The hardness of a material is the single most important parameter when designing contacts. The harder material wears down the softer when in contact with each other. In mechanics, hardness is defined as the resistance of a material to permanent deformation during application of load.
Traditionally, hardness is measured on a relative scale such as Mohs or Vickers hardness. Each material is assigned a numerical value from 1 to 10 based on its relative hardness in Mohs scale. Recent advancement in nanoindentation technology allows the measurement of hardness over a variety of materials and is defined in terms of the area imprinted on a material for a given applied load. The hardness, H, from the nanoindentation method is calculated as:
Here, P is the applied load, A is the indentation area. There are different scales of measurement based on materials such as Shore hardness, Vickers hardness, Mohs hardness, and Knoop hardness to mention few. All of these represent the relative hardness of the material with reference to a standard sample. Nanoindentation removes the ambiguity in different scales by providing a physical measurement in terms of absolute hardness.
Unlike elastic modulus which is an intrinsic property of materials, hardness shows size dependence in materials with near surface hardness being different from the bulk hardness. The continuous stiffness measurement is an excellent technique providing depth dependence measurements to study the size effect of hardness in various materials. The size dependence of hardness can be understood by comparing two alloys made up of same material but different grain sizes. The finer grains lead to higher hardness in most materials because of the compactness of the packing.