Structural ceramic components are successfully used in ball bearings or turbochargers. Important advantages of ceramics for such applications are their high corrosion- and wear resistance. Those properties are a prerequisite for a successful implementation of silicon nitride as balls in bearings. There is a certain risk of brittle failure of structural ceramic parts caused by the growth of preexisting cracks. During application ceramic bearing balls are predominantly loaded by contact stresses. Minor overload may lead to the well-known Hertzian cracks. These cracks are located on the surface and often remain invisible. Increasing the contact load causes a cone shaped crack to propagate. This cone shaped cracks lead to the total failure of the bulk material. The specific ring crack initiation force for a certain contact situation defines a major point for lifetime prediction of structural ceramic components Because of the fact, that crack propagation in brittle materials causes ultrasonic emission, the ring crack initiation can be measured by sensitive ultrasonic microphones. Conventional testing methods for ceramics always lead to total failure, while the initiation of the first ring cracks is missed in such tests. In this diploma thesis the initiation of Hertzian ring cracks is characterized for several contact situations. Flat plates are indented with spheres of different diameter. The crack initiation force is measured and the Hertz theory is applied to calculate crack initiation stresses. The influence of machining is studied by performing experiments on ground and polished plates. Further the contact between two spheres is analyzed to give a relation between the force for total failure of the balls and the ring crack initiation force.