Why do razors go blunt, but diamonds don't?

07 March 2017

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Question

Why do razors go blunt, but diamonds don't?

Answer

Chris Smith put this question to materials scientist Anna Ploszajski from University College London...

Anna - This is a brilliant question and it’s archetypal material science, so I’m in my element here.

A razor blade is usually made of a metal and it’s usually made of stainless steel. Now metals are made of crystals, funnily enough. What we mean by that is that in a crystal all of the atoms are lined up in a very 3D orderly pattern. Now in a single razor blade there are billions of tiny, tiny, different crystals. We call these crystals actually grains, and under a microscope they’ll look like crazy paving, so all of the different grains are orientated crystals, orientated different to each other.

At the very tip of a razor blade, every time that collides with a hair, it creates a little dent because the atoms at the tip are not very strongly bonded together, and so even the force of a hair colliding with them will cause them to reshuffle and over time this causes blunting.

The reason I was talking about the grains and the tiny crazy paving crystals earlier was that the points at which they meet are the weakest points in the material. So the fact that there are lots and lots of these grain boundaries, means that a razor blade is more likely to have its atoms disrupted by even something as gentle as a hair.

In contrast, a diamond - diamonds are made of carbon and these carbon atoms are bonded very, very strongly to four others. This is a very stable and a very strong structure. Diamond is actually the hardest material that we have, and it’s down to these carbon bonds. In the case of a diamond that wouldn’t go blunt, it’s probably also because it’s a single crystal so it doesn’t have any of those grain boundaries, and so no weak points in its atomic structure.

Chris - Basically, if I keep rubbing a diamond up and down a piece of glass for example, I shouldn’t get any loss or damage of the diamond crystal. It’ll be the glass that pays the price all the time?

Anna - Absolutely. So what we’re looking at here is the hardness of the materials and diamond is the hardest material that we have. So it would always wear away the glass rather than the diamond, yes.

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