森田 昇

This paper describes the basic conditions for the brittle-ductile transition in the ultraprecision machining of brittle materials in order to explain the deformation and fracture mechanism of a sub-surface layer induced by the microinteraction between a tool and a material. The temperature and stress distribution were simulated by a finite element method using a indenter sliding contact model in diamond cutting of monocrystal silicon. From this analysis, the characteristics of high temperature and stress field during the microcutting of brittle materials, and also the deformation and fracture behavior were clearly understood. The stable ductile mode machining of brittle materials is achieved by controlling the depth of cut, cutting speed and lubrication.

Defocusing of a laser beam is one of the most important factors to control the feature of drilling holes in ceramic materials. This suggests that laser beam energy is delivered and focused on the bottom of the drilling hole by multi-reflection from the internal wall surface of the hole. The deep hole drilling process of silicon nitride ceramics was simulated by a newly developed analysis model. The ray tracing method for a laser power intensity distribution analysis and difference method for thermal analysis were combined. The drilling hole is processed mainly by energy reflected from the internal wall surface of the hole. In this process laser beam intensity becomes higher in the center of the hole since the laser beam is focused by the wall-reflecting effect. The defocusing position from the material surface has direct effects upon the drilling process, which depends on the reflection angle of a laser beam at the internal wall surface of the hole. The reflection angle controls the wall-focusing effect and determines the hole shape.

This paper describes the influence of tool angles, especially rake angles, on the surface integrity of mirror cutting material. In this study, strain distribution in damaged layer is measured by a newly developed method. This method is applied electron beam lithography to drawing Au micro grids on the surface of oxygen free copper. Orthogonal cutting of specimen using various rake angle tools are carried out using a micro cutting device. It was found that strain distribution had close connection with tool geometry, and that highly compressed strain to vertical direction from machined surface was shown on the condition of negative rake angle tool. This method is effective to visualize cutting mechanism and material deformation at microscopic structure. ? 1994, The Japan Society for Precision Engineering. All rights reserved.

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 laser-processed 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.

The laser-induced thermal shock test and thermal stress analysis by a finite element method were carried out for explaining the generation and propagation behavior of thermal cracks in ceramics such as silicon nitride, alumina and zirconia caused by high-power-laser irradiation. Different features of crack propagation were observed depending on the ceramic species, probably depending on their thermal properties. Maximum tensile stress in every axis direction was observed at certain locations, where the stress intensity factor was also largest. It was found that cracks might generate right under the center of a heat source circle and at the outside of the circle, and might propagate from right under surface towards the radial direction. The initial point of cracks seems to be grain boundary phases which soften at high temperatures.

This paper describes a mechanism of deep hole drilling of covalent bond-ceramics using a pulsed YAG laser. The deep hole drilling process was simulated by the newly developed analysis method. It was combined the ray tracing method for a laser power intensity distribution analysis and difference method for a thermal analysis. It assumed that laser beam energy was delivered by reflection from the internal wall surface of a hole in the ray tracing model. Sublimation process of silicon nitride was considered during the thermal analysis. The deep drilling hole was processed mainly by energy reflected from the internal wall surface of the hole. In this process laser power intensity became higher in the center of the hole since the laser beam was focused by the wall-reflecting effect.

During the machining of brittle materials, the brittle-ductile transition (B-D-T) is related to the machining conditions. In this paper, micro-cutting is carried out to investigate the effect of B-D-T on the surface integrity of silicon single crystals. Surface texture, residual stress and crystal properties of the machined surface are evaluated with a surface roughness tester, scanning electron microscopy, atomic force microscopy and micro-Raman spectroscope. Results are as follows : (1) For the ductile mode cutting surface, the surface roughness of 0.005 ?m Ra, compressive residual stress of 23.0 MPa, surface topography and crystal properties are nearly the same as that of the polishing surface. (2) For the brittle mode cutting surface, the value of the surface roughness Ra is much larger than that of the polishing surface, as is the compressive residual stress at the value of 172.5 MPa. (3) The value of the critical material removal rate, a Vc, forductile mode cutting, as well as the critical depth of cut, becomes smaller with the increase of feedrate. ? 1993, The Japan Society of Mechanical Engineers. All rights reserved.

This study analyzes the effect of temperature and load rate on brittle-ductile transition (B-D- T). The purpose is to control grinding conditions on the basis of an understanding of the micro mechanism of B-D-T. In the study, both indentation and scratching experiments were carried out under various temperatures. The specimen is heated by CO2laser beam up to 750°C. The relationships among crack length, temperature and load rate are discussed. The load rate is larger under conditions of low temperature, high speed and heavy load. Larger load rate will cause occurrence of brittle fracture. As a result, it is clarified that B-D-T is affected directly by a specimen's temperature and load rate in addition to the critical depth of cut. ? 1992, The Japan Society of Mechanical Engineers. All rights reserved.

This paper describes the dynamic behavior of laser-produced ionizing vapor in laser-processing the covalent bond-ceramics. The distribution of laser-ionized vapor is determined by the measurements of electrical potential using newly developed small pin-type electric probes. The desity of ionized vapor is highest around a laser beam axis above the workpiece. The distribution and motion of ionized vapor is successfully controlled under the external electric field. ? 1992, The Japan Society for Precision Engineering. All rights reserved.

To achieve a damage-free grinding surface of brittle materials, the characteristics of brittle-ductile transition must be realized and the surface texture of the grinding wheel must be adequately controlled. In the study, a grinding wheel is truncated by grinding cast iron after dressing. The peak-valley value of the cross-sectional profile of the truncated wheel is much better than the dressed one. Moreover, after analyzing the surface texture results from various experiments, it is found that the statistic mean value of the critical depth of cut dcis approximately 40 nm by the truncated wheel with up cut grinding. The percentage of fracture surface is decreased with the decrease in the depth of cut in each case. The data points on the skewnesskurtosis plot of ductile mode grinding surface are relatively concentrated compared to the fracture mode surface. ? 1992, The Japan Society of Mechanical Engineers. All rights reserved.

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 laser-processed 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.