A Study on Femtosecond Laser Ablation and Fracture Toughness of Silicon Nitride Ceramics

Silicon nitride is a commonly used ceramic for bearing balls and rollers in hybrid bearings due to its low density and high wear resistance. A limiting factor for the lifetime of silicon nitride balls is surface spalling due to crack propagation. Therefore, it is essential to measure the fracture toughness of these parts. Standardized tests to determine the fracture toughness use prismatic bend bars which can be machined from balls. Such bars probe the properties of the ball interior. These properties may not reflect the properties near the surface due to inhomogeneities in microstructure and/or residual stresses. In this study, first investigations are made to explore a new method, capable of testing the fracture toughness near the surface of balls.
To measure the fracture toughness, usually, cracks are introduced into the tensile loaded region of a fracture toughness specimens by various methods like pre-cracking, with the help of hardness indentations, or by honing notches with small tip radii. Some of these pre-cracking methods are difficult and require experience. Therefore, the introduction of sharp notches with the help of a femtosecond laser were investigated. The reason for using a femtosecond laser is to remove material with minimal heat generation. Parameters like fluence, repetitions and more were varied systematically and the resulting notches were examined microscopically.
The results from the systematic laser-parameter study were used to identify optimal parameters for the creation of deep V-notches and semi-elliptical surface V-notches with small notch root radii in prismatic bend bars. The laser-notched specimens were used to determine the fracture toughness of selected silicon nitride materials. These results were compared to measurements made with standardized testing methods and excellent agreement was obtained.
The establishment of the generation of crack-like features with a femtosecond laser in silicon nitride bending specimen is the basis for a new method capable of testing not only machined specimens but actual components, such as bearing balls. In a further step, the notched-ball test, which loads the surface of a ball with tensile stresses, can be modified to act as a fracture toughness test using such laser machined notches.