Re: What is the hardest metal?

Sandra and Lance,

Wow, that's a tough question! I could tell you "it hasn't been invented yet" but that's the easy way out. (Maybe you could become materials engineers someday and develop the hardest metal!)

Let's start with what do we mean by hard. A good general rule is that if material A can scratch material B, material A is harder. (Test this yourself with a rock and an old piece of wood. The rock can scratch the wood so the rock is harder. What can scratch rock?) Another way to test for hardness is to press on the material using a small ball or upside down pyramid with a known force and see how deep of a dent we can make. The shallower the dent, the harder the material. Brinell, Rockwell, Vickers, and Knoop are four different scales to describe relative hardness. The higher the number on the scale the harder the material.

Where do we look to find material properties? One place is in books on materials. Other resources can be found on the internet and can be used as a guide. One such place is at:
http://www.matweb.com/search.htm

Here you can get material properties by giving the material name or you can specify a property and get the names of materials. A good materials engineer never takes this information to heart, however. It is only a guide. A good engineer always tests his/her material and finds out for sure how hard it is.

One of the hardest metals I could find was Hafnium Carbide. It has a Knoop hardness value of 2300! That's pretty hard. The only problem is it comes as a powder so it would require some fancy processing to make something with it. Tool steel is pretty hard and is easier to make something with, but it's Knoop Hardness is only about 850 - less than half as hard. To learn more about Hafnium go here:
http://www.periodic.lanl.gov/elements/72.html

This is from the Los Alamos National Labratory. They say that Hafnium Carbide is the "most refractory binary composition known", which I think is a fancy way to say it doesn't melt easily (Melting point for Hafnium Carbide is 3900 degrees Celsius!) A high melting temperature suggests a high hardness value (but not always).

I hope I answered your question. It is hard to know for sure what is the hardest metal but I think we are pretty close. Every day engineers are working on ways to make metals harder, stronger or lighter - the information is always changing.

Sincerely,
Steven Miller
Undergrad - Mechanical Engineering
San Diego State University