Measuring the effects of heat treatment on SiC/SiC ceramic matrix composites using Raman spectroscopy

The evolution of residual stresses found within a silicon carbide/silicon carbide (SiC/SiC) ceramic matrix composite through thermal treatments was investigated using Raman microspectroscopy. Constituent stress states were measured before, during, and after exposures ranging from 900 to 1300°C for varying times between 1 and 60 minutes. Silicon carbide particles in the as-received condition exhibited average hydrostatic tensile stresses of approximately 300 MPa when measured at room temperature before and after heat treatment. The room temperature Raman profile of the silicon matrix was altered in both shape and location with heat treatment cycles due to increasing activation of boron within the silicon lattice as heat treatment temperatures increased. By accounting for boron activation in the silicon–boron system, little to no permanent change of any constituent stresses were observed, and the silicon matrix subsequently exhibited a complimentary average hydrostatic compressive stress of approximately 300 MPa at room temperature, measured before and after heat treatment. This result builds upon previous literature and offers increased insight into boron activation phenomena measured through Raman spectroscopy methods.     

Formation of Ti3SiC2 interphase coating on SiCf/SiC composite by electrophoretic deposition

To improve the oxidation resistance of SiC composites at high temperature, the feasibility of using Ti₃SiC₂ coated via electrophoretic deposition (EPD) as a SiC fiber reinforced SiC composite interphase material was studied. Through fiber pullout, Ti₃SiC₂, due to its lamellar structure, has the possibility of improving the fracture toughness of SiC_f/SiC composites. In this study, Ti₃SiC₂ coating was produced by EPD on SiC fiber; using Ti₃SiC₂‐coated SiC fabric, SiC_f/SiC composite was fabricated by hot pressing. Platelet Ti₃SiC₂ powder pulverized into nanoparticles through high‐energy wet ball milling was uniformly coated on the SiC fiber in a direction in which the basal plane of the particles was parallel to the fiber. In a 3‐point bending test of the SiC_f/SiC composite using Ti₃SiC₂‐coated SiC fabric, the SiC_f/SiC composite exhibited brittle fracture behavior, but an abrupt slope change in the strength‐displacement curve was observed during loading due to the Ti₃SiC₂ interphase. On the fracture surface, delamination between each layer of SiC fabric was observed.     

Cf/SiC复合材料的制备及性能的研究

采用料浆渗积-有机前躯体裂解工艺制备碳纤维增强碳化硅陶瓷基复合材料.制备材料的抗弯强度达283 MPa,断裂韧性达12.1 MPa·m1/2.     

低温合成Ti3SiC2陶瓷

采用机械合金化和放电等离子烧结技术制备了纯度较高的Ti3SiC2陶瓷,研究了微量Al对Ti3SiC2的机械合金化和放电等离子烧结过程的影响.结果表明:添加适量的Al可以显著提高机械合金化及放电等离子烧结产物中Ti3SiC2的含量,并显著降低高纯度Ti3SiC2的烧结温度.机械合金化10h,成分为3Ti/Si/2C/0.2Al(摩尔比)的混合粉体,经850℃放电等离子烧结可获得质量分数(下同)高达96%的Ti3SiC2块体,烧结温度提高到1 100℃,可获得纯度为99.3%、相对密度高达98.9%的Ti3SiC2致密块体.     

Effect of BN nanoparticle content in SiC matrix on microstructure and mechanical properties of SiC/SiC composites

BN-nanoparticle-containing SiC-matrix-based composites comprising SiC fibers and lacking a fiber/matrix interface (SiC/BN + SiC composites) were fabricated by spark plasma sintering (SPS) at 1800°C for 10 min under 50 MPa in Ar. The content of added BN nanoparticles was varied from 0 to 50 vol.%. The mechanical properties of the SiC/BN + SiC composites were investigated thoroughly. The SiC/BN + SiC composites with a BN nanoparticle content of 50 vol.%, which had a bulk density of 2.73 g/cm³ and an open porosity of 5.8%, exhibited quasiductile fracture behavior, as indicated by a short nonlinear region and significantly shorter fiber pullouts owing to the relatively high modulus. The composites also exhibited high strength as well as bending, proportional limit stress, and ultimate tensile strength values of 496 ± 13, 251 ± 30, and 301 MPa ± 56 MPa, respectively, under ambient conditions. The SiC fibers with contents of BN nanoparticles above 30 vol.% were not severely damaged during SPS and adhered to the matrix to form a relatively weak fiber/matrix interface.     

Simultaneous hardening and toughening of a high-entropy (NbTaZrW)C ceramic carbide using SiC particle

Previously, we have found that (NbTaZrW)C exhibits a good combination of nanohardness and toughness. In this report, we explore the possibility to further increase the overall properties of this high-entropy carbide ceramic (HECC) through introducing SiC particle (SiC_P). To this end, a series of (NbTaZrW)C–xSiC ceramic composites (x = 0/5/15/30/50 vol.%) were fabricated using spark plasma sintering (SPS), their microstructure and mechanical properties were characterized. Our results reveal a grain refinement effects of SiC_P, an agglomeration of SiC_P with (1 0 0) plane preferentially perpendicular to the SPS-pressing direction and the formation of a transition region with various stoichiometric ratio of (NbTaZrW)_xC_1−x in the (NbTaZrW)C–SiC_P vicinity. The elastic modulus, microhardness, and flexural strength of the HECCs show tight positive relations with the SiC_P content and the beneficial effect of SiC_P to the fracture toughness of (NbTaZrW)C becomes evident once the content of SiC_P reaches 30 vol.%. Altogether, (NbTaZrW)C–50%SiC, which has a microhardness of 22 GPa, a flexural strength of 455 MPa, and an indentation fracture toughness of 6.54 MPa m^1/2, presents the optimal combination of mechanical properties among the investigated composites. Mechanistically, the strengthening effect of SiC_P introduction arises from the intrinsic high hardness of SiC_P and the SiC_P-induced grain refinement and the toughening effect is mainly associated with crack bridging mechanism.     

碳纤维增强SiC陶瓷复合材料的研究进展

碳纤维增强SiC陶瓷基复合材料具有良好的高温力学性能，是航空航天和能源等领域新的高温结构材料研究的热点之一．本文回顾了增强体碳纤维的发展，对材料的成型制备工艺，材料的抗氧化涂层研究进展和现有的一些应用做了综述，并展望了碳纤维增强SiC陶瓷基复合材料以后的研究重点及发展前景．     

Optimizing the fiber push-out method to evaluate interfacial failure in SiC/BN/SiC ceramic matrix composites

The investigation of several parameters during fiber push-out micromechanical tests on the interfacial shear strength (ISS) of the BN interphase in SiC_f/SiC ceramic matrix composites (CMC) was undertaken to optimize experimental work. The SiC_f/SiC composites—candidate materials for jet engine components—were manufactured with varying fiber types and interlayer thicknesses. Experimental parameters explored included analyzing the effect of sample thickness on the success rate of micromechanical tests, the effect of fiber local environment whether at tow-level (intra-tow variability in ISS) or CMC architecture-level (inter-tow variability), the effect of nanoindenter flat-punch tip size, and the effect of the interphase thickness itself. Over 1000 fiber push-outs were performed and analyzed in this work—with data presented as cumulative distribution functions to compare and contrast samples. It was found that the ISS measured was strongly and statistically influenced by the underlying fiber roughness (interphase adherence), as well as its local fiber environment (e.g., number of nearest neighbors) only if the thickness of the interphase itself surpassed a threshold of 200 nm. Finally for thinner interphases, limited value was added to the CMC as the ISS measured was high and there was no effect from any local environment.     

放电等离子烧结工艺合成Ti3SiC2的研究

以元素单质粉为原料，当原料配比为n(Ti):n(Si):n(Al):n(C)=3:(1.2-x):x:2，其中：x=0.05-0.2时，在1200－1250℃温度下经放电等离子烧结成功制备了高纯、致密Ti3SiC2固溶体材料。原料中掺加适量Al能改善Ti3SiC2的合成反应并提高制备材料的纯度。当x=0.2时，所合成的固溶体形貌为板状结晶，分子式近似为Ti3Si0.8Al0.2C2，晶格参数a=0.3069nm,c=1.767nm。在1250℃温度下烧结，得到平均厚度达5μm，发育完善均匀的致密多晶体材料。材料Vickers硬度为3.5-5.5GPa，具有与石墨相似的加工性能。     

Intermediate temperature oxidative strength degradation of a SiC/SiNC composite with a polymer‐derived matrix

The article describes an experimental investigation of oxidative degradation in mechanical performance of a SiC fiber‐reinforced composite with a SiCN matrix produced by polymer infiltration and pyrolysis. Tensile stress rupture and retained strength tests were performed at 800°C in dry air and in water vapor. Fracture surfaces were examined to determine the degree of fiber pull‐out and constituent oxidation and to measure radii of representative fiber fracture mirrors. The results indicate that degradation in tows adjacent to cut surfaces occurs equally rapidly in water vapor with or without application of stress; regions in the composite interior and near as‐processed (uncut) surfaces appear far less affected. Similar effects are evident but less pronounced in dry air. Although oxidation of fiber coatings is observed in some cases, collectively the results suggest that fiber degradation is the main mechanism leading to reduced composite strength.     

Densification and characterization of rapid carbothermal synthesized boron carbide

Submicrometer boron carbide powders were synthesized using rapid carbothermal reduction (RCR) method. Synthesized boron carbide powders had smaller particle size, lower free carbon, and high density of twins compared to commercial samples. Powders were sintered using spark plasma sintering at different temperatures and dwell times to compare sintering behavior. Synthesized boron carbide powders reached >99% TD at lower temperature and shorter dwell times compared to commercial powders. Improved microhardness observed in the densified RCR samples was likely caused by the combination of higher purity, better stoichiometry control, finer grain size, and a higher density of twin boundaries.     

Dispersion and densification of nano Si–(Al)–C powder with amorphous/nanocrystalline bimodal microstructure

The dispersion behavior and densification of nano Si–(Al)–C powder with amorphous/nanocrystalline bimodal microstructure were investigated. The Si–C powders synthesized by a mechanical alloying (MA) process had a near‐spherical shape with an average particle size of 170 nm. A solid loading of 62 vol% was achieved using polyethyleneimine (PEI) as a dispersant. The optimum dispersant amount was 1 wt% based on zeta potential, sedimentation, and viscosity analysis data. The high zeta potential value (73 mV) compared with that of the commercially available SiC (65 mV) was caused by modified surface properties and consequent promotion of the cationic dispersant adsorption. A Si–Al–C slurry containing 6.5 wt% of sintering additives with a solid loading of 60 vol% was also prepared. The relative density of the dried Si–Al–C slurry was 63.3% without additional compaction, which could be densified at 1650°C at a pressure of 20 MPa using a spark plasma sintering furnace.     

High-temperature oxidation of BN-coated sylramic SiC fibers in dry and wet atmosphere

The oxidation behavior of SiC Sylramic fibers coated with chemically vapor deposited Si-doped boron nitride (BN) was investigated at temperatures between 800 and 1200°C in dry and wet O₂ atmospheres. Thermogravimetric analysis was used to study the oxidation kinetics of the fiber and the influence of the BN layer and the environment. The morphology and composition of the thermally grown oxide scale was determined posttest by scanning electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectrometry. This study gives new insights into the synergistic effects of BN and water vapor on the oxidation behavior SiC Sylramic fibers. The vulnerability of the BN fiber interphase and the behavior of the fiber under conditions relevant to high-temperature turbine applications are discussed.     

Analysis of SiC/SiC composites for energy applications at ambient conditions

Plain weave planar and biaxially braided tubular SiC/SiC CMCs are evaluated in tension and four-point bending, respectively, at ambient conditions. Custom-designed fixtures for CMC testing are developed for each loading mode and are coupled with three-dimensional digital image correlation. Stereoscopic image correlation analysis reveals crack initiation and failure sites to provide insight into stress redistribution mechanisms. Scanning electron microscopy is performed postmortem to determine the influence of microstructural features on crack initiation and failure. Crack spacing is measured in situ by stereoscopic image correlation and confirmed by SEM measurements to relate to underlying tow-tow crossing points. Triangulated surface heights of plain weave tow architecture are used to determine that subtle differences in neighboring transverse tow angle, which vary within a range of ±4° from horizontal, have no significant effect on final fracture location. The results presented reaffirm the state of current SiC/SiC CMCs developed for energy applications and will help to further improve SiC/SiC and other CMCs.     

Thermodynamic Behavior of Copper-Coated Silicon Carbide Particles during Conventional Heating and Spark Plasma Sintering

75Cu·25SiC (vol%) compacts were prepared using copper-coated SiC particles and spark plasma sintering (SPS). The preliminary thermal performance of the coated particles was determined using simultaneous DSC-TG-MS measurement. Characterization of compacts using XRD and SEM techniques was conducted to investigate the physical and chemical changes during the SPS operation. It was found that CuO decomposed at 850° and 500°C during conventional heating and SPS, respectively. Cu₂O facilitated the densification of Cu/SiC composites. The optimized sintering temperature of Cu/SiC composites using SPS was ∼730°C. The compacts showed improved hardness because of the SiC reinforcement.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Crack opening behavior in ceramic matrix composites

The evolution of matrix cracks in a melt‐infiltrated SiC/SiC ceramic matrix composite (CMC) under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. CMC modeling and life prediction strongly depend a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time‐temperature‐environment‐stress), and the interactions of matrix cracks with fibers and interfaces. In this work, strain relaxation due to matrix cracking, the relationship between CODs and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross‐section. This calls into question the modeling assumption of through‐cracks for all loading conditions and fiber architectures, which can obscure oxidation mechanisms that are active in realistic cracking conditions. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMCs at stresses well below the proportional limit.     

Microstress in the matrix of a melt‐infiltrated SiC/SiC ceramic matrix composite

Microstress in the SiC: Si matrix of a ceramic matrix composite (CMC) has been characterized, using Raman spectroscopy. The matrix of the composite was manufactured using liquid melt infiltration, and has about 20% unreacted free silicon. During the processing of the composite, the unreacted free silicon expands 11 vol% when transforming from liquid to solid. This crystallization expansion creates compressive microstress in the silicon phase of the matrix, which ranges from 2.4 to 3.1 GPa, and tensile microstress in the SiC of the matrix which ranges from 0.24 to 0.75 GPa. The microstress varies significantly with position, following a normal distribution.     

Multiple nonlinear regression model of cutting force for C/SiC composites by laser-assisted micromachining

Based on the response surface methodology (RSM), laser-assisted micromachining (LAMM) experiments of carbon fiber reinforced silicon carbide matrix(C/SiC) composites have been carried out to obtain the data of three-direction cutting forces Fx, Fy, Fz, and result-force F in the cutting process. A stepwise regression method is used to screen out the factors that significantly affected the cutting force, and hence a multivariate nonlinear mathematical regression model has been established. Through the analysis of variance and regression analysis, the R2 of the regression model is the cutting force Fx: 0.975, the cutting force Fy: 0.970, the cutting force Fz: 0.972, the resultant force F: 0.987, and the probability p > F-values are all less than 0.001. The model analysis results show that the main effects and interactions of the three factors of laser power, cutting speed, and cutting depth significantly influence the cutting force. The main effect of the most considerable influence on the resultant F is cutting depth, followed by cutting speed and laser power, and the interactive effect is cutting speed and cutting depth. The experimental results show that the model prediction results are in good agreement with the experimental results.     

High thermal conductivity of spark plasma sintered silicon carbide ceramics with yttria and scandia

A fully dense SiC ceramic with a room‐temperature thermal conductivity of 262 W·(m·K)^?1 was obtained via spark plasma sintering β‐SiC powder containing 0.79 vol% Y₂O₃‐Sc₂O₃. High‐resolution transmission electron microscopy revealed two different SiC‐SiC boundaries, that is, amorphous and clean boundaries, in addition to a fully crystallized junction phase. A high thermal conductivity was attributed to a low lattice oxygen content and the presence of clean SiC‐SiC boundaries.