SiC晶须增强SiCf/SiC复合材料的力学性能

以连续SiC纤维为增强体,采用前驱体浸渍裂解工艺,在复合材料基体中引入SiC晶须制备出多级增强的SiCf/SiC-SiCw复合材料,并采用化学气相渗透工艺在SiC晶须表面制备BN界面层,研究了SiC晶须及其表面BN界面层对复合材料的性能影响.结果表明:在复合材料中引入SiC晶须后,由于晶须的拔出、桥连及裂纹偏转等作用增加了裂纹在基体中传递时的能量消耗,使SiCf/SiC复合材料的压缩强度有明显提高,当引入体积分数为20％的SiC晶须时,复合材料压缩强度提高了22.6％,可达673.9 MPa.通过化学气相渗透工艺在SiC晶须表面制备BN界面层后,复合材料的拉伸强度、弯曲强度和断裂韧度分别为414.0,800.3 MPa和22.2 MPa·m1/2,较SiC晶须表面无界面层时分别提高了13.9％,8.8％和19.0％.     

Mechanical properties and microstructure in sintered and HIPed SiC particle/Si3N4 composites

Pressureless sintered (PLS) and gas-pressure sintered (GPS) Si₃N₄, PLS and GPS SiC particle/Si₃N₄ composites, and PLS + HIP and GPS + HIP SiC particle/Si₃N₄ composites were produced. Investigation of their mechanical properties showed that PLS + HIP and GPS + HIP composites, containing SiC particles in the beta-silicon nitride grains, yield higher bending strength, although its fracture toughness remains at the same level. This is attributed to the fact that the added SiC particles inhibit excessive growth of beta-Si₃N₄ grains without changing the fracture behaviour. However, this investigation also found precipitation during the reaction between SiC and nitrogen in gas pressure sintering, resulting in a low Young's modulus and low density in the GPS composite.     

反应烧结制备Si3N4/SiC复相陶瓷及其力学性能研究

以两种不同配比Y₂O₃/Al₂O₃ (A, 2:3; B, 3:1, 总量15 wt%)为烧结助剂, 通过添加不同质量分数的SiC粉体,反应烧结制备了高强度的氮化硅/碳化硅复相陶瓷。并对材料的相组成、相对密度、显微结构和力学性能进行了分析。结果表明: 在1700℃保温2 h情况下, 烧结助剂A 与B对应的样品中α-Si₃N₄相全部转化为β-Si₃N₄; 添加5wt% SiC, 烧结助剂A对应样品的相对密度达到最大值94.8%, 且抗弯强度为521.8 MPa, 相对于不添加SiC样品的抗弯强度(338.7 MPa)提高了约54.1%。SiC能有效改善氮化硅基陶瓷力学性能, 且Si₃N₄/SiC复相陶瓷断裂以沿晶断裂方式为主。     

Toughness enhancement by polycarbosilane coating on SiC whiskers incorporated in Si3N4 matrix composite

Si₃N₄ matrix composite was fabricated by hot pressing with 20% SiC whiskers coated with polycarbosilane (PCS). The preceramic polymer on the whiskers was pyrolysed during sintering to form a carbon-rich layer at the whisker/matrix interface. Mechanical properties were measured, and compared to those of the composites with whiskers purified with HCl and HF. Elastic modulus and bending strength of the composite with PCS-coated whiskers were lower than those of the composites with other whiskers. Fracture toughness was measured by single-edge notched beam (SENB) and single-edge precracked beam (SEPB) methods. The toughness, including crack-growth resistance measured by the SEPB method, increased from 7.2 MPam^1/2 to 7.9 MPam^1/2 by PCS-coating on the whisker, while the toughness measured by the SENB method decreased from 6.5 MPam^1/2 to 5.7 MPam^1/2. The layer derived from PCS facilitated debonding at the whisker/matrix interface and activated the wake-toughening. Optical microscopic observation of the crack propagation near the interface confirmed enhancement of interfacial debonding by the PCS-coating.     

Mechanical properties of Al2O3 and Al2O3 + ZrO2 ceramics reinforced by SiC whiskers

The effect of the SiC whisker content on the mechanical properties of Al₂O₃ and Al₂O₃ + 20 vol% ZrO₂ (2 mol% Y₂O₃) ceramic composites has been investigated. It is shown that the strength and fracture toughness of the composites are increased by the addition of 0–30 vol% SiC whiskers with only one exception that 30 vol% SiC whisker leads to a decrease in the flexure strength. The addition of 20 vol% ZrO₂ (2 mol% Y₂O₃) significantly improves the mechanical properties of the Al₂O₃ + SiC whisker (SiC_w) composites and the t-m phase transformation of ZrO₂ is enhanced by the residual stresses caused by the thermal incompatibility between the SiC_w and the matrix. The toughening effect of both SiC whiskers and the t-m phase transformation of ZrO₂ (2 mol% Y₂O₃) is shown to be additive, but the addition of ZrO₂ decreases the strengthening effect of the SiC whiskers.     

SiC晶须与Si_3N_4颗粒强韧MoSi_2复合材料

采用湿法混合和热压工艺制备了不同Si3N4(p)和SiC(w)体积含量的MoSi2基复合材料,研究了复合材料的显微组织,晶粒大小,硬度、断裂韧性和抗弯强度.结果表明,复合材料的晶粒比纯MoSi2明显细化,且随着强化相添加量的增加而减小,抗弯强度和断裂韧性均大幅度提高,其中MoSi2-20%SiC(w)-20%Si3N4(p)复合材料具有较好的综合力学性能,断裂韧性和抗弯强度分别427 Mpa和10.4 Mpa·m1/2.复合材料的强化机制为细晶强化和弥散强化,韧化机制为细晶韧化和裂纹偏转与分支韧化.     

纳米SiC纤维改性短切碳纤维增强Si_3N_4陶瓷介电响应及吸波性能

以甲基三氯硅烷为原料,采用催化化学气相沉积(CCVD)工艺在短切碳纤维(C_(fd))表面制备了纳米SiC纤维(nano SiC_f)改性层,并采用凝胶注模-无压烧结工艺制备了nano SiC_f-C_(fd)/Si_3N_4和C_(fd)/Si_3N_4复合材料。使用矢量网络分析仪研究了nano SiC_f-C_(fd)和C_(fd)对Si_3N_4陶瓷在X波段(8.2~12.4GHz)的介电响应和吸波性能的影响。结果表明:nano SiC_f-C_(fd)/Si_3N_4和C_(fd)/Si_3N_4复合材料的复介电常数和介电损耗角正切值(tanδ)均随纤维添加量增加而增大;相同纤维含量时,nano SiC_f-C_(fd)/Si_3N_4复合材料的介电常数实部比C_(fd)/Si_3N_4复合材料有所降低,但损耗角正切升高。反射损耗结果表明:nano SiC_f-C_(fd)/Si_3N_4复合材料拥有更优的电磁波吸收效果。nano SiC_f-C_(fd)含量为2wt%、d=2.5mm时,出现最大吸收峰-14.95dB,反射损耗优于-5dB,波段频宽达3.5GHz。nano SiC_f界面改性能有效提高C_(fd)/Si_3N_4复合材料的吸波性能。     

Si3N4／SiC纳米复合陶瓷的微观结构

利用ＪＥＭ２０００ＥＸⅡ高分辨电镜和ＨＦ２０００冷场发射枪透射电镜对Ｓｉ３Ｎ４ＳｉＣ纳米笔合陶瓷材料的微观组织，结构和成分进行了研究。结果表明，ＳｉＣ颗粒弥散分布基体相β－Ｓｉ３Ｎ４晶内和晶界，晶内ＳｉＣ颗粒与基体相的界面结构有三种类型；１）直接结合的的界面；２）完全非晶态的界面；３）混合型的界面，晶间ＳｉＣ颗粒与基体相的界面大部分是直接结合的。     

Preparation of Si3N4 whiskers by reaction of wheat husks with NH3

-Si₃N₄ whiskers that are 1–10 mm long and 0.5–1.1 m thick were obtained by the reaction of wheat husks with NH₃ at 1250–1450 °C. A maximum whisker yield of about 30% was obtained at 1450 °C with the addition of an iron impurity. Whiskers with 1.3–2.2 m thickness (average 1.6 m) were obtained by the addition of an H₂S impurity. Thin whiskers with periodic thick and thin diameters were also obtained.     

Strength,toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol% SiC whisker-reinforced Si₃N₄ material were determined as a function of temperature from 25 to 1400°C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100°C; however, appreciable strength degradation started at 1200°C and reached a maximum at 1400°C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPam^1/2. It was also observed that the composite material exhibited no rising R-curve behaviour at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si₃N₄ matrix did not provide any favourable effects on strength, toughness and R-curve behaviour.     

Microstructure and mechanical properties of barium aluminosilicate glass-ceramic matrix composites reinforced with SiC whiskers

BAS glass-ceramic composites reinforced with different volume fractions (0, 10, 20, 30, 40 vol%) of SiC whiskers were successfully fabricated by a hot-pressing method. The microstructure, whisker/matrix interface structure, phase constitution and mechanical properties of the composites have been systematically studied by means of SEM, TEM, XRD techniques as well as three-point bending tests. It was demonstrated that the incorporation of SiC whiskers could significantly increase the flexural strength and fracture toughness of BAS glass-ceramic matrixes. The celsian seeds can effectively promote the hexacelsian-to-celsian transformation in BaAl₂Si₂O₈. The active Al₂O₃ added to the BAS matrix obviously reduced the amount of SiO₂ in the matrix and formed needle-like mullite. The high temperature strengths of the composites were also investigated.     

Fabrication,mechanical properties and microstructure analysis of Si3N4/SiC nanocomposite

Ceramic nanocomposites, Si₃N₄ matrix reinforced with nano-sized SiC particles, were fabricated by hot pressing the mixture of Si₃N₄ and SiC fine powders with different sintering additives. Distinguishable increase in fracture strength at low and high temperatures was obtained by adding nano-sized SiC particles in Si₃N₄ with Al₂O₃ and/or Y₂O₃. Si₃N₄/SiC nanocomposite added with Al₂O₃ and Y₂O₃ demonstrated the maximum strength of 1.9 GPa with average strength of 1.7 GPa. Fracture strength of room temperature was retained up to 1400 as 1 GPa in the sample with addition of 30 nm SiC and 4 wt% Y₂O₃. Striking observation in this nanocomposite is that SiC particles at grain boundary are directly bonded to Si₃N₄ grain without glassy phases. Thus, significant improvement in high temperature strength in this nanocomposite can be attributed to inhibition of grain boundary sliding and cavity formation primarily by intergranular SiC particles, besides crystallization of grain boundary phase.     

烧结工艺对Si3N4-SiC材料性能的影响

本文采用氮化硅作为碳化硅材料的结合相的方法制备氮化硅结合碳化硅耐火材料，通过对氮化硅结合碳化硅的烧结工艺过程的探讨，推导了理论密度、气孔率与生坯密度的关系及影响材料氮化率的关系式，分析了材料性能与烧结工艺参数之间的关系。