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Abstract
Aluminum optics are widely used in modern optical systems because of high specific stiffness and high reflectance. Magnetorheological finishing (MRF) provides a highly deterministic technology for high precision aluminum optics fabrication. However, the contamination layer will generate on the surface and bring difficulties for the subsequent processes, which highly limit the fabrication efficiency and precision. In this study, characteristics of the contamination layer and its formation process are firstly revealed through experimental and theoretical methods. Impurities such as abrasives are embedded into the aluminum substrate causing increasing surface hardness. The influence of the contaminant layer on machining accuracy and machining efficiency is analyzed in this study. Based on the analysis, ion beam sputtering (IBS) is induced as a contamination layer modification method. Impurities will be preferential sputtered during the process. Surface hardness and brightness will restore to the state before MRF. Moreover, the thickness of the contamination layer reduces dynamically during IBS because of the bombardment-induced Gibbsian segregation and sputter yield amplification mechanism. Consequently, we proposed a combined technique that includes MRF, IBS and smoothing polishing. Comparative experiments are performed on an elliptical shape plane surface. The results indicate that the efficiency has been increased sevenfold and surface precision is also highly improved. Our research will promote the application of aluminum optics to the visible and even ultraviolet band.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The rapid development of Nanosat has increased the demand for light and compact optical system, which stimulate the improvement of optical components design and optimizing of optical materials selection [1–6]. Aluminum mirrors with complex curve surfaces are currently believed to perfectly meets the above requirements and have a very broad prospect in the applications of aerospace optical systems [7–9]. Usually, Single point diamond turning (SPDT) is typically employed in the ultra-precision machining of aluminum alloy optics because of advantages of high efficiency and ability to process complex surface [10–14]. Currently, the applied optical frequency band of imaging aluminum optics is moving from infrared (IR) and far infrared (FIR) to visible (VIS) and ultra violet (UV). Within these optical systems, aluminum optics require surface profile referred as low spatial frequency (LSF) error and surface roughness referred as high spatial frequency (HSF) error to attain nanometers simultaneously [15,16]. To date, SPDT lathes with higher kinematic accuracy and dynamic stiffness, such as Precitech Nanoform 700, are upgrading rapidly. However, the machining precision will not exceed the accuracy of machine tools due to characteristics of maternal processing. The machining precision determine that the applied waveband of optics processed by SPDT is mostly concentrated on mid-infrared and infrared. Also, inevitable periodic turning marks will deteriorate roughness, cause light scattering, and finally reduce the quality of optical imaging [17–20]. All these problems make SPDT subject to a variety restriction on visible quality aluminum optics processing.
Magnetorheological finishing (MRF) is a highly stable and commonly used method for aluminum optics manufacture, which can efficiently eliminate LSF error and periodic turning marks after SPDT [21]. Belonging to deterministic optical fabrication methods, MRF break through the restriction of maternal processing, which can significantly improve fabrication precision [22,23]. By optimizing the process, MRF can fabricate freeform aluminum mirrors for visible light applications, which possess form accuracy of 0.025μm RMS. However, the properties of softness, chemical activity, temperature sensitivity make aluminum difficult to manufacture with high precision and ultra-smooth. Along with excellent process precision, an inherent contamination layer will generate during MRF process as shown in Fig. 1, which will cause the reduction of reflectivity and surface quality. Some studies are conducted to solve the problem. By optimizing aluminum materials, Cheng et al. [24] obtained the aluminum surface with surface roughness of 4 nm. Through the optimization of process parameters such as magnetic field strength, Ge et al. [25] improved the surface roughness to 5.593 nm after MRF. However, even with better surface roughness, the contamination layer still exists on the surface. Thus, some researchers look for other methods to remove contamination layer instead of improving MRF process. Zhao et al. [26] effectively removed contamination layer by using femtosecond laser. Though achieving good results, this method is too expensive to be widely promoted.
To remove contamination layer and improve surface quality, smoothing polishing (SP) is usually used as subsequent procedure after MRF and the final procedure of aluminum optics manufacture [27,28]. The SP process, which has a long-standing history, belongs to traditional polishing in nature. Based on CMP mechanisms, by strictly controlling the machining status, a rigid or semirigid polish pad is used to smoothing the high-spatial error. However, there are several issues exiting in this procedure during aluminum optics manufacture. First, existed edge effects and uneven pressure distribution lead to undesired material removal, so long-time SP process will cause misconvergence of surface profile [29,30]. Second, polishing abrasive aggregation and adhesion on the surface lead to surface contamination and scratches, so SP process will cause deterioration of surface quality surface [31]. Once the issue appears, the whole manufacture process will start over again. Repeated iteration reduces the machining efficiency. It is believed that the contamination layer is inevitable and SP is the unreplaceable and effectively method to improve surface qualities. Thus, most researches about contamination removal are concentrated on optimization of SP process to alleviate the above issues [32,33]. There is great lack of attention to characteristics of contamination layer and its formation process, which makes the mechanism remain unclear. Moreover, most researches only concentrate on single issues which can’t effectively improve the polishing process of multi-factor coupling. In order to meet the requirement of machining precision, the most used processing think is to improve the precision of the previous process (MRF) to provide machining allowance for SP, which quite limit the improvement of fabrication precision and efficiency of aluminum optics. Hence, the characteristics of contamination layer and more efficient removal methods need to be studied urgently.
Recently, electron and ion beam treatment are considered highly promising for surface modification and surface cleaning [34–36]. Ion beam sputtering (IBS) is one of the advanced methods of surface treatment, which shows efficient surface cleaning of glass materials such as fused silicon [37–39]. It is noted that IBS can eliminate impurities and improve surface qualities and reflectivity without destroying the crystal structure of substrates, surface form and inducing extra contaminations [35]. Moreover, with steady dwell time matrix, IBS can further improve the surface form precision. However, seldom works are conducted on metal materials such as aluminum.
In this work, IBS is firstly introduced in aluminum optics fabrication process. An SP experiment is conducted on section 2 to address shortcomings of process. In section 3, the characteristics of contamination layer and its formation process are firstly revealed through experimental and theoretical methods. The shortcomings of SP are also revealed by theoretical methods exhaustively. In Section 4, IBS is introduced and its removal mechanism is revealed by experimental and theoretical methods. Finally, a high efficiency aluminum optics machining process is introduced in section 5 based on the above analysis. The results of this study will be beneficial for application of IBS in the field of aluminum optics manufacture and will significantly improve the machining efficiency and precision of aluminum optics.
2. SP experiments
2.1 Experiment details
A SP experiment is processed on an aluminum 6061 optic surface, which is an elliptical shape plane surface. This sample is pre-processed by SPDT and further polished by MRF to improve surface shape accuracy. Subsequently, the SP experiment is performed in our self-developed four-axis CCOS CNC polishing systems. The SP equipment is composed of a pressure applying module, a polishing pad (including pad base and lap), slurry, etc. The parameters we choose are shown in Table 1.
Table 1. Parameters of SP process
In order to maintain surface profile, a uniform raster scan is conducted as pad path. One processing cycle will cost nearly 3 hours.
2.2 Experiment results
The SP process cost 19 times iterations and about 56 h to polishing this samples. After polishing, the contamination layer is fully removed and surface restores to its brightness state. The initial surface residual error is 69.926 nm RMS. After 19 times iterations, it deteriorates to 98.167 RMS. During SP process, the surface shape continues deteriorating and shows oscillations. Figure 2 shows surface profiles during SP. The surface profile is measured by Interferometer (Zygo VeriFire MST) with 1000×1000 CCD pixels. Full caliber data is measured. To ensure repeatability and accuracy, the measurement of each profile was repeated at least five times. The difference of RMS value between repeated measurements should be less than 4 nm. Fabricated by MRF, the original surface error in Fig. 2(a) shows obvious warped edge. Nearly most of the contact processing technology including MRF will cause edge effects. Though edge effects can be effectively compensated in MRF, many factors such as position error, stability of removal function need to be controlled precisely to achieve that goal. Most of the time, algorithm optimization can only reduce instead of eliminate the edge effect. During SP process, the edge effect become more obvious. The topography of the edges varies greatly (transformation between edge collapse and edge warping) as shown in Fig. 2(b)-(d). Thus, a long-time SP process will make surface shape difficult to maintain.
Fig. 2. Experimental results of SP of the elliptical surface, (a) original MRF surface error, (b) surface error after 6 iterations, (c) surface error after 12 iterations, (d) surface error after 19 iterations.
To make clearly investigation on edge evolution, the horizontal profile lines during the SP is presented in Fig. 3. Because of compensation process in MRF, warped edge in Fig. 3(a) is not very steep. However, in SP process, when the pad moves to the edge of the workpiece, the pressure changes greatly, which results in the material removal efficiency of the edge deviating greatly from the theoretical value. Hence, the edge is steeper in Fig. 3(b)-(d) which reveals an evident edge effect. Specially, in Fig. 3(c), the edge topography matches well with theoretical pressure distribution of pad partly bareness [29].
Fig. 3. Evolution of corresponding horizontal profile lines of SP in Fig. 2, (a) original MRF surface, (b) surface after 6 iterations, (c) surface after 12 iterations, (d) surface after 19 iterations.
Figure 4 shows Super-Resolution Microscope (SRM, VHX-5000) images of scratches that generated on the aluminum surface after first SP. As shown in Fig. 4(b). the width of scratches is around 3μm, which can be clearly observed on the surface. During the process, abrasives at nano scale aggregate to a larger size and lead to severely issues of scratches comparing to polishing of SPDT aluminum surface.
It is obvious that the effects of SP process are shown in two aspects: figuring the low-frequency surface error and smoothing the mid-high frequency surface error. Summarizing the previously experimental results, the efficiency of SP is relatively low, which seriously limit the application of SP process. Moreover, long-time SP process will cause deterioration of surface profile and surface scratches.
3. Theoretical analysis
3.1 Characteristics of contamination layer
In the aluminum optics manufacture process, the purpose of SP process is to remove the contamination layer without deteriorating initial surface profile. Thus, the characteristics of contamination layer are studied firstly by applying Depth-Sensing Indentation (DSI, CSM UNHT + MCT), Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS, ION-TOF GmbH TOF.SIMS5-100) and X-ray photoelectron spectroscopy (XPS, ULVAC-PHI PHI5000 VersaprobeIII XPS). Figure 5 shows the corresponding test results of contamination layer. A stable hardness is achieved in depth of around 100 nm as shown in Fig. 5(a) which reveals that the thickness of contamination layer is about 100 nm. The conclusion can also be supported by TOF-SIMS result in Fig. 5(b). There is a distinct turning point at 100 nm in the signal intensity figure of Fe element. Moreover, the TOF-SIMS tests are carried in different surface position with different polishing time. The same results are achieved which we can presume that the contamination layer has uniform thickness regardless of polishing time. The XPS results in Fig. 5(c) and (d) shows that the main elements tested in the contamination layer are O Al Fe Ce. Specifically, element Fe and Ce only exist in elemental state and oxidation state respectively. The test results indicate that Fe powders and abrasives are directly embedded into substrate without chemical reaction. Embedded particles will impede machining deformation dislocation according to Orowan equation which can be expressed as follow [40]:
where L is average mean particle spacing and it decreases with particle concentration. In Fig. 5(c), particle concentration decreases with test depth which means the surface hardness will decrease with depth. The analysis matches well with test results in Fig. 5(a). When test depth exceeding 100 nm, the DSI probe reaches the substrate and hardness value will maintain a steady value.Fig. 5. Testing results of contamination layer, (a) DSI results, (b) TOF-SIMS results (depth distribution of Fe element signal intensity), (c) XPS results (depth distribution of atomic concentration of all elements), (d) XPS results (depth distribution of atomic concentration of chemical states).
3.2 Analysis of removal efficiency
Based on the Greenwood-Williamson (GW) theory, material removal process can be analyzed on micro-scale [41]. GM theory which is based on Hertz contact theory divide the contact surface into Random rough surface and Nominal flat surface. Through MRF process, we can assume that the surface micro-topology is at a isotropous state. The material removal capability can be measured by removal rate of a single abrasive. For simplification, this paper only discusses the removal rate of a single abrasive. Volume removal rate of single abrasive can be expressed as:
where K is wear constant, V is the relative speed of abrasive and workpiece, and A is the cross-sectional area of a single abrasive particle in a workpiece that can be expressed as where a is contact radius and r is abrasive radius. According to plastic contact theory, the Rv can be expressed as:As showed in the Eq. (4), for chosen abrasive and pad, the removal rate is mainly affected by coefficients Be and C which correspond to the hardness of the workpiece and pad pressure respectively. By normalization, a function of removal efficiency, pad pressure and surface hardness are presented in Fig. 6. Work pressure is strongly associated with parameter d which represents the contact degree of pad and work surface. The higher the value of d, the less pressure the pad is applied. With steady Brinell hardness, removal efficiency decreases as the pressure decreases. With the same work pressure, the removal efficiency nearly reduces by 2/3 when the surface Brinell hardness goes up twice. Thus, the removal efficiency of contamination layer is relatively low because of its higher hardness.
3.3 Analysis of scratch generation
Usually, to maintain surface quality and avoid scratches, abrasives chosen in aluminum polishing has nanoscale size. However, aggregation is a commonly issues for nanoscale abrasives. Several methods such as reduce concentration of the slurry or raise ambient temperature can alleviate aggregation problems in aluminum polishing [31]. But, for contamination layer polishing, abrasives aggregation problem is more severe and can hardly be solved. A comparative experiment is conducted to reveal the phenomenon. Two aluminum surfaces (both are processed by SPDT and one is further polished by MRF to generate contamination layer) are immersed in commonly used polishing fluid to monitor the particle aggregation. The results are shown in Fig. 7. After 10 min of soaking, apparent particle aggregation appears on the contamination surface while on obvious arrogation on turning surface as shown in Fig. 7(a). Moreover, the agglomerated particles adhere to the surface and are hardly removed. SRM is used to observed the agglomerated particles as shown in Fig. 7(b). The size of agglomerated particles exceeds 100μm. With applied pressure, visible scratches can easily be generated.
Fig. 7. Comparative immersion experiment between SPDT aluminum surface and MRF surface, (a) comparison of particle aggregation state after 10 min soaking, (b) SRM image of agglomerated particles.
When the fluid environment meets certain conditions, the nucleation reaction occurs and causes abrasives aggregation. There are two types of nucleation process: homogeneous nucleation and heterogeneous nucleation. The change of the free energy of the system for homogeneous nucleation ΔGo and heterogeneous nucleation ΔGe can be written as [42]:
where V and S are the volume and superficial area of the crystal embryo respectively. ΔGV is difference in free energy between solid and liquid phases per unit volume. σ is the surface energy per unit area. -ΔGd represents the energy released when abrasives attach on the impurities. Due to the existence of -ΔGd, heterogeneous nucleation will have lower critical nucleation energy, which will greatly promote the nucleation process. Comparing with substrate, the contamination layer contains lots of impurities such as abrasives and Fe powders that will cause heterogeneous nucleation and more obvious abrasives aggregation phenomenon, which is quite fits the experimental results in Fig. 7.Summarizing the previous experiments and theoretical analysis, during MRF, the Fe powders and polishing abrasives will embed into the surface without changing chemical states, which will generate contamination layer with uniform thickness of 100 nm. Surface hardness increases because of the embedded impurities which leads to removal efficiency decreasing. Moreover, the impurities in contamination layer will prompt abrasives aggregation leading to surface scratches during SP. Obviously, the contamination layer need to be eliminated before SP.
4. Introduction of IBS
4.1 IBS experiment details
Admittedly, IBS is the highest precision process because of its stable, non-contact and contamination-free characteristics. When the atoms of work surface receive enough energy, it will be sputtered from the surface. In order to verifies the eliminated effects, an IBS experiments are performed in our self-developed IBS systems (KDIBF650L-VT) under the bombardment of Ar+ ions at normal incidence with the work pressure of 2.5×10-3 Pa. IBS parameters are shown in Table 2.
Table 2. Parameters of IBS process
Eight spots with different removal depth of 50nm-200 nm are sputtered on an aluminum surface after MRF.
4.2. Experimental results
It is obvious that the grey contamination layer is removed and the surface is restored to its former state as shown in Fig. 8(a) and (b). In Fig. 8(d), only the depth of first three points didn’t exceed the depth of contamination layer. It can be clear seen in Fig. 8(b) that the three point on the right share the same surface state. The corresponding roughness evolution results are shown in Fig. 9(b), (c), (d). The original roughness of MRF surface is 17.373 nm. Because of the chemical reaction and embedded particles, surface roughness deteriorates during the MRF process. After IBS process, the surface roughness improved to 14.487 nm. During IBS, the energy transferred to atoms by ion beam can not only lead to atoms sputtering but also cause thermal diffusion which will smooth the surface. Comparing with aluminum substrate, contamination layer has relatively lower sputtered rate. The thermal diffusion will be more significant during the IBS leading to roughness improvement. As for the remaining five sputtered points, obvious whitish hues are observed on the center area. It is because that the sputtered depth reaches aluminum substrate for the latter five points leading to micro-morphology evolution which is revealed in our previous researches [43]. Corresponding roughness present a trend of deterioration. With removal depth of 190 nm, the surface roughness deteriorates to 19.347 nm.
Fig. 8. Experimental results of IBS experiment, (a)(b) appearance of MRF surface after IBS, (c) diagram of material removal of eight spots, (d) corresponding line profile of material removal.
Fig. 9. Roughness evolution of different removal depth of IBS on MRF surface, (a) original MRF surface, IBS sputtered depth of (b) 50 nm, (c) 75 nm, (d) 100 nm, (e) 125 nm, (f) 150 nm, (g) 160 nm, (h) 175 nm, (i) 190 nm.
4.3. Discussion
An interesting phenomenon is noticed that even when the removal is 50 nm (removal depth is not exceeding depth of contamination layer), the surface can still restore to its former state as shown in Fig. 8(a). Thus, XPS test is repeated on a contamination layer with sputtered depth of 50 nm. As shown in Fig. 10(a) and (b), the type and chemical state of elements remain unchanged during IBS. The atomic concentration of aluminum reaches a stable value at depth of around 10 nm. It can be deduced that contamination layer is fully removed. As for impurities (Fe and Ce), which are the factors that leads to variation of color and surface properties, the variation with depth is shown in Fig. 10(c) and (d). In Fig. 10(c), the black dotted line indicates the sputtering depth of 50 nm. The blue line represents the distribution of Fe with depth in the contamination layer while the red line represents distribution in the contamination layer treated by IBS. With the same depth, the atomic concentration level is greatly reduced in IBS treated sample. The phenomenon is more obvious for Ce as shown in Fig. 10(d). From the testing results, the depth of contamination layer shows a phenomenon of dynamic reduction during IBS.
Fig. 10. XPS results of contamination layer with removal depth of 50 nm, (a) depth distribution of atomic concentration of all elements, (b) depth distribution of atomic concentration of chemical states, (c) comparison of atomic concentration of Fe between original MRF contamination surface and contamination surface with IBS sputtered depth of 50 nm, (d) comparison of atomic concentration of Ce between original MRF contamination surface and contamination surface with IBS sputtered depth of 50 nm.
In order to verifies our analysis, a hardness test experiment is conducted. Firstly, an oblique groove with uniform change in depth is generated on the contamination layer by IBS. The working parameters in Table 2 are applied. The maximum removal depth is controlled in around 100 nm. A Micro-Hardness Tester (HT, Matsuzawa MMT-X7B) is used for harness testing. Three points are tested at each depth. The average of three hardness values is taken as the final hardness of corresponding depth. As shown in Fig. 11, the hardness of contamination layer (removal depth of 0 nm) is 121HV. The hardness decreases rapidly with removal depth. A stable hardness of 109HV is achieved at removal depth of 40 nm, which is close to the hardness of aluminum substrate as shown in Table 3.
Table 3. Properties of aluminum substrate
Based on above test results, impurities such as Fe and Ce in the subsurface will dynamically float to the surface during IBS and will be preferential sputtered from the contamination layer, which will lead to restoration of surface optical state (roughness and reflectivity) and surface mechanical properties (hardness). Unlike commonly used materials such as single-crystalline silicon or fused silicon, contamination layer generated by MRF on the aluminum substrate contains quite amounts of impurities. When the ions bombard surface, vacant and interstitial atoms will generate because of collision cascade, which will cause surface segregation of internal atoms [44,45]. The phenomenon is referred as Bombardment-Induced Gibbsian Segregation (BIGS). In many former researches, the phenomenon of heavy atoms segregating and aggregating on the surface is observed [46]. In our experiment, Fe and Ce segregation quite tally with former researches. Moreover, because of the high mass ratio (Fe: Al=2:1, Ce: Al=5:1), the sputter yield amplification (SYA) phenomenon can’t be ignored [47–50]. Binary sputtering theory introduced by Bradley points out that SYA only occurs if the heavier atomic species has a relatively low concentration-on the order of 10% or less and an instability will be induced. Combining with our experimental results, the heavy ion species will flow to the trough area on the surface. With higher deposition energy on trough area, a higher sputtering rate will be achieved for heavy atoms such as Fe and Ce.
Consequently, even removal depth is not exceeding 100 nm (depth of contamination layer), the impurities can be eliminated by IBS and surface mechanical hardness will restore to its former state. Impurities will segregate on the surface and be preferential sputtered. Due to thermal diffusion, surface roughness will be improved by IBS. However, when the removal depth reaching substrate, surface roughness will deteriorate.
5. Combine fabrication technique
5.1 Technique description
According to advantages of IBS, we add it to the process flow and propose a new processing technology. During the first processing phase, the MRF is employed to rapidly correct low-frequency errors and SPDT marks with high convergence and high materials removal rate. The processing parameters adopted in the experiment are wheel speed Vω=180 rpm, fluid flux Jf=130 L/h, current for the magnetic field Im=5 A, and plunge depth Hp=0.31 mm. Subsequently, we use IBS to eliminate the contamination layers. The processing parameters adopted in the IBS experiment are beam energy Eion=700 eV, beam current J=15 mA, beam diameter d=30 mm. In order to achieve uniform material removal, a uniform raster scan is chosen as tool path. Scanning speed is V=400 mm/min to acquire material removal depth of around 40 nm. Finally, a SP process is applied as the final ultra-precision fabrication of aluminum optics and the same parameters described in Table 1 are applied.
5.2 Experiment certification
To demonstrate the feasibility and the advantages of this combined fabrication technique, the polishing experiments are performed on the same elliptical plane surface. The elliptical surface with a surface form in Fig. 2(d) is firstly polished by MRF. As shown in Fig. 12(a), an obvious warped edge can still be observed on the surface. The surface form restores to 729.744 nm PV and 66.407 nm RMS, which is quite close to the results in Fig. 2(a). In order to remove contamination layer, IBS is used in the subsequent process. After IBS, the surface error doesn’t vary much because of the uniform scan path. Finally, a short time SP process is applied for further smooth the elliptical surface. As shown in Fig. 12(c), the final surface error is 741.418 nm PV and 31.464 nm RMS, which is slightly improved comparing to surface after MRF.
Fig. 12. Experimental results of combined fabrication technique for elliptical surface, (a) original surface error after MRF, (b) surface error after IBS, (c) surface error after SP.
Table 4 shows the evolution process of the surface error during the combined fabrication technique. Within only 8.5 h processing, the contamination layer is fully removed and surface restore to its former brightness. The machining efficiency has been increased sevenfold comparing with the 56 h direct SP. As the processing time decreases, the issues during machining such as scratches and edge effects can be easier controlled. Thus, the surface error doesn’t deteriorate. The experimental results indicate that this combined technique possesses high machining efficiency and machining accuracy, which demonstrates the feasibility of our proposed method for figuring a high-precision aluminum surface.
Table 4. Evolution of the Surface Error of the Combined Machining Process
6. Conclusion
Contamination layer on aluminum optics generated by MRF will cause severely deterioration of reflectivity and surface qualities. SP provides an effectively technology for removing contamination layer and restoring surface qualities. However, the fabrication is a time-consuming process. Scratches and surface form deterioration will be generated because of the physicochemical characteristic of aluminum. The properties and formation process of contamination layer is firstly studied in this research. TOF-SIMS and DSI results reveal that thickness of contamination layer is stable at about 100 nm despite of machining time. As shown in XPS tests, polishing abrasives and Fe powders are embedded into aluminum substrates without chemical state variation during MRF, causing reinforcement of surface mechanical properties. Therefore, we proposed IBS as a contamination layer removal method. Benefit from BIGS and SYA, impurities will be preferential removed. Depth of contamination layer show a state of dynamic reduction during IBS. Consequently, a highly efficient and high precision combined fabrication technique is proposed. An elliptical plane surface is taken as a demonstration of feasibility and the advantages of combined fabrication technique. Without deterioration of surface profile, contamination layer is rapidly removed. The efficiency has been increased sevenfold. This combined technique possesses high machining efficiency and machining accuracy, which will promote the application of aluminum optics to visible and even ultraviolet band.
Funding
National Natural Science Foundation of China (51991371); Postgraduate Scientific Innovation Fund of Hunan Province.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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