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目前红外晶体材料的非球面加工方法主要通过超精密切削成形,然后采用CMP工艺提升面形精度和表面质量[11]。但由于硒化锌晶体具有高脆性、低断裂韧性和各向异性的特点[12],在对其进行超精密切削时,晶体表面极易沿着不同的晶向断裂,产生裂纹和缺陷,并且由于其各向异性的特点,导致其加工时断裂的方向也不固定,很难保证加工后表面质量。硒化锌晶体除了高脆性之外,其质地也较软, CMP抛光虽然也能加工出高质量光学表面,但加工精度难以保证。ZnSe晶体材料的特性如表1所示。
表 1 硒化锌的特性和性能
Table 1. Characteristic and properties of ZnSe
Material Type Crystal
structureHv/
GPaGrain
size/μmDensity/
g·mm−3ZnSe Polycrystalline Cubic 0.9±0.05 43±9 5.26 磁流变抛光(MRF)作为一种超精密柔性加工技术,其利用磁流变抛光液在梯度磁场下形成的柔性磨头对工件实现剪切去除,且单个抛光颗粒对工件表面作用压强较小[13],所以磁流变抛光加工的材料有几乎没有亚表面损伤[14]。MRF抛光同时还具有加工确定性高、表面粗糙度低、加工面形精度高等特点[15],可以实现对多种材料及不同面型光学元件的纳米精度的加工。采用磁流变抛光实现对硒化锌元件的超精密抛光需要研发特殊的磁流变抛光液体,若采用常规的抛光液进行抛光,虽然抛光效率极高,但是材料内部的晶粒结构会表现的异常明显,呈类似于橘皮状,抛光后表面的粗糙度较差[16],进而导致后续抛光难度加大,抛光周期增加。因此,研发适用于硒化锌材料的磁流变抛光液是实现磁流变抛光对其高精度高质量加工的关键。
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磁流变抛光液通常由微米级铁粉颗粒、抛光粉、基载液(水或油)、化学添加剂组成。对于红外材料的抛光通常采用水基磁流变抛光液体[15]。同样地,文中也选择去离子水为载液,由于为了防止铁粉颗粒生锈,磁流变抛光液体的pH通常需要调节至强碱性(pH=11左右),但当pH较高时磁流变抛光表面的粗糙度较差,然而当pH接近中性甚至酸性的情况下,磁流变抛光液体中铁粉颗粒将很快生锈,无法保证长时间抛光使用。
因此,在保证磁流变抛光液中铁粉不生锈的前提下,通过优化液体选择了将基载液的pH调节至9.3,磁流变抛光液的主要成分如表2所示。
表 2 磁流变抛光液组成成分
Table 2. Composition of MR polishing fluid
Ingredient Deionized water Carbonyl iron powder (CIPs) Glycerin Rust inhibitor Sodium carbonate Citric acid Auxiliaries polish powder Volume fraction Other 35% 3% 0.8% 2% 0.6% 0.2% 0.2% 所用铁粉的粒径为D50=3 μm羰基铁粉,铁粉类型为高纯羰基铁粉。图1为羟基铁粉的扫描电镜(SEM)图片。
为了获取较好的表面粗糙度,选择六种不同的纳米抛光粉,配制六种磁流变抛光液体进行去除函数实验,获取去除效率和表面粗糙度的最佳平衡。
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为了测试不同磁流变抛光液体ZnSe抛光时的去除效率和表面粗糙度,采用自主研发的磁流变抛光设备进行去除函数,加工参数如表3所示。
表 3 工艺参数
Table 3. Process parameter
Parameter Wheel
diameter/mmRotate
speed/r·min−1Penetration
depth/mmMagnetic
fieldValue 160 120 0.8 340 mT 试验件采用的是两块直径为50 mm的典型的多晶ZnSe材料,其参数如表1所示。两块试验件均为平面,并且经过预抛光后其初始面形误差RMS小于15 nm,初始表面粗糙度为3.5 nm,图2为ZnSe材料的初始粗糙度检测结果,可以看出预抛光的表面仍有存在明显的晶粒分布。
为测试不同磁流变抛光液体的抛光性能,采用开展去除函数实验的方法进行研究,实验过程按照表2所示的参数,并控制单点的驻留时间为10 s,测试完成采用zygo激光干涉仪及白光干涉仪测量液体的材料去除效率及抛光表面粗糙度,进而评估液体抛光性能。图3为去除函数实验过程图。
具体抛光液体参数及实验结果如表4所示。
表 4 不同抛光液成分的去除函数实验结果
Table 4. Experimental results of removal function for different polishing liquid compositions
MR polishing fluid series Abrasive type Removal rate/μm·min−1 Roughness/nm 1 single crystal diamond/100 nm 5.2 9 2 Alumina/100 nm 1.8 4 3 Alumina/800 nm 3.6 8.5 4 Polycrystalline diamond/100 nm 2.88 10 5 Silicon oxide/100 nm 0.3 1.8 6 Cerium oxide/100 nm 0.36 2.5 通过表4的实验结果可以看出,在抛光ZnSe晶体材料时候,不同尺寸和类型的抛光粉对ZnSe抛光的材料去除效率和抛光后表面粗糙度具有显著影响。图4是6种不同实验所得的去除函数检测图。
根据表4实验结果可以得出图5的实验结果曲线,可以看出抛光过程中材料去除效率与粗糙度成正比关系,即材料去除效率越高,抛光表面的粗糙度越大,同时粒径对材料去除效率与抛光粉粒径及种类有关。
图 5 不同磁流变抛光液体对ZnSe 抛光的材料去除效率和粗糙度
Figure 5. Material removal efficiency and roughness of ZnSe polished by different magnetorheological polishing liquids
从图5可以看出,使用1号抛光液时材料的去除效率为5.2 μm/min,加工后的粗糙度为9 nm;使用2号抛光液时材料的去除效率为1.8 μm/min,加工后的粗糙度为4 nm;使用3号抛光液时材料的去除效率为3.6 μm/min,加工后的粗糙度为8.5 nm;使用4号抛光液时材料的去除效率为2.88 μm/min,加工后的粗糙度为10 nm;使用5号抛光液时材料的去除效率为0.3 μm/min,加工后的粗糙度为1.8 nm;使用6号抛光液时材料的去除效率为0.36 μm/min,加工后的粗糙度为2.5 nm。使用1号抛光液的去除效率虽然最高,但是同时加工后的表面粗糙度也很高。使用3、4号抛光液时加工后的表面粗糙度超过8 nm,导致后续抛光难度加大,精度无法保证。使用5、6号抛光液进行磁流变抛光时,虽然有较好的表面粗糙度,但是去除效率过低。结合加工效率及加工质量综合考虑,2号抛光液的材料去除效率和表面质量最合适。图6为在使用2号抛光液进行磁流变抛光后的ZnSe材料的表面粗糙度检测结果图,粗糙度为3.832 nm。
图 6 使用2号磁流变液加工后材料的粗糙度
Figure 6. Roughness of the material after processing with No. 2 magnetorheological fluid
虽然使用磁流变抛光技术可以获得较高精度的光学表面,但是磁流变抛光后晶体表面仍然存在磁流变抛光后特有的表面划痕的情况,且粗糙度变差,需要结合传统抛光方法来消除表面微观划痕和提升表面粗糙度。因此,在磁流变抛光结束后继续采用传统抛光数控抛光(CCOS)方法进行最终的精抛光。
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对于非球面硫化锌的抛光采用小磨头配合抛光垫,对其进行超精密抛光,硫化锌晶体超精密抛光所用抛光垫对抛光效率和表面质量均有重要的影响,表5为所用抛光垫的参数,图7为实验所用抛光垫实物图。
表 5 抛光垫参数
Table 5. Polishing pad parameters
Material Polyurethane Color Black Thickness/mm 0.8±0.1 Shore hardness (C) 81±5 Density/g·cm−3 0.5±0.1 Compression ratio (1.6±0.5)% CCOS抛光过程所用的抛光液体为碱性氧化硅抛光液, pH=10,其中二氧化硅颗粒呈球形,粒径为100 nm。将其与抛光垫配合使用,将加工正压力控制在0.05~0.1 MPa之间,进行最后的精抛光,经过30 min均匀抛光,表面粗糙度达到了1.57 nm,与抛光前相比,粗糙度精度在短时间内得到了明显改善。图8为使用2号抛光液进行磁流变抛光后,再对材料进行CCOS加工后最终ZnSe材料的表面粗糙度检测结果。
综上所述,对于硒化锌晶体的高精度高效抛光,可选用磁流变抛光结合传统抛光模式进行组合加工。先通过磁流变技术进行抛光,再通过CCOS对其面形进行快速修正。对一块口径为50 mm的硒化锌进行组合抛光实验,通过正交实验选取合适的磁流变抛光液,对其进行磁流变抛光,抛光后粗糙度为3.832 nm,再通过CCOS进行30 min的快速抛光使其粗糙度达到1.57 nm,粗糙度得到了明显改善。该组合加工方法可以有效地提高硒化锌的抛光质量及抛光效率,抛光后的硒化锌光学元件粗糙度可达到可见光波段使用精度,为硒化锌光学元件的广泛应用提供了有效的加工指导。
High-efficiency and high-quality combined polishing method of zinc selenide crystal (invited)
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摘要: 硒化锌晶体作为常用的红外晶体材料,广泛应用于红外光学系统中。为了提高硒化锌晶体的加工质量及加工效率,提出了将磁流变抛光(MRF)与传统数控抛光(CCOS)技术相结合的方法,通过多组正交实验配置硒化锌晶体的磁流变抛光液,对一块口径为50 mm的硒化锌晶体展开磁流变抛光,再针对磁流变抛光后的表面痕迹进行传统数控抛光,在正压力为0.05~0.1 MPa范围内,经过30 min均匀抛光,硒化锌晶体的表面粗糙度由3.832 nm降低到1.57 nm,粗糙度得到明显改善。该方法有效提高了非球面硒化锌晶体的加工效率并改善了加工后的表面质量,对硒化锌晶体的非球面超精密加工具有重要的参考价值。Abstract: As an excellent infrared crystal material, zinc selenide crystal is widely used in infrared optical systems. In order to improve the processing quality and processing efficiency of zinc selenide crystal, a method combining magnetorheological polishing (MRF) and traditional numerical control polishing (CCOS) technology was proposed, and the magnetic current of zinc selenide crystal was configured through multiple sets of orthogonal experiments. Change the polishing liquid, carry out magnetorheological polishing on a zinc selenide crystal with a diameter of 50 mm, and then perform traditional numerical control polishing on the surface traces after magnetorheological polishing. The positive pressure is in the range of 0.05-0.1 MPa. Uniform polishing after 30 minites, the surface roughness of the zinc selenide crystal was reduced from 3.832 nm to 1.57 nm, and the roughness was significantly improved. The method effectively improves the processing efficiency of aspheric zinc selenide crystals and improves the surface quality after processing, and has important reference value for aspheric ultra-precision processing of zinc selenide crystals.
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Key words:
- optical manufacturing /
- aspheric zinc selenide /
- magnetorheological finishing /
- roughness
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表 1 硒化锌的特性和性能
Table 1. Characteristic and properties of ZnSe
Material Type Crystal
structureHv/
GPaGrain
size/μmDensity/
g·mm−3ZnSe Polycrystalline Cubic 0.9±0.05 43±9 5.26 表 2 磁流变抛光液组成成分
Table 2. Composition of MR polishing fluid
Ingredient Deionized water Carbonyl iron powder (CIPs) Glycerin Rust inhibitor Sodium carbonate Citric acid Auxiliaries polish powder Volume fraction Other 35% 3% 0.8% 2% 0.6% 0.2% 0.2% 表 3 工艺参数
Table 3. Process parameter
Parameter Wheel
diameter/mmRotate
speed/r·min−1Penetration
depth/mmMagnetic
fieldValue 160 120 0.8 340 mT 表 4 不同抛光液成分的去除函数实验结果
Table 4. Experimental results of removal function for different polishing liquid compositions
MR polishing fluid series Abrasive type Removal rate/μm·min−1 Roughness/nm 1 single crystal diamond/100 nm 5.2 9 2 Alumina/100 nm 1.8 4 3 Alumina/800 nm 3.6 8.5 4 Polycrystalline diamond/100 nm 2.88 10 5 Silicon oxide/100 nm 0.3 1.8 6 Cerium oxide/100 nm 0.36 2.5 表 5 抛光垫参数
Table 5. Polishing pad parameters
Material Polyurethane Color Black Thickness/mm 0.8±0.1 Shore hardness (C) 81±5 Density/g·cm−3 0.5±0.1 Compression ratio (1.6±0.5)% -
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