Eli5: Why do shaving razor blades become dull?


I saw a video once of how a razor blade starts getting dull after just one shave. It doesn’t seem like it should be the case if it’s literally just cutting hair.

In: 254

It’s because the blade edge is so fine. Yes they’re made of metal, but when the metal edge is [microscopically thin](https://i.dailymail.co.uk/1s/2020/08/07/09/31669388-8600793-image-a-38_1596787839546.jpg), then even something like hair can dent it after lopping through a few thousand (1 shave).

Check this out they even have microscope-video of a blade in action:



It’s because they are engineered to do so. If razor blades never dulled, then you’d never have to buy more, and the company would go out of business.

You may have noticed your kitchen knives dull too, even after cutting through soft things like vegetables. That’s because to have a sharp edge you need a very thin edge. At a microscopic level, the thinner the steel becomes, the easier it is to bend. The edge rolls in on itself just slightly, but it’s enough to be misaligned and no longer cut properly. That’s what happens when you notice the knife doesn’t “bite in” anymore, you struggle to pierce even tomatoes and such.

The quick and usual remedy for this, of course, is to use another knife or a honing rod and grind the edge of the dull knife across it. You may have seen chefs or waiters use this in restaurants. That tool isn’t actually hard enough to shave metal and sharpen the knife, and neither is another knife. Instead, what that does is it helps bend the rolled edge of the blade into position. If the knife is of good enough quality, it should restore most of its sharpness.

Now, imagine a blade that’s many times smaller and thinner. This same process happens. But you have no way to individually realign the edges of blades that are this small. And no, harder steel wouldn’t help here, because the blade is too thin, it would just chip. A rolled edge still cuts, but a chipped edge wouldn’t, and would most likely scrape your skin or even cut you. It’s preferable to have a softer edge that rolls instead of chipping.

Add into this the fact that human hair is actually pretty strong itself. Keratin, the stuff hair is made from, is among the toughest non-mineral structures out there, alongside chitin. Pound for pound, I’m pretty sure it’s stronger than steel. That is, a human hair would be stronger than a steel string of the same thickness. You can see how on a microscopic level this is a tough problem to solve.

Even electric shavers need their blades replaced every 2 years or so because they dull out. And this despite having many more blades, and thicker, than regular razors.

The same reason a knife becomes dull “just by cutting a tomato”. A tomato is obviously less hard then the knife and so is hair. But the cutting edge is extremely fine – and repeatedly hitting something, even a softer material, will warp it, first at micro then at macro level. That will eventually wear out and deform the edge, which needs to be re-formed.

Whenever two materials contact each other, there will always be a tiny but that wears off. Even something like dripping water will erode away stone. It just takes a lot of time.
The only way to slow this is to use materials that are very hard.
Now you might think steel is very hard, but with a very sharp blade, that cutting edge is actually quite thin. That makes is not so tough actually, so the contact it makes with hairs will start to dull it right away.