Why do the edges of a steel razor dull from cutting far softer materials? The micromechanical conditions that lead to this phenomenon have been revealed through a series of "realistic shaving" experiments, the findings of which suggest ways for current blade technology to be improved, toward lengthening the lifetimes of commercial disposable razors and thus lessening their environmental burden. It's an everyday observation: The sharpest steel razor dulls quickly after a few short shaves. The tempered edge of the most finely honed chef's knife can become practically unusable after cutting tomatoes and potatoes. However, how steel loses it edge upon cutting significantly softer materials like hair - despite being more than 50 times harder - remains poorly understood. While it's generally been assumed that the degradation of a sharpened edge is due to basic wear mechanisms like edge rounding or cracking of a steel blade's brittle and hard coating, such explanations do not account for the underlying structural complexity of the interaction between the steel and the material it's meant to cut, nor for the dynamics of their co-deformation. Using martensitic stainless-steel blades - those typical of standard commercial disposable safety razor cartridges - and human hair, Gianluca Roscioli and colleagues performed a series of cutting experiments and in situ electron microscopy to observe the evolution of blade wear during realistic shaving conditions. Roscioli et al. found that differences in cutting angle due to hair bending, microstructural variation in the blades edge, and the location of these variations relative to the hair as it was being cut all worked in concert to cause a blade's edge to fail. The findings suggest that the design of cutting blades could be improved by implementing more homogeneous microstructures at the cutting edge, which could be realized using nanostructured alloys, for example.
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Journal
Science