Hardness and Toughness
Location: Outside U.S.
Date: Spring 2010
What is the difference between hardness and toughness?
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)
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.
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".
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
I would like to credit Dr. Michael Rauscher, an expert in materials
science, for the content of this response.
Hope this helps,
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Update: June 2012