PyC Interphase Research Articles

Effect of interphase on mechanical properties and microstructures of 3D Cf/SiBCN composites at elevated temperatures

AbstractInterphase between the fibers and matrix plays a key role on the properties of fiber reinforced composites. In this work, the effect of interphase on mechanical properties and microstructures of 3D Cf/SiBCN composites at elevated temperatures was investigated. When PyC interphase is used, flexural strength and elastic modulus of the Cf/SiBCN composites decrease seriously at 1600°C (92 ± 15 MPa, 12 ± 2 GPa), compared with the properties at room temperature (371 ± 31 MPa, 31 ± 2 GPa). While, the flexural strength and elastic modulus of Cf/SiBCN composites with PyC/SiC multilayered interphase at 1600°C are as high as 330 ± 7 MPa and 30 ± 2 GPa, respectively, which are 97% and 73% of the values at room temperature (341 ± 20 MPa, 41 ± 2 GPa). To clarify the effect mechanism of the interphase on mechanical properties of the Cf/SiBCN composites at elevated temperature, interfacial bonding strength (IFBS) and microstructures of the composites were investigated in detail. It reveals that the PyC/SiC multilayered interphase can retard the SiBCN matrix degradation at elevated temperature, leading to the high strength retention of the composites at 1600°C.

Oxidation resistance of NITE-SiC/SiC composites with/without CVD-SiC environmental barrier coating

In this study, the oxidation resistance of nuclear grade graphite (IG-430U) and NITE-SiC/SiC composites with/without CVD-SiC outer coating were evaluated at various temperatures (400–1000 °C) under high oxygen contents (O2 1% and O2 21%). The effect of CVD-SiC outer coating on the oxidation of NITE-SiC/SiC composite was also investigated in the atmosphere of partial oxygen. The weight change of NITE-SiC/SiC composites was mainly dominated by the active oxidation at the interphase of PyC. The weight change speed of these materials significantly decreased with increasing the oxidation time. The oxygen concentration was not sensitive for the oxidation behavior of NITE-SiC/SiC composites, because these materials were affected by oxygen supply rate and CO2 gas evaporation rate through fine channels between fiber and matrix. The PyC interphase of NITE-SiC/SiC composites with a CVD-SiC outer coating remained after the oxidation test. It could be confirmed that the CVD-SiC environmental coating on the surface of NITE-SiC/SiC was very effective for the suppression of oxidation weight loss.

Thermal conductivities of plain woven C/SiC composite: Micromechanical model considering PyC interphase thermal conductance and manufacture-induced voids

In this paper a micromechanical model considering PyC interphase thermal conductance and manufacture-induced voids is proposed to predict the thermal conductivities of plain woven C/SiC composite. This model is based upon the analysis of the representative volume element (RVE) models of composite. The modeling strategy starts with a geometrical description and finite element discretization on two scales consisting of the fiber yarn modeling (fiber-scale) followed by a woven fabric modeling (yarn-scale). The PyC interphase is introduced in the fiber-scale modeling while the large voids at the intersection of orthogonal yarns are included in the yarn-scale modeling. Experiments are conducted to measure the thermal conductivities of plain woven C/SiC samples from room temperature to 800 °C temperature. The satisfied agreement with experimental data has highlighted the predictive capability of the proposed micromechanical model. Finally, a parametric study is performed using the presented model to investigate the effects of PyC interphase thermal conductance and manufacture-induced voids on the thermal conductivities of composite.

Microstructure and mechanical properties of carbon/carbon composites with PyC/SiC/TiC multilayer interphases

Carbon/carbon composites with PyC/SiC/TiC multilayer interphases (CCs-PST) have been successfully prepared by a joint process of chemical vapor deposition and carbothermal reduction. Effect of the Ti(OC4H9)4/C6H4(OH)2 molar ratio on the morphology of TiC particles was investigated and the ratio was optimized as 8:1. When the Ti(OC4H9)4/C6H4(OH)2 molar ratio was 8:1, many homogeneously distributed TiC nanoparticles with the sizes of 100–500nm on the fibers were observed. The structural evolution of CCs-PST was discussed and the mechanical properties of as-prepared materials were investigated by flexural and interlaminar shear tests. The resulted composites demonstrated a PyC and SiC mixed inner interphase with the thickness of 0.5–1μm and a TiC outer interphase with a thickness about 0.5µm. Flexural strength of 201.45 ± 5.27MPa and modulus of 21.21 ± 1.58GPa showed a 41.7% and 7.83% improvement respectively as compared with that of the neat CCs. The interlaminar shear strength of CCs-PST was 66.71 ± 4.87MPa, which was 51.20% higher than that of the CCs. The improved mechanical properties were attributed to the enhanced interface bond between fibers and matrix induced by the PST.

Micro-mechanical evaluation of SiC-SiC composite interphase properties and debond mechanisms

Abstract SiC-SiC composites exhibit exceptional high temperature strength and oxidation properties making them an advantageous choice for accident tolerant nuclear fuel cladding. In the present work, small scale mechanical testing along with AFM and TEM analysis were employed to evaluate PyC interphase properties that play a key role in the overall mechanical behavior of the composite. The Mohr-Coulomb formulation allowed for the extraction of the internal friction coefficient and debonding shear strength as a function of the PyC layer thickness, an additional parameter. These results have led to re-evaluation of the Mohr-Coulomb failure criterion and adjustment via a new phenomenological equation.

Thermal shock resistance and hoop strength of triplex silicon carbide composite tubes

AbstractMulti‐layered SiC composites have been considered as a nuclear fuel cladding material of light water reactors, LWRs, because of their excellent high temperature strength and corrosion resistance under accident conditions. During a design basis accident of a LWR such as a loss‐of‐coolant accident, the peak temperature of the fuel clad rapidly increases as the production of decay heat continues. The emergency core cooling systems then automatically supply the reactor core with emergency cooling water. The fuel clad consequently suffers from thermal shock. In this study, the structural integrity of multi‐layered SiC composite tubes after thermal shock was investigated. Several kinds of multi‐layered SiC composite tubes consisting of CVD SiC and CVI SiCf/SiC were water‐quenched from 1200°C to room temperature. The triplex SiC composite tube retained its tubular geometry during quenching. The strength degradation after thermal shock was <13% for the specimens with a PyC interphase. The residual stress distribution within the tubes during thermal shock was evaluated by a finite element method.

Properties and microstructure evolution of Cf/SiC composites fabricated by polymer impregnation and pyrolysis (PIP) with liquid polycarbosilane

In the present study, a novel liquid polycarbosilane (LPCS) with a ceramic yield as high as 83% was applied to develop 3D needle-punched Cf/SiC composites via polymer impregnation and pyrolysis process (PIP). The cross-link and ceramization processes of LPCS were studied in detail by FT-IR and TG-DSC; a compact ceramic was obtained when LPCS was firstly cured at 120°C before pyrolysis. It was found that the LPCS-Cf/SiC composites possessed a higher density (2.13g/cm3) than that of the PCS-Cf/SiC composites even though the PIP cycle for densification was obviously reduced, which means a higher densification efficiency. Logically, the LPCS-Cf/SiC composites exhibited superior mechanical properties. The shorter length and rougher surfaces of pulled-out fibers indicated the LPCS-Cf/SiC composites to possess a stronger bonding between matrix and PyC interphase compared with the PCS-Cf/SiC composites.

Effects of SiC/HfC ratios on the ablation and mechanical properties of 3D Cf/HfC-SiC composites

Effects of SiC/HfC ratios on the ablation and mechanical properties of 3D Cf/HfC–SiC composites by precursor impregnation and pyrolysis (PIP) process were investigated systematically. Both strength (flexural and compressive strength) and modulus increase as the SiC/HfC ratio are improved. The compact and stiff HfC-SiC matrix in addition to the carbon fiber and PyC interphase with less reaction damage accounts for the improved mechanical properties of Cf/HfC-SiC with higher SiC/HfC ratios. Meanwhile, both weight loss and erosion depth of Cf/HfC-SiC are improved with the increased SiC/HfC ratios. Therefore, in order to balance the ablation and mechanical properties, an appropriate SiC/HfC ratio should be considered.

Ablation and mechanical properties of 3D braided C/ZrC–SiC composites with various SiC/ZrC ratios

Three-dimensional (3D) braided C/ZrC–SiC composites with various SiC/ZrC ratios were fabricated by precursor infiltration and pyrolysis (PIP) process. The effects of SiC/ZrC ratios on the ablation and mechanical properties were investigated systematically. The flexural strength of 3D C/ZrC–SiC grows with the increase of SiC/ZrC ratios. The compact and stiff ZrC-SiC matrix, moreover, the carbon fiber and PyC interphase with the less reaction damage account for the improved flexural strength of C/ZrC–SiC with higher SiC/ZrC ratios. As for the ablation properties, both mass and linear ablation rates drop with the decrease of SiC/ZrC ratios at the beginning. However, the ablation rates start to increase with the further decrease of SiC/ZrC ratio, where the high open porosity deteriorates the ablation properties. Herein, in order to obtain a balance between ablation and mechanical properties, an appropriate SiC/ZrC ratio should be considered.

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) Cf/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the Cf/ZrC–SiC composites without and with a thin PyC interlayer of 0.15µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2µm. The highest flexural strength of 272.3±29.0MPa and fracture toughness of 10.4±0.7MPam1/2 were achieved for the composites with a 1.2µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the Cf/ZrC–SiC composites slightly and the ZrO2 scale acts as an anti-ablation component during the testing.

Effects of thermal oxidation on microwave-absorbing and mechanical properties of SiCf/SiC composites with PyC interphase

The SiCf/SiC composites containing PyC interphase were prepared by chemical vapor infiltration process. The influences of thermal oxidation on the complex permittivity and microwave absorption properties of SiCf/SiC composites were investigated in the frequency range of 8.2–12.4 GHz. Both the real and imaginary parts of the complex permittivity decreased after thermal oxidation. The composites after 100 h thermal oxidation showed that reflection loss exceeded −10 dB in the frequency of 9.7–11.9 GHz and the minimum value was −11.4 dB at 11.0 GHz. The flexural strength of composites decreased but fracture behavior was improved obviously after thermal oxidation. These results indicate that the SiCf/SiC composites containing PyC interphase after thermal oxidation possess good microwave absorbing property and fracture behavior.

Mechanical and electromagnetic shielding properties of SiCf/SiC composites fabricated by combined CVI and PIP process

2.5D KD-I SiCf/SiC composites were fabricated by combined chemical vapor infiltration (CVI) and precursor infiltration-pyrolysis (PIP) processes for electromagnetic interference (EMI) shielding applications. The effects of PyC interphase and Al2O3 filler on mechanical and EMI shielding properties of SiCf/SiC composites were studied. The PyC interphase effectively improves the accommodation of the fibers and matrix, and Al2O3 filler compensates for matrix shrinkage and acts as a strengthening phase to improve the capability of bearing force for the composites. Thus, the composites with PyC interphase and Al2O3 filler possess the maximum flexural strength and fracture toughness with the values of 313MPa and 12.7MPam1/2. Owing to the existence of excess carbon and PyC interphase, the composites also maintain the highest SET and SEA values of 30 and 20dB, respectively, indicating its advantages as a kind of structural and functional materials. The superior SEA value is mainly attributed to the repeated reflection and dissipation of electromagnetic wave between the interfaces of PIP-SiC matrix and Al2O3 particles.

Mechanical and dielectric properties of 2.5D SiCf/SiC–Al2O3 composites prepared via precursor infiltration and pyrolysis

2.5D KD-1 SiC fabrics were employed to fabricate SiCf/SiC composites by a modified polymer infiltration and pyrolysis (PIP) process, using Al2O3 as inert filler. The effects of Al2O3 content on the microstructure, mechanical and dielectric properties of SiCf/SiC–Al2O3 composites were investigated. The results reveal that the infiltration efficiency is remarkably improved by incorporating the Al2O3 filler into the PIP process. The Al2O3 filler can effectively compensate for volume shrinkage to form dense matrix and act as strengthening phase to deflect microcracks, thus a maximum flexural strength of 186MPa is obtained when the Al2O3 content is 10wt%, which is 75% higher than that of composites without filler (106MPa). Both ε' and ε″ of complex permittivity for the composites decrease as Al2O3 content increases from 0 to 20wt%. With the incorporation of PyC interphase, the flexural strength of composites with 10wt% Al2O3 filler is improved from 186 to 234MPa, and its complex permittivity increases from 20.5-i15.5 to 43-i37.

Influence of dip-coated boron nitride interphase on mechanical and dielectric properties of SiCf/SiC composites

The utilization of urea and boric acid solutions as precursors via dip-coating method for synthesis of BN coatings on KD-I SiC fibers was demonstrated. The effects of precursor compositions of BN on coatings microstructure, coatings morphology, mechanical and dielectric properties of SiCf/SiC composites were investigated. XRD, FT-IR and Raman spectra results show that the crystalline degree of t-BN decreases with urea and boric acid mole ratios increase from 3:1 to 12:1. The surface of as prepared BN coatings is smooth and its thickness is uniform when the ratio is higher than 6:1. From mechanical properties measurements, the higher mole ratio helps to improve the flexural strength and fracture ductility of the composites. The flexural strength and failure displacement of SiCf/SiC composites with 0.2μm thickness BN interphase reach 278MPa and 0.29mm, which are both much higher than those of the composites without interphase, whose ultimate values are only 191MPa and 0.13mm, respectively. The flexural strength retention of the composites with BN interphase is higher than that of the composites with PyC interphase below 800°C and there is no significant difference at higher temperatures. The BN coatings have little influence on the complex permittivity of SiC fabrics.

Effects of thermal oxidation on electromagnetic interference shielding properties of SiCf/SiC composites

Abstract SiC f /SiC composites containing PyC interphase were prepared by chemical vapor infiltration and their properties were investigated for electromagnetic interference (EMI) shielding applications in the frequency of 8.2–12.4 GHz. The room temperature direct current electrical conductivity ( σ dc ) and EMI shielding effectiveness (SE) were tested after thermal oxidation for various time at 900 °C. It was found that the EMI was shielded mainly by absorption for both as-received and oxidized SiC f /SiC composites. Both σ dc and SE of SiC f /SiC composites decrease after thermal oxidation at 900 °C. It was proposed that the free carbon, including PyC interphase and free carbon in matrix and fibers, mainly determines the EMI SE of composites. The decrease of σ dc and SE of composites after oxidation are attributed to the consumption of free carbon. The SE of SiC f /SiC composites with a thickness of 2 mm remains above 15 dB after thermal oxidation for 4 h in air at 900 °C.

Fabrication and Properties of 3‐D Cf/SiC–ZrC Composites, Using ZrC Precursor and Polycarbosilane

Three‐dimensional (3‐D) needled Cf/ZrC–SiC composites were successfully fabricated by polymer infiltration and pyrolysis combined with ZrC precursor impregnation. The microstructure and mechanical properties of the composites were studied using XRD, SEM and three‐point bending test. The composite with PyC interphase between fiber and matrix had a bulk density of 2.20 g/cm3, an open porosity of 13%, and a bending force of 356 N, and exhibited a nonbrittle failure behavior due to propagation and deflection of cracks, and fracture and pullout of fibers. The mass lose and linear recession rate of the 3‐D needled Cf/SiC–ZrC composite during oxy‐propane torch test were 0.010 g/s and 0.001 mm/s, respectively. The formation of ZrSiO4 melt on the surface of the composite contributed mainly the excellent high‐temperature property.

Fabrication of Environmentally Resistant NITE-SiC/SiC Composites

NITE-SiC, SiC/SiC qualification of environmental resistance in various conditions is on-going toward early utilization in advanced energy and aero-space systems. Multi-layered SiC/SiC composites for preventing environmental attacks to pyrocarbon interphase was provided. Thermal exposure test in air and liquid metal(Pb and Li-Pb) compatibility test were carried out. It was confirmed the significant loss of PyC interphase in SiC/SiC composites. In case of Li-Pb-layered SiC/SiC composites, an attack of air and liquid metal has been sucessfuly supressed by surface SiC layer

Mechanical properties of advanced SiC fiber composites irradiated at very high temperatures

Six different composite materials of various near-stoichiometric silicon carbide (SiC) fiber reinforcements and pyrolytic carbon or SiC/pyrolytic carbon multilayer interphases were neutron-irradiated to ∼6×1025n/m2 (E>0.1MeV) at nominal temperatures of 800°C and 1300°C, and tested for tensile properties at room temperature. Only insignificant or very minor modifications to the tensile strength were realized. However, statistical analysis on relatively large specimen populations revealed minor but statistically significant strength degradation for some composites. From 50 to 150nm appeared to be within the optimum PyC interphase thickness range for the SiC fibers used in terms of tensile properties. The misfit stresses present in the unirradiated samples were significantly reduced after irradiation. The change in misfit stress may be attributed to the irradiation-induced modification of coefficient of thermal expansion or potential differential swelling between the fibers and the matrix. True matrix cracking stress estimated from the proportional limit stress and misfit stress did not appear to degrade by neutron irradiation.

プリフォーム高密度化処理によるSiC/SiC複合材料の微細組織および強度特性への影響

As an advanced liquid phase sintering method of SiC/SiC composites production, Nano-Infiltration and Transient Eutectic-phase (NITE) process has been intensively studied, where large volumetric shrinkage during sintering process is an issue to be solved. The objective of this work is to investigate the effects of preform densification on microstructure and mechanical properties of SiC/SiC composites fabricated by hot-press of preforms. The significant improvement seen in preform is the suppression of macro-pores at inter-/intra- fiber-bundles. Finally, this is contributing the reduction of volumetric shrinkage to suppress fiber damage and micro pore formation and eliminate macro pore formation after sintering. And, the composites fabricated presents microstructure homogeneity. The improvement in structure is reflected in high flexural strength. However, pseudo-ductility was worth than anticipated. The brittle behavior of the composites is interpreted to be caused by the damage of PyC interphase.

Microstructure and Mechanical Properties of SiC and Carbon Hybrid Fiber Reinforced SiC Matrix Composite

Silicon carbide (SiC) matrix composite reinforced by both SiC and carbon fibers ([SiC–C]/pyrolytic carbon [PyC]/SiC) was fabricated by chemical vapor infiltration for reducing matrix microcracks. Microstructure, mechanical properties, and oxidation resistance of the composite were compared with those of C/PyC/SiC and C/PyCHT/SiC composites in which PyC interphase was untreated and heat‐treated at 1800°C in argon, respectively. Compared with the C/PyC/SiC composite, (SiC–C)/PyC/SiC composite shows considerable improvements in flexural strength, fracture toughness, and oxidation resistance. The different mechanical behaviors of the three composites were analyzed based on the He and Hutchinson's model.