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Crack-free processing of hot-pressed silicon nitride ceramics using pulsed YAG laser
micromachining process of copper taking into consideration of the roundness of the tool edge. An FEM model is also developed to analyze the flow of cutting heat and the temperature distribution within both the workpiece and the tool based on the stress and the material flow within the workpiece calculated. Orthogonal micromachining experiments are carried out by employing both a micromachining equipment installed within SEM (Scanning Electron Microscope) and an ultraprecision fly cutting machine. The results of the FEM analysis are compared with the experimental results. 19 Refs. 16. Morita, N. Crack-free processing of hot-pressed silicon nitride ceramics using pulsed YAG laser. Proceedings of the International Conference on Machining of Advanced Materials - NIST Special Publication 847. 1993 Jun:
517-526. This paper describes the effect of crack-free laser processing of hot-pressed silicon nitride ceramics on fracture strength and residual stress. The fracture strength of the processed test pieces was determined by three-point bending tests. The residual stress in the processed surface was characterized by X-ray diffraction measurement. The strength of the laserprocessed workpieces was reduced to 90-80% compared with that of the ground workpieces because of subsurface layer damage, which included the heat-affected layer and the residual stress layer. In particular, the latter greatly affected the fracture strength because the higher compressive residual stress layer generated during grinding was released by laser processing. It is concluded that the newly developed crack-free processing technique has great industrial possibilities. 15 Refs. 17. Weisenhorn, Albrecht L.; Khorsandi, Mitra; Kasas, Sandor; Gotzos, Vassilis; Butt, Hans-Juergen. Deformation and height anomaly of soft surfaces studied with an AFM. Nanotechnology. 1993 Apr; 4(2): 106-113. The atomic force microscope (AFM) can easily image 'hard' sample surfaces with atomic or molecular resolution. For 'soft' samples, such as polymers or biological objects, this resolution power is very difficult to reach, because the AFM tip can cause large deformation. This deformation is revealed in the touching regime of the force-versus-distance curve, or more directly in the force-versus-indentation curves. To obtain the force-versus-indentation curve of a soft sample, its force-versus-distance curve is corrected with the force-versus-distance curve measured with the same cantilever on a hard sample. We have measured the force-versus-indentation curves of different elastomers (polyurethane), rubber, cartilage, and living cells and deduced a parabolic tip shape from these curves. We have also calculated the radius of curvature of the AFM tip to be 50-100 nm. The calculated ranges of the local Young's moduli E are 0.6-2.4 MPa for rubber, 0.16-0.6 MPa for cartilage, and 0.0130.15 MPa for a living cell. This means that an applied force of 1-10 pN is required to obtain high-resolution images of a living cell with about 1 nm vertical deformation and only a few nm 2 area of contact. Therefore, understanding the deformation mechanism is not only important for determining local elasticity, but also to understand the 'height' information and the resolution limits of AFM images. Deformation without strong adhesive forces causes a soft sample on a hard substrate to appear thinner, a fact which has already been observed in many AFM images. Furthermore, if deformation of the soft sample occurs the applied force is overestimated. Therefore, high-resolution imaging of soft samples remains a challenge since the applied force needs to be in the pN range. 77 Refs. 18. Sparks, R. G.; Paesler, M. A. Depth profiling of residual stress along interrupted test cuts in machined germanium crystals. Proceedings of the International Conference on Machining of Advanced Materials - NIST Special Publication 847; 1993 Jun. 1993: 303-315. Residual stress profiles in machined germanium have been determined by measuring the longitudinal optical phonon spectra using micro-Raman spectroscopy. Lateral spatial resolution of 1 14m results from this direct non-destructive technique, By measuring the Raman spectra with several probe wavelengths, axial resolution of 10 nm is obtained through differential absorption profiling, or DAP. The stress field in these machined brittle semiconductors is characterized by a surface layer under compression, an abrupt compressive-to-tensile transition, and a deeper-lying region of tension which ultimately relaxes in the unstressed bulk. The abrupt transition is indicative of a change from plastic to elastic deformation. The DAP technique has been applied to interrupted test cuts in diamond-turned germanium to reveal that fractured regions of the machined surface possess higher tensile stress that occurs at shallower depths than unfractured regions. A qualitative mechanism of the machining process consistent with these residual stress data is presented. 19 Refs 19. Shimura, H.; Hashimoto, F.; Yoshioka, J. Design of ultraprecision grinding machines for ductile mode grinding of brittle materials. Proceedings of the International Conference on Machining of Advanced Materials - NIST Special Publication 847. 1993 Jun: 317-323. The field of ductile mode grinding of brittle materials using grinding wheels with a grain size larger than the ductile-brittle transition value dc is defined as nano-grinding technology. In the present paper, new design concepts are introduced for a vertical spindle surface grinding machine, aiming at developing an ultraprecision and high stiffness machine tool for finishing of silicon wafer with 8 inch diameter. This concept is also applied to develop an ultraprecision centerless grinding machine. Both ultraprecision grinding machines have been built and evaluated through grinding tests. Effects of application of these concepts on motion accuracy and stiffness are shown by experimental results. 6 Refs. 20. Ellingson, W. A.; Ayaz, D. M.; Brada, M. P.; O'Connell, W. Detection of subsurface defects in machined silicon nitride ceramics by optical scattering methods. Proceedings of the International Conference on Machining of Advanced Materials - NIST Special Publication 847. 1993 Jun: 147-157. Silicon nitride (Si3N4) materials from two manufacturers, in the form of flexure test bars with surface finishes that were ground and polished (0.03 p.m Ra) and unidirectionally ground (0.07 i~m Ra) and contained simulated subsurface defects were studied by optical Fourier scattering methods. Subsurface defects were obtained by either ultrasonically drilling 750 t~m-diameter holes to within 75-105 p.m of the surface or by grinding slots parallel to and 75-105 t~m below the surface. Because of the unique optical transmission properties of many ceramic materials, including those in this study, subsurface defects, such as those likely to be caused by machining-induced damage, may be detected by appropriate optical methods. In this study s-polarized He-Ne laser light (X = 0.6328 Ilm) was used as the light source. Specular reflections were detected by an optical Fourier method that 68
,JANUARY 1994 VOL 16 NO
517-526. This paper describes the effect of crack-free laser processing of hot-pressed silicon nitride ceramics on fracture strength and residual stress. The fracture strength of the processed test pieces was determined by three-point bending tests. The residual stress in the processed surface was characterized by X-ray diffraction measurement. The strength of the laserprocessed workpieces was reduced to 90-80% compared with that of the ground workpieces because of subsurface layer damage, which included the heat-affected layer and the residual stress layer. In particular, the latter greatly affected the fracture strength because the higher compressive residual stress layer generated during grinding was released by laser processing. It is concluded that the newly developed crack-free processing technique has great industrial possibilities. 15 Refs. 17. Weisenhorn, Albrecht L.; Khorsandi, Mitra; Kasas, Sandor; Gotzos, Vassilis; Butt, Hans-Juergen. Deformation and height anomaly of soft surfaces studied with an AFM. Nanotechnology. 1993 Apr; 4(2): 106-113. The atomic force microscope (AFM) can easily image 'hard' sample surfaces with atomic or molecular resolution. For 'soft' samples, such as polymers or biological objects, this resolution power is very difficult to reach, because the AFM tip can cause large deformation. This deformation is revealed in the touching regime of the force-versus-distance curve, or more directly in the force-versus-indentation curves. To obtain the force-versus-indentation curve of a soft sample, its force-versus-distance curve is corrected with the force-versus-distance curve measured with the same cantilever on a hard sample. We have measured the force-versus-indentation curves of different elastomers (polyurethane), rubber, cartilage, and living cells and deduced a parabolic tip shape from these curves. We have also calculated the radius of curvature of the AFM tip to be 50-100 nm. The calculated ranges of the local Young's moduli E are 0.6-2.4 MPa for rubber, 0.16-0.6 MPa for cartilage, and 0.0130.15 MPa for a living cell. This means that an applied force of 1-10 pN is required to obtain high-resolution images of a living cell with about 1 nm vertical deformation and only a few nm 2 area of contact. Therefore, understanding the deformation mechanism is not only important for determining local elasticity, but also to understand the 'height' information and the resolution limits of AFM images. Deformation without strong adhesive forces causes a soft sample on a hard substrate to appear thinner, a fact which has already been observed in many AFM images. Furthermore, if deformation of the soft sample occurs the applied force is overestimated. Therefore, high-resolution imaging of soft samples remains a challenge since the applied force needs to be in the pN range. 77 Refs. 18. Sparks, R. G.; Paesler, M. A. Depth profiling of residual stress along interrupted test cuts in machined germanium crystals. Proceedings of the International Conference on Machining of Advanced Materials - NIST Special Publication 847; 1993 Jun. 1993: 303-315. Residual stress profiles in machined germanium have been determined by measuring the longitudinal optical phonon spectra using micro-Raman spectroscopy. Lateral spatial resolution of 1 14m results from this direct non-destructive technique, By measuring the Raman spectra with several probe wavelengths, axial resolution of 10 nm is obtained through differential absorption profiling, or DAP. The stress field in these machined brittle semiconductors is characterized by a surface layer under compression, an abrupt compressive-to-tensile transition, and a deeper-lying region of tension which ultimately relaxes in the unstressed bulk. The abrupt transition is indicative of a change from plastic to elastic deformation. The DAP technique has been applied to interrupted test cuts in diamond-turned germanium to reveal that fractured regions of the machined surface possess higher tensile stress that occurs at shallower depths than unfractured regions. A qualitative mechanism of the machining process consistent with these residual stress data is presented. 19 Refs 19. Shimura, H.; Hashimoto, F.; Yoshioka, J. Design of ultraprecision grinding machines for ductile mode grinding of brittle materials. Proceedings of the International Conference on Machining of Advanced Materials - NIST Special Publication 847. 1993 Jun: 317-323. The field of ductile mode grinding of brittle materials using grinding wheels with a grain size larger than the ductile-brittle transition value dc is defined as nano-grinding technology. In the present paper, new design concepts are introduced for a vertical spindle surface grinding machine, aiming at developing an ultraprecision and high stiffness machine tool for finishing of silicon wafer with 8 inch diameter. This concept is also applied to develop an ultraprecision centerless grinding machine. Both ultraprecision grinding machines have been built and evaluated through grinding tests. Effects of application of these concepts on motion accuracy and stiffness are shown by experimental results. 6 Refs. 20. Ellingson, W. A.; Ayaz, D. M.; Brada, M. P.; O'Connell, W. Detection of subsurface defects in machined silicon nitride ceramics by optical scattering methods. Proceedings of the International Conference on Machining of Advanced Materials - NIST Special Publication 847. 1993 Jun: 147-157. Silicon nitride (Si3N4) materials from two manufacturers, in the form of flexure test bars with surface finishes that were ground and polished (0.03 p.m Ra) and unidirectionally ground (0.07 i~m Ra) and contained simulated subsurface defects were studied by optical Fourier scattering methods. Subsurface defects were obtained by either ultrasonically drilling 750 t~m-diameter holes to within 75-105 p.m of the surface or by grinding slots parallel to and 75-105 t~m below the surface. Because of the unique optical transmission properties of many ceramic materials, including those in this study, subsurface defects, such as those likely to be caused by machining-induced damage, may be detected by appropriate optical methods. In this study s-polarized He-Ne laser light (X = 0.6328 Ilm) was used as the light source. Specular reflections were detected by an optical Fourier method that 68
,JANUARY 1994 VOL 16 NO