Microstructure characteristics of silicon carbide coatings fabricated on C/C composites by plasma spraying technology

A functional gradient SiC coating on C/C composites has been developed using a novel process which is the combination of plasma spraying technology with reaction-formed heat-treatment. Microstructure observation and phase identification of the SiC coatings were analyzed by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Experimental results showed that a uniform silicon coating was deposited on C/C composite by plasma spraying technology. The reaction between the silicon coating and C/C substrate occurred during the heat-treatment at temperature of 1450 °C and 1600 °C in argon environment, respectively. A continuous SiC coating was formed on the surface of the C/C substrate. And a layer of SiC/C convention layer was formed on the near-surface area of the substrate, which was resulted from the molten silicon penetrating into the open pores and consequently reacting with the C/C composites. The thickness of the formed SiC coatings was closely related to the original silicon coatings.     

Fabrication and performance of mini SiC/SiC composites with an electrophoresis-deposited BN fiber/matrix interphase

The feasibility of fabricating a BN matrix/fiber interphase of SiC/SiC composites via electrophoresis deposition (EPD) was investigated based on the simplicity and non-destructiveness of the process and the excellent interfacial modification effects of BN. The BN suspension and SiC fiber surface properties were both adjusted to generate suitable conditions for the EPD process of the BN interphase. Next, the deposition dynamics and mechanism were studied under different deposition voltages and time, and the relationship between the deposition morphology of the BN interphase and mechanical properties of the fabricated mini SiC/SiC composites were also discussed. After oxidation at high temperature (600–1000 ℃), the mechanical properties of the mini SiC/SiC composites were studied to verify the oxidation resistance effect of the EPD-deposited BN interphase, whose oxidation resistance mechanism was briefly analyzed as well.     

Wet-chemical coating of silicon carbide fibers with hexagonal boron nitride layers

Hexagonal BN fiber coatings and BN powders were prepared by pyrolysis of the raw materials boric acid and urea in an atmosphere consisting of hydrogen and nitrogen. The powders were used to determine the appropriate mixing ratio of the raw materials to produce BN with the desired composition and crystal structure. The pyrolysis of boric acid and urea in a molar mixing ratio of 1:2 resulted in a BN that was hexagonal and had a near-stoichiometric composition.To prepare a solution for the coating of fibers, boric acid and urea were dissolved in an ethanol-water mixture. The coating was then applied to SiC filaments using a continuous roll-to-roll dip-coating process. It could be shown by SEM/EDS that BN layers were applied to the fibers. No significant bridging in the fiber bundle was found. Furthermore, it could be demonstrated by grazing incidence x-ray diffraction that the layers were crystalline.     

脉冲电沉积纳米镍-碳化硅复合镀层的性能

分别采用直流(DC)和换向脉冲电流(PRC)电沉积法制得纳米Ni-SiC复合镀层。采用X射线衍射仪、扫描电镜、能谱仪对比研究了纯Ni镀层和Ni-SiC复合镀层的微观结构、宏观残余应力、表面形貌及成分。用浸泡法研究了不同镀层在3.5%(质量分数)NaCl和10%(体积分数)H2SO4溶液中的腐蚀行为。结果表明,脉冲电沉积能改变镀层的微观结构,有效提高镀层硬度,降低宏观残余应力。脉冲电沉积所得到的纯Ni镀层和纳米Ni-SiC复合镀层在3.5%NaCl及10%H2SO4溶液中的耐蚀性均优于直流镀层。脉冲镀层在3.5%NaCl溶液中受腐蚀很轻,主要腐蚀形态为点蚀,而在10%H2SO4溶液中,SiC粒子作为增强相使镀层的耐腐蚀性进一步提高。     

Fabrication of nanoporous silicon carbide fibres by thermal treatment

Abstract

Nanoporous silicon carbide fibres were prepared by curing and heat treatment of melt spun polycarbosilane (PCS) fibres. During the curing process, green PCS fibres were thermally oxidised at the temperature between 180 and 220°C and time between 2 and 10 h for cross-linking among the molecule chains in the PCS and controlling the oxygen concentration and distribution. After thermal oxidation, fibres were heat-treated between 1200 and 1600°C for the conversion to SiC phase. About 15–20 wt-% of oxygen was analysed after heat treatment at 1200°C and it can be possible to pyrolyse without melting or deformation of fibre. At a temperature above 1400°C, the uniform distribution of nanopores was observed on the fibre surface, and the size of pores was increased with curing and heating condition. This type of nanoporous SiC fibre is expected to be a good candidate for high temperature catalyst or catalytic supports.     

Modeling environmentally induced property degradation of SiC/BN/SiC ceramic matrix composites

The degradation of SiC‐based ceramic matrix composites (CMCs) in conditions typical of gas turbine engine operation proceeds via the stress rupture of fiber bundles. The degradation is accelerated when oxygen and water invade the composite through matrix microcracks and react with fiber coatings and the fibers themselves. We review micromechanical models of the main rate‐determining phenomena involved, including the diffusion of gases and reaction products through matrix microcracks, oxidation of SiC (in both matrix and fibers) leading to the loss of stiffness and strength in exposed fibers, the formation of oxide scale on SiC fiber and along matrix crack surfaces that cause the partial closure of microcracks, and the concomitant and synergistic loss of BN fiber coatings. The micromechanical models could be formulated as time‐dependent coupled differential equations in time, which must be solved dynamically, e.g., as an iterated user‐defined material element, within a finite element simulation. A paradigm is thus established for incorporating the time‐dependent evolution of local material properties according to the local environmental and stress conditions that exist within a material, in a simulation of the damage evolution of a composite component. We exemplify the calibration of typical micromechanical degradation models using thermodynamic data for the oxidation and/or volatilization of BN and SiC by oxygen and water, mechanical test data for the rate of stress rupture of SiC fibers, and kinetic data for the processes involved in gas permeation through microcracks. We discuss approaches for validating computational simulations that include the micromechanical models of environmental degradation. A special challenge is achieving validated predictions of trends with temperature, which are expected to vary in a complex manner during use.     

反应烧结碳化硅材料的抗热震性能研究

通过对比不同温差热震后材料的残余强度 ,对反应烧结碳化硅材料的抗热震性能进行了研究。结果表明 :反应烧结碳化硅材料的抗热震性能与显微组织密切相关 ,低游离硅含量与小粒径的反应烧结碳化硅材料具有较好的抗热震断裂性能 ,而高游离硅含量或大碳化硅粒径的材料具有相对优异的抗热震损伤性。对反应烧结碳化硅材料的抗热震性与显微组织的关系进行了探讨。     

通过SiC纳米颗粒的原位合成制备氧化铝/碳化硅复合材料

通过一种包含SiC纳米微粒原位合成的新技术制备氧化铝/碳化硅复合材料。在水悬浮液中制备氧化铝粉末和碳化硅前体混合物。通过冷冻破碎和氩气中1 750℃、4h的无压烧结得到的小颗粒,通过冷等静压将其制成坯体。在烧结形成莫来石的过程中加入SiC。SiC颗粒主要存在于莫来石内部和氧化铝基质的颗粒中。     

An in situ phosphatizing coating on 2024 T3 aluminum coupons

Toxic chemicals, such as chrome, are commonly used in the application of conversion coatings to various metal surfaces. In situ phosphatizing coatings (ISPCs) are an innovative approach for eliminating the requirement of a conversion coating which ends the need of toxic materials used in a multi-step coating practice. An ISPC is formulated by predispersing an in situ phosphatizing reagent (ISPR) into a paint system. In this study, an ISPR, an arylphosphonic acid, is used in a polyester–melamine paint to react in situ with the metal surface and to provide the acidic catalyst needed, while thermally curing the paint. A second polyester–melamine paint system is used as the control that uses the standard catalyst para-toluenesulfonic acid (p-TSA) to catalyze the cross-linking reaction in the paint. These two paint systems are applied to bare 2024 T3 Al panels and to chromated 2024 T3 Al panels. The coated panels are treated in a corroding media for 2,400 h and monitored periodically using electrochemical impedance spectroscopy (EIS). Results of EIS data and the corresponding electrical equivalent circuit (EEC) show that the ISPC applied to both the chromated and untreated Al panels provide superior corrosion protection. At low frequency (0.01 Hz), the panels coated with the ISPC show 10 000 times more resistance than both the chromated and bare Al panels coated with the control polyester–melamine paint. The corresponding EEC shows that the panels with the control paint applied have double the amount of electrical components (resistors and constant phase elements). A physical interpretation is suggested for the EEC. The paint adhesion to the Al surface and the corrosion behavior are tested by salt-water immersion and salt fog. After the panels are exposed to the salt solution, a pressure tape is applied to test the adhesion. The results show that the ISPC applied on the chromated Al adheres the best. The simultaneous reaction of the ISPR catalyzing the curing of the paint and forming the metal–phosphate layer is the reason for the superior paint performance.     

镍-碳化钨纳米复合镀层的制备与性能

采用电镀的方法制备出Ni-WC纳米复合镀层,镀液组成为:NiSO4·7H2O 250 g/L,NiCl2·6H2O 30 g/L,H3BO3 30 g/L,光亮剂0.1 g/L,纳米WC颗粒5～ 30 g/L,表面活性剂及分散剂适量.研究了温度、电流密度及pH对复合镀层外观的影响,得到最佳电镀工艺条件为:温度50～55...     

Study on growth factors of SiC whisker in situ in SiCW/SiC composites based on selective laser sintering technology

In this study, SiC_W/SiC composite material was prepared through selective laser sintering (SLS), the effect of material parameters (pretreatment of PCS, content of PCS, type of catalyst and content of catalyst) on the growth number of SiC whisker was investigated. The results indicate that when the temperature variable of PCS pretreatment was 260 °C or 300 °C, the number of in situ generated SiC whisker was noticeably higher than that pretreated at 140/180/200/240 °C. The catalytic effect of ferrocene was superior than pure Fe, α-Al₂O₃ and Fe(NO₃)₃. Under this experimental condition, the best parameter was 10 wt.% of PCS and 4 wt.% of ferrocene. This find not only provides a research basis for the preparation of SiC_W/SiC composites prepared by SLS technology, but also perfects basic datas for the preparation process of SiC_W/SiC composites.     

锌-碳化硅复合镀层的制备及性能

以钢板为基体,在普通氯化物镀锌液中加入碳化硅制得Zn-SiC 复合镀层。研究了电流密度、温度以及 SiC、氯化铵的质量浓度对镀层耐蚀性和显微硬度的影响,得到制备 Zn-SiC 复合镀层的较佳工艺条件:电流密度 0.5～1.0 A/dm2,温度 20～25℃,SiC 10～11 g/L,氯化铵 250～260 g/L。在较佳工艺下,Zn-SiC 复合镀层中 SiC 的质量分数为 0.75%,耐蚀性优于纯锌镀层,镀层中 SiC 的存在有利于生成晶粒细小、致密且显微硬度较高的镀层。     

Creep strain recovery of an in situ spinel-forming refractory castable under loading/unloading compressive creep conditions

Creep strain recovery after unloading has been well studied for metals and certain ceramic composites; however, it has not yet been investigated for ordinary ceramic refractories applied in industrial furnaces. The present study explores the question whether creep strain recovery can be observed in ordinary ceramic refractories to justify its consideration in the design of such refractories and refractory linings. To this end, the dependence of creep strain recovery on different loading conditions was investigated for a high-alumina in situ spinel-forming castable, commonly used as refractory lining of steel ladles in secondary steel metallurgy. Several loading/unloading compressive creep tests were performed at 1300 °C for different loading histories. Creep strain recovery was observed to occur and it was significantly affected by the holding time and degree of unloading. A longer holding time for the loading period was found to increase the internal stress, which is the driving force for creep strain recovery. In addition, the findings indicate that a higher excess of internal stress over external stress after unloading induces higher strain recovery.     

SiC nanowire-toughened MoSi2-WSi2-SiC-Si multiphase coating for improved oxidation resistance of C/C composites

To improve the oxidation resistance of carbon/carbon (C/C) composites at high temperature, a SiC nanowire-toughened MoSi₂-WSi₂-SiC-Si multiphase coating was prepared by chemical vapor deposition (CVD) and pack cementation. The microstructure, mechanical properties and oxidation resistance of the coating were investigated. After the introduction of SiC nanowires, the elastic modulus, hardness, and fracture toughness of the MoSi₂-WSi₂-SiC-Si coating were increased by 25.48%, 4.09% and 45.03%, respectively. The weight loss of the coated sample with SiC nanowires was deceased from 4.83–2.08% after thermal shock between 1773 K and room temperature for 30 cycles and the weight loss is only 3.24% after isothermal oxidation at 1773 K in air for 82 h. The good oxidation resistance of the coating is mainly attributed to that SiC nanowires can effectively inhibit the propagation of cracks in the coating by the toughening mechanisms including bridging and pull-out.     

Mechanical,structural and oxidation resistance enhancement of carbon foam by in situ grown SiC nanowires

A method for processing carbon foams containing both silicon carbide (SiC) nanowires and bulk SiC and silicon nitride (Si3N4) phases has been developed by reaction of powder mixtures containing precursors for carbon, sacrificial template, silicon (Si), short carbon fibers (SCF) and activated carbon (AC). In situ growth of Si nanowires during pyrolysis of the foam at 1000 °C under N2 changed the foam׳s microstructure by covering the porous skeleton inside and out. In situ-grown SiC nanowires were found smoothly curved with diameters ranging around two main modes at 30 and 500 nm while their lengths were up to several tens of micrometers. SCF were found effectively mixed and well-bonded to pore walls. Following density, porosity and pore size distribution analyses, the heat-treated (HT) foam was densified using a chemical vapor infiltration (CVI) process. Thereafter, density increased from 0.62 to 1.30 g/cm³ while flexural strength increased from 29.3 to 49.1 MPa. The latter increase was attributed to the densification process as well as to low surface defects, presence of SCF and coating, by SiC nanowires, of the entire SiC matrix porous structure. The foam׳s oxidation resistance improved significantly from 58 to 84 wt% residual mass of the heat treated and densified sample. The growth mechanism of Si nanowires was supported by the vapor–liquid–solid mechanism developed under pyrolysis conditions of novolac and reducing environment of coal cover.     

Sintering behaviour of silicon carbide matrix composites reinforced with multilayer graphene

The scope of this paper includes preparation and characterisation of dense silicon carbide matrix composites reinforced with multilayer graphene (MLG). Application of graphene as a reinforcement phase should simultaneously improve mechanical properties of SiC matrix composites and act as one of the sintering activators. In the present work the mechanical properties and the microstructure changes of samples sintered with different additions of graphene (0.5, 1, 2, 3, 4 wt%) and boron (0.3, 1 and 2 wt%) were examined. The composites were consolidated at two different temperatures (1800 °C and 1900 °C) using the Spark Plasma Sintering method (SPS). Reference samples with the addition of graphite as a source of carbon (1 and 3 wt%) were also sintered in the same conditions. The abovementioned amounts of graphite are an optimal content which is essential to obtain high density of samples [1], [2], [3], [4], [5], [6], [7], [8], [9]. The influence of MLG on density, mechanical properties and phase structure of the sintered samples were investigated. A high rate of densification for the composites with 0.3 wt% of B and 1 wt% of MLG sintered at 1900 °C was observed. Moreover, these composites showed the highest average of microhardness (2663 HV0.5) and single-phase structure.     

Mechanical and tribological properties of silicon carbide coating on Inconel alloy from liquid pre-ceramic precursor

Liquid polycarbosilane (LPCS) derived hard coatings of silicon carbide (SiC) were deposited on Inconel alloy at three different moderately high temperatures by chemical vapour deposition. The deposited films were characterized by X-ray diffractometry and Field emission scanning electron microscopy. Liquid PCS yielded a mixture of α-SiC and β-SiC during decomposition having uniform round-shaped particles of dimension around 200–300 nm without extensive cracking and few discrete shaped particles were also found to form at higher temperature (i.e. 1100 °C and 1200 °C) deposited films. The coated samples showed substantial increment in hardness and fracture toughness as compared to the uncoated sample. The fracture toughness (K_IC) values of the deposited films were in the range of 6.7–10.7 MPa(m)^1/2. The tribological properties and hardness of the films were also found to vary with deposition temperature. The scratch tracks of the films revealed that brittle failures occurred in all SiC coated substrates.     

Stressed-oxidation behavior of 2D CVI SiC/BN/SiC at elevated temperature in air

The stressed-oxidation behaviors of 2D woven SiCf/BN/SiC composites were investigated at 950 °C and 1100 °C in air. The different proportions (60%–90%) of the ultimate tensile strength (UTS) at corresponding temperatures were chosen as constant stress. The stressed-oxidation experiments were taken to failure or interrupted (240h). The UTS decreases by 20.75% at 950 °C and 30.71% at 1100 °C. The composites did not fail during stressed oxidation when subjected to constant stress corresponding to the initial linear and the beginning of nonlinear segments of the tensile curve, above which the composites failed with a maximum failure life of about 10h. Fiber degradation due to the thermal exposure and the fiber cracks caused by the oxidation of BN interface coating and SiC fiber could be responsible for the strength degradation and failure of the composites during stressed oxidation.     

Occurrence forms of major impurity elements in silicon carbide

In this study, the forms of occurrence of impurity elements in silicon carbide (SiC) with different particle sizes were systematically investigated using a combination of X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-OES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The experimental results showed that in SiC powders prepared using the Acheson process, the contents of O, free-Si, free-C, and Fe impurities are high, and those of other trace impurity elements follow the order: Ti > Al > Ni > V. Moreover, O impurities mainly cover the SiC particle surface in the form of amorphous SiO₂. Fe impurities exist around SiC particles as Fe₂O₃, Fe_xSi_y, Fe_xSi_yTi_z, and Fe_xAl_ySi_z phases. Impurity elements are often introduced during the smelting of SiC and/or during milling or subsequent storage.     

Improved oxidation resistance of expanded graphite through nano SiC coating

Expanded graphite with nano SiC and amorphous SiC_xO_y coating was successfully prepared through pyrolysing silane coupling agent (SCA), where the grafting of SCA dominated the final products. The results show that mainly amorphous SiC_xO_y coating covers expanded graphite at 1000 °C, regardless of the SCA concentration. In comparison, nano SiC coating can be synthesized at 1200 °C depending on the good dispersion of SCA (with a SCA concentration of 50 vol%). The formed SiC coating contributes to much higher peak oxidation temperature (812.1 °C) than 678.0 °C of the pure expanded graphite. Meanwhile, the oxidation activation energies of expanded graphite are remarkably improved from 149.15 kJ/mol to 176.16 kJ/mol (based on Kissinger method), attributing to the derived nano SiC and SiC_xO_y coating.