O3, 0-4% CaO, 0-12% SrO, and 0-10%
ZnO.

3. A glass-ceramic body according to claim 1 which is crystallized in situ through internal nucleation and crystal-lization of a precursor glass body containing at least one nucleant selected from the group in at least the indicated minimum proportion of 0.1% Si, 10% Nb2O5, and 5% TiO2.

4. A glass-ceramic according to claim 2 prepared by firing a finely-divided, precursor glass frit wherein the mole ratio Al2O3:(MgO+CaO+ZnO+BaO+SrO)<1.

5. A glass-ceramic body according to claim 2 prepared by firing a finely-divided, precursor glass frit containing more than 1% CaO and wherein the mole ratio Al2O3:(MgO+BaO+SrO) may be ?1.

6. A method for making a glass-ceramic body exhibiting high strength and a use temperature up to 1300°C and higher, wherein Ba-osumilite constitutes the predominant crystal phase, comprising the steps of:
(a) melting a batch for a glass consisting essentially, expressed in terms of weight percent on the oxide basis, of SiO2 51-68 Al2O3 17.5-30 MgO 5-12 BaO 3.5-15 Si 0-1 Nb2O5 0-20 Ta2O5 0-10 TiO2 0-12 (b) simultaneously cooling said melt and forming a glass body of a desired configuration therefrom; and (c) exposing said glass body to a temperature between about 803°-1325°C for a period of time sufficient to crystallize said body in situ.

7. A method according to claim 6 wherein said glass also contains up to 15% total of the following materials in the indicated proportions of 0-3% Al2O3, 0-4% CaO, 0-12% SrO, and 0-10% ZnO.

8. A method according to claim 6 wherein said glass body is internally nucleated and crystallized in situ to a glass-ceramic, said glass containing at least one nucleant selected from the group in at least the indicated minimum proportion of 0.1% Si, 10% Nb2O5, and 5% TiO2.

9. A method according to claim 7 wherein said glass body is finely divided glass frit having a mole ratio Al2O3:
(MgO+CaO+ZnO+BaO+SrO) ?1.

10. A method according to claim 7 wherein said glass body is finely-divided glass frit containing more than 1% CaO
and wherein the mole ratio Al2O3:(MgO+BaO+SrO)?1.

11. A carbon and/or silicon carbide fiber reinforced glass ceramic composite exhibiting high strength and a use temperature up to 1300°C and higher comprising carbon and/or silicon carbide fibers implanted within a glass-ceramic matrix, said matrix consisting essentially, expressed in terms of weight percent on the oxide basis, of SiO2 51-68 Al2O3 17.5-30 MgO 5-12 BaO 3.5-15 Si 0-1 Nb2O5 0-20 Ta2O5 0-10 ZrO2 0-4 TiO2 0-12

12. A composite according to claim 11 wherein said matrix also contains up to 15% total of the following materials in the indicated proportion of 0-3% Al2O3, 0-4% CaO, 0-12% SrO, and 0-10% ZnO.

13. A composite according to claim 11 wherein said matrix is crystallized in situ through internal nucleation and crystallization of a precursor glass body containing at least one nucleant selected from the group in the indicated minimum proportions of 0.1% Si, 10% Nb2O5, and 5% TiO2.

14. A composite according to claim 12 wherein said matrix is prepared by firing a finely-divided, precursor glass frit wherein the mole ratio Al2O3:(MgO+CaO+ZnO+BaO+Sr)<l.

15. A composite according to claim 12 wherein said matrix is prepared by firing a finely-divided, precursor glass frit containing more than 1% CaO and wherein the mole ratio Al2O3:(MgO+BaO+SrO) may be ?1.

16. A composite according to claim 12 demonstrating oxidation resistance up to 1300°C and higher wherein said matrix contains 0.5-3% As2O3.

17. A composite according to claim 11 reinforced with carbon fibers wherein said matrix contains 5-8% TiO2 as a nucleating agent.