What's the sharpest knife on earth?

  1. I was thinking about the principle of energy density and how sharp knives cut so easily because their surface areas are tiny and thus any force applied to them is imparted on this tiny area.

    So my question arises, does anyone know just how sharp the sharpest knife/ sword is? Lets no discuss microscopic knives, but rather macroscopic cutting instruments such as samurai swords. I would make a wild guess at a surface width of 10 microns, am I crazy?
     
  2. jcsd
  3. laser knife count?
     
  4. nope. no lasers.
     
  5. I've heard a titanium alloy knife that is the sharpest. It's very hard. But if that's the case, a diamond knife should be number one.
     
  6. As mentioned, a diamond knife should be number one. And I'm sure you can get pretty freaking sharp, but at the expense of the knife's durability. Sharper knives tend to be more brittle.
     
  7. Maybe this is what you're looking for:

    http://en.wikipedia.org/wiki/Damascus_steel

    Microns? Pff. Think nanometers =D Who knew we actually managed to produce and use nanomaterials 1000 years ago? They still aren't sure if its properties can be attributed to the nanotubes, but still, that seems to be very sharp for a weapon.
     
  8. Wikipedia saves the day yet again. :approve:
     
  9. Danger

    Danger 9,879
    Gold Member

    I'm not sure of the accuracy, but the guy certainly seems to know his stuff.
    http://lithiccastinglab.com/gallery-pages/2001julykanzichimp.htm
     
  10. Thanks guys, most interesting posts.
     
  11. Synthetic diamond scalpel blades are commercially available and well liked by surgeons. Contrary to what you might think they stay sharp much better than steel and chipping doesnt seem to be a problem. Quoted sharpness is 3 nm and achieved with plasma polishing. This is like 30 atoms wide at 1 angstrom per atom.

    Outside diamond, cracked glass edges have been used for years as ultramicrotome blades for cutting sections as thin as 200angstroms for electronmicroscopy. These would be similar sharpness to the obsidian blades quoted above by Danger. However the glass blades can cut less than 100 sections before geting blunt even on soft tissue. cutting bone is not possible. 40 yrs ago there were expensive diamond blades available that lasted well and would cut bone ok, but the glass ones were sharper and would cut the thinnest sections. The latest diamond blades are obviously sharper than glass if the 3 nm is correct.
     
  12. I saw something on Discovery channel where eye surgeons are using flint (obsidion)blades because they are sharper then any steel.
     
  13. Very nice post. 3nm is very impressive.
     
  14. Obsidian Is one of the sharpest things there is.......it can be up to 5 times sharper than steel.....An Obsidian blade can reach almost molecular thickness
     
    Last edited: Jul 10, 2007
  15. sharpest Knife

    I'd say Wootz should be pretty close.

    Diamond of course but you'd need to need to find a big enough diamond.

    My guess is a carbon nanotube edge might be worth a try.

    I wonder which non-steel superalloy might be as sharp or sharper than wootz while being possible to actually make (forge, grind or laser cut...) and not too brittle.
     
  16. stewartcs

    stewartcs 2,279
    Science Advisor

    Come on guys, eveyone knows the Ginsu is the sharpest! :rofl:
     
  17. When talking about the sharpness measured in atomic widths, you have to look at the molecular structure.

    Diamonds are cubic. This means two things
    1. They have distinct 90 degree cleavage (look it up if you think breasts. Check out the geology related cleavage article if you want to know more) in all 3 dimensions. This means that the edge will be flat because a large number of layers will just slog off of the cutting edge.(if you have a big diamond in your/your spouse's ring, you will want to be careful not to knock it on anything hard. It's easier to break in two than a piece of glass the same size. If the atoms are jolted just slightly out of place, the atomic repulsion will do the rest and your diamond will cleave.)

    2. The two sides of the blade will always want to be 90 degrees from each other, but you may be able to grind it down to around 45. The repulsion of each carbon atom to each next will keep you from getting a stable edge.

    Obsidian is a glass. Glasses being amorphous, do not have any cleavage so easy to shatter along. It does however have conchoidal cleavage, though its comparatively weak.
    This actually works to a glass knifes advantage. If you take some obsidian and knap off two sides, you can get the two sides to be around 33 degrees or even less depending on how fast the glass cooled and what minerals it is made up of. The concoidal cleavage means a smooth cutting surface, and the amorphous, but bonded structure, means that you can get down to a molecule thick. These blades are actually MORE durable AND SHARPER than a diamond blade. Try it. Break a peace of glass and see how easy it is for that glass to cut you. Some man made glass is specifically designed NOT to have these edges, so get a non shatter proof glass. Im not liable when you get cut or worse, you were warned.

    Obsidian, specifically, has been used for a very long time in the medical profession and was the prized tip for spears. We have had the sharpest knives on earth since before we could write. Now days they are used in ocular surgeries when lasers are not available. As far as the famed diamond blades are concerned, its their HARDNESS (mineralogical term. look up Mohs hardness scale) that gives them their notoriety. If your mineral is harder on Mohs scale than mine, you can cut mine. If mine is harder, your just going to blunt your edge. Now days, we harvest non jewler grade diamonds and artificial diamonds to COAT blades with. To cut glass, jewels or just anything you want a precise cut on, you use diamond because it will keep its edge the longest, seeing as its the hardest. There's also nothing it cant cut, while glass which can be sharper, is only a 5.5 on this scale to 10 (diamond). Glass will loose its edge on about anything.

    It'd make a wicked blade till you hit something other than flesh though!
     
  18. turbo

    turbo 7,365
    Gold Member

    I learned how to knap obsidian blades from an instruction set and some materials and basic tools gotten from an anthropologist. The flakes are incredibly sharp. He sold flakes to a company that mounted them in autoclave-able SS handles. Those scalpels were very popular with plastic surgeons. They cut cells rather than tear them like metal blades can do, so there was a lot less scarring and swelling, and shorter recovery-times. All very good things when you are cutting up movie stars and millionaires for a living.

    Edit: Necroposting can be fun!
     
  19. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    People doing dissections on insects used to make their own blades by snapping a steel wire in a flame, but it had to be old fashioned town-gas (CO) it doesn't work with methane.

    You can make tips for scanning electron microscopes by just cutting a brittle wire.
     
  20. That is very interesting turbo. One can even cut a single cell in two, and cleanly!
     
  21. :) funnyyyyyyyyyyyyyyy and trueeeeeeeeeee.
     
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