Hardness
:
For the mathematical concept of the difficulty of proving a conjecture, solving an equation, etc., see computational complexity theory. For water hardness, see hard water.In
materials science,
hardness is the characteristic of a
solid material expressing its resistance to permanent deformation. Hardness can be measured on the
Mohs scale or various other scales.
There are three principal
operational definitions of hardness:#Scratch hardness#Indentation hardness#Rebound, dynamic or absolute hardness
In
mineralogy,
hardness commonly refers to a material's ability to penetrate softer materials. An object made of a
hard material will scratch an object made of a
softer material. Scratch hardness is usually measured on the
Mohs scale of mineral hardness. Pure
diamond is the hardest known natural mineral substance and will scratch any other material. Diamond is therefore used to cut other diamonds; in particular, higher-grade diamonds are used to cut lower-grade diamonds.
The hardest substance known today is
aggregated diamond nanorods, with a hardness 1.11 times diamond.
Estimates from proposed molecular structure indicate the hardness of
beta carbon nitride should also be greater than diamond (but less than
ultrahard fullerite). This material has not yet been successfully synthesized.
In the
December 4 2005 issue of
The Jerusalem Post, Professors Eli Altus, Harold Basch and Shmaryahu Hoz, with doctoral student Lior Itzhaki
report the discovery of a
polyyne that is 40 times harder than diamond. It is a superhard molecular rod, comprised of
acetylene units.
Primarily used in
engineering and
metallurgy, indentation hardness seeks to characterise a material's hardness, i.e. its resistance to permanent, and in particular
plastic, deformation. It is usually measured by loading an indenter of specified geometry onto the material and measuring the dimensions of the resulting indentation.
There are several alternative definitions of indentation hardness, the most common of which are:
|
A Vickers hardness tester |
*
Brinell hardness test (HB)
*
Janka Wood Hardness Rating*
Knoop hardness test (HK) or microhardness test, for measurement over small areas
*
Meyer hardness test*
Rockwell hardness test (HR), principally used in the
USA*
Shore durometer hardness, used for polymers
*
Vickers hardness test (HV), has one of the widest scales
*
Barcol hardness test, for composite materials, scale from 0 to 100There is, in general, no simple relationship between the results of different hardness tests. Though there are
practical conversion tables for hard steels, for example, some materials show qualitatively different behaviours under the various measurement methods.
Hardness increases with decreasing
grain size. This is known as the
Hall-Petch effect. However, below a critical grain-size, hardness decreases with decreases grain size. This is known as the inverse Hall-Petch effect.
For measuring hardness of nanograined materials,
nanoindentation is used.
Also known as
dynamic or
absolute hardness, rebound hardness measures the height of rebound of an indenter dropped onto a material using an instrument known as a
scleroscope.One scale that measures rebound hardness is the
Bennett Hardness Scale.
*Dieter, George E.
Mechanical Metallurgy (SI Metric Adaptation). Maidenhead, UK: McGraw-Hill Education, 1989. ISBN 0071004068.
*
Tensile strength*
Toughness*
Yield (engineering)*
Young's modulus*
Softness*
Soft matter*
Hard matter*
Nanoindentation*
An introduction to materials hardness*
Engineering Stress-strain Curve
