The 3D Characterisation Of Microscale Indentations On Silicon Nitride

Silicon nitride is a high hardness structural ceramic that is often used for the production of ball-bearing components due to its good mechanical properties. However, during manufacturing, microscale pits (called “star features”) can form on the surface of ball components. In this project, microscale pits, on silicon nitride balls are characterised in three-dimensions using a combination of surface and sub-surface characterisation techniques. Investigations into low load (0.5 kg and 1 kg) Vickers micro-indentations on silicon nitride are first carried out to ascertain which techniques are most applicable to evaluation of star features. This includes the determination of key microstructural parameters including microstructure, cracking distribution and residual stress. High resolution SEM imaging and Raman spectroscopy of indentations reveal a primarily intergranular fracture mode for cracking, with surface tensile stresses of ~ 1 GPa at radial crack tips for 1 kg indentations.
Micro-XCT resolves sub-surface half-penny, lateral, and radial cracking present after high load 5 kg Vickers indentation into silicon nitride. Focused ion beam (FIB) tomography revealed the presence of half-penny and lateral cracking in low load (0.5 kg and 1 kg) indentations with c/a ratios < 2. The initiation depth of half-penny and lateral cracking has been used to approximate the volume of the compressive stress region that exists beneath the indentation impression.
FIB tomography of star features on silicon nitride balls reveals half-penny cracking that has not observed using UV fluorescence microscopy. The lateral extension of lateral cracking from the centre of star features observed via FIB tomography has been found to correlate well with UV fluorescence imaging. In addition, lateral cracking is found to be located at the same depth as adjacent regions of missing material, implying that material removal initiates from lateral cracking. Uplifted zones of up to ~ 0.8 μm vertical displacement were also found to exist surrounding star features.
Overall it is found that 3D characterisation methodologies, including FIB tomography, optical profilometry and micro-XCT, are highly effective in the quantification of tribological surface damage in silicon nitride, giving new insights into the distribution and role of surface deformation and fracture.