Depending on the background of a given individual or situation, the concept of material hardness can be described and interpreted in many different ways.
The property of hardness can be defined as either a measure of material strength or wear resistance. In the product design world, engineers will commonly specify a minimum material hardness when strength is a concern because hardness is proportional to strength and because hardness is easily measured. Specifying material hardness is also crucial when differing materials slide against one another. In such a situation, the engineer or designer will specify material hardness to dictate which components endure and which components sacrificially wear away. For example, a shaft turning against a stationary bearing material would normally be harder than the replaceable bearing material that supports it.
Hardness can also be defined as the measure of a material's resistance to permanent deformation. A material's strength and resistance to permanent deformation is fundamentally based on the underlying structure of the material of interest. For metals, material hardness is a function of many micro-structural features including crystallographic structure, grain size, alloying, cold work history, heat treatment, second phase particles, etc.
Hardness testing is commonly used in a failure analysis investigation because the test is simple to perform, is relatively inexpensive, and creates minimal damage to the article of interest. Test results provide invaluable insight into material suitability for the application.
The standardized tests used to measure material hardness are as varied as its interpretation and can be classified into three major categories: scratch tests, dynamic indentation, and static indentation. These three test categories will be summarized and discussed in articles that follow.