Microstructure and ablation behaviors of a novel gradient C/C-ZrC-SiC composite fabricated by an improved reactive melt infiltration

An improved reactive melt infiltration (RMI) route using Zr, Si tablet as infiltrant was developed in order to obtain high-performance and low-cost C/C-ZrC-SiC composite with well defined structure. Two other RMI routes using Zr, Si mixed powders and alloy were also performed for comparison. Effects of different infiltration routes on the microstructure and ablation behavior were investigated. Results showed that C/C-ZrC-SiC composite prepared by Zr, Si tablets developed a dense gradient microstructure that content of ZrC ceramic increased gradually along the infiltration direction, while that of SiC ceramic decreased. Composites prepared by Zr, Si mixed powders and alloy showed a homogeneous microstructure containing more SiC ceramic. In addition, two interface patterns were observed at the carbon/ceramic interfaces: continuous SiC layer and ZrC, SiC mixed layers. It should be due to the arising of stable Si molten pool in the tablet. Among all as-prepared samples, after exposing to the oxyacetylene flame for 60 s at 2500 °C, C/C-ZrC-SiC composite infiltrated by Zr, Si tablet exhibited the best ablation property owing to its unique gradient structure.     

Effects of zirconium addition on microstructure and ablation resistance of carbon fibre reinforced carbon and SiC ceramic matrix composite prepared by reactive melt infiltration

Abstract

Carbon fibre reinforced C and SiC binary ceramic matrix composites (C/C–SiC) were fabricated by a quick and low cost reactive melt infiltration (RMI) method with Si–Zr25 and Si melts. Effects of zirconium addition in infiltrated Si melt on microstructure and ablation resistance of the composite were investigated. The composite by Si–Zr25 melt infiltration was composed of SiC, ZrC, C and a little amount of ZrSi₂ without residual silicon, overcoming the problem of residual silicon in C/C–SiC composite by Si RMI. Compared with the composite by Si melt infiltration, the ablation resistance of the composite by Si–Zr25 was greatly improved by zirconium addition due to ZrO₂ and SiO₂ protecting layer formed during ablation.     

Studying the wettability of Si and eutectic Si-Zr alloy on carbon and silicon carbide by sessile drop experiments

The contact angles of two different systems, molten silicon and a eutectic Si-8 at. pct Zr alloy and their evolution over timeon vitreous carbon and polycrystalline silicon carbide (SiC) substrates were investigated at 1500°C under vacuum, as well as in argon using the sessile drop technique. The contact angle and microstructure of the liquid droplet/solid substrate interface were studied to understand fundamental features of reactive wetting as it pertains to the infiltration process of silicon and silicon alloys into carbon or C/SiC preforms. Both pure Si and theeutectic alloy showed good wettability onvitreous carbon and SiC characterized by equilibrium contact angles between 29° and 39°. Theeutectic alloy showed a higher initial contact angle and slower spreading as compared to that of pure Si. On vitreous carbon bothsilicon and the eutecticalloy formed SiC at the interface, while no reaction was observed on the SiC substrates.     

Fabrication of Cf/ZrC–SiC composites using Zr–8.8Si alloy by melt infiltration

Cf/ZrC–SiC composites were fabricated by melt infiltration at 1800 °C using Zr–8.8Si alloy and carbon felt preforms. Microstructural analysis showed the formation of both ZrC and SiC phases in the matrix, in which ZrC acted as a main composition of the resulting composites. The results showed that carbon matrix reacted preferentially with Si of Zr–8.8Si alloy, which caused the formation of SiC first and then ZrC. The designed carbon coating by pyrolysis prevented the severe reaction between fibers and the melt. The composites could be more dense and uniform with the bending strength of 53.3 MPa, when preforms had a high open porosity (47.2%) with small size pores (10–40 μm).     

Inhibition of liquid Si infiltration into carbon-carbon composites by the addition of Al to the Si slurry pre-coating: mechanism analysis

The mechanism of the inhibition of liquid Si infiltration (LSI) into a two-dimensional carbon-carbon composite (2D-C/C) by the addition of Al to the Si slurry pre-coating was investigated. It was shown by means of a vapor treatment experiment designed intentionally that the surface composition of the inner pores beneath the Si slurry pre-coating before the occurrence of LSI was pure carbon and SiC, while before the occurrence of the LSI with the Si-6 wt.%Al slurry pre-coating, the surface composition of the inner pores was Al₄C₃, SiC and a small amount of pure carbon. The formation of the SiC and the Al₄C₃ was the result of the evaporation of almost all the Al additive and a little Si during the heating. For reactive infiltrations, reactions at the vapor-liquid-solid triple line are believed to affect the final infiltration depth. Faster reactions at the triple line lead to faster infiltration velocity and hence deeper reactive infiltration. The reaction at the triple line for the LSI with the Si-6 wt.%Al slurry pre-coating was mainly between liquid Si and the surface Al₄C₃, which was probably slower than the reaction of liquid Si with the pure carbon at the triple line corresponding to the LSI with the Si slurry pre-coating. Therefore, the extent of the penetration of liquid Si during the LSI with Si-6 wt.%Al slurry pre-coating was lower than that with the Si slurry pre-coating.     

Liquid metal infiltration of silicon based alloys into porous carbonaceous materials. Part I: Modelling of channel filling and reaction phase formation

A relation for an adequate pore fraction needed to obtain residual Si and C free composites via reactive Si-X alloy infiltration is presented. The volume ratio of SiC and carbonaceous phase, the composition of the infiltrating liquid and the apparent density of the preform are used as design entities. The approach allows identifying combinations of these design entities leading to desirable microstructures, e.g. those free of residual silicon or free of excess graphite. The approach gives further access to important post infiltration characteristics like propensity of the various phases.An idealising mathematical model describing the reactive flow of Si-X alloy in a single micron sized capillary channel of carbon as well as in carbonaceous preforms is presented. The model is further expanded to evaluate the infiltration depth in porous carbonaceous preform for a given composition of Si-X alloy and infiltration temperature. The model is presented for both isothermal and non-isothermal cases.The analysis is formulated in general terms and is hence applicable to a large variety of Si-C-refractory metal systems of potential interest.     

Formation mechanism of Si-Y-C ceramic matrix by reactive melt infiltration using Si-Y alloy and properties of C/Si-Y-C composites

Herein, Si-Y eutectic alloy were introduced into porous C/C preform by reactive melt infiltration (RMI) to prepare C/Si-Y-C composite. Phase compositions and their distributions in the as-prepared composites were investigated. Result indicated that four main regions were found in the typical zone in Si-Y-C matrix, i.e., amorphous carbon, polycrystalline SiC doped with YSi2, amorphous SiC and single crystal YSi2. Based on the reaction between Si-Y alloy and C/C preform and microstructural observations, a model regarding to microstructure formation mechanism was proposed to reveal reaction process. Moreover, improved flexural strength, fracture toughness, thermal diffusivity and thermal conductivity of C/Si-Y-C composite were achieved compared to C/C-SiC.     

Capillary infiltration studies of liquids into 3D-stitched C–C preforms: Part A: Internal pore characterization by solvent infiltration,mercury porosimetry,and permeability studies

Mathematical modeling of silicon infiltration in porous carbon–carbon (C–C) preforms is the key to fabricate liquid silicon infiltration based carbon–silicon carbide (C–SiC) composite components. Existing models for silicon infiltration are based on straight capillaries. For interconnected capillary systems, e.g. as in 3D-stitched C–C preforms these show large deviations when compared with experimental observations. The aim of the present study is to develop a mathematical model suitable for silicon infiltration in 3D-stitched C–C preforms. The work is being presented in two parts: A and B. This part (Part A) describes the experimental details pertaining to the fabrication of the C–C preforms and their pore structure characterization by mercury porosimetry, infiltration of solvents by capillary rise, and by permeability studies. A two-pore capillary infiltration model termed as modified Washburn equation has been proposed. It has been validated by experimental data of solvent infiltration. The same model correlates silicon infiltration observations as well (Part B).     

Preparation and formation mechanism of C/C–SiC composites using polymer-Si slurry reactive melt infiltration

This study proposes a polymer-metal slurry reactive melt infiltration (RMI) method to overcome the limitations of conventional RMI in modifying irregular geometric carbon–carbon (C/C) preforms. Herein, polycarbosilane (PCS), polysiloxane, phenol-formaldehyde, and epoxy resin, which were introduced to prepare slurries with Si powder, and subsequently used to modify cylindrical C/C preforms into C/C–SiC composites. Results show that the PCS–Si slurry has the best RMI capability, by which, a cylindrical C/C preform (1.35 g·cm⁻³) was modified successfully to into a dense C/C–SiC composite (1.92 g·cm⁻³). PCS plays a vital role in fixing the coating to prevent it from falling off the surface of the C/C preform in PCS–Si slurry RMI. Both of the degree of densification and flexural strength of the C/C–SiC composites increase with an increase in the thickness of the PCS–Si slurry coating. The overreaction of the PCS–Si slurry RMI was effectively suppressed because the content of Si powder is reasonably controlled in the PCS–Si slurry coating. Moreover, nozzle-shaped C/C composites were successfully modified into a C/C–SiC composite for the first time using PCS–Si slurry RMI.     

Interphase degradation of three‐dimensional Cf/SiC–ZrC–ZrB2 composites fabricated via reactive melt infiltration

Layer‐structured interphase, existing between carbon fiber and ultrahigh‐temperature ceramics (UHTCs) matrix, is an indispensable component for carbon fiber reinforced UHTCs matrix composites (C_f/UHTCs). For C_f/UHTCs fabricated by reactive melt infiltration (RMI), the interphase inevitably suffers degradation due to the interaction with the reactive melt. Here, C_f/SiC–ZrC–ZrB₂ composite was fabricated by reactive infiltration of ZrSi₂ melt into sol‐gel prepared C_f/B₄C–C preform. (PyC–SiC)₂ interphase was deposited on the fiber to investigate the degradation mechanism under ZrSi₂ melt. It was revealed that the degraded interphase exhibited typical features of Zr aggregation and SiC residuals. Moreover, the Zr species diffused across the interphase and formed nanosized ZrC phase inside the fiber. A hybrid mechanisms of chemical reaction and physical melting were proposed to reveal the degradation mechanism according to characterization results and heat conduction calculations. Based on the degradation mechanism, a potential solution to mitigate interphase degradation is also put forward.     

C/C–SiC composite prepared by Si–10Zr alloyed melt infiltration

A high performance and low cost C/C–SiC composite was prepared by Si–10Zr alloyed melt infiltration. Carbon fiber felt was firstly densified by pyrolytic carbon using chemical vapor infiltration to obtain a porous C/C preform. The eutectic Si–Zr alloyed melt (Zr: 10 at.%, Si: 90 at.%) was then infiltrated into the porous preform at 1450 °C to prepare the C/C–SiC composite. Due to the in situ reaction between the pyrolytic carbon and the Si–Zr alloy, SiC, ZrSi₂ and ZrC phases were formed, the formation and distribution of which were investigated by thermodynamics. The as-received C/C–SiC composite, with the flexural strength of 353.6 MPa, displayed a pseudo-ductile fracture behavior. Compared with the C/C preform and C/C composite of high density, the C/C–SiC composite presented improved oxidation resistance, which lost 36.5% of its weight whereas the C/C preform lost all its weight and the high density C/C composite lost 84% of its weight after 20 min oxidation in air at 1400 °C. ZrO₂, ZrSiO₄ and SiO₂ were formed on the surface of the C/C–SiC composite, which effectively protected the composite from oxidation.     

Capillary Infiltration Rates into Porous Media with Applications to Silcomp Processing

The rate of capillary rise of a liquid into a porous medium made up of consolidated particulates is analyzed. The infiltration distance is parabolic in time and can be modeled using the Washburn analysis. The effective pore radius is measured to be one to two orders of magnitude smaller than the particle size, particle spacing, and the median pore size as measured by mercury porosimetry. This result is interpreted using a modification of the Washburn model which models the porous medium as a single pore with varying diameter. Using a two-sized single pore model, the predicted infiltration rate is consistent with the measured values. In applying the two-sized single pore model to the reactive capillary infiltration of silicon into a carbonaceous preform in the Silcomp process, the effect of pore closure by the conversion of carbon to SiC is predicted. Using pore closure dimensions measured in a partial infiltration experiment, a decrease in the infiltration rate constant is predicted and is consistent with the measured infiltration rate.     

Vapor-mediated melt infiltration for synthesizing SiC composite matrices

A two-step synthesis of SiC involving initial exposure of carbon surfaces to Si vapor, followed by Si melt infiltration, is investigated in this article with emphases on the mechanisms, kinetics, and microstructure evolution. Interrupted differential thermal analysis of amorphous C and Si powder mixtures and microstructure characterization are used to generate insight into the stages of the reaction. Exposure to Si vapor yields a SiC layer with nano-scale porosity driven by the volume change associated with the reaction. This forms a continuous pore network that promotes subsequent melt access to the reaction front with the C. While the pores remain open, the vapor phase reaction proceeds at a nearly constant rate and exhibits a strong temperature sensitivity, the latter due in part to the temperature sensitivity of the Si vapor pressure. The implications for enhancing the reactive melt infiltration process and fabrication of SiC matrices for ceramic composites are discussed.     

Properties of Si/SiC ceramic composite subjected to chemical vapour infiltration

Si/SiC ceramic composite was prepared by infiltration of liquid silicon into carbon preforms that was made from cotton fabric and phenolic resin. This composite was subjected to the chemical vapour infiltration (CVI), using methyltrichlorosilane as a precursor gas. The effect of infiltration time on densification and mechanical properties was studied. Results show a significant improvement in density by pore closure. Flexural strength increases with increasing infiltration time. However, beyond 60 h of infiltration, the strength improvement was insignificant. The high temperature oxidation resistance of the above ceramics was also studied. The CVI treated samples show considerable resistance to oxidation compared to untreated samples. Thermogravimetric analysis also confirmed the better oxidation resistance of the CVI treated samples.     

Gel reactive melt infiltration: A new method for large-sized complex-shaped C/C components ceramic modification

To widen the applications of conventional reactive melt infiltration (RMI) in large-sized complex-shaped C/C components, an ingenious process of gel-RMI (GRMI) was proposed in this study. The arching C/CSiC composite was prepared successfully using GRMI method with polycarbosilane (PCS)Si90Zr10 (Si: 90 at.%; Zr:10 at.%) sol. The porosity rate of the C/C preform decreased from 18.5% to 2.9%, while the density was raised from 1.40 g·cm⁻³ to 2.05 g·cm⁻³ after GRMI. The reason why C/C preform has been significantly densified is as follow: the PCS in PCS-Si90Zr10 sol formed SiC aerogel skeleton after pyrolysis, and then the Si90Zr10 powders were melted and released from the SiC aerogel into the C/C preform body when the temperature reached the melting point of Si90Zr10 alloy. The obtained C/CSiC composite showed a pseudo-ductile rupture characteristic distinguished from that of the C/C preform, and its bending strength was significantly improved from 104.2 MPa of the C/C preform to 258.8 MPa. The C/CSiC composite had a far lower mass ablation rate of 0.75 mg·s⁻¹ than that of C/C preform, 23.30 mg·s⁻¹. Moreover, the GRMI was preliminarily applied in ceramic modifying nozzle-like C/C preform, and the result showed that the nozzle-like C/C preform was successfully densified from 1.3 g cm⁻³ to 1.96 g cm⁻³. The GRMI process has great potential in ceramic modifying large-sized complex-shaped C/C components.     

Reaction Mechanism and Microstructure Development of Zr–Si Alloyed Melt‐Infiltrated ZrC‐Modified C/C Composite

ZrC ceramic modified‐C/C composite is prepared by a quick and low‐cost reactive melt infiltration process with a Zr‐Si8.8 alloy. Reaction kinetics and mechanism of pyrolytic carbon with the infiltrated Zr‐Si8.8 alloy are investigated. A continuous ZrC layer is found to be formed around pyrolytic carbon due to the in situ reaction and its thickness parabolically increases with an increase in reaction time period. Zr concentration in the alloyed melts decreases due to the reaction between Zr and pyrolytic carbon and Zr₂Si phase precipitates from the residual alloyed melts. A model for the growth of ZrC layer is established to describe the reaction kinetics of pyrolytic carbon with Zr‐Si8.8 alloy. The calculated thickness of the reaction‐formed ZrC layer is in good agreement with the experimental data. Based on the Arrhenius equation, the activation energy of the reaction between carbon and Zr‐Si8.8 alloy is 313.2 KJ/mol, smaller than that of the reaction between carbon and pure zirconium. The microstructure of the reactive melt‐infiltrated ZrC‐modified C/C composite is characterized by optical microscope, SEM, EDS, XRD, and TEM. The mechanism of the reaction between pyrolytic carbon and Zr‐Si8.8 alloy is discussed on the basis of the characterization results and phase diagram. A schematic is proposed to understand the reaction mechanism between pyrolytic carbon and Zr‐Si8.8 alloy and microstructure development of the ZrC‐modified C/C composite.     

Liquid metal infiltration of silicon based alloys into porous carbonaceous materials Part-III: Experimental verification of conversion products and infiltration depth by infiltration of Si-Zr alloy into mixed SiC/graphite preforms

Porous carbonaceous preforms, made from graphite and silicon carbide (SiC) powders, with varying graphite powder mass fractions and particle sizes, were infiltrated at 1500 °C and 1700 °C by Si-8 at. pct Zr alloy to produce dense Si-Zr-SiC composites. The experiments were performed in a graphite chamber vacuum furnace at 10^?2 mbar. The most desirable results were obtained for preforms composed of a mixture of graphite and SiC powders, with preforms containing 15–20 % mass fraction of graphite and infiltrated at 1500 °C. The banding of the Zr-rich phase observed in the cross-sections of SiC-C pre- forms infiltrated by the Si-Zr alloy may help in decoding the reactive infiltration process with binary alloys.     

Microstructure and flexural strength of C/HfC-ZrC-SiC composites prepared by reactive melt infiltration method

C/HfC-ZrC-SiC composites were fabricated via reactive melt infiltration (RMI) of the mixed HfSi₂ and ZrSi₂ alloys. The microstructure, infiltration behavior of the hybrid silicide alloys infiltrating C/C composites, and flexural strength of C/HfC-ZrC-SiC composites was studied. Inside composites, there were more Hf-rich (Hf, Zr)C phases distributed in the exterior region, while more SiC and Zr-rich (Zr, Hf)Si₂ in the interior region. There was compositional segregation in (Hf, Zr)C, with the HfC content decreasing from the exterior region to interior region. The RMI process was performed at different temperatures to investigate the structural evolution, and a model for the reactive melt infiltration of the mixed HfSi₂ and ZrSi₂ alloys into C/C composites was established. Compared with C/HfC-SiC and C/ZrC-SiC prepared by same process, C/HfC-ZrC-SiC had the highest flexural strength of 247Mpa and 213Mpa after oxidation at 1200 ℃ for 15 min. Both the unoxidized and oxidized samples presented a pseudo-plastic fracture behavior.     

C/C–SiC composite fabricated via PCS–Si slurry reactive melt infiltration

The slurry reactive melt infiltration (RMI) method was used to overcome the shortcomings of the traditional RMI method utilized in preparing the large-irregular shaped C/C preform. C/C–SiC composite was successfully fabricated via PCS–Si slurry RMI. Results show that it has a favorable physical bonding between the PCS–Si slurry and the C/C preform. After RMI, most of the Si in the PCS–Si slurry coating infiltrated into the C/C preform, the density of the C/C preform increased from 1.24 to 1.52g cm⁻³, and the open porosity decreased from 27.22% to 18.04%. SiC was formed on the surface as well as in the pores of the C/C preform. The as-received C/C–SiC composite showed a pseudo-ductile fracture behavior with a flexural strength of 76.4MPa. The mass ablation rate of the C/C–SiC composite is 0.34 mg s⁻¹, exhibiting much better ablation resistance than the C/C preform with a mass ablation rate of 1.80 mg s⁻¹.     

Reactive melt infiltrated Cf/SiC composites with robust matrix derived from novel engineered pyrolytic carbon structure

Needle-punched C_f/SiC composites were fabricated by a novel pore tuned reactive melt infiltration (RMI) process. The novel hierarchically porous carbon structure in the fiber preform with the porosity well open to liquid silicon was engineered by impregnation of phenolic resin with addition of a pore former. Neither residual bulk carbon nor residual bulk silicon is detected in the matrix of the C_f/SiC composites prepared by the pore tuned RMI, indicating that a robust matrix with homogenous SiC can be formed. The composite prepared by the pore tuned RMI exhibits a tensile strength of 159±5 MPa, which is 46% higher than that without addition of pore former.