We are all familiar with the phenomenon: the razor blade, made with the hardest steel available, stops cutting after a few uses; the sharpest edge of the most refined knife can become unusable – at least from the chef’s point of view – after slicing a few tomatoes and potatoes.
We still don’t quite understand how steel loses its edge when cutting materials that are much softer than itself, such as human hair, which is more than 50 times softer than steel.
Why the knife loses its edge
Although it is generally assumed that the degradation of a sharp edge is due to basic wear mechanisms, such as wire rounding or cracks at the edge of the brittle and hard coating of a steel blade, these explanations do not cover the structural complexity underlying the interaction between the steel and the material it cuts, nor the dynamics of its deformation during cutting.
To heal this tooth in scientific understanding, Gianluca Roscioli and his colleagues at MIT built a cutting simulator – a device that simulates the conditions of a razor cut by humans or a knife cutting some material – and placed it inside an electron microscope. scan to observe the process in its most intimate details.
They also filmed everything with ultra-fast cameras and even analyzed the molecular composition of the blades, to try to find out why soft materials – they worked with human hair, potatoes and cheese – can blind blades and knives, even though the blades are much harder and stronger than cut materials – the team used martensitic stainless steel blades.
Why the blade goes blind
The analysis revealed that, during cutting, the blades develop tiny bumps and recesses in the cutting edge. And these roughnesses arise due to the steel’s hardening process.
These tiny cracks tend to occur at the boundary between microscopic areas of steel that have slightly different properties – they are called steel granules.
The images show that when the blade cuts through the hair, these cracks tend to increase, with the amplitude of the crack depending on the angle between the blade and the hair and whether the hair meets the blade at a point where there is one of these boundaries between granules.
In other words, it is not that there is a wear of the blade on the wire: the blade literally breaks, opening cracks between its granules, causing teeth and splinters to appear.
Nanostructured steel
“Our main goal was to understand a problem that more or less everyone is aware of: Why blades become useless when interacting with much softer materials. We discovered the main ingredients of failure, which allowed us to determine a new processing path to manufacture blades that can last longer”
Researcher Cem Tasan.
And the team also offers some tips on how to preserve the sharpness of a blade. For example, when slicing vegetables, cut as close to a 90º angle as possible.
This is because electron microscopy images and super slow motion films showed that no cracks and teeth appeared in the steel when the hair was cut perpendicular to the blade. When the hair was free to bend, however, the chips appeared much more often – they appeared mainly where the edge of the blade met the sides of the hair.
The results suggest that the design of the cutting blades can be improved with the implementation of more homogeneous microstructures on the cutting edge, which can be done with nanostructured alloys of steel , for example, in which the granules are smaller.
Bibliography:
Article: How hair deforms steel
Authors: Gianluca Roscioli, Seyedeh Mohadeseh, Taheri-Mousavi, Cemal Cem Tasan
Magazine: Science
Vol .: 369 Issue 6504 689-694
DOI: 10.1126 / science.aba9490
Discussion about this post