Advances in Sintering Additives for High-quality AlON Transparent Ceramics

Transparent spinel-type aluminum oxynitride (γ-AlON) ceramics have emerged as a highly promising material for military protection (e.g., infrared windows, armor materials) and civilian optics (e.g., lenses, semiconductor devices) due to their excellent optical properties (high transmittance, broad transmission band), outstanding chemical stability, and superior mechanical strength. Transparent ceramics require not only high optical transmittance but also high mechanical properties, which is the fundamental prerequisites for the practical application of AlON ceramics.The incorporation of sintering aids is a critical strategy in fabricating transparent AlON ceramics, as optimizing their type and content allows precise control over microstructural evolution during sintering, including grain nucleation and growth, phase distribution, and overall densification. Consequently, these microstructural modifications directly influence the ceramic’s properties, such as, optical transmittance, mechanical strength, and chemical stability, enabling their effective regulation and optimization for advanced applications. This paper comprehensively reviews the research progress on sintering aids for preparing high- quality AlON transparent ceramics. The sintering aids are categorized into rare earth oxides (Y₂O₃, La₂O₃, and Pr₂O₃), alkaline earth oxides (CaCO₃and MgO), and silicon - based compounds (SiO₂ and Si₃N₄). The review delves into the effects of these aids on the optical and mechanical properties of AlON transparent ceramics and details their mechanisms in promoting densification, optimizing grain size distribution, suppressing phase decomposition, and lowering sintering temperature. Optimal sintering aids facilitate pore elimination and suppress abnormal grain growth via liquid-phase formation or pinning effects, thereby enhancing optical transmittance while simultaneously improving mechanical properties (hardness, fracture toughness, and strength) through microstructural refinement. The mechanisms of sintering aids in AlON densification vary significantly depending on their chemical nature. Specifically, rare-earth additives predominantly facilitate liquid-phase sintering, alkali metals induce grain-boundary pinning effects, while silicon-based compounds primarily form solid solutions. This review also analyzes the existing problems and challenges in current research and looks forward to future research directions, aiming to provide theoretical guidance and technical reference for the preparation of high-performance AlON transparent ceramics.