Ductile behavior of glass–ceramics with precipitated CaF2, xonotlite, and cristobalite

Glass with a chemical composition of 69.7SiO2–7.9CaO–5.5Na2O–3.5K2O–1.4Al2O3–12CaF2 (mol %) was crystallized via a simple heat treatment. CaF2 and xonotlite precipitated above the glass transition temperature, followed by the precipitation of α-cristobalite above 900 °C. Thermal expansion curve of the glass–ceramics containing α-cristobalite exhibited a steep increase around 270 °C due to α–β phase transition of cristobalite. Ball indentation test on the glass–ceramics resulted in a clear circular dent in the glass–ceramics with precipitated cristobalite but only ring cracks in the glass–ceramics without cristobalite. X-ray computed tomography using synchrotron radiation revealed the formation of microcracks around cristobalite, with a greater number of microcracks observed after ball indentation. Therefore, it was concluded that the stress caused by a large volume change during the α–β phase transition of cristobalite induced the generation of microcracks in the glass–ceramics during cooling, eventually leading to ductile behavior. The glass–ceramics also exhibited ductile behavior in the bending test that was used to measure fracture toughness. The obtained findings offer valuable insights into the fabrication of a new type of glass–ceramics with ductile behavior, which is expected to expand the application scope of glass–ceramics.