Particle Impact Damage in Silicon Nitride

The evolution of particle-impact-induced fracture damage in hot-pressed (HP) silicon nitride was established by accelerating single 2.4-mm-diameter tungsten carbide spheres against polished HP Si₃N₄ surfaces. Threshold velocities for ring, cone, and radial cracks were determined and the corresponding threshold stress for ring cracking was obtained from an elastic stress analysis. Particle size had significant effects on the threshold velocities for the inelastic impression and the various crack types. Loading rate had little effect on the threshold stress for ring cracks; rate effects on other crack types could not be assessed because the quasistatic indenter failed at stresses less than those required to invoke other crack types. A 20-μm-thick oxide scale had little influence on morphology and extent of damage but was removed easily at low velocities, suggesting higher erosion rates for Si₃N₄ in oxidizing environments. Damage phenomenology in 85% dense reaction-bonded Si₃N₄ was similar to that in HP material; however, all stages of damage occurred at substantially lower velocities.