Simulation and experiment of double grits interacting scratch for optical glass BK7

The elastic-plastic transition regime and brittle-ductile transition regime in scratch process for optical glass BK7 were analyzed based on the Hertzian equation and the stress ratio theory which was proposed by Wei. The interacting scratch process for optical glass BK7 with the grit interval distance as the variable was simulated by the ABAQUS software of finite element simulation based on the energy fracture theory. Double grits interacting scratch test for optical glass BK7 was carried out on the DMG ULTRASONIC 70-5 linear, by which the reliability of finite element simulation was verified. The surface morphology of the workpiece was analyzed by scanning electron microscopy (SEM), which showed that the width of groove increased obviously with the increase of scratch depth and the grit interval distance. Results of the width of groove were consistent with the simulation results. The subsurface damage layer was analyzed by the method of HF acid etching, which showed that there was an area of cracks intersecting. The scratching force was measured by the threedimensional dynamometer of KISTLER, which showed that the second scratching force increased with the increase of scratching depth and the grit interval distance. The force in the second scratch was smaller than that in the first time, which was consistent with the Griffith fracture theory.     

光学元件亚表层损伤检测和规律研究

为了分析光学元件的加工工艺与亚表层损伤的关系,验证亚表层损伤/表面粗糙度比例预测模型的正确性,探索一种能够快速检测亚表层损伤的方法.文中基于激光散射和共聚焦显微成像理论,分析了共聚焦显微系统测量亚表层损伤的可行性;对三种不同工艺加工的光学表面,利用OPTELICS S130五波长共聚焦显微镜获得光学表面亚表层损伤的三维立体图和纵向数据图,在TalySurf CCI上测量了光学表面的粗糙度,分析了加工过程中磨粒直径变化规律对表面粗糙度和亚表层损伤深度的影响,验证了亚表层损伤深度和表面粗糙度之间呈单调递增了的非线性关系.研究结果表明,共聚焦显微成像技术可以实现亚表层的非破坏性定量检测.     

Development of an ultrahard nanotwinned cBN micro tool for cutting hardened steel

Binderless nanotwinned cubic boron nitride (nt-cBN) synthesized from onion-structured BN precursors under high pressure and high temperature shows a very fine microstructure consisting of densely lamellar nanotwins (average thickness of 4 nm) within nanograins. The unique nanotwinned microstructure offers high hardness, wear resistance, fracture toughness, and thermal stability which are essential for advanced cBN tool materials. Thus, a circular micro tool of nt-cBN was fabricated using femtosecond laser contour machining followed by focused ion beam precision milling. Thereafter turning tests were performed on hardened steel using the studied micro tool. To evaluate the cutting performance, the machined surface quality and subsurface damage of the hardened steel were characterized. The wear mechanism of the nt-cBN micro tool was also investigated. It is found that the fabricated nt-cBN micro tool can generate high quality surface with surface roughness less than 7 nm and nanograin subsurface of about 500 nm deep. In addition, abrasive wear is found to be the dominant wear mechanism of the nt-cBN micro tool in turning hardened steel. These results indicate that nt-cBN has outstanding potential for ultra-precision cutting hardened steel.     

Determination of efficient superfinishing conditions for mirror surface finishing of titanium

To determine mirror surface finishing conditions and efficient and economical superfinishing conditions for pure titanium and titanium alloys, an abrasive film is used when superfinishing is performed under varying conditions. These conditions include the workpiece rotation speed, the oscillation speed, the contact pressure of the roller, the hardness of the roller, and the type of abrasive film. The superfinishing device is applied to polishing a thin and long cylindrical bar. A micro-finishing film and a lapping film were used as abrasive films. Al₂O₃ grains or SiC grains were used as abrasives. The surface roughness of a polished workpiece was measured using a stylus-type surface-roughness measuring instrument. As a result, the conditions to improve the polishing surface efficiently include high values for the workpiece rotation speed, oscillation speed and contact pressure. The roller hardness has no effect on the efficient polishing conditions. The mirror finish of a surface can be created using lapping film of 3 μm with Al₂O₃ grains after polishing to a steady surface roughness under the efficient polishing conditions.     

光学元件研磨加工裂纹形成过程数值模拟

针对传统有限元方法过度依赖于网格,计算时磨粒和工件接触区域的网格畸变严重,很难模拟脆性材料加工中材料内部裂纹的形成过程这一问题,利用光滑粒子流体动力学法建立单个磨粒切削加工过程的FE/SPH耦合有限元模型,对熔石英材料研磨过程进行数值模拟,为脆性材料切削过程的仿真提供了新途径.系统分析研磨加工中亚表层裂纹的形成过程以及切削参数对亚表层裂纹深度的影响规律.仿真结果表明:磨粒刚开始切入工件时材料处于弹/塑性变形阶段,随后在磨粒的挤压及撕扯作用下,材料内部产生大量微裂纹,微裂纹的合并、连通和扩展最终形成了平行于工件表面的横向裂纹和垂直于工件表面的纵向微裂纹,导致工件材料的脆性断裂去除.     

Effect of counterface materials on friction and wear behavior of double glow glasma discharge surface alloying on Ti22Al25Nb alloy

Samples of surface chromising layer were prepared by the double glow plasma discharge technique. X-ray diffraction and X-ray photoelectron spectroscopy(XPS) analysis of different elements confirmed the formation of chrome in the layer. Their tribological properties were investigated by pin-on-disk tribometer. Silicon nitride, GCr15, and nickel-based alloy were selected as counterface materials. Results indicated that the lowest friction coefficients and wear rate were obtained when substrate and chromising layer against nickel-based alloy, and tribological properties of chromising layer were better than those of substrate. The highest friction and wear rate were samples against silicon nitride alloys. In the case of three rubbing pairs, the unchangeable materials against different hardness counterfaces leaded to different wear mechanisms. Samples against silicon nitride exhibited abrasive mechanism, and when GCr15 and nickel-based alloy were used as counterface, transfer film and glaze layer formed on the contact surface, which played the main role in decreasing friction and wear. Funded by the Natural Science Foundation of Jiangsu Province China(No.BK2005128), and the Scientific Research Foundation of Nanjing University of Information and Technology     

Fracture toughness and fatigue crack growth analysis of 7050-T7451 alloy thick plate with different thicknesses

The strength,fracture toughness and fatigue crack growth resistance of 7050-T7451 aluminum alloy plate with different thicknesses(35 mm and 160 mm) were investigated by means of optical microscope,scanning electron microscope and transmission electron microscope.The results show that thicker plate has lower strength and fracture toughness but higher fatigue crack growth resistance,by comparison to the thinner plate.The drop of strength is mainly attributed to grain coarsening in the thicker plate,and the increased degree of recrystallization results in the loss of KIC.However,the coarsened grains in the thicker plate make cracks deflected and closure effect enhanced due to surface roughness increased.For both of plates,in the fracture surface subjected plain strain,a transition from transgranular dimpled fracture to intergranular dimpled fracture is observed during the fracture process.     

硬脆性材料塑性电泳磨削试验研究

利用超微磨粒的电泳特性,实现硬脆性材料的塑性去除磨削方式,试验结果表明,采用自进给电泳磨削加工技术,明显改善硬脆性材料的表面粗糙度和破损率。     

熔石英亚表面微缺陷原位表征及损伤阈值研究

通过对熔石英表面和亚表面划痕的原位测量,研究划痕的表面均方根(RMS)粗糙度、宽度和深度在HF溶液中刻蚀不同时间后的变化规律,测试了不同刻蚀时间下熔石英的损伤阈值。实验结果表明:随刻蚀时间的增加,表面RMS、划痕深度及宽度的总体变化趋势是增加的;熔石英的损伤阈值随刻蚀时间的增加,在1～10min时间段呈增加趋势,在20～40 min时间段呈下降趋势,而在60～120 min时间段先增加后降低。综合熔石英划痕的微观形貌损伤阈值的测量结果认为,刻蚀10 min时效果最佳。     

磨粉机轧辊失效机理及提高使用寿命的途径

本文探讨了磨辊的磨损失效机理。用扫描电镜分析磨粉机轧辊的磨面和磨屑,提供了疲劳脱落和不均匀磨损的证据。侧磨试样发现磨辊亚表层显微裂纹,珠光体抑制渗碳体中裂纹的扩展。 经变质处理,在硬度变化不大情况下提高铸铁冲击韧性。磨损试验表明,相对耐磨性随韧性提高而提高,硬度与相对耐磨性并非线性关系,硬度与韧性相匹配的材料具有良好的抗磨性,装机试验比原材料磨辊使用寿命提高30％。     

Nondestructive testing for crack of tunnel lining using GPR

Lining craze, as a common engineering damage, affects the bearing capacity and the safety of tunnels. To improve the capabilities of the surface crack detection in the tunnel lining, ground penetrating radar (GPR) was employed. The principle and method of GPR for cracks investigation of tunnel lining were expounded. As an application example, some field measurements in order to detect the depth of cracks for the tunnel lining were achieved in a new tunnel. All GPR investigations were made with the antennae of 1 GHz. The cracks of concrete structure were located, and the distributing and depth of cracks are inspected and estimated by GPR. The results of investigation show GPR is accurate and efficient to detect cracks of tunnel lining. Foundation item: Projects(50204012; 50490274) supported by the National Natural Science Foundation of China; project supported by the Postdoctoral Science Foundation of China     

Relationship among wear-resistance of three-body abrasion, substructure and property in martensite steels

The effects of subsurface hardness on wear-resistance of martensitic steel 20Cr, 40CrSi, 60Mn, T8 and T10 in three-body abrasion under static load was investigated. It shows that the characteristic of the subsurface hardness distribution and the abrasive wear resistance is related to the substructure near the worn surface. The substructure of the tested martensite steel has an apparent relationship with the carbon content and steels with moderate carbon content and hardness exhibit good resistance to abrasive wear. The competition of the work-hardening effect and the temper softening effect, which resulted from deformation and friction heat generating during abrasive wear is considered to be a main reason for the relation among wear-resistance, hardness and substructure. At the test conditions, the wear-resistance of 40CrSi is the best.     

高温力学性能测试SiC夹具加工工艺研究

工程陶瓷材料具有引人注目的特性:高硬度、高耐热性、化学惰性、热传导率低、断裂韧度低等。然而,这些特性使得陶瓷材料的机械加工,不论是磨削加工或非磨削加工都非常困难,在磨削过程中,工件表面将产生缺陷或裂纹,进而影响工件的强度及可靠性。本文讨论了在SiC夹具制造过程中,各种因素对加工的影响。     

自由曲面模具磁力研磨的试验研究

基于三杆五自由度并联机床进行了对自由曲面模具磁力研磨的试验研究.实验分析了利用球型磨头对工件磁力研磨时,磁场强度、研磨间隙、磨粒粒度等因素对自由曲面模具表面研磨质量的影响.利用五自由度并联机床不仅可以去除自由曲面模具表面的切削残留痕迹,降低模具的表面粗糙度,还解决了传统手工研磨方式所引起的工件研磨质量不一致的缺陷.拓宽了三杆五自由度并联机床的应用范围.     

非固结面裂隙深度对爆破振动影响试验研究

鉴于岩体中存在大量裂隙,为了探索非固结面裂隙对爆破振动效应的影响,通过建立波在裂隙岩体中传播的砂浆模型,对节理裂隙岩体进行模拟试验,研究分析了岩体裂隙深度对应力波传播规律的影响。研究表明：由于非固结面裂隙因素影响,裂隙深度对垂直方向质点峰值的影响较大,质点垂向振动峰值变小,裂隙深度和质点峰值大小不是反线性比例关系;裂隙深度对三个单独方向（垂向、径向和切向）主频率大小的影响并不明显,但其垂向频域分布主要集中在中高频,径向和切向主要集中在低频频域范围内。     

Homogeneity of microstructure and Vickers hardness in cold closed-die forged spur-bevel gear of 20Cr Mn Ti alloy

Cold closed-die forging is a suitable process to produce spur-bevel gears due to its advantages, such as saving materials and time, reducing costs, increasing die life and improving the quality of the product. The homogeneity of microstructure of cold closed-die forged gears can highly affect their service performance. The homogeneity of microstructure and Vickers hardness in cold closed-die forged gear of 20 Cr Mn Ti alloy is comprehensively studied by using optical microscopy and Vickers hardness tester. The results show that the distribution homogeneity of the aspect ratio of grain and Vickers hardness is the same. In the circumferential direction of the gear tooth, the distribution of the aspect ratio of grain and Vickers hardness is inhomogeneous and they gradually decrease from the surface to the center of the tooth. In the radial direction, the distribution of the aspect ratio of grain and Vickers hardness is inhomogeneous on the surface of the gear tooth; while it is relatively homogeneous in the center of the gear tooth. In the axial direction of the gear tooth, the distribution of the aspect ratio of grain and Vickers hardness is relatively homogeneous from the small-end to the large-end of the gear tooth.     

Effect of bond-coat surface roughness on failure mechanism and lifetime of air plasma spraying thermal barrier coatings

This work investigates the failure mechanism of an air plasma spraying(APS) thermal barrier coatings(TBCs) with various bond coat(BC) surface toughness under constant temperature loading condition. Results show that the increased BC surface roughness decreased the lifetime. Moreover, the relationship between the TGO depth and the lifetime can be well described by linear law.The failure mechanism of the TBCs samples was studied by cross-sectional morphologies observation and using the finite element method(FEM). Cracks in the TBCs samples with higher BC surface roughness are initiated from the defects on the BC caused by Al depletion in the BC layer. However, cracks in the samples of TBCs with lower BC surface roughness condition preferentially occur at the TC-TGO interface. This finding could be attributed to the high mismatch stresses in the TC-TGO interface.     

添加微细铬铁粉对烧结钢摩擦磨损性能的影响

研究了加入不同含量的微细铬铁粉对烧结钢干摩擦磨损性能的影响,并借助于扫描电镜观察分析其磨损形貌,探讨摩擦磨损机制。研究结果表明:添加微细铬铁粉可改善烧结钢的强度、硬度和摩擦磨损性能,铬的质量分数为1.5%时,耐磨性最佳。磨损造成一定厚度的塑性变形,硬度较高的材料塑性变形层较薄。磨损早期,磨粒磨损是主导机制,磨损后期,由于塑性变形导致亚表层产生裂纹,进而发生的剥层磨损是主导磨损机制。     

Effect of anions on preparation of ultrafine α-Al2O3 powder

Ultrafine alumina power was obtained by calcining the precursor at 1 200 ℃ for 2 h, which was prepared by homogeneous precipitation method using aluminium salts and urea as raw materials. The effects of anions on the morphology, particle size, surface area and configuration of the precursors were studied. The results show that the reactions of urea with aluminium nitrate and aluminium chloride result in agglomerates gels with bad filtering performance, the morphology is fibrillar. Aluminium sulphate-urea reactions result in the direct formation of amorphous powders with good filtering performance, of which morphology are regular spherical particles with larger granularity and smaller surface area. The reaction of mutual compound of aluminium sulphate and aluminium nitrate with molar ratio of 40:60 with urea can produce precursor with good filtering performance, spherical morphology, and uniform granularity distribution in the particle size range of 2-3 μm.     

Wear mechanisms and hardness thresholds under repeated impact contact loading

Experiments were performed on 20 different types of materials to analyze the impact contact wear and to establish the wear mechanism map. The wear mechanism has been categorized into three prominent regions(delamination wear, quasi-nano wear and lamination-like wear) and is governed by two hardness thresholds(H_(dq) and H_(ql)). When the material hardness HcpH_(dq), the delamination wear is the dominant wear mechanism; when H_(cp)H_(ql), the mechanism is lamination-like wear; however, when H_(cp) is between Hdq and H_(ql), the mechanism is determined to be quasi-nano wear. H_(dq) is determined to be the threshold hardness between delamination wear and quasi-nano wear that reflects the cracks being produced in the subsurface layer, whereas, H_(ql) is termed threshold hardness between quasi-nano wear and lamination-like wear with formation of nanostructure within the surface layer.