In addition to structural applications, modern high-performance ceramics are widely used as functional components due to their special electrical, optical or magnetic properties. Voltage-dependent electrical resistors, so-called varistors based on zinc oxide, represent one area of application. Industrially used varistors consist of doped, polycrystalline zinc oxide, whose characteristic electrical properties are determined by grain boundary effects and thus by the microstructure. In addition to their electrical properties, functional ceramic materials have to withstand high mechanical stresses both during manufacture and processing and during operation. To fulfill the functional tasks, the components must be able to withstand the mechanical requirements. The fracture toughness and strength are the relevant mechanical parameters for assessing the usability of a functional ceramic material.
In this work, these essential mechanical properties are to be determined on four industrially used varistor materials with different doping. Plates with thicknesses in the range between 400 and 800µm were chosen to investigate sample geometries close to the component. The standardized SEVNB and SEPB methods were used to determine the fracture toughness. To measure the strength, the “ball on three balls”- test was performed firstly at room temperature and secondly at 400°C to obtain strength information at elevated temperatures. The measurements were accompanied by microstructure and fracture surface studies using optical and scanning electron microscopy.
The results showed that the measured fracture toughness using the SEPB method is higher than the values of the SEVNB method. The strength decreases for all investigated variants in the 400°C tests compared to the room temperature tests. For one varistor variant, the strength was also measured at 200°C in an intermediate step, and contrary to expectations, the strength increased compared to the room temperature tests. The variant with the smallest grain size showed the highest strength.