Hardness

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Physics

Diagram of a stress-strain curve, showing the relationship between stress (force applied per unit area) and strain or deformation of a ductile metal.

In solid mechanics, solids generally have three responses to force, depending on the amount of force and the type of material:

• They exhibit elasticity—the ability to temporarily change shape, but return to the original shape when the pressure is removed. "Hardness" in the elastic range—a small temporary change in shape for a given force—is known as stiffness in the case of a given object, or a high elastic modulus in the case of a material.
• They exhibit plasticity—the ability to permanently change shape in response to the force, but remain in one piece. The yield strength is the point at which elastic deformation gives way to plastic deformation. Deformation in the plastic range is non-linear, and is described by the stress-strain curve. This response produces the observed properties of scratch and indentation hardness, as described and measured in materials science. Some materials exhibit both elasticity and viscosity when undergoing plastic deformation; this is called viscoelasticity.
• They fracture—split into two or more pieces. The "ultimate strength" or toughness of an object is the point at which fracture occurs.

Strength is a measure of the extent of a material's elastic range, or elastic and plastic ranges together. This is quantified as compressive strength, shear strength, tensile strength depending on the direction of the forces involved. Ultimate strength is measure of the maximum strain a material can withstand.

Brittleness, in technical usage, is the tendency of a material to fracture with very little or no detectable deformation beforehand. Thus in technical terms, a material can be both brittle and strong. In everyday usage "brittleness" usually refers to the tendency to fracture under a small amount of force, which exhibits both brittleness and a lack of strength (in the technical sense). For brittle materials, yield strength and ultimate strength are the same, because they do not experience detectable plastic deformation. The opposite of brittleness is ductility.

The toughness of a material is the maximum amount of energy it can absorb before fracturing, which is different than the amount of force that can be applied. Toughness tends to be small for brittle materials, because it is elastic and plastic deformations that allow materials to absorb large amounts of energy.

Materials whose properties are different in different directions (because of an asymmetrical crystal structure) are referred to as anisotropic.

See also

• Hardness of ceramics

Other strengthening mechanisms

• Grain boundary strengthening
• Precipitation hardening
• Solid solution strengthening
• Work Hardening

Hard materials

• Ceramics
• Composites
• Metals
• Semiconductors
• Superhard materials

References

Bibliography

• Dieter, George E. (1989). Mechanical Metallurgy (SI Metric Adaptation ed.). Maidenhead, UK: McGraw-Hill Education. ISBN ISBN 0-07-100406-8. {{cite book}}: Check |isbn= value: invalid character (help)
• Malzbender, J (2003). "Comment on hardness definitions". Journal of the European Ceramics Society. 23: 1355. doi:10.1016/S0955-2219(02)00354-0.

External links

• An introduction to materials hardness
• Guidelines to hardness testing