Guiling Liu

Removal Behaviors of Different SiC Ceramics during Polishing

Comparative experiments were conducted to reveal the removal behaviors of three kinds of silicon carbide (SiC) ceramics during polishing and the effects of ceramic microstructure on the surface quality were also reported. Experimental results show that the second phase in SiC ceramics plays an important role in the surface quality when its size is large enough. The surface quality is enslaved to the formation of steps at interfaces between second phase and SiC matrix that results from different elastic modulus and hardness between two phases. Under 3 μm abrasive grains polishing condition, different SiC ceramics show different removal mechanisms. With decreasing abrasive grain size, all of different SiC ceramics exhibit a ductile removal mode, which decreases surface roughness efficiently.

Manufacture of large-scale lightweight SiC mirror for space

Large-scale lightweight silicon carbide (SiC) mirrors were manufactured for space. Sintered SiC (SSiC) ceramic was adopted as the material to manufacture these mirrors. Complex structure designed for highly weight reduction and installation requirements was near-net-shape formed on SiC green body by digital machining technique before the high temperature sintering process. The dimensional accuracy of thin ribs and faceplate can be precisely controlled above 99.5%. During sintering process, the temperature distribution was kept uniform enough to avoid residual stress and deformation in the whole furnace. Isotropic shrinkage occurs during densification from SiC green body to ceramic with a fluctuation less than 0.3%, which is the dimension error of the final size as well. Mirror surface with low surface roughness, high shape accuracy and reflectivity was finished by polishing and plating. Moreover, large-scale lightweight SSiC mirror was demonstrated to be suitable for space use by tests simulating launch conditions and space environments.

Research progress of optical fabrication and surface-microstructure modification of SiC

SiC has become the best candidate material for space mirror and optical devices due to a series of favorable physical and chemical properties. Fine surface optical quality with the surface roughness (RMS) less than 1nm is necessary for fine optical application. However, various defects are present in SiC ceramics, and it is very difficult to polish SiC ceramic matrix with the 1nm RMS. Surface modification of SiC ceramics must be done on the SiC substrate. Four kinds of surface-modification routes including the hot pressed glass, the C/SiC clapping, SiC clapping, and Si clapping on SiC surface have been reported and reviewed here. The methods of surface modification, the mechanism of preparation, and the disadvantages and advantages are focused on in this paper. In our view, PVD Si is the best choice for surface modification of SiC mirror.

Investigation on the laser ablation of SiC ceramics using micro‐Raman mapping technique

Research on the laser ablation behavior of SiC ceramics has great significance for the improvement of their anti-laser ability as high-performance mirrors in space and lasers, or the laser surface micro-machining technology as electronic components in micro-electron mechanical systems (MEMS). In this work, the laser ablation of SiC ceramics has been performed by using laser pulses of 12 ns duration at 1064 nm. The laser induced damage threshold (LIDT) below 0.1 J/cm2 was obtained by 1-on-1 mode and its damage morphology appeared in the form of “burning crater” with a clear boundary. Micro-Raman mapping technique was first introduced in our study on the laser ablation mechanisms of SiC surface by identifying physical and chemical changes between uninjured and laser-ablated areas. It has been concluded that during the ablation process, SiC surface mainly underwent decomposition to the elemental Si and C, accompanied by some transformation of crystal orientation. The oxidation of SiC also took place but only in small amount on the edges of target region, while there was no hint of SiO2 in the center with higher energy density, maybe because of deficiency of O2 atmosphere in the ablated area, elimination of SiO2 by carbon at 1505 °C, or evaporating at 2230 °C.

Nucleation and Growth Mechanism of Si Amorphous Film Deposited by PIAD

The nanoscale Si films with the thickness of 2 nm, 5 nm, 10 nm, and 20 nm were deposited by plasma ion assisted deposition (PIAD) on glass substrate, in order to investigate the initial stage and the nucleation and growth mechanism of the Si film. The atomic force microscopy (AFM) was used to investigate the surface topography of the as-deposited Si film. The initial nucleation and growth process of the film was described. The continuous film had been already formed when the film thickness was 10 nm. The growth of the deposited Si film accorded with the Volmer-Weber growth mode.