Why are ceramics transparent?

Transparent ceramic, stronger than Gorilla Glass

Teaser image source: Forschungszentrum Jülich / Hiltrud Moitroux

Scientists at Forschungszentrum Jülich have developed a new coating process that specifically improves the mechanical and optical properties of transparent ceramics. This enabled them to double the break resistance.

Source: Research Center Jülich / Hiltrud Moitroux

Thanks to a wafer-thin coating, the surface of a transparent ceramic is pressed together. This hinders the formation of cracks.

Because of their special stability, transparent ceramics can be used in areas where conventional glass reaches its limits - for example in industry as a scratch- and heat-resistant viewing window in high-temperature ovens. Because they are also permeable to short- and long-wave radiation, the material is also suitable for lenses in UV lithography or for sensors in the field of infrared imaging. As a material for optical lenses, they are also of interest for cameras and smartphones.

Difficult to manufacture

The manufacture of transparent ceramics is difficult, however; it requires high temperatures and precisely defined conditions, as otherwise defects can occur in the material. These in turn mean that the strength values ​​are often far below the theoretically possible limits. "Even the smallest irregularities on the surface can lead to cracks appearing, which expand under certain loads - for example when bending - so that the component breaks", Olivier Guillon is quoted in a press release. He is head of the Materials Synthesis and Manufacturing Processes department at the Jülich Institute for Energy and Climate Research. That is why he and his team thought about a functional coating. The aim was to transfer the concept of reinforced glass to transparent ceramics for the first time. "Similar to gorilla glass, which is widely used in cell phones, the surface of transparent ceramics is supposed to be stabilized by applying a thin layer of another material." This process, known as tempering, is a significant improvement in both the mechanical and the mechanical properties optical properties to be expected.

Fewer cracks thanks to the surface being pressed together

To test their concept, the scientists chose a transparent ceramic made of zirconium dioxide. With the help of so-called gas phase coating, they applied a layer of yttrium dioxide that was only a few micrometers thick on the heated surface. Martin Bram, head of the working group, explains: "Due to the different thermal properties of the two materials - zirconium dioxide expands more than yttrium dioxide when heated - a high compressive stress arises when the coating cools." This ensures that the surface of the component is pressed together thereby preventing the formation of cracks. Correspondingly, the ceramic treated in this way becomes more stable and its surface is more resistant to scratching. The process also improves the optical properties because it reduces the light reflections on the surface, which means that more light can penetrate the ceramic.

Easily applicable for mass production

The new method doubles or even triples the fracture resistance of transparent ceramics. According to the announcement, these values ​​are far above the breaking resistance of toughened glasses. Depending on the choice and combination of materials, layer thicknesses and process parameters, mechanical and optical properties can also be maximized in a targeted manner. The process can therefore theoretically be used as a post-treatment on finished workpieces, requires only a small amount of material and can be carried out within hours, making it suitable for industrial mass production. (mt / pd)