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What is hardness?

What is hardness?

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Image showing how pressure can cause an object to change shape Hardness is a measure of how resistant a material is to changing its shape when pressure is applied.
A knife can cut through butter as the steel in the knife is harder than the butter For one object to be able to cut another, it needs to be harder. For example, a table knife can easily cut through butter, because the knife is harder than the butter.
Image to show that a knife cannot cut a stone because the stone is generally harder than the knife If you used the same knife to try to cut a piece of stone, however, you would have a lot less success. This is because the stone is harder than the knife.

Measuring hardness

Moh's scale, a measure for hardness Hardness is measured on the Mohs scale, and is given a number between 0 and 10, with 10 being the hardest.
A bottle of talc which is one of the softest materials on the Moh's scale Talc is a very soft substance, measuring only 1 on the Mohs scale.
Image of a fingernail which measures 2.5 on the Moh's scale You could easily cut into a piece of talc with your fingernail, which measures 2.5 on the Mohs scale.
Magnetite in its naturally occuring form. This is the same material that forms on a file during the black oxide coating process. Magnetite, the compound that is created during black oxide coating, has a hardness of 5.5 on the Mohs scale.
A steel girder, which has a hardness of 5.5 on the Moh's scale Steel also clocks in at 5.5 without any added alloying elements.
Wonkee Donkee explains that the steel used in files is harder because it is alloyed with both carbon and chromium
The element chromium which occurs in concentrations of 5 to 7 per cent in chrome-alloy steel Chromium, the first of the two alloying elements used in the creation of the tool steel used in files, has a hardness of 8 on the Mohs scale. This is the first of two elements that increases the steel’s hardness.
A lump of carbon which is used in the steel alloying process during file making The second alloying element, carbon, has a low hardness under normal conditions.

Graphite, a type of carbon used as lead in pencils, only has a hardness of 1 or 2.

A diamond which has a hardness of 10 on the Moh's scale However, when carbon is heated to a high temperature under high pressure, it crystallises into diamond, which tops the Mohs scale with a score of 10.
Image comparing the hardness of steel and high carbon chromium steel on the Moh's scale As carbon steel is heated, the carbon content of the material adds to the hardness, instead of subtracting from it. While it doesn’t make it as hard as diamond, it works with the chromium to bring it up to a hardness of around 9!

Black oxide coating on steel files

Image to show that high carbon chromium steel is harder than magnetite, which is used in black oxide coating As you can see, the steel used in files is much harder than black oxide, which is why it is not often used as a protective coating. The file is tougher than it is!

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