The most difficult problem in the processing of quartz glass for optics is the ablation of the glass surface. The main component of optical glass is silicate, which will be hydrolyzed when it encounters water or water steam, resulting in ablation.
The essence of hydrolysis is the exchange between H+ in water and alkali metal ions on glass surface. As a result, the hydrogen ions in the water were reduced and the OH- ions were increased. At the same time, a layer of silica gel film was formed on the glass surface. As a result of the increase of OH- ions, the alkaline environment of the glass liquid is continuously enhanced, resulting in the formation of highly concentrated alkaline liquid, which reacts with H2SiO3.
The formation of alkaline substances increases again, and the cycle leads to increased ablation. At the same time, due to the porous cracking structure of the silica gel layer, OH- ions continue to erode to the glass layer, especially the material containing less silicon and poor chemical stability, the density and fastness of the silica gel film layer is poor, which further intensifies the erosion of OH-.
The hydrolysis process almost runs through the whole process of quartz glass for optics, whether it is grinding, core and other processes or processes, will occur in varying degrees. There are many forms of hydrolysis, or external conditions that aggravate the degree of hydrolysis, such as alkaline corrosion, salt corrosion, temperature corrosion and so on.
The ablative problems and causes of quartz glass for optics surface are analyzed. The grinding process is taken as an example to illustrate how alkaline environment and processing method accelerate hydrolysis. Usually, the grinding powder with CeO2 as the main component will not cause corrosion to the lens, but in the grinding process, the grinding liquid is prepared by water and grinding powder, so the initial pH value of the newly prepared grinding liquid is jointly determined by the acid and alkali of the water and grinding powder, which is generally alkaline. As mentioned above, the glass will produce hydrolysis reaction when it meets water, and the generated H2SiO3 is in a gel state and attached to the surface of the glass plays a protective role and prevents the reaction from proceeding. Meanwhile, part of H2SiO3 will decompose and the generated SiO2 attached to the surface of the glass can also slow down the hydrolysis reaction and play a protective role.