What does the term "hardness" measure in materials?

The term "hardness" specifically measures a material's resistance to deformation or scratching. This property is crucial in many applications because it indicates how well a material can withstand mechanical forces without being permanently deformed. Hardness can vary significantly between different materials and is often assessed using standardized testing methods, such as the Mohs scale or Rockwell hardness test.

Understanding hardness is essential when selecting materials for various applications, as it provides insight into their performance and longevity under stress. In contrast, the other choices pertain to distinct material properties: durability against corrosion involves a material's ability to resist chemical degradation; flexibility refers to how easily a material can bend without breaking; and thermal expansion rate describes how a material reacts to temperature changes. None of these aspects relate directly to the concept of hardness.