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 edge of a razor blade is very thin. And some research suggests that microcracks in the metal and the act of shaving hair causes the metal to chip and dull.
https://www.npr.org/2020/08/06/898577234/cutting-edge-research-shows-how-hair-dulls-razor-blades
At really tiny scale a blade can have defects. Even with hair being soft compared to a blade, at that scale the force exerted on the blade can cause it to bend. This introduces something called stress intensification.
Defect will cause the bending to chip, and those chip is what dulls the blade.
For rusting, those are also defects that can directly contribute to the dulling, but has nothing to do with hair
Chemical bonding on metal of shaving creams, sloughed off skin, hair fragments. Build up reduces edge into rounded irregular shape.
A very simple way of looking at the mohs scale is that it’s a scale of comparison. If steel leaves scratches on something but doesn’t recieve any scratches in return then the steel is significantly more scratch resistant (read: hard) than that something. The gimmick is that the steel was still scratched, but the ratio of damage between the steel and the something favored the steel significantly to the point that the steel *looked* undamaged to our eyes but the not-steel *looked* damaged. Small differences in hardness can result in extreme differences in practical scratch resistance though, so scratch testing typically pretty unambiguous.
But a razor edge on a blade is extremely fine (it has to be to cut things well) meaning that small damage done to that edge by softer things it encounters can add up pretty quickly.
The difference in hardness between the blade and the thing being cut does determine how quickly the blade wears down and loses its edge, but essentially any blade will eventually lose that nice sharp edge no matter what you’re cutting if you cut enough things. Hair is actually pretty hard for an organic material so it can cause damage to steel blades fairly quickly.
There are a lot more details to this of course, entire textbooks worth. The subtleties of this general principle are both highly elusive and very material-specific. It’s practically a whole field of mechanical engineering at this point and we’re still discovering details about how these interactions.
Mohs scale is not used for hardness (unless maybe you’re a geologist?). There’s a bunch of other hardness scales. Rockwell, Brunel, Vickers ,Knoop, etc. And the worst part is they’re not even comparable to each other really.
Hardness seems to be exceptionally difficult in metallurgy. Even within those scales there’s sort of sub scales. Rockwell B/C , brunel has a tungsten one.
The Mohs scale measures hardness. Hardness just tells you whether or not one material will scratch another; that is not what dulling is
Dulling is what happens when a thin edge (like a blade) folds over itself. That’s related to the material of the edge, the thinness of it, and the amount of force being applied to it.
I could push my blade flat through water, and it would eventually dull it; It’s the metal being too malleable at that thickness that causes it to dull.
Now, could you make it out of a less malleable material? Totally. But malleability is a sliding scale; The opposite of malleable is brittle.
I’d prefer my blades not hold an edge indefinitely, if the alternative is that they can snap or shatter under pressure.
On an unrelated note tho, keep in mind that hair is not weak. Your hair and fingernails are made of keratin; That’s the same thing that reptile scales, hooves and horns are made of; It’s very strong.
The issue is that our hair and nails are really thin. Similar case with glassware; Glassware doesn’t shatter because glass is weak, it shatters because we make it incredibly thin.
Same reason water slowly erodes rock. Just an analogy to go on top of answers from other people smarter than me
There’s a couple of things going on here, which have mostly been mentioned, I’ll just try to sum up.
First, the geometry of the hair and the blades leads to high resolved shear stresses. To avoid giving you an esoteric lecture on materials science, think of the atoms in the razor blade like a tree trunk- the direction that you apply stress changes how much it takes to fracture, and to fully fracture something you have to break the atomic bonds in all the grains. In large parts, the grains in the metal, analogous to grains in wood, are randomly oriented, and so it takes a lot of stress to fracture- each grain must be subjected to enough stress that unideal geometry can be overcome. With thin blades, it’s much easier to meet, and so micro chipping can occur, and the bending of the hairs means that the geometry becomes favorable in a way that cutting stiffer materials does not. This is a huge oversimplification, I recommend reading the study linked here:
https://physicsworld.com/a/bending-hairs-and-compliant-microstructures-make-razor-blades-dull/
Secondly, a lot of dulling, in all consumer edges, is a corrosion process. Stainless steel is awesome, but it is a bit of a misnomer- it greatly slows down corrosion, but you can’t really stop it without controlling the environment. This is made worse by the fact that hardened stainlesses push the definition of stainless, and some commercial grades don’t even contain the 12% of chromium which is supposed to designate a steel as stainless- this is a result of the mechanism which causes hardness in steels, which is a bit outside an ELI5. Suffice to say, dry your blades, and the edges will more slowly convert to rust and wear away.
Finally, wear is a probabilistic process. When atoms interact, there’s always a chance, even a small one, that they will pick up each other. With enough time and replacement, vinyl can wear through diamond, although you’d have to constantly clean and replace the ‘cutting’ surface constantly. While wear is obviously heavily influenced by the relative hardness of the materials, this can and does lead to gradual rounding of edges, particularly thin ones.
Hopefully this summarized some of the mechanisms in an understandable way, if you have any other questions about materials science or metallurgy, feel free to shoot me a DM, I love talking about my field.
Ultimately, for the same reason that you can’t cut a block of cheese with a single sheet of aluminum foil. Razor blades are as sharp as they are because the blade edge is very very fine, meaning the edge is very thin metal. Hair is soft in relation to steel, but still resists being cut, and this resistance bends the thin edge of metal over time.
I’ve seen it mentioned already but wanted to emphasize because of how crazy I found it to be.
The main cause of dulling is the blade rusting. I began giving my razors a shake, a wipe or two on a towel and then blowing on them once. Takes about 20 seconds or less. The lifespan of my razors increased DRAMATICALLY.
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