We rely on sharp blades in our daily lives for domestic purposes, medical use, the food and catering sector, agricultural and industrial applications and everything in between. There are no corners of our society that do not benefit from the ready availability of sharp blades. Coupled inextricably with the demand for sharp blades in our homes and work places, is the age-old, relentless battle we wage against the dulling (wear) of our sharp edges.
Some blades, like knives, we strive to maintain in tip top shape and they require regular attention to keep their edges suitably sharp. Other blades are treated as consumables and are regularly replaced when they lose their sharpness. Stainless steel cutting edges like those on a razor blade, begin with an extremely sharp edge to which a coating layer may be added, which is harder than the steel itself, like diamond, amorphous diamond or diamond-like carbon. These coatings function to augment the strength of the blade and improve their corrosion resistance. The hard coatings applied to the razor’s cutting edge permit thinner edges and allow lower cutting forces to be applied.
Cutting edges like knives and scalpels require regular sharpening to maintain their sharpness but consumables like shaving razors are quickly disposed of when their edge becomes dull. This universal loss of sharpness that occurs on precisely engineered blades surprisingly occurs after cutting materials that are a great deal softer (~50x) than the blades themselves.
This phenomenon has been examined in close detail to determine how a razor blade can be damaged just by cutting human hair. It has been discovered that shaving hair deforms the blade in a more complex way than first thought. Shaving doesn’t simply wear down the edge with use over time. Studies at MIT revealed that even a single strand of hair can cause the edge of the razor blade to chip under certain conditions. Further examination showed that once an initial crack is formed, the blade becomes susceptible to the formation of further chips and cracks which quickly build up and soon render the razor’s edge dull.
It has been determined that the microscopic structure of the blade plays a critical role in the chipping of the blade edge. Logically, the blade is more susceptible to chipping and cracking if the microstructure of the steel is not uniform. Along with defects in the microscopic structure of the steel, another contributing factor to initiating the cracks is the blade’s approaching angle to the strands of hair.
Exploring the microstructure of metals gives clues as to what governs their eventual deformation. Studies of the shaving process have included the imaging of the razor’s edge after each shave, by scanning electron microscope. This process has enabled engineers to track how the blade’s edge wore down and dulled over time.
The scanning electron microscope imaging has confirmed that it isn’t so much wear that dulls the blade, but the formation of chips and cracks along the razor’s edge. The chipping was confined to certain locations along the blade and occurred mainly when the hair being cut was bending. However, it became apparent from the imaging that the cutting of the hair at an angle perpendicular to the blade did not cause the chipping.
Further studies revealed that there were at least three factors at play, all combining to cause chipping of the blade edge. Failure was most likely to occur when the blade struck the hair at an angle, when the blade was composed of heterogenous steel and when the edge of the strand of hair met the heterogenous steel blade at a weak point in its structure. This condition is known as stress intensification, which can occur when the hard material’s structure already has microcracks. The initial microcracks in the heterogenous steel enable the cracks to grow into chips, even if the stress is applied by a soft material.
With a greater understanding of the complexity of the factors that are at play in the blunting of a sharp edge, solutions to the problem can be postulated. The sharpness of a kitchen knife might be preserved longer if the cook cuts straight down into foods, rather than slicing on an angle. Also, blade manufacturers could investigate the advantages of using more homogenous materials to produce more chip-resistant blades that will maintain their sharpness longer.
The goal, therefore, is to manufacture more chip-resistant blades, so they last longer. One of the keys to achieving this is to process the steel used in the blades into a more homogenous form.
Further information: G. Roscioli el al., “How hair deforms steel,” Science (2020). science.sciencemag.org/cgi/doi … 1126/science.aba9490
Keywords: sharp blades, blunt, dull edge, heterogenous steel, wear,
homogenous materials, stress intensification, blade deformation,
sharpness, knives, razor blades.
Founder of TriboNet, Editor, PhD (Tribology), Tribology Scientist at ASML, The Netherlands. Expertise in lubrication, friction, wear and contact mechanics with emphasis on modeling. Creator of Tribology Simulator.