Electromagnetic wave absorption and mechanical properties of silicon carbide fibers reinforced silicon nitride matrix composites

It is difficult for ceramic matrix composites to combine good electromagnetic wave (EMW) absorption properties (reflection coefficient, RC less than -7 dB in X band) and good mechanical properties (flexural strength more than 300 MPa and fracture toughness more than 10 M P·m^1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si₃N₄ matrix composites (SiC_f/Si₃N₄) were designed and fabricated via chemical vapor infiltration technique. Effects of conductivity on EM wave absorbing properties and fiber/matrix bonding strength on mechanical properties were studied. The SiC_f/Si₃N₄ composite, having a relatively low conductivity (its conduction loss is about 33% of the total dielectric loss) has good EMW absorption properties, i.e. a relative complex permittivity of about 9.2-j6.4 at 10 GHz and an RC lower than ?7.2 dB in the whole X band. Its low relative complex permittivity matches impedances between composites and air better, and its strong polarization relaxation loss ability help it to absorb more EM wave energy. Moreover, with a suitably strong fiber/matrix bonding strength, the composite can transfer load more effectively from matrix to fibers, resulting in a higher flexural strength (380 MPa) and fracture toughness (12.9 MPa?m^1/2).     

Pressureless sintered silicon carbide matrix with a new quaternary additive for fully ceramic microencapsulated fuels

This study suggests a new additive composition based on AlN–Y₂O₃–Sc₂O₃–MgO to achieve successful densification of SiC without applied pressure at a temperature as low as 1850 °C. The typical sintered density, flexural strength, fracture toughness, and hardness of the SiC ceramics sintered at 1850 °C without applied pressure were determined as 98.3%, 510 MPa, 6.9 MPa·m^1/2, and 24.7 GPa, respectively.Fully ceramic microencapsulated (FCM) fuels containing 37 vol% tristructural isotropic (TRISO) particles could be successfully sintered at 1850 °C using the above matrix without applied pressure. The residual porosity of the SiC matrix in the FCM fuels was only 1.6%. TRISO particles were not damaged during processing, which included cold isostatic pressing under 204 MPa and sintering at 1850 °C for 2 h in an argon atmosphere. The thermal conductivity of the pressureless sintered FCM pellet with 37 vol% TRISO particles was 44.4 Wm^?1 K^?1 at room temperature.     

Tribo-mechanical properties of composites based on polyoxymethylene reinforced with basalt fiber and silicon carbide whiskers

This paper presents an analysis of the hybrid reinforcement of polyoxymethylene composites. Basalt fibers and monocrystalline silicon carbide fibers were used as reinforcement. Basic tests of mechanical properties were carried out, such as the static tensile and flexural test. The tests were repeated under external factors, such as the influence of water aging and a wide range of exploitation temperatures. The materials were also subjected to tribological tests, that is, determination of the friction coefficient and the specific wear rate. Strength tests revealed an increase in the stiffness of the material as well as a reduction the friction coefficient and abrasive wear. The addition of monocrystalline fibers significantly limited water absorption, stabilized the strength properties in the water environment as well as provided better material's resistance to dynamic impact.     

Micromechanical properties and microstructural evolution of Amosic-3 SiC/SiC composites irradiated by silicon ions

In this work, Amosic-3 SiC/SiC composites were irradiated to 10 dpa and 115 dpa with 300 keV Si ions at 300 °C. To evaluate its irradiation behaviour and investigate the underlying mechanism, nanoindentation, AFM, Raman and electron microscopy were utilized. Nanoindentation showed that although micromechanical properties declined after irradiation, hardness and Young’s modulus were maintained better under 115 dpa. AFM manifested differential swelling among PyC interface, fiber and matrix and SEM showed irradiation-induced partial interface debonding, which are both more obvious under 115 dpa. TEM revealed the generation and proliferation of amorphous regions, which is according with the decline and broadening of peaks in Raman spectra. The material was almost completely amorphous after irradiated to 10 dpa while recrystallization occurred under 115 dpa. All results mentioned above contribute to the decline of hardness and Young’s modulus and may explain why the micromechanical degradation was more significant under 10 dpa.     

Advances in modifications and high-temperature applications of silicon carbide ceramic matrix composites in aerospace: A focused review

This article is a detailed review of the measures to modify the high-temperature mechanical properties of silicon carbide ceramic matrix composites (SiC CMCs), namely toughness, high-temperature stability and wear resistance. Additionally, it briefly describes the common processing methods of the SiC CMCs and their application in the high-temperature field of aerospace. The advantages and disadvantages of various existing processing and molding methods for the SiC CMCs are also discussed. The high-temperature mechanical properties of the SiC CMCs are mainly affected by the properties of the matrix, added phase and interface. It is crucial to reduce the crystal defects of the matrix and select a suitable enhancement phase for an elevated performance. Moreover, it is important to improve the bonding at the interface between the enhancement phase and the matrix. This review is expected to provide useful information for the subsequent development of complex SiC CMCs for high-temperature applications.     

Effect of heat transfer channels on thermal conductivity of silicon carbide composites reinforced with pitch-based carbon fibers

In this study, silicon carbide (SiC) composites reinforced with pitch-based carbon fibers and composed of heat transfer channels were fabricated by combining chemical vapor infiltration and reactive melting infiltration method. It was observed that the internal heat conduction skeleton of pitch-based carbon fibers was sequentially formed. The thermal conductivities from room temperature to 500 °C along through-thickness direction and in-plane direction were investigated. The results showed that C_pf/SiC composites with heat transfer channels possessed excellent thermal conductvity in two directions, and the thermal conductivity increased with increasing volume content of heat transfer channels. The thermal conductivity in through-thickness direction reached 38.89 W/(m·K), and that for in-plane direction reached 112.42 W/(m·K). Theoretical calculations were empolyed to study the temperature dependence of the C_pf/SiC composites. The variations in slope A′ and intercept B′ values of fitted curves were in good agreement with the experimental results. To verify the reliablilty of the theoretical model, the C_pf/SiC composites were heated at 1650 °C for 2 h and the thermal conductivity exhibited further improvement due to the formation of more perfect crystalline structure. Thermal conductivity through thickness direction improved to 43.49 W/(m·K), and that in in-plane direction improved to 142.49 W/(m·K), which could be identified by the theoretical model. Finally, the leading edge model was established by using ABAQUS finite element analysis software to evaluate the potential application of the composites. Owing to the outstanding thermal conductivity, the leading edge obtained by using C_pf/SiC composites in this study exhibited lower temperature gradient and a more uniform temperature distribution. Moreover, less thermal stress and displacement were generated during heating process.     

Effect of tungsten carbide,silicon carbide and graphite particulates on the mechanical and microstructural characteristics of AA 5052 hybrid composites

Reinforced aluminium metal matrix composite materials are being used extensively in diverse fields that include aerospace and automobile. In this investigation, we introduce two distinct and novel types of aluminium hybrid composites and characterize their mechanical properties and microstructure. The first type was fabricated by reinforcing aluminium alloy (AA 5052) with tungsten carbide (WC) and graphite particulates and the second type was fabricated by reinforcing AA 5052 with silicon carbide (SiC) and graphite particulates. The composite material was processed through the melt-stir casting method and characterized by analyzing their densities, micro hardness, Charpy impact strength, tensile strength and peak elongation. Melt-stir casting method was chosen due to its cost effectiveness and productivity. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) studies were conducted to analyze thorough mixing of the reinforcements in the aluminium matrix metal. It was found that addition of tungsten carbide and graphite particulates with AA 5052 resulted in an increase in micro hardness and Charpy impact strength by 10.3% and 34.2% respectively, which are found to be better when compared to that of adding SiC and graphite particulates with AA 5052. Moreover, tensile tests revealed that there was a drop in tensile strength for the Al/SiC/graphite composites, while the peak elongation increases for both composites. On the other hand, while adding WC and graphite particulates the tensile strength of the composite improved by 15.12%. Also, the SEM fractographs taken for Al/SiC/graphite composite samples, subjected to Charpy impact and tensile tests revealed the presence of particle fractures and cracks and confirmed the possibility of plastic deformation. The results showed the Al/WC/graphite composites to be the superior among the two fabricated composites in terms of mechanical properties and therefore have good potential for structural applications.     

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.     

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.     

Influence of carbon nanotubes as secondary phase addition on the mechanical properties and amorphization of boron carbide

The mechanical properties and amorphization response of a carbon nanotube (5 wt.%) boron carbide (CNT-B₄C) composite with 1 μm grain size are investigated, and compared to those of coarse-grained (10 μm grain size) and ultrafine-grained (0.3 μm grain size) monolithic boron carbides. The quasi-static and dynamic uniaxial compressive strengths for CNT-B₄C were statistically the same as those of the ultrafine-grained ceramic and higher than the coarse-grained material, contradicting the expected grain size hierarchy (Hall-Petch-type relationship). Addition of CNTs to B₄C resulted in decreased quasi-static hardness compared to the large grain size material; however, dynamic hardness was substantially improved compared to quasi-static values. CNT pullout and crack bridging were observed to be possible toughening mechanisms. Finally, Raman spectroscopy was used to quantify amorphization, and it was concluded that addition of CNTs to boron carbide does not alter the propensity for amorphization, but does improve mechanical properties by enhanced toughening.     

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.