Sciencemadness Discussion Board

What is theory behind mohs hardness, files, sandpapers?

CharlieA - 14-2-2019 at 18:08

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

mayko - 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!

happyfooddance - 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]