Transparent polycrystalline cubic spinels protect and defend

T he first reported work on transparent polycrystalline ceramics goes back to the 1950s. For example, some of the literature on materials discussed in this article, magnesium aluminate spinel and aluminum oxynitride spinels, traces back to late 1950s and early 1960s.1–3 The advantages of these materials over current state-of-the-art materials include • Ease of manufacturing; • Superior mechanical properties, such as modulus, hardness, and strength; • Performance at high-temperature environments; and • Chemical durability. Candidate polycrystalline transparent ceramic compositions include yttrium aluminum garnet and yttria, but aluminum oxynitride (γ-AlON )and magnesium aluminate spinels seem to have established themselves as leading candidates in multiple market segments, such as the military, aerospace, and lasers, mainly because of their durability, availability in large sizes, and cost. Today, γ-AlON and magnesium-spinel are manufactured in large sizes and in large volumes. However, high cost remains a barrier to their deployment as replacements for glasses and some opaque ceramics. Growing demand in current and emerging markets position these materials at the cusp of new commercialization opportunities in terms of volumes and costs. This article reviews the unique properties of these materials that make solving the production challenges a worthwhile endeavor.