Large-scale mirror plays an important role in the field of science and technology. Just because of shortcomings of
traditional materials, silicon carbide is tending to be the most potential material for large-scale mirror.
Large-scale mirror is fabricated by three steps. The first step includes design and analysis of mirror. The second step is
the preparation of mirror. The third step is machining, polishing and coatings.
Some mirrors with sizes from 100 mm to 1m were prepared. The roughness is less than 3 nm rms. The wave-front error
can be less than 1/40 wavelength rms. The reflectivity of the mirrors with Ag films is larger than 95%.
In order to obtain smoother surface, a dense coatings with single phase material should be prepared. In this paper, silicon
coatings were prepared by electronic beam physical vapor deposition and magnetron sputtering. The properties of
coatings-substrate system were tested. The adherence strength of silicon coatings by electronic beam physical vapor
deposition is between 15~20 g, and porosity is about 6%, and the adherence strength of silicon coatings by magnetron
sputtering is between 20~40 g, and porosity is less than 2%. The silicon coatings by magnetron sputtering can be
polished to a smooth surface.
Some silicon carbide mirrors were prepared for the large pointer mirror and the primary mirror of the high resolution
camera. Currently, experiments are under way to test the properties under the simulation space situation.
Swing system used in the guiding system is a kind of special equipment composed by a piece of silicon carbide mirror and a support structure made of Al alloy. In order to connect them without harm to their properties, epoxy resin is used and is solidified at room temperature. The influence of connecting process on the optical property of the mirror in swing system is studied by testing the wave-front error before and after connecting process. The results show that the connecting process under room temperature has little effect on wave-front error. The deformations of the mirror under gravity fields and thermal fields are analyzed by finite element method. The calculated results show that the surface figures under all conditions are satisfied with the requirement. The deformation of the mirror under gravity field paralleling to axial direction is the largest. When there is temperature gradient, the effect of axial temperature field on the mirror surface is much greater than that of radial temperature field.
Silicon Carbide (SiC) has been recognized as a leading material for optical applications. In this paper, grinding of RBSiC using diamond wheels on surface milling machine under various grinding parameters was investigated. Examinations of ground surfaces reveal that surface roughness increases with increase of depth of cut and decreases with increase of burnishing time, but a further prolonged of burnishing time did not work obviously to improve surface quality. Values of surface roughness swing with increase of work-piece rotation speed and exhibit variety with the increase of the cumulate removal volume under different depths of cut. The Vickers hardness of ground RBSiC decreases with the depth of cut. Scanning electron microscopy (SEM) observation revealed that brittle fracture and plastic flow removal mode coexist during grinding process. The percentage of ductile-mode grinding area decreased with increasing of depth of cut.
Some investigations into preparation of silicon coating on SiC mirror by Electronic Beam Physical Vapor Deposition (EB-PVD) are introduced in the paper, which include adhesion strength of coating to substrate, pattern of coating, and residual stress etc. According to the investigations, it was found that only when the temperature of substrate in the optimization temperature range, the excellent thermal-shock resistance and adhesion strength of coating to substrate are obtained. Furthermore, the coating has the column crystal structure. surface finish, deposition rate and gravity direction may have important influences on properties of coating-substrate system.
Reaction Bonded Silicon Carbide (RBSiC) has long been recognized as a promising material for optical applications because of its unique combination of favorable properties and low-cost fabrication. Grinding of silicon carbide is difficult because of its high hardness and brittleness. Grinding often induces surface and subsurface damage, residual stress and other types of damage, which have great influence on the ceramic components for optical application. In this paper, surface integrity, subsurface damage and material removal mechanisms of RBSiC ground using diamond grinding wheel on creep-feed surface grinding machine are investigated. The surface and subsurface are studied with scanning electron microscopy (SEM) and optical microscopy. The effects of grinding conditions on surface and subsurface damage are discussed. This research links the surface roughness, surface and subsurface cracks to grinding parameters and provides valuable insights into the material removal mechanism and the dependence of grind induced damage on grinding conditions.