A controlled scratch test for measuring the elastic property, yield stress and contact stress–strain relationship of a surface

The procedure for a controlled scratch test to measure the elastic property, yield stress and contact stress–strain relationship of a surface is described. A hard sphere scratches the surface at a progressively increasing load. The contact deformation is firstly elastic and then elastic–plastic. The contact width between the sphere and the surface as a function of normal load is measured and the relationship between the contact pressure and scratch strain is obtained. The elastic property and yield stress of the surface are then derived from the curve with good accuracy. The strain hardening ability of the surface can also be estimated from the curve. This method is very valuable for thick coatings and brittle materials in which these mechanical properties are difficult to obtain by other methods.