Surface integrity reflects the properties of a material after it has been subject to some type of manufacturing process or modification. Engineers and product designers often plan projects based on the known characteristics of a particular metal. For example, these designers know that a specific steel alloy offers a set level of strength or hardness. After the material has been modified, these original properties may no longer apply, as many manufacturing processes create a permanent change in the material. Surface integrity helps these individuals determine how a material will change under certain conditions, and what it's new properties are compared to its old ones.
The surface integrity of any material is made up of two basic components. They include the topography and internal surface features of the product. Topography reflects changes on the exterior surface of a material, and includes things like smoothness, bumps or waves, pitting and cracks. Internal features address changes just below the outer surface, such as deformation and changes in strength or hardness. They do not include internal changes deep within the heart of a material, but rather in the layer just below the surface.
Ultimately, most manufacturing processes will have some impact on surface integrity, though this doesn't always have to be the case. Standard lathe work, grinding or milling, when done properly, does not impact surface integrity. When these processes are performed using poor techniques or dull tools, however, they can have a major impact on material properties. Excessive heat, cold or speed or work can also lead to significant changes.
More invasive procedures almost always have some permanent effect on surface integrity. They may include electro treatments, such as plating, which adds a permanent coating to metal, or chemical treatments. Almost any chemical treatment, as well as excessive heat, can alter the material at its molecular level, bringing about irreversible changes to its very structure. Burnishing and other types of deformation also bring about changes, especially when applied to plastics.
Changes in surface integrity can be either positive or negative. Negative changes could mean that the material can no longer be used as intended. For example, a steel column subject to quenching may ultimately be too brittle to support a structure. Positive changes are those that give the material the desired finish or appearance, such as burnishing to smooth out a rough piece of material. Positive changes in surface integrity also include those that improve properties like hardness, strength, or moisture resistance.