Advanced Materials Research Vol. 1017

Abstract: This paper describes the application of loose-abrasive processes to the manufacture of 1.4 meter, off-axis aspheric, hexagonal mirror-segments. These are prototypes for the 39 m European Extremely Large Telescope (E-ELT). The application of active forces to correct the overall form of segments in the telescope, means that the overall form-accuracy achieved in polishing can be less critical than for a non-active mirror. However, it is a requirement that the base-radii and conic constants of mirror-segments are very closely matched, so that the combined image is not degraded. This means that abrasive processes have to operate with respect to an absolute rather than relative datum. Furthermore, there are stringent requirements on mid-spatial frequency defects on segment surfaces, and on edge-roll. These control stray-light, and ultimately detectability of faint astronomical targets. We describe the CNC abrasive techniques we have developed in response. We then demonstrate the success of the approach, which represents the first time ever that segments have been processed entirely in the hexagonal shape:- a milestone in loose-abrasive processing. Finally, we address up-scale for the unprecedented number of segments required for the E-ELT build-phase.

Abstract: This paper investigates the effect of the grinding-induced cyclic heating on the hardened layer properties generated in plunge cylindrical grinding. It was found that by increasing the number of grinding cycles, the hardened layer becomes thicker. The cyclic grinding stressing together with the heating initiates a plastic deformation zone of a highly oriented microstructure. The surface residual stresses of the layer are compressive. Under a lower infeed rate, the tempering of the hardened layer material will occur more severely.

Abstract: In order to improve the efficiency and reduce the cost of sapphire crystal machined by loose abrasive lapping, the fixed-abrasive diamond grinding tools for sapphire grinding were investigated in this paper. Four vitrified-resin composite bond diamond grinding tools with different grain sizes (40μm, 20μm, 7μm, 2.5μm) were developed. The grinding experiments were performed with the developed diamond grinding tools and the grinding performance of four grinding tools were evaluated by comparing the surface roughness and the material removal rate (MRR) of sapphire. The experiment results show that with the increase in grain size, both the MRR and the surface roughness increase. A high-efficiency and high-quality ultra-precision grinding process using diamond grinding tools with different grain sizes was proposed.

Abstract: Diamond abrasive coated wires are core tools for precision multi-wire sawing of mono-crystal ingots for substrate manufacturing, especially for hard materials applications such as SiC, GaN, sapphire and silicon. However, repeated contact by reciprocation motion of wire makes it difficult to design optimized utilization of diamond wires since it makes temporal change in cutting performance due to gradual wear of abrasives. In this paper, the cutting performance of wires are examined based on experimental results with wires having different concentration. The results indicated that the concentration of diamond abrasives have strong relationship with cutting performance and wires with low concentration showed higher material removal characteristics.

Abstract: Micro-cracks on the edge surface of thin glass edge sheet have been identified as a key factor of catastrophic glass breakage. Hence, their removal will strengthen the thin glass substantially. This paper studies the glass edge finishing using magnetorheological finishing (MRF). The thin glass sheet edge is finished by shear force exerted by magnetorheological fluid, which is magnetically held by a specially designed magnetic wheel tool. All micro-cracks can be removed from the edge surface and the surface roughness improves from R_a 0.5 μm to R_a 0.03 μm.

Abstract: A pressure sensor is developed to explain the finishing mechanism in centrifugal disc finishing. The distributions of the flow pressure of mass in the radial direction and the height direction of the barrel are investigated. The flow pressure of mass is higher in the lower layer near the disc than in the upper layer, regardless of the finishing conditions. In the upper layer of the mass and near the side wall of the barrel, the flow pressure of the mass of the wet process is higher than that of the dry process. However, the stock removal of the dry process is larger than that of the wet process.

Abstract: 6H-SiC single crystal substrates are considered to be suitable for thin film growth of semi-conductive GaN with wide energy bandgaps, because the lattice mismatch between 6H-SiC and GaN is quite small compared with sapphire substrates. However, the process of SiC wafer prior to epitaxial growth is quite difficult due to its high hardness and chemical stability. The chemical mechanical polishing (CMP) is typically performed in semiconductor device planarization, but the polishing slurry is not environmental friendly and the polishing efficiency is very low. In this paper, polishing experiments were conducted on 6H-SiC wafers using a flexible fixed abrasive film fabricated by sol-gel technique. Other three fixed abrasive tools bought from the different companies were also chosen in these experiments. Abrasive wear resistance on the different tools was observed with the increase of polishing time. Studies were also made on the surface micrographs and roughness of the 6H-SiC wafers polished by the four different tools in order to evaluate the tool performance. The results indicated that poor wear resistance of abrasive were showed in tool C and D, less abrasive worn out in the sol-gel polishing film and tool B. The surface quality of 6H-SiC wafer polished by the sol-gel polishing film was much better than the other three tools, and it attained the lowest surface roughness Ra=0.003 μm. Keywords: Sol-Gel polishing film; Fixed abrasive tools; 6H-SiC wafers

Abstract: Laser ablation is a novel non-mechanical wheel preparation method for optimizing the treatment costs of superabrasive tools. In this study the thermal effects of picosecond laser radiation on CBN superabrasive grinding wheel surface is analytically and experimentally investigated. The analytical approach is intended to find threshold process parameters for selective ablation of cutting grains and bond material. It has been analytically and experimentally shown that, the extent of material degradation is defined by the maximum surface temperature induced by the laser radiation which is in turn defined by the laser beam energy. It is also suggested that, the depth of laser thermal effects is governed by the relative speed of the laser scanner with respect to the wheel surface.

Abstract: Responding to the demand on the ultra-precision equipment for machining monocrystalline silicon, sapphire, zirconia ceramics and other large-size functional crystal substrates in the microelectronics and optoelectronics manufacturing, this study analyzes the current state-of-the-art of polishing technologies and the technical challenges to achieving high surface quality substrates. In this study, a planetary double-sided polishing machine is designed to solve the problem between achieving high polishing quality and efficiency. The machine is characterized by process stability, large polishing pressure and high polishing speed, etc. which is verified by polishing zirconia substrates with satisfactory results: material removal rate of 5 μm/h and the surface roughness Ra of 1 nm.

Abstract: The phenomenon that bubbles or particles are suspended at the node position of a standing wave of sound is known as acoustic levitation and has recently been applied in the fields of semiconductors, aerospace, and biology [1-3]. By using this phenomenon in this study, a new classification method has been proposed, and a device has been developed to sort objects in accordance with their densities. Unlike the conventional methods as such as centrifugal particle separation or magnetic separation, this method can separate fine particles both of metal and non-metal and without contact easily only using acoustic power. First, we derived the acoustic radiation pressure to be applied to the object from King’s theory, and then we designed and developed a twin-transducer system to apply the required levitation force. The distributions of sound pressure and particle velocity were then visualized. Finally, a series of experiments was conducted to show the capability of classifying the fine abrasive SiO₂ particles and fine iron particles (φ = 50 μm).