It is a paradoxical phenomenon: although razor blades and kitchen knives are made of specially hardened stainless steel, they become dull over time – even if you only use them to cut material that is much softer than steel. Researchers have now investigated how this can be explained and what happens on a microscale. The surprising result: It is not a gradual rounding of the cutting edge due to erosion that dulls the razor blades, but a collection of tiny notches and cracks. They arise when the hair hits a certain microstructure of the steel in the cutting edge at a shallow angle while shaving.
Razor blades actually have all the requirements to be able to cut through a material as soft as human hair easily and without consequences. Because its cutting edge is made of a high-carbon stainless steel whose layered microstructure, the so-called lath martensite, makes it particularly hard. On the razor blade edge there is an even harder carbon coating, which is finished with a thin, friction-inhibiting polymer layer. And yet none of this helps: after a while of shaving, the razor blade inevitably becomes dull and kitchen knives also lose their sharpness over time – even if you haven’t cut much more than cheese or potatoes with them. “It’s really amazing that you can cut something soft like a human hair with something very hard like steel and the steel still gives way,” says senior author Cem Tasan of the Massachusetts Institute of Technology in Cambridge.
Razor blades in the electron microscope
What lies behind this paradoxical failure of the razor blade, he and his colleagues have now investigated in more detail in experiments. First author Gianluca Roscioli first made a self-experiment: he used a new razor blade and shaved it several times – until the edge became blunt. After each pass, he examined the blade in an electron microscope. To his surprise, there was only little wear on the cutting edge in the form of rounding and grinding of the sharp edge. Instead, after a while, the cutting edge began to show more and more small cracks and notches. “These micro-cracks initially spread perpendicular to the edge, before they then changed their direction and led back in an arch – this gave rise to the geometry of the flaking,” report Roscioli and his colleagues. The recordings also revealed that the martensite steel deformed immediately before breaking, while the hard, brittle coating tore immediately.
In order to track down the cause of the micro-notches, the scientists then carried out standardized tests in which they brought hair of different diameters and at different angles in the electron microscope to a firmly clamped razor blade and cut them up. Similar to beard or armpit hair, the hair was also attached at the lower end, but freely movable at the top. “We wanted to know the conditions under which this flaking occurs and what it takes to make the steel give,” explains Tasan. The researchers also wanted to find out why the tiny notches only appeared in a few places on the cutting edge.
Three factors are crucial
It turned out that if the blade cuts the hair exactly at right angles, it remains intact. “In these cases, we did not observe any deformation of the cutting edge or chipping,” the scientists report. “But if we simulated a realistic situation and inclined the cutting edge by 21 degrees, this led to plastic deformation and notches in several places.” The thickness of the hair or the number of hairs, on the other hand, hardly played a role. Another factor came into play here: As the researchers observed, the microcracks mainly occurred where the edges of the hair met the cutting edge. “This allows a single hair to create two notches in a cutting edge – each of them starts on either side of the hair,” say Roscioli and his colleagues. Closer analysis revealed that this occurs whenever the edge of the hair hits one of the softer spots in the steel’s microstructure.
This makes it clear that the paradoxical effect is due to the forces that arise when the blade and hair are at a flat angle and that create tension in the stainless steel. These then strengthen until they fail the microscopic weak points in the stainless steel of the blade. The side edges of the hair develop the greatest destructive force – probably because this is where the hard shell of the hair hits the side. “Because these conditions only come together in a few cases, commercial razor blades only become dull after repeated use,” explain the scientists. Their findings not only reveal the secret of blunt blades, they could also help produce more durable blades in the future. Until then, the advice remains for everyday life: the more vertically you cut the hair or the potato, the gentler it is on the blade.
Source: Gianluca Roscioli (Massachusetts Institute of Technology, Cambridge) et al., Science, doi: 10.1126 / science.aba9490