A material is considered to be brittle if it exhibits low strain at failure when subjected to stress. In other words, unlike ductile materials, it has very little plastic capability and hence no specific yield point. Many optics—both refractive lenses and reflective mirrors—consist of brittle materials. All glasses (fused silica, ULE™, ZERODUR®, BK-7, borosilicate, etc.) as well as most engineering ceramics (zinc sulfide, zinc selenide, germanium, silicon, silicon carbide, etc.) fall into the brittle category.
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