What does the diameter of indentation indicate in hardness testing?

The diameter of the indentation in hardness testing serves as a direct measure of the width of the mark left on the material's surface. This width is critical because it provides a quantitative value that correlates with the hardness of the material. When a specific load is applied using a hardness testing method, such as Rockwell or Vickers, the resulting indentation's size reflects how resistant the material is to deformation under that load.

Materials that are harder typically exhibit smaller indentation diameters when subjected to the same force, while softer materials display larger indentations. This relationship allows for an effective comparison of hardness across different materials, making the diameter of the indentation an essential parameter in determining material properties.