Key Engineering Materials Vol. 625

Abstract: In this paper, a five-axis ultra precision machine tool for fabrication of microstructured surfaces is presented. This machine consists of two rotary axes (C&B) and three linear axes (X&Y&Z). High precision aerostatic bearing and torque motor are adopted in C axis (main spindle) and B axis. X axis and Z axis use the hydrostatic guideway and are driven by linear motors. Y axis is driven by torque motor and precision ball screw. This machine is able to realize multiple processing methods, including ultra precision diamond turning, ultra precision milling, fly-cutting, fast tool servo and slow tool servo diamond turning.Furthermore, a large number of experiment researches are carried out. Some typical microstructure surfaces are manufactured, for sinusoidal grid surface, the surface roughness Ra is 11.9nm, which is machined by slow tool servo diamond turning. Micro pyramid array surface is fabricated by using fly-cutting, which performs well both in the profile accuracy and the repeatability. These experiment researches prove that this ultra precision machine is superior in accuracy and system reliability.

Abstract: 4H-SiC is considered as one of the most promising next-generation semiconductor power-device materials. An atomically flat 4H-SiC surface with a well-ordered step/terrace structure was essential for epitaxial growth or applications in electrical devices. Plasma assisted polishing (PAP), in which the irradiation of atmospheric-pressure water vapor plasma and ceria (CeO₂) abrasive polishing were combined, was successfully applied to the atomic-scale flattening of 4H-SiC. To clarify the atomic-scale flattening mechanism of 4H-SiC in PAP process, investigations of thermal oxidation of 4H-SiC were conducted. Cross-sectional transmission electron microscopy (XTEM) observations revealed that the interfaces between the thermal oxidized oxide layer and SiC were very flat regardless of the thickness of the oxide layer. Dipping in hydrofluoric acid for 10 min and CeO₂ abrasive polishing for 3 h were respectively conducted on a 4H-SiC surface which was thermally oxidized for 2 h. A flat surface was obtained after dipping in HF acid. However, no step/terrace structure, which corresponds to the inclination of the crystal plane, could be observed due to the residual of silicon oxycarbide. A well-ordered step/terrace structure was obtained on the surface polished by CeO₂ abrasive. The step height was about 0.25 nm, which corresponds to a one-bilayer structure of 4H-SiC. The different oxidation rates of Si atoms on the cubic face and Si atoms on the hexagonal face were considered the reason why two kinds of terraces with different width were generated.

Abstract: The Ga-doped zinc-oxides (GZO) as the transparency conductive oxide is the good candidate for substituting ITO. The buffer layer SiO_x could improve the quality of GZO thin film. The atmospheric pressure plasma multi-jets (APPMJ) system with three jets was designed and applied for SiO_x deposition process. The deposition thickness of three jets was 2.5 times higher than that of single jet, and the uniformity was less than 5% for the area 100mm². GZO thin film with SiO_x buffer layer had 3% decreases in resistivity compared to GZO thin film due to the increasing of mobility. The SiO_x/glass fabricated APPMJ system will be a good alternative substrate to bare glass for producing high quality GZO film for advanced electro-optic applications.

Abstract: In this study, we investigated ultra-precision cutting technology used in the production of Al alloy polygon mirrors. We compared characteristics of three Al alloys (Al–Mg, Al–Cu, and Al–Si) when cut with a straight diamond tool. When cutting Al alloys, it is desirable to reduce geometric surface roughness and remove tear-out marks and scratch marks. We investigated the relationship between end cutting edge length, surface roughness, and cutting force of straight diamond tools. In addition, we compared surface roughness and cutting force produced by double facet tools with microfacets at the end cutting edge and on the rake-face side. It was found that both tools have similar cutting characteristics and that Al–Mg has the best optical quality, followed by Al–Cu and Al–Si.

Abstract: At Fraunhofer IPT a test bench has been setup to analyze closed loop control components and their interfaces with focus on their application in ultra-precision machining applications. This paper will present the results of the analysis of feedback and servo drive systems and will give a close inside on the influences of linear scales and analog and digital servo drives for applications in ultra-precision machining. The performed measurements include the analysis of the position accuracy and repeatability (step response test) as well as the determination of the dynamic frequency characteristics (stiffness / compliance) of an air bearing axis. With respect to the aforementioned measurements the tests have been performed under the variation of linear scales (vendor, pitch, signal, sampling frequency, etc.) and servo drives (vendor, switching or linear amplifiers, PWM frequency, control architecture, DC bus voltage, etc.). The paper will give a summary on the results of the analyzed topics, carefully chosen regarding their relevance to ultra-precision machining. Finally, a short outlook to future research work concerning the analysis of CNC controls will be given.

Abstract: In this study, we perform the end-mill process of a difficult-to-cut material (JIS SUS310 stainless steel) and observe it with high performance infrared thermography. Considering the rotating angle of end-mill tool, a pixel temperature in each frame is investigated to obtain the tool temperature variation after cutting of each tooth in end-mill process. The tool temperature distribution can be analyzed at each rotating tool position in end-mill process from imageries, considering the relationship between the time duration of each frame and the rotating speed of an end-mill tool. Moreover, the tool/holder shape and the number of cutting teeth can be seen to affect the cutting temperature because the tool heat capacity and the heat input are different. The examination and analytical results show this method to be effective to estimate the tool temperature in the end-mill process sufficiently.

Abstract: In this paper, a 3-axis stage consisted of a XY stage and Z-axis feeding tool holder is proposed for the application of fast tool servo (FTS). The XY stage actuated by six piezoelectric (PZT) actuators is designed with symmetric flexural hinges featuring low interference motions, high stiffness, and fast response. Numerical design using the finite element method (FEM) was conducted to investigate the steady characteristics (displacement, stiffness, stress, and strain) and dynamic characteristic of resonance frequency. According to calculation results, the major characteristics obtained along XYZ axes are as follows: displacements induced are 10.06, 10.28, and 20.31 μm due to the applied voltage being 50 V; stiffness are 112.84, 110.31, and 223.34 kN/mm; the maximum stresses at the hinges are 9.78, 10.9, and 100.56 N/mm², which are lower than the allowable stress of aluminum used; and the resonant frequencies are 1.0, 0.64, and 0.4 kHz, respectively. Experimental examinations regarding to the resonant frequencies were performed with a maximum deviation of 16% along the Z-axis compared to the simulation result. As a result of the investigation, it is expected that the 3-axis stage can be effectively applied to implement a FTS.

Abstract: Recent advances in measurement systems require positioning systems with high stiffness, accuracy and speed. Piezoelectric actuators which are featured with mechanical simplicity, quick response, and electromagnetic immunity, are often used in precision positioning. It is known that piezoelectric actuators can achieve high positioning accuracy by the stepping mode but low speed. By contrast, the resonance vibration mode will offer high positioning speed, but sacrifices the high inherent position resolution. For the stepping mode, the displacement of the piezoelectric actuator significantly affects the speed, of which larger displacement induces higher speed. For the resonance vibration mode, an elliptical motion of the piezoelectric actuator tip is generated by horizontal and vertical eigenmodes, and the optimal efficiency can be achieved when the two eigenmodes are operated at the same frequency. For the applications of high positioning accuracy and speed, a piezoelectric actuator should be designed by taking these two operation modes into consideration simultaneously. Based on these requirements, the optimal structural dimensions of a piezoelectric actuator are obtained using a genetic algorithm.

Abstract: Neutron beam is a very useful probe for material characterization and non-destructive testing. There are a number of plans to construct neutron instrumens using focusing mirror to improve beam quality with higher intensity and smaller spot size. In this paper, an ellipsoidal mirror made by a metal substrate is proposed. As the metal substrate, electroless nickel is adopted because of its amorphous structure, machinability and relatively large critical angle of neutron reflection. The design of ellipsoidal neutron focusing mirror is made by considering the neutron reflectivity. A mirror is fabricated by ultrahigh precision cutting and 1^st polishing process, and the form accuracy of 5 μm P-V with surface roughness of 0.7 nm Ra is obtained. Through this experimental fabrication, the feasibility of the proposed method is verified.

Abstract: Surface micro-defects of large diameter KDP crystal have great influence on its laser damage threshold. The method of micro-mechanical repairing can inhibit growth of the laser damage obviously. The paper adopts Fourier modal method to periodically process Gaussian repaired contour and established the light intensity modulation model of the Gaussian contour inside the crystal. Based on the model, the paper researches the influences on the light intensity inside the crystal made by Gaussian repaired contour with different width. Calculation results show that when the curve depth is fixed, the maximum relative light intensity decreases gradually as the curve width increases. Through experiments of micro-mechanical repairing and laser irradiation, we find that the laser damage threshold of Gaussian repaired contour with 20μm depth and 400μm width is 3ω, 5J/cm². Meanwhile, experimental results verify the theoretical analysis. Keyword: KDP crystal, Gaussian repaired contour, internal intensity modulation, Fourier modal method, surface defect