Influence of precursor concentration on the densification efficiency and properties of SiC/SiC composites

The SiC/SiC composites were manufactured by polymer precursor impregnation pyrolysis process with near stoichiometric SiC fiber 2D preform as the reinforcing phase, the mixed solution of polycarbosilane (PCS), and xylene as impregnant. The effects of PCS concentration on the densification process, microstructure, and mechanical behavior of SiC/SiC composites were investigated using mechanical property testing, scanning electron microscopy, and other characterization techniques. Results showed the porosity and flexural strength of SiC/SiC composites increased first and then decreased with the increase of PCS concentration. When the concentration of PCS was 55% and 60%, the flexural strength of SiC/SiC composites reached 565.77 and 573.02 MPa, respectively. The mechanical behavior of SiC/SiC composites presented typical pseudoplastic characteristics such as fiber pulling-out, fiber bridging, and interface layer peeling, which would meet the dual requirements of optimizing the matrix and interface structure.     

Microstructure and Mechanical Properties of Compact SiC/SiC Composite Fabricated with an Infiltrative Liquid Precursor

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature and fusion applications. In this study, SiC/SiC composite was fabricated by polymer impregnation and pyrolysis process with LPVCS, a liquid precursor with active Si–H and ‐CH=CH₂ groups. The cross‐link and ceramization processes of LPVCS were studied and SiC/SiC composite was fabricated with LPVCS. The porosity and mechanical properties of the SiC/SiC composite was investigated, and the results indicated that the SiC/SiC composite exhibited low porosity and superior mechanical properties owing to the compact matrix derived from LPVCS.     

Failure mechanism and tensile constitutive model of the unidirectional-laminated Cf/SiC–Al composites

The unidirectional-laminated C_f/SiC–Al composites were prepared by using precursor infiltration and pyrolysis (PIP) and vacuum pressure infiltration processes. Bulk density and open porosity of as-prepared C_f/SiC–Al composites were characterized which showed a large number of pores in the unidirectional-laminated carbon fiber preform were filled with SiC and Aluminum alloy matrix. The uniaxial tensile tests were conducted to study the mechanical properties. The fracture surface and cross-section of tensile specimens were characterized to clarify the failure mechanism. The results showed that under the action of load, the propagation of microcracks in matrix led to interface debonding, fiber fracture and pull-out. According to the stress-displacement behavior and analysis of damage process, the prediction formulas of the linear proportional limit stress value and the tensile strength value were proposed. A bilinear constitutive model was established based on the assumption of the damage process which well characterized constitutive response of the composites.     

In situ growth of a carbon interphase between carbon fibres and a polycarbosilane-derived silicon carbide matrix

An in situ grown carbon interphase between C fibres and a SiC matrix has been produced by impregnation and pyrolysis of a polycarbosilane–xylene solution. The microstructures of the carbon interphase and the effects on the mechanical properties of the C fibre reinforced SiC (C/SiC) composites were investigated using transmission and scanning electron microscopy. It was found that the carbon interphase was a turbostratic carbon with high porosity. The resulting C/SiC composites were found to exhibit improved mechanical properties with respect to the interphase-free composite.     

Manufacture and characterisation of a low cost carbon fibre reinforced C/SiC dual matrix composite

A porous two-dimensional C/C composite was produced via the polymer pyrolysis route using phenolic resin as the matrix precursor and polyacrilonitrile- (PAN-) or pitch-based carbon fibres as reinforcement. The resulting C/C composites were then densified using a modified polysilane followed by pyrolysis to convert the polymer into silicon carbide, sealing the pores in the C/C composite. Aiming to increase the ceramic yield of the infiltrated polysilane and to reduce its volumetric shrinkage during pyrolysis the polymer’s curing behaviour was modified by catalytic addition of 0.1% dicobaltoctacarbonyl [Co₂(CO)₈]. The densification procedure is very efficient in sealing cracks in the C/C composite with SiC. The obtained carbon fibre reinforced C/SiC dual matrix composites were subjected to flexural tests and dynamic mechanical analysis. The flexural and visco-elastic properties of the composite are dominated by the strength of the fibre/matrix interface rather than by the fibre strength or modulus. A correlation between the mechanical loss factor (tan δ) and the fracture behaviour of the composite is suggested.     

Advanced technology for fabrication of reaction-bonded SiC with controlled composition and properties

An advanced fabrication technology of reaction-bonded SiC is developed, which includes the preparation of a C/SiC preform by repeated cycles of phenolic resin impregnation and pyrolysis, followed by infiltration with silicon melt. The use of different number of impregnation stages provides control of carbon content in the preform and the corresponding SiC content in final ceramics. The effect of the impregnation number on the preform characteristics and ceramics composition, thermal and mechanical properties are investigated comprehensively. With an increase of impregnation number up to four, SiC fraction in the ceramics enlarges to 93 vol%, thermal conductivity and Young’s modulus increase to 186 W/(m?K) and 427 GPa respectively, which are superior to most reaction-bonded SiC. Flexural strength (225 MPa) and thermal expansion coefficient (2?10^?6 K^-1) are not dependent on the impregnation number. The obtained results provide an opportunity to design and fabricate reaction-bonded SiC ceramics with a given set of properties.     

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.     

Interaction of fiber matrix bonding in SiC/SiC ceramic matrix composites

Non-oxide ceramic matrix composites (CMC) based on SiC fibers with SiC matrix were fabricated by polymer infiltration and pyrolysis (PIP) and characterized regarding their microstructural features and their mechanical properties. The fiber preform was made using winding technology. During the winding process, the SiC fiber roving was impregnated by a slurry containing SiC powder and sintering additives (Y₂O_3, Al₂O₃ and SiO₂). This already helped to achieve a partial matrix formation during the preform fabrication. In this way, the number of PIP cycles to achieve composites with less than 10% open porosity could be reduced significantly. Additionally, damage-tolerant properties of the composites were obtained by an optimal design of the matrix properties although only uncoated fibers were used. Finally, composites with a strength level of about 500 MPa and a damage-tolerant fracture behavior with about 0.4% strain to failure were obtained.     

Effects of CVI SiC amount and deposition rates on properties of SiCf/SiC composites fabricated by hybrid chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) routes

The SiC_f/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t₀, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t₀, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiC_f/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiC_f/SiC composites for industrial applications.     

Microstructural evolution and mechanical performances of SiC/SiC composites by polymer impregnation/microwave pyrolysis (PIMP) process

SiC/SiC composites were prepared by polymer impregnation/microwave pyrolysis (PIMP) process, and their microstructural evolution and the mechanical performances were characterized. Using non-coated Tyranno SA fiber preforms as reinforcement and impregnation with only allylperhydropolycarbosilane (AHPCS) into the preforms, Tyranno SA/SiC composite (TSA/SiC) with higher density was obtained. While using carbon-coated Tyranno SA fiber preforms, Tyranno SA/C/SiC composite (TSA/C/SiC) with lower density were also fabricated. In this composite, SiC particulate was loaded with polymer precursor (AHPCS) in the first cycle impregnation. Microstructural observation revealed that pore and crack formation was affected by processing conditions. Bending strength was also dependent on the microstructural evolution of the samples. In TSA/SiC composite, relatively strong interfaces contribute to effective load transfer so that higher bending strength could be reached. In the TSA/C/SiC composite, weak interfaces provide a relatively lower strength. Meanwhile, different microstructural evolution and interfacial properties of the composites lead to the variation of the fracture behaviors.     

Novel application of ceramic precursors for the fabrication of composites

Preceramic polymers are enabling the development of a variety of advanced shaping methods which, in turn, make possible new and cost-effective approaches for the fabrication of composite materials. This opens new perspectives for the mass production of composites which might, for example, be used in cost-sensitive areas of application in the machine and automobile industries. In two examples it will be shown how preceramic polymers can be used to obtain both metal matrix composites (MMC) and ceramic matrix composites (CMC). Their properties will be discussed in particular with respect to the usage of a preceramic polymer.The first example shows an approach to manufacturing short-fibre-reinforced CMCs by means of a plastic forming technique which involves mixing of either carbon or SiC fibres, ceramic fillers and a viscous ceramic precursor. The precursor permits a fibre-reinforced ceramic with a low porosity to be obtained. The role of the precursor in the whole process and the resulting material properties will be discussed.The second example shows a method for fabricating porous SiC ceramic preforms which are subsequently infiltrated with aluminium to form a MMC. By using the precursor route, a machinable preform with tailored porosity can be produced. Correlations between precursor, preform and MMC properties will be drawn.     

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.     

模压压力对SiC/SiC复合材料力学性能及力学行为的影响

采用热模压辅助聚合物先驱体浸渍裂解工艺制备了国产近化学计量比SiC纤维增强SiC陶瓷基复合材料,通过阿基米德排水法和SEM技术对SiC/SiC复合材料致密化过程进行表征,采用弯曲强度、拉伸强度和断裂韧性对SiC/SiC复合材料力学性能和力学行为进行评价。研究表明,热模压压力是影响材料结构和性能的重要因素,热模压在提升材料致密度的同时,亦造成纤维的损伤。随着热模压压力的增加,SiC/SiC复合材料力学性能先增加后降低。热模压压力适中时,致密度增加因素占优,材料力学性能较为优异;热模压压力较大时候,热模压操作对纤维性能的损伤因素逐渐凸显,基体致密化和纤维损伤两种作用机制相当。     

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.     

Effects of fabrication processes on the properties of SiC/SiC composites

Precursor infiltration and pyrolysis (PIP) and chemical vapor infiltration (CVI) were used to fabricate SiC/SiC composites on a four-step 3D SiC fibre preform deposited with a pyrolytic carbon interface. The effects of fabrication processes on the microstructure and mechanical properties of the SiC/SiC composites were studied. Results showed the presence of irregular cracks in the matrix of the SiC/SiC composites prepared through PIP, and the crystal structure was amorphous. The room temperature flexural strength and modulus were 873.62 MPa and 98.16 GPa, respectively. The matrix of the SiC/SiC composites prepared through CVI was tightly bonded without cracks, the crystal structure had high crystallinity, and the room temperature bending strength and modulus were 790.79 MPa and 150.32 GPa, respectively. After heat treatment at 1300 °C for 50 h, the flexural strength and modulus retention rate of the SiC/SiC composites prepared through PIP were 50.01% and 61.87%, and those of the composites prepared through CVI were 99.24% and 96.18%, respectively. The mechanism of the evolution of the mechanical properties after heat treatment was examined, and the analysis revealed that it was caused by the different fabrication processes of the SiC matrix. After heat treatment, the SiC crystallites prepared through PIP greatly increased, and the SiOxCy in the matrix decomposed to produce volatile gases SiO and/or CO, ultimately leading to an increase in the number of cracks and porosity in the material and a decrease in the material load-bearing capacity. However, the size of the SiC crystallites prepared through CVI hardly changed, the SiC matrix was tightly bonded without cracks, and the load-bearing capacity only slightly changed.     

Friction and wear properties of C/C–SiC braking composites

Two series of C/C–SiC composites were fabricated via precursor infiltration pyrolysis (PIP) and chemical vapor infiltration (CVI) using porous C/C composites with different original densities as preforms, respectively. The tribological characteristics of C/C–SiC braking composites were investigated by means of MM-1000 type of friction testing machine. The friction and wear behaviors of the two series of composites were compared and the factors that influence the friction and wear properties of C/C–SiC composites were discussed. Results show that the friction and wear properties relate close-knit to the content of SiC and porosity. As the original preform density increasing, the content of SiC and porosity decrease, and then the friction coefficient increases obviously, the braking time and the wear rate both decrease. Preparation techniques play an important role in the tribological properties of C/C–SiC composites. Compared with PIP process, the samples from CVI have a little higher friction coefficient, shorter braking time and higher wear rate.     

Effect of heat treatment on the mechanical properties of PIP–SiC/SiC composites fabricated with a consolidation process

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been considered as candidates for heat resistant and nuclear materials. Three-dimensional (3D) SiC/SiC composites were fabricated by the polymer impregnation and pyrolysis (PIP) method with a consolidation process, mechanical properties of the composites were found to be significantly improved by the consolidation process. The SiC/SiC composites were then heat treated at 1400 °C, 1600 °C and 1800 °C in an inert atmosphere for 1 h, respectively. The effect of heat treatment temperature on the mechanical properties of the composites was investigated, the mechanical properties of the SiC/SiC composites were improved after heat treatment at 1400 °C, and conversely decreased with increased heat treatment temperature. Furthermore, the effect of heat treatment duration on the properties of the SiC/SiC composites was studied, the composites exhibited excellent thermal stability after heat treatment at 1400 °C within 3 h.     

界面类型对三维Nextel 720纤维增韧莫来石陶瓷基复合材料力学性能的影响

采用化学气相渗透工艺在Nextel 720纤维表面制备PyC和PyC／SiC两种涂层，然后以正硅酸乙酯和异丙醇铝作为先驱体，以先驱体浸渗热解法制备三维Nextd 720纤维增韧莫来石陶瓷基复合材料，比较分析了两种涂层复合材料的力学性能和断裂模式。结果表明：具预先涂覆PyC的复合材料中纤维与基体直接接触，发生烧结形成强结合界面，复合材料脆性断裂，三点抗弯强度仅56MPa。PyC／SiC涂层则演化为间隙／SiC复合界面层，SiC成为阻滞纤维与基体接触的阻挡层，间隙保证了纤维拔出，复合材料韧性断裂且三点抗弯强度高达267．2MPa。     

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 C_f/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 C_f/HfC-SiC with higher SiC/HfC ratios. Meanwhile, both weight loss and erosion depth of C_f/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.     

Entwicklung von Siliciumcarbid-Verbundwerkstoffen

Development of silicon carbide composites . This paper describes the production of fibre-reinforced SiC composites applying the methods of chemical vapour impregnation (CVI), reaction bonding by liquid silicon impregnation (RB-SiC), and impregnation with subsequent polymerisation and thermal decomposition of organosilicon compounds. The chemical and physical fundamentals of these processes are discussed. Composites of different fibre/matrix combinations are introduced; the most promising combinations can be selected from the experimental results presented.