极硬纳米孪晶金刚石刀具的飞秒激光蚀除-热化学抛光机理及工艺基础

Nanotwinned diamond (ntD) is a new kind of cutting tool material with much higher hardness and better chemical stability under high temperature than natural single crystal diamond, thus is promising to realize ultra-precision cutting of hard-brittle material. However, it is extremely difficult to machine the ntD bulk into a cutting tool, since extra-high hardness of ntD material makes the common used method of mechanical lapping or polishing infeasible for manufacture of cutting tool. .In order to develop the new ntD tool with excellent physical properties, this project proposes a composite technology of femtosecond pulsed laser ablation and continuous laser-assisted thermo-chemical polishing to finish the contour machining and precision machining of the tool, respectively. The material removal mechanism of ntD during femtosecond pulsed laser processing will be clarified through numerical simulation and experiments. Then the contour machining of ntD tool can be finished with high material removal rate. Furthermore, in the process of laser-assisted thermo-chemical polishing, the phase transition mechanism and removal way of ntD material will be studied through molecular dynamic simulation as well as analyzing the results of polishing disk wear, material removal rate, polishing quality and the critical condition of phase transition. For the sake of removing the polishing damage, this project will study ntD material structure within the damage layer after polishing as well as the oxidation properties of every strong oxidizer. Next both the action of twin crystal structure on the formation of polishing damage and subsequent removal way of damage material will be revealed. Then, high-precision ntD cutting tool without polishing damage can be developed after completing the fundamental process of femtosecond pulsed laser ablation and continuous laser assisted thermo-chemical polishing. Therefore, the research finding in this project is of great significance to carry forward the application of ntD tool material and raise the accuracy grade in cutting of hard-brittle material.

纳米孪晶金刚石(ntD)的硬度和高温化学稳定性均远高于天然单晶金刚石，有望成为胜任硬脆材料超精密切削的新型刀具材料。然而极高的硬度为ntD刀具制造带来了巨大挑战，机械方法不可行。为了突破刀具制造难题，本项目依据材料特性提出飞秒激光轮廓加工及激光辅助热化学抛光的解决思路。通过三维数值仿真及工艺实验阐明飞秒激光对ntD的蚀除机理，进而解决ntD刀具的高效率轮廓加工问题；在研究抛光盘磨损、材料去除率、抛光质量以及热化学反应临界条件的基础上，结合分子动力学仿真揭示ntD的热化学相变机制和材料去除机理；通过分析抛光损伤层组织结构和氧化剂的氧化性能，探明纳米孪晶结构对抛光损伤形成的调控机制和氧化剂作用下损伤层材料的移除机理，在此基础上建立超硬刀具的飞秒激光加工和激光辅助热化学抛光工艺，实现ntD刀具的高精度、无损伤制造。项目成果对于推进ntD刀具材料应用和提高硬脆材料切削精度具有重要的理论和应用价值。

单晶金刚石（SCD）刀具因韧性差、易崩刃等特点很难满足硬脆材料超精密切削的要求，因此新型极硬、高韧的金刚石类超精密切削刀具急需探索。近几年，中国科学家研制成功了极硬的纳米孪晶金刚石（nt-D）材料，它的维氏硬度达到了200GPa，接近于SCD硬度的两倍，并且具有各向同性、无解理特征、高温稳定性好、韧性高（断裂韧性值为9.7~14.8 MPa m0.5，接近于SCD的2~3倍）等优势，然而对nt-D块材进行材料加工进而制造成刀具的难度很大。为此，本项目针对极硬nt-D刀具的制造难题，完成了飞秒激光三维轮廓加工、催化金属化学驱动机械抛光和化学机械抛光的研究。首先建立了飞秒激光加工nt-D材料的三维有限元仿真模型，阐明了飞秒激光作用下nt-D材料亚表层的结构变化机制和损伤形式。建立了次摆线扫描轨迹的脉冲重叠数理论模型，形成了nt-D刀具高效率、高精度的三维轮廓加工工艺。催化金属对nt-D表层碳原子的作用机制为金刚石类型的sp3杂化相变为无序的sp2杂化，完成了催化金属抛光盘的制备。在催化金属化学驱动机械抛光中，提出了nt-D材料在剪切和机械载荷/化学催化协同作用非晶化的多因素耦合去除机理。在研究化学机械抛光氧化剂溶液和抛光工艺的基础上，提出了芬顿试剂作为氧化剂对nt-D表层材料去除的化学反应移除机理。最终建立了超硬nt-D刀具飞秒激光三维轮廓粗加工、催化金属化学驱动机械抛光和化学机械抛光的工艺基础，制造出了刀面粗糙度Ra < 2 nm、切削刃钝圆半径约64 nm的nt-D刀具，且刀面的抛光损伤层几乎完全被去除，实现了nt-D刀具的高精度、无损伤制造。硬脆材料的切削实验表明，nt-D刀具的切削质量、耐磨性和抗冲击性能均优于SCD刀具，提高了硬脆难加工材料的切削精度。此外本项目关于新材料刀具制造工艺和机理的研究也可以为其它纳米结构金刚石类或立方氮化硼类新型人工合成超硬材料刀具的制造提供技术思路和理论依据，从而促进新材料刀具制造技术的进步。