Research On Mechanism And Experiments Of Nanoparticle Colloid Jet Polishing

With the development of science and technology, the requirements for ultra smooth surface in modern optical systems, large-scale integrated circuits and thin-film science and technology, have been increasingly improved. In all of these areas, the demands for the workpiece surface's shape accuracy, surface roughness, as well as sub-surface damage become more and more higher. In order to obtain high transmittance and surface reflectivity, optical components need to have a low surface scattering properties. As for electronic functional elements, they are required in a very low surface roughness while maintaining complete lattice structure. Due to the introduction of metamorphic layer and sub-surface damage to the workpiece surface, the traditional ultra-precision, ultra-smooth surface machining methods, such as mechanical polishing methods, can't meet the requirements of modern science and technology for the ultra-smooth surface.To meet the demands for ultra-smooth surface, there have been a number of new ultra-smooth surface machining methods, such as Magnetorheological Finishing, Fluid Jet Polishing, Ion Beam Figuring, Plasma Chemical Vaporization Machining and Elastic Emission Machining. Elastic Emission Machining utilizes surface reaction between workpiece material and abrasive powder to remove the work material, avoiding the surface / subsurface damage and maintaining integrity of the surface lattice, while access to a very low surface roughness. Based on the removal mechanism of elastic emission machining, this paper presents a new ultra smooth surface processing methods called nanoparticle colloid jet polishing. Nanoparticle colloid jet polishing, which combines of surface chemistry, fluid dynamics theory and optical processing methods, provides a surface processing method that can achieve atomic-scale accuracy. By computer precision control, nanoparticle colloid jet polishing can realize optical components surface shaping, ensuring nano-scale surface roughness and avoiding the introduction of damage to the workpiece surface.Firstly, this paper analyzes the material removal mechanism of nanoparticle colloid polishing. In the colloidal environment, the active groups on workpiece surface will adsorb the OH- ions. The impacting nanoparticles, which have high surface energy and chemical activity, will chemically reacte with the work surface. And then, under the viscous action of the colloid, the nanoparticles together with the surface atoms will be taken away from the workpiece surface. An atomic-scale material removal can be achieved during the above processing.Using quantum chemistry theory, the chemical adsorption process of single-crystal Si surface atoms and the OH- ions in colloid has been studied. On the basis, the chemical reaction between SiO₂ cluster and single crystal Si workpiece surface atoms has been simulated to reveal the removal mechanism of nanoparticle colloid jet polishing. Quantum chemical calculations showed that in nanoparticle colloid jet polishing, both the chemical adsorption of OH- ions and the impacting reaction between nanoparticles and the workpiece interface, will selectively priority to the activity atoms of the work surface, ultimately achieving a flat surface with small surface roughness and better surface morphology.Based on Non-Newtonian fluid constitutive equations, SiO₂ nanoparticles colloid's rheological properties have been studied. With fluid dynamics simulation method, the flow field distribution for micro-jet nozzle has been analysised under the conditions of non-submerged jet. Corresponding processing experiments have been carried out to test the simulation results.Based on the material removal mechanism and characteristics of nanoparticle colloid jet polishing, a nanoparticle colloid jet polishing system has been designed using a special structure polishing fluid containers, in which, fluid and pressure transmission medium are of independent cycles.The nanoparticle colloid jet polishing method has been applied to obtain ultra smooth surface on K9 glass. An ultra smooth surface with roughness less than 1nm Rms has been gotten in nanoparticle colloid jet polishing. The mechanical properties of processed surface have been experimentally investigated. The results show that nanoparticle colloid jet polishing can significantly reduce surface roughness, improve the mechanical properties of the workpiece, such as the elastic modulus and surface hardness. And the power spectral density characterizations of the workpiece surface before and after polishing have been analysied. Power spectral density characterizations results show that workpiece surface ripples will basically be eliminated to achieve a good surface state in nanoparticle colloid jet polishing.The paper also analyzes the effectible factors in nanoparticle colloid jet polishing, such as the distribution of the flow field, jet pressure, pH value and colloid concentration of the nanoparticle colloid. As a result, reasonable ranges of these parameters have been given.