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Article

Effect of H2O2 Treatment on Mechanical and Mechanochemical Properties of Fused Silica

by
Xinqi Liu
1,
Lingyu Yin
2,
Hongtu He
1,*,
Youze Ma
3,
Qiuju Zheng
3,
Laixi Sun
2,
Fang Wang
2,*,
Jiaxin Yu
1 and
Yong Cai
1,*
1
Key Laboratory of Testing Technology for Manufacturing Process in Ministry of Education, State Key Laboratory of Environment-Friendly Energy Materials, Southwest University of Science and Technology, Mianyang 621010, China
2
Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
3
School of Materials Science and Engineering, Qilu University of Technology, Jinan 250353, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2023, 13(13), 7636; https://doi.org/10.3390/app13137636
Submission received: 13 June 2023 / Revised: 24 June 2023 / Accepted: 26 June 2023 / Published: 28 June 2023
Browse Figures
Versions Notes

Abstract

:
The surface properties of fused silica (FS) change after H2O2 treatment, but the surface and subsurface damage behaviors and their mechanisms under various physical contact conditions have not been elucidated yet. This work investigated the effect of H2O2 treatment on mechanical and mechanochemical properties of FS surface. The results show that the hydrophilicity and adsorbed water film thickness of the FS surface increase with the concentration of H2O2 solution. The surface damage, nanowear, and subsurface deformation of FS caused by indentation increase with the concentration of H2O2 solution, while the nanohardness and reduced modulus decrease. Further analysis revealed that the water activity on the FS surface plays a critical role in reducing the mechanical and mechanochemical properties. In addition, the treatment with H2O2 solution on the FS surface shows a weakly corrosive effect, which implies the H2O2 treatment can be an alternative method to remove the surface defects on FS optics.

1. Introduction

Fused silica (FS) has excellent properties, such as high transmittance and high temperature resistance, as well as a low thermal expansion coefficient. As a result, FS has been widely used in high-power laser systems, the semiconductor industry, and other high-tech fields [1,2,3]. During precise machining, handling, transporting, and cleaning, and in the service process, the FS surface is inevitably subjected to physical contact, which causes surface damage. The surface damage is known to affect the performance and usage; for instance, the surface damage of FS caused by chemical mechanical polishing (CMP) can suppress the laser-induced damage threshold (LIDT, which is the maximum laser fluence that a material can withstand without exhibiting any permanent damage), limiting the service lifetime of FS optics used in high-fluence laser systems [4]. The subsurface damage caused by polishing and grinding, or revealed by subsequent cleaning methods, can also act as a precursor that can lead to surface damage under laser irradiation, limiting the material’s optical performance and working lifetime [5]. Thus, to improve the performance and lifetime of FS optics, it is of great importance to understand the damage mechanism of the FS surface under various contact conditions and thus find possible ways to enhance the surface properties of FS.
In humid air, water molecules impinging from the gas phase are inevitably adsorbed on the FS surface, which can potentially alter the physical and chemical properties of the FS surface. It is well-known that without physical contact, FS can react with dissociated OH groups in water to form hydroxylated groups [6]. With physical contact such as indentation and scratching, the surface damage of FS is known to be sensitive to the presence of water [7]. It was reported that the hardness of FS did not change with loading time when the indentation tests were conducted in water-free conditions, such as in toluene, acetonitrile, formamide, and hydrazine, but it decreased with loading time when the indentation tests were conducted in conditions involving water, such as in humid air and liquid water [8]. Further analyses suggested that the adsorbed water on the FS surface can diffuse into the FS subsurface during the indentation process, causing more damage on the FS surface via water-induced hydrolysis reactions of the Si-O-Si network under normal stress conditions [8]. The indentation tests not only showed that the reciprocating scratch (wear) resistance of the FS surface decreased along the surface’s normal direction with environment humidity, but that it also decreased along the tangential direction. This can be explained because the adsorbed water molecules impinging from the gas phase can facilitate the hydrolysis of the Si-O-Si network of FS under shear stress conditions [9].
Apart from the ambient environments conditions such as relative humidity (RH) and temperature, the water reactivity on the material surface also varies with surface hydrophilicity, which essentially alters the contact damage behavior. For instance, the nanowear of hydrophilic single crystalline silicon (100) was more serious than that of hydrophobic and pristine silicon (100) [10]. Moreover, with the increase in RH, the nanowear of hydrophilic silicon increased to a maximum value at ~50% RH and then decreased to no discernible wear in water [11], while the nanowear of hydrophobic silicon increased with RH and the maximum wear was found in water [12]. All these behaviors can be explained by the variation in the adsorbed water layer thickness on the silicon surface, which is dependent on the surface hydrophilicity and RH and influences the formation of interfacial Si-O-Si bonds at sliding interfaces [10,11,12].
When the surface groups of silica are mainly Si-O-Si bonds, the surface is hydrophobic, but when the coverage of Si-OH groups is sufficiently high, the surface is hydrophilic [13]. As silica reacts with H2O, the silica surface becomes hydroxylated where the Si-OH groups can be formed on the silica surface [14]. In addition to pure water, hydrogen peroxide (H2O2) can also react with the silica surface. Recently, it was reported that the nanoscratch resistance, nanohardness, and elastic modulus of the FS surface modified with a lower concentration H2O2 solution (5%) were weakened compared to those of pristine FS, and they were close to those of pristine FS when the concentration was higher (15%), which was attributed to the presence of a thin corroded layer on the surface [15]. This raises an important question about whether the surface hydroxylation and adsorbed water layer thickness may vary after treatment with various H2O2 solutions. However, the modification of the FS surface properties by H2O2 solutions, the further effects on the surface and subsurface damage behavior caused by indentation, and scratch testing have not been studied yet.
In this study, the FS surfaces were treated with various concentrations of H2O2 solution, and the degree of hydroxylation of the FS surface was revealed by the water contact angle, adhesion force, Raman, and sum frequency generation spectra measurements. The nano- and macromechanical properties of various FS surfaces were studied using nanoindentation and Vickers indentation, respectively. The nanowear behavior of various FS surfaces was studied with a diamond tip in humid air. The subsurface damage of various FS surfaces caused by indentation and scratching was revealed by sub-Tg annealing treatments. The obtained results show that the surface hydroxylation and the change in adsorbed water film thickness can significantly affect the mechanical and mechanochemical properties of the FS surface, suggesting H2O2 treatment could be a potential alternative method to modify the surface quality, and mechanical as well as mechanochemical properties, of FS optics used in high-energy laser systems. This research can be helpful for improving the surface quality and lifetime of FS surfaces for various applications.

2. Materials and Methods

FS samples with a thickness of 1 mm were selected as glass substrates. Prior to mechanical tests, the glass substrates were ultrasonically cleaned sequentially in acetone and ethanol solutions for 5 min, and finally rinsed with deionized water before drying with nitrogen gas. To prepare various concentration of H2O2 solutions, the H2O2 solution of 30 wt% (purchased from Chengdu cologne chemicals Co., Ltd., Chengdu, China) was mixed with pure water, and the final concentrations of H2O2 solutions were set as 0 wt%, 5 wt%, 10 wt%, and 20 wt%, where the 0 wt% was prepared as the deionized water only for comparison. The glass samples were immersed in various solutions for 3 h, and then taken out and dried with nitrogen gas for the subsequent experiments.
The chemical composition of treated glass samples was detected by Raman spectroscopy. Using a contact angle tester, the water contact angle of glass surfaces was detected in humid air (RH = 40%); all water contact angle tests were repeated independently at least five times to ensure the repeatability of the experiments. In order to compare the distribution of hydroxyl groups on various FS surfaces, vibrational sum frequency generation (SFG) spectroscopy was employed. The details of the experimental setup of the SFG spectrometer were described elsewhere [16]. Visible pulses (532 nm) and tunable IR pulses (2.5–10 µm) generated with an EKSPLA laser system were spatially and temporally overlapped at the glass surface. The incident angles of visible and IR pulses were 60° and 56°, respectively. The visible and IR pulse energies of the samples were 3 mJ and 1.5 mJ, respectively. The polarization combination for the spectra collected was s for SFG signal, s for visible beam, and p for IR beam (ssp). All the spectra were collected at room temperature with an RH of ~50%.
Nanoindentation tests were performed with a nanoindentation instrument having a Berkovich tip on a nanoindenter at ambient conditions of 22 °C and 40% RH. The nanoindentation tests were carried out with a continuous stiffness measurement, and the maximum penetration depth was set as 50 nm. More than 20 individual nanoindentation tests at diffident positions were carried out, and the average value and standard error were used to ensure the reproducibility of experiments results.
Using an atomic force microscope (AFM), the nanowear tests were conducted with a spherical diamond probe with a curvature radius of 0.54 μm and elastic constant of 254 N/m. During the whole tests, the room temperature and humidity were controlled at 22 °C and 40% RH, respectively. The applied normal load was 60 μN, the sliding speed and distance were 2 μm/s and 4 μm, respectively, and the reciprocating sliding cycles were set as 100. After the nanowear tests, the nanowear marks were in situ imaged with a silicon nitride tip with a radius of curvature of 10 nm in the tapping mode. The force constant of the tip cantilever was 20 N/m, and the scan area was 3 μm × 3 μm. In order to ensure the repeatability of the experiments, all the wear tests were repeated at least 5 times independently.
The Vickers indentation tests were performed using a Vickers hardness tester (HXD-1000TMC/LCD, Shanghai Taiming Optics Co., Ltd., Shanghai, China). The normal load was set as 0.1 N. The loading time, holding time, and unloading time were 5 s, 15 s, and 5 s, respectively. The room humidity and temperature were 40% RH and 22 °C, respectively. The Vickers hardness (HV) of various FS surfaces was calculated by [17]:
H V = ( 1.8544 ) ( 0.102 ) F d 2
where d is the diagonal length of the residual imprint indentation under a given applied normal load F. To reveal the indentation-induced subsurface damage of FS, all the glass samples after Vickers indentation tests were annealed at 0.9 × Tg (K) for 4 h, and the Vickers indentation marks were re-imaged with AFM. To ensure data reproducibility, all Vickers indentation tests under each condition were repeated at least five times independently.

3. Results and Discussion

3.1. Modified Adsorbed Water on Various FS Surfaces

Figure 1a shows the water contact angle of FS surfaces after treatment with various H2O2 solutions for 3 h. It was found that the water contact angle of pristine FS surface is ~66°, which is close to the value reported in previous publication [18]. Moreover, the water contact angle of FS surface does not change when treated with pure water, but it decreases with the concentration of the H2O2 solution, implying the FS surface becomes more hydrophilic when it is treated with higher concentration H2O2 solutions [19]. When treated with H2O2 solutions, the under-coordinated Si and Si-O− groups on FS surface can react with H2O2; as a result, more Si-OH groups are formed on the FS surface [15,20]. This can explain why the hydrophilicity of FS surface increases with the concentration of the H2O2 solution (Figure 1a).
Although the adhesion force (Fa) between the diamond tip and various FS surfaces is relatively low (tens of nN) compared to the applied force for nanowear tests (tens of μN), the adhesion force on the FS interface increases with the concentration of the H2O2 solution (Figure 1b). Generally, the adhesion force between a diamond tip and the FS surface involves the capillary force (Fc) and van der Waals forces (Fvdw), as well as the electrostatic forces and chemical bonding force [21]. Among these surface forces, only the capillary force and van der Waals forces vary with the thickness of the adsorbed water film in humid air, which can be a function of the water contact angle of the diamond tip and the FS surface [21]. Based on the proposed theory, with the decrease in the water contact angle of the FS surface, the thickness of the adsorbed water film on the FS surface increases from ~0.42 nm to ~0.52 nm; as a result, the capillary force increases and van der Waals forces decrease (Table 1). Since the adhesion force on the FS surface increases after treatment in H2O2 solution, the increased hydrophilicity and capillary force on the FS surface should be the main reason for the increased adhesion force (Figure 1b).
Figure 1c displays the Raman spectra of various FS surfaces. It was found that there are two main peaks centered at ~437 cm−1 and ~784 cm−1, which originate from the coupled vibrational modes of the silica random network [22]. The two relatively sharp bands centered at ~496 cm−1 (D1) and ~606 cm−1 (D2) are the typical bands of in-phase breathing motions of oxygen atoms in puckered four-membered and planar three-membered ring structures, respectively [23]. It can be seen that after treatment with various H2O2 solutions, there is no obvious structure evolution of the FS surface (Figure 1c). The peak from 3050 to 3600 cm−1 is a typical O-H stretch region [24], and the accurate peak position depends on the strength of hydrogen bonding. The peak at ~3700 cm−1 could be assigned to the free OH groups on the FS surface. Since there is no significant change in the range of 3100–3800 cm−1, no significant change in the adsorbed water structure on the FS surface occurs [24]. This finding seems to be inconsistent with the results in Figure 1a,b, which is due to the much deeper information (hundreds of nanometers) provided by Raman spectra [25].
Figure 1d shows the SFG spectra of various FS surfaces in 50% RH conditions. Specifically, the lower wavenumber centered at ~3200 cm−1 is typically attributed to a strongly hydrogen-bonded solid-like water, and the higher wavenumber centered at ~3400 cm−1 is attributed to a liquid-like water [26,27]. With the increase in H2O2 concentration, the band signal of 3000–3400 cm−1 gradually became stronger and wider, which can be explained by the increased water film thickness on the FS surface (Table 1) and the increased hydrophilicity (Figure 1a) [28]. In addition, the contribution of the lower wavenumber at ~3200 cm−1 increases with H2O2 concentration. This means that, after treatment with H2O2, the hydrogen-bonding interactions of adsorbed water on the FS surface increase [29]. This is consistent with previous publications where the contribution of the ~3200 cm−1 peak increases with the hydrophilicity of the FS surface [30].

3.2. Suppressed Mechanical Properties of FS Surfaces

Figure 2 displays the nanohardness and reduced modulus of various FS surfaces measured with nanoindentation tests. Note that the maximum penetration depth of the indenter into the FS surface is 50 nm, and the nanohardness and reduced modulus are estimated by the average value between the penetration depth of 30 nm and 50 nm, because the obtained values in the near-surface (<30 nm) are mainly caused by the indentation-size effect [31]. The nanohardness and reduced modulus of the pristine FS surface are ~10 GPa and ~71 GPa, respectively, which are close to the values in previous publications [32,33]. As the H2O2 concentration increases, the nanohardness and reduced modulus of the FS surface begin to decrease, to ~9.5 GPa and ~68 GPa, respectively, as the H2O2 concentration increases to 20%.
Not only are the nanomechanical properties of the FS surface affected, but also the macroscale mechanical response. The macroscale mechanical properties of the FS surface were analyzed with Vickers indentation, and AFM images of Vickers indentation imprints were obtained to further reveal the surface damage behavior of the FS surface, as shown in Figure 3. It was found that the Vickers hardness of the pristine FS surface is ~7.1 GPa, which is close to the hardness value of the original FS surface reported in the previous investigations [33,34]. Note that the Vickers hardness of the FS surface is much smaller than the nanohardness (Figure 2); the discrepancy between the two hardness measures must due to the difference in penetration depth with the two methods, where the penetration depth of the indenter is calculated to be ~0.8 µm under an applied load of 0.1 N, which is much deeper than that in nanoindentation tests (Figure 2).
As the concentration of the H2O2 solution increases from 0% to 20%, the residual indentation depth (volume) of the FS surface increases from ~0.37 µm (~1.2 × 103 µm3) to ~0.5 µm (~1.2 × 103 µm3) (Figure 3b,c), and the corresponding Vickers hardness of the FS surface decreases from ~7.0 GPa to ~6.2 GPa (Figure 3d). This result indicates that the Vickers hardness of the FS surface decreases with the H2O2 solution concentration, where the overall trend is similar to that in nanohardness (Figure 2). There are two possible reasons for the decrease in hardness of the FS surface after treatment in various H2O2 solutions. First, after treatment with H2O2 solutions, the FS surface becomes more hydrophilic and more water molecules can be adsorbed on the FS surface (Table 1); these water molecules are more susceptible to diffusing into the FS subsurface and facilitate the hydrolysis of the Si-O-Si network during the indentation process [8,35], causing more damage to the surface of FS, and thus reducing hardness (Figure 2 and Figure 3). Second, the H2O2 solutions can cause weak surface corrosion, which may lead to the reduction in Si-O bond strength of the FS surface [15]. As a result, the FS surface becomes more penetrable, and more surface damage of FS caused by indentation can occur, resulting in lower hardness. As the H2O2 concentration increases, these effects can induce more surface damage of FS following indentation; thus, the hardness and modulus of FS continue to decrease with increasing H2O2 concentration (Figure 2 and Figure 3).
Figure 4 shows the AFM image and corresponding cross-section of nanowear marks on various FS surfaces. As the H2O2 concentration increases from 0% to 20%, the depth of wear mark on the FS surface increases from ~9.7 nm to ~12.6 nm, and the corresponding wear volume of FS increases from ~1.2 × 103 nm3 to ~1.4 × 103 nm3, indicating an increase in the wear depth and volume of ~27% and ~16%, respectively (Figure 4). The friction force on the FS surface decreases slightly from ~6 µN to ~5 µN (Figure 4d), which is because as the H2O2 concentration increases, the water film thickness on the FS surface increases slightly (Table 1), and this water film can lubricate the sliding interface to some extent [36]. The decrease in friction force and the increase in wear volume with H2O2 concentration suggests the wear mechanism of the FS surface in humid air is not purely a mechanical effect, because a slightly lower wear depth and volume with a lower friction force would be expected with a higher H2O2 concentration if the mechanical effect is the predominate role; in contrast, the data presented in Figure 4 show that the wear depth and volume increase with the H2O2 concentration.
Numerous studies have demonstrated that the nanoscale wear of oxide glass in humid air is governed by mechanochemical reactions involving glass substrate, adsorbed water, and the counter-surface, where the formation and dissociation of interfacial bridging bonds at sliding interfaces play critical roles [36,37,38]. As the diamond tip slides over the FS surface in humid air, the C-O-Si interfacial bridging bonds can be formed [38]. As the FS surface is treated with H2O2 solutions, the FS surface becomes more hydrophilic; thus, there are several roles that can affect the nanowear performance. For example, as more water can be adsorbed on FS surface, the formation of C-O-Si interfacial bridging bonds becomes more difficult, and the nanowear of FS surface is weakened [38]. Alternatively, as more water molecules are adsorbed on the FS surface, it will facilitate the hydrolysis of the Si-O-Si network of the FS surface under shear stress [37,39], causing more nanowear of the FS surface. Moreover, H2O2 solutions can cause weak surface corrosion, which may lead to the reduction in the Si-O bond strength of the FS surface [15]. As a result, the nanowear of the FS surface would be facilitated. Based on the results in Figure 4, it seems that the latter two roles play more important roles since the nanowear of FS surfaces increases with the H2O2 concentration under the given sliding contact conditions (Figure 4).

3.3. Enhanced Subsurface Deformation of FS upon Indentation and Nanowear

Not only has surface deformation of FS been reported in previous studies, but also the indentation-induced subsurface deformation of FS [40,41]. To reveal the effect of H2O2 treatment on the subsurface deformation of various FS surfaces following Vickers indentation, these Vickers indentations were annealed at sub-Tg temperature for 4 h, and re-imaged again using AFM. The densified volume (Vd), shear flow volume (Vp), and densification ratio (or volume recovery ratio, VR) can be calculated as [40,41]:
V d = V i − − V a − + V a + − V i +
V p = V i − − V i − − V a − + V a + − V i + = V i − − V d
V R = V i − − V a − + V a + − V i + V i −
where the subscripts i and a indicate the initial volume and volume after annealing, and the superscripts − and + represent the indentation cavity volume and pile-up volume, with respect to the sample surface, respectively. It was found that as the H2O2 concentration increases from 0% to 20%, the corresponding Vd of the FS surface following indentation increases from ~0.8 × 106 nm3 to ~2.5 × 106 nm3, the shear flow volume Vp of the FS surface decreases from ~0.1 × 106 nm3 to ~0.08 × 106 nm3 (Figure 5c), and the VR of the FS surface increases from ~88% to ~96% (Figure 5d).
The friction-induced subsurface densification can also occur as the normal load is applied along the tangential shear direction [42,43]. Thus, following the same sub-Tg annealing treatments as used for the indentation marks, the nanowear marks on various FS surfaces were also annealed and the corresponding densified volume, shear flow volume, and volume recovery ratio were also calculated, as shown in Figure 6. It was found that as the H2O2 concentration increases from 0% to 20%, the corresponding Vd of the FS surface following nanowear increases from ~1 × 106 nm3 to ~1.4 × 106 nm3, the Vp of the FS surface decreases from ~0.25 × 106 nm3 to ~0.08 × 106 nm3 (Figure 6c), and the VR of the FS surface increases from ~80% to ~93% (Figure 6d).
Based on the results in Figure 5 and Figure 6, it is clear that regardless of the nanoindentation and nanowear tests, both the Vd and VR of the FS surface increase with H2O2 concentration, while the Vp decreases with H2O2 concentration, which indicates that the subsurface densification of the FS surface increases with H2O2 concentration. After treatment with H2O2, the FS surface becomes more hydrophilic (Figure 1), more adsorbed water on the FS surface can facilitate the surface contact damage caused by indentation, and the subsurface can undergo more stress since the penetration depth is deeper while the applied normal load remains the same (Figure 3); thus more subsurface densification of FS would be expected when treated with a higher concentration H2O2 solution (Figure 5). In the case of nanowear, however, the friction force at the sliding interface is decreased slightly (Figure 4), but the surface nanowear and subsurface densification of FS increases with the treated H2O2 concentration (Figure 4 and Figure 6). Since the shear stress that propagates into the subsurface is the direct source of the friction-induced subsurface densification [42], the stress that diffuses into the subsurface must increase. This finding implies that the magnitude of the friction force at the sliding interface is not a good descriptor for the surface nanowear and subsurface densification of glass caused by friction [44], because the friction force is dissipated at the sliding interface, while the surface nanowear and indentation damage of FS is governed by mechanochemical reactions [35,37,38] and the subsurface densification of FS is governed by stress that diffuses into the subsurface region [42,44]. This is consistent with the literature finding that the selective transfer phenomenon can effectively control dislocation movement and thus affects surface wear and damage [45]. In addition, the damage of the FS surface caused by indentation and nanowear increases with H2O2 solution (Figure 3 and Figure 4), accompanying the increased subsurface damage (Figure 5 and Figure 6). Similar trends of the evolution of surface damage with subsurface damage have also been found on borosilicate glass in scratching [46] and wear tests [44].
Generally, the pile-up formation around the nanoindentation or nanoscratch is identified as the direct evidence for the shear flow during the nanoindentation or nanoscratch processes [41]. In the present study, there is no pile-up around the Vickers indentation mark on the FS surface (Figure 3). However, a tiny pile-up formation can be found on the annealed indentation mark on the FS surface, indicating there is some shear flow during the indentation process (Figure 5). The calculated shear flow volume of various FS surfaces is not zero according to Yoshida’s methods (Figure 5). This means the presence of shear flow on the glass surface cannot be simply elucidated from the pile-up formation on pristine indentation topography [47].
The results obtained in the current study imply that even with surface hydroxylation by H2O2 solutions, the surface and subsurface damage of FS can be altered significantly. The nanohardness, nanowear, and subsurface deformation following nanoindentation and nanoscratching of the FS surface in ambient conditions are slightly different than those in previous publications [23,33,40,41], which may due to the slight difference in surface water adsorption on FS surfaces. As the surface chemistry of FS is slightly altered, the water film thickness adsorbed on the FS surface is changed (Table 1), and the contact damage of the FS surface following indentation and nanowear is changed significantly since the mechanochemical reactions involving water play a critical role in the surface damage behavior of the FS surface [8,9,35,36,37]. These findings indicate that the hydrophilicity of the FS surface after various treatments or cleaning procedures should be paid more attention because it can significantly affect the contact damage behaviors.

3.4. Suppressed Topography of Various FS Surfaces

Figure 7 shows the surface morphology and corresponding root mean square (RMS) roughness in a 3 × 3 μm2 scanning area of various FS surfaces. The roughness value of pristine FS is ~0.47 nm. With the increase in H2O2 concentration, the roughness value of the FS surface gradually decreases, and it reduces to ~0.35 nm as the H2O2 concentration increases to 20%, indicating a reduction in roughness of ~25%. Generally, the reduction in surface roughness is caused by the surface corrosion and reduction in surface defects [48,49,50]. Thus, based on the results from Figure 1 and Figure 7, the treatment with the H2O2 solution can not only affect the surface water adsorption and change the hydrophilicity of the FS surface, partial H2O2 may penetrate into the subsurface and corrode the FS surface to some extent. As a result, the tiny structural defects (scratches, cracks, and pits, etc.) originated from the precision process can be removed, causing the decrease in surface roughness.
Previously, it was reported that after traditionally grinding and polishing processes, a polishing redeposition layer with traceless photoactive impurities (such as ceria and iron) can exist on the top surface layer with a thickness of ~50 nm [51]. These surface defects can be migrated or removed by chemical etching [50], reactive ion etching [48], or a combined etching process [49]. The photoactive impurities at the near-surface region of FS can also be leached out of glass during the immersion in strong acid and/or H2O2 solutions [50]. If the traceless photoactive impurities ions are leached out of the FS surface, the atomic sites that were originally taken by impurity ions would be occupied by the diffused water molecules, the FS surface would become more susceptible to damage by indentation and nanoscratching, and the subsurface densification of FS would also become higher since the network on the top surface layer has more propensity to be compacted upon indentation and nanoscratching. Thus, the leaching of impurity ions on the top surface layer and the weak corrosion by H2O2 solution can also explain the increased surface damage and subsurface densification by nanoindentation and in nanowear tests (Figure 3, Figure 4, Figure 5 and Figure 6).
These findings also imply the H2O2 solution can be alternative method for removing the surface defects on FS optics. Previously, the chemical etching [50], reactive ion etching [48], or combined etching [49] methods have been used to remove the surface defects caused by traditional grinding and polishing processes, thus improving the LIDT and lifetime of FS optics. However, these methods are either hazardous for the environment and during handling operations, or difficult to operate or dependent on expensive equipment. In the present study, the surface roughness of FS was reduced using H2O2 solution to some extent and weak surface corrosion of FS occurred (Figure 7). This suggests H2O2 treatment could be a wet shallow etching method used to remove the surface defects and clean the surface of FS optics, which is environmentally friendly, easy to handle, and low cost. For example, the combination of reactive ion etching or annealing treatment with H2O2 treatment of fused silica may be helpful to improving the LIDT of fused silica optics used in high-energy laser systems. Although the data obtained in the present study indicate the mechanical and mechanochemical properties of FS surface after H2O2 treatment may degrade to some extent (Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6), the LIDT of FS optics is governed by the surface roughness as well as the concentration and/or distribution of photoactive impurities at the near-surface region [47,48,49,50], and no direction relationship between the LIDT of FS optics and its mechanical properties has been found [52,53]. Nevertheless, how the LIDT and lifetime of FS optics after H2O2 treatment will be improved remains to be determined, and this will be one of our research topics in future studies.

4. Conclusions

This work investigated the effect of H2O2 treatment on FS surface damage behavior under various physical contact conditions. The experimental results show that the hydrophilicity of the FS surface increases with the concentration of H2O2 solution, which increases the thickness of the adsorbed water film on the glass surface. With the increase in the concentration of the H2O2 solution, the surface damage and nanowear of FS caused by indentation increase. This is because the increase in the thickness of the adsorbed water film can facilitate the surface damage and nanowear by hydrolysis of the Si-O-Si network following indentation, thus causing the decrease in the nanohardness and modulus, as well as the increase in the wear volume of FS. The subsurface densification of FS is increased with the concentration of the H2O2 solution, which is because more stress is transferred to the subsurface regions of FS by indentation and during nanowear tests. Moreover, following the H2O2 solution treatment, the surface roughness of FS is decreased to some extent and weak surface corrosion of FS occurs. This suggests the H2O2 treatment could be a wet shallow etching method, which is environmentally friendly, easy to handle, and low cost, that can be used to clean the surface of FS optics in high-energy laser systems.

Author Contributions

Conceptualization, X.L., L.Y., H.H., Y.M., Q.Z., L.S., F.W., J.Y. and Y.C.; Methodology, X.L., L.Y. and Y.M.; Investigation, H.H., Q.Z., L.S., F.W., J.Y. and Y.C.; resources, H.H., F.W. and Y.C.; writing—original draft preparation, X.L., L.Y. and Y.M.; writing—review and editing, H.H., F.W. and Y.C.; supervision, project administration and funding acquisition, H.H., F.W. and Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

National Natural Science Foundation of China (Grant No. 51975492, 62175222 and 62005258) and Laser Fusion Research Center Funds for Young Talents (RCFPD3-2019-2).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Southwest University of Science and Technology, Research Center of Laser Fusion at China Academy of Engineering Physics, Qilu University of Technology for providing a professional research environment.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 07636 g001 550
Figure 1. (a) Water contact angle measured for various FS surfaces. (b) Adhesion force between the diamond tip and various FS surfaces in humid air. (c) Raman spectra and (d) SFG spectra of various FS surfaces.
Figure 1. (a) Water contact angle measured for various FS surfaces. (b) Adhesion force between the diamond tip and various FS surfaces in humid air. (c) Raman spectra and (d) SFG spectra of various FS surfaces.
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Figure 2. (a) Nanohardness and (b) reduced modulus of various FS surfaces.
Figure 2. (a) Nanohardness and (b) reduced modulus of various FS surfaces.
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Figure 3. (a) AFM images and (b) corresponding cross−section line profiles of Vickers indentation residual imprints on FS surfaces with an applied load of 0.1 N. (c) Residual indentation volume of various FS surfaces. (d) Vickers hardness of various FS surfaces.
Figure 3. (a) AFM images and (b) corresponding cross−section line profiles of Vickers indentation residual imprints on FS surfaces with an applied load of 0.1 N. (c) Residual indentation volume of various FS surfaces. (d) Vickers hardness of various FS surfaces.
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Figure 4. (a) AFM images and (b) corresponding cross−section line profiles of nanowear marks on FS surfaces. (c) Scratch depth and its wear volume of various FS surfaces. (d) Friction force during nanowear of various FS surfaces.
Figure 4. (a) AFM images and (b) corresponding cross−section line profiles of nanowear marks on FS surfaces. (c) Scratch depth and its wear volume of various FS surfaces. (d) Friction force during nanowear of various FS surfaces.
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Figure 5. (a) AFM image and (b) cross−section line profiles of various FS surfaces after sub-Tg annealing treatment (red line). (c) Densified volume and shear flow volume of various FS surfaces during Vickers indentation tests. (d) Recovery ratio of various FS surfaces.
Figure 5. (a) AFM image and (b) cross−section line profiles of various FS surfaces after sub-Tg annealing treatment (red line). (c) Densified volume and shear flow volume of various FS surfaces during Vickers indentation tests. (d) Recovery ratio of various FS surfaces.
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Figure 6. (a) AFM image and (b) cross−section line profiles of various FS surfaces after sub-Tg annealing treatment. (c) Densified volume and shear flow volume of various FS surfaces during nanowear tests. (d) Recovery ratio of various FS surfaces.
Figure 6. (a) AFM image and (b) cross−section line profiles of various FS surfaces after sub-Tg annealing treatment. (c) Densified volume and shear flow volume of various FS surfaces during nanowear tests. (d) Recovery ratio of various FS surfaces.
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Figure 7. (a) AFM surface topography and (b) RMS roughness value of various FS surfaces.
Figure 7. (a) AFM surface topography and (b) RMS roughness value of various FS surfaces.
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Table
Table 1. Comparison of the calculated and measured adhesion forces. Herein, the calculated adhesion forces were determined based on Ref [21]. The measured adhesion forces were tested on various FS surfaces before nanowear tests.
Table 1. Comparison of the calculated and measured adhesion forces. Herein, the calculated adhesion forces were determined based on Ref [21]. The measured adhesion forces were tested on various FS surfaces before nanowear tests.
SamplePristine0%5%10%20%
Water contact angle (°)6666636058
Thickness of water (nm)0.420.420.460.500.52
Fc (nN)59.0959.0970.8982.3189.70
Fvdw (nN)13.0313.0311.7910.8410.33
Calculated Fa (nN)72.1172.1182.6893.16100.03
Measured Fa (nN)78.0178.0189.56103.45112.03
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MDPI and ACS Style

Liu, X.; Yin, L.; He, H.; Ma, Y.; Zheng, Q.; Sun, L.; Wang, F.; Yu, J.; Cai, Y. Effect of H2O2 Treatment on Mechanical and Mechanochemical Properties of Fused Silica. Appl. Sci. 2023, 13, 7636. https://doi.org/10.3390/app13137636

AMA Style

Liu X, Yin L, He H, Ma Y, Zheng Q, Sun L, Wang F, Yu J, Cai Y. Effect of H2O2 Treatment on Mechanical and Mechanochemical Properties of Fused Silica. Applied Sciences. 2023; 13(13):7636. https://doi.org/10.3390/app13137636

Chicago/Turabian Style

Liu, Xinqi, Lingyu Yin, Hongtu He, Youze Ma, Qiuju Zheng, Laixi Sun, Fang Wang, Jiaxin Yu, and Yong Cai. 2023. "Effect of H2O2 Treatment on Mechanical and Mechanochemical Properties of Fused Silica" Applied Sciences 13, no. 13: 7636. https://doi.org/10.3390/app13137636

APA Style

Liu, X., Yin, L., He, H., Ma, Y., Zheng, Q., Sun, L., Wang, F., Yu, J., & Cai, Y. (2023). Effect of H2O2 Treatment on Mechanical and Mechanochemical Properties of Fused Silica. Applied Sciences, 13(13), 7636. https://doi.org/10.3390/app13137636

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