eli5 How does razor blade dull on hairs when razor blades are made of steel and they are much higher on mohs scale?


eli5 How does razor blade dull on hairs when razor blades are made of steel and they are much higher on mohs scale?

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The Mohs scale and other scratch hardness measures tell you about if one material will scratch another. The hairs are not scratching a razer blade, or really abrading it much at all. They cause the sharp edge to [bend and roll over](https://cdn.shopify.com/s/files/1/2111/3997/articles/Knife_Edge_-_Magnified_d18ce694-e441-4098-bfff-7f5dece60878_2048x.jpg?v=1505150624). They don’t need to be particularly strong to do this, because the edge of the blade is necessarily very thin and metals are malleable.

As has been pointed out in the replies to this they also cause parts of the blade to chip. There’s also the issue of rusting if you leave it wet.

This is actually not due to the material but due to the sharpness of the blade. As the blade becomes sharper, the contact area between blade and hair reduces where the force required to cut the hair stays the same. For the same force, a reduced contact area results in higher stress concentration across the contact area on the blade edge and this is sufficient to deform the material. Also as the area reduces, the strength of blade reduces which also helps in deformation of the blade. The angle of approach of this deformed blade towards the hair is changed (from perpendicular to parallel eventually) which causes difficulties in cutting the hair or dullness in blade.

Moh’s scale defines hardness but the dulling of blade happens due to lack of its tensile strength.

There was a scientific study on this subject a few years ago and they found out that microstructures and the angle of cut drives the process of dulling the blade edge.

When a razorblade slices through a hair that is fixed to the skin, the hair bends away from the blade – thus changing the cutting angle. At some angles, the blade is subject to a large shear force that is perpendicular to the sharp edge. The team believe this causes the deformation and chipping of the blade


Bread dough is so soft you can push a finger through a block of it. Aluminum is hard enough to make some tools out of. And yet, if you tape a piece of aluminum foil to the edge of a counter so that it sticks out horizontally and then try to rest a one-pound chunk of bread dough on top of the overhang, the foil will bend and drop the dough on the floor. It’s not unusual at all for a softer material to damage a harder one. It’s more likely if the piece of soft material is bigger or thicker than the piece of hard material, if the soft material gets to attack the hard material repeatedly, and if either of them is moving fast or is pushed very hard.

The edge of a razor blade is much thinner than the hairs it’s cutting through, and it takes many hairs to damage the blade noticeably. And in proportion to the strength of the hairs and blade edge, the force your hand applies to the razor is big enough to not be insignificant.

The Mohs hardness scale is not used by materials scientists and engineers and it not relevant to materials performance.

The tensile strength is a much more relevant property.

The tensile strength of a beard hair is around 150-270 MPa, while the tensile strength of steel is around 400-800MPa.

Due to geometrical factors, stress concentrator etc, beard hair could easily cause yielding and plastic deformation in a razor, dulling it.