Understanding and Improving Ceramic Armour Materials

Ceramic materials are used for both personnel and vehicle armour since they can be very effective at stopping ballistic projectiles by breaking and eroding them. However, such armour is generically fairly heavy and does not have multihit capability due to its fragmentation during impact. The development of new ceramics for armour is further hindered by the limited understanding of the mechanisms involved in their success and therefore what the characteristics of the ideal ceramic should be. Challenges to improving the situation include: the difficulties in defining initial material state (defects and damage); experimental characterisation of short-lived impact events; the fragmented nature of the specimens left behind; and the importance of the whole system to the dynamic failure process (e.g. impact resistance depends inherently on the shape and size of the component unlike conventional materials properties). This hinders the comparison of reports of competing materials from different labs. Previous work has included the use of a range of mechanical testing equipment (from quasi-static, low velocity hydraulic and gas gun actuated split- and direct-impact Hopkinson bar systems to intermediate-, high- and hyper-velocity gas and powder guns) and aimed to quantify the impact response by means of high-speed photography. There have also been attempts to simulate experiments using a range of modelling techniques. However, most of the available literature uses particular techniques and materials in isolation. No work has yet demonstrated the use of an integrated experimental-numerical, multi-scale approach to understanding and predictively modelling deformation and fracture of ceramics subject to impact loading.A holistic approach to developing an understanding of the high strain rate performance of ceramics is proposed. Modelling plays a dual role, which is both to design the experiments employed to understand the materials' behaviour on a macroscopic scale and to give insight into the role of micromechanisms in determining ballistic performance. Model input will be provided by materials property data determined before testing (e.g. basic information such as grain size and hardness and detailed surface and subsurface characterisation of the defect population using advanced microscopy). The models will be developed by comparing their predictions with the output of instrumented laboratory tests covering a wide range of strain rates (10 exp -4 to 10 exp 6 /s), ballistic tests (through DSTL) and with the results of novel post mortem characterisation. The latter will include detailed characterisation of the fragments ejected from the surface and the remnants of the main specimen. This will include assessments of fracture mode and origin using SEM, dislocations and twinning by TEM, particle size distributions by laser scattering and the use of optical luminescence microscopy for the first time in this context to measure residual stresses in the specimens with a spatial resolution of 2um and to measure dislocation densities. This quantitative information can be compared directly with the predictions of the models. This approach will be used on a range of ceramics with systematically differing characteristics and will give comparative information about which microstructural features and mechanical properties successful ceramics possess, as well as enabling the development of a fundamental understanding of the high strain rate performance. The materials used will include both existing armour ceramics and, for the first time, novel nano-grained and nano-composite ceramics which quasi-static tests indicate to have interesting properties for armour applications (e.g. the ability to undergo much more plastic deformation before crack initiation). The outcome will be the ability to design ceramic microstructures and armour systems with improved performance. The final task will be to begin making such structures.