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HOW COME MY RAZOR GETS DULL?

SURELY STEEL IS stronger than hair, right? Indeed, it’s fifty times stronger. Then how come razors don’t stay sharp forever? 

What with one thing and another, this may seem inconsequential. But, in fact, metallurgists have recently discovered complex failure mechanisms at the cutting edge. 

Here are tidbits gleaned from Science magazine’s “How Hair Deforms Steel,” reporting on research performed by Gianluca Roscioli et al. at Massachusetts Institute of Technology.

Hair-splitting Dullness. Brent Grocholski explains in “A Hair-splitting Way to Get Dull,” Science, August 7, 2020: “Whereas edge rounding and brittle cracking of a blade’s hard coating were thought to be responsible, a detailed microstructural investigation by Roscioli et al. shows a different mechanism. A combination of out-of-plane bending, microstructural heterogeneity, and asperities—microscopic chips along the smooth edge—sometimes caused fracture to occur if the conditions lined up. This fracture originated at the hair-edge asperity interface and created chipping that dulled a blade faster than other processes.”

The Steel Razor. Roscioli et al. write, “A typical metallic material used for blades in straight razors, for example, is a carbide-rich lath martensitic stainless steel, honed to a wedge geometry with an angle of 17° and a tip radius of 40 nm to obtain the desired sharpness. This material is coated with even harder materials such as diamond-like carbon and a final polytetrafluoroethylene layer to reduce friction.”

Martensite, by the way, is a very hard form of steel crystalline structure, named for German metallurgist Adolf Martens, 1850–1914. 

For perspective, 50 µm is about 0.002 in.; 5 µm, about 0.0002 in. This and other images from Science, August 7, 2020

Human Hair. The researchers write, “Similarly, human hair is a highly anisotropic composite with a noncircular cross section and an average diameter ranging between 80 and 200 μm [0.003–0.007 in.]. The outer layer is the ~170-MPa hard cuticle that forms a shell with cells arranged like shingles on a roof. The middle layer, the cortex, is three times softer and composed of a hierarchy of fibrils elongated along the hair direction. The medulla is the hollow inner layer and has little mechanical contribution to the cutting response.”

The Mechanism of Dulling. Researchers note, “During shaving, each single hair can be represented as a flexible cantilever, quasi-fixed at the end toward the skin and completely free at the other. In this configuration, the hair is free to bend when the blade approaches it and penetrates in it during the cut, influencing the mode of deformation.”

Both the steel blade and human hair are highly anisotropic; that is, their physical properties vary when measured in different directions (like wood, for example, varying along or across the grain). Thus, modes of deformation, both for hair and blade, are complex. 

Studying It All. The researchers “carried out interrupted tests and in situ electron microscopy cutting experiments with two micromechanical testing setups.” Their investigations revealed that the blade’s variation in martensite structure played an important part in a mixed-mode cracking phenomenon leading to appreciable wear.

Above, the blade at left approaches a hair at an angle. For perspective, 98.16 µm in this electron microscope image is about 0.004 in. Below, chips appear on the blade; 75 µm is about 0.003 in.

Roscioli et al. conclude, “All of these insights suggest the design of hard but more homogeneous microstructures for cutting tools. This can be realized by creating even finer structures at sharp edges, for instance, by further refining martensitic structure, by creating other nanostructured alloys to benefit from size effects, or even by avoiding crystallinity altogether.” 

Enhanced understanding of this could lead to improvements in everything from medicine to manufacturing to food processing to the home—with prematurely dulled razors a thing of the past. ds

© Dennis Simanaitis, SimanaitisSays.com, 2020 

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