Infiltration Of Molten Silicon Research Articles

3D in situ observation of the capillary infiltration of molten silicon in a SiC/SiC composite by X-ray tomography

The Liquid Silicon Infiltration (LSI) process is used to decrease residual porosity of SiC/SiC composite materials. However, it is not fully mastered since the mechanisms involved at 1500 °C under high vacuum are complex to analyze, especially without direct observation. Previous work had demonstrated the feasibility of using X-ray radiography to observe the front rise of silicon in a SiC/SiC composite material during the LSI process. 2D observations, as a first approach, could give a basic understanding of the mechanisms but raised several interrogations due to the superposition of these phenomena along the thickness preventing any quantification. The setup has been improved in order to make X-ray tomography using a fully integrated DC motor in place of the rotation stage commonly used. The sets of X-ray tomographs confirm two successive fillings. First, the molten silicon rapidly and non uniformly invades the accessible intergranular micro porosity of the powder. Then, the liquid slowly fills the remaining isolated powder areas. Once the SiC matrix is fully saturated, the liquid fills the bigger porosities such as cracks and intra yarn macro porosities. In addition, this 3D analysis enabled to give a better comprehension of the non uniform wetting front in the SiC matrix powder. During the 1st step, the accessibility of the powder was known to have a major effect on the speed of progression. Also, the cracks network plays a key role in the filling of the isolated areas in the powder matrix.

Effect of graphite content on the microstructure and mechanical properties of SLS-based RB-SiC ceramics

In this study, reactive bonded (RB)-SiC ceramics were fabricated using a combination of selective laser sintering (SLS) and reactive melt infiltration (RMI). The microstructure and mechanical properties of RB-SiC ceramics were optimized by adjusting the graphite content. The sintered bodies were found to consist of α-SiC, β-SiC, Si and C. The addition of graphite prolonged the reaction time between carbon and silicon, resulting in a greater infiltration of molten silicon and an increased formation of β-SiC. Furthermore, a correlation was observed between the proportion of graphite content and the bulk density and bending strength of RB-SiC ceramics. In the range of 0–10 wt%, the bulk density initially increased and then decreased as the graphite content increased. The maximum density was found to be 2.74 ± 0.15 g/cm3. The flexural strength reached the maximum value at 217.9 ± 16.9 MPa when the graphite content was 5 wt%. However, beyond this point, the flexural strength decreased rapidly. The present research demonstrates the promising potential of utilizing SLS additive manufacturing to fabricate SiC ceramics with exceptional mechanical properties.

3D observations of room temperature capillary infiltration mechanism in SiC compacts

The melt infiltration of a liquid metal is a successful method for densifying metal and ceramic matrix composites (CMC). The reduction of residual porosity of silicon carbide composites by infiltration of molten silicon into the matrix allows to reach interesting mechanical properties. However, fluid progression within the pore network of the granular matrix is a complex phenomenon, driven by physical and thermomechanical mechanisms that are not yet fully understood. This publication focuses on the capillary impregnation at room temperature and highlights important parameters related to the physical aspect of the process. Two different model fluids (n-hexadecane and exo-dicyclopentadiene) were used to reproduce the behaviour of molten silicon alloy. During the infiltration, the monitoring of the infiltrated liquid weight, along with the image acquisition, was used to compare the infiltration difference between several types of samples. An innovative way to interrupt and fix liquid inside samples, based on rapid polymerization, was investigated during capillary rise experiments. The correlation between 2D observations and 3D volumes obtained by tomography outlines the existence of two different flow fronts, the surface flow front (SFF) which often have a flat shape, and a second inside the sample, the volume flow front (VFF), which shows a paraboloid shape.

Diamond reinforced reaction-bonded boron carbide composites: Fabrication, microstructure, mechanical and tribological properties

Reaction-bonded boron carbide composites were fabricated by molten silicon infiltration of diamond-containing boron carbide preforms. The effects of diamond particle size on the microstructure, mechanical and tribological properties of reaction-bonded boron carbide composites were investigated. Fine diamond particles (≤7.7 µm) completely reacted with silicon generating nano silicon carbide particles, which bonded the boron carbide particles forming a 3-D ceramic skeleton and improving the flexural strength of the composites. Coarser diamond (≥24 µm) particles were retained protected by a dense SiC layer generated from the reaction of diamond and molten silicon. A “core-shell” structure with the retained diamond as the “core” and with the reaction formed silicon carbide as the “shell” formed. Relief structures were established on the composites’ surface during the wear process, greatly reducing the friction coefficient and improving the wear resistance.

Synthesis and thermomechanical behavior of SiC/Si compounds derived from wood waste

The traditional method of manufacturing SiC compounds is associated with a serious environmental problem, mainly due to the need for large amounts of energy (generally derived from oil) to reach processing temperatures (typically above 2500 ºC). In addition, the chemical reaction that gives rise to the formation of SiC has CO and CO2 as by-products. Therefore, in this work an alternative method to manufacture SiC/Si composites using waste from the wood industry as the main raw material was developed. SiC/Si composites were fabricated by infiltration of molten silicon into carbon preforms at 1500 °C. The carbon preforms were obtained by pyrolysis (in an inert Ar atmosphere) of four types of resin-carbon mixtures. The carbon used in the mixtures was obtained by pyrolysis of sawdust powder.The mechanical and thermomechanical behavior in uniaxial compression was studied at a constant compression rate of 0.05 mm/min at different temperatures (ambient, 1100 °C and 1400 °C). The maximum resistance values found were in the range of 58 and 384 MPa, while the Young's modulus values were between 40 and 120 GPa. The porosity found in the materials was between 1 and 4%. Finally, the fabricated compounds presented a homogeneous microstructure of interconnected silicon carbide in gray contrast and dispersed and unconnected whitish phases of uniformly distributed silicon.

Joining of SiC Ceramic by Si-C Reaction Bonding Using Organic Resin as Carbon Precursor.

In this study, the joining of silicon carbide (SiC) ceramics was achieved via a Si–C reaction bonding method using the phenolic resin (PF)–MgCl2 system as the carbon precursor. Specifically, by adding MgCl2 to the phenolic resin mixture, the average pore size of the product of carbonization of the PF resin mixture increased from 14 ± 5 nm to 524 ± 21 nm, which was beneficial for the infiltration of molten silicon at high temperature. The microstructure of the joined specimens and the effect of the inert filler on the joint strength were investigated. It was demonstrated that SiC–SiC joints with strong interfacial bonding and high flexural strength could be obtained by the Si–C reaction bonding method using a phenol formaldehyde resin/alcohol sol-gel system as the carbon precursor. The flexural strength of the joined specimens reached the highest value, i.e., 308 ± 27 MPa when the solid loading of the inert filler was 26%. Overall, stable joining of silicon carbide ceramics was achieved by the proposed method, which has significance for realizing the preparation of complex-shaped or large silicon carbide ceramic parts.

A novel MoSi2-rich coating on the SiC-Si coated C/C composites for the preparation and antioxidative properties at 1773 K and 1973 K

To provide a long-term oxidative protection for C/C composites at elevated temperatures, we designed and fabricated a novel MoSi2-rich oxidation protective coating on the surface of SiC-Si coated C/C composites by a combined method of powder reactive sintering and molten silicon infiltration. The phase composition, microstructure and oxidation protective performance of the coating were investigated. The results show that as-prepared MoSi2-rich coating had a dense structure with multiphase mosaic. After oxidation for 581 h at 1773 K in air, the mass loss of the coated specimen was only 1.83%. Particularly, the composite coating can effectively prevent C/C substrate from oxidation for more than 200 h at 1973 K in air, indicating that it possessed outstanding antioxidative capacity due to the rapid formation of the dense SiO2 glass layer in a long-term high-temperature atmosphere.

In situ observation of the capillary infiltration of molten silicon in a SiC/SiC composite by X-ray radiography

Capillary infiltration is an innovative fabrication method for metal and ceramic-matrix composites. SiC/SiC composites can be infiltrated by molten silicon to decrease residual porosity. Physical and chemical mechanisms involved during Liquid Silicon Infiltration (LSI) are complex to analyse. An in situ observation setup for capillary infiltration of molten silicon has been designed for synchrotron observations. The setup reproduces the extreme high temperature and high vacuum conditions used in the LSI process. It is also designed for X-ray observations in synchrotron beamlines and tomography stages. Sets of 2D X-ray absorption radiographs were acquired at high frequency during the LSI process. The study outlines the capillary infiltration mechanisms of molten silicon inside SiC/SiC composites. It proves that full saturation of the composite is not directly achieved after the rise of molten silicon. It is a two step mechanism. First, the infiltration occurs inside the intra granular porosity of the SiC powder matrix. Then, larger scale porosities such as cracks are filled. These phenomena have been discussed previously in the literature but never observed in situ.

TEM characterization of turbostratic and rhombohedral BN interphases synthesized by chemical vapour infiltration in SiC/SiC-Si composites

BN interphases were synthesized by chemical vapour infiltration (CVI) from BCl3 and NH3 at 900 °C and 1200 °C within preforms made of a single 2D woven SiC fibre ply. Then CVI of a SiC matrix followed by molten silicon infiltration resulted in laboratory-made single-ply SiC/SiC-Si composites representative of similar industrial ceramic matrix composites. The interphases and interfaces were characterized by transmission electron microscopy. It has been found that, in the two cases, the interphases are mostly turbostratic BN. Those synthesized at 900 °C are very poorly crystallized and exhibit a quasi-isotropic microstructure. An equiaxed interlayer is obtained prior to the columnar CVI growth of SiC matrix. Interphases synthesized at 1200 °C exhibit high degrees of crystallization and structural anisotropy. The SiC matrix grows directly in columns on BN. For interphase thicknesses of 300 nm and more, triangular-shaped crystallites of rhombohedral BN appear in turbostratic BN and the columnar growth of SiC is disoriented.

Characterization of SiC ceramics with complex porosity by capillary infiltration: Part B – Filling by molten silicon at 1500 °C

In Part A of this study, infiltrations experiments of porous SiC samples by hexadecane with pore-size distributions comprising small and large pores were realized. Two successive stages were identified during the filling of these samples corresponding to the infiltration of the two types of pores. The experimental data were successfully treated with a new analytical function. In Part B, it was found that this function can also be applied to the analysis of the mass gain during molten silicon infiltration at 1500 °C. Prior to silicon infiltration, it was found that the operating temperature induces a shift of the pore size distributions towards larger values. A dissolution-recrystallisation mechanism can also occur during the infiltration of silicon. During the first stage, liquid silicon fills rapidly larger pores than hexadecane. The kinetics are significantly larger with liquid silicon. Consequently, the durations for the complete filling are very short with molten silicon.

Effect of Particle Size Distribution on Microstructure and Mechanical Properties of TiB2-SiC-Si Composites

TiB2-SiC-Si ceramic composites were fabricated based on molten silicon infiltration method. The influence of particle size distribution of TiB2 on the microstructure and mechanical properties of TiB2-SiC-Si ceramic composites was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron universal testing machines, etc. The results showed that the composite consisted of TiB2, SiC and Si phases. The microstructure analysis showed that: the content of residual Si could be controlled less than 16 vol.% in the composite by particle size distribution. When the particle size distribution of TiB2 was 42:32:26, the TiB2-SiC-Si composite showed the optimum properties. The open porosity, volume density, Vickers-hardness, flexural strength and fracture toughness were 0.51%, 3.63 g/cm3, 17 GPa, 275 MPa and 5.7 MPa·m1/2, respectively.

Finite element modeling of reactive liquid silicon infiltration

The LSI process, i.e. the infiltration of molten silicon into porous structures, is one of the most economical techniques for the production of C/C-SiC and C/SiC ceramics. However, despite decades of development, the infiltration behavior affected by phenomena at the infiltration front has not been understood sufficiently. In the present work, a numerical model, based on the finite element method, was developed to simulate the infiltration process. The 3D model includes the penetration of silicon into the porous preform as well as the exothermal reactions at the infiltration front caused by the growth of SiC layers. For model validation, a special measuring furnace was used, enabling in situ optical inspection and weight measurement during liquid silicon infiltration into C/C-preforms in a controlled atmosphere. For the first time, a numerical model could be established which provides a tool to simulate the infiltration kinetics as well as the thermal processes during the LSI process in three dimensions. The model enables the optimization of melt infiltration processes with complex components within reasonable computer times.

Mechanical properties and microstructure of reaction sintering SiC ceramics reinforced with graphene-based fillers

To improve the fracture toughness and bending strength of reaction sintering SiC ceramics, graphene oxide (GO) and reduced graphene oxide (rGO) were selected as fillers to develop reinforced SiC ceramic composites by reactive sintering in the current work. Different amounts (0.5, 1.0, 1.5, or 3.0 wt.%) of graphene-reinforced reaction-bonded silicon carbide (RBSC) composites were fabricated. The mechanical behaviors of the materials were evaluated as a function of the type of graphene source and graphene content. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis showed that graphene was maintained after reactive sintering by liquid infiltration of molten silicon. Both the fracture toughness and bending strength of the RBSC increased with employing graphene-based additives. GO showed the most significant positive effects on improving mechanical performance of RBSC ceramics. The highest fracture toughness of 3.6 MPa m1/2 was obtained at 1.5 wt.% of GO addition. It was 33% higher than that of RBSC without graphene. The highest bending strength corresponded to the composite reinforced with GO content of 1.0 wt.%. It was about 58% higher than that of RBSC ceramic. The relation between phase content and mechanical properties was discussed. The main toughening mechanism was sheet pullout/debonding and the distribution of graphene along grain boundaries.

Preparation and properties of reaction bonded silicon carbide (RB-SiC) ceramics with high SiC percentage by two-step sintering using compound carbon sources

Abstract A novel processing method to prepare silicon carbide ceramics with a high SiC content by two-step reaction bonding was proposed, using a carbon source composed of fine amorphous carbon black and coarse micro-spherical carbon. The inert carbon particles (coarse micro-spherical carbon) remained present after the process of infiltration of molten silicon and were consumed in the reaction with residual silicon at high temperature. With a soaking time of as short as 5 min at 1550 °C, complete infiltration of Si with reaction to the carbon source could be achieved as the first step, and some coarse carbon may remain present due to incomplete reaction. With the increase in the soaking time and temperature in the second step, the contents of residual carbon and silicon decreased simultaneously through their diffusion-controlled reaction, resulting in an improvement of density and hardness. At 1850 °C, the residual carbon decreased to near zero, accompanied by 6% Si resulting from continuous infiltration of Si during the second-step soaking, and a hardness of 25.3 GPa and a Young's modulus of 443 GPa were achieved. It was clearly demonstrated that the sintering process with the compound carbon source was composed of the fast direct reaction in infiltration, and slow reaction through diffusion, which had the potential to obtain the RB-SiC with a very low content of residual Si.

Fundamental Mechanisms With Reactive Infiltration of Silicon Melt Into Carbon Capillaries

The LSI process, that is, the infiltration of molten silicon into porous structures, is one of the most economical techniques for the production of dense C/SiC composites. However, despite decades of development, the phenomena at the infiltration front have not been understood sufficiently. Consequently, a comprehensive model of the reactive infiltration process, which would help to optimize the production process of C/SiC components, has not been established yet. A special thermo‐optical measuring furnace, TOM_ac, enables in situ inspection during liquid silicon infiltration in a controlled atmosphere. Using a novel experimental set‐up, temperatures and atmospheres were varied during silicon melt infiltration into gap capillaries incorporated in glassy carbon. Results clearly demonstrate that the capillary effect is not the rate controlling process as commonly believed but show the importance of gas phase reactions. Besides optical inspection of the infiltration process, quenching experiments combined with SEM analyses of the samples provided a detailed insight into reaction mechanisms and transport processes.

Mechanisms and kinetics during reactive infiltration of molten silicon in porous graphite

Liquid silicon Infiltration (LSI) is a fast and economical process to manufacture SiC-based ceramics. For a better understanding of reactive melt infiltration of liquid silicon, the wetting and infiltration of porous graphite by molten silicon were investigated at 1450, 1500 and 1550 °C for duration comprised between 10 s and 1 h. Infiltrations tests were performed in an argon atmosphere with an inductively heated furnace operating with heating and cooling rates of 300 °C.min−1. The formation and growth of SiC grains were investigated at the outer surface and within graphitic carbon substrates with 11% porosity and a narrow pore size distribution centered at 2 μm. The length of the infiltrated zone and the SiC crystals growth were determined from scanning electron microscopy. Rapid spreading and infiltration of molten silicon are observed from the first 60 s. The growth rate of the interfacial SiC layer obeys a fourth-power law with an activation energy of 260 ± 30 kJ mol−1. Pore filling by SiC is limited by volume diffusion with an activation energy equal to 320 ± 40 kJ mol−1.

Effect of technological parameters on densification of reaction bonded Si/SiC ceramics

Si/SiC composite ceramics was produced by reaction sintering method in process of molten silicon infiltration into porous C/SiC preform fabricated by powder injection molding followed by impregnation with phenolic resin and carbonization. To optimize the ceramics densification process, effect of slurry composition, debinding conditions and the key parameters of all technological stages on the Si/SiC composite characteristics was studied. At the stage of molding the value of solid loading 87.5% was achieved using bimodal SiC powder and paraffin-based binder. It was found that the optimal conditions of fast thermal debinding correspond to the heating rate of 10 °C/min in air. The porous C/SiC ceramic preform carbonized at 1200 °C contained 4% of pyrolytic carbon and ∼25% of open pores. The bulk density of Si/SiC ceramics reached 3.04 g/cm3, silicon carbide content was 83–85 wt.% and residual porosity did not exceed 2%.

Effect of Forming Pressure on Microstructure and Mechanical Properties of B<sub>4</sub>C- SiC-Si Ceramic Composites

B4C-SiC-Si ceramic composites were fabricated based on molten silicon infiltration method. The influence of preforms'forming pressure on the microstructure and mechanical properties of B4C-SiC-Si ceramic composites was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron universal testing machines, etc. The results showed that the ceramic consists of B4C, B12(C,Si,B)3, SiC and Si phases. The microstructure analysis showed that: the volume percent of free silicon decreased with the increase in forming pressures. The Vikers-hardness of B4C-SiC-Si ceramic composites increased, while the bending strength and fracture toughness both increased initially and then decreased with the increase in forming pressures of which the optimal pressure is 200 MPa. The optimum bending strength, fracture toughness and Vikers-hardness of the obtained B4C-SiC-Si ceramic composites are 319±13 MPa, 4.9±0.1 MPa·m1/2and 24±1 GPa, respectively. The volume density and open porosity of the obtained B4C-SiC-Si ceramic composites are 2.58 g/cm3and 0.19 %, respectively.

SiC/Si composites elaboration by capillary infiltration of molten silicon

Infiltration of molten metal into ceramic performs without external pressure is a promising fabrication method for metal-matrix composites. Thus, this research is focused on the kinetic of capillary infiltration in the processing of Si/SiC composites. The calculation of the infiltration kinetics for a liquid in a porous material is possible by applying the modified-Washburn equation which involves the effective diameter of the pores. This equation implies a K value depending on the wetting angle, the density, the viscosity and the surface tension of the liquid at a given temperature. In this study, the main aim was to measure the KSi value of molten silicon infiltration into porous SiC at 1500 °C. On this way, consolidated β-SiC powder compacts were prepared for infiltration experiments. The effects of the SiC particles presintering on the pore size were examined as they can affect molten silicon infiltration. So, the monitoring of the mass increase during hexadecane rise was used to determine the effective diameter of the pores. It was evidenced that an increase of 15.5% of the initial effective diameter occurs. Consequently, a corresponding correction has to be applied to the initial effective diameter of the silicon-infiltrated compacts. Infiltrations of pure molten silicon into SiC compacts were successfully carried out at 1500 °C under high vacuum (5.10−3 mbar) and the average value of KSi was found equal to (5.8 ± 0.4) × 10−6 cm5 s g-2. The effect of molten silicon rise on the porosity was also considered.

Fatigue of three advanced SiC/SiC ceramic matrix composites at 1200°C in air and in steam

AbstractHigh‐temperature mechanical properties and tension‐tension fatigue behavior of three advanced SiC/SiC composites are discussed. The effects of steam on high‐temperature fatigue performance of the ceramic‐matrix composites are evaluated. The three composites consist of a SiC matrix reinforced with laminated, woven SiC (Hi‐Nicalon™) fibers. Composite 1 was processed by chemical vapor infiltration (CVI) of SiC into the Hi‐Nicalon™ fiber preforms coated with boron nitride (BN) fiber coating. Composite 2 had an oxidation inhibited matrix consisting of alternating layers of silicon carbide and boron carbide and was also processed by CVI. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Composite 3 had a melt‐infiltrated (MI) matrix consolidated by combining CVI‐SiC with SiC particulate slurry and molten silicon infiltration. Fiber preforms had a CVI BN fiber coating applied. Tensile stress‐strain behavior of the three composites was investigated and the tensile properties measured at 1200°C. Tension‐tension fatigue behavior was studied for fatigue stresses ranging from 80 to 160 MPa in air and from 60 to 140 MPa in steam. Fatigue run‐out was defined as 2 × 105 cycles. Presence of steam significantly degraded the fatigue performance of the CVI SiC/SiC composite 1 and of the MI SiC/SiC composite 3, but had little influence on the fatigue performance of the SiC/SiC composite 2 with the oxidation inhibited matrix. The retained tensile properties of all specimens that achieved fatigue run‐out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.