US 5,702,654 A · Method of making thermal shock resistant sapphire for IR windows and domes

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.

Claims12

Dependent claims

A process for strengthening a sapphire article having a volume comprising sapphire and having an outer surface, said process comprising the steps of:

(a) diffusing magnesium into at least a portion of said sapphire by exposing said sapphire article to magnesium vapor at a first temperature for a first time period;
(b) heating said sapphire article to a second temperature within the range of about 1800.degree. to 2000.degree. C., in the absence of magnesium vapor, for a second time period within the range of about 2 to 24 hours, thereby causing said magnesium to be substantially homogeneously distributed throughout said volume of said sapphire article;
(c) quenching said sapphire article to thereby cool said sapphire article to a third temperature within the range of about 1200.degree. to 1450.degree. C. over a third time period at a cooling rate of at least about 10.degree. C./minute;
and
(d) annealing said sapphire article at said third temperature for a fourth time period within the range from about 0.1 to 60 hours, thereby causing precipitation of magnesium aluminate spinel crystal particulates comprising magnesium aluminate spinel having said spinel crystal structure, said magnesium aluminate spinel particulates serving to precipitation-harden said sapphire article.

2 more elements

Dependencies are inferred from the claim text.

Method of making thermal shock resistant sapphire for IR windows and domes

Life of this patent

Method of making thermal shock resistant sapphire for IR windows and domes

Life of this patent

Abstract

Claims12