A simple introduction to chemical strengthened glass parameters

Whenever we talk about chemically strengthened glass like gorilla glass or similar, often you will hear these names: CS, DOL and CT. Let's break them down them each to better understand.

 

 

CS(Compressive Stress):

 

It's the "squeeze" on very surface of glass. Think it like a tight, invisible jacket that compresses the outer layer. The compression is to make glass harder to scratch and more resistant to cracks. The higher CS value, the tougher surface feels.

DOL(Depth of layer): It tells you how deep the compressed "jacket" goes into the glass. It's the thickness of strengthened layer measured from the surface inward. The deeper DOL usually means the glass can withstand deeper scratches or minor chips without shattering due to tough layer thicker.

 

DOL(Depth of layer):

 

It tells you how deep the compressed "jacket" goes into the glass. It's the thickness of strengthened layer measured from the surface inward. The deeper DOL usually means the glass can withstand deeper scratches or minor chips without shattering due to tough layer thicker.

 

CT(Central Tension):

 

It's the opposite force happening deep inside the glass. While the outside is being squeezed, the inner core is being stretched(It's the tension). You can consider it's the "balance" force. If the CS is push, the CT is the pull that holds everything together from inside.

 

Then which values related to strength?

 

In actual application, the CS and DOl are the 2 main players to determine how strong the glass feels in your hand. But there's a catch that many peiole don't realize:

 

The longer to strengthen the glass(soaking in chemical bath), the CS doesn't keep going up: In fact, it starts to down. Meanwhile, the DOL keeps growing deeper and deeper. So you can't just chase high CS number, you have to find a sweet spot. In industry, a deep DOL often seem as a better indicator of long term reliability than a super high CS only, since it means the protection runs all the way through.

 

The last but not least, there's another test you will see in datasheets:

 

4PB(4 point bending): It simply a physical test the glass is bent between 4 contact points until it breaks. It measures the actual flexural strength of whole piece: Basically how much it can bend before it breaks. It gives you the actual "endurance" score of the glass.

 

A simple conlusion:

 

CS: Surface squeeze(benefit for scratch and crack resistance)

 

DOL: How deep the squeeze goes(benefit for duraibility against deep damage)

 

CT:Inner stretch(keeps the glass bslanced: If CT too high the glass comes brittle and can exlode on it own)

 

4PB(The final bend test to prove the whole package works

 

So when you will need to evaluate the glass strength, don't just look at the highest CS number, also watch DOL and CT balance too. A slightly lower CS with a much deeper DOL gives a tougher and more reliable glass in daily use.

 

How to measure these numbers?

 

We use special tools to measure CS, DOL and CT instead of by eyes.

 

The most common one is the FSM (Surface Stress Meter). It works by shining a laser into the edge of the glass. The light bends differently depending on how much stress is inside, and the machine reads the light patterns (called interference fringes) to calculate CS and DOL values.

 

CT isn't measured directly: It calculated from CS, DOL, and glass thickness using a formula. The rule is simple: surface squeeze and inner tension always balance each other out.

 

One catch: FSM works best for glass strengthened on both sides (like phone screens). For one-sided strengthening, you need a different machine called a scattered light photoelastic meter.

 

As for the 4PB (4-Point Bending) test, it uses a universal testing machine (like Instron). The glass sits on two bottom supports, two top points push down until it breaks, and the machine records the force. That gives you the actual flexural strength in MPa.

 

Quick summary:

FSM → measures CS and DOL with laser

Formula → calculates CT from CS, DOL, and thickness

Universal tester → does the 4PB bend test for real-world strength