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If you're someone who shaves regularly, you may have noticed your razor blades don't cut as well after just a few uses, and you're familiar with the endless frustration and expense of constantly replacing the darn things. It seems unfair. After all, steel kitchen knives go a bit dull after a few dozen onions, but not nearly as dull as a razor blade, which is also made of steel. But it turns out hair might be kind of like kryptonite for steel.

Steel is an alloy, meaning it's a mix of two different materials, iron and carbon. When they make razor blades, manufacturers start with a thin, flexible strip of steel, which they then subject to a series of heatings and coolings to give those materials a desired structure. Depending on the precise temperatures used, and how quickly the metal is cooled, different combinations of iron-carbon crystals will form - some stronger than others.

Razor blades are made of steel with a martensitic crystal structure: a particular arrangement of iron and carbon atoms that strike the right balance between hardness and flexibility.  All that is to say we know a fair amount about how to make steel, and how to create nice, sharp blades.

So a 2020 study, published in the journal Science, set out to explain why the razor blades we end up with don't do a better job of staying sharp. In it, one of the researchers used a razor to shave their beard, a little at a time. In between each shave, the team used a scanning electron microscope to take pictures of the razor. That's a powerful microscope, often used to image surfaces in detail.

Even before use, up close the razor edge looked a little jagged. But the more it was used, the more damaged the razor got. In some places, the team even saw the blade crack, bend, and eventually chip. It happened pretty quickly, way before the rest of the edge got dull.  All just from shaving hair!

And it turns out that the properties of both steel and hair are to blame. When you look closely, steel razor blades are mostly hard. All that heating and cooling to rearrange the crystals leaves behind an uneven structure with little soft spots mixed in.

In places where soft and hard spots lie next to each other, the razor is particularly vulnerable to cracking and chipping, especially if that border happens to be on the edge, which is never perfectly straight. Then there's hair's inherent bendiness. When blade and hair interact, each exerts a force on the other.  That's basic Newtonian physics. 

When the razor blade cuts a strand of hair head on, all the force from the hair goes straight through the blade, and the hair doesn't stand a chance, it gets sliced with no damage to the blade. But since hair is only held in place on one end, where it meets your skin, it can put up a fight by laying flat. This changes the angle between the razor blade and the hair so that some of the force points up, where the blade is weakest because it's thin.

If one of the soft, hard junctions happens to be there, even worse for the blade. And the hair is hardest on the outside, with a tough outer layer surrounding a softer core. So the part of the blade cutting the edges of the hair gets a double whammy of force, making them the riskiest place for those weak areas to end up.

So while you were shaving, these researchers studied the blade, and hopefully will be applying what they learned to create better razors.  Thanks for watching this episode of SciShow, and a huge thanks to Matthew Brant, this month's president of space, for your continued support. Your help makes SciShow possible, so thanks!

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