Bewertung der Lebensdauer von dünnen Hartstoffschichten: Der Impact-Test als ergänzende Prüfmethode

The impact test is a complementary test method for thin PVD and CVD hard coatings and was also examined beyond the specified tests for industry related layer systems such as, for example, CrN, TiAlN or AlCrBN. In addition to the conventional parameters, such as, layer thickness, hardness and adhesion, which describe a layer system, the result of the impact test gives a statement about the fatigue strength of the layer system. Depending on the application area, the impact test can be operated in a permanent contact or in a pounding mode. The reproducibility of the experiments is good, so that 2–3 impact tests permit a meaningful evaluation. If several tests are carried out on one component, a sufficient distance between the individual imprints is necessary in order to avoid mutual interference. The size of the impact imprint depends on the pressing and the material properties of the layer substrate composite, but not on the number of impacts. The deformation caused by the mechanical stresses within the material lead to coating damages at the edge and / or to expansion cracks in the interior of the impact imprint. The layer system is analyzed either by light-optical or by SEM-EDX with regard to damage. A cross-section of the impact imprint also allows it to make a statement about material damage under the layer system. Depending on the coating material to be tested, a suitable sphere material should be selected. Compared to carbide or ceramic sheres, a steel sphere is less suitable for higher test forces and long test durations. The service life of coatings can be determined on flat as well as on convex curved components