Material removal and micro-roughness in fluid-assisted smoothing of reaction-bonded silicon carbide surfaces

The magnetic fluid-assisted polishing for fuse silica and other optical materials with a high degree of success, and a super-smooth surface (Ra < 1 nm) and subsurface-damage-free layer can be produced. However, the fundamental mechanisms of the process for polishing reaction-bonded silicon carbide (RB-SiC) have not yet been studied in detail. This paper is concerned with the fabrication processability aspect of the RB-SiC components, and investigates results obtained by magnetorheological finishing (MRF) of RB-SiC mirror. It details experimentally the features of different polishing fluids and the characteristics of relative removal rates, analyses the processing limitations of the normal processing techniques and studies the effects of certain processing parameters on surface accuracy. The final surface roughness with an initial value of Ra = 17.58 nm reached 4.03 nm after 15 h of polishing, and then convergent to 1.03 nm after another 7 h fine polishing. Experimental results based on the magnetorheological (MR) fluids show that, a MR fluid containing diamond particles helps to accelerate removal rates. Additionally, by adding a small amount of CeO₂ into the diamond-based MR fluid, it is possible to finish RB-SiC material to a higher level of surface quality.     

Mechanism of material removal in electrical discharge diamond grinding

The paper presents the mechanism of material removal in electrical discharge diamond grinding, which integrates electrical discharge machining and diamond grinding for machining electrically conducting hard materials. The role of current and wheel speed on the material removal rate, the grinding forces and power is studied to elucidate the issue. The spark discharges thermally soften the work material in the grinding zone, and consequently decrease the normal force and the grinding power. The improved grinding performance is due to continuous, in-process dressing and declogging of the wheel surface.     

硅及硅合金对反应烧结SiC耐磨材料性能与结构的影响

本文首先分析了熔渗材料硅及硅合金的润湿性、流动性、烧失率的变化.结果表明:熔渗材料硅在1 430℃,硅铁合金在1 470℃时润湿角小(分别为0°和6°)、流动性大(分别为267%和198%)、烧失率小(均小于15%).同时分析了反应烧结SiC耐磨材料性能,并采用扫描电子显微镜、X射线衍射仪、EDS分别对其显微结构、主晶...     

Carbon nanofiber assisted micro electro discharge machining of reaction-bonded silicon carbide

Carbon nanofiber assisted micro electro discharge machining was proposed and experiments were performed on reaction-bonded silicon carbide. The changes in electro discharging behavior, material removal rate, electrode wear ratio, electrode geometry, spark gap, surface finish, surface topography and surface damage with carbon nanofiber concentration were examined. It has been found that the addition of carbon nanofiber not only improves the electro discharge frequency, material removal rate, discharge gap, but also reduces the electrode wear and electrode tip concavity. Bidirectional material migrations between the electrode and the workpiece surface were detected, and the migration behavior was strongly suppressed by carbon nanofiber addition. Adhesion of carbon nanofibers to the workpiece surface occurs, which contributes to the improvement of electro discharge machinability. These findings provide possibility for high-efficiency precision manufacturing of microstructures on ultra-hard ceramic materials.     

Grinding characteristics,material removal and damage formation mechanisms in high removal rate grinding of silicon carbide

Development of advanced ceramics such as silicon carbide has gained significant importance because of their desirable properties. However, their engineering applications are still limited owing to the limitations in developing damage-free and economical machining techniques. It is often desired to increase the machining rate to improve productivity while maintaining the desired surface integrity. The success of this approach, however, requires a fundamental understanding of the material removal and damage formation mechanism in grinding. In this paper, high removal rate grinding of silicon carbide was investigated with respect to material removal and basic grinding parameters using a diamond grinding wheel. The results showed that the material removal was primarily due to the microfracture and grain dislodgement under the grinding conditioned selected. For grain dislodgement removal mode, the relationship for the removal rate in scratching based on a simple fracture mechanics analysis has been established. This research provides valuable insights into the surface and subsurface integrity and material removal mechanism during high removal rate grinding of silicon carbide.     

半导体工业硅材料加工用金刚石工具的发展

随着我国半导体工业集成电路(IC)业的快速发展,硅材料加工用金刚石工具具有巨大的潜在市吵了半导体加工中使用的各种金刚石工具:其中包括:单晶硅晶锭的裁切与整圆、晶圆线切割、倒圆磨边、磨削、CMP修整器、背面减薄与划片及其最新发展.详细介绍了各种不同类型CMP修整器及其完善与发展.建议应积极研发我国半导体工业硅材料加工用金刚石工具.     

金刚石砂轮磨削氮化硅陶瓷数值仿真及实验研究

针对金刚石砂轮磨粒的尺寸多变化、位置不规则的特点,通过切割正四面体的方法,获得了接近实际的十四面体磨粒模型,并通过其建立了金刚石砂轮模型。采用DEFORM-3D进行金刚石砂轮磨削氮化硅陶瓷数值仿真,分析工件的切屑去除过程以及其间的切削力变化,并分析不同的工艺参数对切削力的影响;同时,在平面磨床上进行磨削氮化硅陶瓷的实验。结果表明:磨削加工和数值仿真的磨削力值变化趋势相同,且差值在10%的误差范围内。最后,结合实际砂轮表面面积密度、磨粒平均个数、出刃高度等测量参数,共同验证了所建立的十四面体磨粒模型和砂轮整体模型的正确性。     

金刚石线切割单晶碳化硅锯切力的实验研究

从锯切力的角度对金刚石线锯锯切单晶SiC材料的加工过程进行了研究。得出了线速度、进给速度、线锯张紧力对锯切力的影响规律。从单位长度线锯材料去除量、锯切比能的角度讨论了锯切工艺对锯切力的影响机理。在金刚石线锯锯切单晶SiC过程中,锯切力随着线速度的增大而减小,随着进给速度的增大而增大,线速度与进给速度对锯切力的综合影响表现为:单位长度线锯材料去除量的增加会增大锯切力。单位长度线锯材料去除量对于金刚石线锯锯切单晶SiC材料的锯切比能具有显著的影响。     

拓扑法计算硅/碳化硅材料的电阻率

用拓扑关系将硅 /碳化硅材料转化为具有 3个不同显微结构单元的整体 ,建立了等效电路 ,结合相关试验 ,计算了不同显微组织的硅 /碳化硅材料的电阻率。得到的理论计算结果与实验值的综合相对误差仅为3.9% ,分析比较了各相含量和分布形态对硅 /碳化硅材料整体电阻率的影响。结果表明 ,提高低电阻率硅的体积含量和其分布连续性 ,可降低材料的整体电阻率。该方法可用来预测要得到所需电阻率材料中应具有的硅含量和分布形态 ,为确定制备材料的显微结构设计提供指导。     

Thermochemical material removal of diamond by solid iron and mischmetal

Solid iron and mischmetal (Ce, La, Fe, etc. alloy) were used to remove material from single crystal diamond. Contact experiments with diamond and mischmetal were carried out at 600°C and 750°C with hold times from 3 to 24 h. Contact experiments with diamond and iron were performed at 850°C with hold times of 3, 4 and 5 h. In the case of mischmetal, material removal was only observed for temperatures of 750°C with hold times of 16 h or more. In these experiments it was found that material was removed only at certain areas of the contact surface whereas for the iron experiments material was removed over the entire contact surface.     

A study of grinding silicon nitride and cemented carbide materials with diamond grinding wheels

This paper presents selected results of the grinding of silicon nitride and cemented carbide materials with diamond grinding wheels, which will in later research be extended to the grinding of ceramic-cemented carbide compound drill tools. In these fundamental experiments four different types of diamond grinding wheels were used in face grinding processes. The diamond grinding wheels vary by the grain size, the grain concentration and the bonding material. The relevant influencing variables such as the cutting and feed speed and the coolant supply method were varied to investigate the effect on grinding of the two different workpiece materials, the brittle silicon nitride workpiece material and the ductile cemented carbide workpiece material. Some factors, which have significant effects, like the radial wear of the grinding wheel, the components of the grinding forces, the normal and the tangential grinding force, and the surface quality of the ground workpieces are discussed in detail.     

Analysis on ductile mode processing of binderless, nano crystalline tungsten carbide through ultra precision diamond turning

Ultra precision diamond turning is usually applied for processing non ferrous metals, plastics and a few single crystal materials. The machining of hard and brittle material, such as nano crystalline, binderless tungsten carbide has only been investigated within a few publications on a theoretical basis and applying nano indenting. Therefore the goal of this paper is to qualify the ultra precision diamond turning technique for processing nano crystalline, binderless tungsten carbide applying the real process conditions identifying optimal parameters. A potential application of this process would be the mold manufacturing for precision glass molding. Within a systematical procedure the ductile to brittle transition is analyzed as well as the tool wear, varying both, tool geometry and processing parameters. Based on material analysis the critical depth of cut for the material was calculated at 165 nm. This value could be validated in the experiments at the optimal cutting speed of 50 m/min with a feed of 1 μm. Least tool wear was observed at a tool radius of 0.4 mm and a rake angle of −20°. The experiments demonstrate the strong influence of the processing conditions on the achieved results.     

Characterisation of nanosurface generation in single-point diamond turning

This paper describes a parametric analysis of nanosurface generation in single-point diamond turning (SPDT). The properties of the surface roughness profiles were extracted and analysed using the power spectrum analysis method. A series of face cutting experiments was undertaken on an aluminium alloy under various cutting conditions. The results indicate that the power spectrum of a surface roughness profile is basically composed of several periodical components that can be correlated to different process parameters and mechanisms of surface generation. Moreover, it is found that the tool feed, tool geometry, spindle error motions and relative vibration between the tool and the workpiece are not the only dominant components contributing to the surface generation in SPDT. Materials swelling and tool interference are other important factors. Based on these findings, relationships are proposed to explain the influence of tool interference on the variation of the spectral components and process parameters. The implications of these findings on the optimisation of the surface quality in SPDT are also discussed.     

高压磨料水射流加工中材料去除机理研究

本文根据脆、塑性断裂理论,通过高压纯水射流和磨料水射流对石材冲蚀试验,对冲蚀凹坑成型机理进行了研究。结果表明,高压纯水射流只有不发散的中心射流对材料具有去除作用。高压磨料水射流对材料表面冲击时形成"倒钟"型孔,材料去除机理是在成穴力、剪切应力和水楔的共同作用下,以脆性和塑性断裂方式实现去除。磨料水射流对材料的冲蚀区分为中心射流区、成穴区和喷砂区,材料的主要体积去除量处在成穴区。磨料水射流加工孔的直径和深度随靶距的增加而增加。磨料水射流从射流中心沿径向至射流边缘流速逐渐降低,磨料浓度逐渐增大,但射流能量最高的部位既不在射流中心也不在磨粒集中的射流边缘,而是在成穴区。     

Grinding of silicon wafers using an ultrafine diamond wheel of a hybrid bond material

An ultrafine diamond wheel of a mesh size of 12,000 was fabricated by using a hybrid bond material, which consists of silicon carbide, silica and alumina. The employment of the newly developed wheel enabled excellent performance during grinding of silicon wafers. An extremely smooth surface of an average roughness of 0.6 nm was achieved. TEM examinations showed that the total thickness of the defected layer was less than 60 nm.     

Measurement of optical surfaces generated by diamond turning

For the manufacture of ultraprecision parts an entirely controlled machining technology is required as well as the capability of a complete evaluation regarding microgeometry and integrity of the generated surfaces. Optical and stylus profilometry are the commonly used technologies for determining surface roughness. Furthermore, atomic force microscopy AFM is frequently applied for 3D characterization of the microtopography.In this paper the application of atomic force microscopy in the field of diamond turning is presented by means of measuring and manufacturing examples. Typical applications are the manufacture of metal mirrors from aluminum, copper and electroless nickel substrates by diamond turning and fly-cutting operations using monocrystalline diamond tools. The atomic force microscope is used for the evaluation of the microtopography of these mirrors, which is decisive for their optical properties and quality. Additionally, the atomic force microscope can be used for measurements of microstructured components and the diamond tools itself. In this case, AFM is not only used for roughness measurement, but also for inspection and verification of the actual, respectively, the produced geometry of the components and the tools. The examples given in this paper illustrate the great potentials of atomic force microscopy in the field of ultraprecision machining.     

Orientation changes of aluminium single crystals in ultra-precision diamond turning

In this paper, the effects of cutting speed on the variation of surface texture and lattice rotation of diamond-turned surfaces were investigated. The {1 1 1} pole figures were determined at various locations by the X-ray diffraction method. The local lattice rotation at various locations on a machined groove by the electron back-scatter diffraction (EBSD) method was also obtained. A simulation of the orientation change was performed and the theoretical prediction was compared with the experimental results.     

Machinability exploration for high-entropy alloy FeCrCoMnNi by ultrasonic vibration-assisted diamond turning

High-entropy alloy (HEA) is an emerging alloy which consists of five or more metallic elements with equimolar concentrations and exhibits excellent mechanical properties at cryogenic temperature. However, its machinability is almost unknown. In this study, high frequency one-dimensional ultrasonic vibration-assisted diamond turning (UVDT) experiments were conducted on an FeCrCoMnNi-based HEA to investigate the micro-nanoscale material removal mechanisms. Compared with conventional diamond turning, UVDT produced thinner chips, lower cutting forces, less tool wear and better surface integrity. Due to the ultrasonic vibration-assisted burnishing effect, surface scratches were significantly eliminated. A freeform surface was test-fabricated with optical-level finish.     

Nanometric cutting mechanism of silicon carbide

Ductile to brittle transition is critical to achieve nanometric surfaces in the ultraprecision diamond cutting of silicon carbide. Although atomic simulations have long been used to better understand this mechanism, the extremely small model scale limits its capability in matching the actual cutting process. To overcome this serious issue, an enhanced molecular dynamics method is proposed in this study, which successfully predicts and clarifies the onset of brittle regime machining, and indicates the essential roles of dislocation and the shear band. The experimental results validate the effectiveness of this modelling approach.     

Damage-free machining of monocrystalline silicon carbide

Cutting tests of monocrystalline SiC, on the surface of which an amorphous layer was preformed by ion implantation, were performed. Ductile-mode machining was observed at a depth of cut smaller than 60 nm. At a depth of cut larger than 60 nm, cracks were observed on the work surface. However, transmission electron micrographs show that crack propagation was obstructed at the interface between the amorphous and crystalline layers even under brittle-mode machining, and no subsurface damage extended into the crystalline layer. The results suggest that the damage-free machining of monocrystalline SiC is possible by surface modification to an amorphous structure.