Strengthening glass by ion exchange

A method of strengthening a glass article by developing compressive stress in a surface layer on the article through an exchange of alkali metal ions in the surface layer at an elevated temperature below the glass strain point, the step of minimizing stress relaxation by carrying out the ion exchange in a glass essentially free from non-bridging oxygen atoms. Glasses having particular utility contain alumina in their compositions in such amount that the number of aluminum atoms in a glass are at least equal to the number of alkali metal ions, or contain both alumina and boric oxide in such amounts that the formula ##EQU1## is satisfied.

FIELD OF THE INVENTION 
The field is strengthening of a glass article by exchanging alkali metal 
ions in the surface of the article at a temperature below the strain point 
of the glass. 
BACKGROUND OF THE INVENTION 
Chemical strengthening of glass articles by ion exchange is well 
documented. The process involves exchange of alkali metal ions from within 
a surface layer on the article with different alkali metal ions from an 
external source. The usual practice is to operate at an elevated 
temperature that is below the glass strain point. In that case, relatively 
large ions enter the glass and replace smaller ions in the glass by 
counter diffusion. This develops compressive stress in the ion-exchanged 
surface layer on the article. In turn, the strength of the article is 
increased, and, consequently, so is its resistance to fracture. 
Early work demonstrated that the rate of ion exchange could be increased by 
increasing the concentration of alkali metal in the glass composition. 
However, an increased amount of ion exchange did not always lead to a 
commensurate increase in strength. This inconsistent behavior was found to 
be caused by relaxation processes in the glass whereby the larger ions 
were accommodated in the glass structure. As a result, the stress created 
by ion exchange was relieved, and the increase in strength lost. 
In many glasses, the rate of stress relaxation increases as the 
concentration of alkali metal oxides in the glass composition increases. 
Unless some means can be found to prevent stress relaxation, the value of 
increasing alkali metal oxide concentration in a glass is seriously 
limited. It is a primary purpose of the present invention to address this 
problem. 
Another facet of the problem arises from the desire to use strengthened 
glass substrates for deposition of thin films of active materials. For 
example, electrically conductive, metal oxide films, as well as 
transistors, may be applied to glass substrates in producing LCD devices. 
However, the presence of alkali metal ions in a glass substrate causes 
contamination of such active materials when they are in intimate contact 
with the glass. The usual mechanism for this contamination is an exchange 
of alkali metal ions initially in the glass for protons found in almost 
any material in contact with moisture. It is a further purpose of the 
invention to solve this problem as well. 
One means of avoiding these problems is to use glass substrates that are 
essentially free of alkali metal oxides. However, the use of such glass 
substrates precludes the possibility of improving the strength of the 
substrate by an exchange of alkali metal ions. Such a solution is, 
therefore, unacceptable. 
The present invention is based on my discovery that, by properly selecting 
compositions for glasses to be strengthened by ion exchange, these 
purposes can be achieved. I have found that glasses having certain 
compositions will permit a rapid exchange of ions while undergoing a 
minimum amount of stress relaxation. I have further found that such 
glasses also exhibit a minimal tendency to undergo proton-alkali exchange. 
This minimizes contamination of materials in contact with a substrate. 
These discoveries permit rapidly exchanging alkali metal ions to produce 
articles, such as substrates, that have a high degree of strength. The 
articles are also adapted to use without danger of alkali contamination of 
sensitive materials applied thereto. 
SUMMARY OF THE INVENTION 
One aspect of the invention is a method of strengthening a glass article by 
developing compressive stress in a surface layer on the article through an 
exchange of alkali metal ions in the surface layer at an elevated 
temperature below the glass strain point, the step of minimizing stress 
relaxation by carrying out the ion exchange in a glass essentially free 
from non-bridging oxygen atoms. 
A further aspect resides in a method of minimizing stress relaxation in a 
glass article having compressive stress developed by exchange of alkali 
metal ions in a surface layer on the article which comprises carrying out 
the ion exchange on a glass essentially free from non-bridging oxygen 
atoms. 
The invention further resides in an ion-exchange strengthened glass article 
composed of a glass selected from a group consisting of glasses having 
compositions containing alumina in such amount that the number of aluminum 
atoms in the glass are at least equal to the number of alkali metal ions 
in the glass, and glasses having compositions containing both alumina and 
boric oxide in such amounts that the formula 
##EQU2## 
is satisfied. 
PRIOR ART 
Prior art of possible interest is described in a separate document.

DESCRIPTION OF THE INVENTION 
The present invention is based on discovery of the effects of glass 
structure on ion exchange. It is particularly based on the effects created 
by the presence of bridging and non-bridging oxygen atoms in such glass 
structures. 
A bridging oxygen forms a bridge between two atoms which it connects by 
highly covalent bonds. A non-bridging oxygen does not connect two atoms by 
covalent bonds. It is connected to only one atom by a covalent bond. Other 
bonding requirements are fulfilled by ionic bonds. 
In pure silica glasses every silicon atom is attached to four different 
oxygen atoms by bonds that have a high degree of covalency. Each oxygen 
atom is, in turn, bonded to two silicon atoms. The covalent bonds do not 
allow appreciable changes in the distances between the silicon and oxygen 
atoms. Because the oxygen atoms are bonded to two neighboring atoms 
(silicon atoms in this case) by covalent bonds, they are said to form a 
bridge and are called bridging oxygen atoms. Each silicon is essentially 
tied into the glass structure at four points by bonds which are not easily 
deformed. Therefore, the structure which results is a very rigid 
structure, and changes in the glass structure caused by stresses occur 
very slowly. 
When alkaline oxides are added to the glass, one non-bridging oxygen is 
introduced into the structure for every cation of alkali introduced. A 
non-bridging oxygen atom is bonded to only one silicon atom by a covalent 
bond. A single negative charge which is localized on such an oxygen atom 
is compensated by a vicinal alkali ion. The distance between a 
non-bridging oxygen atom and an alkali ion is changed rather easily. Thus, 
a silicon atom which is bonded to only three bridging oxygen atoms and to 
one non-bridging oxygen atoms is essentially tied into the glass structure 
at three points. In essence, the presence of non-bridging oxygen atoms can 
be considered to "depolymerize" the glass structure. This causes the 
structure to be less rigid and causes the rate of any relaxation process 
to occur more rapidly. 
The presence of high field strength ions such as aluminum alters the 
structure of alkali silicate glasses in that it inhibits the formation of 
non-bridging oxygen atoms. Aluminum atoms are bonded into the glass by 
four covalent bonds analogously to silicon atoms. The negative charges 
required to compensate for the positively charged alkali ions are not 
localized on single oxygen atoms. They are delocalized over the four 
oxygen atoms to which the aluminum atom is bonded. The aluminum atoms and 
its four neighboring oxygen atoms, in essence, constitute an oxyanion 
analogous to the carbonate or sulfate ions. Thus, the addition of alumina 
to an alkali silicate decreases the degree of depolymerization of the 
glass. Consequently, high concentrations of alkali can be included in the 
glass without substantially increasing relaxation rates if one atom of 
aluminum is included for every atom of alkali. 
The formation of non-bridging oxygen atoms can also be inhibited when the 
number of alkali metal ions (R) exceeds the number of aluminum ions. 
However, sufficient boric oxide must be included so that 
##EQU3## 
A slight excess of alkali metal oxide relative to alumina, plus some boric 
oxide, is desirable. This provides a glass which is melted more easily 
than one containing equal amounts of alkali and alumina and no boric 
oxide. 
It is well known that a glass containing high concentrations of alkali, 
particularly high concentrations of lithium, undergoes rapid ion exchange. 
The value of the present invention resides in part in the discovery that a 
high strength value, obtained by such rapid exchange, can be retained 
after such exchange, if non_bridging oxygen atoms are excluded from the 
system. In other words, stress relaxation is minimal even in glasses 
containing high concentrations of alkali ions. A further feature of the 
invention derives from the discovery that boric oxide can be utilized to 
eliminate non-bridging oxygen atoms without slowing the rate of ion 
exchange and without adversely influencing the chemical durability of the 
glass. This requires that the ratio 
##EQU4## 
is satisfied while the concentration of boric oxide does not exceed 15 
cation percent. In the absence of B.sub.2 O.sub.3, the non-bridging 
oxygens (NBOs) should not be more than 1% of the total oxygens. Another 
feature resides in the discovery that the avoidance of non-bridging oxygen 
atoms strongly increases the resistance to alkali metal ion extraction 
from the glass by a proton-alkali exchange. 
For purposes of the invention, it is preferred that compositions of the 
glasses, as calculated in cationic % on an oxide basis, consist 
essentially of 
______________________________________ 
SiO.sub.2 
35-50% 
Al.sub.2 O.sub.3 
20-28% 
B.sub.2 O.sub.3 
0-10% 
Li.sub.2 O 
8-10% 
Na.sub.2 O 
15-20% 
Li.sub.2 O + Na.sub.2 O 
20-30% 
______________________________________ 
TABLE I sets forth the compositions of several glasses which exemplify the 
invention. The compositions are presented in cation percent, mol percent 
and weight percent, as indicated. Compositions 1-6 illustrate the present 
invention. Compositions 7 and 8 are for comparison glasses having similar 
components in different amounts 
TABLE I 
______________________________________ 
1 2 3 4 5 6 7 8 
______________________________________ 
Cation % 
SiO.sub.2 
40 40 45 40 42.5 45 48.8 50 
Al.sub.2 O.sub.3 
25.6 25 22.5 25.9 27.2 27 19.8 13.1 
ZrO.sub.2 
-- -- -- -- -- -- -- 2.2 
B.sub.2 O.sub.3 
6.3 7.5 7.5 6.3 2.3 -- -- -- 
Li.sub.2 O 
8.1 10 10 10 10 10 15.1 17.8 
Na.sub.2 O 
20 17.5 15 17.9 18 18 16.2 16.8 
MgO -- -- -- -- -- -- 0.9 -- 
Mol % 
SiO.sub.2 
57.1 57.1 62.2 57.1 59.7 62.1 65.2 65.8 
Al.sub.2 O.sub.3 
18.3 17.9 14.5 18.5 19.1 18.6 12.7 8.6 
ZrO.sub.2 
-- -- -- -- -- -- -- 2.9 
B.sub.2 O.sub.3 
4.5 5.4 5.2 4.5 1.6 -- -- -- 
Li.sub.2 O 
5.8 7.1 6.8 7.1 7.0 6.7 10.1 11.7 
Na.sub.2 O 
14.3 12.5 10.3 12.8 12.1 2.4 10.8 11.1 
MgO -- -- -- -- -- -- 0.5 -- 
Wt. % 
SiO.sub.2 
51.4 51.6 56.9 53.7 56.2 
Al.sub.2 O.sub.3 
28.0 27.4 24.1 29.2 28.6 
B.sub.2 O.sub.3 
4.7 5.6 5.5 1.7 -- 
Li.sub.2 O 
2.6 3.2 3.1 3.1 3.1 
Na.sub.2 O 
13.7 11.6 9.8 11.7 11.6 
______________________________________ 
Initial development work involved two pound glass melts for screening 
purposes. For each melt a batch was formulated, mixed and melted overnight 
at 1500.degree. or 1550.degree. C. in a platinum crucible. The glass melt 
was poured into molds to provide bars for measurement. 
Test bars were subjected to ion exchange from different molten salt baths 
and at different temperatures. One bath was 100% sodium nitrate while a 
second was a mixture of 60% potassium nitrate and 40% sodium nitrate. 
These baths were maintained at either 380.degree. C. or 430.degree. C. for 
exchange purposes. Exposure times of 4 hours and 16 hours were employed. 
Because of the large differences in alkali metal ion mobility, the primary 
ion exchange occurred between sodium ions from the salt bath and lithium 
ions from the glass. 
The test bars were cooled and cleaned after the ion exchange. They were 
then measured to ascertain the depth of the ion exchanged layer (DOL) in 
the glass surface. The central tension developed in the unexchanged core 
of the test bar was also measured. 
TABLE II sets forth DOL in microns; CT in psi; and mechanical strength in 
psi for certain of the glasses of TABLE I. 
TABLE II 
______________________________________ 
Cation % 1 2 7 
______________________________________ 
DOL 196 185 225 
CT 2800 3300 3400 
Strength 66,700 68,900 
______________________________________ 
For substrate purposes, it is prescribed that the ion-exchange in a surface 
layer on a glass substrate be to a depth (DOL) of at least 150 mm. and 
provide a central tension (CT) in the article of at least 2000 psi. The 
glasses of the present invention, as well as the comparison glasses, 
exceed these criteria. However, the much lower lithia (Li.sub.2 O) content 
in the present glasses entails a much lower batch cost. Also, the 
comparison glasses are much harder to melt because they contain more 
SiO.sub.2, no B.sub.2 O.sub.3 and substantial ZrO.sub.2 as compared to the 
present glasses. 
One use of the present glasses is as substrates in articles such as LCD 
devices. Such use involves deposition of thin films of active material on 
the substrate. Consequently, k is deskable that alkali metal not be 
extracted from the glass substrate since it contaminates a film on the 
surface. Extraction of the alkali metal is possible because water vapor in 
the air may give rise to a ion exchange of a proton with an alkali metal 
ion. A direct test of the resistance of a glass to this type of extraction 
is not practical because the time for such a test is impracticaly long. 
Therefore, a quicker test has been developed that comprises immersing a 
sample of glass in a given volume of distilled water for a given time at 
95.degree. C. The water is then analyzed to determine the concentration of 
the various components of the glass which have been extracted. The 
analysis, of course, measures the total material extracted from the glass 
by any possible mechanism, and not merely that extracted by the 
proton-alkali ion exchange. 
The usefulness of this test for measuring the potential for alkali metal 
extraction during the conditions of use of the glass can be understood 
from the following considerations. If the alkali metal extraction occurred 
exclusively through an ion exchange process, alkali metal ions would be 
the only contaminant in the water after the test. If the extraction 
occurred exclusively through a uniform solution of the glass in the water, 
all the components would be found in the solution in the same ratios in 
which they occurred in the glass. If both mechanisms were operative, the 
relative importance of the two mechanisms could be determined by the 
comparison of the molar percentages of each contaminant in the solution to 
that in the original glass. For example, assume all the components of the 
glass are found in the solution, but the ratio of alkali metal to silica, 
or the ratio of alkali metal to total material, in the extract are 
significantly higher than the corresponding ratio found in the glass. 
Then, one can deduce that the ion exchange mechanism makes a significant 
contribution to the process of alkali extraction. 
TABLE Ill shows extraction data after 1, 3 and 7 days exposure. The data 
represents the ratio in percent of total analyzed alkali metal oxide 
(M.sub.2 O) to total of all glass components. 
TABLE III 
______________________________________ 
1 2 3 4 5 6 7 8 
______________________________________ 
M.sub.2 O % 1 
24.5 20.9 19.7 20.1 25.9 28.4 33.6 66.0 
M.sub.2 O % 3 
21.6 21.4 18.5 23.1 25.9 23.7 -- 29.7 
M.sub.2 O % 7 
22.5 21.8 18.8 22.5 22.7 22.6 21.0 24.8 
______________________________________ 
The extraction data displayed in the table shows that, for the comparison 
glass immersed in water for one day, the ratio exceeds by almost a factor 
of three the ratio in the original glass. This is a clear indication that 
proton alkali exchange is the dominant mechanism of extraction for short 
times. After seven days, the extract has the same composition as the 
original glass, indicating that uniform solution becomes the dominant 
mechanism of extraction after a sufficiently long period of time. This 
sequence of processes is quite typical of glasses which contain a high 
density of non-bridging oxygen atoms. Initially the ion exchange mechanism 
erodes the durability of the glass and then it dissolves uniformly. 
The behavior of the present glasses, which contain no non-bridging oxygen 
atoms, is distinctly different. The molar ratio of alkali to total extract 
after one day of immersion is, within experimental error, equal to that in 
the initial glass. This indicates the absence of any appreciable 
extraction by the ion exchange process. This deduction is corroborated by 
the observation that the total mount of alkali extracted in one day is 
significantly less than that observed in comparison glasses 7 and 8. The 
extraction data obtained after seven days shows that the total material 
extracted from the glasses of the instant invention is no less than that 
extracted from comparison glasses, but this merely indicates that these 
glasses do not resist uniform solution in hot liquid water any more 
strongly than do other glasses. However, in use the glasses will not be 
exposed to liquid water. The important point is that ion exchange of a 
proton and an alkali ion does not occur so that extraction of alkali from 
these new glasses by a low level of water vapor in the air is not 
possible.