The fabrication and mechanical properties of SiC/SiC composites prepared by SLS combined with PIP

Traditionally, SiC components with complex shapes are very difficult or even impossible to fabricate. This paper aims to develop a new manufacturing process, combining selective laser sintering (SLS), cold isostatic pressing (CIP) and polymer infiltration pyrolysis (PIP), to manufacture complex silicon carbide parts and improve the mechanical properties of silicon carbide ceramic parts. The density and porosity of SiC/SiC composites were measured. Furthermore, the mechanical properties of the specimens with cold isostatic pressing and the specimens without cold isostatic pressing were compared. The bending strength of the specimens with cold isostatic pressing was 201?MPa, and the elastic modulus was 1.27?GPa. And, the bending strength of the specimens without cold isostatic pressing was 142?MPa, and the elastic modulus was 0.88?GPa. Increasing the density of SiC/SiC can enhance the mechanical properties of SiC/SiC composites.     

Significance of modification of slurry infiltration process for the precursor impregnation and pyrolysis process of SiCf/SiC composites

Several intermediate steps were applied before the precursor infiltration and pyrolysis process to improve the infiltration of SiC slurry for promoting the infiltration of SiC slurry into fiber voids. These steps include sonication, popping, electrophoretic deposition, vacuum infiltration and cold isostatic pressing (CIP). The intermediate processes, especially popping and CIP, had a beneficial effect on green density enhancement and improving the homogeneous infiltration of the slurry into fiber fabrics. The density of the SiC_fiber/SiC_filler green body was 2.20 g/cm³, which corresponded to 68 % of relative density. The SiC_f/SiC composite has a high density of 2.65 g/cm³ after seven PIP cycles.     

Mechanical,thermal, and dielectric properties of SiCf/SiC composites reinforced with electrospun SiC fibers by PIP

Electrospun unidirectional SiC fibers reinforced SiC_f/SiC composites (e-SiC_f/SiC) were prepared with ∼10% volume fraction by polymer infiltration and pyrolysis (PIP) process. Pyrolysis temperature was varied to investigate the changes in microstructures, mechanical, thermal, and dielectric properties of e-SiC_f/SiC composites. The composites prepared at 1100 °C exhibit the highest flexural strength of 286.0 ± 33.9 MPa, then reduced at 1300 °C, mainly due to the degradation of electrospun SiC fibers, increased porosity, and reaction-controlled interfacial bonding. The thermal conductivity of e-SiC_f/SiC prepared at 1300 °C reached 2.663 W/(m∙K). The dielectric properties of e-SiC_f/SiC composites were also investigated and the complex permittivities increase with raising pyrolysis temperature. The e-SiC_f/SiC composites prepared at 1300 °C exhibited EMI shielding effectiveness exceeding 24 dB over the whole X band. The electrospun SiC fibers reinforced SiC_f/SiC composites can serve as a potential material for structural components and EMI shielding applications in the future.     

Effects of in-situ carbon layer and volume fraction of SiC fiber on the mechanical properties and fracture behaviors of SiCf/SiC composites fabricated by a modified PIP method

Unidirectional SiC_f/SiC composites (UD SiC_f/SiC composites) with excellent mechanical properties were successfully fabricated by a modified PIP method which involved the preparation of film-like matrix containing carbon layer with a low concentration PCS solution followed by the rapid densification of composites with a high concentration PCS solution. Carbon layers were in-situ formed and alternating with SiC layers in the as-received matrix. The unique microstructure endows the composites with appropriate interfacial bonding state, good load transfer ability of interphase and matrix and load bearing ability of fiber, and great crack deflection capacity, which ensures the synergy of high strength and toughness of composites. It is also found that the fiber volume fraction in the preform makes a non-negligible effect on the distribution of interphase and matrix, of which the reasonable adjustment can be utilized to optimize the mechanical properties of composites. Compared with the composites only using high concentration PCS solution, the UD SiC_f/SiC composites prepared by the modified PIP method exhibit superior mechanical properties. Ultrahigh flexural strength of 1318.5 ± 158.3 MPa and fracture toughness of 47.6 ± 5.6 MPa·m^1/2 were achieved at the fiber volume fraction of 30%.     

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 C_f/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-C_f/SiC composites possessed a higher density (2.13 g/cm³) than that of the PCS-C_f/SiC composites even though the PIP cycle for densification was obviously reduced, which means a higher densification efficiency. Logically, the LPCS-C_f/SiC composites exhibited superior mechanical properties. The shorter length and rougher surfaces of pulled-out fibers indicated the LPCS-C_f/SiC composites to possess a stronger bonding between matrix and PyC interphase compared with the PCS-C_f/SiC composites.     

Mechanical properties of the SiCf/SiC composites reinforced with KD-I and KD-II fibers fabricated assisted by a microwave heating method

SiC_f/SiC composites using KD-I and KD-II SiC fibers braided preforms as the reinforcements were fabricated by applying the polymer impregnation and pyrolysis (PIP) technique with a microwave heating assistance. The microwave heating temperature was 1100 °C, 1200 °C, 1300 °C, and 1400 °C, respectively. Microstructures, flexure properties, and fracture behaviors of the composites were investigated. The KDIISiC_f/SiC composites exhibited higher flexure properties and improved non-brittle fracture characteristics than those of the KD-ISiC_f/SiC composites. The differences in the flexural properties, fracture behaviors and microstructures between the KD-I and KDIISiC_f/SiC composites were discussed based on the tensile properties of the SiC filaments and the interfacial bonding statues in the composites.     

High-performance 3D SiC/PyC/SiC composites fabricated by an optimized PIP process with a new precursor and a thermal molding method

To improve the efficiency of the polymer impregnation and pyrolysis (PIP) process and the mechanical properties for SiC/SiC composites, 3-dimensional (3D) SiC/SiC were fabricated by a PIP process with a new precursor polymer and the thermal molding method. Liquid polyvinylcarbosilane (LPVCS) with active Si–H and –CHåCH₂ groups was adopted as the SiC matrix precursor. The SiC/SiC composites with superior mechanical properties were efficiently fabricated. The fiber volume of the SiC/SiC was 50.4%. The bulk density and porosity of the SiC/SiC composites were 2.16 g cm⁻³ and 15.4% respectively. The flexural strength and fracture toughness of the SiC/SiC composites were 637.5 MPa and 29.8 MPa m^1/2 respectively. The influences of LPVCS and molding pressure on the performances of the SiC/SiC composites were discussed in-depth.     

Mechanical and dielectric properties of SiCf/BN/SiBCN composites via different synthesis technologies

SiC fibers reinforced SiBCN ceramic matrix composites (SiC_f/BN/SiBCN composites) were synthesized by direct chemical vapor infiltration (CVI), polymer infiltration pyrolysis (PIP) or chemical vapor infiltration combined with polymer infiltration pyrolysis (CVI + PIP). It is shown that the insertion of a continuous and dense SiBCN matrix via the CVI process improves the flexural strength and modulus. Interface debonding and fiber pullout happened with 50–100 nm BN interface in CVI and CVI + PIP SiC_f/BN/SiBCN composites. The relative complex permittivity was measured in X-band. Higher ε′′ values in CVI-containing composites can be observed, which can be attributed to the accumulation of C and SiC phases and a multilayer matrix. Strong electromagnetic wave attenuation ability was obtained with high dielectric loss.     

Effect of pyrolysis temperature on the mechanical evolution of SiCf/SiC composites fabricated by PIP

Three types of SiC_f/SiC composites with a four-step three-dimensional SiC fibre preform and pyrocarbon interface fabricated via precursor infiltration and pyrolysis at 1100 °C, 1300 °C, and 1500 °C were heat-treated at 1300 °C under argon atmosphere for 50 h. The effects of the pyrolysis temperature on the microstructural and mechanical properties of the SiC_f/SiC composites were studied. With an increase in the pyrolysis temperature, the SiC crystallite size of the as-fabricated composites increased from 3.4 to 6.4 nm, and the flexural strength decreased from 742 ± 45 to 467 ± 38 MPa. After heat treatment, all the samples exhibited lower mechanical properties, accompanied by grain growth, mass loss, and the formation of open pores. The degree of mechanical degradation decreased with an increase in the pyrolysis temperature. The composites fabricated at 1500 °C exhibited the highest property retention rates with 90% flexural strength and 98% flexural modulus retained. The mechanism of the mechanical evolution after heat treatment was revealed, which suggested that the thermal stability of the mechanical properties is enhanced by the high crystallinity of the SiC matrix after pyrolysis at higher temperatures.     

Fabrication of 2D C/C-SiC composites using PIP based hybrid process and investigation of mechanical properties degradation under cyclic heating

2D C/C-SiC composites were fabricated using PIP process by repeated impregnations of porous C/C composite preforms with polycarbosilane followed by pyrolysis. Effect of cyclic heating on flexural and shear strength of these composites was studied by exposing the test specimens to oxyacetylene flame for 20 s and cooling by a blast of air. The cyclic heating tests were repeated up to five times. Average flexural and shear strength of the as fabricated composites were about 330 MPa and 14.5 MPa respectively. After five heating and cooling cycles, average flexural and shear strength were reduced to 120 MPa and 5.5 MPa respectively. SEM, XRD, EDAX and XPS studies were also carried out to investigate the causes of strength reduction. Oxidation started preferentially at carbon matrix through the cut ends of the weft fibers. Oxidative damage due to repeatedly heating cooling was found to be much smaller in through-thickness direction due to passive oxidation of SiC matrix while severe damage was observed parallel to the fabric layers.     

Flexural behaviors and microstructures of SiC/SiC composites fabricated by microwave sintering assisted with heat molding process

To improve densification degree and reduce process time, microwave sintering and heat molding method were combined to prepared SiC matrix reinforced SiC (SiC/SiC) composite via polymer infiltration and pyrolysis process (PIP). The effects of heat molding pressures on the densification process, flexural behaviors and failure modes of the fabricated SiC/SiC were examined via scanning electron microscopy (SEM), computed tomography (CT) technique and mercury intrusion test. Results indicate that heat molding process promoted the densification degrees of SiC/SiC and adjusted the interphase bonding between SiC matrix and SiC fibers on the basis of rapid microwave heating. Owing to the appropriate interphase bonding, SiC/SiC composites fabricated under the heat molding pressure of 3 MPa had preferable flexural properties and failure mode.     

Effect of SNNWS content on the microstructure and properties of SNNWS/Si-C-N ceramic composites via PIP

Si-C-N ceramic composites containing well distributed silicon nitride nanowires (SNNWs) were fabricated by die-pressing and precursor infiltration and pyrolysis process at a low temperature. The structure, composition, mechanical and thermophysical properties of the composites were investigated. The results show that the composites consisted of amorphous SiCN, α-Si₃N₄ and α-cristobaslite. The composites with different contents of SNNWs possessed a density of 2.02–2.07 g cm^?3 and open porosity of 7.9–9.9%. SNNWs can effectively restrain the contraction of matrix with a decrease by 25% in linear shrinkage. The composites with 3 wt% SNNWs owned the highest flexural strength (83.7 MPa) and elastic modulus (54.0 GPa) at room temperature, which increase by 13.2% and 62.3% respectively, compared with pure SiCN ceramics. The SNNWs displayed good reinforcement function at high temperature due to the fact that the composites with 7 wt% SNNWs had a 96.8% retention rate of bending strength at 1200 °C. The composites had relatively low coefficient of thermal expansion and thermal diffusivity which were less than 2.2 × 10^?6 K^?1 and 0.62 mm² s^?1, respectively.     

Improvement of high-temperature stability of PIP SiCf/SiC material through in situ grown BNNTs

In this paper, the effect of in situ grown boron nitride nanotubes (BNNTs) and preparation temperature on mechanical behavior of PIP (Precursor Infiltration and Pyrolysis) SiC_f/SiC minicomposites under monotonic and compliance tensile is investigated. In situ BNNTs are grown on the surface of SiC fibers using ball milling–annealing process. Composite elastic modulus, tensile strength, fracture strain, tangent modulus, and loading/unloading inverse tangent modulus (ITM) are obtained and adopted to characterize the mechanical properties of the composites. Microstructures of in situ grown BNNTs and tensile fracture surfaces are observed under scanning electronic microscopic (SEM). For SiC_f/SiC minicomposites with BNNTs, the elastic modulus, tensile strength, and fracture strain are all lower than those of SiC_f/SiC minicomposites without BNNTs, mainly due to high preparation temperature and the oxidation of the PyC interphase during the annealing process. Tensile stress–strain curves of SiC_f/SiC minicomposites with and without BNNTs are predicted using the developed micromechanical constitutive model. The predicted results agreed with experimental data. This work will provide guidance for predicting the service life of SiC_f/SiC composite materials and may enable these materials to become a backbone for thermal structure systems in aerospace applications.     

Mechanical properties and high temperature failure mechanism of carbon fibers reinforced polysilazane-derived SiCN ceramic matrix composites via PIP process

Carbon fibers reinforced SiCN ceramic matrix composites (C/SiCN) were prepared by PIP with homemade polysilazanes (PSN-T03). The PSN-T03 had a viscosity of less than 500 mPa·s and favorable good chemical stability at 80 ℃, which made it quite applicable for PIP process. PSN-T03’ curing reactions happened at temperatures ranging from 149℃ to 205℃, and conversion to amorphous SiCN ceramics could be finished at temperatures above 700℃ with a 70+ wt% ceramic yield. Tensile strength of C/SiCN composites were 230 MPa and 350 MPa respectively at room and high temperature (1400℃), and bending strength reached 380 MPa and 530 MPa. SiCN ceramics began to lose weight at 1400℃ due to the carbothermic reduction of the silicon nitride phase under inert atmosphere, which results in collapse of the mechanical properties of C/SiCN composites above 1400℃.     

Enhanced mechanical and dielectric properties of SiCf/SiC composites with silicon oxycarbide interphase

SiC_f/SiC composites with silicon oxycarbide (SiOC) interphase were successfully prepared using silicone resin as interphase precursor for dip-coating process and polycarbosilane as matrix precursor for PIP process assisted with hot mold pressing. The effects of SiOC interphase on mechanical and dielectric properties were investigated. XRD and Raman spectrum results show that SiOC interphase is composed of silicon oxycarbide and free carbon with a relatively low crystalline degree. The surface morphology of SiC fibers with SiOC interphase is smooth and homogeneous observed by SEM. The flexural strength and failure displacement of SiC_f/SiC composites with SiOC interphase vary with the thickness of interphase and the maximum value of flexural strength is 289 MPa with a failure displacement of 0.39 mm when the thickness of SiOC interphase is 0.25 µm. The complex permittivity of the composites increases from 8.8-i5.7 to 9.8-i8.3 with the interphase thicker.     

Manufacturing 2D carbon-fiber-reinforced SiC matrix composites by slurry infiltration and PIP process

Sub-micrometer SiC particles were firstly added to the preceramic solution in the first infiltration step to enhance the mechanical properties of 2D C_f/SiC composites fabricated via polymer infiltration and pyrolysis (PIP) process. The effects of pyrolysis temperature and SiC-filler content on microstructures and properties of the composites were systematically studied. The results show that the failure stress and fracture toughness increased with the increase of pyrolysis temperature. SiC filler of sub-micron scale infiltrated into the composites increased the mechanical properties. As a result, for the finally fabricated composite infiltrated with a slurry containing 40 wt.% SiC filler, the failure stress was doubled compared to that without SiC filler addition, and the fracture toughness reached ≈10 MPa m^1/2.     

Mechanical and microwave dielectric properties of SiCf/SiC composites with BN interphase prepared by dip-coating process

The BN interphase of SiC fiber-reinforced SiC matrix (SiCf/SiC) composites was fabricated by dip-coating process with boric acid and urea as precursor. The results show that the tensile strength of SiC fiber decreases about 30% after BN coating treatment, but the BN coating has little influence on the electrical resistivity of SiC fiber. Compared with the as-received SiCf/SiC composites, the SiCf/SiC composites with BN interphase exhibit a toughened fracture behavior, and the flexural strength is about 2 times that of the as-received SiCf/SiC composites. From the microstructural analysis, it can be confirmed that the BN interphase plays a key part in weakening interfacial bonding, which can improve the mechanical properties of SiCf/SiC composites remarkably. Owing to the close dielectric properties between SiC and BN, the complex permittivity of SiCf/SiC composites with and without the BN interphase is similar.     

Microstructure and properties of 2D-Cf/SiC composite fabricated by combination of CVI and PIP process with SiC particle as inert fillers

2D-C_f/SiC composite was manufactured by chemical vapor inflation (CVI) combined with polymer impregnation and pyrolysis (PIP) with SiC particle as inert fillers. The effects of CVI processes on SiC morphologies and the properties of composite were investigated. The composites were characterized by XRD, flexural strength test and SEM. The results revealed that uniform SiC coatings and nanowires were prepared when MTS/H₂ ratio of 1:8 was employed, while gradient thick coatings were fabricated as MTS/H₂ ratio of 1:1 was employed. The flexural strength of composites varied from 156 MPa at MTS/H₂ ratio of 1:1 to 233 MPa at MTS/H₂ ratio of 1:8. All of composites exhibited toughness due to significant debonding and pullout of fibers. The laminated structure of coatings on the fibers and nanowires were manufactured by combination of above different CVI process, and the obtained composites showed flexural strength of as high as 248 MPa and impressive toughness.     

Effects of the single layer CVD SiC interphases on the mechanical properties of the SiCf/SiC composites fabricated by PIP process

Owing to the degradation of the mechanical properties of the SiC fiber reinforced SiC matrix (SiCf/SiC) composites with the pyrocarbon (PyC) and BN interphases under oxidation environment and neutron irradiation, single layer SiC interphases prepared by chemical vapor deposition (CVD) process were employed to substitute for them. Effects of the CVD SiC interphases on the mechanical properties and interfacial characteristics of the SiCf/SiC composites fabricated by precursor infiltration and pyrolysis (PIP) process were investigated. Compared with the as-received SiCf/SiC composites, the SiCf/SiC composites with the single layer CVD SiC interphases exhibit an obvious toughened fracture behavior, the flexural strength of which is about 4 times that of the as-received SiCf/SiC composites. From the microstructural analysis, it can be confirmed that the SiC interphases play a key part in protecting the fibers from damage during composite preparation and weakening interfacial bonding, which can provide high in situ fiber strength and appropriate interfacial bonding strength for the SiCf/SiC composites.     

Fabrication and characterization of PIP based C/SiC composites having improved mechanical properties using high modulus M40J carbon fiber as reinforcement

PIP based C/SiC composites are fabricated using high modulus M40J carbon fiber. High ceramic yield polycarbosilane (PCS) was also synthesized in the laboratory and the same was used to infiltrate the fibrous preforms. The infiltrated preforms were pyrolyzed at three different temperatures viz. 1400, 1500 and 1600 °C and termed as set-1, set-2 and set-3. Flexural strength was determined using 3-point bend fixture and the data obtained are analyzed using Weibull distribution. Average flexural strengths were found to be 691±23 MPa, 654.6±24 MPa, and 504±31 MPa for the sets 1, 2 and 3 respectively and the corresponding Weibull moduli were found to be 27.9, 25.5 and 15.6. The composites pyrolyzed at 1400 and 1500 °C, have been found to exhibit extensive fiber pull-out and thus demonstrated pseudo-ductile fracture behavior. A relatively brittle fracture was observed for the composites pyrolyzed at 1600 °C. Area under the flexural stress and displacement curve is found to be in the ratio 1.0:0.92:0.8 for the for the sets 1, 2 and 3 respectively. The effect of the pyrolysis temperature on the mechanical properties is discussed in the light of the microstructure of the composites.