Many of us, without thinking twice, will interchangeably use the words strong, tough, and hard. In everyday conversations it’s no big deal and often just a matter of semantics. However, in a conversation of a metallic product it is a big deal which word you choose to use. In the discipline of metallography and metals analysis hardness, toughness, and strength are three distinct properties yet also share some overlap. Together, the three properties make up what is commonly referred to as metal “strength.” Here’s how these three metal properties are different and how they also overlap and work together:
Hardness: A material’s ability to withstand friction, essentially abrasion resistance, is known as hardness. Diamonds are among the hardest substances known to man, it is incredibly difficult to scratch a diamond. However, while a diamond is hard it is not tough. If you took a hammer to a diamond it would shatter, which demonstrates that not all materials that are hard are also tough. In the world of metal tools, drill bits and grinding discs must be extremely hard to be able to handle high amounts of friction.
Strength: The amount of force necessary for a material to deform. The higher the force required to change the shape of the material, the stronger the material is. Steel is notoriously difficult to pull apart, hence it has a high strength. Silly putty, on the other hand, is not strong at all, and merely requires a child’s touch to quickly deform this material into all sorts of shapes..
Toughness: How well the material can resist fracturing when force is applied. Toughness requires strength as well as ductility, which allows a material to deform before fracturing. Do you consider silly putty to be tough stuff? Under these terms, believe it or not, it actually is relatively tough, as it can stretch and deform rather than break.
It is very common that a single material will have more than one of these properties. These three categories do overlap. Drill bits, which are hard, must also be strong and tough so that they can perform as needed. Anvils are very tough and strong, they have to be with all the abuse that they take, but they are not necessarily hard. Bulletproof glass, on the other hand, is hard and tough but not incredibly strong.
There are also ways to enhance these properties. Metals coated with a ceramic finish can lend hardness to a metal that is already strong and tough. Alternately, carbon infused steel can maintain steel’s properties in the core but take on some of carbon’s properties at the edges.
So the next time you hear someone describe a material as being strong, hard or tough, consider whether or not those are the correct terms. In other words, think like a materials scientist.