The Scientific Reason Why Razors Don’t Stay Sharp for Long
When soft hair hits the blade’s weak points, it causes the metal to chip apart
The blades in everyday razors are made of Martensitic stainless steel, a heat-hardened metal also used in surgical instruments, wrenches and cutlery. So why is it that after only a few weeks of cutting back stubble, razor blades need to be replaced?
According to new research published on Friday in the journal Science, the answer goes beyond predictable wear and tear. The steel doesn’t just become eroded from a sharp edge to a round one. Electron microscopy revealed that when hair pushes against weak points on the metal blade, the steel chips apart. And once it begins to chip, it falls apart more easily. But now that researchers know why razors fail so quickly, they can start to develop steel without the same weaknesses.
"For me, personally, it was both a scientific curiosity, of 'What's going on?' and also aiming to solve an important engineering problem," says Cemal Cem Tasan, a physical metallurgy expert at the Massachusetts Institute of Technology, to NPR’s Nell Greenfieldboyce.
The study began with lead author Gianluca Roscioli’s shaving schedule. Every three days, Roscioli shaved his facial hair with a variety of disposable razors. He brought the razors to the lab for imaging to track how the blades changed with each shave. The imaging revealed something unexpected in every blade: C-shaped chips missing from the sharp edge.
To better understand how something as soft as hair could chip away at a steel blade, the team set up an apparatus to record super close-up video of a razor slicing through hair. The team also analyzed the blade’s molecular makeup, Leah Crane writes for New Scientist.
They found that the steel has a vital weakness caused by the same process that makes the Martensitic steel so sharp in the first place. When the blade is moved across a coarse surface to sharpen it, microcracks form in the metal. And when a hair meets a razor at one of those microcracks, the crack widens, and chips flake off.
But it’s all too small to see with the naked eye.
"The size of the chips are about 1/10 of the diameter of a human hair," Roscioli tells NPR. "The more I shaved, the more chips started appearing on the blade."
Analysis with a computer model showed that several factors affect how much the blade will chip. Hair is especially damaging when it meets the blade not just at a weak point but also at an angle, and when the steel is made of an uneven mix of metals, per a statement.
“We want to design new materials that are better and go longer,” says Tasan to Wired’s Eric Niiler. “This problem of the blade is an excellent example. We are so used to it, you don’t think about it. You use the razor for a few weeks and then move on.”
The researchers have filed a provisional patent for a new razor manufacturing process that will have fewer microcracks. While it would be more expensive, Roscioli tells Wired, it would also last longer, saving money on replacements and reducing the polluting impact of disposable razors.
Materials scientist Jennifer Carter, of Case Western Reserve University, tells NPR that the research is “beautiful” and emphasizes that getting the microscope images was “not a trivial thing.”
University of California, Riverside materials scientist Suveen Mathaudhu tells NPR that while he thought that corrosion could play a larger role than the team found, the results make sense overall. He tells NPR, "It's a good finding that when the hair bends, or the blade interacts with the hair at a certain angle, it can cause chipping.”