Fracture and Deformation

The high-velocity impact resistance of hot-pressed zirconum diboride with 30 volume percent silicon carbide was studied using a combined experimental and computational approach. Test specimens in the form of 2 mm thick polished disks were impacted with -0.8 mm diameter tungsten carbide spheres at velocities up to 320 d s . The intrinsic flexure strength of the specimens was -1000 MPa. The flexure strength retained by impacted specimens decreased linearly with increasing impact velocity, falling to -600 MPa at -290 d s . Above this threshold velocity, the retained flexure strength fell rapidly, with no measurable retained strength for samples impacted at 320 d s . The experimental results suggest gradual strength degradation is associated with the formation of shear and sliding faults under the impact zone at moderate impact velocities. The abrupt decrease in strength above 290 d s is due to cone-crack propagation. Finite element modeling supports the failure mechanism for impact velocities above 290 d s , but fails to provide insight as to the failure mechanism below this velocity. INTRODUCTION Ceramics based on zirconium and hafnium diborides with silicon carbide additions (ZrB2-Sic and HfSz-SiC) are candidate materials for the leading edges of hypervelocity atmospheric re-entry vehicles due to their moderate strength’, high melting temperature and oxidation characteristics2. It is critical to understand the evolution of impact damage in these materials for possible encounters with debris during launch, orbit, or re-entry. A previous impact study was performed on ZrBt-Sic and HfB2-SiC ceramics manufactured during the SHARP B1 and B2 flight experiment^.^ The reported strength of the materials from this era was less than -400 MPa. Recent improvements in processing have lead to intrinsic flexure strengths in excess of 1000 MPa for ZrBz-Sic materials prepared at the University of Missouri Rolla.’ These ceramics have heterogeneous microstructures and fracture toughness values greater than 2.5-3 MPa.m”’.’ The present work focuses on the strength degradation of this ZrB2-Sic material as a function of impact velocity with WC projectiles. Figure 1 is a schematic of the damage expected for a hard sphere impact on a ceramic surface, where the ceramic responds in a classic brittle manner to a predominately elastic stress field! When the projectile makes contact with the specimen, a small contact patch is formed between the surface of the projectile and specimen. Ring 3 Ceramic Engineering and Science Proceedings © 2007 by the American Ceramics Society Rajan Tandon