How is "hardness" defined in material properties?

Hardness is fundamentally defined as the resistance of a material to deformation, particularly to permanent deformation. When we consider hardness in the context of material properties, it typically refers to how well a material can withstand indentation, scratching, or deformation under applied force. This property is critical in applications where materials are subject to wear and tear, as a harder material will effectively protect against such deterioration.

In practical terms, hardness is assessed through various testing methods, such as the Rockwell, Brinell, or Vickers tests, which help quantify a material's ability to resist physical changes when subjected to stress. This definition aligns with a variety of engineering applications, where high hardness can indicate a material's durability and longevity under operational conditions.

The other options address different material properties: ease of shaping pertains to ductility or malleability, resistance to heat relates to thermal properties, and toughness refers to a material's ability to absorb energy and deform plastically without fracturing. While these are all important characteristics, they do not directly define hardness.