Liu Qi, Cheng Jian, Liao Zhirong, Liu Mingyu, Chen Mingjun, Zhao Linjie, Lei Hongqin, Ding Wenyu
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, China.
Centre for Precision Manufacturing, Department of Design Manufacturing & Engineering Management (DMEM), University of Strathclyde, Glasgow G1 1XJ, UK.
Materials (Basel). 2023 Feb 21;16(5):1782. doi: 10.3390/ma16051782.
The micro-defects on KHPO (KDP) optic surfaces are mainly repaired by the micro-milling technique, while it is very easy to introduce brittle cracks on repaired surfaces, as KDP is soft and brittle. To estimate machined surface morphologies, the conventional method is surface roughness, but it fails to distinguish ductile-regime machining from brittle-regime machining directly. To achieve this objective, it is of great significance to explore new evaluation methods to further characterize machined surface morphologies. In this study, the fractal dimension () was introduced to characterize the surface morphologies of soft-brittle KDP crystals machined by micro bell-end milling. The 3D and 2D fractal dimensions of the machined surfaces and their typical cross-sectional contours have been calculated, respectively, based on Box-counting methods, and were further discussed comprehensively by combining the analysis of surface quality and textures. The 3D is identified to have a negative correlation with surface roughness ( and ), meaning the worse the surface quality the smaller the . The circumferential 2D could quantitively characterize the anisotropy of micro-milled surfaces, which could not be analyzed by surface roughness. Normally, there is obvious symmetry of 2D and anisotropy on the micro ball-end milled surfaces generated by ductile-regime machining. However, once the 2D is distributed asymmetrically and the anisotropy becomes weaker, the assessed surface contours would be occupied by brittle cracks and fractures, and corresponding machining processes will be in a brittle regime. This fractal analysis would facilitate the accurate and efficient evaluation of the repaired KDP optics by micro-milling.
磷酸二氢钾（KDP）光学表面上的微缺陷主要通过微铣削技术修复，然而由于KDP质地软且脆，在修复表面很容易产生脆性裂纹。为评估加工表面形貌，传统方法是测量表面粗糙度，但它无法直接区分延性加工状态和脆性加工状态。为实现这一目标，探索新的评估方法以进一步表征加工表面形貌具有重要意义。在本研究中，引入分形维数（）来表征用微球头铣刀加工的软脆KDP晶体的表面形貌。基于盒计数法分别计算了加工表面及其典型横截面轮廓的三维和二维分形维数，并结合表面质量和纹理分析进行了全面讨论。确定三维分形维数与表面粗糙度（和）呈负相关，即表面质量越差，分形维数越小。圆周二维分形维数可以定量表征微铣削表面的各向异性，这是表面粗糙度无法分析的。通常，由延性加工状态产生的微球头铣削表面上二维分形维数具有明显的对称性和各向异性。然而，一旦二维分形维数分布不对称且各向异性变弱，评估的表面轮廓将被脆性裂纹和断裂占据，相应的加工过程将处于脆性状态。这种分形分析将有助于通过微铣削对修复后的KDP光学元件进行准确有效的评估。
