P-177L / K. Kawamoto

Late-News Poster

Development of High Strength Chemically Strengthened Glass
Kosuke Kawamoto, Takashi Murata, Shinkichi Miwa, Masahiko Ohji, Hiroki Yamazaki
Nippon Electric Glass Co., Ltd. Shiga, JapanAbstract
Newly developed chemically strengthened glass, T2X-1, isdemonstrated. T2X-1 has high strength and excellent ionexchange properties. Fracture strength of T2X-1 is 20% higher
than conventional Alumino-silicate glass (CX-01). T2X-1 hastwice as fast ion-exchange speed and durable compressive stressagainst four times more Na+ contamination in molten KNO3
than conventional Alumino-silicate glass (CX-01).
Figure 1. Schematic illustration of ion-exchange.

Author Keywords
Chemically strengthened glass; Cover glass; High strength; Ionexchange property.

1. Objective and Background
It is common for touch panel makers to use chemicallystrengthened glass as cover glass to protect touch panel displays.
Chemical strengthening is a well known process to increase
fracture strength of glass. During the chemical strengthening
process, glass substrate (generally containing Na+ ion) is
immersed in KNO3 molten salt, the Na+ ion site in glass surface
is replaced by the larger ionic radius of K+ ion. The difference inionic radius of K+ and Na+ generates compressive stress on glass
surface (Figure 1, 2). Depth of compressive stress layer isabbreviated to ‘DOL’. Compressive stress value is abbreviatedto ‘CS’.
It is important to form a deep DOL and a high CS to guarantee
high fracture strength of glass, [1][2]. A 40µm and more DOL
and a 700MPa and more CS are recently recommended for thechemically strengthened glass. However, it is not easy to form adeep DOL and high CS in mass production due to the following
reasons. One is stress relaxation during ion-exchange. The
deeper DOL generally requires longer ion-exchange time. Theion-exchange time could be shortened by higher ion-exchange
temperature. However, the higher temperature accelerates thestress relaxation of glass, resulting in a decrease of CS of glass.
The other is a decrease of CS by Na+ ion contamination in
KNO3 molten salt bath. Na+ ions are released from glass surface
into the KNO3 molten salt during the ion-exchange process inmass production. High contamination of Na+ ion in KNO3molten salt prevents the formation of higher CS of thechemically strengthened glass.
T2X-1 is a newly developed glass for chemical strengthening tosolve these issues. T2X-1 is characterized by forming deep DOL
and high CS in mass production. In this work, the fracture
strength and the excellent ion-exchange properties of T2X-1 isdiscussed.

2. Experiments
The fracture strength, the DOL and CS of T2X-1 were
investigated experimentally and compared with conventional
Alumino-silicate glass and soda-lime-silicate glass.

1240 • SID 2014 DIGEST

Figure 2. Schematic illustration of compressive stresslayer.
Table 1. The Properties of T2X-1, CX-01 and SLS
Glass code

T2X-1

CX-01

SLS

Glass-system

Aluminosilicate

Aluminosilicate

Soda-limesilicate

Density (g/cm3)

2.45

2.54

2.5

91

100

90

Young’s
Modulus (GPa)

71

75

71

RefractiveIndex(587.6nm)

1.50

1.52

1.52

Photo-Elastic
Constant(nm/cm/MPa)

29.5

28.0

26.5

DielectricConstant(1MHz)

7.7

7.7-Softening Point
(oC)

860

760

730

Annealing Point(oC)

610

560

550

Strain Point
(oC)

560

520

510ThermalExpansion
Coefficient(×107oC)[30-380 oC]

ISSN 0097-966X/14/4503-1240-$1.00 © 2014 SID

Late-News Poster

Specimens: T2X-1, CX-01(conventional Alumino-silicate)and SLS (Soda-lime silicate glass) were prepared. T2X-1 andCX-01 are manufactured by the over flow down draw method,which is widely used for producing glass substrates for LCD. On
the other hand, SLS is manufactured by the float process, whichis also widely used for producing glass sheet for normal
windows. Thickness of glass specimens are 0.7mm andproperties of each glass are listed in table 1.
Ion-Exchange Properties: DOL and CS of the specimens
treated under the same ion-exchange conditions wereinvestigated. T2X-1, CX-01 and SLS, were ion-exchanged inKNO3 molten salt (Na+ ion concentration: 6000ppm) at 430oC
for 4 hours. DOL and CS of the specimens were determined byusing a surface stress meter FSM-6000 as shown in Table 2.
Three Point Bending Test: Specimens, T2X-1, CX-01 andSLS, of dimensions 135.4×85.2×0.7mm were prepared andindividually ion-exchanged in KNO3 molten salt. T2X-1, CX-01
and SLS were ion-exchanged in 430oC of KNO3 molten salt for
4.5, 8 and 1 hours, respectively. Na+ ion concentration in KNO3
molten salt was 1,000ppm. DOL and CS were shown in Table 2.
A load tester was used for the three-point-bending test (Figure3). Specimens are placed on stationary supports. The specimens
were loaded at a rate of 3mm/min. The fracture strength in the
15th percentile, B15, was calculated. The results were treated byWeibull plot.

P-177L / K. Kawamoto

Table 2. DOL and CS of T2X-1, CX-01 and SLS
Glass CodeIon-ExchangeProperties(430oC-4h,Na:6000ppm

For Three Point
Bending Test
(430oC -4.5,8,1h,
respectively,Na:1000ppm)

T2X-1

CX-01

SLSDOL
[µm]

46

33

23

CS
[MPa]

790

680

300

DOL
[µm]

49

47

11

CS
[MPa]

860

740

550

50
T2X-1
CX-01

45SLSFigure 3. Schematic diagram of three-point-bending test.

3.

Results

Ion-Exchange Properties: Figure 4 shows the DOL and CSof each specimen. The X axis shows CS and Y axis shows DOL.
A plot on the upper right region of the graph indicates excellent
ion-exchange properties.
It is clear that T2X-1 shows excellent ion exchange properties
compared to conventional Alumino-silicate glass (CX-01). The
deepest DOL and the highest CS have been formed under thesame ion-exchange conditions. Only T2X-1 satisfies a 40µm andmore DOL and a 700MPa and more CS in the ion-exchange
condition.

DOL [µm]

40353025200200

400600800

1000

CS [MPa]
Figure 4. DOL and CS of T2X-1, CX-01 and SLS, ion+exchanged in KNO3 molten salt (Na ion concentration:o6000ppm) at 430 C for 4 hours.

SID 2014 DIGEST • 1241

ln(ln(1/(1-F)))

P-177L / K. Kawamoto

Late-News Poster

1.5
1.0
0.5
0.0
-0.5
-1.0

T2X-1
CX-01
SLS-1.5
-2.0
-2.5
-3.0
-3.5
-4.0

The 15th percentile

6.2

6.3

6.4
570MPa610MPa

6.5

6.6

6.7

720MPaln(σ)

Figure 5. Weibull plot of T2X-1, CX-01 and SLS. σ[MPa] is fracture strength. F is failure probability calculated by Mean ranks.
Three Point Bending Test: Figure 5 shows a Weibull plot offracture strength of T2X-1, CX-01 and SLS. The figure showsthat the fracture strength of T2X-1 is clearly higher than that ofCX-01 or SLS. B15 of T2X-1, CX-01 and SLS were 720MPa,610MPa and 570MPa, respectively. B15 of T2X-1 was 1.2 times
higher than that of conventional Alumino-silicate glass (CX-01).

70T2X-1

60CX-01

4. Discussion

SLS50
DOL [µm]

Ion-Exchange Properties (DOL): In this section, thebenefits from the excellent ion-exchange properties of T2X-1
are discussed. Figure 6 shows DOL versus ion-exchange time ofspecimens, ion-exchanged in pure KNO3 molten salt at 430oC
for 1 to 8 hours. Remarkably, T2X-1 shows twice as fast ionexchange speed than conventional Alumino-silicate glass (CX01). T2X-1 achieves 40µm of DOL at 3 hours. On the otherhand, 6 hours and over 8 hours are necessary for CX-01 andSLS, respectively. Ion-exchange time to exceed the same DOL
can be shortened without any change in ion-exchange
temperature by using T2X-1 instead of other glass materials. We
believe that the shortened ion-exchange time of T2X-1 willincrease productivity of ion-exchange process of chemicallystrengthened glass.

403020100246Ion Exchange Time [hours]
Figure 6. DOL of T2X-1, CX-01 and SLS versus ion
exchange time.

1242 • SID 2014 DIGEST

8Late-News Poster

P-177L / K. Kawamoto

1100
T2X-1

CS [MPa]

900

CX-01

exchanged in highly Na+ contaminated KNO3 molten salt. In
other words, CS of T2X-1 stays in a high standard of the recent
recommendation even if the Na+ contamination in KNO3 moltensalt is 4 times more than that of conventional Alumino-silicate
glass (CX-01).
Fracture Strength: It is the most important for a cover glass
to have high fracture strength in order to protect inner devices.
This study showed that B15 of T2X-1 was 20% higher than thatof conventional Alumino-silicate glass (CX-01). Therefore, thestrength of cover glass could be increased by using T2X-1.

SLS

700

5. Conclusion

500

T2X-1 developed by Nippon Electric Glass has the followingfeatures.
•20% higher fracture strength in the 15th percentile
compared with conventional Alumino-silicate glass(CX-01).

•

Twice as fast ion-exchange speed compared withconventional Alumino-silicate glass (CX-01).

•High and stable compressive stress which stays in ahigh standard of the recent recommendation even ifthe Na+ contamination in KNO3 molten salt is 4 times
more than that of conventional Alumino-silicate glass(CX-01).

300

10005000 10000 15000 20000
Na+ ion concentration in KNO3molten salt [ppm]+Figure 7. CS of T2X-1, CX-01 and SLS versus Na ion
concentration in KNO3 molten salt.
Ion-Exchange Properties (CS): Figure 7 shows the CS ofT2X-1, CX-01 and SLS treated under 430oC for 3, 6 and 13hours, respectively, to create about 40µm of DOL versus various
Na+ ion concentrations in KNO3 molten salt (0-20000ppm). CSof T2X-1, CX-01 and SLS were 930MPa, 810MPa and 360MPa,
respectively when treated with pure KNO3 molten salt. T2X-1
can keep high CS of over 700MPa until 15,000ppm, whereas,
CS of CX-01 deteriorates to 700MPa at 4,000ppm. SLS cannot
achieve 700MPa. By using T2X-1, higher CS can be obtained
compared to using other glass materials even when ion-Therefore, T2X-1 has both enough strength for cover glass for
touch panel devices and suitable ion-exchange properties forcover glass mass production. There is the possibility for touch
panel makers to achieve both product quality improvement andcost reduction by using T2X-1.

6. References[1] Sinue Gomez et al, “Designing Strong Glass for MobileDevices”, SID 09 DIGEST, pp1045-1048[2] James J. Price et al, “A Mechanics Framework for IonExchanged Cover Glass with a Deep Compression Layer”,
SID 09 DIGEST, pp1049-1051

SID 2014 DIGEST • 1243