Compressive surface strengthening of brittle materials

A theoretical approach has been put forward for predicting the strengthening of materials by the introduction of surface compressive stresses. An approximate technique was used to determine the closure length of a linear surface crack which extends through the compressive surface layer. The stress intensity factor of the partially closed crack was then determined for the case of an applied tensile stress, with the assumption that the residual surface compressive stress was uniform within the surface layer (step function). The analysis shows that the strengthening depends on the magnitude and depth of the compressive surface stress. It is found that partial crack closure decreases the amount of strengthening compared with that predicted for an open crack, and that for large compressive surface stresses the amount of strengthening can saturate.     

On machining of hard and brittle materials using rotary tool micro-ultrasonic drilling process

The present paper reports on a recently developed rotary tool micro-ultrasonic drilling (RT-MUSD) process. The RT-MUSD process was utilized for machining of micro-holes in zirconia, silicon and glasswork materials. The effects of work material properties on the performance characteristics (material removal rate (MRR), depth of hole and hole overcut) of RT-MUSD process were investigated by varying the power rating, rotation speed, abrasive size and slurry concentration. Additionally, machined micro-holes and tool surface were analyzed considering microscopic images. The experimental results revealed that the MRR and depth of hole increased by increasing the power rating. An increase in rotation speed up to 300 rpm, abrasive size up to #1200 mesh and concentration up to 20% increased the MRR, depth of hole and decreased hole overcut. The maximum machining rate and hole overcut were observed during machining of silicon followed by glass and zirconia. The fracture toughness and hardness of the work material affected the MRR and tool wear, respectively. Pure brittle fracture mode of material removal was observed in all the work materials during RT-MUSD process. Eventually, the RT-MUSD process was optimized using desirability approach and a micro-hole of depth 4355 µm was achieved using optimal parameter settings.     

Fracture statistics of brittle materials with surface cracks

Several different statistical fracture theories are developed for materials with cracks confined to the surface. All assume that crack planes are normal to the surface, but are otherwise randomly oriented. The simplest theory assumes that only the component of stress normal to the crack plane contributes to fracture. This theory is in fair agreement with biaxial fracture data on Pyrex glass obtained by Oh. When the contribution of shear is included in the analysis, the crack shape has to be considered. Several shapes are examined and the corresponding fracture statistics are derived. Two failure criteria are employed. In one the fracture occurs when the maximum tensile stress on some part of the crack surface reaches the intrinsic strength of the material. The other is based on a critical strain energy release rate. The assumption of shear-sensitive cracks leads to improved agreement with experiment, but really good agreement appears to require the assumption that the cracks have a preferred orientation.     

表面改性对纳米蒙脱土/聚酰胺6-66复合材料性能的影响

选取三种不同的纳米蒙脱土（nMMT）（分别为钠基蒙脱土（Na-MMT）、氨基酸改性nMMT（OMMT-A）和CH₃（CH₂）₁₇N（CH₃）[（CH₂CH₂OH）₂]+改性nMMT（OMMT-B））,通过熔融共混法制备了不同改性的纳米蒙脱土/聚酰胺6-66（nMMT/PA6-66）复合材料,研究了不同表面改性对nMMT/PA6-66复合材料的结晶、流变和力学等性能的影响。结果表明,nMMT的加入促进了nMMT/PA6-66复合材料中γ晶的形成,提高了复合材料的结晶温度,但加入OMMT-B后OMMT-B/PA6-66复合材料的异相成核作用效率有一定程度减弱;同时,OMMT-B能更好地改善PA6-66的储能模量,提高PA6-66的流动性。力学性能测试表明,nMMT提高了nMMT/PA6-66复合材料的强度,降低了复合材料的韧性,但效果不同。其中,加入OMMT-B后OMMT-B/PA6-66复合材料的韧性几乎保持不变,拉伸强度和弯曲强度相对于纯PA6-66分别提高了26%和28%,表现出最佳的综合力学性能。综合研究结果表明,不同表面改性nMMT对PA6-66性能的影响主要取决于改性剂和PA6-66分子链之间相互作用的强弱。      

表面改性方式对纳米氧化锌紫外吸收性能影响研究

纳米氧化锌是一种重要的无机紫外吸收剂,在应用时需要对其进行表面改性和分散,本文研究了不同的改性方式对纳米氧化锌紫外吸收性能的影响,分析研究了其不同的改性机理.结果表明原位改性是一种较好的改性方式,但对纳米氧化锌的外观形貌产生了影响,主要原因是聚乙二醇作为模板剂和表面活性剂影响了其前驱体氢氧化锌的形貌,而纳米氧化锌粉体表面改性是通过聚乙二醇的空间位阻作用使纳米氧化锌分散程度提高.使用同一种改性剂对纳米氧化锌原位改性和对纳米氧化锌粉体改性,对其紫外吸收性能没有大的影响.     

Fatigue analysis of brittle materials using indentation flaws

A two-part study has been made of the fatigue characteristics of brittle solids using controlled indentation flaws. In this part a general theory is developed, with explicit consideration being given to the role played by residual contact stresses in the fracture mechanics to failure. The distinctive feature of the formulation is a stress intensity factor for well-defined indentation cracks, suitably modified to incorporate the residual component. Taken in conjunction with a standard power-law crack velocity function, this leads to a differential equation for the dynamic fatigue response of a given material/ environment system. Reduced variables are then introduced to facilitate generation of universal fatigue curves, determined uniquely by the crack velocity exponent,n. A scheme for using these curves to evaluate basic fracture parameters from strength data is outlined. In this way the foundation is laid for lifetime predictions of prospective brittle components, as well as for reconstruction of the crack velocity function. One of the major advantages of the analysis is the manner in which the residual stress parameters are accommodated in the normalized fracture mechanics equations: whereas it is understood thatall strength data are to be taken from test pieces in their as-indented state, so making it unnecessary to have to resort to inconvenient stress-removal procedures between the contact and failure stages of testing,a priori knowledge of the residual stress level is not required. The method is proposed as an economical route to materials evaluation and offers physical insight into the behaviour of natural flaws.     

Corrosion degradation and prevention by surface modification of biometallic materials

Metals, in addition to ceramics and polymers, are important class of materials considered for replacement of non-functional parts in the body. Stainless steel 316, titanium and titanium alloys, Co-Cr, and nitinol shape memory alloys are the most frequently used metallic materials. These alloys are prone to corrosion in various extents. This review briefly discusses the important biomaterials, their properties, and the physiological environment to which these materials are exposed. Corrosion performance of currently used metallic materials has been assessed and threat to the biocompatibility from corrosion products/metal ions is discussed. The possible preventive measures to improve corrosion resistance by surface modification and to increase the bioactivity of the metallic surfaces have also been discussed. Importance of the formation of oxide layers on the metal surface, another aspect of corrosion process, has been correlated with the host response. The gap areas and future direction of research are also outlined in the paper.     

Fatigue analysis of brittle materials using indentation flaws

The results of an experimental dynamic fatigue study on glass-ceramic specimens containing indentation flaws are analysed in terms of the theory developed in Part 1. A Vickers indenter is used to introduce the flaws, and a conventional four-point bend apparatus to break the specimens. Base-line data for testing the essential theoretical predictions and for evaluating key material/environment parameters are obtained from polished surfaces, i.e. surfaces prepared to a sufficient finish to ensure removal of any pre-existing spurious stresses. The fatigue tests are carried out in water. Inert strength tests in dry nitrogen are used to calibrate appropriate equilibrium fracture parameters, with dummy indentations on selected control specimens providing a convenient measure of the critical crack dimensions at failure. Regression analysis of the dynamic fatigue data yields values for apparent kinetic parameters, which are converted to true kinetic parameters via the transformation equations of Part I. Regeneration of the fatigue function from the theory using the parameters thus determined gives a curve which passes closely through the experimental data points, thereby providing a self-consistent check of the formalism. The implications of the results in relation to the use of macroscopic fracture parameters in the prediction of strength properties for materials with small-scale flaws is an important adjunct to this work. Finally, a recommended procedure for the general testing of dynamic fatigue properties of ceramics using indentation flaws is described.     

纳米材料在涂料中的改性和应用

着重介绍纳米材料在涂料中的改性与应用,论述了纳米材料的选择及分散方法,研究表明,纳米材料改性涂料的各项性能显著提高.     

高能机械化学法制备微纳米Mo—Si粉体

以Mo粉和Si粉为原料,通过自行设计的高能机械化学球磨机在室温下制备出了微纳米Mo-Si粉体,利用X射线衍射（XRD）和扫描电子显微镜（SEM）进行物相分析和粒度分析,结果表明,在不同的反应阶段会生成MoSi2和Mo5Si3,并且生成物之间相互有一定的转化,反应生成率接近100％,制备出的微纳米Mo—Si粒度在100nm左右,在Mo和Si的高能机械化学反应中,适当地提高球料比和转速可以加快反应的进程。     

SMA丝表面纳米SiO_2改性对SMA/环氧树脂复合材料界面粘结强度的影响

为了研究形状记忆合金(SMA)丝增强环氧树脂复合材料的界面粘结行为,首先通过单纤维拔出试验测定了SMA/环氧树脂界面的粘结强度,重点考察了埋入深度对界面极限粘结强度及其拔出行为的影响。然后,结合ABAQUS有限元分析方法,利用基于表面内聚力行为的单元对SMA丝拔出过程中应力分布随拔出时间的变化关系进行了模拟。最后,针对SMA/环氧树脂复合材料界面粘结强度较弱的缺陷,提出了利用纳米SiO2改性SMA丝表面提升材料界面粘结强度的方法,并通过拔出试验进行了验证。结果表明:随着埋入深度从1.0cm增加到1.5cm和2.0cm,最大拔出载荷显著增加,平均界面粘结强度却逐渐下降。当纤维埋入深度为2.0cm时,在0.300s时临界脱粘出现。利用在SMA表面涂覆纳米SiO2颗粒的方法可以增加纤维的表面粗糙度,进而有效提高SMA丝增强环氧树脂复合材料的临界拔出强度。研究结论为SMA丝在实际工程领域中的应用提供了理论指导。     

Mirau干涉型微纳台阶高度测量系统的研究

为了实现纳米量级到微米量级的微观三维台阶样板高度的快速测量,在普通光学显微镜的基础上改造完成了一台微纳台阶高度测量装置,设计了整体硬件结构,编写了测量控制软件和数据处理软件,并结合Hilbert变换和小波变换实现三维表面形貌测量。使用不同高度的台阶样品进行测试,测试结果表明:系统测量准确度较高,测量重复性较好,垂直测量范围大于50 μm。     

Micro/nano engineering on stainless steel substrates to produce superhydrophobic surfaces

Creating micro-/nano-scale topography on material surfaces to change their wetting properties has been a subject of much interest in recent years. Wenzel in 1936 and Cassie and Baxter in 1944 proposed that by microscopically increasing the surface roughness of a substrate, it is possible to increase its hydrophobicity. This paper reports the fabrication of micro-textured surfaces and nano-textured surfaces, and the combination of both on stainless steel substrates by sandblasting, thermal evaporation of aluminum, and aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). Meanwhile, fluorinated carbon films were used to change the chemical composition of the surfaces to render the surfaces more hydrophobic. These surface modifications were investigated to create superhydrophobic surfaces on stainless steel substrates. The topography resulting from these surface modifications was analyzed by scanning electron microscopy and surface profilometry. The wetting properties of these surfaces were characterized by water contact angle measurement. The results of this study show that superhydrophobic surfaces can be produced by either micro-scale surface texturing or nano-scale surface texturing, or the combination of both, after fluorinated carbon film deposition.     

高能机械化学法制备铁铝微／纳米粉体材料的研究

采用微米级的铁粉和铝粉,按Fe：A1原子比72：28进行配料,磨料为氧化锆（Zr02）陶瓷球,在试验中分别将球料比设为8：1,12：l以及15：1,将转速设为l000r／min,l200r／min以及1400r／min,通过x射线衍射仪（XRD）和扫描电子显微镜（SEM）分析各球磨条件对反应过程的影响,研究表明,在机械球磨过程中,塑性原材料生成新相的过程主要是反复撞击、压缩和剪切过程,相较脆性材料更易于发生颗粒团聚。试验成功地制备出铁铝金属间化合物,并且最终得到最佳试验条件为转速1200r／min,球料比12：1。转化率为93％以上。     

基于紫外-臭氧辐射的高分子材料表面改性研究进展

随着高分子复合材料的广泛应用,界面性能成为影响其综合表现的重要因素。紫外-臭氧辐射法(UVO)是一种简单经济的表面改性工艺,可激发高分子表面分子链,从而赋予其不同于本体性能的表面特性(如表面润湿性、与其他材料的界面相容性等),也为进一步的界面偶联提供反应位点。本文首先概述了国内外学者在电子、生物医疗等领域中采用UVO工艺改性不同种类高分子材料(聚酯类高分子材料、聚烯烃高分子材料、有机硅高分子材料、合成橡胶、纺织材料)表面的研究现状及相应研究成果;在此基础上,梳理了改性高分子材料表面与不同种类硅烷偶联剂(甲基丙烯酰氧基、环氧基、氨基、巯基硅烷偶联剂)的反应条件及复合材料界面性能;最后,对UVO工艺未来可能的应用空间进行了展望。     

金红石型纳米TiO2紫外屏蔽性能的相关应用及改性研究

金红石型纳米TiO2对可见光的透射性强,具有极好的吸收、散射紫外线能力,可作为紫外屏蔽剂广泛应用于涂料、化妆品、纺织、木材加工等行业.但纳米TiO2同时也具有光催化性能,是有机基体发生光氧化的催化剂,且在有机基体中的分散性和稳定性不好,需经表面处理以解决这些问题.本文针对金红石型纳米TiO2防紫外性能的相关应用、存在的...     

Effect of disturbed surface layer parameters on the structural strength of brittle nonmetallic materials

Strength of Materials -