Hardness is a measure of a material's ability to resist plastic deformation. In other words, it is a measure of how resistant material is to denting or scratching. Diamond, for example, is a very hard material. It is extremely difficult to dent or scratch a diamond. In contrast, it is very easy to scratch or dent most plastics. As shown in the diagram below, hardness increases from the very soft plastics to the incredibly hard diamond with most other materials ranging between.
A common method for measuring the hardness of a material is outlined in the figure below. A very hard sphere is pushed with a set force into the material. The resulting indent is measured for width and depth. A harder material will have a smaller width and depth, i.e., smaller indentation. Larger hardness results in a high resistance to deformation from compressive loads, i.e., resistance to scratches and dents, and better wear properties.
Now that you have been introduced to the concept of hardness, please go to your e-textbook and finish the reading for this chapter (pages 90 to 97 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University). When finished with the reading proceed to the next web page.