Reaction-sintered Silicon Carbide Research Articles

Ultra-precision diamond finishing of reaction-sintered silicon carbide enhanced by vibration-assisted photocatalytic oxidation

RS-SiC faces limitations in surface accuracy with conventional finishing processes due to its high mechanical hardness and multiphase nature. In this study, vibration-assisted photocatalytic oxidation is introduced to enhance the efficiency and precision of RS-SiC diamond polishing. By utilizing the photocatalytic reaction, the hard heterogeneous SiC phase and Si phase on the RS-SiC surface are transformed into a "softer" homogeneous oxide layer that can be removed easily. The introduction of vibration increases the relative velocity between the workpiece and the abrasive and prevents the aggregation of the photocatalyst, thus improving polishing efficiency. By reducing the abrasive size to the nanometer scale, the boundary between the Si phase and SiC phase can be smoothed out to further improve the workpiece surface finish. Using TiO2 as the photocatalyst, H2O2 as the oxidant, 25 nm diamond as the abrasive, and introducing vibration, an ultra-smooth surface with a surface roughness of 0.195 nm in Ra is obtained by the finishing method proposed in this paper. The proposed photocatalysis/vibration-assisted finishing approach offers a novel method for ultra-smooth polishing reaction-sintered silicon carbide.

Effect of the Carbon Component on the Strength of Reaction-Sintered Silicon–Carbide Ceramics

Effect of the Carbon Component on the Strength of Reaction-Sintered Silicon–Carbide Ceramics

Study of the Structural, Physical, and Mechanical Characteristics of Reaction-Sintered Silicon Carbide Ceramics

Study of the Structural, Physical, and Mechanical Characteristics of Reaction-Sintered Silicon Carbide Ceramics

METHODS OF FORMING COMPLEX PROFILE PRODUCTS FROM HEAT-RESISTANT CERAMIC MATERIALS BASED ON SILICON CARBIDE

The possibility of obtaining complex-shaped mechanical engineering products based on reaction-sintered silicon carbide using two methods - mechanical processing of polymerized blanks before sintering and hot slip molding under pressure, has been studied. The obtained silicon carbide ceramic materials are characterized by a low density of about 3.04…3.07 g/cm3, a porosity of ? 1 % and a bending strength of 320…360 MPa. Materials based on silicon carbide modified with boron carbide have a density of 2.72 g/cm3, a porosity of about 1 % and a bending strength of 280 MPa.

Effect of the grain composition of the initial silicon carbide powder on the structure and properties of reaction-sintered silicon carbide

The paper shows the effect of different dispersed compositions of initial silicon carbide powders on the microstructure, strength, and crack resistance of reaction-sintered silicon carbide.

Born-Oppenheimer Molecular Dynamics of OH Radical in the SiC Grains in the Reaction-Sintered Silicon Carbide

Born-oppenheimer molecular dynamics is an effective method to analyze machining mechanism of some advanced manufacturing techniques, which was introduced to investigate the absorption of OH radical on different β-SiC surfaces in the reaction-sintered silicon carbide. During the plasma oxidation-assisted polishing of reaction-sintered silicon carbide, absorption of the OH radical in the SiC grains was one of the most important process, because hardness of the SiC grains was obviously higher than that of the Si grains. the born-oppenheimer molecular dynamics model to investigate the absorption process of the OH radical was constructed firstly, which could form the foundation for the further simulation and analysis. Secondly, absorption of the OH radicals in the Si-face, those in the C-face, and those in the asymmetric face were investigated successively, which included the absorption process and the oxide product. Finally, comparisons of the absorption of the OH radical in the different faces were conducted, which aimed to further understand the plasma oxidation-assisted polishing of reaction-sintered silicon carbide. Simulation result of born-oppenheimer molecular dynamics was propitious to promote the machining level of reaction-sintered silicon carbide and promote its practical application.

Determination of the Temperature Coefficient of Linear Expansion of Materials Based on Silicon Carbide

Information on the temperature coefficient of linear expansion (TCLE) of materials based on silicon carbide is presented. It is shown that the different polytypes 3C (cubic modification) and 4H and 6H (hexagonal modifications) are characterized by different thermal expansions, the difference between the values of which increases with increasing temperature. The TCLEs of reaction-sintered, liquid-phase-sintered, and hot-pressed silicon carbide materials are determined in the range 20 – 1800°C.

Research of properties of protective coating applied to the surface of reaction-sintered ceramic materials

The study describes the properties of the protective coating deposited on the surface of the reaction-sintered silicon carbide and molybdenum disilicide. The technology of increasing the protective ability of the coating of products deposited on the surface on the basis of reactive sintered carbide of silicon and molybdenum disilicide, which operate in an oxidizing environment at high temperature and a sharp change of thermal regime, is investigated. The obtained results showed that the presence of a protective slip layer significantly increases the stability of the deposited silicoboride coating, thus blocking the interaction of silicon hexaboride with the environment, slowing down almost all diffusion processes at the transition of the interaction of diffusion. It has been established that the simultaneous use of both diffusion and slurry coatings enables maximum protection of reaction-sintered ceramic materials based on silicon carbide and molybdenum disilicide against high-temperature gas corrosion. The developed coating ensures maximum resistance to repeated changes in temperature conditions, while cyclic changes destroy products of silicon carbide and molybdenum disilicide without applied protective coating. The proposed protective coating can be recommended for the protection of reaction-sintered ceramic materials operated in high temperatures.

Complex elements of aerospace equipment from reaction silicon carbide ceramics

The results of studying the process of obtaining complex-profile elements of the substrate of mirrors of optical telescopes from reaction-sintered silicon carbide ceramics are presented. It is shown that the strength of silicon carbide ceramics depends on the dispersion of the silicon carbide powder and on the temperature of reaction sintering. An increase in the sintering temperature from 1500 to 1650 °C leads to an increase in strength by 60 MPa, and to 1800 °C – to a decrease in strength by 40 MPa. An increase in strength is explained by a decrease in free silicon and an increase in the content of secondary silicon carbide, a decrease in strength is explained by an increase in the size of carbide grains. The study of the influence of the modes of soldering of hexagonal elements to obtain a complex-profile element of the substrate of the mirror of an optical telescope on the strength of the soldered seam showed that the introduction of silicon carbide powder 7 μm in size and amorphous boron in an amount of 6 % into the solder composition based on silicon carbide has a positive effect on the strength of the soldered seam. Tests of the brazed specimens at three-point bending showed that fracture occurs along the body of the specimens being brazed, and not the brazed seam. The structure of the brazed joint depends on the composition of the braze alloy and the gap between the samples to be brazed.

The effect of the dispersed composition of SiC on the physical and mechanical properties of reactionsintered silicon carbide

We obtain dense ceramic materials of silicon carbide by reaction sintering, with high mechanical properties. The effect of different dispersed composition of silicon carbide powders on the microstructure, grain size after sintering, as well as physical and mechanical properties is shown.

Effect of SiC Dispersed Composition on Physical and Mechanical Properties of Reaction-Sintered Silicon Carbide

Dense ceramic materials of silicon carbide with high mechanical properties are obtained by reaction sintering. The effect of different dispersed compositions of silicon carbide powders on the microstructure, grain size after sintering, and physical and mechanical properties is shown.

Structurization of Reaction-Sintered Silicon Carbide Modified by Chromium Carbide

The structurization and phase composition of reaction-sintered silicon carbide (RSSC) modified by chromium carbide are examined. When hexagonal silicon carbide is used, the modification of RSSC by chromium carbide stabilizes the cubic lattice of secondary silicon carbide. A solid solution of silicon in chromium carbide is formed when the samples are impregnated with liquid silicon. In the impregnation of porous samples, silicon influences the consolidation of chromium carbide particles because they are displaced by liquid silicon. Depending on the particle size and features of starting silicon carbide, RSSC is characterized by various structural modifications. A hexagonal 6H structure shows up in RSSC produced from the nanosized cubic solid solution of carbon in silicon carbide.

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 RS-SiC 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.

Study of influence of the removal depth of the silicon modification layer on grating structures in reaction-sintered silicon carbide substrate and improvement method.

The influence of the removal depth of a silicon modification layer on grating structures and mirrors is studied. The removal depth 6-14μm is the optimization result for Si-modified reaction-sintered silicon carbide (RS-SiC) used as mirror substrates, but the removal depth 9-12μm is the optimization result for Si-modified RS-SiC used as grating substrates. The diffraction efficiency and stray light of the gratings fabricated in the Si-modified RS-SiC substrates with removal depth 9-12μm is 90.5%-94% and 5.30×10-7-5.45×10-7, respectively. Additionally, the number and scale of high-reflection points can be used as the basis for judging the removal depth of the Si-modified RS-SiC used as grating substrates. These results and the regularity have guiding significance for the application of Si-modified RS-SiC as a microstructural substrate.

Optimization of Gas Composition Used in Plasma Chemical Vaporization Machining for Figuring of Reaction-Sintered Silicon Carbide with Low Surface Roughness

In recent years, reaction-sintered silicon carbide (RS-SiC) has been of interest in many engineering fields because of its excellent properties, such as its light weight, high rigidity, high heat conductance and low coefficient of thermal expansion. However, RS-SiC is difficult to machine owing to its high hardness and chemical inertness and because it contains multiple components. To overcome the problem of the poor machinability of RS-SiC in conventional machining, the application of atmospheric-pressure plasma chemical vaporization machining (AP-PCVM) to RS-SiC was proposed. As a highly efficient and damage-free figuring technique, AP-PCVM has been widely applied for the figuring of single-component materials, such as Si, SiC, quartz crystal wafers, and so forth. However, it has not been applied to RS-SiC since it is composed of multiple components. In this study, we investigated the AP-PCVM etching characteristics for RS-SiC by optimizing the gas composition. It was found that the different etching rates of the different components led to a large surface roughness. A smooth surface was obtained by applying the optimum gas composition, for which the etching rate of the Si component was equal to that of the SiC component.

Investigation on the formation of projections and cracks in anodic oxidation of reaction-sintered silicon carbide

Among the present oxidation-assisted polishing (OAP) technique, anodic oxidation is a promising method to process reaction-sintered silicon carbide (RS-SiC) with the assistance of abrasive polishing. Projections and cracks are inevitably formed in the anodic oxidation of RS-SiC for the volume expansion force. Evolvements of the oxide morphologies along with different oxidation voltage and different oxidation time are investigated by scanning electron microscope (SEM). It can be found that when the oxidation time stays the same, size of the projections and cracks on the oxidized RS-SiC sample is gradually enlarged. Meanwhile, when the oxidation voltage stays the same, size of the projections and cracks on the oxidized RS-SiC sample is also enlarged. Details of projections and cracks on the oxidized RS-SiC sample are obtained by the SEM with high magnification. Based on the theoretical derivation, it can be concluded that the formations of projections and cracks depend on the inner volume expansion force and the resultant force generated from other grains. Research on formation of projections and cracks in anodic oxidation of RS-SiC can promote the process level of RS-SiC products.

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

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.

A critical energy model for brittle–ductile transition in grinding considering wheel speed and chip thickness effects

The ability to predict the critical depth for ductile-mode grinding of brittle materials is important to grinding process optimization and quality control. The traditional models for predicting the critical depth are mainly concerned with the material properties without considering the operation parameters. This article presents a new critical energy model for brittle–ductile transition by considering the strain rate effect brought by the grinding wheel speed and chip thickness. The experiments will be conducted through a high-speed diamond grinder on reaction-sintered silicon carbide materials under different grinding speed and chip thickness. Through detailed analysis of the strain rate effect on the dynamic fracture toughness, a new fracture toughness model will be established based on the Johnson–Holmquist material model (JH-2) and calibrated through experiments based on the indentation fracture mechanics. Then, the new critical model for brittle–ductile transition will be established by introducing the dynamic facture toughness model considering the wheel speed and chip thickness. According to scanning electron microscope observations, the results show that ductile-mode grinding can be obtained through a combination of higher grinding speed and smaller chip thickness. Moreover, the critical value for ductile grinding of brittle materials can be improved through the elevation of the grinding speed or reduction in the chip thickness.

Comparative analysis on surface property in anodic oxidation polishing of reaction-sintered silicon carbide and single-crystal 4H silicon carbide

For effective machining of difficult-to-machine materials, such as reaction-sintered silicon carbide (RS-SiC) and single-crystal 4H silicon carbide (4H-SiC), a novel polishing technique named anodic oxidation polishing was proposed, which combined with the anodic oxidation of substrate and slurry polishing of oxide. By scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDX) observation and atomic force microscopy analysis, both the anodic oxidation behaviors of RS-SiC and 4H-SiC were investigated. Through comparison of the surfaces before and after hydrofluoric acid etching of the oxidized samples by the scanning white light interferometry (SWLI) measurement, the relationships between oxidation depth and oxidation time were obtained, and the calculated oxidation rate for RS-SiC was 5.3 nm/s and that for 4H-SiC was 5.8 nm/s based on the linear Deal–Grove model. Through anodic oxidation polishing of RS-SiC substrate and 4H-SiC substrate, respectively, the surface roughness rms obtained by SWLI was improved to 2.103 nm for RS-SiC and to 0.892 nm for 4H-SiC. Experimental results indicate that anodic oxidation polishing is an effective method for the machining of RS-SiC and 4H-SiC samples, which would improve the process level of SiC substrates and promote the application of SiC products in the fields of optics, ceramics, semiconductors, electronics, and so on.

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

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.