Hardness is a measure of the materials resistance to scratching, abrasion and indentation. The hardness of a rubber describes how firm or soft the material is.
Hardness is an important factor in product design and is often one of the first properties to be considered during material selection. The desired hardness of a material will be based on its intended application.
Hard materials are chosen to achieve a high resistance to surface penetration. Harder materials will be more durable and tend to offer more resistance to wear either by friction or erosion by steam, oil, and water. This makes hard rubbers a suitable choice for applications such as caster wheels or applications in the construction industry. Hard materials should also be selected in situations where high pressure is used. Softer compounds on the other hand are often chosen for gaskets and O-rings as they stretch easier and seal better on rough surfaces. For a gasket the material must be as soft as possible in order to completely fill the space between the two surfaces, but it must also be strong enough to resist the lateral forces acting on the gasket. The hardness of a rubber can be used as an indication of the materials resistance to bending or its stiffness.
There are three types of material hardness which can be measured:
- Indentation hardness – The resistance of the material to localised plastic deformation.
- Rebound/dynamic hardness – The elastic hardness of a material. The rebound hardness measures the energy absorbed by the material on impact which is returned to the indenter. In this case the material regains its original shape and is not permanently deformed.
- Scratch hardness – The ability of a material to resist surface scratches.
Each material can have different values for all three categories of hardness. Usually, the indentation hardness is what is referred to as the “material hardness”. There are various tests which can be carried out to measure the hardness of engineering materials and the hardness can be expressed on many scales such as Brinell (HB), Rockwell Hardness, Leeb hardness value or Vickers Hardness (HV). The hardness of elastomers is usually determined using the Shore Hardness Scales or the IRHD scale. On all scales a higher number indicates a harder material.