Study on biomimetic staggered composite for better thermal shock resistance

Inspired by the fact that biological composites have the excellent resistance to mechanical shock, we systematically studied the thermal shock resistance of biomimetic staggered composite via analytical, computational and experimental approaches. The effective stiffness, coefficient of thermal expansion and maximum thermal stress predicted by our theoretical model agree well with finite element simulations. By sacrificing part of stiffness, an optimized microstructure can be obtained, in which the maximum thermal stress of the composites can be much lower than that of homogenous hard material. Finally, the thermal shock experiment on the material system of glass–epoxy were done, and the result supports our conclusions. This study provides an alternating way for material design to achieve high thermal shock resistance.     

New approach to MoSi2/SiC intermetallic-ceramic composite with B4C

The effects of SiC and B₄C additives in the MoSi₂ matrix on the microstructures and mechanical properties at room temperature were investigated. Their coefficients of thermal expansion (CTE) were also evaluated up to 1200°C by a thermal mechanical analysis (TMA). The experimental results show that the Mo₂B₅ reinforced phase was formed in situ in the hot-pressed MoSi₂/SiC/B₄C composites. Both the Mo₂B₅ phase and the SiC phase significantly improved the mechanical behavior of MoSi₂. Besides, the SiC with a high content up to 40 vol% could be added into the MoSi₂ composite with the B₄C additive. As a result, a dense and homogenous MoSi₂/SiC/B₄C composite was obtained, which possessed a relatively high bending strength and fracture toughness. Meanwhile, the CTE of the MoSi₂/SiC/B₄C composites linearly decreased with the increasing SiC content, which dropped to 21% at 1200°C in comparison with the pure MoSi₂ when adding 40 vol% SiC. This MoSi₂/SiC/B₄C composite system is very important for developing new applications at elevated temperature, particularly for high-temperature coating applications.     

Formation of MoSi2-SiC composite powder by reacting Si3N4 with molybdenum and carbon

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of B 4 C-ZrB 2 -SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

液硅渗透法制备SiBC改性C/CSiC复合材料

为了降低液硅渗透法制备C/C-SiC复合材料中残留Si的含量, 采用浆料浸渗结合液硅渗透工艺制备B₁₂(C, Si, B)₃改性C/C-SiC复合材料。通过分析不同比例B₄C-Si体系在不同温度的反应产物, 确定了B₁₂(C, Si, B)₃的生成条件。结果表明: B₄C和Si在1300℃开始反应, 生成少量B₁₂(C, Si, B)₃和SiC, 且B₁₂(C, Si, B)₃的生成量随反应温度的升高而增加; 当B₄C和Si的摩尔比为3:1、 反应温度为1500℃ 时, 产物为B₁₂(C, Si, B)₃和SiC; 液硅渗透法制备的C/C-SiC复合材料相组成为非晶态C、 β-SiC和B₁₂(C, Si, B)₃, 未见残留Si。     

Mechanical behavior of B4C particulate-reinforced 7091 aluminum composite

The incorporation of B₄C particulates into 7091 Al alloy can improve the tensile modulus, yield strength and tensile strength of the monolithic matrix alloy. However, the ductility and fracture toughness need to be improved through optimization of processing parameters. The fracture was primarily along article/matrix interface.     

碳／碳复合材料活塞的制造与性能

介绍了用Ｃ／Ｃ复合材料制造的航模发动机活塞的新方法，并研究了制备工艺，对用相同方法制造的短梁试样进行了性能测试和微观组织分析。实验结果表明：所制得活塞的密度达到１．４３ｇ／ｃｍ３，剪切强度最高达到１６．８９ＭＰａ，弹性模量７．５５ＧＰａ，断裂强度２５．５２ＭＰａ，其试样断口为纤维拔出型。     

Microhardness and high-velocity impact resistance of HfB2/SiC and ZrB2/SiC composites

The results of Vickers microhardness and high-velocity impact tests on monolithic ZrB₂/SiC and HfB₂/SiC ultra-high temperature ceramic (UHTC) composites are presented. The UHTC materials exhibit fracture behavior typical of ceramics under indentation and impact loading. The materials are relatively hard with microhardness values of about 15 to 20 GPa. Cracks were observed to extend from the corners of indentations. Impacts of stainless steel and tungsten carbide spheres, with diameters in the 500 to 800 micron range and velocities of 200 to 300 m/s, produced minimal plastic deformation but significant radial and ring cracking at the impact sites. Impacts of micron-scale iron particles traveling at 1 to 3 km/s produced essentially no surface damage.     

Friction and wear behaviors of B4C/6061Al composite

The friction and wear behaviors of B₄C/6061Al composite were studied by considering the effect of sliding time, applied load, sliding velocity and heat treatment. The results show that, when the sliding time, applied load and sliding velocity reach critical values (namely 120 min, 30 N and 240 r min⁻¹, respectively), the mass loss and friction coefficient (COF) increase significantly. Severe delamination wear is the main wear mechanism after sliding for 120 min and under an applied load of 30 N. While fretting wear happens at a sliding velocity of 240 r min⁻¹. After solution-treated at 550 °C for 1 h and then aged at 180 °C for 15 h, the composite shows the highest wear resistance owing to the precipitation of β″ (Mg₂Si) phases in the matrix and the strong interface bonding between B₄C particles and the matrix alloy.     

Evaluation of thermal shock resistance of cordierite honeycombs

A comparative study on thermal shock resistance (TSR) of extruded cordierite honeycombs is presented. TSR is an important property that predicts the life of these products in thermal environments used for automobile pollution control as catalytic converter or as diesel particulate filter. TSR was experimentally studied by quenching (descending test) the heated samples to water or by heating (ascending test) with an oxyhydrogen flame along with crack detection by acoustic emission (AE) method. TSR was also calculated by using coefficient of thermal expansion (CTE), modulus of elasticity (MOE) and modulus of rupture (MOR) of the honeycomb samples. Cordierite honeycombs of 200 and 400 cpsi were used for the above study. It was observed that the trends of TSR were same for both the experimental methods as well as by calculation. The ascending test method showed lower TSR values compared to water quench method due to early detection of cracks by AE. Finite element method (FEM) was also used to evaluate the thermal stress distribution in solid cordierite using thermal shock test data. It was observed that the maximum thermal stress calculated by FEM was lower than the strength of the material; therefore, the solid cordierite did not fail during such tests.     

Rheological properties of Al2O3-SiC whisker composite suspensions

The rheological properties of both aqueous and non-aqueous composite suspensions were investigated together with a characterization of the colloidal stability of the separate components. The colloidally stable, concentrated aqueous SiC_w and Al₂O₃-SiC_w composite suspensions displayed strong shear thinning followed by a severe, sometimes discontinuous, shear thickening at a critical shear rate. The non-aqueous composite suspensions, containing weakly flocculated SiC_w, only showed a continuous shear thinning behaviour. The variations in the steady shear behaviour were related to the differences in the colloidal stability and possible shear induced effects on the suspension structure. It was possible to fit the volume fraction dependence of both the SiC_w and the Al₂O₃ suspensions to a modified Krieger-Dougherty model which yields values of the maximum volume fraction; _m(Al₂O₃)=0.61, _m(SiC_w)=0.28. The viscosity of the composite suspensions were successfully predicted from the Farris theory using the rheological data for the separate components.     

纳米增强金属陶瓷的组织及热冲击性能

为了进一步提高金属陶瓷的力学性能及其刀具的高速切削性能, 研究了纳米增强金属陶瓷的显微组织特征和热冲击性能。扫描电镜和透射电镜分析表明: 组织中陶瓷相呈现典型的芯2壳结构特征; 芯为TiC 或Ti (C ,N) , 而壳则为( Ti ,Mo ,W) (C ,N) 固溶体。纳米增强金属陶瓷机制为细晶强化、弥散强化和固溶强化。热冲击实验表明: 随着热循环的进行, 材料中孔洞的数量、孔洞的尺寸以及微裂纹的尺寸随之增大; 同时, 热循环过程中出现的表面氧化、裂纹长大、偏转以及桥接现象也很显著。与常规Ti (C ,N) 金属陶瓷相比, 纳米增强金属陶瓷的抗热冲击性能明显提高。