Scratches are usually associated with a single occurrence in the specimen surface. Normal scratching includes deformation such as plowing, but might also involve material cracking, peeling and removal. In comparison, marring is the term used to describe relatively fine surface scratches generally characterized by shallow damage, distributed over a relatively large area that typically spoils the appearance of a surface coating. It may be convenient to consider mar resistance as a surface property, whereas scratch or abrasion resistance involves the body of the material as well.
scratch testing consists of using a hard metal or diamond spherical tipped indenter (typically with a radius of 200 μm) to apply an increasing load on the coating surface continuously while the sample is displaced at a constant velocity. Scratching of the surface results in increasing elastic and plastic deformation until extensive spalling of the coating from the substrate occurs at some critical load (Lc). In general, the critical load is determined by acoustic emission, friction force measurements, or optical microscopy.
For laboratory scratch and mar tests, a stylus with a defined geometry is drawn across a specimen surface at a known speed and with a known force. The objective of most scratch or mar resistance studies is to determine the behavior of the material under specific test conditions, establish a relative ranking of similar materials, or to determine the failure limit of a surface coating. This information can then be used to better understand material surface properties and performance characteristics.
