The instant invention is directed toward the production of an alkali metal-free glass-ceramic body wherein the predominant, and preferably sole, crystal phase is a calcium fluorophlogopite. The body is strong, chemically durable, exhibits excellent electrical properties, and has a base composition consisting essentially, expressed in weight percent on the oxide basis, of about PA0 CaO: 5-20 PA0 MgO: 15-25 PA0 Al.sub.2 O.sub.3 : 5-20 PA0 SiO.sub.2 : 35-60 PA0 F: 5-15 which is nucleated with 0.5-3.5% BaO+SrO, consisting of 0-3.5% BaO and 0-2.5% SrO, or, if BaO and/or SrO are absent, with 8-15% TiO.sub.2. The preferred products exhibit a white, translucent appearance and excellent resistance to staining.

BACKGROUND OF THE INVENTION 
The production of glass-ceramic articles containing synthetic micas as the 
predominant crystal phase is well known to the art. Hence, whereas 
naturally occurring micas are typically hydroxyl silicates, micas formed 
synthetically have customarily involved replacing the hydroxyl group 
within the crystal lattice with fluorine. Those crystals, often termed 
fluormicas, have been developed in glass-ceramic articles and, although 
such fine-grained, polycrystalline articles do not exhibit the single 
crystal capability of flexibility, they can, however, demonstrate 
excellent dielectric properties, thermal stability, and mechanical 
machinability. 
In general, the structure of fluormica has been deemed to be defined by the 
postulated structural formula X.sub.0.5-1 Y.sub.2-3 Z.sub.4 O.sub.10 
F.sub.2, wherein X designates cations of relatively large size, i.e., 
having an ionic radius of about 1.0-1.6 A, Y represents somewhat smaller 
cations, i.e., having an ionic radius of about 0.6-0.9 A, and Z signifies 
small cations, i.e., having an ionic radius of about 0.3-0.5 A, which 
coordinate to four oxygens. In general, the X cations will normally be 
potassium, but other large alkali metal ions such as Na.sup.+, Rb.sup.+, 
and Cs.sup.+ and, more rarely, alkaline earth metal ions such as 
Ca.sup.+2, Sr.sup.+2, and Ba.sup.+2 may be substituted in whole or in part 
for the potassium ions. The Y cations will commonly be selected from the 
group of Mg.sup.+2, Li.sup.+, and Al.sup.+3 ions, and the Z cations will 
be selected from the group of Si.sup.+4, Al.sup.+3, and B.sup.+3 . 
The development of glass-ceramic bodies capable of being shaped utilizing 
hand and machine tools was disclosed in U.S. Pat. No. 3,689,293. Those 
mechanically machinable glass-ceramic bodies contained synthetic fluormica 
crystals and consisted essentially, expressed in terms of weight percent 
on the oxide basis, of about 25-60% SiO.sub.2, 15-35% R.sub.2 O.sub.3, 
wherein R.sub.2 O.sub.3 consists of 3-15% B.sub.2 O.sub.3 and 5-25% 
Al.sub.2 O.sub.3, 2-20% R.sub.2 O, wherein R.sub.2 O consists of 0-15% 
Na.sub.2 O, 0-15% K.sub.2 O, 0-15% Rb.sub.2 O, and 0-20% Cs.sub.2 O, 4-20% 
F, and 6-25% MgO+Li.sub.2 O, consisting of 4-25% MgO+0-7% Li.sub.2 O. 
X-ray diffraction analyses of those products indicated that the basic mica 
structure consisted of a fluorophlogopite solid solution, this solid 
solution being posited to be encompassed within three components, viz., 
normal fluorophlogopite, KMg.sub.3 AlSi.sub.3 O.sub.10 F.sub.2, boron 
fluorophlogopite, KMg.sub.3 BSi.sub.3 O.sub.10 F.sub.2, and a subpotassic 
aluminous phlogopite thought to approximate K.sub.0.5 Mg.sub.2 Al.sub.0.83 
BSi.sub.3 O.sub.10 F.sub.2. 
U.S. Pat. No. 3,756,838 describes the preparation of glass-ceramic articles 
wherein the predominant crystal glass is an alkali metal-free fluormica. 
The articles consist essentially, expressed in terms of weight percent on 
the oxide basis, of about 30-65% SiO.sub.2, 5-26% Al.sub.2 O.sub.3, 10-35% 
MgO, 3-30% RO, wherein RO consists of 3-30% SrO and 0-25% BaO, and 3-15% 
F. The patent notes that up to several percent individually of a number of 
metal oxides may optionally be included, but the total of all such 
additions will not exceed 10% by weight. Those additions were selected 
from the group of As.sub.2 O.sub.3, B.sub.2 O.sub.3, BeO, CaO, Fe.sub.2 
O.sub.3, La.sub.2 O.sub.3, MnO, PbO, P.sub.2 O.sub.5, Sb.sub.2 O.sub.3, 
SnO.sub.2, TiO.sub.2, ZnO, and ZrO.sub.2. K.sub.2 O, Rb.sub.2 O and 
Cs.sub.2 O will be avoided because of their ready substitution for BaO and 
SrO. 
The products are mechanically machinable and contain fluormica solid 
solutions varying over the range of RMg.sub.2.5 AlSi.sub.3 O.sub.10 
F.sub.2 and R.sub.0.5 MgAlSi.sub.3 O.sub.10 F.sub.2. It was observed that 
the presence of Sr.sup.+2 ions was necessary in the initial batch to 
stabilize precursor glass formations. Thus, the complete substitution of 
Ba.sup.+2 ions for Sr.sup.+2 ions causes the melt to quickly and 
spontaneously devitrify as it is being cooled. Where less than 5% BaO is 
included in the compositions, the glass-ceramic bodies swell when 
contacted with water, leading to subsequent disintegration. Moreover, 
Sr.sup.+2 -containing fluormicas and their intermediates with Ba.sup.+2 
additions are quite prone to develop cracks as the precursor glass body is 
crystallized in situ during heat treatment thereof. Thus, the articles 
almost invariably develop concentric cracks, the origin of which is not 
well understood. A study of the crystallization process at progressive 
stages has augured the hypothesis that a crystallizing front advances from 
the side of the glass body toward the center, causing the concentric 
cracks to appear due to density differences between the mica crystals and 
the residual glass. 
Yet, because of the absence of alkali metals from the compositions, the 
electrical properties of the alkaline earth metal fluormica glass-ceramics 
are far superior to those glass-ceramics containing "conventional" 
fluorophlogopite crystals. Furthermore, the mechanical strengths of the 
alkaline earth metal fluormica glass-ceramics appear to be consistently 
greater than those demonstrated by those glass-ceramics containing alkali 
metal fluormicas. 
OBJECTIVES OF THE INVENTION 
The primary objective of the present invention is to provide an essentially 
alkali metal-free, alkaline earth metal-containing fluormica glass-ceramic 
body which is highly crystalline, is machinable with hand and machine 
tools, and exhibits high mechanical strength, good resistance to attack by 
acids and alkalies, and excellent dielectric properties. 
Another objective is to provide a method for preparing such a glass-ceramic 
body. 
A specific objective is to provide such a glass-ceramic body wherein the 
crystals are very fine-grained such that the body exhibits a white, 
translucent appearance and excellent resistance to staining such as to be 
aesthetically pleasing and of practical utility in dental restorations. 
SUMMARY OF THE INVENTION 
Those objectives can be achieved in a highly crystalline, glass-ceramic 
article wherein the predominant, and most preferably essentially the sole, 
crystal phase is a calcium fluorophlogopite. Thus, the inventive 
glass-ceramic article will be substantially alkali metal-free and have an 
overall base composition, expressed in terms of weight percent on the 
oxide basis, of about 35-60% SiO.sub.2, 5-20% Al.sub.2 O.sub.3, 15-35% 
MgO, 5-20% CaO, and 5-15% F which is nucleated with 0.5-4.0% SrO+BaO, 
consisting of 0-3.0% SrO and 0-4.0% BaO, or, if SrO and/or BaO are absent, 
with 8-15% TiO.sub.2. 
In preparing the inventive article, a batch of the necessary ingredients to 
produce a parent glass of a desired composition is melted, the melt is 
simultaneously cooled to a temperature at least within the transformation 
range and a glass article of a desired configuration shaped therefrom, and 
the glass article is exposed to temperatures within the interval of 
800.degree.-1100.degree. C. for a sufficient period of time to effect 
crystallization in situ within the articles. The transformation range has 
been defined as that temperature at which a liquid melt is deemed to have 
been converted into an amorphous solid, that temperature customarily being 
considered to lie in the vicinity of the annealing point of the glass. 
Where desired, the precursor glass article may be cooled to room 
temperature for inspection prior to being heat treated to develop 
crystallization therein. Because the crystallization heat treatment is 
time and temperature dependent, only brief dwell periods will be required 
at the upper extreme of the temperature range, e.g., 0.25 hour; whereas, 
at temperatures near the lower end of the range, much longer exposure 
periods may be necessary to promote high crystallinity, e.g., 24 hours and 
more. 
The preferred heat treatment practice comprehends a two-step process. 
Hence, the parent glass article is first heated to a temperature somewhat 
above the transformation range thereof, viz., about 
600.degree.-700.degree. C., and held within that temperature zone for a 
period of time sufficient to insure substantial nucleation and initiate 
incipient crystal development. Thereafter, the nucleated glass article is 
heated to a temperature between about 1000.degree.-1100.degree. C. and 
maintained within that interval for a sufficient length of time to 
generate growth of crystals on the nuclei and achieve high crystallinity. 
A glass prepared from a composition having the approximate stoichimetry of 
calcium fluorophlogopite is extremely stable against devitrification. 
Thus, the glass does not nucleate internally and, upon heating to 
temperatures approaching the softening point of the glass for extended 
periods of time, crystallization begins at the surfaces of the glass 
article and grows inwardly in an oriented fashion to meet at the center of 
the article. 
The stability of the glass is such that an addition of at least 8% 
TiO.sub.2 and, most preferably, at least 10% TiO.sub.2 is demanded to 
provide satisfactory nucleation. The incorporation of such large 
quantities of TiO.sub.2 leads to the growth of rutile crystals in the 
final product which dilute the desired physical properties inherently 
imparted via the calcium fluorophlogopite crystals. However, the inclusion 
of only about 0.5% BaO and/or SrO is adequate to yield well-nucleated, 
translucent, fine-grained, highly crystalline glass-ceramic bodies. 
Accordingly, the preferred products utilize BaO and/or SrO nucleation. 
In the BaO and/or SrO nucleated products, the crystallization, as 
investigated via X-ray diffraction analyses, consisted of essentially a 
single phase, viz., calcium fluorophlogopite. The crystals exhibited an 
aspect ratio of about 1:2, as viewed utilizing replica electron microscope 
techniques. X-ray diffraction patterns and electron microscope 
examinations determined the essential absence of any secondary crystal 
phase and the amount of residual glass was estimated to not exceed about 
10% by volume. 
Where heat treating temperatures in excess of 1100.degree. C. are used, 
partial melting and crystallization of the fluormica phase take place. 
Hence, the total crystal content decreases significantly with a consequent 
substantial increase in glass phase, and X-ray diffraction analyses 
identified the substantial presence of norbergite crystals (Mg.sub.2 
SiO.sub.4.MgF.sub.2).

DESCRIPTION OF PREFERRED EMBODIMENTS 
Table I reports a number of glass compositions, expressed in terms of parts 
by weight on the oxide basis, which, when heat treated in accordance with 
the parameters of this inventive method, were crystallized in situ to 
relatively highly crystalline, glass-ceramic bodies. The actual batch 
ingredients may comprise any materials, either the oxides or other 
compounds, which, when melted together, will be converted into the desired 
oxide in the proper proportions. Because it is not known with which 
cation(s) the fluoride is combined, it is simply reported as MgF.sub.2, 
the batch ingredient utilized to supply the fluoride. It will be 
appreciated, of course, that other fluoride compounds, e.g., AlF.sub.3, 
can be employed to furnish the desired fluoride content. Inasmuch as the 
sum of the individual constituents totals or approximately totals 100, for 
all practical purposes the values recited in Table I may be considered to 
reflect weight percent. 
The batch materials were compounded, ballmilled together to assist in 
achieving a homogeneous melt, and thereafter melted in closed platinum 
crucibles for about 5 hours in a furnace operating at about 1450.degree. 
C. The melts were poured into steel molds to form glass slabs having 
dimensions of about 6".times.6".times.0.5" and the slabs immediately 
transferred to an annealer operating at about 600.degree. C. Visual 
observation of the slabs showed them to be essentially clear. 
Volatilization of fluoride from the melts was determined to be relatively 
low, i.e., less than about 15%. 
TABLE I 
______________________________________ 
1 2 3 4 5 6 
______________________________________ 
SiO.sub.2 
44.0 43.6 43.5 42.4 41.4 40.5 
Al.sub.2 O.sub.3 
12.5 12.3 12.3 12.0 11.7 11.5 
MgO 18.7 18.6 18.5 18.1 17.8 17.3 
MgF.sub.2 
16.8 16.5 16.5 16.1 15.7 15.4 
CaO 5.6 5.6 6.8 6.6 6.5 6.3 
SrO 2.5 -- -- -- -- -- 
BaO -- 3.4 -- -- -- -- 
TiO.sub.2 
-- -- 2.4 4.7 6.9 9.0 
______________________________________ 
7 8 9 10 11 
______________________________________ 
SiO.sub.2 
44.3 44.0 43.8 44.4 43.9 
Al.sub.2 O.sub.3 
12.6 12.5 12.4 12.6 12.5 
MgO 18.9 18.8 18.7 18.9 18.7 
MgF.sub.2 
16.8 16.7 16.6 16.8 16.7 
CaO 6.7 6.4 6.1 6.5 6.7 
SrO -- -- -- 0.84 -- 
BaO 0.85 1.7 2.6 -- 1.7 
______________________________________ 
After annealing and visual inspection for glass quality, the slabs were 
introduced into an electrically-fired furnace and subjected to the 
following heat treatment schedule: heated at about 5.degree. C./minute to 
625.degree. C., maintained at that temperature for four hours, heated at 
about 5.degree. C./minute to 1000.degree. C., held at that temperature for 
four hours, and then cooled at furnace rate to room temperature. Cooling 
at furnace rate contemplates cutting off the electric current to the 
furnace and allowing the furnace to cool to room temperature with the 
crystallized article retained therewithin. This rate has been estimated to 
average 3.degree.-5.degree. C./minute. 
Table II reports a visual description of the glass-ceramic body, the 
crystal phases present therein as identified through X-ray diffraction 
analyses, a qualitative appraisal of the mechanical strength of each 
article, and modulus of rupture values where measured. All of the 
crystallized bodies were translucent-to-opaque white and demonstrated 
excellent machinability. 
TABLE II 
__________________________________________________________________________ 
Example 
No. Visual Description 
Crystal Phases 
Strength 
Modulus of Rupture 
__________________________________________________________________________ 
1 Fine-grained, 
Fluormica 
Fairly -- 
translucent Strong 
2 Fine-grained, 
Fluormica 
Strong 21,000 psi 
translucent 
3 Coarse-grained, 
Fluormica + 
Quite -- 
opaque white 
minor rutile 
Weak 
4 Coarse-grained 
Fluormica + 
Weak -- 
opaque white 
rutile 
5 Coarse-to-medium- 
Fluormica + 
Stronger than 
-- 
grained, opaque white 
rutile 4 
6 Medium-grained, 
Fluormica + 
Stronger than 
12,000 psi 
opaque white 
rutile 5 
7 Fine-grained, 
Fluormica 
Rather 17,000 psi 
translucent Strong 
8 Fine-grained, 
Fluormica 
Strong -- 
translucent 
9 Fine-grained, 
Fluormica 
Strong -- 
translucent 
10 Fine-grained, 
Fluormica 
Strong -- 
translucent 
11 Fine-grained, 
Fluormica 
Strong -- 
translucent 
__________________________________________________________________________ 
In Table II, the term "fine-grained" designated crystals having average 
diameters of less than about one micron. "Medium-grained" signifies 
crystal diameters in the range of about one to five microns. 
"Coarse-grained" indicates crystal diameters greater than about five 
microns. It will be observed that the translucency and strength of the 
crystallized articles are directly related to the size of the crystal, the 
coarser-grained bodies exhibiting lower strengths. The presence of 
coarse-grained crystals is due to poor nucleation. Hence, the need for at 
least about 8% TiO.sub.2. A modulus of rupture at least 12,000 psi has 
been deemed a desirable minimum with levels in excess of 15,000 psi being 
much preferred. 
Table III records a group of electrical property measurements conducted on 
samples of Example 9 crystallized in accordance with the heat treatment 
schedule employed for the products of Table II. Dielectric strengths in 
excess of 2 KV/mil were measured on samples of 10 mil (0.01") thickness. 
As can be seen from Table III, electrical resistivities (.rho.) varied 
from greater than 15 to less than 9 over a range of temperatures, and loss 
tangents (tan .perspectiveto.) and dielectric constants (K.sup.1) ranged 
between 0.0001-0.005 and 6.77-6.90, respectively, over the interval of 
room temperature (25.degree. C.) to 200.degree. C. between frequencies of 
10.sup.2 -10.sup.5 Hz. 
TABLE III 
__________________________________________________________________________ 
10.sup.2 Hz 
10.sup.3 Hz 
10.sup.4 Hz 
10.sup.5 Hz 
Temp .degree.C. 
Log .rho. 
tan .delta. 
K.sup.1 
tan .delta. 
K.sup.1 
tan .delta. 
K.sup.1 
tan .delta. 
K.sup.1 
__________________________________________________________________________ 
25 -- 0.001 
6.80 
0.0001 
6.77 
0.0004 
6.77 
0.0007 
6.77 
108 15.04 
0.001 
6.82 
0.0002 
6.80 
0.0004 
6.80 
0.0005 
6.80 
197 13.29 
0.005 
6.90 
0.0014 
6.85 
0.0008 
6.84 
0.0007 
6.84 
299 11.33 
0.078 
7.28 
0.0203 
6.99 
0.0049 
6.92 
0.0017 
6.90 
398 9.93 
0.561 
9.88 
0.1468 
7.61 
0.0364 
7.13 
0.0090 
6.99 
498 8.87 
2.123 
17.61 
0.6280 
10.14 
0.1615 
7.73 
0.0400 
7.20 
__________________________________________________________________________ 
To illustrate the resistance of the inventive materials to various 
reagents, standard chemical durability tests were conducted on samples of 
Example 10, crystallized in accordance with the heat treatment schedule 
reported for Table II above. The data are tabulated below in Table IV 
along with those demonstrated by MACOR.RTM. brand material, an alkali 
metal-containing machinable glass-ceramic product marketed by Corning 
Glass Works, Corning, N.Y., under Code 9658, having a composition within 
U.S. Pat. No. 3,689,293, supra. The four reagents utilized were: (1) a 5% 
by weight aqueous solution of HCl; (2) distilled water; (3) a 0.02 N 
aqueous solution of Na.sub.2 CO.sub.3 ; and (4) a 5% by weight aqueous 
solution of NaOH. Each liquid was at a temperature of 95.degree. C. and 
the samples were immersed therein for 24 hours in the HCl solution, 24 
hours in the distilled water, 6 hours in the Na.sub.2 CO.sub.3 solution, 
and 6 hours in the NaOH solution. Weight loss of each sample is recorded 
in terms of mg/cm.sup.2 and a visual description of each sample after 
removal from the liquid is provided. 
TABLE IV 
______________________________________ 
Example 10 Code 9658 
Weight Weight 
Loss Appearance Loss Appearance 
______________________________________ 
HCl 79 Chalky 110 Chalky 
H.sub.2 O 
0.01 Yellow Specks 
0.01 Chalky 
Na.sub.2 CO.sub.3 
0.01 No Change 0.13 Chalky 
NaOH 0.46 No Change 12 Chalky 
______________________________________ 
As can be observed from the table, the alkali metal-free, alkaline earth 
metal-containing fluormica product exhibits chemical durability superior 
to that of the conventional alkali metal-containing fluorophlogopite 
glass-ceramics. 
Finally, in contrast to strontium-containing fluormicas, the inventive 
calcium-containing fluorophlogopites do not swell when immersed into 
water. Also, contrary to the general experience with barium-containing 
fluormicas, the inventive materials do not develop cracks during the 
crystallization process. 
The compositions demonstrating a preferred combination of high mechanical 
strength, exceptional resistance to attack by acids and alkalies, 
excellent dielectric properties, and very desirable white, translucent 
appearance are encompassed within the base ranges of 40-50% SiO.sub.2, 
10-15% Al.sub.2 O.sub.3, 20-30% MgO, 5-10% CaO, and 5-10% F, with the most 
preferred base composition following the stoichiometric formula Ca.sub.0.5 
Mg.sub.3 AlSi.sub.3 O.sub.10 F.sub.2 which converts to the approximate 
weight percent of 44.9% SiO.sub.2, 20.0% MgO, 15.4% MgF.sub.2, 12.7% 
Al.sub.2 O.sub.3, and 7.0% CaO.