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Name: Mohamed
Status: student
Grade: 9-12
Location: Outside U.S.
Date: Spring 2010



Question:
What is the difference between hardness and toughness?


Replies:
Mohamed,

In technical terms, hardness and toughness are used as qualitative descriptors for how much energy has to be put into a material in order to change its shape. For example, if we were to pull on a bit of plastic along a line (just in one dimension), and the plastic stretches for a bit before breaking, we want to be able to describe in words wether it took a lot of energy, wether the plastic stretched a lot, how much energy was required to break the plastic, etc.

Hardness describes how much energy it takes to deform (stretch, compress, bend, etc.) a material. If the material takes a lot of energy to change only a little, it is said to be hard. Conversely, if only a little amount of energy is needed to make a lot of shape change, then the material is soft. Metals would be considered hard, chewing gum would be soft.

Toughness describes how much total energy has to be used before a material breaks. If the material takes a lot of energy (it may change shape) before breaking, then it is a tough material. If only a little energy is needed to break the material it is weak or brittle (depending on wether the material yielded or not).

There are many words like these in common usage. Mostly it is a way for a scientist to communicate an idea to someone for whom numbers would not mean so much. These words might be used in combination also. Glass might be considered hard and brittle (takes a lot of energy to make small changes in shape, and breaks easily with just a little energy). Rubber might be considered soft, strong, and tough. Soft because it changes shape with a little energy, strong, because it does not yield (get to a point where it suddenly stretches) easily, and tough because it takes a lot of energy to break.

Remember though that these are just qualitative words used to make conversation easier and without using actual experimental numbers.

Greg (Roberto Gregorius)
Canisius College


Hi Mohamed,

While "hardness" is easy to define, "toughness" is a little more difficult. Perhaps the best way to explain this is by examples. Think of a steel hammer, and a diamond. The diamond is obviously very hard. However it is not tough. The steel hammer, on the other hand, is not particularly hard (you can easily scratch it with many hard substances, including the diamond). However the steel hammer is certainly tough. You can hammer things with great force, and the hammer is never damaged. In fact, if you hit a diamond with the hammer, using only very moderate force, the hammer will survive without damage, but the diamond (even though it is very hard) will be smashed into many pieces.

Hardened steel chisels, for example, can easily be heat treated to a very high degree of hardness. Just heat the chisel to a red heat, then plunge it into a water bath. The result will be a chisel that is so hard, it will even scratch glass. Unfortunately, however, that chisel will also be essentially useless. When hit with a hammer, it will tend to shatter. To add toughness (at the expense of slightly reduced hardness), the chisel is "tempered" by heating it to several hundred degrees (not to red heat) and allowing it to cool. The result will be a tool that is still hard enough to be useful (but not so extremely hard as before), but which is now tough enough that whacking it with a hammer will not damage it at all.

Hardness is a property that is necessary for cutting tools. They must be harder than the material they are cutting. Toughness, on the other hand, is the property that allows an object to withstand impacts without damage. You might think of "tough" and "brittle" as direct opposites, just as "hard" and "soft" are opposites.

Regards,
Bob Wilson


Simply put, hardness is the resistance to penetration the surface of a material has. If you were to subject two materials to the same force applied by the same shape, such as a small ball bearing, the one that the ball bearing pressed into the least is the harder of the two (hardness testers do exactly this, using different forces and indentor shapes and sizes for the different hardness scales).

Toughness is the total amount of energy a material can absorb before fracture. So something very resilient, like rubber as an extreme example, would be a very "tough" material, even though it might not be very "strong".

David Brandt


Hi Mohamed,

Toughness is the energy absorption needed to permanently deform an object. In mathematical terms it is the area under the stress vs strain curve to failure. For example, rubber is very tough (you can put a lot of energy into and it still returns to its original shape).

Hardness is resistance to plastic (irreversible) deformation. Elastic deformation is where an object bends, but returns to its original state -- plastic deformation is where it does not return to its original shape. In practice, people use the term hardness for a lot of different things. The classical definition is "what can it scratch without being scratched itself", but that does not give insight into the meaning. In specific fields, people talk about hardness in terms of specific types of plastic deformation -- such as resistance to being penetrated.

I would like to credit Dr. Michael Rauscher, an expert in materials science, for the content of this response.

Hope this helps,
Burr Zimmerman



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