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研究采用由国晶辉公司由CVD法生产的ZnSe多晶作为工件,工件直径和厚度分别为42 mm和5 mm。ZnSe晶体材料性能如表1所示。在超精密车床(IL300,Inolite)上进行了槽切和端面车削实验。在生产加工中选用大圆弧半径刀具会与非球面、衍射面等功能表面发生严重干涉,无法加工指定面型且刀具磨损严重[20],因此文中选用小圆弧半径刀具进行。为确定ZnSe晶体脆塑转变深度,分别采用两把不同前角的天然单晶金刚石刀具以分析刀具前角对脆塑转变深度的影响,ZnSe晶体材料特性如表2所示[21]。首先将工件装夹在弹性夹具上,随后弹性夹具通过真空吸附固定于空气主轴。通过万分表反馈调节工件径向圆跳动,确保切削工程平稳,车削实验装置及工件局部放大分别如图1(a)和1(b)所示。
Material ZnSe Grain size (typical) /μm 70 Yang’s modulus /GPa 79 Hardness /GPa 1.2 Fracture toughness/MPa·m0.5 0.9 Poisson ratio 0.28 Density /g·cm−3 5.3 表 2 刀具参数及实验条件
Table 2. Cutting tool parameters and experimental condition
No. Parameters Values 1 Nose radius/mm 1.15 (tool Ⅰ), 1.12 (tool Ⅱ) 2 Rank angle/(°) −25 (tool Ⅰ), 0 (tool Ⅱ) 3 Clearance angle/(°) 10 (tool Ⅰ, Ⅱ) 4 Clearance angle/(°) 10 (tool Ⅰ, Ⅱ) 5 Cutting speed/m·s−1 8.3×10−3 (PCT), 2.88 (FTT) 6 Depth of cut/μm 0-2 (PCT), 3 (FTT) 7 Feed rate/μm·rev−1 0.5, 1, 1.5 and 2 (FTT) 图 1 超精密车削ZnSe晶体实验设备及切削实验示意图。 (a)超精密车床;(b)工件放大图;(c)槽切实验示意图;(d)端面车削实验示意图
Figure 1. Experimental step-ups and schematic of cutting experiments. (a) the ultra-precion lathe; (b) enlarged view of workpiece; (c) the machined groove of plunge-cutting test; (d) schematic of machined surface for face-turning test
槽切实验(Plunge-cutting test, PCT)如图1(c)所示,刀具以恒定切削速度在长度为2 mm的切削路径上切削深度由0 nm线性增加到2 μm。已有研究表明[9],切削速度对脆塑转变深度的影响最小。考虑到机床稳定性,将切削速度设置为500 mm/min (8.3×10−3 m/s),分别进行5次试验以排除偶然因素干扰。进给率对工件表面和亚表面损伤有极大影响[21],相较而言切削深度的影响可以忽略不计[18]。因此采用单因素实验法进行端面车削实验(Face-turning test, FTT)以确定进给率对工件表面质量及材料去除机理的影响。在工件边沿至工件回转中心依次设置四个切削区域其进给率分别为0.5、1.0、1.5、2.0 μm/rev,其中每个切削区域宽度为3 mm,切削区域间距均为1 mm,切削区域分布如图1(d)所示。为排除线速度对实验结果的影响,通过调节主轴转速以保持所有切削区域线速度恒定。考虑机床超精密切削稳定性须控制主轴转速低于5000 RPM即最内侧切削区域主轴转速,因此最终确定切削线速度为2.8 m/s。超精密切削实验均在24 ℃、54%湿度和标准大气压下进行,刀具参数及实验条件如表2所示。
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采用场发射扫描电子显微镜(FESEM,FEI Nova NanoSEM 450)观察已加工表面和切屑形貌。工件三维形貌信息采用白光干涉仪(WykoNT 9100,Vecco)测量,通过分析工件表面轮廓信息以量化评估工件表面质量。为揭示ZnSe晶体材料去除机制,主要针对不同进给率下ZnSe晶体已加工表面和切屑进行物相结构分析。因此选用激光共聚焦拉曼光谱仪(HR 800, HORIBA)进行无损检测,其激光输出功率为50 mW,波长为532 nm,曝光时间为5 s,选用100倍物镜观测,激光光斑聚焦尺寸约为1 μm。
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脆性材料发生脆塑转变是实现塑性域切削的前提,因此确定脆塑转变临界深度也成为超精密切削的研究主题[22]。目前,鲜有关于ZnSe晶体塑性域切削的报道。在变深度槽切实验中,选用两种前角刀具分别重复进行5次划刻实验。采用FESEM观测由两种刀具划刻的工件表面,切削深度由零线性增加到数十纳米并最终达微米尺度,由此可观察到不同切深下材料去除方式对工件表面质量的影响。采用刀具I和刀具II划刻后的工件表面脆塑转变区域放大图分别如图2(a)、2(c)和图2(b)、2(d)所示,结果表明沟槽内部存在两个明显不同的区域,分别对应ZnSe晶体的塑性和脆性去除模式。以图2(b)为例,在塑性域,ZnSe晶体主要通过塑性流动而非脆性断裂实现去除,因此工件表面光滑。在脆性域,沟槽表面并非仅存在由表面断裂引起的凹坑,同时还观测到工件表面发生部分塑性切削观察到部分光滑区域。该现象与单晶锗、单晶硅槽切过程在脆性域仅发生脆性断裂不同,主要与ZnSe晶体固有的多晶结构相关[3],即由于晶粒取向各异在特定晶粒上发射塑性变形而其他晶粒则发生脆性变形。根据刀具与工件几何关系,脆塑转变深度dc可由公式(1)计算:
图 2 ZnSe晶体典型加工沟槽FESEM图像。(a) 0°前角刀具加工;(b) −25°前角刀具加工;(c) 放大区域I;(d) 放大区域II
Figure 2. Typical FESEM image of ZnSe crystal machined groove by using. (a) tool I with 0° rank angle; (b) tool II with −25° rank angle; (c) Enlarged region I; (d) Enlarged region II
$${d_c} = \left(R - \frac{1}{2}\sqrt {4{R^2} - {w^2}} \right) \cdot \cos (\alpha )$$ (1) 式中:R和α分别为刀具半径和前角;w为裂纹首次形核并扩展到表面的沟槽宽度,由IMAGJ软件分析沟槽FESEM图像获取。如图3所示为不同刀具槽切实验测得ZnSe晶体脆塑转变临界深度,明显可见采用前角为负25°的刀具II可更好地抑制脆性断裂。采用刀具I和刀具II获得的脆塑转变深度平均值分别为74.9、104.4 nm,后者较前者提升30%。较大的脆塑转变临界深度为实现延性加工提供了较宽松的约束条件,在超精密切削加工中有利于获得较高的材料去除率和加工效率。因此,后续仅选用刀具II进行端面车削实验。
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超精密切削过程中材料去除机理对工件表面质量起着决定性影响。由于切削尺度在纳米和微米尺度存在尺寸效应,超精密切削中材料去除机理与金属切削过程由剪切滑移主导不同[23]。刀具尖端产生应力集中,工件材料在一定条件下可通过塑性流动而非脆性断裂实现去除[23]。如图4所示为超精密切削过程中刀具和工件的接触示意图,通过切削运动去除两次进给间的未变形切削区域(图4中蓝色区域),最大未变形切削厚度hmax可根据刀具工件几何关系及切削参数求出,如公式(2)所示:
$${h_{\max }} = R - \sqrt {{R^2} + {f^2} - 2f\sqrt {2R{a_p} - a_p^2} } $$ (2) 式中:R、f和ap分别为刀具半径、进给率和切削深度。材料去除机制与最大未变形切削厚度相关,即最大变形切削厚度小于脆塑转变深度,材料通过塑性域去除。当最大未变形切削厚度大于脆塑转变深度dc,由脆性断裂主导了材料的变形过程,在已加工表面留下微坑和裂纹。由此通过分析比较hmax和dc,即可确定材料去除模式,如公式(3)所示:
$$\left\{ \begin{array}{l} {h_{\max }} < {d_c},\;{\rm{ Ductile}}\;{\rm{ regime}} \\ {h_{\max }} \geqslant {d_c},\;{\rm{ Brittle}}\;{\rm{ regime}} \end{array} \right.$$ (3) Bifino等[24]的研究表明,dc取决于工件材料物理特性和加工条件,dc可通过公式(4)求出:
$${d_c} = \beta \left( {\frac{E}{H}} \right){\left( {\frac{{{K_{ic}}}}{H}} \right)^2}$$ (4) 式中:β为常数,与切削条件相关;H、E和Kic分别是硬度、杨氏模量和断裂韧性,其取值如表1所示。Bifino指出对硬脆材料,β等于0.15[24],但并不适用于软脆性材料ZnSe晶体,因此考虑到ZnSe晶体材料特性修正几何常数为6.21×10−3。联立公式(2)~(4)即可确定不同加工条件下ZnSe晶体材料去除方式。如图5所示绘制了hmax的二维等高线图,hmax随进给速度和切削深度的增加而增大,当hmax小于dc即可求出在本研究条件下实现塑性域加工的切削参数设置,其中当进给率分别为2、1.5、1、0.5 μm/rev时,hmax分别对应为144.51、108.71、72.69、36.46 nm,由此可预测前两组切削区域位于脆性域,而后两组则处于塑性域。
Study on the material removal mechanism of ZnSe crystal via ultra-precision diamond turning
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摘要: 硒化锌晶体在红外成像与激光系统中有着广泛的应用,作为典型软脆性材料,其材料去除机理目前尚不清晰,获得超光滑表面仍极具挑战。文中采用槽切法研究刀具负前角对硒化锌晶体脆塑转变临界深度的影响。通过分析最大未变形切削厚度随切削参数变化规律,提出实现硒化锌晶体塑性域切削的理论模型。借助场发射扫描电子显微镜、白光干涉仪和拉曼光谱仪,系统分析了进给率对工件表面粗糙度、表面完整性及亚表面损伤的影响,提出表面缺陷形成机理,进而揭示硒化锌晶体材料去除机理。Abstract: ZnSe crystal has been widely used in infrared imaging and laser systems. However, as a typical soft-brittle material, the material removal mechanism in ultra-precision diamond turning process has not been clarified, it is still challenging to obtain nano-smoothed surface. In the study, the effect of tool ranke angle on ductile-brittle transition depth of ZnSe crystal have been investigated through novel plunge-cutting tests. The ductile regime machining model was revealed by comparing the maximum undeformed chip thickness and ductile-brittle transition depth. With the aid of FESEM, white light interferometer, and Raman spectrometer, the effect of feed rate on surface roughness, surface quality, phase transition and subsurface damage were systematically investigated. The surface defects formation mechanism was proposed. Furthermore, the material removal mechanism of ZnSe crystal in ultra-precision diamond turning process have been revealed.
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图 1 超精密车削ZnSe晶体实验设备及切削实验示意图。 (a)超精密车床;(b)工件放大图;(c)槽切实验示意图;(d)端面车削实验示意图
Figure 1. Experimental step-ups and schematic of cutting experiments. (a) the ultra-precion lathe; (b) enlarged view of workpiece; (c) the machined groove of plunge-cutting test; (d) schematic of machined surface for face-turning test
Material ZnSe Grain size (typical) /μm 70 Yang’s modulus /GPa 79 Hardness /GPa 1.2 Fracture toughness/MPa·m0.5 0.9 Poisson ratio 0.28 Density /g·cm−3 5.3 表 2 刀具参数及实验条件
Table 2. Cutting tool parameters and experimental condition
No. Parameters Values 1 Nose radius/mm 1.15 (tool Ⅰ), 1.12 (tool Ⅱ) 2 Rank angle/(°) −25 (tool Ⅰ), 0 (tool Ⅱ) 3 Clearance angle/(°) 10 (tool Ⅰ, Ⅱ) 4 Clearance angle/(°) 10 (tool Ⅰ, Ⅱ) 5 Cutting speed/m·s−1 8.3×10−3 (PCT), 2.88 (FTT) 6 Depth of cut/μm 0-2 (PCT), 3 (FTT) 7 Feed rate/μm·rev−1 0.5, 1, 1.5 and 2 (FTT) -
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