液压升降舞台的设计【含CAD图纸+PDF图】
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传统涂料和现代涂料之间摩擦性质的对比1Grindiability comparison between conventional and nanostructured material coatings.Bi Zhang, Xianbing Liu, Zhaohui Deng and Jian MengDepartment of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USAEmail: zhangengr.uconn.eduABSTRACT This paper compares the grindability of conventionaland nanostructured material coatings in terms ofnormal grinding force, specific grinding energy, surfacefinish and surface topography. Material removalmechanism is correlated with the microstructures of thematerials such as material grain size. The effect of thedecreasing material grain size in nanostructuredmaterials on the grindability is studied.1 INTRODUCTION In grinding of ceramics, the effects of grindingprocess parameters have been extensively studied (e.g.,Kirchner and Conway, 1985;Tnshoff and Brinksmeier,1988; Blake et al., 1988). It is of equal importance tostudy the influence of material microstructure. One ofthe material microstructures is the grain size, whichinfluences the mechanical properties, such as hardnessand toughness, and therefore the grindability ofceramics. Few works have been done on this aspect. Intheir work, Roth and Tnshoff (1993) studied thegrindability of alumina with different grain sizes increep feed grinding and conventional surface grinding.Both hardness and toughness for n-Al2O3/13TiO2 andn-WC/12Co coatings are found higher than theirconventional counterparts due to the reduced grain sizeand richer binder phases. Hardness of a material is itsability to resist plastic deformation. Plastic deformationis induced by the dislocation movement. The richer binder phases in nanostructured materials constrainmaterial flow and therefore plastic deformation. Unlike in the conventional materials, the increase of hardness in nanostructured materials does not lead to thedecrease of toughness due to more bridging ligaments,higher in-situ flow stress and higher rupture strength (Jia, et al., 1998). The difference in hardness and toughness between nanostructured and conventional materials can be expected to influence the grindability of their coatings. The large quantities of voids, cracks and microcracks induced by the thermal spray process greatly influence the properties of coatings made of these materials. The difference in hardness and toughness between conventional and nanostructured material coatings are not as much as in their bulk counterparts. Table 1 shows the typical properties of conventional and nanostructured WC/12Co and Al2O3/13TiO2 coatings. Normally, grindability is evaluated based on material removal rate, grinding force, surface finish and integrity of ground samples. In this paper, normal grinding force, specific grinding energy and surface finish are compared for nanostructured and conventional coatings. In addition, the scanning electronic microscopy (SEM) is used to assess the effects of grain sizes on the material removal mechanisms. Table 1 Typical Properties of the Conventional and Nanostructured WC/12Co and Al2O3/13TiO2 Coatings.c-WC/12Con-WC/12Coc-Al2O3/13TiO2n-Al2O3/13TiO2Bonding strength, MPa 82.7 89.6 15.5 20.7Powder grain size, mm 1.3 0.04 2.5 0.05Mass density, g/cm3 14.2 14.5 3.5-4.0 3.7-4.1Vickers hardness, GPa 12.00 12.50 10.44 10.57Toughness, MPa m1/2 16.0 16.5 3.3 3.52 EXPERIMENTAL CONFIGURATION2.1 Sample preparation and characterization Conventional and nanostructured material coatings were made on low carbon steel substrates of dimensions of 25 75 4 mm3 that were cleaned and blasted before thermal spray. The conventional and nanostructured WC/12Co coatings were produced using the high velocity oxygen fuel method, and the conventional and nanostructured Al2O3/13TiO2 coatings were prepared by the plasma thermal spray method. All the coatings had a thickness of around 0.5mm. The coated samples were cut into 25 4 4 mm3 for grinding. Fig.1 shows the SEM observations of c/n-WC/12Co coatings. Fig.1 (b) indicates that the smaller grains of WC are bonded together by the binder material cobalt and a large quantity of porosities can be observed in both conventional and nanostructured WC/12Co coatings. There are no obvious cracks found in conventional and nanostructured WC/12Co coatings. Fig.2 shows the typical surface features of the thermally sprayed conventional and nanostructured Al2O3/13TiO2 coatings: pores, cracks, microcracks and segmented structures formed by the connected microcracks perpendicular to the coating surface. Priorto the formal grinding test, the coatings were preground 2 with a diamond wheel of a mean grit size of 15 mm under minimum loading to avoid damaging the coatings. This preparation process was effective in getting rid of the random influence from the thermal spray process and making the samples more uniform, although it was time-consuming and effortsdemanding.2.2 Grinding experiments Grinding experiments were conducted on a precision grinding machine (Dover Model 956-S) with the computer numerical control (CNC). The machine had aerostatic bearings for its spindle and x, y, z slideways. The spindle had an axial run-out of 0.05 mm and the three slideways had a straightness error of 0.1 mm/25mm. A laser interferometer was equipped to the machine that formed feedback loops for the x, y, z slideways with a resolution of 0.07 mm. The loop stiffness of the machine was measured to be 50 N/mm. In this study, a diamond grinding wheel SD600N100V (600V) was used to grind the coatings under different conditions, and the ground samples were compared. The wheel speed was set to 33 m/s or 3500 rpm. In order to investigate the effect of material removal rate (MRR) on residual stresses, depths of cut were set at 2, 5, 15 and 30 mm and feedrates at 1, 4, and 8 mm/s for the grinding experiments. Water-based synthetic solution (ITW fluid products Group, Rustlick G-10066D) was used as grinding coolant.2.3 Post-grinding evaluation A surface profilometer (Federal Products, Surfanalyzer 5000) was used to measure surface finish (Ra) of ground coatings along the directions perpendicular to the grinding direction. An SEM (JOEL, Model JSM 840) was used to observe the surfaces of the ground coatings. One issue in SEM observations was to differentiate grinding damage from the coating defects. A large quantity of defects such as voids, unmolten particles, cracks and microcracks were identified in the as-sprayed coatings (Fig.1 and Fig.2). Because some of these defects can be easily mistaken as grinding damage, SEM examinations of the assprayed coatings were conducted to identify the defects from the spray process. It can be found that the voids from the thermal spray process normally appeared with smooth edges. From Fig.1 and Fig.2 the cracks or microcracks on the as-sprayed coatings were connected to each other without obvious directionality. With the as-sprayed coatings as a reference, the grinding damage was identified.3 RESULTS AND DISCUSSIONS3.1 Comparison of normal grinding force The normal grinding forces are important in characterizing a grinding process. Fig.3 compares the normal grinding forces in grinding c/n-Al2O3/13TiO2 and c/n-WC/12Co coatings at the same grinding conditions. The normal grinding force is higher for n-Al2O3/13TiO2 than for its conventional counterpart. One can also observe that the break-in force for n-Al2O3/13TiO2 is larger. This shows that the resistance to wearing for n-Al2O3/13TiO2 is higher due to its enhanced mechanical properties such as hardness and toughness when compared to its conventional counterpart. A similar trend is observed in grinding c/n-WC/12Co coatings: higher grinding force and break-in force for nanostructured coatings. Fig.3 also shows that the difference between the grinding forces for the nanostructured and conventional coatings becomes smaller at a large wheel depth of cut. This means that the material grain size exerts stronger influence on the grinding force at a low material removal rate. When the wheel depth of cut or material removal rate increases, the influence of grain size becomes second to that of grinding process parameters.3.2 Comparison of tangential grinding force andspecific grinding energy Tangential grinding force is much smaller than normal grinding force due to large negative rake angles of abrasive grits in grinding. The grinding force ratio(a) c-WC/12Co (b) n-WC/12Co(a) c-Al2O3/13TiO2 (b) n-Al2O3/13TiO2Fig. 2 SEM observations of as-sprayed c/n-Al2O3/13TiO2 coatings. Fig. 1 SEM observations of as-sprayed c/n-WC/12Co coatings.222222000000 _mm 222222000000 _mm1200 _ _mm 1200 _ mm3 indicates the relative magnitude of the normal grinding force Fn to the tangential grinding force Ft and is defined astnFFl = (1) Fig.4 shows the grinding force ratio vs. wheel depth of cut for the four coatings. The grinding force ratio is higher for c/n-Al2O3/13TiO2 coatings than for c/n-WC/12Co coatings. c/n-Al2O3/13TiO2 coatings are more brittle than c/n-WC/12Co coatings. Under the same grinding conditions, brittle fracture is more obvious for c/n-Al2O3/13TiO2 coatings. The dominant ductile flow in grinding c/n-WC/12Co coatings results in a relatively high tangential grinding force and therefore a lower grinding force ratio. It is observed that the grinding force ratio of the n-Al2O3/13TiO2 coatings is distinctly different from that of the c-Al2O3/13TiO2 coatings while the difference for the grinding force ratios of n-WC/12Co and c-WC/12Co coatings is insignificant. The grinding force ratios for the four coatings decrease with the increase of material removal rate or wheel depth of cut. On the other hand, the grinding force ratios for the four coatings change over a relatively narrow range, which suggests that the material removal mechanism does not change much for the given range of the wheel depth of cut. The specific grinding energy U is defined as the energy required to remove a unit volume of material, which is derived from the tangential grinding force,ft cW d vF vU = (2) where vc the grinding speed; W the width of a workpiece; d the wheel depth of cut; vf is feedrate. Fig.5 presents the effect of grain size on the specific grinding energy and the change of specific grinding energy with wheel depth of cut. The specific grinding (a) Conventional and nanostructured Al2O3/13TiO2. (b) Conventional and nanostructured WC/12Co. Fig. 3 Comparison of normal grinding force in grinding conventional and nanostructured coatings.0 10 20 3002468Depth of cut, mmNormal grinding force, N/mm2Nano.Conv.Wheel speed: 33 m/sFeedrate: 4 mm/sWheel: 600V0 10 20 3002468Depth of cut, mmNormal grinding force, N/mm2Nano.Conv.Wheel speed: 33 m/sFeedrate: 4 mm/sWheel: 600VFig. 4 Comparison of grinding force ratio.00 10 20 3036912Depth of cut, mmGrinding force ratio, ln-WC/12Coc-WC/12Con-Al2O3/13TiO2c-Al2O3/13TiO2Wheel speed: 33 m/sFeedrate: 4 mm/sWheel: 600V Fig. 5 Comparison of specific grinding energy.0 10 20 300.00.51.01.52.02.5Depth of cut, mmSpecific grinding energy, 103 J/mm3n-WC/12Coc-WC/12Con-Al2O3/13TiO2c-Al2O3/13TiO2Wheel speed: 33 m/sFeedrate: 4 mm/sWheel: 600V4 Energy for four coatings decreases with the wheel depth of cut and asymptotically reaches a limit. The high value of specific grinding energy at small depth of cut suggests that only a part of the energy is associated with the chip formation (Malkin, 1989). Generally, the specific grinding energy consists of chip-forming energy Uch, sliding energy Upl and plowing energy Usl, ch pl sl U =U +U +U (3) Except Uch, the rest of the specific grinding energy is attributed to sliding and plowing between the workpiece and abrasive grits at a small depth of cut. At a larger depth of cut, sliding becomes insignificant and chip formation commen. However, plowing still exists, which reflects by the grinding marks and material pile-up on the ground surface. Only Uch is actually used in removing material and forming new surface. Theoretically, the asymptotical limit in Fig.5 is Uch. Relative flat curves for c/n-Al2O3/13TiO2 coatings in Fig.5 suggest that the energy expended in plowing is not dominant due to their high brittleness. The reduced grain size in nanostructured coatings apparently increases the specific grinding energy. More energy is needed for plowing due to enhanced hardness in nanostructured material coatings. The higher toughness also means more energy required for new surfaceformation in grinding.3.3 Comparison of surface roughness Surface roughness was measured to characterize the ground coatings. Fig.6 shows that the influence of grain size on surface roughness of both ground coatings is significant. Opposite to the grinding force and specific grinding energy, the reduced grain size results in the decrease of the surface roughness for both nanostructured coatings, which can be explained by material removal mechanism. As observed in SEM photos (Fig.7 and Fig.8), brittle fracture dominates in grinding c-Al2O3/13TiO2 coatings while ductile flow plays a main role in grinding n-Al2O3/13TiO2 coatings. Although ductile flow is the major material removal mechanism in grinding both c/n-WC/12Co coatings, the observed transgranular fracture may partially contribute to surface roughness in c-WC/12Co coatings.Similar to the grinding force, surface roughness for ground conventional and nanostructured coatings are closer to each other at a larger wheel depth of cut, which means that the effect of reduced grain size in nanostructured coatings disappears at a higher material removal rate.3.4 SEM surface observations and comparison Fig.7 shows the SEM observations of ground c/n-WC/12Co coatings under the same grinding conditions. The ground c-WC/12Co coating surface is more segmented and larger WC grains can be observed when compared to the ground n-WC/12Co coating surface. The ground n-WC/12Co coating surface is completely covered with a layer of plastically deformed material and the WC grain boundary is hardly observable. The comparison of the SEM surface observations of ground c/n-Al2O3/13TiO2 coatings is shown in Fig.8. Although the defects from thermal spray process are observable, the sound and smooth surface of ground n- Fig. 6 Comparison of surface roughness.n-WC/12Coc-WC/12Con-Al2O3/13TiO2c-Al2O3/13TiO2Wheel speed: 33 m/sFeedrate: 4 mm/sWheel: 600V0 10 20 3000.30.60.91.21.5Depth of cut, mmSurface roughness Ra, 102 nm(a) n-WC/12Co (b) c-WC/12CoFig. 7 SEM observations of ground c/n-WC/12Cocoatings.(a) n-Al2O3/13TiO2 (b) c-Al2O3/13TiO2Fig. 8 SEM observations of ground c/n-Al2O3/13TiO2 coatings.2121210 _mm 2121210 _mm2121210 _mm 2121210 _mm5 Al2O3/13TiO2 coating suggests that ductile flow is a predominant material removal mechanism. Brittle fracture results in rough and fractured surface of ground c-Al2O3/13TiO2 coatings. Chipping and transgranular fracture dominate the surface of ground c-Al2O3/13TiO2 coating. The surface observations explain the above difference in roughness.4 CONCLUSIONS From the comparisons done on grinding force, specific grinding energy, surface finish and surface topography of nanostructured and conventional coatings, it is concluded that the grain size plays a significant role in material removal for grinding.Grinding force, break-in force and specific grinding energy vary inversely with the grain size while the surface roughness increases with the grain size. Both ductile flow and brittle fracture occur during grinding. The grain size influences the extent of ductile flow in grinding, which dominates the final appearance of ground surface. The reduced grain size and richer binder phases enhance both hardness and toughness in nanostructured materials, and therefore influence the grinding of these materials. However, at a higher material removal rate, the influence of grain size becomes insignificant.REFERENCES1 H.P.Kirchner and J.C.Conway. “Mechanisms of material removal and damage penetration during single point grinding of ceramics”. Machining of Ceramic Materials and Components, ASME, NewYork, Vol.17, 1985, pp.55-612 H.K.Tnshoff and E.Brinksmeier. “Abrasives and their influences on force temperature and surface”. Proc. of SME Intl Grinding Conf., Philadelphia,1990, pp.10-123 P.Blake, T.Bifano, T.Dow and R.O.Scattergood. “Precision machining of ceramic materials”. Ceramic Bulletin, Vol.67, No.6, 1988, pp.1038-10444 P.Roth and H.K.Tnshoff. “Influence of microstructure on grindability of alumina ceramics”. Proceedings of the InternationalConference on Machining of Advanced Materials, Gaithersburg MD, July 1993, pp.247-2615 K.Jia, T.E.Fischer and B.Gallois. “Microstructure, hardness and toughness of nanostructured and conventional WC-Co composites”. Nanostructured Materials, Vol.10, No.5, 1998, pp. 875-8916 S.Malkin. “Grinding technology, theory and application of machining with abrasives”. Ellis Horwood Limited, Chichester, England, 1989一、 传统涂料和现代涂料之间摩擦性质的对比毕章 刘小兵 邓朝晖 和建盟都是美国CTO6292机械工业部门和各大学的领导。电邮:zhangenger.ucom.edu简介: 这篇文章对比了传统材料在固定的摩擦力和空间摩擦力作用下的表面摩擦性质。材料成型机械是和微结构理论(材料的粒度)相联系的。材料粒度的减少对变形材料的摩擦性质的影响正在研究之中。1.说明陶瓷制品的摩擦过程影响已经在深入的研究之中。例如:Kirchner 和Brinksmeier,1998Blake,1998)摩擦对材料微型结构的影响同等重要。材料的微型结构就是粒度,它将影响机械性能。例如:硬度和韧性。所以也将影响到陶瓷的摩擦性质,在这方面几乎还未取得研究上的突破。 Both和Tonshoff在1993年研究了不同粒度的铝在滑动摩擦和静摩擦方面的摩擦性质。对n-Al2O3/13TiO2和n-WC/12Co两种涂料来说,其硬度和韧性比同等或粒度大的其他材料来说要髙的多。在塑性变形条件下,材料的硬度是不变的。由于固有位置的移动,塑性变形将会被削弱。粒度越大,越会对材料的移动和塑性变形的束缚。和传统材料不同的是,材料硬度的降低并不会导致材料韧度的降低。(由于更大的纤维化,更大的流动压力和更高的挠曲力)(Jia,1998)对传统材料和现代材料来说,硬度和韧度的差别是涂料的摩擦性质得的影响。大量的砂眼,裂纹和微裂纹是由于膨胀过程对材料中各成分的比率产生了很大的影响。对传统材料和现代材料来说,硬度和韧度的不同并不仅仅是由于体积的不同。表1说明了传统材料和现代材料(Wc/12Co和Al2O3/13TiO2)两种涂料的成分比率。一般来说,摩擦性质可根据材料的滑移率,材料所承受的摩擦力,样本表面的结合率作出预测。在这篇报告中,传统的摩擦力,空间摩擦能是作为传统材料和现代材料对照。除此之外,微电子扫描技术被用来预料粒度对材料滑移机械的影响。表1:传统材料和现代材料(Wc/12Co和Al2O3/13TiO2)的百分比C-Wc/12Co n-Wc/12Co C-Al2O3/13TiO2 n-Al2O3/13TiO2边界力:82.7 89.6 15.5 20.7 (MPa)粒度:1.3 0.04 2.5 0.05 (um)物体密度:14.2 14.5 3.54.0 3.74.1 (g/cm3)维氏硬度:12.00 12.50 10.44 10.57 (GPa)韧度:1/2 16.0 16.5 3.3 3.5 (MPa/m)2结构试验2.1预加工特征图表 传统材料和现代材料都是由低碳钢制成,其晶格大小为25754mm3,晶格在热膨胀之前将会爆炸。传统材料和现代材料(Wc/12Co)是用高压氧流的方法生产的。传统材料和现代材料(Al2O3/13TiO2)是用等离子下热膨胀的方法预制的。所有的涂层都有大约0.5mm厚的硬壳。材料样本被削减为2544mm3。 表1说明了用微电子扫描技术对c/n-Wc/12Co涂料的观察结果。表1(b)揭示了小粒度的Wc在材料钴的边界结合在一边。用微电子扫描技术在传统材料和现代材料(Wc/12Co)中可观察到大量的裂纹。表2说明了在热膨胀作用下,传统材料和现代材料(Al2O3/13TiO2)的典型表面特征:孔隙,裂纹,微裂纹和与材料微裂纹垂直相交的部分结构。在现有的摩擦测试以前,为了降低材料的毁坏,外表是带有15um的金钢粒的砂轮。尽管这种准备过程在热膨胀的影响下非常有效,但要浪费时间和精力。2.2摩擦试验 摩擦试验是用计算机对精密摩擦机械(Dover Model 956-S)进行了大量的控制而完成的。这台机器在它的测量轴及X Y Z 坐标方向上都有空间静止齿轮。测量轴在轴向由0.05um的窜动,三坐标轴向上有0.1um-25mm的直线度误差。机器上安装的激光干涉仪能够对Xx Y Z轴向上0.07um的回路误差进行反馈。机器上这种回路的刚度为50N/um。 在此项研究中,用金刚摩擦轮(5D 600N 100V)在不同的环境下(和样本比较而言)摩擦涂料。轮速设置为33m/s或3500r/min。为了预测在残余应力作用下材料滑移率的影响,切深设置为2,5,15,30 um补偿率为1,4,8mm/s(在摩擦试验中)用冷却液作为水系统。2.3后摩擦预测 轮廓曲线用于测量材料在摩擦方向上的成型表面微电子探测仪(JOEL Model Jsm840)用于观察材料表面。微电子探测仪的观察部件能够在材缺陷中区分出摩擦损坏。 大量的材料缺陷,例如:砂眼,熔融粒子,裂纹和微观裂纹,在飞溅中都能被检测出来。 因为其中的一些缺陷很容易被误认为摩擦毁坏,所以微电子探测仪能够在飞溅材料的检测中探测出这些缺陷。热膨胀过程中的毛孔一般呈现光滑的边缘。表1和表2中,飞溅材料的裂纹和微裂纹彼此相连。依据摩擦材料,摩擦破坏将能被检测出来。3研究成果与发现3.1普通摩擦力的对比普通摩擦力在表征摩擦过程中非常重要。表3对c/n-Al2O3/13TiO2和c/n-Wc/12Co在相同摩擦环境下作用的普通摩擦力进行了对比。对n-Al2O3/13TiO2来说,普通摩擦力要高于传统的配对物。人们也观察到:对n-Al2O3/13TiO2来说,抱刹力要大的多。它表明了:n-Al2O3/13TiO2和传统的配对物相比增加了机械性能(硬度和韧度),所以n-Al2O3/13TiO2得到广泛的应用。据观测,摩擦中c/n-Al2O3/13TiO2具有类似的趋势:在大的切深下,摩擦力对传统材料和现代材料的不同影响变得非常小。这也表明在非常低的材料滑移率下,材料粒度对摩擦力的影响非常大。当切深或材料滑移率增大时,对摩擦过程干扰仪来说,切深的影响成为次要因素。由于在摩擦时大的负前角的从存在,切向摩擦力比普通摩擦力小的多。摩擦力如下:(a)c-Wc/12Co (b) n-Wc/12Co (a) c-AL2O3/13TiO2 (b) n-Al2O3/13TiO2 表2:微电子探测仪观测到了飞溅
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