Modelling the Surface Hardness Testing of a Nitrided Layer

The surface hardness of nitrided coatings is usually determined over a wide load range, typically between 0.1 and 200 N. The surface hardness measured at different loads characterizes the load-bearing and deformation capacity of the layer, which is highly dependent on the thickness of the layer, i.e. the distribution of the hardness (depth hardness) measured in the direction perpendicular to the outer surface. In practical measurements, the relationship between surface and depth hardness distribution is affected by a variety of confusing effects. Small loads, which cause indentation depths comparable to the grain size, cause the so-called indentation size effect (ISE), and surface quality, surface machining and its residual stress, internal inhomogeneities, texture and the possible presence of inclusions also modify the measured hardness. The use of a mathematical model of hardness measurement provides the possibility to study the relationship between depth and surface hardness distribution, free from these confusing effects.