金刚石固结磨料研磨K9玻璃的研究

为提高光学材料的研磨效率与质量,提出一种亲水性固结磨料研磨方法.采用图形转移与UV固化工艺,将粒径为5～10 μm的金刚石磨料固结于亲水性光固化树脂中,制备固结磨料研磨抛光垫(FAP).选取工件的材料去除率(MRR)和表面粗糙度(Sa)来评价研磨的加工性能.对比研究了在相同粒径磨粒下的游离磨料研磨、固结磨料丸片研磨、及亲水性FAP研磨三种不同方法对K9光学玻璃的加工性能.实验结果表明:采用FAP研磨K9玻璃,MRR为350 nm/min,表面粗糙度Sa为3.24 nm,达到了精研的加工效率和抛光的表面质量.提出了固结磨料抛光丸片和亲水性FAP的加工模型,以及亲水性FAP的自修整机理.     

Deterministic Microgrinding, Lapping, and Polishing of Glass–Ceramics

We report a series of microgrinding and polishing experiments on glass–ceramics. Microgrinding includes deterministic microgrinding (two-body abrasion at fixed infeed rate) and loose abrasive lapping (three-body abrasion at fixed pressure). We correlate material mechanical properties (Young's modulus, hardness, fracture toughness) and chemical properties (chemical susceptibility or mass loss under chemical attack) with the quality of the resulting surface (surface microroughness and surface grinding-induced residual stresses). We compare deterministic microgrinding (at fixed infeed) and loose abrasive microgrinding (at fixed pressure) in terms of material removal rates and resulting surface quality.     

注塑件残余应力影响因素的模拟实验研究

在分析注塑过程中残余应力产生机理和原因的基础上，应用软件模拟实验方法，研究了不同成型工艺条件对聚苯乙烯平板注塑件残余应力的影响，分别给出了沿厚度方向的残余应力和固化压力分布。研究发现：残余应力在厚向上呈拉-压-拉（表层-次表层-中间层）分布，并随熔体温度和保压压力升高而增大，固化压力是决定注塑件厚向残余应力分布的一个主要因素。     

Surface Integrity Effects on the Fracture Resistance of Electrical-Discharge-Machined WC–Co Cemented Carbides

The flexural strength evolution for two WC–16 vol% Co cemented carbides, with different mean carbide size, subjected to sequential and upgrading electrical-discharge machining (EDM) is studied. It is compared with the fracture behavior exhibited by a reference surface finish condition, attained through conventional mechanical grinding and polishing using diamond as abrasive. Considering that rupture is related to existing defects, either introduced during sample elaboration or induced by machining, a detailed fractographic examination by scanning electron microscopy is conducted to discern fracture origins. The experimental findings indicate that the flexural strength of WC–Co hardmetals may be strongly affected by EDM, depending on the correlation existing between natural defects, as given by particular microstructural parameters, and EDM-induced flaws. An analysis of the results using a linear–elastic fracture mechanics approach permits one to establish a clear connection between surface integrity and fracture resistance. Quantitative discrepancies between the estimated and the experimentally measured critical flaw sizes for all the EDM-related grades are rationalized through the existence of local residual tensile stresses of considerable magnitude at the shaped surface. Release of these stresses through final mechanical and annealing treatments is pointed out as a quite effective alternative for improving the fracture behavior of WC–Co cemented carbides shaped by EDM.     

Nano-precision diamond cutting tools achieved by mechanical lapping versus thermo-mechanical lapping

A comparison of lapping qualities of nano-precision diamond cutting tools achieved by the mechanical lapping versus thermo-mechanical lapping is carried out in this work. The experimental results indicate that in mechanical lapping, the removal rate is in the nanometric level, as evidenced by the groove depth ranging from several to tens nanometers. And as a result, a satisfied cutting edge radius is sharpened, ranging from 35 nm to 50 nm. In thermo-mechanical lapping, the material removal takes place not only on the surface apexes but also on the valley bottoms of grooves inherited in the previous mechanical lapping. However, the rake face has a damage layer induced by the previous mechanical lapping, and then the removal rate on the bottoms is slightly lower than that on the apexes in the initial lapping stage, as demonstrated by the lessened groove depth. Meanwhile, the successive departing of carbon atoms from diamond crystal lattice one by one is always necessary for the thermo-mechanical lapping, and resultantly, its removal rate is at atomic scale. As expected, a more sharpened cutting edge radius of less than 10 nm can be achieved. Moreover, the mechanical lapping is capable of finishing a surface roughness of 0.8 nm in Ra, but about 2.0 nm of thermo-mechanical lapping. Such difference implies that the achieved cutting edge radius is independent of the finally finished surface roughness but mostly decided by the dominant removal mode of lapped surface layer, i.e. its corresponding removal rate.     

Heat treatment of thin carbon films and the effect on residual stress, modulus, thermal expansion and microstructure

Thin carbon films are used to hermetically seal and improve the performance of devices exposed to extreme conditions. Such films, which are deposited by chemical vapor deposition, develop residual thermal stresses due to a mismatch in the coefficient of thermal expansion between the film and substrate. Residual stresses reduce the adhesion of the film, and are a common cause of coating failure. This work investigates heat treatment as a potential technique to reduce residual stresses in thin carbon films. The magnitude of the residual stress has been challenging to measure due to the associated size scales and mechanical properties. In this study, experimental measurements of mechanical properties and residual stresses in thin carbon films are performed using nanoindentation and Raman spectroscopy. The results relate surface residual stresses to film thickness and heat treatment temperature. The approach presented in this study is a nondestructive and non-intrusive method for measuring residual surface stress and properties in thin films, and is ideal for small or curved-surface specimens such as optical fibers and other photonic devices.     

Cr3+ microspectroscopy measurements and modelling of local variations in surface grinding stresses in polycrystalline alumina

Surface residual stresses caused by grinding and polishing of alumina are thought to influence materials properties but have previously been measured only by low spatial resolution techniques which sample average stresses. In this work confocal Cr³⁺ fluorescence microscopy has been used to investigate the spatial distribution of the residual stresses. A model for the residual stresses, accounting for both surface plastic deformation and “pullout” of material from the surface by brittle fracture, was developed to help in analysing the results. After coarse diamond grinding, the results showed that the residual stresses fluctuate greatly with position. Large tensile stresses (up to ～600 MPa) were found below the plastically deformed surface layer in regions between the “pullouts”. These tensile stresses are expected to aid crack propagation and further surface pullout. They arise because pullout removes parts of the plastically deformed surface layer. The stresses beneath the pullout sites themselves were compressive, but the largest compressive stresses (≈?1.5 GPa) were within the plastically deformed surface regions and extended to a depth of 1.3 μm. The plastically deformed surface layer was much shallower following polishing with 3 μm diamond paste but the compressive stress within it was of similar magnitude to that in the plastically deformed surface layer caused by grinding.     

Development of warpage and residual stresses in film insert molded parts

Residual stresses, bending moments, and warpage of film insert molded (FIM) parts were investigated by experimental and numerical analyses. Thermally induced residual stresses in FIM parts were predicted by numerical simulations with both commercial and house codes. Bending moments and warpage of FIM tensile specimens were calculated numerically and compared with experimental results. Thermally induced residual stresses were predicted by utilizing a one‐dimensional thermoelastic model where constant material properties are assumed. The residual stress distribution depended remarkably on the Biot number and the heat was removed rapidly through the surface resulting in high residual stresses. Asymmetric residual stresses generated by nonuniform cooling of the part provoked nonuniform shrinkage and warpage of the molded tensile specimen. It was found that the numerically calculated bending moment is in good agreement with the experimental results. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Contact accuracy and orientations affecting the lapped tool sharpness of diamond cutting tools by mechanical lapping

In this paper, a brittle–ductile transition lapping mechanism is proposed for the mechanical lapping of single crystal diamond cutting tools. The critical depths of cut for brittle–ductile transition and the dynamic critical tensile stress are regarded as the references to analyze the influence of contact accuracy between the rapid rotating scaife and lapped tool surface and the effects of tool face orientation on the sharpened cutting edge radius, respectively. The experimental results indicate that the vibrations of lapping setup, surface quality of scaife, dynamic balance and motion accuracy of the spindle, lapping compression force and lapping velocity all have enormous influences on the contact accuracy so as to affect the lapped cutting edge radius of diamond cutting tools. Under the optimal settings of each influencing factor for a robust contact accuracy, a perfect diamond cutting tool is sharpened in ductile lapping mode with a cutting edge radius of 30–40 nm and a surface roughness Ra of 0.7 nm on the tool rake face. On the other side, different orientation settings of tool faces enable diamond cutting tools edge to have different micro mechanical strength. Under the same configurations of the lapping parameters, the higher the micro mechanical strength of tool cutting edge is, the smaller the sharpened cutting edge radius would be.     

Prediction of edge and tunnelling crack formation in layered ceramics using a stress-energy fracture criterion

A coupled stress-energy criterion is utilized to predict initiation of both edge and tunnelling cracks in layered ceramics containing thermal residual stresses. Edge (surface) cracks may originate in layers having high compressive in-plane stresses while tunnelling (internal) cracks may form in layers with high tensile in-plane stresses. This work investigates the influence of both the residual stresses magnitude and layer thickness on the formation of surface cracks and provides a design map defining safe regions where no cracks will be present in the sintered multilayer architecture upon reaching the room temperature. Necessary stress and energy inputs to evaluate the coupled criterion are calculated using the finite element method. Simulation results are validated with experimental observations on sample architectures fabricated with layers of various thicknesses and in-plane thermal residual stresses. The good agreement demonstrates the potential of the stress-energy coupled criterion for designing crack-free multi-layered ceramic architectures.     

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.     

Contribution of residual stress to the strength of abrasive ground alumina

Residual stresses are introduced into the surface region of ceramics during abrasive grinding. The presence of the residual stresses can affect the strength of the ground specimens. In the present study, a methodology is proposed to determine the contribution of the residual stresses to the strength of the ground specimens. The method uses Weibull statistics to evaluate the crack size distribution before and after annealing. If the crack size distribution is not changed after annealing, the amount of the strength reduction is attributed to the effect of residual stresses as verified by direct measurements of the residual stress at the surface.     

Strength Distributions of a Quench-Strengthened Aluminosilicate Ceramic

The quench strengthening of an aluminosilicate ceramic body was studied by measuring the strengths and determining the strength distributions, estimating the residual compressive stresses at the surface by measuring the bending moments, and directly observing the surface crack patterns of quench-strengthened specimens. When considered in combination, these related results enable the assessment of the individual contributions of the residual compressive surface stresses and the changes in the flaw populations to the quench strengthening phenomenon. The quench strengthening phenomenon is discussed as a combination of those features of the process and a general schematic diagram of the phenomenon is presented.     

Residual stresses in polymers and their effect on mechanical behavior

Plates of bisphenol-A polycarbonate and poly(methyl methacrylate) have been quenched in ice water from temperatures slightly above their glass transition temperatures. Residual stresses are thus created, Measurement of these residual stresses has been accomplished by the “layer removal” method and the stress distributions through the thickness are presented. Compressive stresses, approximately 3000 psi, exist at the surface while tensile stresses-of at least 1000 psi exist in the interior. It is shown that these residual stresses can influence the notched Izod impact strengths for polycarbonates. The mechanism is thought to be suppression of craze initiation in advance of the notch due to the presence of residual compressive stresses for specimens notched prior to quenching. In the case of poly(methyl methacrylate), it is shown that compressive residual stresses at the surface can cause plastic yielding to occur in bending experiments resulting in permanent deformation and greater energy absorption.     

Residual stresses and cracking in large ceramic injection mouldings subjected to different solidification schedules

In ceramic injection moulding, the moulding dimensions and the residual stresses are related to the hold pressure history during solidification in the cavity. In conventional moulding, the residual stresses are generally compressive at the surface. In this work, an insulated sprue device was made. It allows prolonged pressure transmission to the moulding and this can result in residual tensile stresses at the surface of thick section mouldings. At very low holding pressures (<9 MPa) or under conditions of premature gate solidification, a reversal of the sign of surface stresses occurs with compressive stresses near the surface. The appearance of cracks during binder removal was related to these residual stresses. For 25 mm thick mouldings which were subjected to a low but persistent hold pressure of 5 MPa no defects developed during the binder removal stage.     

Determination of Surface Residual Stresses in Brittle Materials by Hertzian Indentation: Theory and Experiment

Hertzian indentation has been used to determine the surface residual stress levels in brittle materials. In this method, a hard sphere is pressed into the surface of the material: at a critical load a preexisting surface-breaking crack in the neighborhood of the contact will propagate. There is a threshold load below which no such crack, of whatever size, can be propagated. The presence of a residual stress in the surface will lead to a shift in this threshold load. The effects of residual stresses on the minimum load to produce Hertzian fracture are predicted for alumina and glass, assuming that the variation of the residual stress over the length of the crack is small. Two methods of analysis (one approximate, one more general) are presented that enable the residual stress to be calculated from the shift in threshold load; the only further information required is a knowledge of the radius of the sphere, the elastic constants of the sphere and substrate, and also the fracture toughness of the substrate (or use of a stress-free specimen as a reference). No measurement of any crack length is necessary. Experimental results are presented for the residual stress levels determined in glass strengthened by ion exchange. Indenting balls of a variety of materials with a range of elastic mismatch to the glass substrate were used, so as to evaluate the effects of elastic mismatch and interfacial frictional tractions on the results obtained. The results obtained by Hertzian indentation are consistent with residual stress levels determined by differential surface refractometry. We also present results on alumina specimens with induced surface stresses.     

Grinding performance improvement of silicon nitride ceramics by utilizing nanofluids

Silicon nitride ceramics are widely used as advanced structural components because of their excellent thermal and mechanical properties at ambient and elevated temperatures. In manufacturing industries, grinding is an efficient and productive technique for finishing ceramic workpieces. However, high wheel-workpiece friction and the extreme hardness associated with silicon nitride cause large heat generation during grinding. The heat produced during grinding impairs the workpiece quality by inducing surface and sub-surface damages, tensile residual stresses etc. The damages can critically limit the applications of ground ceramic components. Extensive experimental studies have been carried out to find the effect of dry and nano MQL (Graphite, WS₂ and MoS₂) grinding conditions on silicon nitride using resin bonded diamond wheel at different parametric (wheel speed, depth of cut and table speed) combinations. Results indicate that the use of nanofluids considerably improve the process performance in terms of grinding forces, surface finish and sub-surface damage. The ground surface is characterized by optical microscopy, SEM/EDX and XRD.     

Residual stresses in polymers. II. Their effect on mechanical behavior

The distribution of density and tensile properties in quenched modified poly(phenylene oxide) specimens was investigated. Quenching was carried out from temperature level above T_g to below T_g temperatures. Simultaneous to buildup of residual stresses, profiles of density and tensile properties were observed. The profiles were obtained using the layer removal technique, which was found not to affect the measured properties. Quenching of the material results in a steep density gradient in the surface layers. Correspondingly, the tensile modulus increases significantly from the surface to the inner layers and so are also the ultimate tensile properties. This behavior could be accounted for neither by the conventional packing volume approach nor by superposition of internal and external stresses. However, observations of the fracture surfaces are very supportive and indicate that the fracture initiation sites are influenced by the residual stresses. Hence, the mechanical behavior is strongly affected by both density and residual stresses profile. Density is the controlling factor in determining the elastic properties whereas residual stresses determine the ultimate strength and fracture mechanism.     

Optimization of Residual Stresses in the Surface Regions of Injection Moldings

The present study focused on the optimization of the injection molding process parameters to minimize thermal residual stresses in the surface regions of the polystyrene and high density polyethylene parts. Process parameters such as melt temperature, mold temperature and cooling time were considered as variables and their effects on residual stresses in surface regions of the parts were investigated by utilizing design of experiment (DOE), Taguchi and analysis of variance (ANOVA) methods. As a result, the most important parameters for residual stresses in surface regions of the PS and HDPE parts were found melt temperature and mold temperature, respectively.     

Effect of Hot Extrusion, Other Constituents, and Temperature on the Strength and Fracture of Polycrystalline MgO

Improved agreement was confirmed between the Petch intercept and single-crystal yield stresses at 22°C. Hot-extruded MgO crystal specimens (recrystallized with no obvious grain boundary phases or residual porosity), stressed parallel with the resultant (100) axial texture (1) gave the highest and least-scattered strength–grain size results at 22°C, (2) showed direct fractographic evidence of microplastic initiated fracture at 22°C and showed macroscopic yield at 1315° and especially 1540°C, and (3) fractured entirely via transgranular cleavage, except for intergranular failure initiation from one or a few grain boundary surfaces exposed on the subsequent fracture surface, mainly at 1540°C. Hot-extruded, hot-pressed MgO billets gave comparable strength when fracture initiated transgranularly, but lower strength when fracture initiated from one or especially a few grain boundary surfaces exposed on the fracture (with residual pores). The extent and frequency of such boundary fracture increased with test temperature. While oxide additions of ≤5% or impurities in hot-pressed or hot-extruded MgO can make limited strength increases at larger grain sizes, those having limited solubility can limit strength at finer grain sizes, as can coarser surface finish (the latter especially at 22°C). Overall, MgO strength is seen as a balance between flaw and microplastic controlled failure, with several parameters shifting the balance.