eli5 How does razor blade dull on hairs when razor blades are made of steel and they are much higher on mohs scale?
In: 4692
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.
Latest Answers