P-60 / H. Ikeda

High Edge Strength Glass for Mobile Devices
Hikaru Ikeda, Kiyotaka Kinoshita, Mutumu Fukada, Kosuke Kawamoto, Takashi Murata,Ken Choju, Masahiko Ohji and Hiroki Yamazaki
Nippon Electric Glass co., Ltd. Shiga, Japanhkikeda@neg.co.jp
Abstract
A new evaluation method of the “Edge Impact Test” has beendeveloped. Breakage of cover glass from the edge is enabled toevaluate quantitatively by this method. It is found that resistanceto edge impact damage has a correlation with the CT of glass,and a high CS is more effective to improve resistance to the edge
impact damage than deep DOL.

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

1. Objectives and BackgroundTouch panel manufacturers utilize chemically strengthened glass
as cover glass for mobile devices, in order to keep theirmechanical durability. Chemical strengthening is a well-known
process to improve the fracture strength of glass. This process
generates compressive stress layers on glass surfaces and tensilestress on the inside of glass (Fig. 1). [1], [2] Compressive stressvalue and depth of compressive stress layer are abbreviated to“CS” and “DOL”, respectively.

Therefore, it is important for cover glass to improve resistanceto edge impact damage.
Generally, fracture strength of glass is evaluated by the Ball
Drop Test, 3-Point Bending Test and other methods. Fig.2 (a)(b) show crack patterns of breakage caused by these generaltests. The tests are applied to cover glass. Fig2 (c) shows crack
patters of breakage caused by dropping impact. The test isapplied to cove glass on simulated mobile device. Crack patternof breakage caused by these general tests is different from thetypical crack pattern of breakage from drop impact. Previously,
tests to make an impact to edge have not been carried out somuch.
Therefore, Nippon Electric Glass Co., Ltd. devised an evaluation
method for the edge strength of glass, to develop a newchemically strengthened glass having high edge strength. In thisstudy, a new evaluation method “Edge-Impact Test” isintroduced and the direction of development of next generation
chemically strengthened glass is reported.

2. Experimental procedure
2-1 Specimens
T2X-1(Alkali-alumino-silicate glass produced by Nippon
Electric Glass Co., Ltd.) with a thickness of 0.55mm
manufactured by the overflow-down-draw method and glass incurrent development were prepared for measurement. The
properties of T2X-1 are listed in Table 1.

Figure 1. Compressive stress layer schematic illustration.
The cover glass of mobile devices demands to be unbreakable,
but breakage sometimes occurs in daily use. Various types ofglass have been proposed [3] [4]. However, the issue ofbreakage of cover glass is not yet solved. That is because themain reason for the breakage of cover glass of mobile devices
has been misunderstood. After our research into the causes ofthe breakage of cover glass of mobile devices, it has been foundthat, breakage from the edge is dominant. Breakage from theedge is caused by the drop impact to the edge of mobile devices.
(a)Specimens of dimensions 22×30×0.55mm were prepared andtheir edges were chamfered with a #800 diamond wheel.
Specimens were ion-exchanged using KNO3/NaNO3 mixed
molten salt. The temperature of molten salt, ion-exchange timeand concentration of K+ ion in molten salt were adjusted toobtain the target CS and DOL. CS and DOL were determinedusing a surface stress meter (Orihara Seisakujo, FSM-6000).

(b)
(c)

Figure 2. Crack patterns of breakage by (a) Ball Drop Test, (b) 3-Point Bending Test and (c) Typical Dropping Impact.

ISSN 0097-966X/15/4503-1365-$1.00 © 2015 SID

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Table 1. Properties of T2X-1.
Glass code
Density [g/cm3]
C.T.E (30-380oC) [x10-7/K]
Young’s Modulus [GPa]Refractive Index (587.6 nm)
Photo-Elastic Constant [nm/cm/MPa]
Dielectric Constant (1MHz)
Softening Point [oC]
Annealing Point [oC]
Strain Point [oC]

Relationship between the fracture height and CS: Specimens
were ion-exchanged using KNO3/NaNO3 mixed molten salt toobtain various CS and 35μm of DOL. The CS and DOL of thespecimens are shown in Table 2 (No. 3, 9~12).

T2X-1
2.45
91
71
1.50
29.5
7.7860610
560

3. Results
3-1 Confirming the reproducibility of breakage /Breakage after Edge Impact Test
Fig. 4 shows a crack pattern of breakage from the edge of glassby Edge Impact Test. Compared with a typical crack pattern ofbreakage from dropping impact (Fig. 2 (c)), both crack patterns
are very similar. This result indicates that the Edge Impact Test
can reproduce practical breakage.

2-2 New evaluation method of “Edge Impact Test”
The Edge Impact Test method was newly developed to evaluate
resistance to edge impact damage. A schematic illustration ofthe evaluation system is shown in Fig. 3. Specimens of glass arefixed in a holder in a state where the edge is exposed. Apendulum arm tip, impact head fitted with a SUS cylinder(φ5mm), is lifted up to a predetermined position. And it isreleased and hits on the edge of specimen. If the specimen does
not break, the position of the arm is raised step by step. The test
continues until the breakage of the specimen, then the height of
the impact head when the specimen broke is recorded. The
results are treated by Weibull plot. The fracture height in the63rd percentile, B63, is calculated and this decides the edge
strength of the glass. The crack pattern of breakage by Edge
Impact Test is investigated.

Figure 4. Crack pattern of breakage after Edge-Impact
Test.

3-2 Resistance to the edge impact damage
Relationship between the fracture height and DOL: Fig.
5 shows the B63 fracture height of the Edge Impact Test versus
DOL. It is clear that the B63 fracture height of specimens wereincreased with increasing DOL.
Relationship between the fracture height and CS: Fig. 6shows the B63 fracture height of the Edge Impact Test versus
CS. It is clear that the B63 fracture height of specimens wereincreased with increasing CS.

Figure 3. Schematic illustration of Edge Impact Test.

2-3 Edge Impact Test
The following specimens are prepared to investigate therelationships between chemical strengthening characteristics andfracture height.
Relationship between the fracture height and DOL: On thepremise of actual production, specimens were ion-exchanged
using KNO3/NaNO3 mixed molten salt to obtain 2 different CS(1000MPa and 700MPa) and various DOL. The CS and DOL ofthe specimens are shown in Table 2 (No. 1~8).

Figure 5. Relationship between B63 fracture height andDOL of specimens.

Table 2. CS and DOL of Specimens.
No.

12345678910111213

CS [MPa]

1000995

990

995

1005

695

690

700

990

850

750

650560DOL [μm]

2631364245304049363635

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Figure 6. Relationship between B63 fracture height andCS of specimens.

Figure 7. Relationship between B63 fracture height andCT of specimens.

4. Discussion

5. Summary

The experimental results indicate that resistance to edge impact
damage will be improved by increasing CS and DOL of glass.
Central tension, CT, on the inside of glass is increased withincreased CS and DOL. CT is a function of CS, DOL and thethickness of the glass substrate, t, as expressed in equation (a).

A new evaluation method of “Edge Impact Test” is devised andbreakage of cover glass from the edge can be quantitatively
evaluated by the method. Resistance to edge impact damage hasa correlation with the CT of glass. High CS of glass, whencompared with deep DOL, is more effective to improveresistance to edge impact.
Now, Nippon Electric Glass Co., Ltd. is developing new glass
which is able to obtain a high CS.

Therefore, the effective to improve resistance to edge impact
damage of either CS or DOL was investigated. Fig. 7 shows theB63 fracture height of the Edge Impact Test versus CT. The B63
fracture height of cover glass increased with increasing CT.
However, excess CT of glass can cause disadvantages to selfdestruction and weakness against surface damage.. Comparing
the same CT samples, the higher CS samples were found thestronger resistance to edge impact damage. From this result, thedirection of development of high edge strength glass for mobile
devices is founded.

6. References[1] Kosuke Kawamoto et al, “Development of HighStrength Chemically Strengthened Glass”, SID 2014
DIGEST, pp1240-1243[2] Kosuke Kawamoto et al, “Development of HighStrength Chemically Strengthened Glass”, ISNOG
2014 final program, A2-7 09-1001, p60[3] Shusaku Akiba et al, “Novel Fracture Resistant Glass for
Mobile Display Cover”, SID 2014 DIGEST, pp693-696[4] Sinue Gomez et al, “Designing Strong Glass for Mobile
Devices”, SID 09 DIGEST, pp1045-1048

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