Particle hardness

Particle hardness describes the resistance of an individual particle to mechanical stresses such as indentation, scratching, breaking and grinding. This includes indentation, scratching, breaking and grinding. Particle hardness is a material-specific property. It is closely related to the crystal structure, the bonding conditions, the porosity and any binder matrix.

In materials science, hardness is traditionally determined on compact solids. Common methods include indentation tests such as the Vickers, Brinell or Knoop hardness tests, as well as scratch hardness tests based on the Mohs scale.

These methods require a continuous material. If, however, the concept of hardness is applied to individual particles, micro- or nanomechanical hardness is particularly relevant. This can be measured using instrumented indentation tests or AFM-based measurement methods.

In bulk solids and process engineering, however, particle hardness is often defined functionally. Hard particles resist comminution and abrasion. Soft or brittle particles, on the other hand, fracture even under low shear or impact loads. This has a direct impact on particle destruction in mixers, conveyors and mills. Particle hardness also affects dust generation and changes in particle size distribution during process operation.

Of particular technical importance is the distinction between the hardness of the particle substance and the mechanical stability of agglomerates. Porous or spray-dried agglomerates, for example, may consist of mechanically hard primary particles. As agglomerates, however, they are generally soft and prone to disintegration. It is therefore necessary to distinguish between crystal, particle and agglomerate hardness. These hierarchical levels should be considered separately when designing mixing, conveying and comminution processes.