What is hardness a measurement of?

Hardness is primarily concerned with the resistance of a material to surface indentation. This property is crucial in determining how well a material can withstand wear, abrasion, and deformation when subjected to external forces or pressures.

When testing for hardness, methods such as the Mohs scale, Brinell, Rockwell, and Vickers tests are typically employed, each assessing how deeply a hard object can penetrate the material surface. The greater the resistance to indentation, the harder the material is considered to be. This characteristic is important in applications where materials are subjected to mechanical stresses and need to maintain their structural integrity.

In contrast, the other options mention different physical properties that are not directly related to the concept of hardness. While resistance to compression, bending, or thermal change are significant factors in materials science, they do not specifically measure the material's response to indentation, which is the defining factor of hardness. This delineation is what makes the concept of hardness unique and specifically aligned with the measurement provided in the correct answer.