Method of making thermal shock resistant sapphire for ir windows and

Abstract

Single crystal sapphire is conventionally employed for mid-wave IR windows and domes exposed to high aerodynamic heating environments. Single crystal sapphire, however, suffers a loss of compressive strength in the c-axis of 95% on heating from ambient to 800.degree. C. This loss of compressive strength on heating results in poor thermal shock resistance. Poor thermal shock resistance jeopardizes the reliability of the windows and domes. The process of the present invention strengthens sapphire and improves its resistance to thermal shock by introducing magnesium (Mg) ions into the crystal lattice of sapphire to form a solid solution. The solid solution increases the strength of the sapphire. Additionally, a larger increase in strength results from converting the Mg into second phase precipitates comprising magnesium aluminate spinel. The formation of the second phase particulates results in precipitation-hardening or precipitation-strengthening due to the volume change on forming the magnesium aluminate spinel. The second phase particulates comprising magnesium aluminate spinel are held to such a small size that the second phase particulates impart increased fracture strength while not significantly degrading the IR transmission when compared with untreated sapphire. Consequently, precipitation-hardening with magnesium aluminate spinel improves the compressive strength of sapphire while retaining high thermal conductivity and IR transmission.