Damage Mechanisms Perspective on Superior Ballistic Performance of Spinel over Sapphire

Abstract

Sapphire and magnesium spinel are two commonly used transparent armor materials. Although it is commonly believed that better mechanical properties often result in better field performance, the ballistic performance of spinel is reported to be superior to sapphire despite its inferior mechanical properties. The above discrepancy has been rationalized on the basis of mechanisms of deformation during impact loading. After reviewing the reported ballistic performance results on spinel and sapphire, we will discuss the relevant mechanical properties and their inability to explain the observed paradoxical behavior. We will then present differences in static (15 s duration) and dynamic (100 μs duration) indentation fracture characteristics and compare these observations to fracture modes witnessed during ball-on-rod impact tests. Lastly, the energy absorbed in various deformation mechanisms for each material is compared to draw a rationale for the intriguing behavior of spinel. Fracture along crystallographic planes in sapphire during ball impact allows large fragments to be ejected causing a significant loss in structural integrity and deeper projectile penetration, whereas the mixed mode fracture in spinel produces fine debris which opposes the incoming projectile and causes severe deceleration and erosion of the ball. Thus a mechanism based rationale has been provided to describe the superior performance of spinel over sapphire.