What are the three types of hardness? Hardness in general is the amount of resistance a material has to any kind of deformation from an outside source. The three main types of hardness include: indentation hardness, scratch hardness, and rebound hardness. Indentation hardness is the resistance a material has to deformation from a consistently applied force. The higher the indentation hardness, the greater ability to not have any resulting deformation from applied compression. Scratch hardness is the degree of resistance one material has when it is subjected to friction caused by another material. Materials that are less impacted by this scratching will have higher scratch hardness. Finally, rebound hardness is the amount of bounce that occurs when an outside object is dropped on the material in question. Often tested with a diamond-tipped hammer, a material with higher rebound hardness will lead to a higher bounce when the hammer is dropped.