Why Shaving Blades Become Useless After Cutting Human Hair

For a long time scientists have been fascinated with one problem when it concerns blades. Although blades are made of stainless steel and have edges that are razor-sharp, to further strengthen them they are coated with diamond-like carbon, but a material that is 50 times softer than a blade such as a human hair can be able to make the blade useless over time. From a logical point of view, this should not be the case.

 

Intrigued by this problem, the engineers at MIT’s department of Material Science and Engineering have come up with an innovative solution. These engineers concern themselves daily with exploring the microstructure of materials in order to design and make new materials that could be able to have exceptional damage-resistance properties. The lead researcher, Gianluca Roscioli, an MIT graduate student, came up with his idea when he was shaving his own hair.

After noticing that his blades tend to get dull with time after shaving, he decided to take images of the blades after each shaving activity. He took these images with a scanning electron microscope (SEM), scanning the blade’s edge in order to track how the blade wore down over time. What he discovered showed that the process is much more complex than a simple wear over time. He noticed very little wear and rounding out at the edges but instead realized that chips were being formed around certain regions of the razor’s edge. These led him to ask himself: Under what conditions do these chipping take place, and what are the ingredients for a strengthened blade to fail after shaving a material as soft as human hair?

To answer these questions conclusively he built an apparatus that was designed to fit inside an SEM and he used it to take samples of his shaving and that of his colleagues. They found that there were some conditions that might cause the edges of a blade to chip and as the chipping proceeds with time, it will cause the blade to get dull. The conditions depend on the blade’s microstructure. If the blade is heterogeneous or the microscopic structure is not uniform, the blade will be more prone to chipping. Also, the angle at which the cutting was done was found to be significant. Therefore, they found that shaving at right angles were better than lower angles. Finally, the presence of defects in the steel’s microstructure was another factor that played a role in initiating cracks on the blade’s edge. Chipping was found to be more prominent when the human hair met the blade at a weak point in the blade’s heterogeneous structure.

These conditions illustrate a mechanism that is well known in engineering - stress intensification. This is the intensification of the stress applied to a material because the structure of the material has microcracks. Once an initial microcrack has formed, the material’s heterogeneous structure enabled these cracks to easily grow to become chips. Therefore, even though the material might be fifty times stronger than what it is cutting, the heterogeneity of the material can increase the stress on it, making cracks to intensify.

The implications of this discovery is immense. It will save money and costs to the average user of shaving blades because it will offer clues on how the edges of a blade can be preserved, and give manufacturers the opportunity to make better blades or cutting materials by using more homogenous materials.

The engineers have already taken their discovery one step further. They have filed a provisional patent on a process to manipulate steel into a more homogenous form, with the hope that they could use this process to build longer-lasting, and more chip-resistant blades.

Material for this post was taken from the MIT news website.