Structural ceramics are used in rolling bearings because of their excellent characteristics (e.g. hardness, corrosion and abrasion resistance). Here, silicon nitride has especially proven itself in ball bearings. The international bearing manufacturer SKF developed together with the Institute of Structural and Functional Ceramics new methods for the mechanical characterization of ceramic rolling elements for hybrid bearings. When rolling over a crack, strong shear stresses (mode II) may occur. This creates so-called KII-stress intensity factors in the component at the crack tip, which need to be considered in a reliability analysis. In this project a test bench has been developed to perform toughness tests on a reference material under very pure mode II- and mixed-mode pre-loads in addition to the standard mode I tests to determine the fracture toughness. For the load test of the samples the asymmetrical four-point bending test was selected, by which the entire region between pure mode I and mode II stress may be examined. Furthermore, it is also possible to generate negative KI-values. For the introduction of cracks in the bending beams, the standardized Surface Crack in Flexure-method and also the standardized single edge V-notch beam-method were used. With these methods, a wide range of different crack geometries is covered. Various mixed-mode hypotheses were used to evaluate the measured data (failure curve or crack propagation angle) and were compared with the predictions of the hypotheses in order to find out which one shows the best accordance with the measured results. It showed that with the hypothesis of the non-coplanar maximum energy release rate, the results with KI>0 can be predicted the most accurately. However, for KI