Laboratory-made Al2O3 and spinel ceramics were investigated so as to enable ballistic tests with a variation of individual influences while keeping other factors constant. The study was aimed at answering the questions about influences of different microstructures and of basic mechanical properties and included also a systematic investigation of the effects of backing materials. Contradicting findings of the past are explained by the observation that the ballistic impact stability of ceramics and single crystals with different backings (steel, aluminium, glass) is governed by a strict hierarchy of few major influences: (1) Top priority is the mode of ceramic fragmentation governed by microstructural features and by the dynamic stiffness of the ceramic/backing target; these influences also affect the relative importance of dwell and penetration. (2a) On a lower rank, Young's modulus of the ceramic is responsible for projectile damage during dwell but the importance of this influence depends on the priority of ceramic fragmentation (2b) On penetration, the abrasive benefit of a high ceramic hardness depends on the size of the ceramic debris, i.e. on priority (1). In contrast, all average strength data are weakly correlated with the ballistic efficiency.
