Influence of microstructure on the impact failure of alumina

In this paper, we detail a study involving the characterization of two grades of alumina (CoorsTelc Engineered Ceramics AD-85 and AD-995) and subsequent impact experiments with tungsten carbide spheres at 400 m/s. Through this work, we hope to understand the relationship between microstructure and the high strain rate behaviors of advanced ceramics. Initial characterization of microstructure with scanning electron microscopy, electron backscatter diffraction, and energy dispersive X-ray spectroscopy determined grain size, and the elemental character of the materials and the inclusions present. AD-85 alumina was found to have smaller alumina grains and more non-alumina inclusions than AD-995 alumina. Compression testing with in-situ visualization for crack speeds demonstrated that the strength of AD-85 increased from 2.0 +/- 0.1 GPa at 10(-3) s(-1) to 2.7 +/- 0.3 GPa at 500 s(-1) (crack speeds of 1800 +/- 600 m/s). The AD-995 had strength increases of 2.4 +/- 0.2 GPa at 10(-3) s(-1) to 3.6 +/- 0.5 GPa at 500 s(-1) (crack speeds of 2200 +/- 400 m/s). The greater rate sensitivity and higher crack speed of the AD-995 is related to microstructure and mechanical properties. Impact experiments used spherical tungsten carbide projectiles fired from a smoothbore powder gun into alumina discs held in place with polycarbonate holders that provided no lateral confinement and minimal back face confinement. The use of simultaneous high-speed video, photon-doppler velocimetry, and flash X-ray during these impact experiments produced information on the cracking and microbending of the target in the first 10 mu s of impact, with a number of significant events happening within the first 5 mu s. At 400 m/s, AD-85 alumina suffered penetration by the projectile while AD-995 alumina defeated the projectile. The data allows for comparison of event timing such as peak back face velocity and the onset of radial cracking, demonstrating that during an impact event, interface defeat occurs within the first 5 mu s. The data collected and analysis done also suggests that the failure of the projectile can couple with the target via the transmission of shear waves and contribute to failure, which has implications for future modelling and design of improved protection systems.