氮化硅轴承球表面层残余应力的形成机理及试验研究

本文论述了烧结热应力及研磨加工对氮化硅轴承球表面残余应力的影响机理.通过比容积测定,对氮化硅轴承球表面残余应力分布进行了分析计算,并就研磨加工对氮化硅轴承球表面残余应力的影响进行了试验研究.     

Experimental Investigation on Effects of Wire Electro Discharge Machining of Ti<Subscript>50</Subscript>Ni<Subscript>45</Subscript>Co<Subscript>5</Subscript> Shape Memory Alloys

TiNiCo shape memory alloy is most popular shape memory alloy for biomedical applications due to their outstanding properties such as shape memory effect, pseudoelasticity and transformation temperature. Machining of such kind of alloys is very difficult through conventional machining process is very difficult because they may affect their internal properties of these alloys. However conventional machining processes give poor surface quality during the machining hence non-conventional machining processes such as (wire electro discharge machining, water jet machining and electro discharge machining etc.) are more suitable for machining of such kind of alloy. From the literature it has been found that Wire electro discharge machining (WEDM) is more suitable non-conventional machining process for such kind of alloy. Present study exhibits the effects WEDM characteristics of Ti₅₀Ni₄₅Co₅ shape memory alloy. L-9 orthogonal array has been created by using Taguchi as a design method for machining of selected alloy and machined surface characterization has been carried out at the optimized process parameters with respect to microstructures, surface topography, microhardness, XRD analysis and residual stresses. To find the optimum setting of the input process parameters a couple of optimization techniques are used, namely principal component analysis (PCA) and Gray relational analysis (GRA) technique. 125μs pulse on time (T_on), 35μs pulse off time (T_off) and 40V servo voltage (SV) were found as an optimal setting for the higher material removal rate (MRR) with better surface roughness (SR) in the present study. Moreover, characterization of the machined surface is performed with respect to microstructures, surface topography, microhardness analysis, XRD and residual stresses. Harder surface observed near the cutting edge and TiNio₃ Tio₂ and CuZn were noticed on the surface of machined component through XRD analysis. However, compressive residual stress has been noticed on the machined surface during WEDM process.     

Indentation Behavior of Ion-Exchanged Glasses

Indentation fracture mechanics was extended to include the effect of a thin layer of residual stress on the identation strength of brittle materials. The proposed theory was used to predict the residual stress values for an ion-exchanged glass. For flaws placed before the exchange, considerable strengthening was observed, but the value of the surface stress predicted was considerably underestimated. For flaws placed after the exchange, there was no strengthening and the value of the surface stress was predicted to be zero. The failure of the indentation analysis indicates that it has to be modified for accurate stress determination. Thin layers of residual stress were found to retard the initiation of surface damage, but their influence on the strength after damage initiation was minimal.     

Study on mechanism of chip formation of grinding plasma-sprayed alumina ceramic coating

The mechanisms of ductile–brittle transition and surface/subsurface crack damage during the grinding of plasma–sprayed alumina ceramic coatings were investigated in an experiment and simulation on single diamond abrasive grain cutting. We observed that the brittle damage modes of alumina ceramic include boundary cracks, median cracks and lateral fractures. The normal force of the abrasive grain results in the initiation of median cracks, whereas the tangential force of the abrasive grain results in the propagation of median cracks in the direction of the abrasive grain cutting. Some cracks propagate downward to form machined surface cracks, whereas others propagate to the unmachined surface of the workpiece to produce brittle removal. Owing to the alternating tensile and compressive stresses, the material in contact with the top of the abrasive grain fractures continuously, forming the main morphology of the machined surface. The geometry and cutting depth of the abrasive grain have a significant influence on the ductile–brittle transition, whereas the cutting speed of the abrasive grain have no significant influence. On one hand, the stress concentration at the pore defects result in crack propagation to the deep layer; on the other hand, it reduces the local strength of the surface material, produces brittle fracturing, and interrupts crack propagation. The pores exposed on the machined surface and the broken morphology around them are important factors for reducing the surface roughness. Experimental observations show that the machined surface morphology of the alumina ceramic coating is composed of brittle fracturing, ductile cutting and plowing, cracks, original pores, and unmelted particles.     

A step toward molecular-level understanding of the toughness of glassy plastics

Rigid, glassy plastics differ over a wide range in the amount of energy they can absorb before fracturing under the influence of an applied stress. It is here proposed that molecular structure influences energy absorption more significantly by determining the onset of fracture than by its effect on the energy-dissipating processes themselves. Emphasis has been placed, therefore, on finding a relationship between molecular structure and the onset of fracture. Since many relationships between molecular structure and bulk mechanical response have already been established, it should be a useful step to relate the onset of fracture to bulk mechanical responses. The hypothesis is developed that the appropriate bulk property is the stress-deformation function of the material. The observed stress-deformation function is of such a nature that a highly non-homogeneous deformation is produced at any point of stress concentration. Typically, this is a narrow region in which the material is highly deformed while the deformation in adjacent material is still slight. Fracture initiation is postulated to occur in such a region of high deformation. Differences among polymers in the shape of the stress-deformation function are demonstrated experimentally. These differences correlate with tough-brittle behavior. An explanation is proposed in terms of the probability of fracture initiation.     

Al2O3陶瓷表面加工残余应力的研究

讨论了四种不同的工艺对Al2O3-TiC复相陶瓷进行表面处理，以改变其表面残余应力分布状态。利用扫描电子显嫩镜(SEM)观察试样的表面形貌；采用非破坏性的X射线衍射sin^2ψ法，对不同工艺条件下的残余应力进行了测试研究。结果表明：在本研究条件下，经平面磨处理后．试样表面赴于拉应力状态；经工具磨处理后，试样表面处于压应力状态；经抛光处理后，残余应力值均有所减小。这对于提高陶瓷材料的强度和耐磨性具有重要的意义。     

ZTM陶瓷表面机加工损伤的初步研究

本文对热压烧结ＺＴＭ／ＳｉＣｐ陶瓷的表面机加工损伤及其对材料强度的影响进行了研究，发现机加工在陶瓷表面引入两类损伤，即机加工裂纹及机加工表面残余应力。材料在机加工后强度下降较大，而机加工后的退火处理则可以使强度有所提高。     

影响接插件电镀金层分布的主要因素

就接插件中接触体的基体形状、电镀电源、电镀方式以及镀件装载量对镀金层在镀件表面分布的影响进行了分析。总结出选择较低的电流密度,适当的镀件装载量,采用双向脉冲电源和新型的滚筒或振动装置,以及针对不同结构的接触体采用不同的工艺流程等方法,可以促进接插件接触体镀金层的均匀分布。     

不同的造孔剂对陶瓷结合剂cBN砂轮性能的影响

砂轮中具有适量的气孔可以在磨削的过程中起良好的冷却、容屑排屑及促使砂轮自锐的作用。选用四种不同的造孔剂,在相同的试验条件下,通过添加不同含量的造孔剂,对比和分析了造孔剂的种类与用量对结合剂强度及砂轮性能的影响。试验结果为:添加4种不同的造孔剂均对结合剂的强度有一定的影响,造孔剂的含量越高,结合剂的强度下降幅度越大。氧化铝空心球及空心玻璃微球造出的气孔比较均匀,且孔隙率比较容易控制,制备出的砂轮在磨削的过程中容易产生较多的气孔,兼顾了砂轮的强度与锋利性。以硫酸铵为造孔剂制备的砂轮气孔比较细小而均匀,而且气孔之间是连通的,但硫酸铵对结合剂的强度影响较大,不宜过多添加。以碳粉为造孔剂制备的砂轮中有轻微的碳粉残留,对砂轮的强度影响较大。     

Temperature-dependent fracture strength and the effect of oxidation for ZrB2-SiC ceramics

Using the stress distribution of the body containing a spherical inclusion, the stress intensity factor at the tip of the annular flaw emanating from the inclusion is formulized. Since the thermal expansion coefficient of matrix and inclusion is not matched, the residual stress is also taken into account. Introducing into the proposed temperature-dependent fracture surface energy or fracture toughness, the temperature-dependent fracture strength for ZrB₂-SiC is obtained. The influence of oxidation on the fracture strength is also discussed and the analysis reveals that the oxidation has significant effect on the fracture strength under some circumstances. The calculated results are compared with the experimental data and they have very good consistency.     

Study of Materials Deformation in Nanometric Cutting by Large-scale Molecular Dynamics Simulations

Nanometric cutting involves materials removal and deformation evolution in the surface at nanometer scale. At this length scale, atomistic simulation is a very useful tool to study the cutting process. In this study, large-scale molecular dynamics (MD) simulations with the model size up to 10 millions atoms have been performed to study three-dimensional nanometric cutting of copper. The EAM potential and Morse potential are used, respectively, to compute the interaction between workpiece atoms and the interactions between workpiece atoms and tool atoms. The material behavior, surface and subsurface deformation, dislocation movement, and cutting forces during the cutting processes are studied. We show that the MD simulation model of nanometric cutting has to be large enough to eliminate the boundary effect. Moreover, the cutting speed and the cutting depth have to be considered in determining a suitable model size for the MD simulations. We have observed that the nanometric cutting process is accompanied with complex material deformation, dislocation formation, and movement. We find that as the cutting depth decreases, the tangential cutting force decreases faster than the normal cutting force. The simulation results reveal that as the cutting depth decreases, the specific cutting force increases, i.e., “size effect” exists in nanometric cutting.     

Investigation on ultrasonic vibration-assisted femtosecond laser polishing of C/SiC composites

This paper proposed a novel ultrasonic vibration-assisted femtosecond laser polishing method for C/SiC composites. The effect of near-field convection enhancement of ultrasonic vibration can improve the cooling of ablated particles and reduce their tendency of bonding to the material surface, reducing surface oxidation and improving the machined surface quality, removal depth and material removal rate. Through optimizing defocusing distance and scanning speed, a specific relationship between ultrasonic amplitude, pulse energy density, and spot overlap rate was established, obtaining a smooth and flat surface without defects. The residual stress of carbon fibers was investigated, and found that the coupling effect of ultrasonic energy and laser energy fields can enhance the residual compressive stress of carbon fibers. The formation of typical features of fiber fracture and pulling-out, banded pits, voids and deposition, was explained. This paper proposes new research ideas for better understanding of the removal mechanism of C/SiC composites using ultrasonic vibration-assisted femtosecond laser.     

Residual stress trend in thermal barrier coatings in through thickness direction measured by photoluminescence piezospectroscopy

Abstract

Photoluminescence piezospectroscopy (PLPS) has been used to determine residual stresses in sapphire, alumina in the yttria stablised zirconia (YSZ)/Al₂O₃ composite and alumina in thermal barrier coatings (TBCs). The TBC of YSZ containing 0·5?wt-% alumina has been produced using electron beam physical vapour deposition. The stress profile through the TBC thickness was measured using a depth sensing method. Reasonable residual stress profiles have been obtained using PLPS with the confocal system for all three material systems. Measurements of TBCs suggest that stress distribution in a TBC system is not uniform in general. However, uniform stress distribution has been found in some positions where damage in TBCs might occur.     

Impact strength of polymers 2: The effect of cold working and residual stress in polycarbonates

The influence of cold working on the toughness improvement in glassy amorphous polycarbonates was studied. Cold working processes, namely rolling and. Steckel rolling were used to produce thickness reductions up to 40 percent in flat-strip specimens. The notched Izod impact strength and tensile properties were measured as a function of strip thickness reduction. It was shown that the toughness enhancement in polycarbonates cold worked to low thickness reductions was due to the residual stress state present as opposed to molecular orientation which becomes significant at higher degrees of cold work. Residual stress measurements were made by using the layer removal technique. Residual tensile stresses as high as 2100 psi were present in 1/4-in. cold-rolled polycarbonate at the surface. The maximum stress in the center of the specimen was 1100 psi in compression. The residual stresses at the surface decreased with increasing thickness reduction. The residual stress state for Steckel rolled. 1/2-in. polycarbonate was also measured and found to be more complex than for the thinner samples, The results demonstrated that surface tensile stresses and interior compressive stresses can produce large values of impact strength if the notch is to be machined after cold working. Thus, the values of impact strength measured from the notch Izod specimen are sensitive to the residual stress state in the polymer. This behavior is in contrast to earlier studies on thermally quenched material in which the material was quenched after notching. The thermal quenching produced surface compressive stresses which were also present at the notch tip. The presence of compressive residual stresses at the center of the notch suppressed the formation of a craze leading to toughness enhancement in cold worked polycarbonate strips. It is shown that by control of residual stresses in polycarbonate, strips at least 1/2 in. in thickness can be made to exhibit ductile failure in the notched Izod impact test.     

(Ni-P)-TiO2复合化学镀层的制备

将TiO2颗粒引入Ni-P合金镀液,采用化学镀的方法在黄铜上制备了(Ni-P)-TiO2复合镀层.利用扫描电镜、X-射线能谱仪、X-射线衍射仪和比表面积测试等检测手段,对(Ni-P)-TiO2复合镀层的形貌、化学组成、相结构以及其比表面积进行了分析.研究结果表明:(Ni-P)-TiO2镀层表面为均匀分布绒丝状复合物;T...     

Grinding characteristics of cBN-WC-10Co composites

In order to explore the grinding characteristics of cBN-WC-10Co composites, the grinding experiment with a resin bond diamond grinding wheel was carried out. The grinding forces, surface roughness, surface morphology and residual stress were investigated. It was found that the material removal mechanism of cBN-WC-10Co was the combination of the brittle fracture of cBN particles, ductile removal of Co phase, plastic deformation, grain dislodgement and grain crush of WC grains. The brittle removal model resulted in a lower specific grinding energy. The main contributor to the surface roughness was cBN particles. Some cBN particles over the surface of cBN-WC-10Co composites were fractured or pulled out and then formed cavities with different depths, this led to a rougher surface. The surface roughness was increased but the specific grinding energy decreased with an increase of the maximum undeformed chip thickness. A high-level residual compressive stress was induced at WC phase and it was increased with an increase of the depth of cut. The depth of cut has more significant influence on the grinding forces than the table speed or the wheel speed.     

Compressive Surface Strengthening of Brittle Materials by a Residual Stress Distribution

A theoretical approach is presented for predicting the strengthening of brittle materials subjected to a residual stress distribution represented by a polynomial series. In the approach, the stress intensity factor for a surface crack is derived incorporating the effect of crack closure. The crack-closure distance is then calculated using an approximate approach which allows the strengthening due to the residual stresses to be estimated. Illustrating the approach using residual stresses typical of tempering, it was found the approach agreed well with previous work. The influence of partial crack closure was found to give higher values of the stress intensity factor than would be calculated if the crack were assumed to be open. This effect decreases the amount of strengthening predicted and gives a wide range of conditions for which subcritical crack-growth processes can occur. For the example of tempering it was also found that these are conditions when weakening or spontaneous failure of the body can occur.     

激光熔蚀内部孔穴形态及相态变化

采用分子动力学方法对皮秒脉冲激光在晶体材料内部熔蚀出空穴的过程进行了模拟研究。通过记录整个烧蚀过程中粒子速度和位置的变化得到了材料形态变化的原子图像;同时,对粒子动能和受力进行统计分析得到了整个材料内部的应力传导和温度变化分布,并利用P-T相图对特定位置处的材料相态变化进行了分析。结果表明：仅处于特定能量密度范围内的皮秒脉冲激光可在晶体材料内部加工出孔穴;激光能量在烧蚀点处的累积以及转化会导致材料内部出现极强的应力波,并且应力的传递将导致烧蚀点处熔池中材料的喷溅与分离,并使材料内部产生结构缺陷;此外,在分析材料内部的温度分布时可以发现激光烧蚀过程中的热影响区极小,并且大部分在加热过程中被破坏的晶格结构在材料冷却后能得到恢复,但晶体材料中仍会存在分散的片状晶格缺陷。     

Functional gradient coating of alumina on net shaped zirconia implant: Improved strength,aging resistance,and role of residual stress

Aging of zirconia in vivo has been widely discussed as a potential cause of implant degradation over time. Hydrothermal degradation is sensitive to composition, process conditions, and microstructure leading to an emphasis on secondary phases, and grain boundary engineering for aging resistance. However, surface coatings, resultant residual stress, and associated physical constraint for phase stabilization are insufficiently explored. Herein a novel ceramic dough processing facilitated the formation of a functional gradient alumina coating (20–50 µm) below the critical thickness, on net-shaped green zirconia dental implant while preserving the fine machined threads. Residual stress (~ ?0.8 GPa) after sintering improved the characteristic strength by ~ 45% with a simultaneous contribution to profound phase preservation after in vitro aging. Thus, the compositional gradient coating on green zirconia components using alumina-based slurries is a facile surface modification technique to inhibit moisture-induced aging.     

Theoretical model of warping deformation during self-rotating grinding of YAG wafers

YAG wafers are the most host laser crystals used for high-power lasers, which are usually machined by grinding to meet the required accuracy for laser components. Warping deformation induced by the residual stress is one of the main damages for YAG wafers after the grinding process, which will seriously decrease the service accuracy and life of the lasers. Developing theoretical model of warping deformation is of great significance to achieving the ultra-precision machining of YAG wafers. The cutting depth of single abrasive and grinding force in self-rotating grinding were investigated by considering the kinematic trajectory of abrasives, brittle-to-ductile transition, elastic mechanics, elastic deformation of the grinding wheel and strain rate effect. A theoretical model of warping deformation in self-rotating grinding of YAG wafers was developed based on the cutting depth and grinding force. The influence of subsurface damage and residual stress on warping deformation was analyzed based on the theoretical model and finite element simulation. Self-rotating grinding tests of YAG wafers were performed, and the results showed that the warping deformation decreased as the wheel rotational speed increased, and increased as the abrasive size, workpiece rotational speed and feed speed increased. The experimental results agreed well with the simulated results of the theoretical model, indicating that the theoretical model can accurately predict the warping deformation induced by self-rotating grinding process. This work will not only enhance the understanding of the essence of the wafer warping induced by ultra-precision machining, but also provide a guide for optimizing the processing parameters in self-rotating grinding of YAG wafers.