C/C复合材料抗氧化耐高温SiC陶瓷涂层的研究

采用高温反应法和PVD法在SiC工业合成炉内制备了C/C复合材料耐高温抗氧化SiC陶瓷涂层.用XRD、SEM对其物相组成和显微结构进行了表征与分析,讨论了涂层的形成机理,并研究了其高温氧化性能.研究结果表明,所制备的陶瓷涂层主要由α-SiCβ-SiC组成,晶粒发育完整,涂层表面致密、无裂纹,且与碳基体结合紧密,涂层厚度约600μm,涂层抗氧化性良好,在1500℃空气中氧化10h失重约为0.3%.     

基体改性对SiC/SiC复合材料高温抗氧化性能的影响

采用先驱体浸渍-裂解工艺结合三种基体改性方式制备了SiC/SiC复合材料,通过形貌分析和力学性能测试,分析了基体改性对Si C/SiC复合材料高温抗氧化性能的影响。研究表明,经1200℃静态空气氧化100h后,三种基体改性的复合材料弯曲强度几乎没有下降,氧化200h后,弯曲强度保留率均可达到80%;氧化300h后,复合材料内部结构没有氧化现象,表面区域界面层的氧化程度降低。改性基体中的B元素氧化生成液相封填SiC涂层表面,延缓了SiC涂层的氧化进程,并阻止氧化介质进入复合材料内部,保护纤维和界面层,从而使SiC/SiC复合材料的长时静态高温抗氧化性能明显提高。     

自愈合基体改性SiC/SiC复合材料的制备与性能研究

为了提高SiC/SiC复合材料的自愈合性能,借助扫描电镜、X射线衍射及抗氧化性能测试等手段分别对硼硅酸盐改性SiC/SiC复合材料进行微观形貌、相组成分析及抗氧化性能分析。研究结果表明:在1100℃下氧化30h后,通过ZnO:B_2O_3:SiO_2含量比为4:2:4的硼硅酸盐改性后试样失重率由未改性的2.41%降为0.89%,硼硅酸盐玻璃表面出现明显的裂纹自愈合现象,基体内部也存在自愈合基体包裹纤维的现象,但其弯曲强度由未改性的240 MPa降为225 MPa。     

水热温度对SiC–C/C复合材料表面水热电泳沉积SiCn–MoSi2复合抗氧化涂层的影响

采用水热电泳沉积法在SiC–C/C复合材料表面制备了纳米碳化硅和二硅化钼的复相(SiCn–MoSi2)抗氧化涂层。采用X射线衍射和扫描电子显微镜等对制备涂层的晶相组成、表面及断面微观结构进行了表征。研究了水热温度对制备涂层的结构及高温抗氧化性能的影响,分析了涂层在1 600℃静态氧化行为及失效机理。结果表明:外涂层主要由MoSi2和β-SiC晶相组成。复相外涂层的致密程度、厚度及抗氧化性能随着水热温度的升高而提高。SiCn–MoSi2/SiC复合涂层具有较好的抗氧化和抗热震能力,在1 600℃氧化80 h后氧化质量损失为3.6×10–3 g/cm2。复合涂层在1 600℃的氧化失效主要是由于经过长时间氧化后SiO2玻璃膜层不能及时有效填补涂层中的缺陷,涂层中出现贯穿性的裂纹和孔洞导致的。     

SiCf/SiC复合材料高温抗氧化研究进展

连续SiC纤维增韧SiC陶瓷基复合材料(SiCf/SiC CMCs)具有低密度、优异的高温力学性能和抗氧化性能,在航空发动机热端部件上具有广阔的应用前景,具备提高发动机推重比和使用温度、减轻无效重量、简化系统结构等显著优势.延长SiCf/SiC复合材料在航空发动机高温氧化环境下的服役寿命是当前需要解决的难题.本文从纤维、界面相、基体、表面涂层四个方面综述了SiCf/SiC复合材料高温抗氧化研究进展.采用多元多层自愈合界面相、对基体进行改性以及采用表面自愈合整体涂层都可以有效提高SiCf/SiC复合材料在高温氧化环境中的使用稳定性和寿命.     

C/C复合材料SiC-SiO_2/ZrO_2-SiC复合抗氧化涂层

为了提高C/C复合材料在高温有氧环境的抗氧化性,在SiC抗氧化涂层防护的基础上,采用气相沉积法及溶胶凝胶吸附冷凝热蒸汽法在C/C复合材料表面制备出了SiC-SiO2/ZrO2-SiC复合涂层。利用扫描电镜、能谱质谱测试及X射线衍射等检测方法对涂层各层进行了分析。结果表明,溶胶吸附ZrCl4蒸汽法制备ZrO2涂层,不仅能够在高温自动修复单层SiC涂层的裂纹缺陷,还起到了在制备外层SiC涂层过程中缓冲应力的作用。这种多层复合涂层在高温下具有良好的抗氧化性,在1 800℃等离子焰动态空气氧化120 s后,计算得出该涂层失重速率仅为0.4 g/(m2·s),表明该涂层具有卓越的抗氧化性。     

碳/碳复合材料表面SiC/c-AlPO_4复合涂层及其抗氧化机制

采用二次包埋法和水热电泳沉积法相结合的工艺在碳纤维增强碳复合材料表面制得SiC/方石英型磷酸铝(cristobalite aluminum phosphate,c-AlPO4)复合涂层。借助X射线衍射仪和扫描电镜对复合涂层的晶相组成和显微结构进行了表征。研究了复合涂层的高温氧化性能,讨论了复合涂层氧化、失效的机理。结果表明:复合涂层具有双层结构,包埋的SiC内层主要由α-SiC,β-SiC和少量的游离硅组成,外层由c-AlPO4颗粒构成,内外层结合紧密。复合涂层在1300～1500℃范围内具有良好的抗氧化性能,其氧化激活能为117.2kJ/mol,氧化过程主要受氧在c-AlPO4层中的体扩散速率所控制;氧化气体逸出留下的孔洞是复合涂层防氧化失效的主要原因。     

多孔隔热炭材料用SiC复合涂层的制备及抗氧化性能研究

多孔炭毡是优异的高温隔热保温材料,但使用过程中的微氧化问题将直接导致材料的氧化失效。本文采用刷涂反应法,在炭毡表面依次制备了预炭层及SiC复合涂层,借助X射线衍射仪和扫描电子显微镜分析涂层的物相组成以及微观结构,探究了在1 200℃、1 400℃反应烧结制备的SiC复合涂层的结构和抗氧化性能差异。结果表明:在大气环境进行加速老化的抗氧化测试中,800℃氧化25 min后,炭毡的失重率为40%,1 200℃制备的SiC涂层具有较好的抗氧化性能,失重率仅为14.9%;1 000℃氧化25 min后,炭毡的失重率达70%,1 200℃、1 400℃制备的SiC复合涂层抗氧化性能差别不明显,氧化失重率约为16.9%。     

C/SiC复合材料表面化学气相沉积涂覆SiC涂层及其抗氧化性能

采用化学气相沉积法,在1 100 ℃,在碳纤维增强碳化硅复合材料表面制备SiC涂层,研究了涂层连续沉积和分4次沉积(每次沉积时间为6 h)所制备的SiC涂层的微观结构和涂层样品的氧化性能.结果表明:两种SiC涂层的厚度均约为40 μm,且4次沉积制备的SiC涂层为一个连续的整体.涂层连续沉积时,表面只出现裸露裂纹;分4次沉积制备时,表面出现大量边缘有SiC生长锥的附着裂纹,附着裂纹在高温氧化时易发生自愈合.与连续涂层样品相比,4次涂层能显著提高C/SiC样品的抗氧化性能.4次涂层样品经1 400 ℃,50 h氧化后,质量损失为0.88%,质量损失速率稳定在6.30 × 10-5 g/(cm2?h),且4次涂层样品具有优异的抗热震性能.     

水热温度对SiC-C/C复合材料表面水热电泳沉积SiCn-MoSi2复合抗氧化涂层的影响

采用水热电泳沉积法在SiC–C/C复合材料表面制备了纳米碳化硅和二硅化钼的复相(SiCn–MoSi2)抗氧化涂层。采用X射线衍射和扫描电子显微镜等对制备涂层的晶相组成、表面及断面微观结构进行了表征。研究了水热温度对制备涂层的结构及高温抗氧化性能的影响,分析了涂层在1 600℃静态氧化行为及失效机理。结果表明:外涂层主要由MoSi2和β-SiC晶相组成。复相外涂层的致密程度、厚度及抗氧化性能随着水热温度的升高而提高。SiCn–MoSi2/SiC复合涂层具有较好的抗氧化和抗热震能力,在1 600℃氧化80 h后氧化质量损失为3.6×10–3 g/cm2。复合涂层在1 600℃的氧化失效主要是由于经过长时间氧化后SiO2玻璃膜层不能及时有效填补涂层中的缺陷,涂层中出现贯穿性的裂纹和孔洞导致的。     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.     

Strengthening/toughening of 2D C/SiC composites by coating

SiC and SiC_w/SiC coatings were prepared on two-dimensional carbon fiber reinforced silicon carbide ceramic matrix composites (2D C/SiC), and strengthening/toughening of the composite by the coatings was investigated. After coating, the density of the C/SiC composites was increased effectively and the mechanical properties were improved significantly. Compared with SiC coating, SiC_w/SiC coating showed the more significant effect on strength/toughness of the composites. Coatings had two effects: surface strengthening and matrix strengthening. The latter was the dominant effect. The surface strengthening can increase the crack initiation stress, while the matrix strengthening can enhance the crack propagation resistance. The former effect increased the strength and the latter effect increased the toughness.     

Interface structure and wetting behaviour of Cu/Ti3SiC2 system

Wetting behaviour of a Cu/Ti₃SiC₂ system was investigated by the sessile drop technique under a vacuum atmosphere. Contact angles between Cu and Ti₃SiC₂ changed from 95 to 15° as temperatures increased from 1089 to 1270°C. Two distinct reaction layers consisting of different contents of Cu, TiC_x, Ti₃SiC₂ and Cu_xSi_y intermetallics were formed at the interface of Cu and Ti₃SiC₂. The formation of the interface layers contributes to the improvement of the wettability of the system. The dissolution of Si from theTi₃SiC₂ into the molten Cu at high temperature plays a dominant role in the wetting behaviour of Cu/Ti₃SiC₂ systems.     

Fabrication of SiCN-Sc2Si2O7 coatings on C/SiC composites at low temperatures

SiCN-Sc₂Si₂O₇ environmental barrier coatings were fabricated on the surface of C/SiC composites at low temperatures by adding Li₂CO₃ as sintering aids. With this addition, the fabrication temperature could be lowered about 100-200 °C. The shrinkage of the polysilazane-Sc₂Si₂O₇ bars with and without Li₂CO₃ was tested by dilatometer. The results indicate that the shrinkage speed of the polysilazane-Sc₂Si₂O₇ bar with Li₂CO₃ is faster than the one without Li₂CO₃, indicating that the Li₂CO₃ greatly promotes the sintering of polysilazane-Sc₂Si₂O₇. Water-vapor corrosion behavior of the SiCN-Sc₂Si₂O₇ coated C/SiC composites was carried out at 1250 °C. The results reveal that the SiCN-Sc₂Si₂O₇ coatings can effectively protect the C/SiC composites. The corrosion resistance of SiCN-Sc₂Si₂O₇ coatings is not degraded by adding Li₂CO₃.     

Si3N4 with comparable permeability to SiC

Porous sintered reaction-bonded silicon nitrides (SRBSNs) with comparable permeability to SiC were fabricated using presintered Si-additive mixture granules. By increasing the granule strength through the adjustment of the presintering conditions, the strengthened sturdiness of the granules led to an increase in the pore channel size. Porous SRBSNs with ≥60% porosity were achieved without employing a pore former due to the formation of intragranular narrow pore channels as well as intergranular wide pore channels. As the specific pore area of the developed SRBSN is nearly 26 times that of SiC with similar permeability, superior filtering efficiency for nano-sized particulate matter is expected.     

In situ preparation of SiC/Si3N4-NW composite powders by combustion synthesis

Large-scale composite powders containing silicon carbide (SiC) particles and silicon nitride nanowires (Si₃N₄-NWs) were synthesized in situ by combustion synthesis (CS). In this process, a mixture of silicon, carbon black, polytetrafluoroethylene (PTFE) and a small amount of iron powders was used as the precursor. The products were characterized by XRD, SEM, EDS and TEM. The particles are equiaxed with diameters in the micron range, and the in situ formed nanowires are straight with uniform diameters of 20-350 nm and lengths of tens of microns. The Si₃N₄-NWs are characterized to be α-phase single crystals grown along the [1 0 1] or [1 0 0] direction. VLS and SLGS processes are proposed as the growth mechanisms of the nanowires. The as-synthesized powders have great potential for use in the preparation of high-performance SiC/Si₃N₄-NW composites.     

Characterization of partially densified 3D Cf/SiC composites by using mercury intrusion porosimetry and nitrogen sorption

The microstructure of partially densified three-dimensional carbon fiber fabrics reinforced silicon carbide (C_f/SiC) composites are characterized by both mercury intrusion porosimetry (MIP) and isothermal nitrogen sorption (INS). By comparison, MIP is preferable to the characterization for its wide effective probing ranges. Based upon multiple measurements, in the C_f/SiC composite, exists a complicated three-dimensional porous network formed by the interconnecting pores and necks with various sizes, diverse shapes and rough surfaces.     

Mechanical properties of SiCf/SiC composites with alternating PyC/BN multilayer interfaces

Alternating pyrolytic carbon/boron nitride (PyC/BN)_n multilayer coatings were applied to the KD–II silicon carbide (SiC) fibres by chemical vapour deposition technique to fabricate continuous SiC fibre-reinforced SiC matrix (SiC_f/SiC) composites with improved flexural strength and fracture toughness. Three-dimensional SiC_f/SiC composites with different interfaces were fabricated by polymer infiltration and pyrolysis process. The microstructure of the coating was characterised by scanning electron microscopy, X–photoelectron spectroscopy and transmission electron microscopy. The interfacial shear strength was determined by the single-fibre push-out test. Single-edge notched beam (SENB) test and three-point bending test were used to evaluate the influence of multilayer interfaces on the mechanical properties of SiC_f/SiC composites. The results indicated that the (PyC/BN)_n multilayer interface led to optimum flexural strength and fracture toughness of 566.0?MPa and 21.5?MPa?m^1/2, respectively, thus the fracture toughness of the composites was significantly improved.     

Alumina/silicon carbide composites fabricated via in situ synthesis of nano-sized SiC particles

Alumina/silicon carbide composites have been fabricated by a new technique involving the in situ synthesis of nano-sized SiC particles. A mixture of alumina powder and silicon carbide precursors was prepared in an aqueous suspension. Green bodies were formed by cold isostatic pressing of granules obtained by freeze granulation, and pressureless sintered at 1750 °C for 4 h in an argon atmosphere. Mullite (10–20 vol%) formed in addition to SiC during sintering. The SiC particles were located predominantly to the interior of the mullite and alumina matrix grains.     

Effect of sintering atmosphere on the grain growth and hardness of SiC/polysilazane ceramic composites

Silicon carbide (SiC) monoliths were synthesised using nano-size SiC powder mixed with/without polysilazane by hot pressing at 1750°C for 1?h under an applied pressure of 20?MPa in N₂ or Ar atmosphere. The effects of polysilazane and sintering atmosphere on the microstructure and hardness of SiC were examined. The grain sizes of the SiC ceramics sintered in N₂ atmosphere with and without the polysilazane were 161 and 605?nm, while the density for those samples were 96.5 and 98.1%, respectively. It was shown that Si₂N₂O was formed for the SiC/polysilazane composite and sintered in N₂. In addition, the sample mixed with polysilazane followed by sintering in N₂ atmosphere revealed a quite high hardness in spite of its relatively low density. It was suggested that Si₂N₂O phase played an important role for the inhibition of grain and subsequent high hardness.