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Why is sapphire used for scratch-resistant optical windows?

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advanced ceramics Steps in doctor blading, a tape-casting process employed in the production of ceramic films. Ceramic powder and solvent are mixed to form a slurry, which is treated with various additives and binders, homogenized, and then pumped directly to a tape-casting machine. There the slurry is continuously cast onto the surface of a moving carrier film. The edge of a smooth knife, generally called a doctor blade, spreads the slurry onto the carrier film at a specified thickness, thereby generating a flexible tape. Heat lamps gently evaporate the solvent, and the dry tape is peeled away from the carrier film and rolled onto a take-up reel for additional processing.

Sapphire is used for scratch-resistant optical windows because it effectively combines high optical transparency with exceptional scratch resistance. As a single-crystal form of aluminum oxide ( $A l_{2} O_{3}$ ), sapphire possesses great hardness, making it a durable choice for applications such as supermarket checkout windows where surface integrity is essential.12