Hardness and damage associated with pointed indentations in armor ceramics

It is empirically known that an armor ceramic should be as hard as or harder than the projectile it intends to defeat. Quasi-static indentation testing is one of the most widely utilized techniques for determining the hardness of armor ceramics. Hardness measurements can also be used to generate other property values (fracture toughness, elastic properties and even the yield strength) that may be relevant to ballistic performance. While the indentation methodologies are simple and straightforward the resultant hardness values for ceramic materials can be influenced by the indenter geometry, indentation load, loading rate, specimen surface finish and ceramic microstructure. This presentation will summarize the results of a study to determine the hardness of a variety of armor-grade ceramics (Al2O3, B 4C, SiC, and WC) with different indenter geometries (Vickers and Knoop) over a range of indentation loads (0.98N to 98N) and discuss the implications for armor ceramics. The resulting data strongly indicates that the best means of determining the hardness of armor ceramics is the use of 19.6N Knoop indentations.; While the hardness data and the subsequent analysis clearly support the use of the Knoop methodology to determine the hardness, it does not take into account the response of the ceramic to the indentation process. One response that is continually overlooked is the role of damage and cracking. A detailed understanding of the damage and cracking that occurs during indentation may provide valuable insights to the ballistic performance of the armor ceramics during the earliest stages of impact. To explore this material response a detailed examination of the topography of the indents and the damage and cracking that develops underneath both Knoop and Vickers indentations in a WC and SiC armor ceramic was conducted. The analysis shows that while the same types of cracks are produced in both materials the magnitude of these cracks as well as the type of damage that is produced during the indentation process is significantly different. This information is related to the ballistic performance of each material.