Corning Gorilla Glass Code 2318 Chemical Strengthening Specification

Corning? Gorilla? Glass (Code 2318)

Chemical Strengthening Specification

Applications Note

Issued: June 2010

Corning Gorilla? Glass is a sheet glass that provides high compressive stress and a deep depth of layer on the surface when chemically strengthened. This glass can be utilized in applications such as touch screen panel covers, mobile phone display windows, and display windows for other handheld devices.

Chemical tempering strengthens the glass by developing a compressive stress on the surface. This is achieved by “stuffing” larger sized ions into the glass surface. During the chemical tempering process, the glass is submersed in a bath of molten salt at prescribed temperatures. Larger ions in the bath diffuse into the glass and replace smaller ions in the glass. Once the glass is removed from the bath and cooled, a compressive stress develops resulting in a stronger glass.

The degree of chemical tempering is measured by the magnitude of compressive stresses (CS) and the depth of the compressive stress layer (also called depth of layer, or DOL). The CS and the DOL are dependent on the time the glass is left in the salt bath, the temperature of the salt bath and the composition of salt bath. Generally, ion exchanging at a higher temperature causes a deeper DOL while the CS is slightly lower due to a slight relaxation process.

The compression layer acts as a protective armor with the magnitude of compressive stress and the depth of compression layer corresponding to the strength and thickness of the armor. Hence, the magnitude of CS and DOL determine the performance of the glass. However, practical considerations govern how high the CS and DOL can be in a given application. These considerations are:

1.The degree of tensile stress developed in the interior of the glass

2.The manufacturability

3.Material limitations for maximum CS

compression

(CS)

tension (CT)

Glass

Figure 1.

A schematic of the stress profile across the thickness in the glass shows that a compressive stress in the surface is balanced by tensile stress in the interior

Figure 2.

The specifications for CS and DOL can be visualized in the CS/DOL space.

When exchange of ions causes a compressive stress to build up in the surface, a corresponding level of tensile stress has to develop in the interior for the object to be in mechanical equilibrium. The tensile stress induced in the glass is representative of the energy stored in the glass after ion-exchange. Too high a level of stress is undesirable and needs to be controlled. This is done through a maximum CT limit.

The stress distribution in the glass is schematically illustrated in Figure 1. The tensile stress in the interior is approximated by the following formula:

DOL

T DOL CS CT

2Where CT is the central tensile stress, CS is the compressive stress, and T is the thickness of glass. It must be noted that a linear stress profile is assumed in the derivation of this formula. In actuality, the stress profile is non-linear resulting in some amount of error. For this reason, the calculated CT is used as a guide and to set specification.

Thus, the specification is defined by a minimum CS, minimum DOL and a maximum CT. This is illustrated in Figure 2 as a triangular CS vs. DOL space. Within this space, the performance is not constant, but changes as one go to higher DOL or higher CS. If the CS and DOL fall within this triangular specification space, the strengthened glass is expected to exceed a minimum level of performance denoted by the lowest CS and lowest DOL.

During manufacture DOL can be kept fairly constant as it is dependent on the temperature, time and temperature/time history during heat-up and cool down. By properly calibrating the temperatures in the tanks and following a good process control, a fairly tight DOL can be achieved.

The control of CS is not as straight forward. A tank with fresh KNO 3 will enable the highest CS to be developed.Since the strengthening process exchanges sodium for potassium, it is inevitable that the sodium content in the salt bath increase with time. This increase in sodium levels decreases the maximum CS that can be

attained as more glass is processed in the salt bath. When the CS reaches a minimum level, the salt needs to be drained and refilled with fresh salt. Thus, the specification for minimum CS affects the manufacturability of the glass.

Although, the maximum CS denoted by the Figure 2 goes up to 950 MPa, in reality this maximum depends on the glass being processed. Code 2318 Gorilla? glass generally develops a compressive stress lower than 800 MPa. For lesser glass thicknesses, the maximum CS developed can be lower than 750 MPa.

The guideline for minimum DOL and CS is based on several factors:

1.Studies have shown that well controlled edge machining will leave behind flaws in the 20 – 30 um

range. Having a DOL of >40 um ensures that these machining flaws are fully enveloped by the

compressive stress. Also, increased DOL will provide for increased resistance to breakage due to

damage introduced during use.

2.With the maximum CT defined, it is important to have enough process window for manufacturing.

This becomes especially true at lower thicknesses. The minimum CS/DOL levels are suggested based on having enough process space for manufacturability.

A series of component level tests such as four-point flexural strength, ring-on-ring strength test, abraded ring-on-ring retained strength test, crack initiation load on point contact, and scratch performance were used to verify these specifications

The specification in Figure 2 is written in such a way that the glass is processed at minimum performance defined by a combination of the lowest CS and lowest DOL, (600 MPa and 40 um). If an increased minimum level of performance is desired, either the minimum CS or minimum DOL can be increased. An increase in CS of about 50 MPa is equivalent to the increase of DOL by about 0.04 mm. Thus, a line joining 600 MPa/44 um and 650 MPa/40 um would define a equal level of performance. A more elegant way is to define an arbitrary minimum CT, that approximates this line. Figure 3 shows this line at CT > 43 MPa.

The specification as defined in Figure 3 will allow the process to run with a target defined by line ABCD. At point A, a target DOL is defined that will ensure compliance of DOL specification. Since, the salt is fresh, the highest CS is obtained. As more and more glass is processed in this salt bath, the CS decreases to point B. At this point, the target DOL needs to be increased to point C, so that the bath can be run further until point D.

Figure 3. A minimum CT will avoid parts with lowest CS AND lowest DOL thereby

effectively increasing the minimum performance level while not affecting

manufacturability. The line ABCD suggests possible target levels of CS/DOL for

manufacturing.

The introduction of minimum CT will introduce a level of complexity to the strengthening process. With proper process control, it is possible achieve a higher level of performance, while not compromising on the manufacturability.

The maximum CT values are experimentally derived and depend on the thickness of glass. Of these various specifications, the maximum CT value cannot be changed and should be closely observed, as exceeding the maximum CT value may increase the possibility of frangible behavior by the glass. This requirement for maximum CT will limit the minimum CS and minimum DOL that can be specified. Table 1 shows suggested minimum CS, minimum DOL, maximum CT and an optional minimum CT values. When designing the glass for a device, the device manufacturer should choose these values based on the level of performance desired, keeping in mind that a key aspect of the design of the strengthened glass article includes proper selection of the magnitude of compressive stress and the depth of compression layer appropriate for the application.

Glass Thickness

(mm)Minimum CS

(MPa)

Minimum DOL

(μm)

Maximum CT

(MPa)

Optional Minimum CT

(MPa)

0.505503575.051.0

0.555503571.044.0

0.606004068.050.0

0.656004064.846.0

0.706004062.042.0

0.756004057.036.0

0.806004054.034.0

0.906004052.032.0

0.956004048.028.4

1.006004045.025.5

1.106004041.023.2

1.206004038.021.3

1.306004033.018.2

1.506004021.013.5

Table 1: Suggested Specifications for chemical strengthening

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