Fabrication and mechanical properties of carbon fibers/lithium aluminosilicate ceramic matrix composites reinforced by in-situ growth SiC nanowires

To improve the mechanical properties of carbon fibers/lithium aluminosilicate (C_f/LAS) composites, C_f/LAS with in-situ grown SiC nanowires (SiC_nw-C_f/LAS) were prepared by chemical vapor phase reaction, precursor impregnation, and hot press sintering, consecutively. The effect of multi-scaled reinforcements (micro-scaled C_f and nano-scaled SiC_nw) on the mechanical properties was investigated. The phase composition, microstructure and fracture surface of the composites were characterized by XRD, Raman Spectrum, SEM, and TEM. The morphology of SiC_nw has a close relation with the content of Si. Microstructure analysis suggests that the growth of SiC nanowires depends on the VLS mechanism. The multi-scale reinforcement formed by C_f and SiC_nw can significantly improve the mechanical properties of C_f/LAS. The bending strength of SiC_nw-C_f/LAS reaches to 597 MPa, achieving an increase of 19% to C_f/LAS. Moreover, the samples show a maximum fracture toughness of 11.01 MPa m^1/2, achieving an increase of 46.4% to C_f/LAS. Through analysis of the fracture surface, the improved mechanical properties could be attributed to the multi-scaled reinforcements by the pull-out and debonding of C_f and SiC_nw from the composites.     

SiC/SiC复合材料及其应用

日本开发的Nicalon和Tyranno两种品牌的SiC纤维占有世界上绝对性的市场份额。SiC/SiC复合材料典型的界面层是500 nm厚的单层热解碳(PyC)涂层或多层(PyC-SiC)n涂层,在湿度燃烧环境及中高温条件下界面层的稳定性是应用研究的重点。SiC/SiC复合材料,包括CVI-SiC基体和日本开发的Tyranno hex和NITE-SiC基体等,具有耐高温、耐氧化性和耐辐射性的特点,在航空涡轮发动机部件、航天热结构部件及核聚变反应堆炉第一壁材料等方面正开展工程研制应用。     

Microstructure and mechanical properties of hot-pressed SiC nanofiber reinforced SiC composites

This study reports on the preparation and mechanical properties of a novel SiC_nf/SiC composite. The single crystal SiC nanofiber(SiC_nf) reinforced SiC ceramic matrix composites (CMC) were successfully fabricated by hot pressing the mixture of β-SiC powders, SiC_nf and Al–B–C powder. The effects of SiC_nf mass fraction as well as the hot-pressing temperature on the microstructure and mechanical properties of SiC_nf/SiC CMC were systematically investigated. The results demonstrated that the 15 wt% SiC_nf/SiC CMC obtained by hot pressing (HP) at 1850 °C with 30 MPa for 60 min possessed the maximum flexural strength and fracture toughness of 678.2 MPa and 8.33 MPa m^1/2, respectively. The nanofibers pull out, nanofibers bridging and cracks deflection were found by scanning electron microscopy, which are believed can strengthen and toughen the SiC_nf/SiC CMC via consuming plenty of the fracture energy. Besides, although the relative density of the prepared SiC_nf/SiC CMC further increased with the sintering temperature rose to 1900 °C, the further coarsend composites grains results in the deterioration of the mechanical properties for the obtained composites compared to 1850 °C.     

Simultaneously enhancing mechanical and microwave absorption properties of Cf/SiC composites via SiC nanowires additions

C_f/SiC composite is an excellent structural and functional material, silicon carbide nanowires (SiCnws) are not only a toughening material but also a great application in the field of microwave absorption. In this study, SiCnws are grown on the surface of carbon fiber (C_f) by polymer impregnation and pyrolysis, and the SiC matrix was prepared by chemical vapor osmosis method. The SiCnws are introduced to enhance the mechanical and microwave absorption properties simultaneously. After 3 impregnations, the flexural strength of the composite was 107.35 ± 10 MPa. When the thickness is 1.86 mm, the minimum reflection loss value is ?41.08 dB, and the effective absorption bandwidth (RL ≤ ?10 dB) is 3.86 GHz. Furthermore, the microwave absorption mechanism of the material is discussed. This work provides a new method to prepare lightweight, stable and high-performance microwave absorption materials, and these materials are expected to be used in high temperature environments.     

Effect of in situ grown SiC nanowires on microstructure and mechanical properties of C/SiC composites

Hexagonal-shaped SiC nanowires were in situ formed in C/SiC composites with ferrocene as catalyst in the densification process of polymer impregnation and pyrolysis. The effect of SiC nanowires on microstructure and properties of the composites were studied. The results show that the in situ formed SiC nanowires were hexagonal, mostly with diamer of about 250 nm, and grew by the vapor–liquid–solid (VLS) mechanism. The C/SiC composite with nanowires shows higher bulk density and flexural strength than the one with no SiC nanowires, and the high temperature flexural strength behavior of C/SiC composites with SiC nanowires was evaluated.     

Microstructure and mechanical properties of B4C–TiB2–SiC composites toughened by composite structural toughening phases

B₄C–TiB₂–SiC composites toughened by composite structural toughening phases, which are the units of (TiB₂–SiC) composite, were fabricated through reactive hot pressing with B₄C, TiC, and Si as raw materials. The units of (TiB₂–SiC) composite with the size of 10‐20 μm are composed of interlocking TiB₂ and SiC with the size of 1‐5 μm. The addition of TiC and Si can effectively promote the sintering of B₄C ceramics. The relative densities of all the B₄C composites with different contents of TiB₂ and SiC are close to completely dense (98.9%‐99.4%), thereby resulting in superior hardness (33.1‐36.2 GPa). With the increase in the content of TiB₂ and SiC, the already improved fracture toughness of the B₄C composite continuously increases (5.3‐6.5 MPa·m^1/2), but the flexure strength initially increases and then decreases. When cracks cross the units of the (TiB₂–SiC) composite, the cracks deflect along the interior boundary of TiB₂ and SiC inside the units. As the crack growth path is lengthened, the crack propagation direction is changed, thereby consuming more crack extension energy. The cumulative contributions improve the fracture toughness of the B₄C composite. Therefore, the composite structural toughening units of the (TiB₂–SiC) composite play an important role in reinforcing the fracture toughness of the composites.     

Mechanical properties degradation and microstructure evolution of 2D-SiCf / SiC composites in combustion gas environment

Based on the turbine high-temperature combustion gas simulation test platform, the long-term combustion gas environment exposure test of the 2D plain woven SiC_f/BN/SiC composites under two combustion conditions was carried out. Uniaxial tensile test, fracture morphology characterization and non-destructive testing analysis revealed the degradation and microstructure evolution of composites after exposure to combustion gas environment. The results show that the degradation of 2D-SiC_f/SiC composites after exposure to combustion gas environment is manifested as a decrease in static toughness, and the interphase transition is the mesoscopic cause of the decrease in static toughness of the composite.     

Tensile and stressed-oxidation behavior of 2D SiC/BN/SiC composites at Intermediate Temperatures in Air

The stressed-oxidation behavior of 2D CVI SiC/BN/SiC composites was studied at intermediate temperatures (800 °C) in air. The ultimate tensile strength (UTS) was acquired to determine the constant stress. The results show that the UTS at intermediate temperature is 14.3 % lower than that at room temperature. The strain-time curves at all stress levels show a deceleration stage and a stable stage. The stressed-oxidation rupture life decreases from 5.4 h to 0.9 h when the stress increases from 60 % to 90 % of the UTS. The element composition and fracture morphologies of the composites were also analyzed. The results show that the oxidation degree increases as the rupture time increases or constant stress decreases. Fiber degradation and interface defects caused by component oxidation induced local fiber failure and ultimate rupture of the composites, which may be attributed to strength degradation at intermediate temperatures and rupture of the composites during stress oxidation.     

Effect of co-deposited SiC nanowires and carbon nanotubes on oxidation resistance for SiC-coated C/C composites

To protect carbon/carbon composites (C/Cs) against oxidation, SiC coating toughened by SiC nanowires (SiCNWs) and carbon nanotubes (CNTs) hybrid nano-reinforcements was prepared on C/Cs by a two-step technique involving electrophoretic co-deposition and reactive melt infiltration. Co-deposited SiCNWs and CNTs with different shapes including straight-line, fusiform, curved and bamboo dispersed uniformly on the surface of C/Cs forming three-dimensional networks, which efficiently refined the SiC grains and meanwhile suppressed the cracking deflection of the coating during the fabrication process. The presence of SiCNWs and CNTs contributed to the formation of continuous glass layer during oxidation, while toughed the coating by introducing toughing methods such as bridging effect, crack deflection and nanowire pull out. Results showed that after oxidation for 45 h at 1773 K, the weight loss percentage of SiC coated specimen was 1.35%, while the weight gain percentage of the SiCNWs/CNTs reinforced SiC coating was 0.03052% due to the formation of continuous glass layer. After being exposed for 100 h, the weight loss percentage of the SiCNWs/CNTs reinforced SiC coating was 1.08%, which is relatively low.     

Polymer-derived SiC ceramic aerogels with in-situ growth of SiC nanowires

Herein, the SiC ceramic aerogels with in-situ growth of SiC nanowires (SiC_w/SiC CAs) have been synthesized by polymer‐derived ceramics (PDCs) method. The morphology, microstructure, and phase composition of the as-prepared samples were systematically investigated through SEM, XRD, TEM, Raman spectrum, FT-IR spectrum, and XPS spectrum techniques. The results showed that the as-obtained SiC_w has a diameter of about 80 nm and a length of 1–3.5 μm. In addition, the formation mechanism and evolution process of growth SiC_w were systematically studied using a VLS growth mechanisms. The way in this work could be expanded to synthesize other Si-based porous ceramic aerogel nanostructed with nanowires.     

Mechanical properties and strengthening mechanism of SiCf/SiC mini-composites modified by SiC nanowires

The effects of the SiC nanowires (SiCNWs) and PyC interface layers on the mechanical and anti-oxidation properties of SiC fiber (SiC_f)/SiC composites were investigated. To achieve this, the PyC layer was coated on the SiC_f using a chemical vapour infiltration (CVI) method. Then, SiCNWs were successfully coated on the surface of SiC_f/PyC using the electrophoretic deposition method. Finally, a thin PyC layer was coated on the surface of SiC_f/PyC/SiCNWs. Three mini-composites, SiC_f/PyC/SiC, SiC_f/PyC/SiCNWs/SiC, and SiC_f/PyC/SiCNWs/PyC/SiC, were fabricated using the typical precursor infiltration and pyrolysis method. The morphologies of the samples were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. Tensile and single-fibre push-out tests were carried out to investigate the mechanical performance and interfacial shear strength of the composites before and after oxidization at 1200 °C. The results revealed that the SiC_f/PyC/SiCNWs/SiC composites showed the best mechanical and anti-oxidation performance among all the composites investigated. The strengthening and toughening is mainly achieved by SiCNWs optimization of the interfacial bonding strength of the composite and its own nano-toughening. On the basis of the results, the effects of SiCNWs on the oxidation process and retardation mechanism of the SiC_f/SiC mini-composites were investigated.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

Strengthening mechanism of SiC nanowires on microhardness of AZ91D-based composites

In this paper, the microhardness of the AZ91D matrix in C_f-SiCNWs/AZ91D composites is investigated. It is found that introducing SiCNWs into composites can increase the microhardness of the AZ91D matrix close to the SiCNWs (the vertical distance does not exceed 100 μm from the SiCNWs), which is as high as 84.34 HV and is 36.78 % higher than that of the alloy far away from the SiCNWs. The SiCNWs/AZ91D zone has the highest average microhardness (103.76Hv), which is 68.28 % higher than that of the AZ91D alloy in the C_f-SiCNWs/AZ91D composite. The increase in microhardness of AZ91D matrix is attributed to the synergistic effect of several reasons, including the increased dislocation density, the uneven dispersion of aluminum element caused by SiCNWs, as well as the nanocrystals of Mg₁₇Al₁₂ precipitates.     

Short SiC fiber/Ti3SiC2 MAX phase composites: Fabrication and creep evaluation

The compressive creep of silicon carbide fiber reinforced Ti₃SiC₂ MAX phase with both fine and coarse microstructure was investigated in the temperature range of 1000-1300°C. Comparison of only steady-state creep was done to understand the response of fabricated composite materials toward creep deformation. It was demonstrated that the fibers are more effective in reducing the creep rates for the coarse microstructure by an increase in activation energy compared to the variant with a finer microstructure, being partly a result of the enhanced creep rates for the microstructure with larger grain size. Grain boundary sliding along with fiber fracture appears to be the main creep mechanism for most of the tested temperature range. However, there are indications for a changed creep mechanism for the fine microstructure for the lowest testing temperature. Local pores are formed to accommodate differences in strain related to creeping matrix and predominantly elastically deformed fibers during creep. Microstructural analysis was done on the material before and after creep to understand the deformation mechanics.     

Oxidation and erosion resistance of multi-layer SiC nanowires reinforced SiC coating prepared by CVD on C/C composites in static and aerodynamic oxidation environments

A multi-layer SiC nanowires reinforced SiC (SiCnws-SiC) coating was prepared in-situ on carbon/carbon (C/C) composites by three chemical vapor deposition (CVD) processes. The microstructure and phase composition of the nanowires fabricated on the first-layer SiCnws-SiC coating and the coatings were examined by SEM, TEM, and XRD. The bamboo-like SiC nanowires with a 50?nm diameter and a length of several tens of micrometers are straight, randomly orientated and distributed like a net on the first-layer SiCnws-SiC coating. The growth direction is [111], and the growth mechanism is VS. The multi-layer SiCnws-SiC coating has three layers: the thickness of the first-layer is roughly 400?µm, and the outer two layers are about 200?µm. Each layer has a sandwich structure. The isothermal oxidation and erosion resistance of the multi-layer SiCnws-SiC coating were investigated in an electrical furnace and a high temperature wind tunnel. The results indicated that the weight loss of the multi-layer SiCnws-SiC coated C/C composites was only 1.8% after oxidation in static air at 1773?K for 361?h. Further, the coated sample failed due to fracture of the coating at the clamping position (i.e. 80?mm) after erosion at 1873?K for 130?h in the wind tunnel. The weight loss of the coated C/C composites occurred due to the formation of penetrating cracks in the coating during the oxidation thermal shock. The maximum bending moment and the larger clamping force caused the coating fracture and resulted in intense oxidation of the substrate and the failure of the specimen.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Enhancement of the microwave absorption properties of PyC-SiCf/SiC composites by electrophoretic deposition of SiC nanowires on SiC fibers

The employment of coating technique on the silicon carbide fibers plays a pivotal role in preparing SiC fiber-reinforced SiC composites (SiC_f/SiC) toward electromagnetic wave absorption applications. In this work, SiC nanowires (SiCNWs) are successfully deposited onto the pyrolytic carbon (PyC) coated SiC fibers by an electrophoretic deposition method, and subsequently densified by chemical vapor infiltration to obtain SiCNWs/PyC-SiC_f/SiC composites. The results reveal that the introduction of SiCNWs could markedly enhance the microwave absorption properties of PyC-SiC_f/SiC composites. Owing to the increasing of SiCNWs loading, the minimum reflection loss of composites raises up to −58.5 dB in the SiCNWs/PyC-SiC_f/SiC composites with an effective absorption bandwidth (reflection loss ≤ −10 dB) of 6.13 GHz. The remarkable enhancement of electromagnetic wave absorption performances is mainly attributed to the improved dielectric loss ability, impedance matching and multiple reflections. This work provides a novel strategy in preparing SiC_f/SiC composites with excellent electromagnetic wave absorption properties.     

Insights into internal oxidation of SiC/BN/SiC composites

The article presents new observations of the physical manifestations of internal oxidation and volatilization in SiC/BN/SiC composites. The observations are made on both unbroken and broken minicomposite specimens before and after 12 h exposures at 1000°C in dry air with 10 ppm water vapor. The observations are enabled by a sample preparation method involving ion-mill sectioning and polishing. Complementary analyses of volatilization and closure of resulting gaps are also presented. The observations show that BN is generally consumed in two stages: (i) through reaction with oxygen along the interfaces with both the fiber and the matrix, producing two concentric annular pockets of borosilicate glass and an intervening annulus of progressively thinning BN; and (ii) subsequent volatilization, through the reaction of boria with trace amounts of water vapor in the environment to form borohydroxide gases. The spatial extent to which these processes proceed is governed by a competition between the outward diffusion of reaction gases through both matrix cracks and interface gaps produced by boria volatilization, and the formation of oxides on the newly exposed surfaces of fibers, matrix, and coating.     

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.     

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.