This study presents a novel abrasive-free jet process that can be used to mitigate the lattice misaligned structure induced by mechanical stresses in potassium dihydrogen phosphate (KDP) crystal subsurface. This method makes use of a thermodynamically and kinetically stable ionic liquid microemulsion that contains nanometer range water droplets evenly dispersed in the non-aqueous carrier liquid. The sprayed out nanoscale droplets remove material through microscale dissolution without introducing new residual stress. Grazing incidence X-ray diffraction was used to evaluate the subsurface structure and to validate the mitigating performance. The experimental results show that the novel mitigation method can effectively reduce the thickness of the deformed layer, as well as reduce the surface roughness. This approach ultimately provides a potential path to extend the lifetime of highly valuable and difficult to grow large KDP crystals.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Inertial confinement fusion (ICF) is one of the most effective ways to solve the problem of energy deficiency. 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 that 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 [1–3]. However, comparing with the theoretical laser-induced damage threshold (LIDT) (147–200 J/cm2) , the current lower LIDT of KDP crystal (< 40 J/cm2) [5–7] has immensely limited the effective application and development of high-energy laser systems. Currently, single-point diamond turning (SPDT) is used for precise KDP manufacturing, but this is limited by the resulting SPDT marks, scratches, cracks and microstructure changes on the crystal surface/subsurface . The surface structural defects and the change of lattice orientation at the subsurface can induce absorption and scattering of the localized light, which would sharply reduce the LIDT, affect the optical transmission and finally shorten the lifetime of KDP crystal [9–11]. Actually, improved laser damage resistance of optics (such as KDP, fused silica etc.) optical surfaces has been a quest for use in high peak/power laser systems . For fused silica, laser damage initiation density has been reduced significantly using improved finishing processes and post-fabrication laser mitigation techniques, such as HF-based etching , ion beam etching  and CO2 laser mitigation .
For KDP, however, there is no a relatively perfect technology to remove or mitigate the subsurface damage (SSD) due to its soft texture, high brittleness, ready deliquescence and sensitivity to temperature . Magnetorheological finishing (MRF), a flexible finishing technology with a low normal force, has been attempted to reduce SSD and polish KDP [17–19]. However, MRF is not well suited for KDP treatment 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 . Li et al [20, 21] have attempted to solve this problem by applying ion beam figuring (IBF), but found that IBF processing imposed a high-temperature gradient field that would generate cracks or breaks. Chemical mechanical polishing (CMP) technology based on water dissolution 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.
Abrasive jet process (AJP), as a novel deterministic precision manufacturing technique, has been widely used in polishing other optical glass, metals, ceramics, etc [23–25]. According to the reported literature, however, AJP has not been applied to KDP polishing due to the issue of embedding of particles, as described above for MRF. Therefore, based on the water solubility characteristics of KDP crystal, in this work, we proposed using novel abrasive-free jet process (AFJP) to achieve abrasive-free and no-residue mitigation of SSD for KDP. This method 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 microscale dissolution without introducing new residual stress.
In this paper, a novel abrasive-free jet mitigation method for KDP crystal is presented, with the purpose of mitigating SSD without the embedding of particles and the introducing of defects. The feasibility of KDP AFJP is firstly described. The removal mechanism of the AFJP for KDP is then considered. Finally, experimental verifications are presented to evaluate the technical feasibility of the method.
2. Experiments and method
The KDP crystal specimens used in this study were produced with the rapid growth technique by the State Key Laboratory of Crystal Materials, Shandong University, China . In this study, a common IL, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), was used to prepare water/oil microemulsions, following a similar method as reported previously . Surfactant Triton X-100 (TX-100) was dissolved in bmimPF6 under magnetic agitation for 5 min, and then deionized water was dropped into the solution. The IL microemulsion used had a composition of 60 wt% bmimFP6 IL, 3 wt% (40g/L) deionized water, and 37 wt% surfactant TX-100.
2.2 Feasibility tests
A preliminary study on the feasibility of material removal through experiments of compatibility tests, controllability of removal was firstly investigated. 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. The marked KDP was placed in two abrasive-free jet fluids, namely polyethylene glycol (PEG)-200 containing 40 g/L of water and the other is the IL microemulsion also containing 40 g/L of water, and then its surface was observed by means of a ZEISS Auriga scanning electron microscope (SEM) after soaking for 14 h. The controllability experiments of material removal were investigated with the above two abrasive-free jet fluids under the same jet conditions, and the morphologies of the resulting spots were examined by means of a Taylor Hobson CCI lite white light interferometer.
2.3 Experimental details
KDP AFJP experiments were conducted on precision equipment along three linear axes (i.e., X, Y, and Z axes). 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 on the X-axis to obtain movements in the X-Y directions. 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.
The grazing incidence X-ray diffraction (GIXD) was used to precisely evaluate the subsurface structure of a KDP crystal and to validate the mitigating performance. The GIXD observation was carried out using Bruker D8 Discover X-ray diffraction (XRD) apparatus with Cu Ka radiation.
3. Results and discussion
Figures 1(b) and 1(c) show the results of compatibility tests. For the PEG-200 water system, there were serious etching pits compared with that before treatment (Fig. 1(a)) not only in the areas around the mark but also in the natural surface (Fig. 1(b)). In general, the areas around the mark are the easiest to be dissolved because here there are many defects and the chemical reactivity is high. However, the experimental results (Fig. 1(c)) suggested that dissolution around the mark or other areas on the KDP surface almost did not occur, since the long-chain surfactant coating on the water droplets directly avoids exposure of the KDP surface to water through a steric hindrance effect.
Figures 2(a) and 2(b) show the 2D morphology features of jet spots generated with the PEG-200 water system and the IL microemulsion respectively. A ‘W’-shaped profile could be observed for a jet spot generated with the PEG-200 water 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. 2(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.
In a static state, the water contained in the droplets was separated from the KDP by the long chain surfactant without jetting. Once this barrier was ruptured by the jet flow, the water in the droplets can contact and remove KDP at the impingement interface, as shown in Fig. 3. The microscale removal effects could not only keep smooth surfaces but also avoid KDP re-deposition. Since the jet velocity was relatively large (about 20 m/s), the contact time between a water droplet and KDP surface was very short. The shorter contact time results in the less material removal amount for each water droplet. Therefore, the dissolved KDP was miscible in a large number of nanoscale water droplets and did not concentrate on the KDP surface, which avoids the re-deposition of dissolved KDP and obtains controllable material removal.
According to the AFJP experimental results, It could be found that the jet spot generated by AFJP, was of an approximately Gaussian shape (Fig. 4(e)) instead of ‘W’-shaped generated by traditional AJP (Fig. 4(c)). Although the material removal mechanism of AFJP is different to that of traditional AJP, the simulated velocity field distribution of AFJP obtained with ANSYS Fluent software is similar to that of traditional AJP (Fig. 4(a)). The fluid jet can be divided into the free jet region, the impingement region, and the wall jet region . In addition, Peng  has demonstrated that small particles follow the fluid streamlines very closely, and larger particles will deviate more from the fluid streamlines. In this works, the radii of the droplets are in the nanometer range (<50 nm), therefore the water droplets also follow the streamlines very closely. For traditional AJP, the maximum of material removal is not in the center of the polishing region, although there is the largest impingement pressure in the center as shown in Fig. 4(b). For AFJP, the removal mechanism involves dissolution in water droplets to complete material removal rather than the shear stress of abrasive particles as shown in Fig. 4(d). According to our recent works , the removal rate of a water droplet is given by:Eq. (1) that the removal includes contact removal mainly in the impingement region and slipping removal mainly in the wall shear region. The contact removal dominates material removal in the center of the jet region (Impingement region), while the slipping removal is dominant in another region (Wall jet region). Therefore, there is no tip in the center of the impingement region (Fig. 4(e)).
The GIXD is a modern nondestructive analysis method that has been widely used in residual stress determination [32, 33], characterization of films [34, 35], structural analysis of materials [36, 37] etc. The principle of GIXD is shown in Fig. 5. A monochromatic X-ray beam irradiates the sample surface with an angle of grazing incidence α, and the detector is placed in a horizontal plane parallel to the film surface to collect diffraction peaks from lattice planes . This technique performs a depth profiling of the sample by varying the X-ray beam incidence angle with respect to the sample surface. For GIXD pattern, the X-ray penetration depth t is described by 40, 41] has demonstrated the GIXD can be used to assess microstructure changes in KDP crystals induced by mechanical stresses.
GIXD spectra taken at different angles of incidence for the KDP cut by the SPDT are shown in Fig. 6(a). For the single crystal, when diffraction planes are not in agreement to the detected surfaces, the diffraction peak will not appear in XRD diffraction pattern as shown in the inserted figure of Fig. 6(a). However, there were two diffraction peaks in the surface structures of the machined KDP crystal under the GIXD pattern as shown in Fig. 6(a). The diffraction angle (2θ) corresponding to the first peak (211) did not change with increasing grazing incidence angle (α), which suggested that the first peak represented re-deposition layer (Beilby layer) in the KDP surface. The presence of different diffraction peaks with the variation of α implied that the subsurface structure of the KDP crystal after machining has become a lattice misaligned structure. It indicated the second peaks corresponding to (112), (220), (202) etc. represented microstructure changes (Deformed layer) in the KDP subsurface induced by mechanical stresses.
The water molecules in the atmosphere were always being absorbed on KDP surface and then the microscale dissolution occurred [42, 43], especially the higher the humidity was, the more positive reaction there was. The absorption reaction would result in a very thin polycrystalline layer, namely Beiby layer which ranged in thickness from a few nanometers to a μm , was formed as time goes on as shown in Fig. 6(b). The plane (112) was the main diffraction plane in Beiby layer and thus its diffraction peak always occurred no matter how the α changed. However, for the deformed layer induced by mechanical stresses has become a lattice misaligned structure. The structure had a close relationship with the slip systems of a KDP . The shear stresses induced by machining made the material deform more easily along these slip systems. Since these slip planes still remained certain orientation relation, their diffraction peaks appeared in turn with the increase of X-ray incidence depth, as shown in Fig. 6(a). In addition, the peak intensities of the two layers appeared as one falls, another rises with the increase of X-ray incidence depth. The X-ray mainly went through the KDP bulk at α>5°, the two peaks gradually grow less, and at last, disappeared entirely (Fig. 6(a)). It indicated the bulk material of the KDP remains as a single crystal after surface machining.
According to the principle of GIXD, The intensity of the diffracted beam I is also affected by incidence angle α, which can be described by Eq. (3), the smaller the peak intensity is, the thinner the lattice misaligned structure is, at the same α. Figure 7 shows the GXID analysis result of KDP crystal surface before and after AFJP at different α. It can be seen that the mitigation treatment has lowered the intensity of peaks except for α = 0.5°. Since the X-ray almost traveled along the surface at α = 0.5°, the anomaly about α = 0.5° probably derived from the difference of surface morphology features before and after AFJP, as shown in the inserted figures of Fig. 8.
The removal process of AFJP mainly involves jet impingement effects and microscale removal effects on KDP surfaces. The jet impingement effects provide an impact force that can keep the sprayed out nanoscale water droplets contacting with KDP surfaces. Because the interfacial tension between bmimPF6 and H2O is small (about 11.06 mN/m at 25 °C) , the KDP is dissolved by small water droplet just need a lower pressure to destroy the surfactant coating layer in the contact interface. Classic breakup in nanodroplet does not occur at a small impinging velocity . Therefore, once the water droplet leaves the contact area, the destroyed coating layer will realign to a whole water droplet. A lower pressure can avoid the damage of KDP surface induced by a large impact force. Therefore, in the AFJP process, since the small droplets size and low jet pressure, the water droplets are dragged and then impact the surface with much lower kinetic energies.
Figure 8 separately shows the analytical results of the peak intensity of the deformed layer at different α. The peak intensities obviously decreased after AFJP mitigation. Since material removal is through dissolution instead of impact force, KDP AFJP will not produce new SSD. The surface roughness Sq by SPDT, which generated turning grooves on the surfaces, was 15.56 nm. This was reduced to 10.23 nm after AFJP, as shown in the inserted figures of Fig. 8. In addition, the tendency of reducing roughness has been demonstrated in our recent works . Therefore, the method cannot only mitigate the SSD of a KDP but also reduce surface roughness to a certain extent.
In this work, a novel abrasive-free jet process for KDP has been presented. The aim has been to overcome the shortcomings associated with traditional surface treatment methods, such as residual particles with MRF, a heating effect with IBF. 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. Experimental results have proven that only the water-in-oil microemulsion constitutes as an appropriate abrasive jet fluid, providing much better material removal compared to the water/oil miscible fluid system. The GIXD was used to evaluate the subsurface structure changes and the GIXD analytical results show that this method can effectively reduce the deformed layer induced by mechanical stresses, as well as can reduce the surface roughness. Hence, AFJP would seem to be a promising method for mitigating SSD of a KDP without a surface residue or sub-surface defects and the concept of the abrasive-free jet process may also provide a reference for the jet treatment of other optical materials.
National Natural Science Foundation of China (No.51575501, 51202228); the Science Challenge Project (No.JCKY2016212A506-0503); and the CAEP Foundation (Grant No.2015B0203030, 2015B0203028).
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