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.     

反应烧结碳化硅材料的高温氧化

研究了反应烧结碳化硅材料 (RB SiC) 90 0℃的氧化过程以及制备参数和掺杂元素对氧化过程的影响。结果表明 :在 90 0℃氧化时 ,除石油焦加入量较高的反应烧结碳化硅材料在氧化初期表现为质量损失外 ,其余均表现为质量增加 ,并且质量增加量与氧化时间遵循抛物线规律。掺杂Al和Ni元素可以提高碳化硅材料的高温抗氧化能力。氧化过程对反应烧结碳化硅材料的导电性能影响不大。对碳化硅材料的氧化机制及影响因素进行了分析和讨论。     

Brittle–ductile transition during diamond turning of single crystal silicon carbide

In this experimental study, diamond turning of single crystal 6H-SiC was performed at a cutting speed of 1 m/s on an ultra-precision diamond turning machine (Moore Nanotech 350 UPL) to elucidate the microscopic origin of ductile-regime machining. Distilled water (pH value 7) was used as a preferred coolant during the course of machining in order to improve the tribological performance. A high magnification scanning electron microscope (SEM FIB- FEI Quanta 3D FEG) was used to examine the cutting tool before and after the machining. A surface finish of Ra=9.2 nm, better than any previously reported value on SiC was obtained. Also, tremendously high cutting resistance was offered by SiC resulting in the observation of significant wear marks on the cutting tool just after 1 km of cutting length. It was found out through a DXR Raman microscope that similar to other classical brittle materials (silicon, germanium, etc.) an occurrence of brittle-ductile transition is responsible for the ductile-regime machining of 6H-SiC. It has also been demonstrated that the structural phase transformations associated with the diamond turning of brittle materials which are normally considered as a prerequisite to ductile-regime machining, may not be observed during ductile-regime machining of polycrystalline materials.     

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。     

Microstructural analysis of reaction-bonded silicon nitride

Reaction-bonded silicon nitride contains at least two different crystallographic forms of Si₃N₄ and several different microconstituents and morphologies. Application of a comprehensive study using x-ray and electron diffraction, electron microprobe analysis, optical metallography, and scanning and transmission electron microscopy has allowed a complete microstructural characterization to be achieved.     

钎焊金刚石线锯切割单晶硅材料时的断裂行为研究

利用铜基钎料钎焊制备小直径金刚石线锯,观察线锯形貌,测试线锯力学性能,并在不同切割参数下对单晶硅材料进行切割,记录观察切割过程中的线锯受力,观察线锯断口形貌,探讨小直径钎焊金刚石线锯线锯过程中断裂行为及断裂机理。研究结果表明,线锯的断裂主要有正常疲劳断裂和非正常拉断。     

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。     

Transmission electron microscopy of reaction-bonded silicon nitride

Reaction-bonded silicon nitride was prepared for transmission electron microscopy by ion beam milling. The structures of three morphological forms were investigated: α-needles, α-matte, and β-grains. The α-needles were found to either have helical bands of impurity segregation or crystalline cores surrounded by noncrystalline sheaths. The α-matte consisted of fine grains (<1 μm) which were usually dislocation free while the β grains were larger (1–5 μm) and contained dislocations in the as-reacted condition.     

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。     

The material removal mechanism in mechanical lapping of diamond cutting tools

In order to reveal the surface layer removal nature and explain the anisotropy of material removal rate in mechanical lapping single crystal diamond cutting tools, a brittle-ductile transition lapping mechanism is proposed. And then, the dynamic critical depths of cut for brittle-ductile transition in different directions on different planes can be calculated. The lapped surface layer of diamond cutting tool will be removed in plastic mode as long as the embedding depth of diamond grit into the lapped surface is less than the corresponding critical depth of cut. Lapping experiments on the named (110) plane and (100) plane are carried out and the lapped surfaces are measured with atomic force microscope (AFM). The results show that all the lapped surfaces of diamond cutting tools consist of plastic grooves in nanometric scale and the maximal groove depths have prominent anisotropy in different orientations and on different planes, which are consistent with the critical depths of cut well. Therefore, the material removal rate anisotropy of lapped surface layer can be analyzed by comparing the critical depths of cut on different crystallographic planes and in different orientations of the identical plane quantitatively.     

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.     

金刚石线锯切割碳化硅陶瓷的机理与工艺研究

通过金刚石线锯切割碳化硅陶瓷的单因素试验设计,研究线速度、进给速度、进给速度与线速度比值R等工艺参数对其表面质量的影响规律,并通过超景深三维显微镜、精密粗糙度轮廓仪对其表面形貌、表面粗糙度进行观察和测量.结果表明:当线速度从0.4 m/s增加到1.3 m/s时,碳化硅陶瓷的表面形貌明显改善,进给方向和线锯方向的表面粗糙...     

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.     

Vitreous bond silicon carbide wheel for grinding of silicon nitride

This study investigates the grinding of sintered silicon nitride using a SiC wheel with a fine abrasive grit size and dense vitreous bond. The difference of hardness between the green SiC abrasive and sintered Si₃N₄ workpiece (25.5 vs. 13.7 GPa) is small. Large grinding forces, particularly the specific tangential grinding forces, are observed in SiC grinding of Si₃N₄. The measured specific grinding energy is high, 400–6000 J/mm³, and follows an inverse relationship relative to the maximum uncut chip thickness as observed in other grinding studies. The SiC wheel wears fast in grinding Si₃N₄. The G-ratio varies from 2 to 12. Two unique features in SiC grinding of Si₃N₄ are the trend of increasing G-ratio at higher material removal rate and the excellent surface integrity, with 0.04–0.1 μm R_a and no visible surface damage. For a specific material removal rate, surface cracks along the grinding direction are generated on the ground surface. The problem of chatter vibration was identified at high material removal rates. Periodic and uneven wheel loading marks and clusters of workpiece surface cracks across the grinding direction could be observed at high material removal rates. This study demonstrates that the SiC grinding wheel can be utilized for precision form grinding of Si₃N₄ to achieve good surface integrity under a limited material removal rate.