Single and Polycrystalline Transparent Ceramic Armor with Different Crystal Structure Andreas Krell, ‡, * ,† Elmar Strassburger, § Thomas Hutzler, ‡ and Jens Klimke ‡ ‡ Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), Dresden 01277, Germany § Fraunhofer Ernst Mach Institute for High-Speed Dynamics (EMI), Kandern 79400, Germany Contrary to the moderate ballistic advantage of Al 2 O 3 ceram- ics over MgAl 2 O 4 polycrystals, the present study shows a reverse ratio of the ballistic strength of alumina and spinel sin- gle crystals: Spinel single crystals outperform sapphire and exhibit a similarly high stability as submicrometer Al 2 O 3 ceramics. The results correlate with different cleavage of single crystalline spinel and sapphire, changing the fragmentation on ballistic impact. I. Introduction T HE benefits of transparent ceramics over armor glass are caused by their higher hardness and stiffness (the high Young’s modulus). The ballistic comparison of highly pure polycrystalline ceramics with their single crystalline counterparts is, however, more difficult. In perforation tests comparing glass with ceramic/glass targets, armor piercing projectiles (7.62 mm 9 51, 850 m/s) were stopped by armor glass or glass-ceramics at areal densities of 130–160 kg/m 2 , whereas 2 mm thin front tiles of submicrometer polycrystal- line Al 2 O 3 or of fine-grained spinel reduced the required weight to 70 kg/m 2 . 1 Depth-of-penetration (DoP) studies revealed a limited, but significant advantage of all Al 2 O 3 ceramics over spinel grades independent of grain sizes (0.35–70 lm) and of the backing (aluminum, glass). 2 Com- pared with this similar performance of the polycrystals, sapphire single crystals with different orientations [(0001), (1120)] turned out to be less stable 1,2 : In perforation tests with almost constant backing thickness (i.e., at constant total 70 kg/m 2 of the composite window), the thickness of the front tiles had to be increased from 2 to 4 mm when poly- crystalline Al 2 O 3 or spinel ceramics were replaced by sap- phire—in spite of higher values of all basic mechanical parameters of sapphire compared with the spinel ceramics and without general mechanical deficits versus sintered alumina. DoP investigations confirmed this performance: a higher DoP was always observed with sapphire than with Al 2 O 3 polycrystals independent of ceramic grain sizes (0.6–12 lm) or of the orientations of the single crystals which do not exhibit any disadvantage in their hardness or Young’s modulus compared, e.g., with 12 lm Al 2 O 3 ceramics. 2 The one explanation was, therefore, the assumption of a different fragmentation of polycrystalline ceramics and single crystals with strong impact on projectile damage both during dwell and penetration. In mineralogy, trigonal sapphire is regarded as a noncleaving crystal with reference to ideally cleaving materials as, e.g., rock salt (halite, cubic NaCl) or calcite (trigonal CaCO 3 ). Sapphire, however, does not show macroscopic cleavage, but exhibits an extreme anisotropy of its specific fracture energy (and of the critical stress intensity K Ic ) with lowest values for three almost orthogonal to each other {1012} planes 3–5 , confirmed also on dynamic impact. 6 Thus, compared with Al 2 O 3 ceramics, sapphire is subject to more intense cracking with closer spacing not only on static indentation but also on ballistic impact: High-speed records observed smaller (i.e., lighter) fragments of sapphire associated with a lower degree of projectile damage. 2,7 These observations are well understood assuming that the ballistic strength of polycrystals and of sapphire is governed by wear processes subject to a hierarchy of influences with a top priority of the mode of ceramic fragmentation (depend- ing on the microstructure and on the dynamic stiffness of the target). 1,2 Within this frame, the lower ballistic strength of sapphire is a consequence of its fracture anisotropy. This consideration leads to the question about other single crystals which could associate a perfect transparency similar to sapphire and a similar hardness with more isotropic frac- ture to enable on ballistic impact a fragmentation similar to polycrystalline ceramics. Generally, less anisotropy is typical for cubic crystals. However, apart from diamond most cubic transparent and colorless crystals which are available from an industrial manufacture exhibit low hardness values. Two exceptions with a similar hardness as sapphire are aluminum oxynitride (“AlON”) and spinel (MgOÁnAl 2 O 3 , n ~ 1–3). It was, therefore, the objective of this study to investigate the ballistic aptitude of spinel single crystals as transparent armor. II. Experimental Procedure: Materials and Methods Costs of spinel syntheses for sintering powders or for crystal growth are governed by expensive Mg sources. Therefore, the present investigation was performed with the same low- Mg single crystals close to MgOÁ3Al 2 O 3 from a previous study 8 although the hardness decreases with higher Al con- tents. 8 In the case of a success with this Al-rich spinel, other compositions will probably work similarly or even better. Single crystalline 60 mm 9 60 mm 9 4.45 mm tiles cut with (111) orientation were compared with 4–mm-thick sapphire of same areal weight; some of the tests were run with 2.2–mm-thick spinel single crystals. Table I compares basic mechanical properties of these spinel single crystals with sapphire of different orientations and with the polycrys- talline reference ceramics of the ballistic tests (sizes: sintered spinel ~85 mm 9 85 mm, single and polycrystalline Al 2 O 3 ~100 mm 9 100 mm). With exception of some of the sap- phire data (Young’s moduli by Wachtmann et al., 9 K Ic W. Chen—contributing editor Manuscript No. 33155. Received May 6, 2013; approved July 3, 2013. *Member, The American Ceramic Society. † Author to whom correspondence should be addressed. e-mail: Andreas.Krell@ikts. fraunhofer.de 1 J. Am. Ceram. Soc., 1–4 (2013) DOI: 10.1111/jace.12530 © 2013 The American Ceramic Society J ournal Rapid Communication