Effect of contact time parameter on wettability and interface phenomena of B4C substrate by Ni–Cr–Si–Fe–B–C filler alloy

In the present research, the wettability of boron carbide ceramic by BNi-1 filler alloy at various contact times from 10 to 40 min has been studied. The results of sessile drop wetting tests showed that the BNi-1 filler alloy could spread well on the B₄C surface at 10–40 min. With the increase of the contact time from the lowest time (10 min) to the highest time (40 min), the contact angle stably reduced, showing the enhancement of the spreading. However, by the increase of the contact time from 30 to 40 min, a slight change was observed in the wetting angle (from 21° to 19°). Overall, the appropriate spreading behavior of BNi-1 filler alloy on the B₄C substrate can be attributed to the tendency of nickel for the reaction with B₄C along with the simultaneous availability of silicon and chromium in the composition of this alloy. The maximum wetting angle of 48° was attained for the specimen with 10 min contact time and the minimum angle of 19° was achieved for the specimen with 40 min contact time. Due to the results, different compounds such as Ni₄B₃, CrB₂, CrB, SiC, and Ni₂Si have been observed at the system's interface. Moreover, the higher contact times can lead to the intensification of the system's interactions which can subsequently result in the higher penetration of the elements, the reacted area enlargement, and the formation of diverse microstructures and phases. The wetting experiments' results confirmed the spreading ratio calculations.     

Thermal plasma synthesis of Si/SiC nanoparticles from silicon and activated carbon powders

In this study, Si/SiC nanocomposites were synthesized by non-transferred arc thermal plasma processing of micron-sized SiC powder. First, micron-sized SiC was synthesized by solid-state method where waste silicon (Si) and activated carbon (C) powder were used as precursor materials. The effect of Si/C mole ratio and solid-state synthesis temperature on structural and phase formation of SiC was investigated. Diffraction pattern confirmed the formation of SiC at 1300 °C. High C content was required for the synthesis of pure SiC as Si remained unreacted when Si/C mole ratio was below 1/1.5. Highly agglomerated micron-size (0.6–10 µm) SiC particles were formed after solid-state synthesis. Thermal plasma processing of solid-state synthesized micron-sized SiC resulted into the formation of uniformly dispersed (20–60 nm) Si/SiC nanoparticles. It was proposed that Si/SiC nanocomposites were formed due to partial decomposition of SiC during high temperature plasma processing. The formation of Si/SiC nanoparticles from micron-sized SiC was resulted from dissociation of grains from their grain boundary during plasma processing.     

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.     

Wettability of Silica Substrates by Silver–Copper Based Brazing Alloys inVacuo

The sessile drop method has been used to determine the time dependence of the contact angle at 850°C in vacuo for Ag–28 wt% Cu, Ag–35 wt% Cu–1.5 wt% Ti, and Ag–27 wt% Cu–12 wt% In–2 wt% Ti on vitreous and devitrified fused quartz substrates. Nonwetting behavior (θ > 90°) was observed for Ag–28 wt% Cu on both substrates with no evident effect of time at temperature. The silica substrate structure, whether crystalline or amorphous, as well as its surface condition, whether smooth or rough, made no significant difference. In contrast, with Ag–35 wt% Cu–1.5 wt% Ti and Ag–27 wt% Cu–12 wt% In–2 wt% Ti the contact angle continuously decreased with time for both silica substrates, and the structure and surface condition of the substrates had a negligible effect in the case of Ag–27 wt% Cu–12 wt% In–2 wt% Ti, which produced essentially the same contact angles on both silica substrates at a given time of hold at 850°C. The contact angles produced by Ag–35 wt% Cu–1.5 wt% Ti on devitrified fused quartz were consistently higher than those produced on the vitreous substrates, with increasing holding time at 850°C. This is attributable to the presence of extensive cracks in the α-cristobalite layer at the surface of the devitrified substrates, which obstruct wetting and spreading. These results, when correlated with the wettability of preoxidized silicon carbide by the same alloys reported in previous work, could account for the adverse effect on wetting of the high-temperature silica films formed on the surface of the SiC in that work.     

Nucleation and growth of SiC at the interface between molten Si and graphite

In the present work, the evolution of the SiC layer formed at the interface between liquid silicon and solid carbon is studied using a diffusion couple configuration. Reaction conditions were isothermal, with a temperature of 1450 °C maintained from 2 min to 4 h. The rapid heating and cooling of the Si–C diffusion couple specimens were achieved using a Pulse-Electric Current Sintering system. Crystallographic, compositional, and phase distribution data obtained after different reaction times were used to develop a two-stage model for SiC growth at the interface between molten Si and C. Initially, the formation of SiC at the interface is governed by diffusion of C into the molten Si and dissolution/reprecipitation of formed SiC nuclei. These nuclei further grow into larger SiC grains at the Si–C interface and this initial stage is successfully modeled using the Johnson-Mehl-Avrami-Kolmogorov model. Once a continuous SiC layer forms at the Si–C interface, the growth of SiC is controlled by the diffusion of C through the SiC layer, which can be modeled using a power rate law. However, the nature of this diffusion is difficult to determine with certainty since the rate laws for both grain boundary and bulk diffusion fit experimental data equally well.     

Liquid metal infiltration of silicon based alloys into porous carbonaceous materials. Part II: Experimental verification of modelling approaches by infiltration of Si-Zr alloy into idealized microchannels

In this work, the mechanisms leading to the pore closure in reactive melt infiltration (RMI) of carbon by pure silicon and a near eutectic Si-8 at-pct Zr alloy at 1500 and 1700 °C under vacuum were studied. Various geometrical configurations of microchannels were fabricated via laser ablation of glassy carbon plates. The micron size capillary channels allowed simplifying the complicated porosity distribution in the infiltration of powder or fibres based porous preform while keeping the physical dimensions in the range of where the physical phenomenon of pore closure takes place. The extent of infiltration was analysed by means of X-ray radiography. For RMI of pure Si, the widely accepted decrease in capillary radius by the formation of a solid state SiC layer by the reaction of liquid Si and C was observed, but did not lead to closure and it is hence not the infiltration limiting step in channels as small as 10 μm. However, in the case of the Si-Zr alloy infiltration, another mechanism of pore closure was observed, namely the precipitation of zirconium silicides at the infiltration front, due to Zr enrichment in the alloy by the continuous consumption of Si for the formation of SiC.     

An Original Way to Investigate Silver Migration Through Silicon Carbide Coating in TRISO Particles

Extensive release of the metastable silver nuclide Ag^110m from fully intact tristructural‐isotropic (TRISO) particles raises concerns over safety of advanced nuclear reactors. In this study, we propose a new model to interpret the silver migration mechanism in SiC based on experimental observations from both Ag/SiC composite pellets and TRISO particles with an entrapped silver layer. For the Ag/SiC composite pellets heat treated at 1450°C, silicon was detected in the silver phase, amorphous carbon was found, and new β‐SiC had formed at the Ag/SiC interface. The results indicate that Ag in its liquid state reacts with SiC by forming a Ag–Si alloy. Carbon precipitates as a second phase or reacts with the Ag–Si alloy to form new SiC. Results from the heat‐treated TRISO particles trapping Ag show that Ag penetrates through the SiC layer and is present in either “finger‐shape” or “wedge shape” at the SiC grain boundaries. Ag was also found inside abnormally large SiC grains at the trailing edge between Ag and SiC, indicating the recrystallization of SiC. A dissolution‐reaction model was proposed to explain Ag migration through SiC, and this model is supported by thermodynamic calculations.     

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).     

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%.     

Ti_3SiC_2耐氢氟酸腐蚀性研究

在Ar气氛中,以TiH_2/Si/2Ti C复合粉体为原料,利用无压烧结技术在1500°C下保温3 h成功制备出高纯Ti_3SiC_2,并利用氢氟酸对Ti_3SiC_2粉体进行刻蚀,研究其耐腐蚀性。XRD检测结果表明,在常压下Ti_3SiC_2样品经氢氟酸腐蚀前后的物相没有发生变化,且不随反应时间和温度的变化而发生变化。但是,扫描电镜图片显示样品中存在一些二维片层和腐蚀孔洞,这表明HF与Ti_3SiC_2发生了部分刻蚀反应。由于Ti_3SiC_2与酸的反应活性依赖于Si与酸的反应活性,而Si与HF在常压下反应较慢,因此Ti_3SiC_2与HF反应较困难。然而,Ti_3SiC_2与HF在180°C水热条件下则能完全反应,晶体结构遭到破坏,这表明Ti_3SiC_2在常温常压下对HF具有良好的耐腐蚀性,而在水热条件下Ti_3SiC_2易受HF的腐蚀。     

Hardness of nanocomposite a-C:Si films deposited by magnetron sputtering

Hydrogen-free a-C:Si films with Si concentration from 3 to 70 at.% were prepared by magnetron co-sputtering of pure graphite and silicon at room temperature. Mechanical properties (hardness, intrinsic stress), film composition (EPMA and XPS) and film structure (electron diffraction, Raman spectra) were investigated in dependence on Si concentration, substrate bias and deposition temperature. The film hardness was maximal for ∼ 45 at.% of Si and deposition temperatures 600 and 800 °C. Reflection electron diffraction indicated an amorphous structure of all the films. Raman spectra showed that the films in the range of 35–70 at.% of Si always contain three bands corresponding to the Si, SiC and C clusters. Photoelectron spectra showed dependency of Si–C bond formation on preparation conditions. In the films close to the stoichiometric SiC composition, the surface and sub-surface carbon atoms exhibited dominantly sp³ bonds. Thus, the maximal hardness was observed in nanocomposite a-C:Si films with a small excess of carbon atoms.     

Polymer derived SiC environmental barrier coatings with superwetting properties

Surfaces with superwetting capabilities can be used for corrosion protection, self-cleaning and bio-fouling protection amongst other applications. In this work, we present a method to produce a SiC coating with an almost superhydrophobic behavior exhibiting water contact angles of 145±3°. Ceramic coatings were produced by the pyrolysis of polycarbosilane as a preceramic precursor of SiC. Aluminum and carbon powders were used as active and passive fillers to compensate for the volume shrinkage of polycarbosilane during pyrolysis. The effects of particle size (Al particles ranging from 0.8 to 10 µm) and concentration 10–30% wt.) C and Al of both fillers were studied to produce defect-free ceramic coatings. We have observed that the fillers used not only affected the microstructure but also the surface roughness. We show that the addition of carbon fillers can increase the water contact angle of the ceramic from 42° up to 141°. The combination of carbon and aluminum fillers resulted in water contact angles up to 145°.     

Template‐assisted hydrophobic porous silica membrane: A purifier sieve for organic solvents

Self‐supporting, interconnected, porous inorganic (silicon/carbon) membranes were obtained by the calcination of composite membranes at 900 °C with a porous polymer template obtained by binary‐phase solid‐state photopolymerization at ?60 °C with acrylate monomers followed by chemical vapor deposition. The physicochemical properties of all of the membranes were characterized by scanning electron microscopy, energy‐dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, X‐ray powder diffraction (XRD), thermogravimetric analysis (TGA), and the measurement of water contact angles, water uptake, and percentage of porosity. The membranes showed contact angles ranging from 127 to 130 ± 0.5°, which were close to those of superhydrophobicity. The EDS, FTIR spectroscopy, XRD, and TGA results confirmed the formation of the Si–C composite structure after calcination and an increase in the thermal stability. The polymer and composite membranes were found to be hydrophilic, whereas Si–C was hydrophobic. The poly(ethylene glycol) diacrylate derived Si–C membrane showed a good absorption efficiency for tinted toluene from water compared to the others. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45822.     

The role of an SiC interlayer at a graphite–silicon liquid interface in the solution growth of SiC crystals

SiC crystal growth using the top seeded solution growth (TSSG) method involves the precipitation of solid SiC from carbon that is dissolved in a silicon melt. The growth rate of SiC is strongly influenced by the solubility of C in liquid Si, which is quite low. In this study, the dissolution of C from graphite to the Si melt was explored by observing the formation of an SiC interlayer at a graphite – Si liquid interface. The SiC interlayer was observed to become thickened during the several hours needed to reach a certain thickness at 1500 °C. Assuming that the SiC interlayer is a direct C source, a pre-formed SiC layer was coated on the graphite crucible to evaluate its effect on the concentration of C in the Si melt. As a result, the concentration of C in the Si melt increased within a short time, especially at low temperatures. By applying the SiC coated crucible to the TSSG process for SiC crystal growth, we confirmed that the development of a pre-formed SiC layer enhanced the growth rate of SiC crystals, especially at the initial stage of crystal growth at low temperatures.     

Deposition Rate,Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

Silicon carbide (SiC) coatings were produced on carbon/carbon (C/C) composites substrates using chemical vapor deposition (CVD) at different temperatures (1100°C, 1200°C, and 1300°C). The deposition rate was found to increase with deposition temperature from 1100°C to 1200°C. From 1200°C to 1300°C, the deposition rate decreased. SiC coating produced at 1200°C exhibited a strong (111) texture compared with the coatings produced at other temperatures. Both hardness and Young's modulus were also found to be higher in the coating produced at 1200°C. The variation in mechanical properties with the increase in temperature from 1100°C to 1300°C showed a direct correlation with the change in deposition rate and (111) texture. Microstructure analysis shows that the change in CVD temperature leads to the change in grain size, crystallinity, and density of stacking faults of SiC coatings, which appears to have no significant effect on mechanical properties of SiC compared with the texture observed in SiC coating. For the coating deposited at 1200°C, both the hardness and Young's modulus increased gradually from the substrate/coating interface to the top surface. The nonuniformity of mechanical properties along the cross‐section of the coating is attributed to the nonuniform microstructure.     

Oxidation protection of C/C composites with a multilayer coating of SiC and Si + SiC + SiC nanowires

An oxidation protective Si–SiC coating with randomly oriented SiC nanowires was prepared on the SiC-coated carbon/carbon (C/C) composites by a two-step technique. First, a porous network of SiC nanowires was produced using chemical vapor deposition. This material was subjected to pack cementation to infiltrate the porous layer with a mixture of Si and SiC. The nanowires in the coating could efficiently suppress the cracking of the coating by various toughening mechanisms including nanowire pullout, nanowire bridging, microcrack deflection and good interaction between nanowire/matrix interface. The results of thermogravimetric analysis and thermal shock showed that the coating had excellent oxidation protection for C/C composites between room temperature and 1500 °C. These results were confirmed by two additional oxidation experiments conducted at temperature of 900 and 1400 °C, which demonstrated that the coating could efficiently protect C/C composites from oxidation at 900 °C for more than 313 h or at 1400 °C for more than 112 h.     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

Performance of the silicon carbide coating under erosion wear by erosion by solid particles

In this article, the phenomenon of erosion by solid particles on the silicon carbide coating (SiC) deposited on AISI 304 stainless steel substrates was analyzed. The specimens used were 25 mm square and 3 mm thick, using 300–450 μm silicon carbide as abrasive particles. Experimental tests were performed on an apparatus developed in accordance with some parameters of the ASTM G76-95 standard. Four angles of impact at 30°, 45°, 60°, and 90° are contemplated with an approximate particle velocity of 25 ± 2 m/s with a maximum exposure time of 10 min per specimen, taking measurements of weight intervals every 2 min to determine the mass loss. The wear mechanisms that were identified to small angles were: plastic deformation, displacement of material, and plow mechanisms. While at higher impact angles, the mechanisms were mainly: cutting, pitting, fractures, and cracks. It was observed that the rate of erosion depends on the angle of incidence of the abrasive particles. The results indicated that a higher damage zone was obtained at 30° of impact angle; on the other hand, at an angle of 90° there was less damage.     

Fabrication of cylindrical SiCf/Si/SiC-based composite by electrophoretic deposition and liquid silicon infiltration

Cylindrical SiC-based composites composed of inner Si/SiC reticulated foam and outer Si-infiltrated SiC fiber-reinforced SiC (SiC_f/Si/SiC) skin were fabricated by the electrophoretic deposition of matrix particles into SiC fabrics followed by Si-infiltration for high temperature heat exchanger applications. An electrophoretic deposition combined with ultrasonication was used to fabricate a tubular SiC_f/SiC skin layer, which infiltrated SiC and carbon particles effectively into the voids of SiC fabrics by minimizing the surface sealing effect. After liquid silicon infiltration at 1550 °C, the composite revealed a density of 2.75 g/cm³ along with a well-joined interface between the inside Si/SiC foam and outer SiC_f/Si/SiC skin layer. The results also showed that the skin layer, which was composed of 81.4 wt% β-SiC, 17.2 wt% Si and 1.4 wt% SiO₂, exhibited a gastight dense microstructure and the flexural strength of 192.3 MPa.     

Interface design in 3D-SiC/Al-Si-Mg interpenetrating composite fabricated by pressureless infiltration

3D-SiC/Al-Si-Mg interpenetrating composites (IPCs) were fabricated by pressureless infiltration method. Interfaces in the 3D-SiC/Al-Si-Mg IPCs were modificated by using two different kinds of aluminum alloy Al-15Si-10Mg and Al-9Si-6Mg to infiltrate into 3D-SiC performs and different treated 3D-SiC preforms unoxidized or preoxidized in air at 1000?°C, 1100?°C and 1200?°C for 2?h respectively. Results showed that desired interfaces can be achieved in both IPCs made with those two aluminum alloys, as demonstrated by their excellent comprehensive properties. When the Al-15Si-10Mg alloy with excessive Si content is used for infiltration, interfaces in 3D-SiC/Al-Si-Mg IPC fabricated with the unoxidized 3D-SiC preform are directly bonded through atomic matching without any interfacial reaction and the composite has the properties of a thermal conductivity (TC) of 224.5?W/(m?°C), a thermal expansion coefficient (CTE) (RT ~ 300?°C) of 7.04?×?10^?6/°C and a bending strength (BS) of 277?MPa. When the Al-9Si-6Mg alloy with a lower Si content is used for infiltration, interface zone with a thickness around 200?nm forms in the 3D-SiC/Al-Si-Mg IPC fabricated with the 3D-SiC preform preoxidized at 1000?°C. The reaction-bonded interface is composed of AlN and MgAl₂O₄ which have better interface affinity with SiC and can isolate SiC effectively from liquid Al against the formation of detrimental Al₄C₃ phase. The composite has the properties of a TC of 219.5?W/(m °C), a CTE (RT ~ 300?°C) of 7.66?×?10^?6/°C and a BS of 318?MPa.