Investigation of ductile-to-brittle transitions on ultra-precision machining of single crystal silicon

近年来，硅、玻璃等脆性材料的纳米级加工研究引起了人们的关注。具有塑性变形的材料去除，称为延性模式加工，已通过非常小的切割深度实现。当表面裂纹随着切削深度的增加而出现时，通常被认为是脆性加工模式。本实验采用的工件材料为（100）面单晶硅。刀具为单晶维氏金刚石金字塔压头，顶角为136°。超精密车床由用于工件旋转的静压轴承主轴和用于刀具进给的滚子轴承滑块组成。在150 ~ 1500 m/min的恒定切削速度和0.03 mm/转的恒定进给速度下，由中心向外进行工件的面车削加工，形成螺旋形格栅。通过在工件和卡盘表面之间部分插入的垫片使安装的工件产生倾斜度，以产生不同深度的凹槽。通过对表面裂纹和次表面裂纹的观测，研究了裂纹的韧脆过渡点。扫描力显微镜用于测量沟槽深度和观察表面裂纹。在斜切后刻蚀的表面上，测量表面到亚表面裂纹最深点的距离。在某些情况下，切削槽下也会产生亚表面裂纹，即使深度小于基于表面裂纹的韧脆过渡点。结果表明，由地下裂纹确定的韧脆过渡点较表面裂纹确定的韧脆过渡点浅。因此，对表面下裂纹的评价尤为重要。

The studies on nano-scale machining of brittle materials such as silicon and glass, have come into notice in recent years. A material removal with plastic deformation, known as ductile mode machining, has been achieved by a very small depth of cut. When surface cracks occurred with being increased the depth of cut, the machining has been commonly considered to be brittle mode. The workpiece materials used in the present experiment was single crystal silicon with (100) surface. A single crystal Vickers diamond pyramid indentor with an apex angle of 136° was used as the cutting tool. The ultra precision turning machine consists of an aerostatic bearing spindle for workpiece revolution and a roller bearing slide for tool feed. Face-turning of the workpiece was done from the center to the outside with a constant cutting speed between 150 m/min and 1500 m/min and a constant feed rate of 0.03 mm/revolution, and a spiral groves were formed. An inclination was given to the mounted workpiece by a partly inserted spacer between the workpiece and the chuck surface in order to generate various depth of grooves. The ductile-brittle transition points were examined by the observations not only on the surface cracks but also on the subsurface cracks. The scanning force microscope is used for measuring the depth of the grooves and observing the surface cracks. The distance from the surfaces to the deepest point of the subsurface cracks on obliquely sectioned and then etched surfaces are measured by the. The subsurface cracks are generated under the cutting grooves in some case, even if the depth is less than the ductile-brittle transition point based on the surface cracks. It is detected that the ductile-brittle transition point determined by the subsurface cracks is shallower than that determined by the surface cracks. Therefore the evaluation of the subsurface cracks would be important especially in finishing.