改性SiC短切纤维体积分数对SiC_(sf)/LAS复合材料介电性能的影响

以改性SiC短切纤维为添加剂,用热压法制备了SiCsf/LAS复合材料,考察了纤维的显微结构以及纤维和复合材料在8.2～12.4GHz频率范围内的微波介电性能。结果表明,当SiC纤维的体积分数为2.92时,纤维混合体介电常数实部、虚部最大,分别为38～25、40～20。高复介电常数的SiC纤维使SiCsf/LAS复合材料比LAS具有更高的介电常数,材料中无富碳界面层的形成。当SiC纤维的体积分数为3时,复合材料介电常数实部、虚部以及损耗角正切值最大,其均值分别为58、25和0.45。     

纤维氧化处理对SiCsf/SiC复合材料力学性能的影响

以Y2O3、Al2O3为烧结助剂,采用无压烧结法制备短碳化硅纤维(2～4mm)增强碳化硅(ShortSiCfiberreinforcedSiCcomposite,SiCsf/SiC)复合材料,研究了纤维氧化处理对SiCsf/SiC复合材料结构及力学性能的影响。采用X射线衍射(XRD)、扫描电镜(SEM)以及力学性能试验机对材料进行结构表征和力学性能测试。结果表明:纤维氧化处理后,复合材料的弯曲强度和断裂韧性均有大幅提高。当纤维含量达到5wt%时,复合材料断裂韧性为5.41MPa.m1/2,与原始纤维增强SiC样品相比,提高了6.5%;与无纤维增强SiC样品相比,提高了27%。扫描电镜显示纤维氧化处理后,纤维与基体结合紧密。     

竹炭/SiC复合材料结构及其吸波性能

以Si粉、竹炭为原料,采用包埋法制备具有类蜂窝结构的竹炭(bamboo carbon,BC)/SiC复合材料。结果表明:BC/SiC复合材料主要由β–SiC相、少量α–SiC相和非晶碳组成。BC/SiC复合材料呈蜂巢状多孔结构,孔内壁分布着直径大小不同、相互熔结连接的SiC三维聚集体结构层,其断裂韧性为18.8 MPa·m1/2,弯曲强度为34.5 MPa。BC/SiC复合材料形成的两相界面,提高了BC/SiC复合材料的吸波性能:介电常数实部最大值为9.14,虚部最大值为2.06;样品厚度为2.5 mm时,在10.7 GHz处,最低反射系数为–10.16 dB;反射系数–8 dB的有效吸收带宽达2.1 GHz。     

竹炭/SiC复合材料结构及其吸波性能

以Si粉、竹炭为原料,采用包埋法制备具有类蜂窝结构的竹炭(bamboo carbon,BC)/SiC复合材料。结果表明:BC/SiC复合材料主要由β–SiC相、少量α–SiC相和非晶碳组成。BC/SiC复合材料呈蜂巢状多孔结构,孔内壁分布着直径大小不同、相互熔结连接的SiC三维聚集体结构层,其断裂韧性为18.8 MPa·m1/2,弯曲强度为34.5 MPa。BC/SiC复合材料形成的两相界面,提高了BC/SiC复合材料的吸波性能:介电常数实部最大值为9.14,虚部最大值为2.06;样品厚度为2.5 mm时,在10.7 GHz处,最低反射系数为–10.16 dB;反射系数–8 dB的有效吸收带宽达2.1 GHz。     

竹炭/SiC复合材料结构及其吸波性能

以Si粉、竹炭为原料,采用包埋法制备具有类蜂窝结构的竹炭(bamboo carbon,BC)/SiC复合材料。结果表明:BC/SiC复合材料主要由β–SiC相、少量α–SiC相和非晶碳组成。BC/SiC复合材料呈蜂巢状多孔结构,孔内壁分布着直径大小不同、相互熔结连接的SiC三维聚集体结构层,其断裂韧性为18.8 MPa·m1/2,弯曲强度为34.5 MPa。BC/SiC复合材料形成的两相界面,提高了BC/SiC复合材料的吸波性能:介电常数实部最大值为9.14,虚部最大值为2.06;样品厚度为2.5 mm时,在10.7 GHz处,最低反射系数为–10.16 dB;反射系数–8 dB的有效吸收带宽达2.1 GHz。     

竹炭/SiC复合材料结构及其吸波性能

以Si粉、竹炭为原料,采用包埋法制备具有类蜂窝结构的竹炭(bamboo carbon,BC)/SiC复合材料。结果表明:BC/SiC复合材料主要由β-SiC相、少量α-SiC相和非晶碳组成。BC/SiC复合材料呈蜂巢状多孔结构,孔内壁分布着直径不同、相互熔结连接的SiC三维聚集体结构层,其断裂韧性为18.8 MPa·m~(1/2),弯曲强度为34.5 MPa。BC/SiC复合材料形成的两相界面,提高了BC/SiC复合材料的吸波性能:介电常数实部最大值为9.14,虚部最大值为2.06;样品厚度为2.5 mm时,在10.7 GHz处,最低反射系数为–10.16 dB;反射系数–8 dB的有效吸收带宽达2.1 GHz。     

纤维增强铝硅酸盐玻璃陶瓷复合材料的摩擦学特性研究

在UMT-2微观磨损试验机（USA）上研究了SiC纤维增强铝硅酸盐玻璃陶瓷复合材料的摩擦学特性,且对摩擦表面进行了SEM观察和分析。研究结果表明：随着SiC纤维含量的增加,摩擦系数逐渐降低,但变化幅度较小。而当纤维含量（体积）低于25％时,复合材料的磨损量明显降低,而显微硬度却有较大提高;超过25％时,继续增加纤维的含量会导致复合材料耐磨性下降。SiC纤维增强铝硅酸盐玻璃陶瓷复合材料的摩擦系数随着载荷的增大显现先增大后减小的趋势,并且在载荷140N时达到最大值,而磨损量随着载荷的增大而增加。复合材料的主要磨损失效形式为磨粒磨损。     

热压条件对短切SiC_f/LAS复合材料介电/力学性能的影响

采用热压烧结法制备出致密度超过90%的短切SiCf增强LAS玻璃陶瓷基复合材料,讨论了热压温度与压力对复合材料性能的影响。力学测试表明,当热压温度由1200℃上升到1280℃,复合材料断裂强度从124MPa下降到80MPa,断裂韧性从3.27MPa.m1/2下降到2.92MPa.m1/2;当热压压力由20MPa提高到34MPa,复合材料断裂强度从124MPa下降到86MPa,断裂韧性从3.27MPa.m1/2下降到3.00MPa.m1/2。断口形貌SEM观察结果表明,因纤维掺入过量,φ(SiCf)=36%,使纤维与基体结合较差;而过高的热压温度与压力会使界面反应加剧,破坏纤维强度及纤维与基体的结合。介电性能测试表明,在8～12GHz频率,复合材料复介电常数的实部ε′由基体的7.6增大到10～70,虚部ε″由基体的0.34增大到60～160,介电损耗tgδ由基体的0.04增大到2～20,并具有明显的频散效应。而且,随热压温度升高或者热压压力的增加,复合材料ε′增大,而ε″与tgδ减小。复合材料具有成为电损耗型宽带微波吸收材料的潜力。     

SiC纤维/LAS玻璃陶瓷的热压工艺研究

研究了Nicalon SiC纤维/LAS玻璃陶瓷复合材料的热压工艺。经探索试验后,进行了热压温度、时间、压力及在950℃析晶处理时间四因素三水平L_s(3~4)正交实验。分析了这些因素对复合材料密度、强度、韧性及从复合材料中萃取出来的纤维强度的影响,获得了最佳工艺参数及强度、韧性分别为612MPa及15MPa·m~(1/2)的Nicalon SiC纤维/LAS玻璃陶瓷复合材料。     

短切碳纤维含量对Csf/SiC复合材料力学性能的影响

以Si作为主要烧结助剂,采用热压烧结法制备了短切碳纤维-碳化硅(short carbon fiber reinforced SiC composite,Csf/SiC)复合材料.采用X射线衍射仪、扫描电镜、硬度仪以及力学性能试验机等,研究了Csf含量对所制备材料的结构、组成、形貌及复合材料的弯曲强度、Vickers硬度和断裂韧性的影响.结果表明:采用热压法能制备出致密且Csf分布均匀的Csf/SiC复合材料.Csf/SiC复合材料的弯曲强度随Csf含量增加先增大后减小,含15%(体积分数,下同)Csf的Csf/SiC样品强度最高,达到466MPa,并且Csf含量小于30%的Csf/SiC样品强度高于无纤维SiC材料.材料的Vickers硬度随Csf含量增加而降低.Csf/SiC样品的断裂韧性随Csf含量增加而逐渐增大,Csf含量为53%时,达到最大为5.5MPa·m1/2,与无纤维SiC样品相比,增加近2倍.     

Additive manufacturing of Csf/SiC composites with high fiber content by direct ink writing and liquid silicon infiltration

Direct ink writing (DIW) provides a new route to produce SiC-based composites with complex structure. In this study, we additive manufactured short carbon fiber reinforced SiC ceramic matrix composites (Csf/SiC composites) with different short carbon fiber content through direct ink writing combined with liquid silicon infiltration (LSI). The effects of short carbon fiber content on the microstructure and mechanical properties of the DIW green parts and the final Csf/SiC composites were investigated. The results showed that the Csf content played an important role in maintaining the structure of the green parts. As the Csf content increases, the dimension deviation ratio of the sample decreased at all stages. With the Csf content of 40 vol%, the final Csf/SiC composite had low free Si content and high β-SiC content. The maximum density, tensile strength and bending strength of the Csf/SiC composites were 2.88 ± 0.06 g/cm³, 53.68 MPa and 253.63 MPa respectively. It is believed that this study can give some understanding for the additive manufacturing of fiber reinforced ceramic matrix composites.     

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.     

Effects of a coupling agent on the mechanical and thermal properties of ultrahigh molecular weight polyethylene/nano silicon carbide composites

Ultra‐high‐molecular‐weight polyethylene (UHMWPE)/nano silicon carbide (nano‐SiC) composites were prepared by compression molding. The effects of a coupling agent and the content of the filler on the filler dispersion and the mechanical and thermal properties of the composites were investigated. The results show that the mechanical properties of the composites first increased and then decreased with increasing SiC content. The macromolecular coupling agent exhibited a much better reinforcing effect than the small‐molecule coupling agent. The tensile strength of the composites with 3‐aminopropyltriethoxysilane (KH550), γ‐methacryloxypropyltrimethoxysilane (KH570), and silicone powders reached its maximum value when the silicon carbide (SiC) content was 3%. We found that a web of the UHMWPE/SiC/coupling agent was formed and played a significant role in improving the heat resistance of the composites. In addition, appropriate amounts of SiC could increase the crystallinity of UHMWPE via a process of heterogeneous nucleation. The comprehensive performance of the KH550/silicone/SiC/UHMWPE composites was the best. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Fabricating thick-section carbon fiber/silicon carbide composites by machining-aided chemical vapor infiltration

Chemical vapor infiltration (CVI) is a prominent process for fabricating carbon fiber/silicon carbide (C/SiC) composites. However, the preparation of enclosed-structure or thick-section C/SiC composites/components with CVI remains a challenge, since the difficulty of densification increases. Here, machining-aided CVI (MACVI) is designed, in which infiltration-assisting holes are utilized (machined) to increase matrix deposition. To validate the approach, thick-section (10 mm thick) C/SiC composites were fabricated by MACVI. Porosity analysis and microstructure characterization were performed on the fabricated MACVI C/SiC composites and their CVI counterparts, showing a density increase up to 12.7% and a porosity decrease up to 32.1%. The mechanical behavior of the fabricated MACVI C/SiC composites was characterized, showing an increase of flexural strength by a factor of 1.72 at most. Besides, the toughness also largely increases. Both the porosity decrease and the strength and toughness increase brought by MACVI demonstrate its effectiveness for fabricating stronger and tougher enclosed-structure or thick-section ceramic matrix composites/components.     

Effect of Carbon and Silicon Carbide/Carbon Interlayers on the Mechanical Behavior of Tyranno-SA-Fiber-Reinforced Silicon Carbide-Matrix Composites

Several CVI-SiC/SiC composites were fabricated and the mechanical properties were investigated using unloading–reloading tensile tests. The composites were reinforced with a new Tyranno-SA fiber (2-D, plain-woven). Various carbon and SiC/C layers were deposited as fiber/matrix interlayers by the isothermal CVI process. The Tyranno-SA/SiC composites exhibited high proportional limit stress (∼120 MPa) and relatively small strain-to-failure. The tensile stress/strain curves exhibited features corresponding to strong interfacial shear and sliding resistance, and indicated failures of all the composites before matrix-cracking saturation was achieved. Fiber/matrix debonding and relatively short fiber pullouts were observed on the fracture surfaces. The ultimate tensile strength displayed an increasing trend with increasing carbon layer thickness up to 100 nm. Further improvement of the mechanical properties of Tyranno-SA/SiC composites is expected with more suitable interlayer structures.     

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.     

Effect of oxidation treatment on KD–II SiC fiber–reinforced SiC composites

Continuous silicon carbide fiber–reinforced silicon carbide matrix (SiC_f/SiC) composites have developed into a promising candidate for structural materials for high–temperature applications in aerospace engine systems. This is due to their advantageous properties, such as low density, high hardness and strength, and excellent high temperature and oxidation resistance. In this study, SiC_f/SiC composites were fabricated via polymer infiltration and pyrolysis (PIP) with the lower–oxygen–content KD–II SiC fiber as the reinforcement; a mixture of 2,4,6,8–tetravinyl–2,4,6,8–tetramethylcyclotetrasiloxane (V4) and liquid polycarbosilane (LPCS), known as LPVCS, was used as the precursor; while pyrolytic carbon (PyC) was used as the interface. The effects of oxidation treatment at different temperatures on morphology, structure, composition, and mechanical properties of the KD–II SiC fibers, SiC matrix from LPVCS precursor conversion, and SiC_f/SiC composites were comprehensively investigated. The results revealed that the oxidation treatment greatly impacted the mechanical properties of the SiC fiber, thereby significantly influencing the mechanical properties of the SiC_f/SiC composite. After oxidation at 1300 °C for 1 h, the strength retention rates of the fiber and composite were 41% and 49%, respectively. In terms of the phase structure, oxidation treatment had little effect on the SiC fiber, while greatly influencing the SiC matrix. A weak peak corresponding to silica (SiO₂) appeared after high–temperature treatment of the fiber; however, oxidation treatment of the matrix led to the appearance of a very strong diffraction peak that corresponds to SiO₂. The analysis of the morphology and composition indicated cracking of the fiber surface after oxidation treatment, which was increasingly obvious with the increase in the oxidation treatment temperature. The elemental composition of the fiber surface changed significantly, with drastically decreased carbon element content and sharply increased oxygen element content.