摘要
针对高导热性聚晶金刚石开展了水导激光加工机理及技术研究。提出了水导激光烧蚀阈值的计算方法,揭示了烧蚀机理,阐明了加工参数对表面形貌、加工质量和加工效率的影响。研究结果表明:水导激光作用下的临界烧蚀条件与初始激光功率成正相关,且烧蚀阈值超过了100 J/cm2;烧蚀机理主要为金刚石在激光能量诱导下形成具有微裂纹的石墨/缺陷层,该层在水射流的冲刷下解理去除,使相变层下方的金刚石相裸露。在一定范围内提高激光功率和扫描速度有利于降低加工表面粗糙度和提高材料去除率,最佳加工参数为激光功率P=14 W和扫描速度v=8 mm/s,此时表面粗糙度和材料去除率分别为538.5 nm±21.1 nm和18.4×10^(-3)mm^(3)/s。完成了聚晶金刚石表面导热微流道阵列的制备,其加工尺寸的一致性较高,单片加工耗时仅约为90 s。
Objective Diamond is an ideal thermal-conductive material for application of high-frequency and high-power components,which possesses high heat conductivity,low coefficient of thermal expansion,and superb heat stability.However,machining of diamond materials has remained a long-term challenge due to its extreme mechanical property.Water-jet guided laser(WJGL),as a novel laser machining technology,has advantages in precise and efficient material removal of diamond.First,laser fluence is confined in the water jet,forming a uniform distribution rather than a Gaussian distribution in dry laser.Focus plane is absent,and ablation taper can be improved.Second,water flow directly acts on the machining surface,which is beneficial for decreasing heat-affected zone and washing out ablation debris.Therefore,WJGL is suitable for machining diamond and fabricating related components.Nevertheless,there is still a research gap in this field,especially in ablation mechanism and fabrication techniques.This work aims to fill in this gap by exploring critical ablation condition,material removal mechanism,and precision/efficiency-related outcomes in WJGL machining of polycrystalline diamond.Fabrication of thermal-conductive micro-channel component was also tested.Methods Experiments were carried out on 5-axis WJGL machining equipment with an Nd∶YAG laser,which has a wavelength of 532 nm and a pulse width of 210 ns.Machining process was operated with the following parameters:repetition rate of 6 kHz,water-jet pressure of 300 bar,nozzle diameter of 50μm,gas(He)flow rate of 1.5 L/min(under standard conditions),laser power of 12‒20 W,and scanning speed of 2‒10 mm/s.Polycrystalline diamond bulks(synthesized via chemical vapor deposition)were the machining samples with a size of 2.5 mm×2.5 mm×0.5 mm.The samples were pre-polished to a surface roughness Sa<10 nm.Both cutting and grooving testing were performed.Results and Discussions The critical ablation threshold of the polycrystalline diamond under WJGL machining cannot be calculated using the traditional method of dry laser,because the laser power decreases with its transmission under water.A modified calculating equation for WJGL machining was proposed,which was related to absorption coefficient of water and underwater depth.As shown in Fig.2,the ablation threshold of WJGL linearly increases with the increase of the initial laser power.Moreover,the ablation thresholds exceed 100 J/cm2,which are 3‒5 times higher than those of dry laser machining.Then,the ablation surface was observed with microscope,as shown in Fig.3.The machining morphology presents periodic cut marks,formed by the radial impact of water jet during each scanning process.Moreover,both bright and dark regions can be recognized on the ablation surface.The bright region seems like uncovered diamond grain,featuring a one-piece construction with grain boundary.The dark region possesses micro-holes with crack propagation.By further analyzing with Raman spectrum,the bright and dark regions are confirmed to be diamond phase and graphitic/defective phase,respectively.Then,the ablation mechanism was proposed,as shown in Fig.4.Under the influence of WJGL,diamond undergoes softening and even sublimation,resulting in graphite/defect layers with microcracks.Subsequently,under the scouring and impact of water jet,cleavage occurs.Part of the underlying diamond grains can be found on the processed surface,which is beneficial for improving the thermal conductivity of the processed surface.The influences of laser power on machining morphology and surface roughness were investigated,as shown in Fig.5.Bright region expands with the increase of laser power,indicating high power is beneficial to diamond exposing.However,increasing power is disadvantageous to reduce cut marks.Surface roughness Sa significantly decreases when laser power reaches,and then slightly increases.Sz fluctuates at 14‒20 W with minimal value at 14 W and 18 W.Grooving tests were performed to investigate machining efficiency,as shown in Fig.6.Machining width and depth are both positively correlated with laser power,while width is more sensitive.Material removal rate(MRR)gradually increases with increasing laser power.Based on surface roughness and MRR,laser power of 14 W is regarded as the suitable parameter.The influences of scanning speed on machining morphology and surface roughness were investigated,as shown in Fig.7.The machining morphology is similar to the experimental results of laser power.Increasing scanning speed is beneficial to decrease surface roughness when it is less than 8 mm/s.Both Sa and Sz follow this trend.Grooving tests with different scanning speeds were shown in Fig.8.MRR is positively correlated with scanning speed,and corresponding influence is higher than that of laser power.The best parameter combination is laser power of 14 W and scanning speed of 8 mm/s,where Sa and MRR reach optimal values of 538.5 nm±21.1 nm and 18.4×10-3 mm3/s,respectively.Using the optimal parameters,we machined a thermal-conductive micro-channel array on the polycrystalline diamond surface,as shown in Fig.9.The structural width and depth are measured to be 85μm±3μm and 300μm±20μm,respectively.The machining duration for a single piece is only about 90 s.Conclusions The ablation threshold for WJGL machining of polycrystalline diamond is positively correlated with the initial laser power,and significantly higher than that of dry laser.The material removal mechanism for WJGL machining of polycrystalline diamond involves the diamond phase softening and transforming into graphitic/defective phase,which is then washed away by the water jet.Increasing both laser power and scanning speed within a certain extent can significantly reduce the machining surface roughness.The material removal rate is positively correlated with both factors.The optimal machining parameters are laser power of 14 W and scanning speed of 8 mm/s,resulting in Sa and MRR of 538.5 nm±21.1 nm and 18.4×10^(-3) mm^(3)/s,respectively.With the optimal parameters,a precise thermal-conductive micro-channel array can be fabricated on the polycrystalline diamond surface,and the machining efficiency is very high.
作者
陈世龙
张宇
刘德辉
靳田野
陈俊云
王真
黄传真
Chen Shilong;Zhang Yu;Liu Dehui;Jin Tianye;Chen Junyun;Wang Zhen;Huang Chuanzhen(School of Mechanical Engineering,Yanshan University,Qinhuangdao 066004,Hebei,China;Center for High Pressure Science(CHiPS),State Key Laboratory of Metastable Materials Science and Technology,Yanshan University,Qinhuangdao 066004,Hebei,China)
出处
《中国激光》
北大核心
2025年第14期223-232,共10页
Chinese Journal of Lasers
基金
国家自然科学基金(52205499,52275466,U22A20195)。
关键词
水导激光
聚晶金刚石
表面粗糙度
材料去除率
微流道
water-jet guided laser
polycrystalline diamond
surface roughness
material removal rate
micro-channel
作者简介
通信作者:靳田野,jintianye@ysu.edu.cn;通信作者:陈俊云,sophiacjy@ysu.edu.cn。
