Study On Adhesive Mechanism Of Contamination On Fused Silica Surfaceand Characterization Method Of Cleanliness

Fused silica, which is commonly used in the Inertial Confinement Fusion device, has played an important role in the process of laser path transmission and laser focus. The cleanliness of fused silica surface decided the laser damage threshold of optical elements, which affect the service life of the laser device. One of the main sources of pollutants is the organic molecules which introduced in the precision and ultra-precision machining process, and did not completely removed after cleaning. From experimental observation, we found that the pollution area was point shaped. Due to the existence of dangling bonds in the nanoscale holes, the organic molecules can easily absorbed on the surface. However, the mechanism of adsorption cannot be cleared up only by experimental method, and we need to study at the molecular level. Thus, molecular dynamics method is used to analyze the adhesion mechanism of organic molecules on the amorphous silica surface, and the characterization method of fused silica surface cleanliness are also improved through the experimental data. The completion of thesis has a great significance of improving the purity of optical element surface.First, the fused silica’s molecular model was established according to the structural characteristics of amorphous materials. The process of silicone oil adsorbed on the fused silica surface and detached in the water medium were simulated and detailedly described through the instantaneous system conformation, the height of mass center, the density distribution of oil molecules. The formation of different directions of water channels was vital for oil detachment. The specific ring potential well, which is related to the number of atoms and the average radius in silica atomic rings, was put forward to explain the phenomenon of oil molecules residue on the substrate surface.Then, the process of water droplet absorbed on the fused silica surface was studied by using molecular dynamics simulation. Through the analysis of different proportion of surface hydroxy and silane influence on wettability, the relation between contact angle values and the degree of pollution on the fused silica surface were defined. Through the building of hydrophilic and hydrophobic rough surfaces of the fused silica, the influence of surface structure on wettability was studied.Finally, using different washing method to clean grouping fused silica samples which were machined by the same batch. Through the contact angle test, the cleaning effect of each sample was evaluated and the role of each cleaning process was assessed. At the same time, based on the functional relation between the water contact angle and the silicone oil pollution degree of fused silica surface, the characterization method of cleanliness was proposed. We also analyzed the factors which might lead to contact angle measurement error from various angle s.