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Knives are normally used to cut softer objects. So the edge of the knifes blade is harder than the substance it cuts through.

A blades geometry is more complex, but let's simplify it to the blade getting thinner towards the edge, ending at some thickness with a roughly round end.

So a hard substance pushes away a softer substance. I can see that on the first use, some irregularities break away, but it should become stable soon.

If the hard substance, like steel, has a crystalline structure of small crystallites, some of them may break away - but that is not fundamentally different.

But if a knife becomes dull on repeated use, the hard surface must be eroded somehow. Is that caused by grains of hard substances that are present as contamination in the soft substance? Or does a soft substance gliding over a considerably harder substance some other side effect other that gliding?

Why do most or all knifes become dull from repeated use?

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  • $\begingroup$ Rubber seals can and do wear grooves in hardened steel ie crankshafts... $\endgroup$ – user207455 Jun 7 at 10:51
  • $\begingroup$ Consider that at microscopic level surely atoms and "flakes" are lost even without contact to other object. Considert that even the old Kilogram sample loose weight. I cannot be technical but besides ablation the material probably redistributes or deforms resulting in broader edge, like a triangular section changing into a more rounded even rectangular one. There are rubber bands used to sharpen razors (they look like tick rubber at least), so they can ablate/deform the steel and this is beneficial if governed by the barber through the proper angle. $\endgroup$ – Alchimista Jun 7 at 11:02
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there are several factors at work here, as follows. I will assume a metal blade.

First of all, the very edge of a super-sharp knife is mechanically weak, in that the sharper it has been made, the less material there is to back up the edge against deformations. This means that if the edge hits a hard object head-on or nearly so, it will deform into a rounded shape.

Second, the edge of a sharp blade is in a precarious position with respect to chemical attack, in that it has a high surface area-to-volume ratio. This means that unless the material from which the blade is made is resistant to chemical attack, the edge of the blade will get eaten away. This is the reason stainless steel razors last so much longer than the carbon steel blades that they replaced.

Note that it is possible to make a blade self-sharpening by sputtering onto one side of it a very thin layer of a very hard metal. as the softer material of the blade erodes away, it continually exposes that harder material right at the edge of the blade which furnishes the cutting action.

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  • $\begingroup$ Thank you: I had never understood self-sharpening before. Stropping a razor, then, must be bending the edge back into shape rather than grinding it, because a strop is soft and can’t grind. $\endgroup$ – Martin Kochanski Jun 8 at 5:38
  • $\begingroup$ the strop can be thought of as an extremely mild abrasive, just enough to burnish off the very tip of a razor. Note that a dull razor has a rounded-off edge; straightening the kinks out of it will not make it sharp again. $\endgroup$ – niels nielsen Jun 8 at 5:47

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