纳米ZrO2与微米Al2O3复合陶瓷的断裂模式

研究了两种微米Al2O3与纳米ZrO2复合陶瓷的裂纹扩展过程与显微结构的关系.结果表明,Al2O3晶粒内部形成纳米级或亚微米级ZrO2颗粒,是复合陶瓷的断裂模式从沿晶断裂向穿晶断裂转化的主因.ZrO2含量较低有利于Al2O3晶界迁移包裹纳米ZrO2形成内晶结构;而ZrO2含量较高使主晶相长大受到抑止,不利于形成内晶结构,趋向于沿晶断裂.裂纹穿晶扩展需要的驱动力比沿晶断裂大,故裂纹扩展阻力曲线的上升趋势更加显著.裂纹穿晶扩展路径主要取决于内晶颗粒产生的弹性应力场的性质.     

Mo2C含量对Ti(C,N)基陶瓷力学性能和显微结构的影响

采用热压烧结工艺制备了添加微量Cr3C2的Ti(C0.5N0.5)-(Ni-Co)-Mo2C-Cr3C2系Ti(C,N）基金属陶瓷。分析三种不同Mo2C含量材料的力学性能、断口形貌和磨削表面压痕裂纹扩展情况，研究表明：材料的断裂均以沿晶断裂为主；材料的抗弯强度与Rim相有关，8wt%Mo2C含量的金属陶瓷Rim相厚度适中，抗弯强度高；0.6wt%Mo2C含量的金属陶瓷的扩展裂纹短，裂纹扩展发生偏转，断裂韧性高。     

纳米TiC颗粒弥散增强超细晶钨基复合材料的组织结构与力学性能

采用高能球磨和热压烧结的方法成功制备了纳米TiC颗粒弥散增强超细晶W基复合材料,并对其组织结构、室温力学性能进行了研究.研究结果表明,当纳米TiC颗粒含量较小时,高能球磨可以使TiC颗粒均匀分散到W基体中,烧结后,TiC颗粒尺寸约100nm,当纳米TiC颗粒含量较高时,局部出现团聚现象;纳米TiC的加入强烈的阻碍了W晶粒的长大并使复合材料的断裂模式由沿晶断裂为主向穿晶断裂为主转变,提高了材料的力学性能;在TiC含量为1%(质量分数,下同)时,材料的致密度、维氏显微硬度、弹性模量、抗弯强度分别达到98.4%、4.33、396GPa、1065MPa.纳米TiC颗粒对复合材料的强化机制主要是细晶强化和晶界强化.     

h-BN/Si3N4陶瓷复合材料的断裂行为及断裂韧性

以亚微米级α-Si3N4和h-BN粉末为原料,Y2O3-Al2O3为助烧剂,采用热压烧结制备了h-BN/Si3N4陶瓷复合材料.研究3h-BN含量对h-BN/Si3N4陶瓷复合材料断裂韧性及其断裂行为的影响.结果表明:随着h-BN含量增加,柱状β-Si3N4晶粒的直径和长径比均下降;未加h-BN时,β-Si3N4陶瓷以沿晶断裂为主,添加体积含量为6%和8%的h-BN后,复合材料出现明显的沿晶和穿晶断裂,而添加10%h-BN的陶瓷复合材料则以沿晶断裂为主.随着h-BN含量增加,h-BN/Si3N4陶瓷复合材料的断裂韧性下降,但由于h-BN颗粒对裂纹扩展的影响,因而其下降程度不大.     

TiB_2/WC/h-BN自润滑陶瓷材料的制备及力学性能

通过添加固体润滑剂h-BN,采用真空热压烧结工艺制备TiB2/WC/h-BN自润滑陶瓷材料,对其密度和力学性能进行测试,并采用XRD和SEM对材料的物相和显微结构进行分析。结果表明:TiB2/WC/h-BN自润滑陶瓷材料的相对密度为97.5%,抗弯强度、断裂韧性和维氏硬度分别为652MPa、4.5MPa·m1/2和14.8GPa。片状结构的固体润滑剂hBN颗粒保存完整,分布均匀,而且可有效抑制基体TiB2晶粒的长大。材料的断裂模式是穿晶/沿晶断裂的混合型,以穿晶断裂为主。材料的增韧机理以裂纹偏转和裂纹桥联为主。TiB2/WC/h-BN自润滑陶瓷材料的摩擦因数低于0.3,与其他添加h-BN的自润滑陶瓷材料相比,具有良好的力学性能和摩擦性能。     

陶瓷刀具材料断口形貌及裂纹扩展的分形特征

通过热压烧结工艺,制备了一种高性能的Si3N4基陶瓷刀具材料。利用X射线衍射仪和扫描电子显微镜分别对材料物相组成、微观形貌及裂纹扩展路径进行了分析。借助图像处理技术和分形理论,计算了断口形貌及裂纹扩展路径的分形维数,并揭示材料断裂机制。研究表明,Si3N4基陶瓷刀具材料表现为穿晶/沿晶的混合断裂模式,其裂纹扩展方式主要是偏转和桥联,断口形貌及裂纹扩展均具有明显的分形特征。当材料断口形貌越粗糙,裂纹扩展路线越不规则,分形维数值增大,表明断口微观结构的粗糙程度、裂纹扩展路线的不规则程度可用分形维数来刻画。     

柠檬酸盐聚合物前驱体法制备锆钛酸铅纳米复相陶瓷研究

采用聚合物前驱体法成功制备出PZT/ZrO2纳米复相陶瓷。烧结过程中ZrO2相从固溶体中析出,制备出内晶型纳米复相陶瓷。对物相、组成和微观结构进行了分析和研究。随ZrO2的加入量增加断口从沿晶穿晶混合断裂变为穿晶断裂。     

TiN/O''''-sialon复相陶瓷的常温导电性能研究

对原位TiN/O'-sialon纳米复相陶瓷(NTS)和采用"二步法"制备的TiN/O'-sialon复相陶瓷(TS)的常温导电性能进行了对比研究,并对材料TS进行了放电加工.研究结果表明,初始原料中20%(质量分数)和25%(质量分数)左右的TiO2加入量是决定材料NTS和TS中TiN能否形成导电网络的最低TiO2加入量,该导电临界值同基相O'-sialon与导电相TiN的颗粒尺寸比有关,此时相应材料的电阻率为1.6×10-2和1.8×10-2 Ω·cm,满足放电加工的需要.烧结温度升高,两种材料的电阻率略有降低.随放电加工速度的增加,材料TS加工表面粗糙度明显增加.     

物理金相三级人员试题参考答案

二　问答题2 4 .沿晶断裂是裂纹沿晶界扩展而造成金属材料的脆断。在失效分析中常见的有以下几种类型的沿晶断裂形式 :(1)晶界沉淀相造成的沿晶断裂。晶界上不连续的沉淀相在外力作用下 ,在其周围首先形成微孔 ,后经长大连接成为晶界裂纹 ,最后造成沿晶断裂 ,此时为沿晶韧窝断裂。若晶界上沉淀相粒子形成脆性网状薄膜 ,此时断裂为脆性薄膜分裂型。(2 )杂质元素偏聚造成的沿晶脆断。杂质元素Ｐ ,Ｓ ,Ｓｂ ,Ａｓ和Ｓｎ等在晶界上存在并达到一定浓度时 ,它们将降低晶界内聚能 ,从而引起脆断。如高强度低合金钢的回火脆性就是典型例子。(3)由环…     

烧结温度对20%ZrO_2(3Y)/Al_2O_3复相陶瓷力学性能和微观结构的影响

20%纳米ZrO2(3Y)粉末加入到高纯亚微米Al2O3粉中,采用高压干压成型方法和恒速升温多阶段短保温烧结方法制备出不同烧结温度下的复相陶瓷。研究烧结温度对复相陶瓷力学性能的影响,通过XRD,EDS和SEM对复相陶瓷进行元素组成和微观结构分析。结果表明:烧结温度在很大程度上影响着复相陶瓷的力学性能和微观结构,常压烧结1600℃保温8h时,相对密度、维氏硬度和断裂韧性达到最大,分别为98.6%,18.54GPa和9.3MPa·m1/2,而基体晶粒尺寸为1.4~8.1μm,ZrO2相变量为34.6%。1600℃下复相陶瓷具有优质的微观结构,断裂方式为沿晶-穿晶混合断裂模式。ZrO2(3Y)粉体的加入,从相变增韧、内晶型颗粒增韧和裂纹偏转等多个方面提高了复相陶瓷的断裂韧性。     

The Fabrication and Properties of the Squeeze-Cast TiN/Al Composites

Co-continuous TiN/Al composites with different volume fractions of Al phase have been fabricated by the squeeze-casting method. TiN porous ceramics with different porosities were fabricated through carbothermal reduction by changing the content of TiN and were used as preforms. The outstanding mechanical properties were attributed to the absence of excessive interface reaction between TiN and Al for the co-continuous TiN/Al composites. With the increase of Al content in the composites, the flexural strength and the microhardness decreased, and the fracture toughness increased. The strengthening and toughening mechanism of composites included dislocation strengthening, ductile rupture, crack deflection, and secondary cracks.     

The contributions of microstructural characteristics and residual stress distribution to mechanical properties of AlN/W composite system

AlN–W composites were fabricated by pressureless sintering. The residual stress and mechanical properties were investigated for AlN–W composites with different tungsten contents. The fracture strength decreased and the fracture toughness increased with increase of the tungsten phase from 5 vol% to 20 vol%. It was thought that the decrease of strength would be due to the effects of relative density and the grain boundary phases. The crack tortuosity of indented specimens was gradually marked as a function of secondary phase content. In the residual stress measurement, the matrix phase of composites exhibited compressive stress distribution, which was increased with the addition of tungsten. This tendency was consistent with the results predicted by the thermoelastic stress model. From the results of the crack propagation path and the development of fracture toughness, it was thought that the residual compressive stress of matrix phase must have contributed to the development of crack propagation resistance.     

Improved fracture toughness and fatigue life of carbon fiber reinforced epoxy composite due to incorporation of rubber nanoparticles

In this study, core–shell rubber (CSR) nanoparticles with approximate particle size of 35 nm were used as a modifier for the epoxy polymer. The effects of various CSR contents in the epoxy matrix on mode I interlaminar fracture toughness, tensile strength, and fatigue life of the carbon fabric reinforced epoxy (CF/EP) composites were investigated. The experimental results showed that the mode I interlaminar fracture toughness at crack initiation and propagation significantly improved by 71.21 and 58.47 %, respectively, when 8.0 wt% CSR was dispersed in the epoxy matrix. The fatigue life of the modified CF/EP composites at all of CSR contents dramatically increased 75–100 times longer than that of the unmodified CF/EP composites at high cycle fatigue while tensile strength slightly increased by about 10 %. Field emission scanning electron microcopy (FESEM) observations of the fracture surfaces were conducted to explain failure mechanisms of CSR addition to the CF/EP composites. The evidences of the rubber nanoparticle debonding, plastic void growth, and microshear banding were credited for delaying the onset of matrix crack, and reducing the crack growth rate, as a result, attributed to increase in the mechanical properties of the CF/EP composites.     

反应烧结法制备(AlN,TiN)-Al2O3复合材料的研究

以Ti,Al,Al₂O₃为初始粉料,通过750~800℃氮气保护下的中温焙烧,然后在1420~1550℃在氮气氛下反应烧结,制备了不同配比的(AlN,TiN)-Al₂O₃复合材料。研究了组成及烧结工艺对复合材料力学性能、显微结构等的影响。用XRD,SEM等方法分析粉体及烧结体的相组成及微观结构。分析结果表明:AlN,TiN的形成,有助于材料的致密化并使其力学性能提高。组成为20wt%(Al,Ti)-Al₂O₃的粉体在1520℃、30MPa、保温、保压30min热压烧结条件下,与N₂气反应可得到硬度(HRA)为 94.1的高硬度的(AlN,TiN)-Al₂O₃复合材料,该材料的抗弯强度为687 MPa,断裂韧性(K_IC)为6.5MPa·m1/2。     

A method for testing interlaminar dynamic fracture toughness of polymeric composites

Static and dynamic Mode I delamination fracture in two polymeric fiber composites was studied using a WIF test method. The dynamic test was conducted on a Split Hopkinson Pressure Bar apparatus. Crack speeds up to 1000 m/s were achieved. Dynamic fracture and crack propagation were modeled by the finite element method. Dynamic initiation fracture toughness of S2/8552 and IM7/977-3 composites were obtained. The dynamic fracture toughness of IM7/977-3 associated with the high speed propagating crack was extracted from the finite element simulation based on the measured data. It was found that the dynamic fracture toughness of the delamination crack propagating at a speed up to 1000 m/s approximately equals the static fracture toughness.     

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of ?31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of ?53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.     

利用微米划痕研究TiN涂层的失效机理

利用微米划痕仪和扫描电子显微镜,研究了TiN涂层的失效过程,分析了摩擦系数波动的机理,并评估了涂层的断裂韧性。结果表明:低载荷阶段,TiN涂层发生剪切滑移,划痕轨迹边缘出现了由反向V形裂纹到平行裂纹,再到正向V形裂纹的裂纹形式转变;随着法向载荷的增大,TiN涂层发生阶梯式剥落。摩擦系数的波动主要由于表面形貌和涂层失效的影响,不同的加载方式,摩擦系数有一定差别。采用微米划痕可以较准确地测试TiN涂层的断裂韧性,与压痕断裂法和有限元模拟法的测试结果相吻合。     

Effect of compressive transverse normal stress on mode II fracture toughness in polymeric composites

Effect of transverse normal stress on mode II fracture toughness of unidirectional fiber reinforced composites was studied experimentally in conjunction with finite element analyses. Mode II fracture tests were conducted on the S2/8552 glass/epoxy composite using off-axis specimens with a through thickness crack. The finite element method was employed to perform stress analyses from which mode II fracture toughness was extracted. In the analysis, crack surface contact friction effect was considered. It was found that the transverse normal compressive stress has significant effect on mode II fracture toughness of the composite. Moreover, the fracture toughness measured using the off-axis specimen was found to be quite different from that evaluated using the conventional end notched flexural (ENF) specimen in three-point bending. It was found that mode II fracture toughness cannot be characterized by the crack tip singular shear stress alone; nonsingular stresses ahead of the crack tip appear to have substantial influence on the apparent mode II fracture toughness of the composite.     

Short-fiber/particle hybrid reinforcement: Effects on fracture toughness and fatigue crack growth of metal matrix composites

We have studied the effects of short-fiber/particle hybrid reinforcement on fracture toughness and fatigue crack growth in metal matrix composites. Reinforcement hybridization was achieved by a hybrid preform process, and composites were fabricated by the squeeze casting method. Al6061 matrix alloy and four composites having different short-fiber/particle ratio were tested. The fracture toughness (K_IC) and the fatigue threshold (ΔK_th) increased with increasing particle contents, whereas the Paris’ exponent (m) was insensitive to the short-fiber:particle ratio. These results emerged as a shift of the crack growth curve which implies on enhanced crack resistance over the entire stress intensity factor range. The positive aspect of particulate reinforcement is advocated by comparison of microstructural variables, and by observation of the crack path and surfaces. The characteristics of hybrid composites in damage tolerance are emphasized.     

(SiCP/Cu)-铜箔叠层复合材料的制备与组织性能

采用浸涂法和热压烧结法制备了（SiC_P/Cu）-铜箔叠层复合材料,研究了SiC_P含量对材料组织结构、拉伸性能和断裂韧性的影响。结果表明,制备的（SiC_P/Cu）-铜箔叠层复合材料层间厚度均匀,界面结合力良好,增强颗粒SiC能够弥散分布于黏结相中和界面处。随着SiC_P体积分数的增加,（SiC_P/Cu）-铜箔叠层复合材料的抗拉强度和屈服强度都先增加后降低,当SiC_P的体积分数为20vol%（总体积为100）时,其抗拉强度和屈服强度达到最大值,分别为226.5 MPa和113.1 MPa,断裂方式主要为韧性断裂和部分脆性解理断裂。裂纹扩展方向平行于层界面时,材料的断裂韧性随SiC_P体积分数的增加略有减小,SiC_P体积分数为15%时达到最大值16.96 MPa·m^1/2;裂纹扩展方向垂直于层界面时,（SiC_P/Cu）-铜箔叠层复合材料的断裂韧性随SiC_P体积分数的增加逐渐减小,SiC_P体积分数为15%时达到最大值12.51 MPa·m^1/2。