Foreign object damage behavior in a silicon nitride ceramic by spherical projectiles of steels and brass

Assessments of foreign object damage (FOD) of a commercial, gas-turbine grade, in situ toughened silicon nitride ceramic (AS800) were made using different projectile materials at ambient temperature. AS800 flexure target specimens rigidly supported were impacted at their centers in a velocity range from 100 to 450 m/s by spherical projectiles with a diameter of 1.59 mm. Three different projectile materials were used including hardened steel, annealed steel, and brass. Post-impact strength of each target specimen impacted was determined as a function of impact velocity to appraise the severity of local impact damage. For a given impact velocity, the extent of FOD was greatest for hardened steel projectiles, least for brass projectiles, and intermediate for annealed steel projectiles. The key material parameter affecting FOD the most was identified as the hardness (or yield stress) of projectile materials. Prediction of impact force as a function of impact velocity for each projectile material was made based on a quasi-static plastic model incorporated with the average 'contact yield pressure' determined from static indentation testing.