Xinmin Shen

Mechanism analysis on finishing of reaction-sintered silicon carbide by combination of water vapor plasma oxidation and ceria slurry polishing

Abstract. Reaction-sintered silicon carbide (RS-SiC), which is considered as a promising mirror material for space telescope systems, requires a high surface property. An ultrasmooth surface with a Ra surface roughness of 0.480 nm was obtained after water vapor plasma oxidation for 90 min followed by ceria slurry polishing for 40 min. The oxidation process of RS-SiC by water vapor plasma was analyzed based on the Deal–Grove model, and the theoretical calculation results are consistent with the measured data obtained by scanning white light interferometer (SWLI), scanning electron microscopy/energy-dispersive x-ray, and atomic force microscope. The polishing process of oxidized RS-SiC by ceria slurry was investigated according to the Preston equation, which would theoretically forecast the evolutions of RS-SiC surfaces along with the increasing of polishing time, and it was experimentally verified by comparing the surface roughnesses obtained by SWLI and the surface morphologies obtained by SEM. The mechanism analysis on the finishing of RS-SiC would be effective for the optimization of water vapor plasma oxidation parameters and ceria slurry polishing parameters, which will promote the application of RS-SiC substrates by improving the surface property obtained by the oxidation-assisted polishing method.

Efficient processing of reaction-sintered silicon carbide by anodically oxidation-assisted polishing

Abstract. Reaction-sintered silicon carbide (RS-SiC) is a promising optical material for the space telescope systems. Anodically oxidation-assisted polishing is a method to machine RS-SiC. The electrolyte used in this study is a mixture of hydrogen peroxide (H2O2) and hydrochloric acid (HCl), and the oxidation potential has two modes: constant potential and high-frequency-square-wave potential. Oxide morphologies are compared by scanning electron microscope/energy dispersive x-ray spectroscopy and scanning white-light interferometer. The results indicate that anodic oxidation under constant potential can not only obtain a relatively smooth surface but also be propitious to obtain high material removal rate. The oxidation depth in anodic oxidation under constant potential is calculated by comparing surface morphologies before and after hydrofluoric acid etching. The theoretical oxidation rate is 5.3  nm/s based on the linear Deal–Grove model. Polishing of the oxidized RS-SiC is conducted to validate the machinability of the oxide layer. The obtained surface roughness root-mean-square is around 4.5 nm. Thus, anodically oxidation-assisted polishing can be considered as an efficient method, which can fill the performance gap between the rough figuring and fine finishing of RS-SiC. It can improve the machining quality of RS-SiC parts and promote the application of RS-SiC products.

Research on optimal process parameters in thermally oxidation-assisted polishing of reaction-sintered silicon carbide

Reaction-sintered silicon carbide (RS-SiC) has been widely used in space telescope systems for its excellent physical and mechanical properties. Thermally oxidation-assisted polishing is a practical machining method to obtain RS-SiC parts with high precision, and the research focus is optimization of process parameters, because there are bumpy structures on the oxidized RS-SiC. By atomic force microscopy (AFM) detection, the distributions of oxides on the oxidized RS-SiC sample are quantitative analyzed when the thermal oxidation time is 5min, 30min, and 60min, and the calculated average differences of oxide heights between the initial Si grains and SiC grains are 10.7nm, 25.1nm, and 35.2nm, respectively. Meanwhile, the volume expansion coefficient in oxidation of Si/SiC to SiO2 is 2.257 and 2.194, respectively. Through theoretical derivation based on the Deal-Grove model, the numerical relationship between differences of oxide heights and thermal oxidation time is obtained. Combining with the material removal rate of oxide by ceria slurry in the abrasive polishing, the obtained surface quality can be precisely forecasted and controlled. The oxidized RS-SiC sample, when the oxidation time is 30min, is polished with different times to verify the theoretical analysis results. When the polishing times are 20min, 30min, and 40min, the obtained differences of oxide heights by the AFM detection are consistent with theoretical calculated results. Research on the optimal process parameters in thermally oxidation-assisted polishing of RS-SiC can improve the process level of RS-SiC sample and promote the application of SiC parts.

Preliminary study on atmospheric-pressure plasma-based chemical dry figuring and finishing of reaction-sintered silicon carbide

Abstract. Reaction-sintered silicon carbide (RS-SiC) is a research focus in the field of optical manufacturing. Atmospheric-pressure plasma-based chemical dry figuring and finishing, which consist of plasma chemical vaporization machining (PCVM) and plasma-assisted polishing (PAP), were applied to improve material removal rate (MRR) in rapid figuring and ameliorate surface quality in fine finishing. Through observing the processed RS-SiC sample in PCVM by scanning white-light interferometer (SWLI), the calculated peak-MRR and volume-MRR were 0.533  μm/min and 2.78×10−3  mm3/min, respectively. The comparisons of surface roughness and morphology of the RS-SiC samples before and after PCVM were obtained by the scanning electron microscope and atomic force microscope. It could be found that the processed RS-SiC surface was deteriorated with surface roughness rms 382.116 nm. The evaluations of surface quality of the processed RS-SiC sample in PAP corresponding to different collocations of autorotation speed and revolution speed were obtained by SWLI measurement. The optimal surface roughness rms of the processed RS-SiC sample in PAP was 2.186 nm. There were no subsurface damages, scratches, or residual stresses on the processed sample in PAP. The results indicate that parameters in PAP should be strictly selected, and the optimal parameters can simultaneously obtain high MRR and smooth surface.

Design and modeling of a walking mechanism for the self-adapting pipeline robot

Self-adapting pipeline robot is widely used in many industrial fields and the walking mechanism is its critical part. According to the actual dimensional demand of a certain pipeline and the requirement of self-adapting pipeline robot, a walking mechanism is designed based on the double-cross structure and modelled through utilizing 3D modeling software. The walking mechanism can adjust its size to adapt to pipeline with different diameters. The workbench can be kept in the center of the pipeline by optimization of size of the double-cross structure. There are two driving wheels in the bottom and one driving wheel in the top to constitute three-point type, which can improve the stability and load capacity of the robot. The H-type driving wheel is adopted to improve stability and reliability of the contact between the wheel and the guide. Through optimizing parameters of the double-cross structure, the designed walking mechanism can meet the requirement of the pipeline with diameters from 480mm to 600mm. Furthermore, constructions of the walking mechanism are modelled by the Solidworks modeling software, which can verify the feasibility of the design. Design and modeling of the walking mechanism are propitious to give technical support to maintenance and repair of the pipeline.

Investigation on machinability of the oxide layer in anodic oxidation of reaction-sintered silicon carbide by pure-water

Oxidation-assisted polishing has been developed as an efficient method for precision machining of reaction-sintered silicon carbide (RS-SiC), and characteristic of the oxide layer is the critical factor to obtain a fine surface property. Machinability of the oxide layer obtained in anodic oxidation of RS-SiC by the pure-water under high-frequency-square-wave potential is investigated by the ceria slurry polishing in this study. Quantitative analysis of surface quality of the oxidized RS-SiC sample is conducted by the scanning white light interferometer (SWLI) measurement. Along with increase of the oxidation time, surface quality of the oxidized RSSiC is changing better at the beginning, and rapidly deteriorated in further oxidation process. Surface qualities of the RS-SiC sample before oxidation, after oxidation, after HF etching, and after abrasive polishing, are compared by the SWLI measurement. Surface roughness rms after anodic oxidation for 60min is 189.004nm. After removing the oxide by ceria slurry polishing, the rms can reach 3.688nm. Meanwhile, there is no visible scratch on the new revealed surface. Therefore, combination of anodic oxidation of RS-SiC by pure water and abrasive polishing of oxide layer by ceria slurry can be considered as an efficient method to machine RS-SiC.

Investigation on the Optimal Energy Recovery System for the Military Hybrid Vehicle Based upon the Comprehensive Evaluation Method

Energy recovery system is considered as the critical component of the novel hybrid vehicle. For the purpose of satisfying the special requirements of military hybrid vehicle, the investigation on the optimal energy recovery system was conducted based upon the comprehensive evaluation method. The investigated energy recovery system included the hybrid electric energy recovery system, the hydraulic hybrid energy recovery system, the high-speed flywheel energy recovery system, the flywheel and hydraulic mixed energy recovery system, and the battery and super capacitor mixed energy recovery system. In the comprehensive evaluation method, the studied parameters consisted of the energy control strategy, energy density, power density, fuel economy, recovery efficiency, seismic capacity, security, economic cost, requirements of working condition, system stability, and the maintainability. Through comprehensive considerations of the characteristics of energy recovery and the requirements of military hybrid vehicle, the weights of each parameter in the comprehensive evaluation method were established by the questionnaires. Meanwhile, through the questionnaires to the experts in the field of energy recovery system, scales of each parameter in the five energy recovery systems were obtained. It could be judged from final scales of the five systems that the hydraulic hybrid energy recovery system was the optimal option for the military hybrid vehicle. Development of the hydraulic hybrid energy recovery system would promote the entire performance of the military hybrid vehicle.

Optimal Design of Experimental Platform for Electronic-Hydraulic Driving Vehicle Based on the Multi-factor Synthetic Evaluation Model

Electronic-hydraulic driving vehicle has the advantages of high energy-utilization efficiency, excellent driving power, fine cross-country performance, and outstanding controllability, which make it a research focus in the field of novel transmission technique. Experimental platform is an important and essential instrument for the research and development of electronic-hydraulic driving vehicle, and its optimal design was conducted based on the multi-factor synthetic evaluation model. Three electronic-hydraulic driving modes were investigated in this study, which included the series type, the parallel type, and the mixed type. The investigated parameters in the multi-factor synthetic evaluation model consisted of volume, mass, system noise, requirement of spatial layout, difficulty in structure array, transformation cost of present car, transmission of gear-box, infinitely variable speed, mechanical transmission mode, difficulty in control, controllability of the driving system, total efficiency of the transmission system, energy integrated management strategy, loss of system energy, energy recovery efficiency, dynamic performance, security, reliability, probability, applicability, and so on. The weight and score of each parameter in the multi-factor synthetic evaluation model had been obtained by the expert scoring system. In order to satisfy the special demand of the military vehicle, it could be found that the series electronic-hydraulic driving mode was the optimal option, which could be judged from the final scores of the three modes according to the multi-factor synthetic evaluation model. Design and construction of the experimental platform would be propitious to advance the further research of the electronic-hydraulic driving vehicle and promote its application in the field of novel transmission vehicle.