Department of Chemical EngineeringUniversity of San Carlos – Technological Center

Nasipit, Talamban, Cebu City

ChE 413N Material Science and Engineering

A Case Study on

Scratches and Failures of Glass Screens of Smartphones

Submitted to:

Engr. May V. TampusInstructor, ChE 413N

Submitted by:

Banaag, Kristian Gregg C.

Navalta, Carl John Louie G.

March 23, 2016

Introduction

This century is the age of gadgets (Strickland, 2016). Today, gadgets are part of

a person’s necessity. These things are very useful in the everyday lives of people. They

use these gadgets for work, education and for security purposes. These gadgets are not

part of luxury anymore.

The use of smartphone in this era is very rampant. Almost all the people you see,

in the airport, shopping malls, parks and in cafes, are using smartphone. They use it to

connect with people around the world through social media, to know the latest news, to

update reports for work and for any other reasons. Smartphones are usually touch

screen gadgets. Through constant use, the screens experience fatigue and fail.

Statement of the Problem

Old school cell phones can call and text. While this can get your message

across, smartphones allow you multiple ways of communicating. Not only can they call,

text and IM, they give you access to email, video calling and video conferencing. Other

concerns that made smartphones advantageous are that it could take photos, store

files, watch videos, play music and even record the pressure and temperature of the

surrounding area.

Smartphone and tablet users’ greatest concern is that their device may be lost or

stolen, while scratches on the screen or body of the device are the most common form

of damage, but are among the least concerning to users. Cell phone screens are made

from a variety of different materials depending on the make and model. Most screens

are made from a durable glass designed not to shatter, but it is still relatively thin in

order to keep the phone lightweight. In some newer touchscreen phones, a material

called Indium Titanium Oxide (ITO) is used in place of the glass. ITO is more

expensive and far less likely to break, but scratching can still occur through accidents

or normal wear and tear.

Most modern smartphones have scratch-resistant glass embedded on their

screens but it isn’t enough to avoid getting scratches. Latest smartphones nowadays

can resist against keys, coins, knife and other sharp objects, however, these hard, gritty

sand that’s found in beaches or floating around the pocket or on the desk and anywhere

else could actually damage and cause a scratch on the screen. These may be tiny bits

of particles but some of its compositions are made of rocks that are harder than modern

glasses used in cellphones nowadays.

Theoretical Background

A. The Screen

The most used part of a smartphone gadget is the screen. The touch screen can

be both resistive and capacitive.

Resistive touch screens are commonly used in automated teller machines

(ATMs) and supermarkets. From its name, the screen literally resists the touch.

Pressing against the screen hardly, shown in figure 1, the screen will slightly bend.

Through bending, the two conductive layers touched on another (McCann, 2012).

Figure 1. Resistive touch screen. Image credit: Chassis Plans

Resistive type of screen has several layers. The two important parts of this type

are the flexible plastic and glass layers that are electrically resistive. The front surface of

the screen panel is a scratch-resistant plastic with coating of a conductive material, that

is typically ITO. The glass layer is also coated with ITO. Both layers face each other and

are separated with a thin gap in between. An electrical resistance is created between

the two layers and the electrical charge runs from top to bottom in one layer and side-to-

side in the other. When a finger or stylus tip presses the outer surface, both the ITO

coating meet, and the resistance of both the layers at point of contact is the touch point

(Tech Explainer, 2016).

Figure 2. Capacitive touch screen. Image credit: Electrotest

Capacitive touch screens on the other hand do not use pressure to create a

change in the flow of electricity. They work with anything that has an electrical charge

such as our fingers (human skin). This type of screen is made from materials like

copper or ITO that store electrical charges in an electrostatic grid of tiny wires (McCann,

2012). When a person touches the capacitive surface of the screen, it changes the local

electrostatic field, and the system continues to monitor the movement of each tiny

capacitor to locate the exact area where the finger is touching. The surface of capacitive

screens is made of glass that are highly sensitive which does not require the use of a

stylus.

B. The Glass

Glasses used in smartphones are designed to be very tough. This toughness is

achieved through very high temperature processing of glass. Usually, synthetic glass-

ceramics are used for smartphone screens.

Glasses are amorphous solid due to the lack of a crystalline structure. The

molecules are not in order, but are arranged more like a liquid yet frozen in place

(Rohrig, 2015). Glasses do not contain planes of atoms that can slip past with each

other, and so stress is not relieved. Excessive stress will form a crack, and the

molecules on the surface of the crack will separate. The intensity of stress increases

and the crack grows which leads to more bonds that will break. This causes the

breaking of the glass.

Ceramics, on the other hand, have the tendency to have a crystalline structure,

and are characterized by the ionic bonds and sometimes contain covalent bonds. When

they crystallize, the strong forces of attraction of opposite charges in the planes of ions

makes it difficult for one plane to slip in another plane. Because of this, ceramics are

brittle materials, and resist compression. But when bent, ceramics can break.

Tougher and stronger glass screens are a combination of glass and ceramics.

Glass-ceramics are formed by by overheating the glass. Through overheating, a portion

of its structure is transformed into a fine-grained crystalline material. According to

American Chemical Society, at least fifty percent of the structure of glass-ceramics are

crystalline, and in some cases, they are more than 95%.

Analysis

A. Manufacturing Process of Soda Lime (Float) Glass

The products of glass industries nowadays are flat glass, container glass, and

pressed and blown glass. The procedures for manufacturing glass are the same for all

products except during forming and finishing. Container glass and pressed and blown

glass, 51 and 25 percent respectively, comprise the total soda-lime glass production,

use pressing, blowing or pressing and blowing to form the desired product. Flat glass,

on the other hand, is formed by float, drawing, or rolling processes. Soda lime glass

which is a type of float glass that is commonly used as a screen for smartphone

industries is also produced via float process.

This soda lime glass is typically made using a wet batch process where the

composition is primarily limestone, soda ash and silica. The typical soda-lime glass

composition is: 73% silica sand, 13% soda ash, 8% limestone, 4% dolomite and 1%

alumina.As the sand, limestone, and soda ash raw materials are received, they are

crushed and stored in separate elevated bins. These materials are then transferred

through a gravity feed system to a weigher and mixer, where the material is mixed with

cullet to ensure homogeneous melting. The mixture is conveyed to a batch storage bin

where it is held until dropped into the feeder to the melting furnace. All equipment used

in handling and preparing the raw material is housed separately from the furnace and is

usually referred to as the batch plant.

Figure 4. Typical manufacturing process of float glass.

The second procedure is to pass the batched raw materials from a mixing silo to

a chambered furnace where they become molten at a temperature of approximately

1500 °C.

The third procedure is to draw the molten glass onto the tin bath and this is the

process called float process. The molten glass  is floated onto a bath of molten tin at a

temperature of about 1100°C. It forms a ribbon with a working width of 3210mm which

is normally between 3 and 25mm thick depending on the company. The glass which is

highly viscous and the tin which is very fluid do not mix and the contact surface between

these two materials is perfectly flat.

The last important procedure is the cooling of the molten glass in the annealing

lehr. On leaving the bath of molten tin, the glass - now at a temperature of 600°C - has

cooled down sufficiently to pass to an annealing chamber called a lehr. The glass is

now hard enough to pass over rollers and is annealed, which modifies the internal

stresses enabling it to be cut and worked in a predictable way and ensuring flatness of

the glass. As both surfaces are fire finished, they need no grinding or polishing.

Fig. 4 Schematic diagram of the float process for making flat glass

Additional information on float process is that glass ribbon, soft enough to be

workable, is fed from a glass-melting furnace and passed between rollers into the float

bath. There, it floats on molten tin under a controlled atmosphere of nitrogen and

hydrogen (N2/H2) that prevents oxidation of the tin. As the bulk of that glass begins to

cool, the surface is heated and polished in order to remove surface blemishes and then

allowed to cool also. The ribbon exits the float bath and passes through the annealing

lehr, where it is cooled uniformly in order to prevent the formation of nonuniform internal

stresses that may warp the glass. The cooled glass is then scored by diamond-tipped

cutters, and individual sheets are separated and stacked.

B. Properties of Soda Lime Glass

Table 1.Typical Properties of Soda Lime Glass

The most noticeable properties of a material is it’s mechanical properties. It is

observed that the density of the material is not that high since it would make the glass

heavier which is not typical for a mobile phone. It is also not that low since lower density

materials tend to be softer and breaks easily. The young’s modulus is 72 GPa which is

an average for glasses and is bendable but requires a bit of force to deform it. The

hardness is based on Mohrs’ scale of mineral hardness and it is found out to be at the

median range which means it could be broken easily by minerals containing SiO2 and

harder minerals. There are also minerals that this glass could cut such as fluorite and

calcium carbonate.

The thermal property of the material especially the thermal conductivity was also

shown since it is important to know the ability of the material to transfer heat. The data

showed that it has a very low thermal conductivity and doesn’t conduct heat that much

which is a standard for all phone screens nowadays. People don’t want to use phones

which conducts heat especially under the heat of the sun since it overheats quickly and

may damage the internal parts in the phone. The coefficient of linear thermal expansion

data is found to be very low and is important that it remains in this region because this

results to the fracture of the glass when the length increases especially when a bezel

(the ring holding the glass in position) is framed on the glass. Knowing the annealing

point or the stress relief point temperature of the glass is also important since this is the

temperature in which the glass will be too hard for significant external deformation

without breaking but is soft enough to relax the internal strains by microscopic flow in

response to the intense stresses they introduce internally. The strain point on the other

hand is the temperature where the microscopic internal flow effectively stops. The

thermal shock resistance for the annealed soda lime glass is quite low and materials

having this thermal shock resistance may deform at a normal hot day. A temperature of

38 °C is almost the same as the normal body temperature which may affect the glass

when a slight change in temperature is introduced.

Another important property to be considered in making glass is its electrical

property. The specific resistivity of the material is considered here since it is usually

used to make resistive type of screens where two screens are used to produce a flow of

electrons in order to have a touch sensitive computer display.

C. Failures

All smartphone screens have typical glass surface damages, it can be scratches,

lateral cracks, dents due to impacts and fracture. These surface damages will cause

glass failure. According to Corning in their failure analysis of broken devices, the major

cause of fracture is impact damage. Fracture occurs because of sharp impacts that will

create flaws. Examples of sharp impacts are keys, pens, gravel and etc. Scratch related

damages can also cause glass failure, where fracture is initiated from a flaw within the

scratch. Over-stress alone will not cause glass failure because new glass manufacturing

technology strengthens glasses which enables glasses to deflect substantially before

failing and failure stresses are not reached.

In smartphone or any other mobile devices, there are requirements that are

considered for glass; tight strength distributions, high retained or abraded strength,

increased resistance to impact damage and greater resistance to scratching.

Older versions of smartphones use soda-lime as a material in their glass screens

that are strengthened by ion exchange (Varner and Wightman, 2012). Soda-lime

glasses are inexpensive and strong after due to the ion exchange process, but is not

much damage-resistant than soda-lime glass that does not undergo ion exchange. This

is because the exchange layer in the glass is thin, no more than 15 microns deep, and

the maximum compressive stress is in the range of 600 MPa. The predominant failure

for this type of glass is mechanical damage caused by sharp impacts.

The surface of soda-lime glass can experience both scratches and individual

impact sites. In the examination of Varner and Wightman, soda-lime glasses in phones

after experiencing service use have typical impacts sites. There is a zone of plastic

deformation from which lateral cracks originates. These lateral cracks form when

contact pressure is applied or due to local residual stress field that accompanies plastic

deformation. When lateral cracks are severe enough, it will intersect the surface and

chips are formed, and it can also be visible.

Solution

Analysis of glass surfaces of smartphones and other hand-held electronic

devices indicates that the predominant failure of glasses are damaged through sharp

contact.

Alkali alumino silicate and soda-lime type glasses are the two glasses that are

used in smartphones glass screens that can be strengthened through ion-exchange.

Compared to soda-lime, alkali alumino silicates will have higher compressive stresses

and deeper compressive layers (Gomez et al., 2011).

According to Gomez et al., strengthening processes in glass will prevent surface

flaws or cracks from propagating when external forces are applied to the surface. The

introduction of surface compressive stress profiles in glass is a well-known approach for

strengthening since glasses are stronger in compression than in tension. To incorporate

residual stresses, ion-exchange or chemical tempering can be used. This is done by

exposing a glass containing alkalis to molten salt baths that contain alkali ions. The ions

in the salt bath are typically larger than those initially in the glass. Some ions in the

glass are replaced by ions in the molten salt bath as a result of chemical potential

differences.

When a smaller ion in the glass is replaced by a larger ion from the salt bath, a

compressive layer is formed in the surface of the glass that produces a compressive

stress. This stress is balanced by a volume below the surface that is under tensile

stress.

There are two properties that are very important in glass strengthening process;

the depth of layer and compressive stress. The depth of layer is the measure of the

thickness of the glass after the ion-exchange process while the compressive stress is

the strength of glass. Large values of depth of layer and compressive stress are desired

to protect the surface of the glass screens from flaws or damage. Alkali alumino silicate

tend to have larger values of depth of layer and compressive stress compared to soda-

lime glasses shown in the figure 3.

Figure 3. Depth of layer vs. compressive stress for soda-lime commercial glasses and alkali alumino silicate glass (Gorilla GlassTM). Source: 71st Conference on Glass Problems

The strength of soda-lime glasses and alkali alumino silicate can also be

compared by ring on ring test done by scratching the surface at various loads. The

strength of soda-lime glass dropped when a scratch load of 0.25 N is applied compared

to alkali alumino silicate glass which retain its strength after 1 N load. The slower drop in

strength indicates a greater damage tolerance. The result of the test is shown below:

Figure 4. Failure as a function of damage induced by scratch. Source: 71st Conference on Glass Problems

Conclusion

Glass strengthening process such as ion-exchange process will help improve the

strength and damage resistance of a glass. Compared to soda-lime glass, alkali

alumino silicate glasses can be ion-exchanged to have a larger depth of layer and

compressive stress. The deeper depth and higher compressive stress is achieved by

alkali alumino silicate glasses. These two properties give superior strength and damage

tolerance. This of great importance in making the glass screens on smartphones intact

and do not fail due to everyday use that exposes the glass surface to damage.

References

Gomez, S., Dejneka, M., Ellison, A., Rossington, K. A Look at the Chemical Strengthening Process: Alkali Alumino Silicate Glasses vs. Soda-Lime Glass. 2011. Glass Research, Corning Incorporated, Corning, NY, USA.

McCann, A. Okay, But How do Touch Screens Actually Work? Retrieved from http://scienceline.org/2012/01/okay-but-how-do-touch-screens-actually-work/ on May 20, 2016.

Rohrig, B. Smartphones: Smart Chemistry. 2015. American Chemical Society. Washington, DC.

Strickland, J. How Gorilla Works. Retrieved from http://electronics.howstuffworks.com/everyday-tech/gorilla-glass.htm on May 20, 2016.

Tech Explainer. Resistive vs. Capacitive Touchscreen. Retrieved from https://techexplainer.wordpress.com/2012/04/02/resistive-vs-capacitive-touchscreen/ on May 20, 2016.

Varner, J., Wightman, M. Fractography of Glasses and Ceramics VI. 2012. The American Ceramic Society. pp 85-88.Wang, J. Corning: Technical Materials. 2008. Corning Incorporated.

BIBLIOGRAPHY Roylance, D. (2008). Mechanical Properties of Materials.

Malou, Z., Hamidouche, M., Bouaouadja, N., Chevalier, J., & Fantozzi, G. (2013). Thermal Shock Resistance of Soda Lime Glass.

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