The surface integrity of a potassium dihydrogen phosphate (KDP) crystal significantly affects the laser damage threshold of the material. However, the detection of the surface integrity of KDP crystals is difficult due to the material’s special properties including soft, brittle, and sensitive to external environments (e.g., humidity, temperature, and applied stress). This results in conventional characterization methods, such as transmission electron microscopy (TEM) and scanning electron microscope (SEM), which cannot be used to study the mechanisms of surface/subsurface damages of KDP crystals. This paper investigates the ultra-precision fly-cutting effect on the surface integrity of KDP crystals. To explore the fundamentals, nanoindentation was used. The results demonstrated that the elastic-plastic deformation of a KDP crystal occurs more easily on a machined surface than on a cleaved (damage-free) surface. The elastic modulus and hardness of the former surface are lower than that of the latter. Additionally, fly-cutting reduces the anisotropy of the elastic modulus and hardness. To explore the mechanisms behind such variations, a novel method to characterize subsurface damage was proposed by using the grazing incidence X-ray diffraction (GIXD) technique. It was identified that the damages induced by fly-cutting are dislocations and lattice misalignments.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Potassium dihydrogen phosphate (KDP) crystals have important applications in the laser ignition facilities for inertial confinement fusion (ICF) [1–3]. In ICF facilities, KDP crystals are used as optical switches and frequency conversion components. However, KDP crystal components are often damaged under the irradiation of high-energy lasers, which significantly reduces the output power of ICF facilities. To improve the laser damage threshold of KDP crystals, the smooth surfaces of the crystals are generally machined using ultra-precision fly-cutting techniques [4,5]. However, note that surface/subsurface damages are inevitably induced in the machining of KDP crystals and that surface/subsurface damages have key effects on the low laser damage thresholds [6–9]. Therefore, the machined surface integrity of KDP crystals should be evaluated and understood in-depth with respect to their mechanical properties.
To date, the mechanical properties of KDP crystals have been measured primarily from polished surfaces. Polishing processes can decrease the effects of surface/subsurface damages on the original mechanical properties of KDP crystals during indentation. Fang et al.  conducted nanoindentation experiments of KDP crystals on the (100) and (001) surfaces polished by conventional means with nonaqueous slurries. They found that KDP crystals have an indentation size effect for both Vickers and Knoop hardness, with indenting loads ranging from 0.24-1.96 N. Additionally, the large-load Vickers hardness is approximately 1.4 ± 0.1 GPa. Lu et al.  measured the mechanical properties of KDP crystals on the (001) and tripler surfaces processed by a polishing method. They reported that the hardness and reduced modulus values of KDP crystals are anisotropic on the polished surfaces. Using the same polishing process, Guo et al.  found from nanoindentation experiments that the nanohardness and elastic modulus of KDP crystals ranged from 1.44-2.61 GPa and 52-123 GPa, respectively, on the doubler surface. To research the effects of deuteration on the KDP mechanical properties, Kucheyev et al.  used water polishing to prepare (001) and (100) surfaces for testing. The material’s deformation behavior is independent of its deuterium content. Multiple pop-in events were observed in the force-displacement curves during indentation loading. The microhardness of KDP crystals was also investigated on polished surfaces . Otherwise, scratching experiments have been widely conducted to study the removal properties of KDP crystals on the sample surface processed by polishing [15–18]. In contrast with polished surfaces, Zhang et al. [19,20] carried out nanoindentation and nanoscratching experiments on the cleaved (damage-free) surfaces of KDP crystals. By utilizing a mechanical stress cleavage method, a damage-free (001) surface can be obtained to conduct experiments. They found that KDP crystals on a cleaved surface without damage have different mechanical properties than KDP crystals on a machined surface. Hence, in summary, existing studies have focused on investigating the intrinsic mechanical properties of KDP crystals on polished and cleaved surfaces. However, the machined surface integrity of KDP crystals has not yet been assessed systematically based on insights regarding mechanical properties, which is a disadvantage when mitigating the laser-induced damage of KDP crystal components. In particular, industrial KDP crystal components are machined by ultra-precision fly-cutting techniques. However, insufficient characterization has been performed of the mechanical properties of the fly-cutting surfaces of KDP crystals.
This paper aims to study the mechanical properties on the machined surface/subsurface of KDP crystals after ultra-precision fly-cutting with the aid of nanoindentation, and to reveal the mechanisms behind these mechanical properties using an analysis of grazing incidence X-ray diffraction (GIXD).
2. Experimental methods
The KDP crystals used in this study were produced with a rapid growth technique at Shandong University in China . Material microstructure information was reported in the literature [22,23]. At room temperature, a KDP crystal has a tetragonal structure and a paraelectric phase with an I-42d space group [22,23]. KDP crystals have low hardness and are very brittle.
The KDP crystal samples are 50×10×13 mm3 in size. Ultra-precision fly-cutting experiments were conducted on the (112) plane, as shown in Fig. 1. Figure 2 shows that a KDP crystal workpiece is clamped with a vacuum chuck on a feed table. A single-point diamond tool with a tool rake angle γo of -45° was used to cut the surfaces of KDP crystals at a cutting speed (v) of 13.2 m/s. The experimental scheme for the ultra-precision fly-cutting was designed as presented in Table 1.
An interferometer (ZYGO’s Verifire XPZ) was used to measure the surface morphology of KDP crystals after ultra-precision fly-cutting. The microstructure of the machined surface/subsurface was detected using a GIXD on an Empyrean X-ray diffractometer (PANalytical, Netherlands) with ceramic tubes and Cu radiation at a wavelength of 0.154 nm. Next, nanoindentation experiments were conducted on the machine of Hysitron TI950. Indenting loads were set to 500 µN, 1000 µN, 2000 µN, 4000 µN, 6000 µN, and 8000 µN. The loading rate was 200 µN/s and the holding time was 5 s. The anisotropy of mechanical properties on the machined surface of KDP crystals was measured under an indenting load of 6000 µN, as shown in Fig. 3. The cleaved (damage-free) surfaces of KDP crystals were produced using the mechanical stress cleavage technology proposed by Zhang et al. [19,20]. Each experiment was repeated five times to obtain reliable data.
3. Results and discussion
3.1 Deformation behavior on machined surface/subsurface
To perform nanoindentation on machined surfaces, the morphologies of machined surfaces were detected with the aid of an interferometer. The surface roughness Ra measured in this experiment is below 13 nm, which according to Miller  is satisfactory to indent these surfaces. Figure 4 shows examples of machined surface morphologies. No cracks or damage are observed for the machined surfaces.
To study the mechanical properties for the machined surfaces of KDP crystals, corresponding nanoindentation experiments were conducted on the cleaved (damage-free) surface. Figure 5 shows that the loading and unloading slopes for machined surface are lower than those for cleaved (damage-free) surface. Additionally, the indented depth of the machined surface is larger than that of cleaved (damage-free) surface. The displacement of the elastic recovery on the machined surface is larger than that of cleaved (damage-free) surface. For example, the displacement of the elastic recovery is 115.16 nm on the machined surface for a depth of cut of 5 µm and feed rate of 60 µm/s, as shown in Fig. 5(b). However, the displacement of elastic recovery is only 93.63 nm on the cleaved surface. The difference may correlate with the residual stress induced by machining. In addition, pop-in events were observed in the load-displacement curves. More pop-in events were observed on machined surfaces than cleaved surfaces, because loading and unloading slopes reflect the plastic and elastic deformations of KDP crystals, respectively. Therefore, the above results indicate that the elastic-plastic deformation of KDP crystals is more easily produced on machined surfaces than cleaved (damage-free) surfaces, and that damage structures and residual stress may exist on the machined surface/subsurfaces.
3.2 Mechanical properties of machined surface/subsurfaces
Elastic modulus and hardness values are important indices for evaluating the elastic-plastic deformation of materials. Figures 6 and 7 show the relationships between the elastic modulus and hardness with load for the various machined surfaces of KDP crystals. The elastic modulus and hardness values on machined surfaces are lower than the values for cleaved (damage-free) surfaces, which is coincident with the load-displacement curves in Fig. 5. This result reveals that machined surface/subsurfaces more easily resist elastic-plastic deformation than cleaved surfaces. Lu et al.  measured the elastic modulus and hardness values of the polished surfaces of KDP crystals, which are also lower than those for damage-free cleaved surfaces, as shown in Figs. 6 and 7. In other words, the machined surface/subsurfaces of KDP crystals had lower mechanical properties than the original materials. It is also implied that for KDP crystals, it is not easy to obtain a damage-free surface via conventional polishing. It is interesting that the mechanical properties of the machined surfaces of metal materials are usually larger than the properties before machining (this is referred to as machining-hardening). In contrast, the conditions of KDP crystals are contrary to those of metal materials, which implies that the surface/subsurface microstructure of KDP crystals were changed. In addition, it can be found from Figs. 6 and 7 that the effects of depth of cut on the mechanical properties of the machined surface/subsurface of KDP crystals are larger than those of feed rate.
Figures 8 and 9 show the anisotropy of the elastic modulus and hardness for the machined and damage-free surfaces of KDP crystals. It is worth noting that the fluctuating amplitudes of elastic modulus and hardness were very small on the machined surface. For example, for the same machined surface, the difference between the maximum and minimum elastic modulus is about 5 GPa, and the difference between the maximum and minimum hardness is about 0.2 GPa. However, the fluctuating amplitudes of elastic modulus and hardness are significant for the damage-free surface. For example, the difference between the maximum and minimum elastic modulus is about 10 GPa, and the difference between the maximum and minimum hardness is about 1.5 GPa. These results indicate that the anisotropy of the elastic modulus and hardness of KDP crystals on machined surfaces is lower than the anisotropy of those values for cleaved surfaces.
3.3 Mechanism for mechanical properties of machined surface/subsurface
The degeneration of the mechanical properties of machined surface/subsurfaces will reduce the laser damage threshold of KDP crystal components. Therefore, it is necessary to investigate the microstructures in the machined surface/subsurfaces of KDP crystals to understand the mechanisms behind the mechanical properties.
For a KDP crystal, it is difficult to use conventional characterization methods (such as TEM) to detect subsurface microstructures, because of special physical properties of the material (softness, brittleness, deliquescence, and sensitivity to temperature). To detect the subsurface characteristics of KDP crystals after ultra-precision machining, a nondestructive detection method, called GIXD, is introduced in this paper . Figure 10 shows the GIXD results for machined surface/subsurfaces under depths of cut of 3 µm and 5 µm at a feed rate of 60 µm/s. The (200), (112), (312), (211), (220), (202), (301), and (310) peaks appear in diffraction patterns, where the diffraction intensity of the (112) peak is the highest. These peaks are identical to the powder diffraction file (PDF) of KDP crystals . However, Fig. 1 shows that only (112) diffraction peaks exist for KDP crystals before machining, which means that the material is a single crystal. Therefore, the diffraction peaks detected in Figs. 10(a) and 10(b) differ from the intrinsic structure of the KDP crystals. This indicates that the surface/subsurface structure of a single KDP crystal was damaged during ultra-precision fly-cutting, as shown in Fig. 11. The damaged structure exhibits dislocation and lattice misalignment [7,27], which significantly decreases the surface/subsurface mechanical properties of KDP crystals. This occurs because ionic, covalent, and hydrogen bonds control the mechanical properties of a KDP single crystal [28–31]. Li et al. [32–34] researched the deformation mechanisms of YAG and GGG single crystals at micro-scale with TEM, and reported that the single crystal structure controlled by ionic, covalent and hydrogen bonds will be destroyed under external stress. Therefore, the appearance of a damaged structure for the machined surface/subsurface of KDP crystals inevitably deteriorates the intrinsic mechanical properties of the material. As a result, the mechanical properties of the machined surface are lower than those of a damage-free surface.
This paper has investigated the effect of ultra-precision fly-cutting on the surface integrity of KDP crystals with the aid of nanoindentation. It has been discovered that the elastic-plastic deformation on a machined surface is easier than that on a damage-free surface. The elastic modulus and hardness of a machined surface are lower than those of a damage-free surface. Fly-cutting reduces the anisotropy of KDP’s properties. It has also been identified that the subsurface damage consists of dislocations and lattice misalignment. These findings will be the fundamentals required for improving the laser damage threshold of KDP crystal components.
National Natural Science Foundation of China (51875137, 51905356); Natural Science Foundation of Heilongjiang Province (E2018033); Australian Research Council (DP170100567).
The authors thank Gang Zeng for detecting the GIXD data in this study.
The authors declare no conflicts of interest.
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