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How is glass hardness defined? I understand that ordinary kitchen knives cannot scratch most toughened glass, such as the ones found on cell phones. However (I once tried) - with enough pressure, one can actually scratch the glass.

So when testing the hardness of glass, is pressure standardized?

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Hardness is the resistance to plastic deformation. What you're observing is brittle fracture.

For example lead is softer than glass, but if I hit a piece of glass with a lead hammer the glass will break. This is because even the relatively soft lead is able to increase the stress on the glass to the point where brittle fracture occurs.

When you press the kitchen knife onto the glass the question is whether the steel will deform before the glass does, which indeed it will. Even so it's possible to raise the local stress under the knife to the point where the glass fails by brittle fracture. If you looked at the resulting scratch with an SEM you'd see it was a trail of microfractures not a plastic flow.

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