〈1210〉-zone fracture anisotropy of sapphire

Single and Polycrystalline Transparent Ceramic Armor with Different Crystal Structure

“…e-mail: Andreas.Krell@ikts. fraunhofer.de adopted from Krell et al 5 and Smith et al 10 ), all parameters in Table I were measured by IKTS Dresden using the approaches outlined in a previous study. 2 The relative density of all sintered ceramics in Table I was ≥99% and that of the spinel ceramics (all transparent grades with an MgOÁAl 2 O 3 composition) was >99.99%.…”

“…Table compares basic mechanical properties of these spinel single crystals with sapphire of different orientations and with the polycrystalline reference ceramics of the ballistic tests (sizes: sintered spinel ~85 mm × 85 mm, single and polycrystalline Al 2 O 3 ~100 mm × 100 mm). With exception of some of the sapphire data (Young's moduli by Wachtmann et al ., K Ic adopted from Krell et al . and Smith et al…”

“…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][4][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.…”

“…Similarly as for a-Al 2 O 3 mineralogists classify the cleavage of spinel as "indistinct" (Wikipedia 11 ) whereas others use (001) microcleavage in (111) spinel substrates for GaN-based LEDs. 12,13 However, without systematic measurements of the orientation dependence of K Ic in spinel single crystals (as previously made for sapphire 5,10 ), speculations about details of the crystallographic basis of the observed fragmentation are not justified.…”

Influence of growth process and crystal defects on sapphire brittleness

“…[MPa•m -½ ] 0.026 [15] 4.54 ± 0.32 [16] 5.62 [17] C 6.04 [11] C 7.0 [18] V 2.43 ± 0,26 [16] 2.51 [11] C…”