Defect-free grinding of silicon nitride at high material removal rate

Owing to the extreme hardness and toughness of sintered silicon nitride (Si₃N₄), the material is used in high stress and/or temperature applications such bearings, turbines, and combustion engines. Unfortunately, the same properties which make it ideal for use also make it particularly difficult to machine -- microcracks, inclusions and spalling are all common. While prior research has shown that it is possible to grind sintered Si₃N₄ without inducing surface damage so long as material is removed entirely under ductile flow, but grind forces associated with ductile Si₃N₄ material flow are so small as to render the material removal rate (MRR) impractical. Prior researchers have attempted to solve the MRR problem through laser-assisted machining. Laser ablation, by inducing a steep thermal gradient, weakens material through surface and subsurface cracks. Grinding of fractured weakened Si₃N₄ has been done at upwards of 50 % higher MRR. There are, however, issues with laser ablation, which prevent its widespread use. Laser ablation severely disrupts the microstructure of Si₃N₄. Because cracks propagate along and through grain boundaries, the irregular morphology makes accurately predicting crack growth from ablation and during subsequent grinding highly problematic. In this proof-of-concept work, researchers determined that it is possible to irradiation weaken Si₃N₄ without cracking it, and the material can be ground defect-free at a highly productive MRR. Findings suggest present laser-assisted machining methods which fracture weaken Si₃N₄ prior to grinding may not be the best way to maximize MRR.