Eli5: Why does ceramic break tempered glass so easily.
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Tempered glass is under very high internal stress from the process of cooling it rapidly.
Ceramics are harder than glass so the impact of the two mostly transfers the force to the glass which causes the stress of the internal glass to exceed its ability to hold its structure. AFAIK.
Ceramic is very hard. Not in the general, colloquial sense of hard, but in the Mohs Hardness Scale. That scale measures whether or not something can scratch something else. Diamond is the hardest on this scale – not because you can’t break diamond. Diamonds are pretty brittle, you can easily crack or shatter them with a hammer. But you can’t *scratch* them.
Tempered glass is made by putting the glass in tension. You cool the glass so that the outside shrinks, compressing the still-hot inside. When the inside then cools, the outside is frozen in place so the inside pulls tightly on it the inside contracts. The glass pulling on itself keeps the molecules from moving, and holds it all together strongly so it’s very hard to break.
By scratching the surface, ceramic breaks the tension, releasing *all* of it throughout the glass.
Tempered glass is like a rubber band stretched out tight. Ceramic is like a razor blade. Once that razor blade nicks the rubber band, it introduces a weak spot that creates a cascading failure and the entire thing tears itself apart.
There is nothing special about ceramic that breaks glass easily. Glass is just very brittle to begin with so if it encounters anything harder than itself, the lack of flexibility in glass causes it to shatter. Ceramic just happens to be a common household material that’s harder than glass but if you had diamond floors that would work just as well.
The other thing not mentioned here is movement.
If you drop say a phone or a computer on a carpet the carpet will move slightly as the object is dropped dissapating the force. If you have say a see through case on your pc and drop it on a hard floor then the floor won’t bend but the metal on the case will and that pinches the glass.
This is why it will still break even if the ceremic never actually touches the glass
Normally, when you apply a lot of force on an object with a sharp tip, the tip will bend. For example, if you try to stab glass with the tip of a kitchen knife, the knife will most likely just bend (obviously don’t try this at home, very high chance of injury)
Ceramic is very non-bendy. This means that when you use a sharp ceramic tip to hit a piece of glass, all of the force gets transferred to the glass (at a very small point) instead of being lost to the bending of the tip. A lot of force on a small point will punch through glass and shatter it, kinda like how bullets work.
In addition, as others have mentioned, ceramic is harder than glass which can allow it to slice into the glass, giving it a better ‘bite’ into the glass instead of just slipping off like a softer (lower Mohs scale material) would do. However this isn’t a full explanation. For example, bullets can shatter glass despite being lower on the Mohs scale.
Diamond is also harder than glass, and can slice glass, but it will shatter before the glass will. Ceramic is much more shatter-resistant than diamond and will stay intact long enough to transfer most of its force into the glass.
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