Mechanical Properties and Impact Resistance of a New Transparent Glass‐Ceramic

The focus in ceramic armour development today is towards improved protection capability combined with an overall reduction in production cost. Because of their brittle nature, ceramics and glasses are susceptible to localized surface damage in the form of cracking when subjected to impact by foreign objects. Much work has been carried out on the dynamic impact behaviour of ceramics and glasses. Tests have been carried out using small ceramic or metallic spheres, impacting at high speed in so-called ballistic tests. In these tests, projectiles are accelerated toward their target by utilizing specialised devices called gas guns. The development of transparent glass-ceramics with a reinforced surface layer is a promising approach to novel armour materials since glass-ceramics combine high strength with improved fracture toughness compared with glass and yet they can be reliably produced using cost-effective processing methods. A number of technical reports have been published in the last forty years regarding different types of transparent glass-ceramics, but very few focused on their use as armour materials. The most attractive advantage of glass-ceramics is the possibility of using standard glass processing methods followed by controlled heat treatment, which is a fast and reliable production method, resulting in almost no residual porosity in the materials. Moreover the melting plus heat-treatment route offers very good reproducibility compared with the alternative method based on powder processing and sintering commonly used in polycrystalline ceramics, which is prone to microstructure inhomogeneity. In the majority of previous investigations, transparent glass-ceramics have been obtained, which exhibit the presence of nanocrystals within an amorphous matrix. This nanostructure (with less than 70 % crystalline phase) guarantees optical transparency since the crystal size is smaller than the wavelength of visible light. The two principal conditions for achieving high transparency are low optical scattering and low atomic absorption in the visible range. Transparent glass-ceramics appear to be an ideal material for advanced armour applications due to their relatively good transparency, low density, relatively high fracture strength as well as their thermal and chemical stability. However, compared with the most popular polycrystalline ceramics (e.g. Al2O3), glass-ceramics exhibit lower fracture toughness. [26–30]