Traditional potassium dihydrogen phosphate (KDP) polishing methods, such as magnetorheological finishing (MRF), ion-beam figuring (IBF), and chemical mechanical polishing (CMP), are limited by either hard-to-remove residual particles, unavoidable heating effects, or an applicability restricted to large-sized KDP. In this paper, we present a novel abrasive-free jet polishing (AFJP) mechanism that can be implemented for abrasive-free and no-residue polishing on KDP surfaces. KDP AFJP makes use of a thermodynamically and kinetically stable ionic liquid (IL) microemulsion that contains nanometer range water droplets evenly dispersed in the non-aqueous carrier liquid. The sprayed out nanoscale water droplets can remove material through dissolution. The feasibility of this approach has first been analyzed through several experiments, namely compatibility tests and assessments of removal controllability and uniformity of the IL microemulsion. The material removal mechanisms in contact removal and slipping removal were then studied. KDP AFJP experiments were then conducted to validate the polishing performance. The experimental results show that an IL microemulsion as an abrasive-free jet can indeed improve the quality of a KDP surface, leaving no residue.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Potassium dihydrogen phosphate (KDP) is a unique nonlinear single-crystal optical material that can serve as a polarization electro-optical switch or as a frequency converter [1–3]. It is widely applied in high-energy laser systems, such as the National Ignition Facility (NIF) in USA, Laser MegaJoule (LMJ) in France, and SG-III Laser Facility in China [4–6]. However, KDP crystal is regarded as very difficult to cut or polish, especially for high-quality requirements, due to its soft texture, high brittleness, ready deliquescence, and sensitivity to temperature [7–9]. Currently, single-point diamond turning (SPDT) is used for precise KDP manufacturing [10–12], but this is limited by the resulting SPDT marks, scratches, and cracks on the crystal surface, which produce a diffraction effect and stray light. Consequently, the surface quality does not meet the requirements of high-energy laser systems [13–15]. With the increasingly urgent need for ultra-precision KDP, many polishing methods have been proposed for enhancing surface accuracy in recent years, such as magnetorheological finishing (MRF), ion-beam figuring (IBF), and chemical mechanical polishing (CMP).
MRF is a flexible polishing technology that uses magnetorheological fluid consisting of carbonyl-ligated iron particles in a basic liquid containing a non-magnetic abrasive. It has been used in finishing other hard optical materials, such as fused silica [16–18]. Menapace , as well as Zhang  and Ji  et al., have independently carried out research on water dissolution polishing of KDP crystal by means of MRF using a non-aqueous MR fluid containing a small amount of water.
However, MRF is not well-suited for KDP polishing because iron particles become embedded in the soft KDP surface, which are very difficult to remove. The residual particles can cause secondary pollution and significantly decrease the threshold for laser damage to KDP crystal by absorbing a sufficient amount of energy to irreversibly modify the KDP surface structure . Some researchers [23, 24] have attempted to solve this problem by applying IBF, but found that IBF processing imposed a high temperature gradient field that would generate cracks or breaks. Nevertheless, IBF could serve to remove powder embedded in the KDP surface and produced a superfine surface to a certain extent.
Wang  and Dong  et al. investigated CMP processing of KDP using a water-in-oil microemulsion. CMP technology based on dissolution in water circumvents the problem of embedding of particles. However, CMP can introduce sub-surface defects at normal pressure, which may lower the laser-induced damage threshold of the KDP. Moreover, it is difficult to ensure a good flow property and removal uniformity due to the high viscosity of the polishing fluid at the KDP surface, which limits broad application of large-sized KDP CMP. The above methods have nevertheless demonstrated the feasibility of water dissolution polishing of KDP crystal, in spite of the aforementioned problems associated with MRF and CMP.
Abrasive jet polishing (AJP) is a novel deterministic precision manufacturing technique. A mixture of water and abrasive particles is delivered by a pump to a converging nozzle of outlet diameter usually between 0.1 and 2.0 mm. The jet impinges on and thereby erodes the target workpiece, thus generating a removal spot [27–29]. Compared with other polishing methods, AJP has the advantages of localized force and cooling of the debris, and of suitability for polishing various complex surfaces (especially steep, concave aspheric surfaces) . As a flexible polishing method capable of processing any material, regardless of its properties, it has been widely used in polishing optical glass, metals, ceramics, etc [31–33].
According to the reported literature, however, AJP has not been applied to KDP polishing because of the issue to embedding of particles, as described above for MRF. Therefore, based on the water solubility characteristics of KDP crystal, the method of abrasive-free jet polishing may be applied to KDP crystal finishing. As for fluid jet polishing, we set out to investigate its potential application for KDP processing, employing abrasive-free jet fluid in place of an abrasive jet to improve KDP surface quality, so that there would be no embedded particles.
In this paper, a novel abrasive-free jet polishing mechanism and method for KDP crystal is presented, with the purpose of improving surface quality without the embedding of particles. The design of the abrasive-free jet fluid is firstly discussed. A preliminary study on the feasibility of KDP polishing through experiments of compatibility tests, controllability of removal, and uniformity of the abrasive-free jet fluid is then described. The surface removal mechanism of the abrasive-free jet polishing (AFJP) for KDP is then considered. Finally, experimental verifications are presented to evaluate the technical feasibility of the method.
2. Design of the abrasive-free jet fluid
In designing the abrasive-free jet fluid for KDP polishing, we considered formulations containing at least two components were compatible with KDP, the AFJP process, and the nozzle. Moreover, the abrasive-free jet fluid should be able to be easily cleaned away without any residue. The components are a non-aqueous carrier liquid and a small amount of water that is evenly dispersed therein. At first, a miscible fluid system comprising polyethylene glycol (PEG)-200 and a small amount of water as an abrasive-free jet fluid was applied for jet polishing of KDP crystal. Unfortunately, however, the jet removal spot was not controllable and the surface clearly showed radial traces of detrimental fluid flow, as shown in Fig. 3(a). Therefore, we had the idea of adding a small amount of water to an appropriate non-aqueous carrier liquid to form a water-in-oil microemulsion as an abrasive-free jet fluid.
A microemulsion is a system of water, oil, and a surfactant that exists as a single optically isotropic, thermodynamically and kinetically stable liquid solution. The droplet dispersion can be either of oil-in-water (o/w) or water-in-oil (w/o) type; in each case, the radii of the droplets are generally in the nanometer range . Microemulsions have been an interesting subject over the past decades, and the range of microemulsion applications spans diverse fields, including the pharmaceutical industry, the biological field, and materials synthesis [35–38].
In selecting an oil for KDP AFJP, it should be nonvolatile, or have low volatility, be nonflammable at room temperature with a high flash point, resistant to a potentially corrosive environment, unreactive towards KDP and the machine components, have low or no toxicity, and, lastly, not be capable of dissolving or adversely affecting the optical surface of a KDP crystal . In general, as a type of oil for a w/o microemulsion, the common alcohols, such as n-butyl alcohol, dodecanol, and decyl alcohol, are quite volatile and have strong odors, although they are compatible with KDP. Hence, alcohols are not well-suited to polish KDP through AFJP.
Ionic liquids (ILs) are organic salts that are liquids at room temperature. They are being increasingly studied as environmentally benign media or catalysts for chemical reactions and new style functional materials with promising applications in many fields, due to their unique and attractive physicochemical properties, including zero volatility, nonflammability, high chemical/thermal stability, and low toxicity [39–41]. Furthermore, in recent years, great attention has been paid to IL microemulsions due to their special physical and chemical properties [42–44]. Therefore, an IL microemulsion is a potential abrasive-free jet fluid for KDP polishing. In this study, a common IL, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), has been used to prepare w/o microemulsions for KDP AFJP.
The addition of a surfactant, such as Triton X-100 (TX-100), is critical for the creation of small-sized droplets, as it decreases the interfacial tension, i.e., the surface energy per unit area, between the oil and water phases of the emulsion.
In contrast to ordinary emulsions, which are kinetically stable but thermodynamically unstable and will undergo phase separation, microemulsions are thermodynamically stable and therefore do not require high inputs of energy or shear conditions for their formation. Therefore, TX-100/H2O/bmimPF6 microemulsions were prepared in two steps in this research, following a similar method as reported previously , as shown in Fig. 1. TX-100 (350 g) was dissolved in bmimPF6 (600 g) under magnetic agitation for 5 min, and then deionized water (50 g) was dropped into the solution.
3. Feasibility study
3.1 Experiments and Method
Compatibility tests were conducted on a 10 mm × 10 mm KDP surface that had been marked with an NHT2 nano-indentation system from CSM Instruments, with a Berkovich diamond indenter. The marked KDP was placed in an IL microemulsion containing 5 wt% water, and then its surface was observed by means of a microscope after soaking for 12 h.
The controllability of material removal was investigated with two abrasive-free jet fluids, namely PEG-200 containing 5 wt% water and the IL microemulsion also containing 5 wt% water. Jet spot experiments under the same conditions of 0.5 MPa and nozzle ϕ 1 mm were carried out with the above two abrasive-free fluids, and the morphologies of the resulting spots were examined by means of a Taylor–Hobson CCI lite with a 2.5 × objective and full resolution.
Removal function stability and uniformity of IL microemulsion as an abrasive-free jet fluid was investigated by producing a series of jet spots on a KDP crystal and analyzing their removal rate. Stability tests of footprints generated were conducted by evaluating uniformity of the cross section contours along the scan direction.
3.2 Experimental results and analysis
Figure 2 shows a comparison of the marked KDP surface before (Fig. 2(a)) and after (Fig. 2(b)) soaking in IL microemulsion. In general, the areas around the mark are the easiest to be dissolved because here there are many defects and the chemical reactivity is higher. However, our results suggested that dissolution around the mark or other areas on the KDP surface did not occur, since the IL microemulsion had a low water content, leading to very small size and rapid Brownian motion of the droplets. In addition, the long-chain surfactant coating on the water droplets directly avoids exposure of the KDP surface to water though a steric hindrance effect.
The controllability of material removal is the key requirement for deterministic polishing. Figure 3 shows the 3D morphology features of jet spots generated with the PEG-200 water system and the IL microemulsion, respectively. For the PEG-200 water system, there was no change on the KDP surface after 20 s of jetting, and the jet spot only appeared when the jetting time was extended to 5 min, as shown in Fig. 3(a). In addition, a ‘W’-shaped profile could be observed for a jet spot generated with the PEG-200 system, and the surface showed many radial traces of jet fluid flow. Because the water was completely dissolved in PEG-200, the KDP was indiscriminately dissolved by water molecules in the jet flow areas, and the dissolution rate of per point was dependent on the distribution of the jet flow field. This indicated use of the water/oil miscible fluid system as an abrasive-free jet fluid to be infeasible. Instead, the jet spot generated by the IL microemulsion, as shown in Fig. 3(b), was of an approximately Gaussian shape, with a smooth surface instead of ‘W’-shaped, and was free from traces of jet fluid flow. This suggested material removal with the IL microemulsion as an abrasive-free jet fluid to be controllable and selective, and the removal mechanism is explored in detail in Section 4.
Figure 4 shows the removal stability results for the IL microemulsion as an abrasive-free jet fluid. The contours of the jet spots are shown at the top of the figure, and the red line is the corresponding cross-section. It is interesting to note that the contours of the jet spots from spot 1 to spot 5 are nearly consistent under the same conditions. Discrepancies in the contours of the respective spots probably derived from fluctuations of the pump and jet machine components. The volumetric removal rates (VRR) from spots 1 to 5 were 1.653, 1.421, 1.408, 2.022, and 1.583 mm3/min, respectively, further indicating the material removal stability with the IL microemulsion as an abrasive-free jet fluid. In fact, the microemulsion was not delaminated or deposited, even under the conditions of high-speed centrifuging. Research by Gao  indicated that the radius of a water droplet in an IL microemulsion is almost linearly related to the bmimPF6-to-surfactant molar ratio. Furthermore, the radii of the water droplets are almost independent of the surfactant concentration, indicating that the droplet interaction is essentially negligible.
When jet tool moves along a single line, the removal track will show a grooved-profile as shown in the inserted figure of Fig. 5. These 6 cross section contours with interval of 10 mm along the scan direction are nearly consistent except for the edge profile (curve 1) as shown in Fig. 5, which further reflects the good stability of footprints generated by AFJP. The fluctuations of the pump probably contributed to the discrepancies in the cross section contours of the track (such as curve 1).
The solubility of KDP in water at 20 °C is 22.6 g per 100 g . The high solubility of KDP in water makes it possible to keep the IL microemulsion water content low and still maintain the ability of the fluid to function without promoting precipitation. For example, a 1 kg IL microemulsion containing 50 g of water (5 wt%) would be capable of holding 11.3 g of KDP. In polishing terms, 2.1 g of KDP would be removed at a VRR of 1.5 mm3/min after 10 h, far less than the amount of 11.3 g of KDP soluble in 50 g of water.
4. Removal mechanism analysis
The removal mechanism of traditional AJP utilizes shear stress to complete material removal when abrasive particles are embedded in the surface under pressure . The material removal mechanism of AFJP is different to that of traditional AJP, but the simulated velocity field distribution of AFJP obtained with ANSYS Fluent software is similar to that of traditional AJP (Fig. 6(a)). In addition, Peng  has demonstrated that small particles (<5 μm) follow the fluid streamlines very closely. The radii of the water droplets are in the nanometer range (<50 nm), therefore the trajectories of water droplets are dependent on the jet velocity field distribution. As shown in Fig. 6(a), the fluid jet polishing can be divided into the free jet region, the impingement region, and the wall jet region . The material removal by erosion and shearing actions in the wall jet region is far greater than that by impinging action in the impingement region, which results in a ‘W’-shaped profile across the polishing area.
However, the removal mechanism of AFJP involves dissolution in water droplets to complete material removal rather than the shear stress of abrasive particles. Jet impingement effects provide an impact force that can keep the sprayed out nanoscale water droplets contacting with KDP surfaces. The water contained in the droplets is separated from the KDP by the long-chain surfactant without jetting. Once this barrier is ruptured by the jet flow, the water in the droplets can contact and remove KDP at the impingement interface, as shown in Fig. 6(b). Such microscale removal effects are beneficial to obtain smooth surfaces. In addition, the dissolved KDP is miscible in a large number of nanoscale water droplets and does not concentrate on the KDP surface, which avoids the re-deposition of dissolved KDP and obtains controllable material removal. For the sake of analysis, the removal action by water droplets could be divided into contact removal (Fig. 6(b)) and slipping removal (Fig. 6(c)). The material removal function can be described as the sum of RP and Rτ, where RP is the material contact removal of water droplets mainly in the impingement region, and Rτ is the slipping removal by erosion and shearing actions mainly in the wall jet region.
The contact removal rate RP and the slipping removal rate Rτ of a water droplet, respectively, can be defined as:
Since the contact time between droplets and KDP surface is extremely short, and the dissolution rate S per point in the contact area is not changed, the removal rate by a water droplet can be expressed as:
The area of contact as a function of the surface pressure and velocity distribution is described by:
Thus, the removal rate of a water droplet is given by:
Supposing that the concentration of droplets is uniform per point in the jet region and the contact area A increases with increasing surface pressure and velocity, then the coefficients kP and kτ reflect the degrees to which various pressures and velocities, respectively, affect the contact area between the water droplets and the KDP surface. Similarly, f(x,P) and f(x,τ) reflect the distributions of surface pressure and velocity, respectively, in the jet region. As shown in Fig. 6(a), the distribution of surface pressure and velocity were similar to those in traditional AJP. There is an empirical function to describe the pressure distribution :
The distribution of surface velocities in jet region reflects the distribution of shear stresses, which can be described by :
Figure 7 shows the normalized removal distribution profile calculated according to Eq. (4), in which the red line denotes the slipping removal rate, the green line denotes the contact removal rate, and the blue lines denote the total removal rates by water droplets at different kP/kτ ratios. The removal distribution profile changes with changing kP/kτ ratio. In particular, the profile in the impingement region is ‘W’-shaped at kP/kτ<20, which is not in agreement with the experimental results. However, the profile in the impingement region shows a Gaussian-like shape, an ideal removal function in deterministic polishing, at kP/kτ = 20. On the other hand, according to the practical jet profile, as shown in Fig. 3(b), the contact removal was at least 20 times the slipping removal in the impingement region. This indicates that contact removal by water droplets dominated the material removal in the impingement region, and hence the ‘W’-shaped removal profile did not arise. Conversely, slipping removal by water droplets would be a major factor in removing material in the wall jet region. Compared to the indiscriminate removal by water in the PEG-200 miscible system as an abrasive-free jet fluid, the material removal with the IL microemulsion as an abrasive-free jet fluid is highly selective and controllable.
5. Polishing experiments and results
Jet polishing experiments were conducted on precision equipment along three linear axes (i.e., X-, Y-, and Z-axes), as shown in Fig. 8. The pressurization device was a gear pump that could be adjusted in the pressure range 0–1.5 MPa at flow rates of 0–2 L/min. The jet nozzle was fixed in the X-axis to obtain movements in the X-Y directions. A Raman spectrometer (HORIBA XploRA, 532 nm laser wavelength) was used to analyze the composition of the processed surface.
The IL microemulsion used had a composition of 60 wt% bmimPF6 IL, 3 wt% deionized water, and 37 wt% surfactant TX-100. The diameter of the nozzle was 1 mm, and a standoff distance of 10 mm was selected. The pressure at the inlet of the nozzle was 0.5 MPa.
Polishing experiments were conducted on a 10 mm × 10 mm region of a KDP crystal of 70 mm × 70 mm × 15 mm. The initial surface roughness was 17.77 nm, as shown in Fig. 8a. Table 1 summarizes the polishing conditions.
The surface roughness Sq by SPDT, which generated turning grooves on the surface, was 17.77 nm. This was reduced to 11.08 nm after jet polishing with an IL microemulsion containing 3 wt% water as an abrasive-free fluid, as shown in Figs. 9(a) and 9(b), respectively, demonstrating good improvement of the roughness.
Figure 10 shows the Raman spectra of KDP crystal surfaces before and after AFJP. It is found that the characteristic peaks of the surface polished by AFJP are the same as those of pure KDP, which is in good agreement with the experimental results of KDP CMP . The absence of any new characteristic peaks reveals that there is no chemical reaction between the KDP crystal and abrasive-free jet fluid. This proves the processed surface is still chemically the same as it was before.
A novel abrasive-free jet-polishing process and mechanism for KDP have been presented. The aim has been to overcome the shortcomings associated with traditional polishing methods, such as residual particles with MRF, sub-surface defects with CMP, and a heating effect with IBM. According to the characteristics of KDP crystal dissolution in water, two types of abrasive-free jet fluid were prepared, one being a water-oil miscible fluid system, and the other being a water-in-oil microemulsion showing nonvolatility, nonflammability, high chemical/thermal stability, and low toxicity. Experimental results have proven that only the water-in-oil microemulsion constitutes an appropriate abrasive-free jet fluid, providing much better material removal in terms of controllability and uniformity compared to the water/oil miscible fluid system. Moreover, the removal profile with the IL microemulsion was Gaussian-like rather than ‘W’-shaped under a vertical jet. Combining theoretical and experimental results, the removal mechanism by water droplets in IL microemulsion could be divided into contact removal and slipping removal. Contact removal dominated the material removal in the impingement region and was at least 20 times greater than slipping removal. Moreover, it has also been confirmed that a KDP surface polished by an IL microemulsion shows reduced roughness. Hence, AFJP would seem to be a promising method for polishing and cleaning KDP without a surface residue or sub-surface defects. In addition, the concept of abrasive-free jet polishing may also provide reference for the jet polishing of other materials. However, there are also challenges associated with KDP AFJP, such as the design and preparation of lower viscosity IL microemulsion, the issue of bubbles in microemulsion in recycling processes, and so on. These problems will be studied and addressed in future work.
National Natural Science Foundation of China (No.51575501, 51202228); Science Challenge Project (No.JCKY2016212A506-0503); CAEP Foundation (Grant No.2015B0203030, 2015B0203028).
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