Papers by Author: Shen Dong

Abstract: In the ultra-precision machining of KDP crystal, there are many factors affecting the surface quality[1-3]. The experiments show that the rake angle and back angle of the tool have significant effects on machined surface roughness. Therefore, an efficient way to improve the surface roughness is to select a proper negative rake angle. In this study, the ANSYS static analysis method was employed to analyze the stress field distribution within the whole cutting region. A finite element simulation model was set up to calculate the residual stresses variation with tool’s angles, which can be considered to select optimal rake and back angles in the ultra-precision machining of KDP crystal. Results show that the optimal tool rake angle and back angle are -49° and 7°, respectively. Finally, by using different tool angles to process KDP crystal and utilizing AFM to analyze the surface roughness, it can be found that the measurement results agree well with what are deduced from theoretical calculation.

Abstract: By focusing femtosecond laser pulses in the bulk of a quartz wafer, sub-surface waveguides were microexploded. The material around the femtosecond laser induced microexplosive zone was densified, which altered local refractive index. Changes in material density can take corresponding variations in nanomechanical properties, which were proved by the depth dependent modulus and hardness variations tested by nanoindentation in the area around the microexplosive zone. Changes in refractive index were correlated with residual strains along radial direction of the waveguide cross section. An equation of residual strain in such area that based on nanoindentation data was set up, so as to discover density and refractive index variations in the adjacent areas of femtosecond laser microexplosive zone indirectly.

Abstract: Indentation tests and single-point scratch tests are probably the simplest methods of measuring the elastic, plastic and fracture behavior of brittle materials. In this paper, the nearsurface mechanical properties of KDP single crystal have been investigated including the elasticity like Young’s modulus E, and the plasticity like the hardness H. These material properties can be used to predict the material responses in optical manufacturing operations. Hardness and elastic modulus on different crystal plane of KDP single crystal have been examined under different loads by nanoindentation test, and the influence of the indentation load on hardness and elastic modulus have been also analyzed systematically. The results show the nanoindentation size effect, that is, the hardness and elastic modulus increase as the indentation load decreases. The hardness and elastic modulus have strong anisotropy in the different crystallographic orientation of the same crystal plane.

Abstract: In this paper, a new generation method for diamond turning non-axisymmetry aspheric mirrors is introduced. A rotary arm that carried a diamond tool combined with fast tool servo was used to replace the straight guides employed by most diamond turning machines. The micro linear feed of the fast tool servo was real-time calculated with the use of high resolution angular feedback on the work spindle and the swing tool. Synchronized motion of the fast tool servo according to rotating angles of the workpiece and the diamond tool produced ultra-precision non-axisymmetry aspheric surfaces. The corresponding mathematical models of NC program are presented.

Abstract: Molecular dynamics is a rapidly developing field of science and has become an established tool for studying the dynamic behavior of material machining. A three-dimensional molecular dynamics (MD) model about the atoms of the diamond cutting tools and the diamond grits is built by using the molecular dynamics. The Tersoff potential function is used to calculate the force and potential energy among the atoms of the diamond tools and the atoms of the diamond grits. The lapping processes at a special cutting depth are simulated. The variety of the specimen potential energy in the lapping process is observed. The mechanism of the diamond micro machining and the form of the surface formation are given by comparing the distribution maps of atoms in initial and cutting states. This study will give a strong support to the diamond cutting tools’ lapping.

Abstract: A three dimensional molecular dynamics model is employed to simulate AFM-based lithography process. To investigate effects of tip geometry, three kinds of tip models are proposed: a cone-shape tip with a hemisphere at the end, a round-edged three sided pyramid tip with a hemisphere at the end and a sharp-edged three sided pyramid tip. These models simulate scratching behaviors of AFM tip at different scratching depths. Results showed that materials removal behavior and scratching forces were significantly affected by tip geometry, depending on the scratching depth and scratching directions. The specific energy using a sharp-edged three sided pyramid tip displayed a different behavior comparing to that using a round-edged three sided pyramid. However, scratching orientations exhibited no effects on the specific energy.

Abstract: An ultra-precision machine is developed by Precision Engineering Research Institute of Harbin Institute of Technology to machine components made of KDP crystal with single point diamond fly cutting technique . A stable ultra-low speed feed of worktable is necessary in the machining process inasmuch as the KDP crystal components to be machined must be high form accuracy, low surface roughness and low surface waveness . This paper analyses the effect on speed stability and positioning accuracy under the control of semi-closed loop and full-closed loop based on the experimental data, and also present a compensating control algorithm of error disturbance feed-forward which enhances the stability of ultra-low speed motion of the semi-closed loop feed control servo system of the machine. The simulation results indicate that the values of the steadystate tracking error decreased to 1/10 after using compensating control algorithm. The P-V value of the aluminum specimen machined by the ultra-precision machine tool was 0.27 wavelengths.

Abstract: The method of single point diamond turning is used to machine KDP crystal. A regression analysis is adopted to construct a prediction model for surface roughness and cutting force, which realizes the purposes of pre-machining design, prediction and control of surface roughness and cutting force. The prediction model is utilized to analyze the influences of feed, cutting speed and depth of cut on the surface roughness and cutting force. And the optimal cutting parameters of KDP crystal on such condition are acquired by optimum design. The optimum estimated values of surface roughness and cutting force are 7.369nm and 0.15N, respectively .Using the optimal cutting parameters, the surface roughness Ra, 7.927nm, and cutting force, 0.19N, are obatained.

Abstract: In this paper, mechanical characteristics of KDP crystal anisotropy are analysed theoretically. Vickers indentation experiments are adopted to validate the variation rule of hardness and fracture toughness in different orientation of KDP crystal plane (100), and a model to calculate critical cutting thickness of brittle-ductile transition is proposed for the KDP crystals. The result shows that, on the crystal plane (100), the minimum value of critical cutting thickness of KDP crystal in brittle-ductile transition appears in the direction [110], but the maximum appears in the direction [010]. Finally, the ultra-precision machining of KDP crystal is performed, and the results agree well with the theoretical conclusions. Super-smooth surface with a roughness RMS of 6.6nm is reached as machined in the crystal direction [010], and 11.2nm to the direction [110].

Abstract: Femtosecond laser pulses are irradiated on a single-side polished single-crystalline silicon wafer. Metamorphic zone appears around the ablated zone and the morphology changes gradually in the metamorphic zone. Typical phenomena of thermal ablation such as melt-resolidification and subtransparent glassy materials occur in the marginal area of the ablated zone. No apparent changes are found among microscopic morphologies of the ablated, metamorphic and unirradiated zones. There are flaws and spallations on the smooth back surface of the wafer, which are caused by the stress inside the sample. Nanomechanical properties of the sample surface hardly change in the small scope of the backside around the ablated zone. While regular changes occur in large scope, which is the conjunct result of such stresses caused by many different collateral damages.