Hardness

In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by pressing or abrasion.

Hardness is dependent on ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity, and viscosity.

Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object.

In order to use it a weight of known mass is added to the scale arm at one of the graduated markings, the tool is then drawn across the test surface.

[2] Indentation hardness measures the resistance of a sample to material deformation due to a constant compression load from a sharp object.

Ultrasonic Contact Impedance (UCI) method determines hardness by measuring the frequency of an oscillating rod.

The rod consists of a metal shaft with vibrating element and a pyramid-shaped diamond mounted on one end.

Ultimate strength is an engineering measure of the maximum load a part of a specific material and geometry can withstand.

[5][6] Some materials are stiffer than diamond (e.g. osmium) but are not harder, and are prone to spalling and flaking in squamose or acicular habits.

In fact, most important metallic properties critical to the manufacturing of today’s goods are determined by the microstructure of a material.

[8] In glasses, hardness seems to depend linearly on the number of topological constraints acting between the atoms of the network.

Dislocations provide a mechanism for planes of atoms to slip and thus a method for plastic or permanent deformation.

Similarly, as more interstitial atoms are added, more pinning points that impede the movements of dislocations are formed.

The latter, which is conventionally obtained via tensile testing, captures the full plasticity response of the material (which is in most cases a metal).

It is in fact a dependence of the (true) von Mises plastic strain on the (true) von Mises stress, but this is readily obtained from a nominal stress – nominal strain curve (in the pre-necking regime), which is the immediate outcome of a tensile test.

This relationship can be used to describe how the material will respond to almost any loading situation, often by using the Finite Element Method (FEM).

However, these are all based on empirical correlations, often specific to particular types of alloy: even with such a limitation, the values obtained are often quite unreliable.

The underlying problem is that metals with a range of combinations of yield stress and work hardening characteristics can exhibit the same hardness number.