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Author: Subject: What is theory behind mohs hardness, files, sandpapers?

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[*] posted on 14-2-2019 at 17:12
What is theory behind mohs hardness, files, sandpapers?

I've heard it hundreds times "ability of substance to abrade or indent each other". But I need more detailed explanation.
Perhaps from some scientist, phd, or experienced person.
Is harder material absolutely (permanently) harder or relatively (gets wasted with usage)?
Here is my personal experience and knowledge.
I think it is absolute, meaning with one harder substance you can abrade/sand unlimited amount of softer substance.
But then I found in manuals, forums, and experience that files, chainsaws, axes, knives get dull with time even if you use them only on soft substances.
So bread or wood is able to make knife or chainsaw or axe dull after long time?
If so, are there any equations or absolute relations that can be used to determine how long will some tool be useful before it gets dull and useless?
Are there certain relative parameters besides long use that impact this that is universal rule for any material? Moisture, temperature, light, air, magnetism...?

Also why nothing happens when I rub my glass bottle against knife? Isn't there supposed to be falling of knife (metal) powder? What is the trick that enables something to be classified as file or sandpaper? Concentrated pressure at certain small area? Could never understand how they make such stuff. Sure, they use crystals and some weird lines, but based on what theory?

I am confused!
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National Hazard

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[*] posted on 14-2-2019 at 18:08

You need to do a little literature research before asking such a general question. See
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International Hazard

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[*] posted on 14-2-2019 at 18:34

It sounds like you might be conflating two different processes/characteristics under the umbrella of "hardness": abrasion and malleability.

Hardness in the context of the Moh's scale refers to the ability of one material to rub pieces of another off. This is what happens when you scratch a piece of glass with a diamond: the harder material breaks off very small pieces of the softer material.

Hardness in the context of a knife refers to its ability to keep or loose its shape: steel is firmer than florist foam, but can be deformed by pressure. That's what happens when you cut a bunch of carrots: The knife slices each carrot, but is very subtly bent by it.

Knife sharpening, on the other hand, often does involve abrading the metal away to reshape it.

"hardness" can mean a lot of other things, too. For example, a diamond will scratch a lot of things, but it is also very brittle; iirc the scratch test is actually not recommended for diamonds because the unresolved thermal stresses created can cause it to shatter!

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National Hazard

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[*] posted on 14-2-2019 at 18:54

Not only what others have said, but also certain materials have an endurance limit and because of this, they can take a certain repeated load an infinite amount of times. Steels are one such material. Aluminum is not. So, if a harder material like a hard aluminum abrades a soft steel repeatedly, eventually the steel will get work hardened, and could do so to a point where it was harder than the aluminum, and thus your "absolute" theory is nonsense.

Like others have said, maybe read some more. Wikipedia and google should be good for basic stuff like the questions you have.

Edit: For what it's worth, many steel tools get softened ("annealed") when heated by say, friction. If you understand these properties better you can practice better use of your tools. For example, rapidly cooling hot blades after use can prevent their becoming too soft. Too hot of a blade being cooled too quickly can have a deleterious effect, though: hardness is acquired but so is brittleness and you have a tool that chips easily. Like a Japanese style knife... This is due to the formation of martensitic crystal structures formed by quenching. Have you ever noticed how easily a file snaps?

[Edited on 2-16-2019 by happyfooddance]
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