Transparent armour ceramics: Spinel vs. Sub-µm Al2O3

Sintered sub-µm Al2O3 is the hardest of all transparent materials for compact windows, and a high hardness is important for the ballistic performance. However, visible light transmission of sintered alumina is limited by birefringent scattering to a real in-line transmission RIT of only 70-75 % of the theoretical maximum (at 0.8-1 mm thickness). These losses increase with thickness whereas a safe ballistic performance requires 1.5-2 mm at least. Slip casting brings RIT closer to the limit with grain sizes of 0.3 µm (at 0.8 mm thickness: 84-93 % of the theoretical maximum) but these 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 fine-grained spinel associates an RIT close to the theoretical maximum with a hardness that approaches sapphire. In first ballistic tests this spinel outperformed sapphire. It is, therefore, suggested that sub-µm Al2O3 may be good for IR windows or as armour for low threat applications (where thinner tiles can be used). For thicker windows, however, the new spinel appears as a most favourable candidate.