Effect of Nb on the mechanical properties of Ti/Al2O3 composite

Ti/Al₂O₃ composite with improved mechanical properties was synthesized by the spark plasma sintering. The effect of Nb on the microstructure of the composite was analyzed by TEM, SEM and so on. The experimental results indicate that the bending strength, fracture toughness, micro-hardness and relative density of the composite are 897.29MPa, 17.38MPa·m^1/2, 17.13GPa and 99.24% respectively when adding 1.5vol Nb. The bending strength is improved by reason of forming dislocation ring and transfering fracture mode from integranular to mixture fracture of intergranular and transgranular. The crack propagating is mainly the deflection bridging. It indicates a reduction of crack driving force and an increase in crack growth resistance, which results in toughness enhanced. Funded by the National Natural Science Foundation of China (No. 50232020) and the Natural Science Foundation of Shandong Province (No. 2002F21)     

Al2O3/(W, Ti)C纳米复合陶瓷材料的力学性能与强韧化机理

采用纳米和亚微米级的α-Al₂O₃,以及微米级的(W,Ti)C粉体为原料,制备了Al₂O₃/(W,Ti)C纳米复合陶瓷材料.在基体Al₂O₃含有体积分数为11%的纳米Al₂O₃时复合材料的抗弯强度和断裂韧性达到最优,其抗弯强度、断裂韧性和硬度分别为840MPa,6.55MPa·m^1/2和20.1GPa.TEM实验表明,纳米颗粒的加入明显抑止了基体晶粒的长大,形成了典型的骨架结构,材料的断裂方式为沿晶断裂和穿晶断裂的混合.内晶型和晶间型第二相颗粒产生的残余应力场、断裂模式的改变和晶粒细化强化促进了复合材料抗弯强度和断裂韧性的提高.     

Microstructure and mechanical properties of CaCO<Subscript>3</Subscript> whisker-reinforced cement

Composite Portland cement (PC) played an important role in various kinds of construction engineering owing to low hydration heat, low-cost, and application of solid industrial waste, but its brittleness and low strength limited its use in stress-bearing locations. The aim of this study is to improve the toughness and fracture resistance by incorporating CaCO₃ whisker in cement matrix. Effect of different content of calcium carbonate whiskers on the mechanical properties of PC was investigated. The results showed that the flexural strength, impact strength and split tensile strength were increased by 39.7%, 39.25% and 36.34% at maximum, respectively. Microstructure and elements of the whiskers in hardened cement were observed and analyzed by SEM/EDS. The mechanisms of the reinforcement of CaCO₃ whisker on cement were also discussed, and the conclusion was that the improvement could be correlated to energy-dissipating processes owing to crack bridging, crack deflection, and whisker pull-out at the crack tips.     

Microstructure and mechanical properties of TiB<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites

Effects of Al2O3 and Ni as the additives on the sinterability, microstructure and mechanical properties were systematic studied. The experimental results show that only a relative density about 96.2% of hot-pressing TiB2-30%Al2O3 can be attained due to the plate-like TiB2 particle and its random orientation and excessive Al2O3 grain growth. When sintering temperature is higher than 1 700 ℃, TiB2 grain growth can be found, which obvious improves flexural strength of TiB2 matrix but decreases toughness. It seems that mechanical properties of TiB2-Al2O3 composites are mainly depended on relative density besides grain growth. otherwise, they will be determined by relative density and TiB2 matrix strength together. Anyway, Al2O3 addition can weaken the grain boundary and thus improve the toughness of the materials. A flexural strength of 529 MPa, Vickers hardness of 24.8 GPa and indentation toughness of 4.56 MPa·m1/2 can be achieved inTiB2-30vol% Al2O3.     

Effect of diatomite additive on the mechanical and dielectric properties of porous SiO<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> composite ceramics

Porous SiO₂-Si₃N₄ composite ceramics with high porosity and excellent mechanical properties were fabricated by pressureless-sintering at relatively low temperature of 1 500 °C using diatomite as pore forming agent. The effects of diatomite on flexural strength, fracture toughness, shrinkage, porosity and phase transformation of the porous ceramics were investigated in detail. Compared with that of the ceramic without adding diatomite, the porosity of the ceramic with 10% diatomite is increased by about 27.4%, the flexural strength and fracture toughness reaches 78.04 MPa and 1.25 MPa·m^1/2, respectively. As the porosity increases, the dielectric constant of porous SiO₂-Si₃N₄ ceramic decreases obviously from 3.65 to 2.95.     

Microstructure and mechanical properties of submicron-grained NiAl-Al2O3 composite prepared by pulse current auxiliary sintering

Dense and submicron-grained NiAl-Al2O3 composite was fabricated by pulse current auxiliary sintering(PCAS).Its microstructure was analyzed by XRD,SEM and TEM,and its mechanical behavior was evaluated through compression test and fracture toughness test.The average grain sizes of NiAl and Al2O3 are about 200 nm and 100 nm respectively.The Al2O3 particles dispersed in NiAl matrix,forming intergranular structure and intragranular structure.During sintering,Al2O3 particles were remarkably spherized due to the unique sintering mechanism of PCAS,which is beneficial to the improvement of toughness.The NiAl-Al2O3 composite exhibits high compressive yield strength,whether at room temperature or elevated temperature.Its room-temperature(23 ℃) and elevated-temperature(1 200 ℃) compressive yield strength are up to 2 050 MPa and 140 MPa,respectively.Meanwhile,its fracture toughness is significantly enhanced,which is up to 8.2 MPa?m1/2.It is suggested that the main strengthening-toughening mechanisms are grain refinement strengthening and Al2O3 dispersion strengthening.The fracture of larger NiAl grain is the transgranular cleavage and this is induced by crack tip deflection and grain boundary weakening which are caused by intergranular and intragranular Al2O3 particles,respectively.     

Effect of sintering additives on properties of Si3N4-BN composites fabricated via die pressing and precursor infiltration and pyrolysis route

The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the α to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3+Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The fl exural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa?m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the fl exural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3+Al2O3 as additives, making it to possess better mechanical properties.     

新型 Al_2O_3 基陶瓷刀具材料颗粒弥散增韧与晶须增韧的协同作用

研制成功了一种新型陶瓷刀具材料——ＳｉＣ晶须（ＳｉＣｗ）增韧和ＳｉＣ颗粒（ＳｉＣｐ）弥散增韧Ａｌ２Ｏ３陶瓷刀具ＪＸ－２－Ｉ,该材料具有良好的抗弯强度和断裂韧性,同Ａ（Ａｌ２Ｏ３）,ＡＰ（Ａｌ２Ｏ３／ＳｉＣｐ）和ＡＷ（Ａｌ２Ｏ３／ＳｉＣｗ）等材料相比较表明,在ＪＸ－２－Ｉ材料中具有明显的增韧叠加效应,详细研究了晶须增韧和颗粒弥散增韧的协同作用,建立了理论模型并进行了实验验证．     

多尺度纤维复合增强水泥基材料的力学性能

为改善水泥基材料抗拉强度低、韧性差以及易开裂等性能缺陷,采用微米级碳酸钙晶须和厘米级短切耐碱玻璃纤维复合增强高性能水泥基材料,并对不同纤维增强水泥基材料的基本力学性能进行研究。结果表明：微观碳酸钙晶须和宏观耐碱玻璃纤维均有利于水泥基材料力学性能的提高,且提高程度与纤维（或晶须）掺量及长度相关;采用碳酸钙晶须和耐碱玻璃纤维复合增强水泥基材料,可分别在微、宏观结构层次上发挥两种纤维的增强优势,增强水泥基材料的抗折和劈拉强度比未增强时最多可分别提高约60%和80%。     

Manufacture Process of 8Y2O3 Stabilized ZrO2 from Nano Powders

The manufacture process of 8 mol% Y2O3 stabilized ZrO2 ( YSZ ) from nano powders, including the forming and sintering stages, was studied. During the forming process of YSZ powders, the relative density of YSZ increases lineally with the forming press, and the sintering linear shrinkage of YSZ to the forming press compiles to the parabola trend. When the forming press exceeding 500MPa, the samples with lower shrinkage and high density were obtained. The sintering temperature of YSZ decreases greatly because of the small size and high active surface of YSZ powders. As a result, the beginning sintering temperature of YSZ made in the experiment is as low as 825℃, and the end sintering temperature is 1300-1350℃ . The relative density of YSZ ceramic by solid sintering at 1300-1350℃ is more than 97% , with little and small pores in the uniform microstructure.     

Effect of metallic content on mechanical property of Ni/（10NiO-NiFe2O4 ） cermets

1 INTRODUCTIONThe use of inert or non-consumable anodes forreplacement of consumable carbon anodes in Hall-H啨roult electrolysis cells for the production of alu-minum has been a technical and commercial goalfor many decades .In the present process ,consumable carbon an-odes are used,andthe anode product is CO2. Withaninert anode ,the cell reaction will be :Al2O3=2Al +23O2(1)The basic requirements for aninert anode are :1) to exhibit a lowcorrosion rate in the high tem-perature melts an…     

Preparation of BN／SiO2 ceramics by PIP method

The precursor infiltration and pyrolysis(PIP) method for preparation of BN/SiO2 composites was used to improve mechanical properties, dielectric properties and feasibility of high temperature dielectric parts with large dimensions and complex shapes. In the processing procedure, the porous BN ceramic matrix was first successfully prepared by compacting the mixed powders of B and BN and then sintering them at a certain temperature under normal pressure of N2. The polycarbosilane(PCS) solution was vacuum infiltrated into porous BN ceramics at the room temperature and then at 800℃ in the air to depolimerize out amorphous SiO2, and sintered further at 1 300℃ in N2 to get BN/SiO2 composites. The microstructure of materials was studied by means of X-ray diffraction and electron probe micro analysis. The thermo-decomposition mechanism of PCS was investigated by a TG-DTA and infrared (IR) spectrum analysis. The flexural strengths were measured by the three-point bending method. The dielectric constant and the loss tangent were measured by the wave-guide method. The results show BN/SiO2 composites were fabricated. The obtained composites posses a flexural strength of 61.96 - 93.31 MPa, the dielectric constant in the range of 3.50 - 3.78 and the order of magnitude of the loss tangent at 10^-3 , which are good for the high tempera turedielectric parts with large size and complex shapes.     

Preparation and properties of 4.25Cu-0.75Ni／NiFe2O4 cermet

4.25Cu-0.75Ni/NiFe₂O₄ cermets were prepared by doping NiFe₂O₄ ceramic matrix with the mixed powders of Cu and Ni or Cu-Ni alloy powder as the electrical conducting metallic elements. The effects of technological parameters, such as the adding modes of metallic elements, the ball milling time, the sintering time and the sintering temperature, on the relative density and resistivity of the cermets were studied. The results show that the resistivity of 4.25Cu-0.75Ni/NiFe₂O₄ cermets decreases with increasing temperature, and has a turning point at 590 °C, which is similar to that of NiFe₂O₄ ceramic. The sintering temperature and adding modes of metallic elements have a great influence on the properties of 4.25Cu-0.75Ni/NiFe₂O₄ cermets. When the sintering temperature increases from 1200 °C to 1300 °C, the relative density increases from 89.86% to 95.33%, and the resistivity at 960 °C decreases from 0.11 Ω · cm to 0.03 Ω · cm, respectively. When the metallic elements are added with the mixed powders of Cu and Ni, the cermets of finely and uniformly dispersed metallic phase, high density and electric conductivity are obtained. The relative density and resistivity at 960 °C are 90.23% and 0.04 Ω · cm respectively for the cermet samples sintered at 1200 °C for 2 h, which are both better than those of the cermets prepared under the same technique conditions but with the metallic elements added as 85Cu-15Ni alloy powders. Foundation item: Project (G1999064903) supported by the National Key Fundamental Research and Development Program of China; project(2001AA335013) supported by the National High Technology Research and Development Program of China; project (50204014) supported by the National Natural Science Foundation of China     

超高韧性水泥基材料的制备技术

分别采用活性粉末混凝土（RPC）和渗浇钢纤维混凝土（SIFCON）两种制备工艺,根据水泥基材料结构的多尺度特征,研究了由碳酸钙晶须和微钢纤维复合增强的超高韧性水泥基材料（Ultra-High-Toughness Cementitious Composite,简称UHTCC）的制备技术,测试UHTCC不同配比的抗压强度、抗折强度、抗弯强度以及单轴拉伸性能,采用折压比、韧性指数等多个指标对UHTCC的韧性进行了评价。试验表明：UHTCC的抗压强度、抗折强度、抗弯强度以及延性和韧性都远高于普通钢纤维混凝土,其抗弯强度最高达65.1 MPa、韧性指数I₂₀最高达49.21,单轴拉伸试验时呈现明显的假应变硬化行为,极限拉应变可达4%~8%。相对而言,利用SIFCON工艺制得的水泥基材料韧性更高。     

碳化硅-二硼化钛复合陶瓷性能研究

采用无压烧结,利用小分子的B、C作为烧结助剂在2180℃获得了相对密度为96％的SiC—TiB2复合材料,XRD分析显示在烧结过程中无新相产生,从烧结体的显微形貌可以看出,在烧结过程中有包晶现象,SiC颗粒有明显长大趋势．大的晶粒存在使烧结体的力学性能降低,主要表现为抗弯强度和断裂韧性不高,分别只有158．6MPa和2．85MPa·m^1/2．     

SiC WHISKER-TOUGHENED ALUMINIUM NITRIDE CERAMICS

Aluminium nitride ceramics containing 15-30 w/o of SiC whiskers can be sintered tofull density(by hot pressing at 1800℃)using2-3 w/o of Y_2O_3,additions.The whiskersincrease the toughness and strength of thecomposite,K_(10) increasing from 2.8 to about5.0 and flexural strength increasing by 30-50%.However,the whiskers must be welldispersed and if the dispersion is notsatisfactory,toughness may increase but thestrength decreases.The hot-pressingtemperature can be reduced by up to 100℃ ifY(NO_3)_3 .5H_20 is used as the sinteringadditive instead of Y_2O_3,but some oxidation ofthe AIN occurs during heating.Isopropan-2-01 is a better dispersing agent thancyclohexane,but again some oxidation of theAIN does occur.The best sample prepared during this work contained 20 w/o of SiC whiskers and 2 w/oY_2O_3 added as Y(NO_3)_3. 5H_2O and mixed inisopropanol.This exhibited a mean strength of453MPa(maximum 522MPa,measured bydisc flexure)and a fracture toughness of5.5MPam~(1/2).     

Dielectric Properties of B2O3 Doped MgTiO3-CaTiO3 System Ceramics

The dielectric ceramics with a main crystal phase of MGTiO3 and additional crystal phase of CaTiO3 were prepared by the conventional electronic ceramics technology .the strucures of MgTiO3 are ilmenitetype,and belong to hexagonal syngony.the ratio of MgTiO3 to Ca TiO3 doping on the dielectric properties of MGTiO3-CaTiO3(MCT)ceranics were inrestigated.the addition of B2O3 decreases the sintering temperatnre and results in rapid desification without obrious negative effect on the Q values of the system(Q=1/tan ).B2O3 exists as liquid phase in the sintering process,promoting the reactions as a singering agent.     

Effect of tin on the reaction synthesis of ternary carbide Ti3AlC2

The effect of tin on synthesis of Ti₃AlC₂ by spark plasma sintering (SPS) from TiC/Ti/Al powders was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for phase identification and microstructure evaluation. The experimental results show that addition of tin can considerably accelerate the synthesis reaction of Ti₃AlC₂ and fully dense, essentially single-phase polycrystalline Ti₃AlC₂ could be successfully obtained by sintering 2TiC/1Ti/1Al/0.2Sn powders at 1200–1250 °C under a pressure of 30 MPa. SEM images show that Ti₃AlC₂ samples in about 2–5 μm thick and 10–25 μm long platelets can be obtained. The fracture toughness and flexural strength of Ti₃AlC₂ were 6.5±0.2 MPa·m^1/2 and 560±10 MPa, respectively. Funded by the National Natural Science Foundation of China (No.20771088, No.50572080) and Doctoral Foundation of Wuhan University of Technology (No.471-38650142)     

烧结温度对Al_2O_3-SiC复合陶瓷力学性能的影响

以Al2O3和SiC为原料,利用热压烧结法制备了Al2O3-SiC复合陶瓷.采用三点弯曲法、单边切口梁法等手段和SEM方法分别测定和分析了该复合陶瓷的抗弯强度、断裂韧性、致密度和断口形貌.结果表明,Al2O3-SiC10wt%复合陶瓷的致密度随热压烧结温度的提高而逐渐提高,最高可达98.42%;抗弯强度随烧结温度的升高而呈上升趋势,在1 800℃时抗弯强度最大为623MPa;断裂韧性明显是随温度的升高加强,特别是在1 850℃烧结时达到最高值7.9MPa·m1/2.材料的断裂方式主要为沿晶断裂,随着烧结温度升高,穿晶断裂所占的比例增大.     

Effect of matrix composition on physical properties of Al<Subscript>3</Subscript>CON in-situ reaction reinforced corundum composite

60% white corundum used for aggregate, 5% aluminium powder for fixed additions and 35% various additives for matrix were prepared for specimens 1#,2#,3#. They were mixed uniformly with the suitable resin as a binder and pressed under pressure of 315 ton forging press, then dried at 200℃ for 24 h. Effects of various additives on 1500 ℃×2 h creep properties of Al3CON reinforced corundum composite were researched. The experimenal results show that creep coefficients of specimens 1#,2#,3# at 1500 ℃×2 h are 1.4×10^- 4, -9.4×10^-4, -22.6×10^-4, respectively. Crushing strength of the slide plate added with suitable additive A after fired at 1500 ℃ ×3 h reaches to 225 MPa, the creep rate is positive all the time from 0% to 0.014% at 1500 ℃ for 2 h. The microstructure result analysis shows that reinforced phases of Al3CON fiber composite have been formed after fired with Al powder in coke at high temperatures for specimen 1#, and the strength of the composite is increased. The hot modulus of rupture is up to 59 MPa at 1400 ℃ and the RUL is obviously higher than that at 1700 ℃. Its service life is two times as that of Al2O3-C slide plate when used in the process of pouring steel. The mechanism of creep rate resistance of the composites can be discovered by means of SEM and EDAX analysis. It is concluded that the active Al3CON and Al2O3 multiphases that were formed by N2 in gas, C, Al and Al2O3 inside the matrix of the composites during in-situ reaction,which gives the composites outstanding creep rate resistance for the dense zone resuiting from Al3CON oxidation that inhibits contraction at the high temperature. Besides, the matrix will turn into the multiphase with high refractoriness, N content and its Al3CON reinforced fiber will further increase accordingly. In addition, Al3CON formed by Al2O3 and C, Al in the matrix with N2 in gas will inhibit the creep rate and also greatly improve the creep rate resistance of the composites.