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Cover Screen What’s Excellent Quality 100% Full Chemically Strengthened & Thermally Tempered?
Cover Screen Tempered
For a piece of bare cover screen glass, it’s fragile that easy break during regular operation, impact, drop or hit. So tempering treatment will be applied on it. But when you deciced to custom a piece of cover glass, you may asked if the cover screen glass should be chemically strengthened or thermally tempered? If you’re first time in this field, maybe get puzzle. For this article, we explain all details about this 2 treatments for your better understand.
Chemically Strengthened
As its name, it uses chemical reaction way to make the glass hardened for thin glass usually 3.0mm below. The principle is to put the cover screen into the salt batch for around 6 hours, the glass surface ion Na+ will be placed by larger size K+ to create a compressive stress layer make the glass surface harder. But the flatness and optical clarity still kept.
When it break, it won’t become small particles but spread longer than thermal tempering.
To different glass substrates, the hardness result also different.
Chemical strengthening furnace.
Thermally Tempered
For thermally tempering, it uses high temperature to heat up the cover glass near to its melting point(become red) and then strong wind to blow to change it structure to create much better hardness 5-6 times than bare glass. When it break, it becomes small particles that won’t get hurt even touch by finger.
Since this kind of treatment method, thicker glass will be required to resistance bend or even blow away and meet the particle requirements, so usually applied 3mm above.
Thermal tempering furnace
What’s The Inspection Methods For Chemically Strengthening and Thermally Tempered on Cover Screen?
Chemically strenthening
Usually, we use FSM to test the CS(Compressive stress) and DOL(Depth of layer) parameters.
For different substrates, the result also different as below:
| Glass Type | Typical CS | Typical DOL | Notes / Process Conditions |
|---|---|---|---|
| Soda-Lime Glass | 600–900 MPa | 5–20 µm | Conventional chemical strengthening; CS improvement limited compared to aluminosilicate glasses |
| Aluminosilicate Glass (general) | 650–950 MPa | 9–17 µm | Patent examples; relationship: DOL ≈ -0.04×CS + 41 to -0.04×CS + 48.6 (CS in MPa, DOL in µm) |
| Aluminosilicate Glass (ultra-thin, one-step) | Higher CS | Shallower DOL | One-step KNO₃ process; K⁺→Na⁺ exchange; higher CS but inferior impact resistance due to shallow DOL |
| Aluminosilicate Glass (ultra-thin, two-step) | Slightly lower CS | Higher DOL | Two-step process; inward shift of K⁺ peak; superior mechanical properties and fracture control |
| Borosilicate Glass Vials (BSGV) | Up to 55% lower than Valor® | Up to 89% lower than Valor® | Trade-off between CS and DOL observed; 450°C/4h gave properties approaching Corning Valor® |
| Lithium Aluminosilicate (LAS) Glass | Varies with post-annealing | Varies with post-annealing | Sub-Tg annealing causes CS decrease and DOL increase due to stress relaxation and alkali-ion redistribution |
| Glass-Ceramic (Li₃PO₄ / Li₄SiO₄ crystal) | ≥400 MPa | ≥70 µm | Haze ≤0.5% (0.7mm); ST limit 18,000–30,000 MPa·µm |
Thermal Tempered
To testify the tempering effect, usually 2 kinds of tests will be applied including:
1.Particle test: Use the impact pen / hammer to hit the glass, when it break draw a 50x50mm square with the biggest size particle locate in center to count particle numbers.
The numbers OK or not depends on different size and thickness(For example thickness 3.0mm the particle numbers should meet 60pcs).
2.IK test: Use steel ball drop from certain height into the glass center(It means IK grade) depends on different thickness 3 times without break.
Which Way Should be Selected On Cover Screen?
1.Normally 3mm below for chemical strengthening and above for thermal tempered.
2.But if you will need safety glass, thermal tempering is the first choice but also depends on actual glass size and thickness.
3.If the cover screen will have high requirement on flatness and optical performance(no distortion allowed), chemically strengthening will be the only choice.