On-machine dry electric discharge truing of diamond wheels for micro-structured surfaces grinding

Precision grinding with diamond wheels gives a promising alternative to achieve high quality micro-structured surfaces on optical molds. However, it is difficult to true these diamond wheels efficiently, because of the remarkable resistance property and the geometrical limitation of small wheel profile. In this paper, an on-machine dry-EDT method to precision shape and prepare diamond wheels with various profiles was proposed for micro-structured surface grinding. Firstly, the fundamental truing errors were analyzed based on the dry-EDT kinematics. And then the capabilities of dry-EDT truing for high abrasive concentration metal bonded diamond wheels were presented. Next, the effects of kinematic parameters variables on trued wheel profile accuracy were investigated. Finally, the micro-structured surfaces on SiC ceramic and tungsten carbide WC were ground by these trued diamond wheels. The experiments results showed that the arc-shaped diamond wheel (diameter of 200 mm) with 4 μm profile error (PV) and 1.0023 mm profile radius, and the V-shaped diamond wheel with 22.5 μm V-tip radius and 120.03° profile angle could be obtained by on-machine dry EDT. The kinematic parameters of dry-EDT have an important influence on truing profile accuracy of diamond wheels, especially for the tip of V-shaped wheel. The subsequent grinding show that the edge radius of V groove array on SiC is less than 2 μm, while the radius of included corner is around 55 μm. The PV error of ground arc groove array on WC is less than 5 μm. The surface roughness of ground micro-structured surface R_a is 142 nm and 97 nm for SiC and WC, respectively.     

Effects of diamond size of CMP conditioner on wafer removal rates and defects for solid (non-porous) CMP pad with micro-holes

Wafer removal rates and defects were investigated for 200 mm tetraethyl orthosilicate (TEOS) oxide chemical mechanical planarization (CMP) processes using two types of CMP pads: a porous pad and a solid pad with micro-holes. An initial CMP test conducted with fumed silica based-slurry and a conditioner with 180 μm diamond revealed that the wafer removal rates by the solid pad with micro-holes were approximately 10% lower than those by the porous pad, but scratch type defects were reduced. In order to increase the removal rate of a solid pad with micro-holes to the comparable level of a regular porous pad without changing process parameters, it was decided to modify conditioner design by using different diamond size from 70 to 130 μm. It was found that wafer removal rates increased from 2973 to 2587 Å/min and defect counts reduced from 5.3 to 1.7 by decreasing the diamond size from 180 to 70 μm in the case of the solid pad with micro-holes. Various pad surface analysis results, including contact area estimation and microscopic observations, also revealed that a smaller diamond conditioner generated the pad texture with finer and more regular pad asperities.     

单晶SiC基片的铜基研磨盘加工特性研究!

采用铜基螺旋槽研磨盘对6H-SiC单晶基片的Si面和C面进行了单面研磨加工,研究研磨压力、研磨盘转速和金刚石磨粒尺寸对SiC基片材料去除率和表面粗糙度的影响。结果表明,单晶SiC的C面和Si面具有明显的差异性,C面更易加工,其材料去除率比Si面大。研磨压力是影响材料去除率和表面粗糙度的主要原因,研磨压力越大,材料去除率越高,但同时表面粗糙度变大,较大的研磨压力会导致划痕的产生。在达到最佳表面粗糙度时,C面加工所需的转速比Si面大。磨粒团聚会严重影响加工表面质量,采用粒度尺寸3 μm的金刚石磨料比采用粒度尺寸1 μm的金刚石效果好,经粒度尺寸3 μm的金刚石磨料研磨加工5 min后,Si面从原始粗糙度R_a 130 nm下降到R_a 5.20 nm,C面下降到R_a 5.49 nm,表面质量较好。      

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.     

Study the characteristics of magnetic finishing with gel abrasive

Given the flexible polishing effect in magnetic abrasive finishing (MAF), the precise and mirrorlike surface can be obtained during this process. However, the abrasives are easily flown away from the working area regardless of what abrasives are used in MAF; this situation will reduce the polished efficiency and induce the pollution problem in the environment. Besides, the abrasives cannot recycle after the finishing process. Therefore, a novel abrasive medium, using the silicone gel to mix the ferromagnetic particles and abrasive, was developed to enhance the disadvantages in MAF. Magnetic finishing with gel abrasive (MFGA) was utilized in this study to polish the cylindrical rod of mold steel; furthermore, this cylindrical rod was fixed in a horizontal chuck that could rotate and vibrate in the axial direction. This study focused on the finishing efficiencies and the surface roughness of the workpieces after MFGA. Moreover, recycling times of gel abrasive were also the main effects that need to be approved. The results demonstrated that surface roughness of the cylinder part was reduced to 0.1 μm Ra from an initial value of 0.677 μm Ra within 10 min, and surface roughness could decrease to 0.038 μm Ra after 30 min in MFGA. Surface roughness reduction in MFGA was 3 times of surface roughness reduction in MAF using the unbonded magnetic abrasive as medium. Roughness improvement rate still remained at a high level of 90% when the same abrasive medium (35 g) was used 15 times to finish 15 workpieces; therefore, this result proved that the gel abrasive had excellent ability for recycling.     

Precision grinding of optical glass with laser micro-structured coarse-grained diamond wheels

This paper presents a series of micro-structured coarse-grained diamond wheels for optical glass surface grinding aiming to improve the grinding performance, especially subsurface damage. The 150 μm grit size, single layer electroplated diamond grinding wheels with different interval micro-groove arrays were manufactured by nanosecond pulsed laser, successfully. The influence of micro-structures on surface roughness and subsurface damage was investigated. Compared with conventional coarse-grained diamond wheel, the subsurface damage depth was reduced effectually from 5 to 1.5 μm, although the better surface roughness was not obtained by the micro-structured coarse-grained diamond wheel. In addition, the surface roughness and subsurface damage depth were both reduced with the decreasing interval of micro-groove arrays.     

ELID grinding characteristics of glass-ceramic materials

Zerodur glass-ceramic materials have been widely used in optical, opto-electronic and precision engineering industries; their efficient ultraprecision machining, with extremely low surface roughness and high form accuracy, is in great demand in those fields. The authors have been conducting studies on realizing high-quality surface and form accuracy of zerodur glass-ceramic materials efficiently by use of electrolytic in-process dressing (ELID) grinding process. This paper proposes a new grinding mode in which top surface and sides of zerodur block were ground by cylindrical surface and side surface of grinding wheel. Grinding experiments were carried out using #1200, #2000 and #4000 diamond cast-iron bond wheels, and grinding characteristics such as grinding performance, ground surface roughness, surface topographies and perpendicularity between ground surfaces were investigated. Experiments on grinding using #4000 wheel successfully produced smooth top surface and side surfaces that were about 10 nmRa in surface roughness, 1.5 μm/□400×400 mm² in flatness and 90°±6″ in perpendicularity. AFM observation of the ground surface also showed that material removal in the ductile mode occurs for fine abrasive wheels. The results showed that grinding was stable without severe clogging for wheels by choosing suitable ELID parameters and grinding conditions.     

Grinding characteristics of micro-abrasive pellet tools fabricated by a LIGA-like process

The purpose of this paper was to investigate the wearing and grinding characteristics of the micro-abrasive pellet tools with 4–6 μm diamond particles fabricated by a LIGA-like process that has micro-lithography with photoresist mold and nickel/diamond composite electroforming. The results showed that when the micro-pellet tool containing partial resist joint with a root on the substrate was designed and fabricated, the tool against alumina sandpaper in wear test showed lesser amount of micro-pulled-out pellets than the tools with flat joint type, which displayed the tool to have better adhesion strength. In addition, when the micro-diamond tools were used to grind silicon wafers, the surface appearance of wafers showed ductile behavior. The surface roughness of wafers ground with increased pellet tool rotation speed became better and R_a=0.05 μm was achieved.     

Precision and micro CVD diamond-coated grinding tools

Both for ultra-precision and for micro-machining diamond is used very often as tool material. The reason is the very high dimensional stability of diamond due to its extreme hardness. Diamond is used for two kinds of machining processes: for cutting, like turning, drilling or milling, as well as for abrasive processes, like grinding. Diamond cutting tools can be made with massive diamond (monocrystal, CVD diamond, PCD) or with diamond coatings. Standard diamond abrasive tools are made by bonding diamond monocrystals onto a base body. A new grinding layer technology is presented: chemical vapour-deposited microcrystalline diamond layers have crystallite tips with very sharp edges that can act for grinding processes. Base body materials and coating technology is presented. Application results of grinding experiments show that very high workpiece quality can be reached, e.g. a roughness Ra of 5 nm with glass workpieces. Truing and recoating techniques are discussed for reuse of worn CVD diamond grinding wheels. Micro grinding tools (abrasive pencils, burrs) can be manufactured with the same coating technology. Very small tools with diameters of 50 μm have been made and successfully tested.     

Shear-thickening polishing method

A shear-thickening polishing (STP) method utilizing the shear thickening mechanism of non-Newtonian power-law fluid based slurry is proposed for curved surface polishing. The STP principle and micro-material removal action are analyzed. The high-performance STP slurry with the shear-thickening rheological behaviors has been prepared. To achieve the material removal mechanism of STP process, based on the Preston formula, fluid dynamics and shear thickening mechanism, the material removal rate (MRR) model is established and the difference of MRR between theoretical and experimental results is 6.12%. The experimental and theoretical tests of STP process are conducted to investigate the influences of polishing velocity, abrasive concentration and grain size on MRR and surface roughness. Compared with Newtonian fluid slurry, STP slurry can achieve much higher MRR and better surface quality due to shear-thickening effect. MRR of Cr12Mo1V1 (die steel) is up to 13.69 μm/h, and surface roughness is reduced from R_a 105.95 nm to R_a 5.1 nm within 0.5 h of processing. This indicates that STP is a promising processing method for precision finishing or polishing.     

Mechanism for material removal in diamond turning of reaction-bonded silicon carbide

Reaction-bonded silicon carbide (RB-SiC) is a new ceramic material that has extremely high strength and hardness. Diamond turning experiments were performed on RB-SiC to investigate the microscopic material removal mechanism. Diamond tools with large nose radii of 10 mm were used for machining. It was found that the surface roughness was not significantly affected by the tool feed rate, but was strongly dependent on the tool rake angle. The mechanism for material removal involved plastic deformation, microfracture and dislodgement of 6H-SiC grains. Raman spectroscopy revealed that the silicon bond component underwent amorphization, while no phase transformation of 6H-SiC grains was observed. Tool wear was also investigated and two types of wear patterns were identified. Under the experimental conditions used, a surface finish of 23 nm Ra was obtained even at an extremely high tool feed rate of 72 μm/rev. This study demonstrates the feasibility of precision machining of RB-SiC by diamond turning at a very high material removal rate.     

固结磨料研磨蓝宝石晶片的工艺优化

为了满足蓝宝石晶片高效低损伤的加工要求,采用亲水性固结磨料研磨垫研磨蓝宝石晶片的工艺,研究基体中碳化硅粒度尺寸、基体类型、金刚石粒度尺寸及研磨液中磨料4个因素对材料去除率和表面粗糙度的影响,并综合优化获得高加工效率和优表面质量的工艺参数。实验结果表明:基体中碳化硅粒度尺寸为10 μm、基体类型为Ⅱ、研磨垫采用F公司粒度尺寸为35~45 μm的金刚石、研磨液中磨料的粒度尺寸为5 μm的碳化硅为最优工艺组合,亲水性固结磨料研磨蓝宝石的材料去除率为431.2 nm/min,表面粗糙度值为R_a 0.140 2 μm。      

The influence of SDS and CTAB surfactants on the surface morphology and surface topography of electroless Ni–P deposits

The effect of surfactants sodium dodecyl sulphate (SDS) and cetyltrimethyl ammonium bromide (CTAB) on the surface roughness, surface morphology and surface topography of electroless nickel (EN)–phosphorus surface protective coating obtained from an alkaline bath is presented in this paper. In this study the influence of surfactant concentrations on the surface roughness of coated samples were investigated. It was observed that the surface roughness, surface morphology and surface topography of Ni–P coating were clearly influenced by the addition of surfactants SDS and CTAB. EN deposits with addition of surfactant SDS and CTAB at a concentration of 0.6 g/l produce a smooth surface and the average roughness (R_a) value is 1.715 μm for SDS and 1.607 μm for CTAB which is less than the R_a value of EN deposit without surfactant addition (1.885 μm). The mean average roughness (R_a) value with addition of surfactant is 1.796 μm.EN deposit with addition of surfactants consists of a significant fraction of particles of nickel. In the presence of SDS, fine nickel particles have dispersed uniformly on the substrate surface resulting in smoother surface finish of the deposited layers. In the presence of CTAB, at lower concentrations (upto 0.6 g/l) coalescence of nickel particles have been deposited on the substrate surface and at the higher concentration (above 0.6 g/l) uniformly improved surface finish of the deposited layer is resulted. The complete experimental details, results obtained and their analysis are presented in this paper.     

Damage-free highly efficient polishing of single-crystal diamond wafer by plasma-assisted polishing

Single-crystal diamond (SCD) is considered to be an ideal material for next-generation power devices. Plasma-assisted polishing (PAP) without using an abrasive was applied to polish SCD fabricated by chemical vapor deposition. Argon-based plasma containing water vapour was used in the PAP to modify the surface of polishing plate and SCD (100), and SCD was polished under a polishing pressure ranging from 10 to 52.6 kPa. Raman spectroscopy measurement showed that there was no residual stress on the polished SCD surface, and a polishing rate of 2.1 μm/h and a surface roughness of 0.13 nm Sq were obtained.     

固结金刚石聚集体磨料垫高效研磨氧化锆陶瓷背板

氧化锆陶瓷背板的高效低损伤研磨加工是其在5G通讯应用中的前提。针对氧化锆材料的硬脆特性造成磨粒磨损严重的特点,以金刚石单晶和聚集体为磨料,制备固结磨料垫（FAP）,并对比研究其加工性能,探索了研磨液中碳化硅磨粒在固结磨料研磨垫自修整过程中的作用机理。结果表明:采用金刚石聚集体作为固结磨料垫的磨料,辅以碳化硅砂浆,能够明显提高研磨速度,改善表面加工质量。采用粒度230/270的金刚石聚集体固结磨料研磨垫,辅以颗粒尺寸3~5 μm的绿碳化硅砂浆,氧化锆陶瓷研磨时材料去除率达2.5 μm/min以上,表面粗糙度值R_a为74.9 nm。      

Residual stress and subsurface damage in machined alumina and alumina/silicon carbide nanocomposite ceramics

We have used TEM and Hertzian indentation to study the interrelation between subsurface damage and residual stress introduced by grinding and diamond polishing surfaces of polycrystalline alumina and 5%SiC/alumina nanocomposites. In all cases a layer of high dislocation density was found near the surface. This varied in thickness from about 300 nm for alumina polished with 1 μm diamond grit to greater than 6 μm for a nanocomposite surface wheel-ground with 150 μm diamond grit. For a given finishing process the nanocomposites showed a greater depth of dislocation activity than alumina. In alumina, extensive basal twinning was found beneath the ground surfaces. Hertzian indentation data indicates a residual compressive stress of about 1500 MPa confined to the dislocation-containing region. Mechanisms for the enhanced dislocation activity in the nanocomposites are discussed.     

Experimentation on MR fluid using a 2-axis wheel tool

This paper is focused on magnetorheological (MR) fluid assistive polishing of optical aspheric components. MR fluid is a functional mixture of non-colloidal magnetic particle of micrometer size suspended in a host fluid, with the special property that its viscosity can be varied by the application of a magnetic field. This paper introduces the basic principles of the methodology and presents experiment results on MR fluids using a 2-axis wheel-shaped tool supporting dual magnetic fields. Mathematical models taking into account the pressure and the tool velocity are derived. The experiments serve to evaluate the effects of process parameters on material removal and performance using a K9 glass parabolic lens of 60 mm diameter as work-piece. It is shown that surface roughness can be reduced from an initial value of 3.8–1.2 nm after 10 min of polishing. The form errors can also be improved from an initial 2.27 μm rms and 7.89 μm peak-to-valley to become 0.36 μm rms and 2.01 μm peak-to-valley after 60 min of polishing.     

Abrasive waterjet cutting of polycrystalline diamond: A preliminary investigation

Although abrasive waterjet (AWJ) machining has been employed in different setups (e.g. through cutting, milling, turning, cleaning) to generate surfaces in various workpiece materials (e.g. metallic alloys, ceramics, composites), up to now there is scarce information on the use of this technology in cutting super-hard materials such as diamond-based materials. The paper reports for the first time on a preliminary study of the capability of AWJ cutting of polycrystalline diamond (PCD) using abrasive media with different hardness, i.e. aluminium oxide (Al₂O₃), silicon carbide (SiC) and diamond. While keeping some operating parameters constant (pump pressure, stand-off distance and size of abrasives) the feed speed has been adjusted to enable full jet penetration for each type of abrasives. It was found that not only the material removal rates vary significantly with the employment of different types of abrasives but also the nozzle wear ratios, with further implications on the kerf quality (width, taper angle) of diamond cut surfaces. Furthermore in-depth studies of the cut surfaces helped to reveal the material removal mechanism when different types of abrasives are employed: Al₂O₃—low intensity erosion; SiC—medium erosion with undesired cracking; diamond—high intensity erosion. The experimental results showed that while Al₂O₃ and SiC abrasive media yield modest material removal rates (comparable with those obtained by electro discharge machining-EDM) the use of diamond abrasives can greatly increase (>200 times) the productivity of AWJ through cutting of PCD test pieces at acceptable roughness (Ra<1.6 μm) and integrity (i.e. crack-free) of the cut surfaces. Despite some limitations (e.g. cost of diamond abrasives, extensive nozzle wear rates) that can be overcome through further developments, it is believed that this preliminary research gives an indication of the capability of the AWJ to profile diamond-based structures for high-value engineering applications where conventional methods (e.g. EDM, laser) cannot be applied or are not productive enough.     

Application of shallow circumferential grooved wheels to creep-feed grinding

A single-point diamond dressing tool was used to cut shallow circumferential groove on aluminum oxide grinding wheels. Creep-feed grinding experiments were then carried out to compare the performance of these grooved wheels with a non-grooved wheel. The results showed that, for the conditions used in this research, a grooved wheel could remove twice as much material as a non-grooved wheel before workpiece burn occurred. The results also showed that a grooved wheel can improve grinding efficiency by reducing the consumed power by up to 61%. Although the use of grooved grinding wheels caused the workpiece surface roughness to increase slightly when compared to a non-grooved wheel, the grooved wheel enabled up to 37% more material to be removed while still maintaining workpiece surface roughness values below 0.3 μm (“fine quality” surface finish), and up to 120% more material to be removed while still maintaining workpiece surface roughness values below 1.6 μm (“average quality” surface finish).     

A study on ultrasonic elliptical vibration cutting of tungsten carbide

Sintered tungsten carbide (WC) is a versatile metal matrix composite (MMC) material widely used in the tool manufacturing industries. Machining of this material with conventional cutting (CC) method is a real challenge compared to other difficult-to-cut materials. Ultrasonic elliptical vibration cutting (UEVC) method is a novel and non-conventional cutting technique which has been successfully applied to machine such intractable materials for the last decade. However, few studies have been conducted on cutting of WC using single point diamond tool (SPDT) applying the UEVC technique. This paper presents an experimental study on UEVC of sintered WC (～15% Co) using polycrystalline diamond (PCD) tools. Firstly, experiments have been carried out to investigate the effect of cutting parameters in the UEVC method in terms of cutting force, flank wear, surface finish while cutting sintered WC. The tests have revealed that the PCD tools in cutting of WC by the UEVC method results in better cutting performance at 4 μm depth of cut (DOC) as compared to both a lower DOC (e.g. 2 μm) and a higher DOC (e.g. 6 or 8 μm). Moreover, the cutting performance improves with the decrease in both the feed rate and cutting speed in the UEVC method like conventional turning (CT) method. A minimum surface roughness, R_a of 0.036 μm has been achieved on an area of about 1257 mm² with the UEVC performance. The CT method has also been employed to compare its cutting performance against the UEVC method. It has been observed that the UEVC method results in better cutting performances in all aspects compared to the CT method. Theoretical analysis on the UEVC method and analysis of the experimental results have been carried out to explain the reasons of better surface finish at 4 μm DOC and better cutting performance of the UEVC method.