What determines how sharp a metal is?

[silverghoul1]

I'm curious of this because I'm not sure if it's density, hardness or some property. For example, why is tungsten sharper than titanium. I know it's all in the edge, but what makes the metals vary, when it comes to edges

[fiveworlds]

Pressure of course. Some materials are too soft and will give under too much pressure. Not just metals but plastics and bones etc. Water as well under high pressure can cut.

[swansont]

As I recall (from a ways back) it has to do with the brittleness/hardness of the material. Soft materials don't hold a good edge but as you harden them they become more brittle.

 

Anecdote: years ago my dad wanted to do some intricate woodworking, and his Craftsman chisels weren't up to the task. Craftsman guarantees (or guaranteed) their tools for life, so they made them out of softer steel, but that meant they wouldn't hold a good edge. The ones that would were more brittle, and would eventually break, so nobody would put that kind of guarantee on them

[StringJunky]

As I recall (from a ways back) it has to do with the brittleness/hardness of the material. Soft materials don't hold a good edge but as you harden them they become more brittle.

 

Anecdote: years ago my dad wanted to do some intricate woodworking, and his Craftsman chisels weren't up to the task. Craftsman guarantees (or guaranteed) their tools for life, so they made them out of softer steel, but that meant they wouldn't hold a good edge. The ones that would were more brittle, and would eventually break, so nobody would put that kind of guarantee on them

You can get Japanese-style chisels that have a thin, very hard face, which holds the edge, backed onto a thicker, softer, more shock-absorbing steel.

 

For sheer maintenance of sharpness, Japanese-tempering styles are the best I've come across.

Edited August 23, 2015 by StringJunky

[Phi for All]

Actually, you can sharpen just about anything solid to a wicked degree, even wood. Sharpness is determined by the method and tools you use to sharpen the metal. Hollow grinding, for example, uses circular grinding to achieve a thinner angle at the very edge. Done carefully, just about any metal can have the same sharp edge, tungsten steel and titanium also.

 

But as others have pointed out, keeping that sharp edge after you've started working with the object is the tough part. And even with the best steel, if you make the edge too razor sharp, the edge will become dull and nicked more easily.

 

It really depends on your application. A short blade for shaving or cutting hair can be razor sharp, but longer, heavier blades have weight and momentum and don't need as sharp an edge. If you made a small razor blade from iridium, an extremely dense metal, it would probably be amazing for shaving hair off the body. But the larger you make the blade, the more brittle it would be. An iridium machete would be very heavy, and would probably shatter the first time you struck a branch with it.

 

Edit to add: An iridium blade would be brittle no matter the size, but for applications like shaving, it's brittleness isn't as much of a consideration. Until you need to sharpen it again.

Edited August 23, 2015 by Phi for All
Clarification

[OptimisticCynic]

Maximum sharpness of a metal is determined by how thin the edge can be. Theoretically this is determined by the size of the atoms or molecules. There is also the factor of the minimum wedge angle, which is determined by the angle(s) at which the atoms connect. More practically, the grain size determines the size of the edge. This is why different treatments of the same metal can result in different degrees of sharpness.