Research On The Physico-Chemical Behaviors In The Nano-field Of Quartz Glass In Ultra-precision Machining Process

Quartz glass is an isotropic amorphous solid material and it has been widely used in the optical field because of its high uniformity in structure,specific and precise optical constants,good transparency,high temperature resistance,corrosion resistance,high strength and hardness.It is the main production material for optical components such as lenses,anti-sputtering sheets and diffraction mirrors in the US National Ignition Facility?NIF?and China Major Inertial Confinement Nuclear Fusion Project?ICF?.Meanwhile,because of its high hardness and brittleness and low fracture toughness,making it one of the most difficult materials to be processed.Surface/subsurface defects are easily generated during processing,such as burns in grinding,microcracks during lapping,making it hard to form nanoscale optical surface.These damages seriously affect the performance such as stability,light transmittance and service life of the optical system.The main processing method of quartz glass is chemical mechanical polishing?CMP?after ultra-precision grinding at present.Chemical-mechanical polishing?CMP?can effectively remove the damage caused by grinding process and improve the surface quality.The surface precision after polishing can reach the nanometer level.CMP is a coupling process of chemical,mechanical,thermal and other factors,and surface materials are removed at micro-scale.It is difficult to study the physico-chemical behaviors of CMP through simple experiments.In chapter 3,the material removal mechanism of quartz glass in CMP process was studied by MD simulations using Reactive Force Field?ReaxFF?.Firstly,the quartz glass model was prepared by annealing quartz crystal at 4000K.The surface material removal process of quartz glass was obtained through the sliding process of abrasive particles on the surface.The results show that H₂O molecules have molecular and dissociation adsorption on the quartz glass surface,and dissociation adsorption is the key to the hydroxylation.Si-O-Si bonds between the abrasive particle and quartz glass surface will form during the sliding process.Under the synergetic action of multiple Si-O-Si bonds and H atoms,the chemical bonds between surface atoms and substrate will be broken and the surface atoms are removed.The processing parameters have a great effect on the material removal rate,removal form and machining properties during the ultra-precision processing of quartz glass.The interface pressure,polishing speed and polishing fluid composition affect the number of interface Si-O-Si bonds,thus affecting the number of removed surface atoms.The number of Si-O-Si bonds also affect the removal form of surface materials,there are single-atom removal mode,chain removal mode and cluster removal mode in the CMP and different machining parameters lead to different removal modes.The effects of temperature during the machining process of quartz glass are also investigated.When the temperature increases from300K to 1500K,the hardness of quartz glass decreases by 53.6%and the critical depth increases from nanometer?9.46nm?to micrometer?234.87nm?.The subsurface damage evolution is more conductive to subsequent removal.Nanoindentation and nano-scratch experiments were carried out on quartz glasses which are modified by KOH and H₂O₂ to verify the influence of surface hydroxylation on the machining properties.The hardness,elastic modulus and surface morphology of quartz glass after scratch under different concentrations of etching solutions are analyzed.The results show that 5%H₂O₂ solution can effectively reduce the hardness of quartz glass and improve its machining properties.