How do I get dull

This is why soft hair makes hard razor blades dull

Human hair is 50 times softer than steel, yet it can destroy the sharp edges of a razor blade. MIT researchers have found out what happens in the process.

Why do razor blades get dull? MIT researchers come up with a surprising answer.

Photo: panthermedia.net/Voyagerix

Razors, scalpels and knives are made from stainless steel, sharpened and coated with even harder materials like diamond-like carbon. However, knives must be sharpened regularly, while razor blades are disposable products.

Engineers at the Massachusetts Institute of Technology (MIT) in Cambridge have investigated why a human hair that is 50 times softer than the blade itself can damage steel. They found out that the processes involved are more complex than previously assumed. It's not just about sheer wear and tear as might be expected. In fact, a single strand of hair can cause the edge of a blade to chip. As soon as a first crack forms, the blade is susceptible to further damage at this point. More cracks will appear and the razor edge will quickly dull.

The blade is more prone to chipping if the microstructure of the steel is not made of a uniform material. The angle of approach of the blade to a strand of hair and defects in the microscopic structure of the steel also play a role.

Metallurgical mysteries

Background: C. Cem Tasan has been studying the microstructure of metals for a long time in order to develop new materials with exceptional resistance to damage. He is professor of metallurgy at MIT; here he heads the Department of Materials Science and Engineering. “We're metallurgists and want to learn what controls the deformation of metals so that we can make better metals,” says Tasan.

To find out why blades become dull when shaving soft human hair, the researcher first carried out some experiments under everyday conditions. He used commercially available disposable razors every morning. After each shave, he used a scanning electron microscope (SEM) to take pictures of the razor blade to see how the blade wore out over time.

Surprisingly, the tests showed very little wear or rounding of the sharp edge over time. Instead, Tasan noticed that chips were forming along certain areas of the razor blade edge.

“This was a mystery to us: We saw splinters, but not everywhere, but only in certain places,” reports the researcher. "We wanted to understand the conditions under which these splinterings take place and what triggering factors are there."

Observe effects on a microscopic level

To answer this question, Gianluca Roscioli, a PhD student in Tasan's research group, developed a micromechanical device for shaving experiments under controlled conditions. His device consists of a movable table with two clamps on each side, one to hold the razor blade and another to anchor strands of hair. Roscioli used blades from commercially available razors. He experimented with different angles and depths of cut to mimic different situations while shaving.

The experimental apparatus was designed so that it fits into a scanning electron microscope. Roscioli took high resolution images of both the hair and the blade while performing cutting experiments. The engineer used both his own hair and hair that came from laboratory colleagues to account for biological differences.

This is how hair destroys the razor blade

Regardless of the thickness of a hair, Roscioli found the same mechanism: hair caused parts of a razor blade to split off, but only in certain places.

Analyzing the SEM images Roscioli took during his experiments, he found that when the hair was cut perpendicular to the blade, no chipping occurred. However, if the hair was able to bend freely, it was more likely to be destroyed.

To find out the conditions under which metallic chips form, the team carried out computer simulations. They modeled a steel blade that cuts a single hair. Parameters such as the cutting angle, the direction of the force exerted when cutting and, above all, the composition of the steel of the blade have been adjusted several times.

There were three criteria with a high probability that metallic chips would be produced. This concerned flat angles between the hair and the blade, metallurgically heterogeneous steel samples and especially the contact of hair with local weak points in the steel.

Homogeneous steel - more durable blades

"Our simulations explain how the heterogeneity in a material can increase the stress, so that a crack develops and grows," says Tasan. He has applied for a patent for the process to bring steel into a more homogeneous form. The engineers hope that this could produce more durable, more permanent blades.

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  • Michael van den Heuvel

    Michael van den Heuvel studied chemistry. Among other things, he works for Medscape, DocCheck, for the University of Munich and for specialist pharmaceutical magazines. He has been a freelance journalist and shareholder of Content Qualitäten since 2017. His topics: chemistry / physical chemistry, energy, environment, AI, medicine / medical technology.