When a scratching tool enters a fine-grained material, an entry section is formed by pure plastic deformation. The length of the entry section strongly depends on the corner radius of the diamond. If the material-specific shear stress is exceeded due to increasing scratching depth, a permanent deformation occurs thrusting aside the material and causing bulgings along the scratched groove. The base of the scratch and the flanks are even and cover a thin layer of plastically deformed material (Figure 3.13a).
Different scratches occur in the material from a critical scratch depth on with the boundary condition linked to it. Along with lateral crack systems observed during penetration tests, a median crack occurs through a succession of semi-elliptical radial cracks at the base that runs vertically to the surface in the direction of the scratch. Similar scratch structures were observed during Vickers indentation tests. Also, V-shaped cracks are formed vertically to the surface and spread. The aperture angle is between 40° and 60° and grows with the increasing distance from the scratch. Obviously, they are crack structures that develop due to the shear stress of the tangential scratching force (shear scratches). If lateral cracks grow until the V-shaped cracks extend vertically to the surface, whole material particles break off on both sides of the scratch.