Advanced spinel and sub-µm Al2O3 for transparent armour applications
Hardness is important for a high ballistic strength, and with HV10 = 20-22 GPa sintered sub-µm Al2O3 is the hardest of all transparent materials for compact windows. However, light transmission through polycrystalline Al2O3 is limited by birefringent scattering losses: high transmissions are known at larger IR wavelengths for grain sizes of about 0.5 µm but the visible real in-line transmission RIT is only 70-75 % of the theoretical maximum at 0.8-1 mm thickness. These losses will be the higher for thicker components whereas a safe ballistic performance requires 1.5-2 mm thickness at least. New technologies bring the transmission closer to the limit associating grain sizes of 0.3 µm with an RIT of 84-93 % of the theoretical maximum (thickness 0.8 mm). However, even these extreme results give again rise to doubt that it will ever be possible to manufacture larger and thicker Al2O3 windows with a sufficiently high transparency. On the other hand, new results are presented for fine-grained spinel with RIT close to the theoreti cal maximum and with a hardness that approaches sapphire. In first ballistic tests this spinel outperformed sapphire of different orientations. It is, therefore , suggested that sub-µm Al2O3 may be a good choice for IR windows or as armour for low threat applications where thinner tiles can be used. Most threats, however, require thicker windows where the new spinel appears as one of the most favourable candidates.
Special issue on transparent ceramics