Fatigue Behavior, Bridging Stresses, and Fatigue Reliability in Silicon Nitride Ceramics

Silicon nitride ceramics doped with rare-earth oxides exhibit excellent hardness, toughness, and strength at elevated temperatures making them attractive materials for replacing cemented carbides in a variety of manufacturing applications such as cutting and rolling tools. One recent example is the application of rolling of high strength alloy wires from steels and nickel-based super-alloys where cemented carbide rolls suffer wear and thermal fatigue cracking, leading to a degradation of wire quality. [1] Furthermore, it has been shown that under moderate loading silicon nitride rolls can give >10 times longer life and improved wire surface quality. [1] However, it has also been shown that the rolls can suffer fatigue failure at higher loadings, for example when rolling wires with high deformation resistance such as the super alloy wire Nicrofer S7020. [1–2] Accordingly the aim of this study is the develop a design tool for predicting the fatigue failure of silicon nitride ceramics. The silicon nitrides with favorable mechanical properties have microstructures with elongated β-phase grains and a glassy intergranular film. The weak film encourages intergranular fracture allowing the formation of grain bridges across the crack wake which helps to reduce the stress intensity felt at the crack tip, K_tip. [3]