Reaction Bonded Silicon Carbide Composite Research Articles

Effect of Femtosecond Laser Processing Parameters on the Ablation Microgrooves of RB-SiC Composites

Because of the high hardness, brittleness, and anisotropy of reaction-bonded silicon carbide composites (RB-SiC), it is challenging to process high-quality textures on their surfaces. With the advantages of high processing accuracy and low processing damage, femtosecond laser processing is the preferred technology for the precision processing of difficult-to-process materials. The present work used a femtosecond laser with a linear scanning path and a spot diameter of 18 µm to process microgrooves on RB-SiC. The influence of different processing parameters on the microgroove profile, dimensions, and ablation rate (AR) was investigated. The ablation width Wa and average ablation depth Da of microgrooves were evaluated, and the various patterns of varying processing parameters were obtained. A model for Wa prediction was developed based on the laser fluence within the finite length (FL). As a result, the experimental values were distributed near the prediction curve with a maximum error of 20.4%, showing an upward trend of gradually decreasing increments. For a single pass, the AR value was mainly determined by the laser energy, which could reach the scale of 106 μm3/s when the laser energy was greater than 50 μJ. For multiple passes, the AR value decreased as the number of passes increased and it finally stabilized. The above research will provide theoretical and experimental support for the high-quality and efficient processing of RB-SiC surface textures.

Laser ablation of RB-SiC composite by femtosecond laser irradiation

Reaction-bonded silicon carbide composites (RB-SiC), consisting of SiC grains and Si matrix, have a complex ablation mechanism as two-phase composites under femtosecond laser irradiation. The ablation threshold of RB-SiC was calculated, which decreases (from 1.01 J/cm2 to 0.56 J/cm2) with increasing pulse number N and finally stabilized. At low fluence (F/Fth ≤ 5) and a low number of pulses, photochemical ablation dominates. For a higher number of pulses and high fluence (5 <F/Fth ≤ 50), photothermal ablation dominates with photochemical ablation. The results show that structures such as pits, convex, and holes exist on the surface of Crater I, while Laser-induced Periodic Surface Structures (LIPSS), and condensation structures exist on the surface of Crater II. In both types of ablation craters, the SiC grain surface produces the Low Spatial Frequency LIPSS (LSFL) with a period of about 900 nm and the High Spatial Frequency LIPSS (HSFL) with a period of about 300 nm. Based on the surface morphology, microstructure, and physical phase analysis of the ablation craters, a preliminary model of the removal mechanism of the ablation craters by femtosecond laser irradiation was established. These findings are expected to provide theoretical and experimental support for the precision processing of RB-SiC by femtosecond laser.

Improvement of toughness of reaction bonded silicon carbide composites reinforced by surface-modified SiC whiskers

To improve the reliability, especially the toughness, of the reaction bonded silicon carbide (RBSC) ceramics, silicon carbide whiskers coated with pyrolytic carbon layer (PyC-SiCw) by chemical vapor deposition (CVD) were introduced into the RBSC ceramics to fabricate the SiCw/RBSC composites in this study. The microstructures and properties of the PyC-SiCw/RBSC composites under different mass fraction of nano carbon black and PyC-SiCw were investigated methodically. As a result, a bending strength of 550 MPa was achieved for the composites with 25 wt% nano carbon black, and the residual silicon decreased to 11.01 vol% from 26.58 vol% compared with the composite of 15 vol% nano carbon black. The fracture toughness of the composites reinforced with 10 wt% PyC-SiCw, reached a high value of 5.28 MPa m1/2, which increased by 39% compared to the RBSC composites with 10 wt% SiCw. The residual Si in the composites deceased below to 7 vol%, resulting from the combined actively reaction of nano carbon black and PyC with more Si. SEM and TEM results illustrated that the SiCw were protected by PyC coating. A thin SiC layer formed of outer surface of whiskers can provide a suitable whisker-matrix interface, which is in favor of crack deflection, SiCw bridging and pullout to improve the bending strength and toughness of the SiCw/RBSC composites.

Effect of boron carbide on the liquid silicon infiltration and performance enhancement of reaction-bonded silicon carbide composites

Reaction-bonded boron carbide (RBBC) is prepared through the process that liquid silicon infiltrating into porous B4C/resin preforms. The reaction between B4C and liquid silicon, structural evolution of RBBC composites and factors affecting the mechanical properties were analyzed. The results showed that the independently distributed B4C particles and newly-formed coarse SiC grains surrounded by banded silicon phase were the main reasons for the performance degradation of RBBC composites. On this basis, we studied the effect of B4C on the liquid silicon infiltration and performance enhancement of reaction-bonded silicon carbide (RBSC) composites. It is an effective and easy method to improve the performance of RBSC composites sharply using refined B4C particle as the reinforcement. The main reasons for the performance enhancement are the reduction of the content and dimensions of residual silicon, increase of the fracture energy and grain refinement.

Microstructural development and factors affecting the performance of a reaction-bonded silicon carbide composite

Abstract Due to its near net-shape forming characteristic, reaction-bonded silicon carbide (RBSC) is an ideal sintering material to prepare complicated structural SiC-based ceramics when combined with selective laser sintering (SLS). With this understanding, various factors affecting RBSC composites and the performance control methods were analyzed in this work. By studying the microstructural development and reaction mechanism during the reaction-bonded process, the main factor affecting the performance was found to be the carbon density of SiC/C preform. The residual silicon content decreased with the increase in the carbon density of preform, thereby improving the performance. When the carbon density of preform exceeded the maximum value, some unreacted carbon existed in the sintered body, resulting in the degradation of performance. In order to break the limit of maximum carbon density of the preform, two effective approaches, namely the slow-release carbon source and new-style structure capillary channel were proposed in this work.

Fabrication and characterization of a new-style structure capillary channel in reaction bonded silicon carbide composites

A new-style structure capillary channel was fabricated by using boron carbide powder mixtures with an appropriate multimodal particle size distribution to promote the liquid silicon infiltration in reaction bonded silicon carbide composites. Two types of core–rim structure were observed and the secondary SiC produced in the siliconisation reaction existed in two forms: nucleating on the original SiC and occupying the original positions of the residual silicon. The size of the secondary SiC in the latter form was in a range of tens to hundreds nanometers. These nano-sized SiC grains and the additive of fine boron carbide particles refined the crystalline structure and broke up the residual silicon phase into small pieces. Using this method, the microstructure was refined and the mechanical properties improved significantly. The lowest residual silicon volume fraction was 4.0% and the flexural strength and fracture toughness reached peak values of 526±21MPa and 6.2±0.4MPam1/2, respectively.

Fabrication and mechanical properties of multi-walled carbon nanotube reinforced reaction bonded silicon carbide composites

Different amounts (0, 1, 3, 5, 10 or 15wt%) of multi-walled carbon nanotubes (MWCNT)–reinforced reaction bonded silicon carbide (RBSC) composites were fabricated by epoxy resin curing and liquid infiltration of molten silicon at 1600°C for 90min. First, the MWCNTs were coated by phenolic resin, which could decompose into amorphous carbon. The in situ formation of the SiC coating via the reaction of amorphous carbon and silicon on the surface of MWCNTs could prevent siliconization. The influence of incorporated MWCNTs on the micro-structure and mechanical properties of RBSC composites was researched. The composites show a higher bending strength of 365MPa and a fracture toughness of 6.9MPam1/2, which are higher than those of monolithic RBSC (a bending strength of 236MPa and a fracture toughness of 3.8MPam1/2). Based on micro-structural measurements, the improved mechanical properties of the MWCNTs/RBSC composite compared with those of the as-fabricated RBSC can be ascribed to three mechanisms: MWCNT pullout, MWCNT bridging and crack deflection.

Effect of carbon particle and carbon fiber on the microstructure and mechanical properties of short fiber reinforced reaction bonded silicon carbide composite

Variation of microstructure and mechanical behavior was investigated with the content increase of carbon particles and carbon fiber in the reaction bonded silicon carbide composites. The composites were prepared by slip casting and liquid silicon infiltration. The bulk density is raised with the increase of carbon black due to the formation of fine β-SiC particles. The flexural strength increases for the reduction of residual Si and the formation of SiC framework; whereas a very high carbon content reduces the flexural strength. The fracture toughness is controlled by the contents of carbon particle and carbon fiber. Thus, fiber debonding, fiber pullout and crack deflection are considered as the main toughening mechanisms. Annealing treatment can effectively improve both the flexural strength and fracture toughness. An increase by 49% of fracture toughness is obtained. A series of structural models are proposed to illustrate the structure changes of carbon fiber.

Zirconia reinforced reaction bonded silicon carbide composites: Microstructure and mechanical properties

Nano-sized zirconia particles were dispersed in a SiC/C suspension to examine the particle influence on microstructure and mechanical properties on the resulting composite. The ZrO2/reaction bonded SiC composite was produced by a combination of slip casting and liquid silicon infiltration. By dispersing zirconia particles into the matrix, the gowth of β-SiC was inhibited and the size of residual silicon was reduced. This result led to an increase of flexural strength with reinforcement ranging from 0 to 20wt.%. Compared with the monolithic RBSC ceramics, the fracture toughness was steadily improved, reaching the peak value of appropriate 4.6MPam1/2 at the zirconia fraction of 25wt.%. The XRD analysis revealed the phase transformation occurred on the fracture surface of the composites. Thus, the transformation of zirconia and the crack deflection are considered as the main toughening mechanisms.

Random chopped fibers in reaction bonded SiC composite: Morphology, etching and reinforcing properties

Random chopped fibers were introduced into SiC/C slurry to improve the mechanical behaviors of reaction bonded SiC ceramics. The distribution, morphology and chemical properties of the siliconized carbon fiber were investigated. The effect of fiber fraction on mechanical behaviors of the reaction bonded silicon carbide composite was studied. The siliconization reaction led to a dense layer of nano and micron SiC particles on the fiber surface. Based on the morphology changes in the acid etching test, a structural model of the siliconized fiber was proposed. The fracture toughness and the flexural strength of the composite were improved by the random chopped fiber, reaching the peak values of 5.1MPam0.5 and 416MPa, respectively, at the fiber fraction of 30vol.%.

Effects of random chopped fiber on the flexural strength and toughness of reaction bonded silicon carbide composite

Chopped carbon fibers were introduced into the SiC/C suspensions to enhance the mechanical behaviors of reaction bonded silicon carbide ceramics. The effects of fiber fraction on the flexural strength and fracture toughness were investigated. No preferred fiber orientation was observed on the fracture surface of the green body. This implies a homogeneous dispersion of chopped fiber in the composite. XRD analysis indicates a complete siliconization of both carbon particles and carbon fiber during liquid silicon infiltration. The flexural strength and fracture toughness increase with chopped fiber ranging from 10vol.% to 30vol.%. This improvement derives from the fiber pullout, fiber debonding as well as crack deflection. The consequent decline at fiber fraction of 40vol.% is resulted from the siliconization of chopped fiber and the increase of residual silicon.

탄소섬유 강화 반응소결 탄화규소의 탄소섬유 첨가량에 따른 기계적 특성 변화

The composite added with surface-coated chopped carbon fiber showed the microstructure of a 3 dimensional discretional arrangements. The fiber reinforced reaction bonded silicon carbide composite, containing the 50 vol% carbon fiber, showed the porosity of 1/2 . As the content of carbon fiber was increased from 0 vol% to 50 vol% in the composite, fracture strength was decreased due to the increase of carbon fiber, which has a less strength than SiC and molten Si. On the other hand, the fracture toughness was increased with increasing the amount of carbon fiber. According to the polished microstructure, carbon fiber was shown to have a random 3 dimensional arrangement. Moreover, the fiber pull-out phenomenon was observed with the fractured surface, which can explain the increased fracture toughness of the composite containing high content of carbon fiber.

Fabrication and characterization of random chopped fiber reinforced reaction bonded silicon carbide composite

This paper deals with the microstructure and mechanical properties of reaction bonded silicon carbide reinforced with random chopped carbon fibers of 3 mm length. The composites were fabricated by dispersing chopped carbon fibers into bimodal SiC/C suspension, forming green body through slip casting, and then reaction sintering at 1700 °C. The effect of the chopped fiber fraction on microstructure and mechanical properties was evaluated. A significant increase of fracture toughness was obtained as the carbon fiber fraction approaches 30 vol.%. The chopped fibers had reacted with liquid silicon during reaction sintering, so little fiber pullout was observed. Crack deflection and bridging is the predominant mechanism for the composite toughening.

Preparation of reaction bonded silicon carbide (RBSC) using boron carbide as an alternative source of carbon

RBSC composites are fully dense materials fabricated by infiltration of compacted mixtures of silicon carbide and carbon by molten silicon. Free carbon is usually added in the form of an organic resin that undergoes subsequent pyrolysis. The environmentally unfriendly pyrolysis process and the presence of residual silicon are serious drawbacks of this process. The study describes an alternative approach that minimizes the residual silicon fraction by making use of a multimodal particle size distribution, in order to increase the green density of the preforms prior infiltration. The addition of boron carbide provides an alternative source of carbon, thereby eliminating the need for pyrolized organic compounds. The residual silicon fraction in the RBSC composites, prepared according to the novel processing route, is significantly reduced. Their mechanical properties, in particular the specific flexural strength is by 15% higher than the value reported for RBSC composites prepared by the conventional approach.

용융 Si 침윤법에 의해 제조된 반응소결 탄소 섬유강화 탄화규소 복합체 제조; I. 탄소 섬유 코팅 방법에 따른 영향

Reaction bonded silicon carbide (RBSC) composite for heat-exchanger was fabricated by molten Si infiltration method. For enforcing fracture toughness to reaction bonded silicon carbide composite, the surface of carbon fiber has coating layer by SiC or pyrocarbon. For SiC layer coating, CVD method was used. And for carbon layer coating, the phenol resin was used. In the case of carbon layer coating, fracture toughness and fracture strength were enhancing to 4.4MPa ·m 1/2 and 279 MPa.