Ductile-brittle transition mechanisms in micro-grinding of silicon nitride

Ductile grinding of brittle materials is essential for high precision applications and to maintain the strength and lifetime of the parts. The critical chip thickness of a brittle material defines a threshold for the lateral cracks' initiation in the workpiece due to the grains' penetration. For ductile grinding, keeping the uncut chip thickness below the critical chip thickness of the brittle material is necessary. This study focuses on the ductile microgrinding of Si3N4 as an advanced ceramic and brittle material. The critical chip thickness and the maximum uncut chip thickness were first calculated based on the material properties, the grinding parameters and the microtopography of the utilized grinding pin and then validated by both diamond grit scratches and microgrinding experiments. Micro-grinding experiments on an inclined workpiece were conducted to investigate the material removal regimes of Si3N4. Grinding forces and surface integrity (surface roughness, surface topography and subsurface damages) induced by different micro grinding parameters and micro-grinding of an inclined workpiece are analyzed in detail. The estimated critical chip thickness and measured maximum uncut chip thickness could be used as an exact guide for achieving the ductile micro-grinding mode. The experiments revealed that the material removal mechanism mainly affects the micro-ground surface integrity. The ductile material removal mode induced no detectable subsurface damage and the surface quality deteriorated by the brittle removal mode.