The indentation size effect (ISE) is the observation that

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

tends to increase as the indent size decreases at small scales. When an indent (any small mark, but usually made with a special tool) is created during material testing, the hardness of the material is not constant. At the small scale, materials will actually be harder than at the macro-scale. For the conventional indentation size effect, the smaller the indentation, the larger the difference in hardness. The effect has been seen through

nanoindentation Nanoindentation, also called instrumented indentation testing, is a variety of indentation hardness tests applied to small volumes. Indentation is perhaps the most commonly applied means of testing the mechanical properties of materials. The nanoi ...

and microindentation measurements at varying depths.

Dislocations In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to sl ...

increase material hardness by increasing

flow stress In materials science the flow stress, typically denoted as Yf (or \sigma_\text), is defined as the instantaneous value of stress required to continue plastically deforming a material - to keep it flowing. It is most commonly, though not exclusivel ...

through dislocation blocking mechanisms. Materials contain statistically stored dislocations (SSD) which are created by homogeneous strain and are dependent upon the material and processing conditions.

Geometrically necessary dislocations Geometrically necessary dislocations are like-signed dislocations needed to accommodate for plastic bending in a crystalline material. They are present when a material's plastic deformation is accompanied by internal plastic strain gradients. They ...

(GND) on the other hand are formed, in addition to the dislocations statistically present, to maintain continuity within the material. These additional geometrically necessary dislocations (GND) further increase the flow stress in the material and therefore the measured hardness. Theory suggests that plastic flow is impacted by both

strain Strain may refer to: Science and technology * Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes * Strain (chemistry), a chemical stress of a molecule * Strain (injury), an injury to a mu ...

and the size of the strain gradient experienced in the material. Smaller indents have higher strain gradients relative to the size of the plastic zone and therefore have a higher measured hardness in some materials. Geometrical_Necessary_Dislocations_during_Indent

Geometrical_Necessary_Dislocations_during_Indent

For practical purposes this effect means that hardness in the low micro and nano regimes cannot be directly compared if measured using different loads. However, the benefit of this effect is that it can be used to measure the effects of strain gradients on

plasticity Plasticity may refer to: Science * Plasticity (physics), in engineering and physics, the propensity of a solid material to undergo permanent deformation under load * Neuroplasticity, in neuroscience, how entire brain structures, and the brain it ...

. Several new plasticity models have been developed using data from indentation size effect studies, which can be applied to high strain gradient situations such as thin films.

References

Hardness tests Materials science {{engineering-stub Plasticity (physics)