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Friction-induced subsurface densification of glass at contact stress far below indentation damage threshold
Acta Materialia 189 (2020) 166 173
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Friction-induced subsurface densification of glass at contact stress far below indentation damage threshold Hongtu Hea,b,*, Seung Ho Hahnc, Jiaxin Yua, Qian Qiaoa, Adri C.T. van Duinc, Seong H. Kimb,** a Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China b Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, PA 16802, USA c Department of Mechanical Engineering, Pennsylvania State University, PA 16802, USA
A R T I C L E
I N F O
Article History: Received 9 October 2019 Revised 5 February 2020 Accepted 4 March 2020 Available online 10 March 2020 Keywords: Borosilicate glass Wear Water Subsurface densification
A B S T R A C T
It is well known that the densification of oxide glass can occur at high contact pressure (typically >5 GPa) under normal indentation conditions. This study reports that when frictional shear is involved, the subsurface densification of glass can occur under low load conditions that would involve completely elastic deformation if the load is applied along the surface normal direction without any interfacial shear. This phenomenon was observed for a borosilicate glass rubbed with a smooth stainless-steel ball in liquid water at a nominal Hertzian contact pressure of 0.5 GPa. Under these frictional conditions, subsurface cracking is completely suppressed, and surface wear occurs through mechanochemical reactions. Since the mechanochemical wear track was sufficiently smooth, it was possible to employ a sub-glass transition temperature (sub-Tg) annealing method to measure the volume recovery of the densified subsurface region. The frictioninduced subsurface densification of the wear track was also confirmed through nanoindentation measurements and dissolution tests in pH 13 aqueous solutions. Molecular dynamics (MD) simulations with a ReaxFF reactive force also suggested that the subsurface structural change can occur readily when friction is involved at low contact pressure conditions. © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
1. Introductions The surface and subsurface damage of silicate glasses have detrimental effects on their mechanical and chemical durability [1 3]. Such damages are often caused by the physical contact with foreign objects. The effects of physical contacts are typically studied in two test modes: (i) indentation or impact along the surface normal direction without tangential shear and (ii) interfacial shear with a counter-surface applying a compressive stress along the surface normal direction and moving along the tangential direction of the surface. The former generally generates a quasi-isostatic stress field in the glass, while the latter produces more complex stress fields due to interfacial friction [4,5]. During their manufacturing and functional operations, glass surfaces are inevitably subject to not only indentations or impacts, but also various frictional contacts [6,7]. Thus, it is critically needed to understand the surface damage mechanisms of silicate glass under interfacial shear conditions. * Corresponding author at: Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China. ** Corresponding author at: Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, PA 16802, USA. E-mail addresses: [email protected] (H. He), [email protected] (S.H. Kim). https://doi.org/10.1016/j.actamat.2020.03.005 1359-6454/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
A majority of mechanical studies in the literature have focused extensively on the surface damage of glasses under normal indentation conditions [8,9]. It is well known that when a high contact load (pressure) is applied with an indenter beyond the elastic deformation regime, glass undergoes two competing plastic mechanisms densification (or structural compaction) and shear flow before eventually cracking or fracture occurs [10]. From the hydrostatic pressure experiments, it was found that the plastic densification of silica glass starts above a threshold value of 10 GPa and saturates at a level of Dr/r0 = 20% (where r is density) above a pressure level of 20 25 GPa; for soda lime silicate glass, the threshold is about 8 GPa and the density change is about 6.3% [11,12]. As a result, the density, refractive index, elastic modulus and hardness of glass also increase [13 15]. Through characterizations of the densified regions with various techniques such as IR [16], Raman [17], and small-angle X-ray scattering (SAXS) [18], it was shown that such changes are accompanied with structural alterations. The local structural change can also substantially increase the dissolution rate of the densified zone compared to that of pristine glass [19]. The densification process can be reverted by thermal annealing at a temperature slightly below the glass transition (Tg) temperature typically, at 0.9 £ Tg for 2 h. The sub-Tg annealing results in volume recovery at the indentation site [20].
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The mechanical scratch test can be performed by moving the glass sample or the indenter laterally while the glass is compressed with the indenter; this introduces tangential shear stress to the indentation region. In a nanoscratch test using a diamond tip on fused silica, borosilicate glass, and phosphate glass, it was found that the scratch mark becomes shallower after the sub-Tg annealing treatment, suggesting the occurrence of the subsurface densification of the scratched region [21]. The extent of densification varies with the applied load, sliding cycles, and humidity conditions [22]. When comparing the nanoindentation and nanoscratch results of silica glass, borosilicate glass, and soda lime silicate glass, it was found that the volume recovery ratios of these glasses are very similar in both test conditions [23]. During the mechanical scratch test, the contact pressure under the nanoindenter tip is extremely high usually >5 GPa; comparable to or higher than its indentation damage threshold (yield strength and/ or hardness) [8,9]. It is questionable if the findings from mechanical tests under such a high contact pressure pertain to numerous frictional contacts made by foreign objects at low contact pressure conditions far below its indentation damage threshold. When the contact pressure is low enough to induce elastic deformation only in the contact region, the glass surface does not get damaged as long as the counter-surface applying the stress moves along the surface normal direction without tangential shear. However, when subjected to interfacial friction, the glass surface can undergo severe mechanical damage in the dry condition, leaving deep and rough wear scars, probably due to interfacial bonding of two surfaces in contact [24,25]. In humid environments, such mechanical damages are suppressed; instead, shear-induced mechanochemical (often called ‘tribochemical’) reactions can occur [26 30]. The chemical reactions occurring in humid conditions involve water molecules adsorbed on the surface and the end result is often the wear of one of the surfaces either the glass itself or the counter-surface or both depending on the applied load, speed, and humidity as well as the glass and counter-surface compositions [26 29]. The mechanochemical wear scar is topographically smooth and closely follows the evolution of the topographic shape of the counter-surface [24,31]. Thus, it was believed that mechanochemical reactions leading to the surface wear take place mostly at the sliding interface where the adsorbed molecules are being sheared by two solid surfaces. Here, an important question that has not been addressed yet is whether subsurface damage can be made while the topmost surface of glass undergoes shear-induced mechanochemical wear reactions in the elastic contact pressure regime. In the case of crystalline silicon surfaces with a nanoscale contact, the subsurface damage is found to be negligible [32]; however, a systematic study has not been done for silicate glass surfaces. This paper focuses on addressing the question of subsurface densification of glass under frictional shear at the contact stress far below its indentation damage threshold of glass. To study this question, a mechanochemical wear track was first generated on a borosilicate glass surface by rubbing with a smooth stainless-steel sphere in liquid water at a nominal Hertzian contact pressure of 0.3 0.5 GPa which is an order of magnitude lower than the indentation hardness and yield strength of the glass. The liquid water environment was chosen because our previous studies showed that it is efficient to prevent any subsurface cracking during the wear tests [26,33]. Then, we investigated the subsurface densification via (i) volume recovery of the wear track upon subTg annealing, (ii) nanoindentation inside the wear track, (iii) chemical etching of the wear track with pH 13 aqueous solution. All these test results indicated that the subsurface densification indeed occurs during the mechanochemical wear process even though the contract stress is in the elastic deformation regime. To support this conclusion from experimental observations, we conducted molecular dynamics (MD) simulations with a ReaxFF
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reactive force field to show the subsurface structural change of a sodium silicate glass during the interfacial shear and thermal annealing. 2. Materials and methods The polished borosilicate glass slide (Schott N-BK7Ò optical glass, purchased from Hefei Kejing Materials Technology Co., Ltd) with a size of 20 mm £ 20 mm £ 2 mm was used as a substrate in the present study. The physical and chemical properties of this borosilicate glass substrate can be found elsewhere [26]. The AISI 316 stainless steel (SS) ball with a diameter of 4 mm was used as a counter-surface for tribological tests. Using a white light scanning profilometry (MFT3000, Rtec, San Jose, CA), the root mean square roughness of the SS ball was measured to be ~11 nm over a 400 mm £ 400 mm area. Using a ball-on-flat reciprocating tribometer (MFT-3000, Rtec, San Jose, CA), friction and wear of the borosilicate glass substrates were tested in deionized (DI) water. Prior to the friction test, both glass and ball surfaces were cleaned by rinsing with ethanol and then water followed by blow-drying with dry nitrogen gas [34]. The sliding speed was 4 mm/s, the length of the reciprocating track was 5 mm, and the total sliding time was 3 min. The normal load during the surface wear test was 1, 2, and 3 N, which corresponds to the nominal Hertzian contact pressure of ~0.35 GPa, ~0.44 GPa, and ~0.52 GPa (on the flat surface before wear), respectively. All wear tests were repeated independently at least 5 times. After wear tests, the wear track of borosilicate glass substrates and the 316-SS ball surfaces were analyzed with optical microscopy (BX51-P, Olympus, Japan) and white light scanning profilometry (MFT-3000, Rtec, San Jose, CA). The nanomechanical properties of the wear-tested surface (inside the wear track) and the original glass surface (outside the wear track) were analyzed using a nanoindenter (G200, Agilent Technologies, Inc., Santa Clara, CA). The reduced modulus and hardness were calculated from force-displacement curves with the Oliver-Pharr model [35]. Then, the wear-tested samples were annealed for 2 h at 520 °C ( 0.9 Tg) [36] and then re-imaged using profilometry. For the simplicity, the wear track before and after the sub-Tg annealing treatment will be referred as ‘pristine’ and ‘annealed’, respectively, hereafter. The pristine and annealed samples were placed in separate Teflon containers filled with 50 mL of 0.1 M NaOH (pH 13) solution at room temperature. Individual samples were retrieved at 0.2, 0.5, 1, 2, and 3 h and rinsed with DI water and blow-dried with nitrogen gas prior to profilometry observations. In order to investigate the subsurface structural changes during the mechanochemical wear, we carried out MD simulations with ReaxFF reactive force fields. Since the ReaxFF-MD model for the sodium silicate glass have been developed sufficiently in recent studies [37,38], we selected the sodium silicate glass (SiO2:Na2O = 70:30 mol%, 3000 atom-system) as a substrate material, and the amorphous silica as the counter surface. Currently, the N-BK7 glass composition cannot be simulated with ReaxFF-MD because the ReaxFF force fields for boron, potassium, and barium are not available. Fig. 1a shows the snapshot of the MD simulation trajectory obtained from the previous mechanochemical wear study [38]. The simulation within the ReaxFF-MD framework were performed through: (i) vertical compression of counter surface into the substrate, (ii) equilibration of the system under normal load, (iii) applying lateral movement to counter surface to induce the shear stress at the interface, and (iv) vertical separation. The contact area of the slab-on-slab geometry system was 11.86 nm2, where the contact pressure of 0.7 GPa was set to applied throughout the equilibration and sliding processes. Note that the uppermost 8 A-thick portion of silica counter surface was defined as the movable rigid part and the constant velocity vector components of v = (vx, vy, vz) = (0, 0, 0.0002), (0.0001, 0, 0), and (0, 0, 0.0002) [A/fs] (£ 105 [m/s]) were applied to this region for the (i), (iii) and (iv) processes, respectively. In this work, we extracted only the
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Fig. 2. (a) Optical profilometry image and (b) line profile of the borosilicate surface after the normal indentation with the 316-SS ball without shear. The applied load was 3 N. In (a), the dashed circle is drawn based on the calculated Hertzian contact diameter at the position where the indentation test was performed.
Fig. 1. (a) Snapshots from the ReaxFF-MD simulations of silica and sodium silicate glass separated after sliding at ~0.7 GPa for 340 ps in the presence on interfacial water molecules at 300 K. Details of simulations can be found in Ref. [38]. Note that the atoms are color-coded based on whether they are originally parts of silica (red and yellow) or sodium silicate (gray and blue) prior to the sliding. (b) Snapshots of the sodium silicate glass substrate of (a) after annealing at 765 K for 250 ps. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sodium silicate substrate from the MD trajectory after sliding (the bottom slab in Fig. 1a) and employed it as the initial configuration of our MD simulation. The initial system was first equilibrated under NVT (constant number of atoms, system volume, and temperature) ensemble at 300 K for 100 ps. The trajectories obtained during this equilibration process were used to analyze the glass network structures (Si-O bond length and Si-O-Si bond angle), which represent the structural data immediately after the mechanochemical wear test. Following the equilibration step, we gradually ramped up the temperature of the system to investigate the effect of annealing to the substrate material. From 300 K, the system temperature was increased at a rate of 5 K/ps until it reached the 90% of Tg determined through MD simulations. At this temperature, the system was further equilibrated for 250 ps and cooled back to 300 K at the same rate. During the annealing process, the periodicity of the system was maintained within the xy-plane while the z direction was set to be non-periodic to allow any adsorbed water molecules on the sodium silicate surface to evaporate during the annealing process. After the system reached back to 300 K, NVT equilibration for another 100 ps was carried out to extract the structural data of the silicate glasses after annealing (Fig. 1b). For all ReaxFF-MD simulations carried out to produce the trajectories for structural analysis, an integration time step of 0.25 fs was used for the NVT ensemble with a Nose-Hoover thermostat (t t=100 fs). ReaxFF reactive potential for Na/Si/O/H description was used within the LAMMPS package and the details on how the initial configuration was obtained from the mechanochemical wear are further elaborated in the Refs. [37,38]. 3. Results and discussion 3.1. Elastic deformation of glass under normal indentation without frictional shear Fig. 2 shows a typical optical microscopy image and line profile of the borosilicate glass surface after compressing with the 316-SS ball in liquid water for 30 s. The applied load was 3 N, which corresponds to the nominal contact pressure of ~520 MPa according to the Hertzian contact theory. No discernible indentation damage can be found within the resolution limit of the optical profilometry. Based on the general relationship between strength and hardness [39,40], the yield strength of borosilicate glass can be estimated as ~2 GPa, which is
about four times larger than the maximum contact pressure used in this study (~0.52 GPa). The results in Fig. 2 confirms that the borosilicate glass surface indented with the 316-SS ball deforms elastically under the contact pressure of 0.52 GPa and fully recovers when the ball is removed without any shear during the indentation. The wear data described in the following sections were obtained at the same or lower contact pressure condition, except that the frictional shear was introduced by sliding the ball on the surface. 3.2. Volume recovery of mechanochemical wear track by sub-Tg annealing Fig. 3a 3c shows the representative optical profilometry images of borosilicate glass surface after wear tests at an applied load of 1 N, 2 N, and 3 N, respectively, in DI water. Although no discernable surface damage is observed without tangential shear of the 316-SS ball on the borosilicate glass surface (Fig. 2), there are noticeable wear marks (material removal) at the same contact stress if the ball is sheared against the glass surface. The wear depth and volume, shown in Fig. 3d and 3e, respectively, increase with the applied load. When the wear region is cut by cracking from the back side of the sample, the cross-sectional image shows no sign of subsurface cracks during the wear test (Fig. S1 in Supporting Information). The subsurface cracking occurs during the wear test in humid air at the same normal load and sliding speed conditions (Fig. S2 in Supporting Information). This supports that in liquid water, wear occurs mainly via mechanochemical reactions [7,24,26]. There is no periodic ripple discernable beyond random topographic features smaller than 2 nm from peak to peak (Fig S3 in Supporting Information). In order to check if there is any subsurface densification in the mechanochemical wear track of borosilicate glass formed while the surface is compressed and sheared at a load that would cause only elastic deformation in the normal indentation test, the wear track was treated with the sub-Tg annealing process [9,20]. Fig. 3d and 3e compare the wear line profile and volume, respectively, of the annealed wear track of borosilicate glass with those of the pristine wear track. After the subsequent sub-Tg annealing, the volume of wear track is smaller compared to the pristine case. The volume recovery was calculated from the difference of the wear volume between the pristine wear track (Vi) and annealed wear track (Va) and plotted in Fig. 3f. The recovered volume upon sub-Tg annealing is about 9.3%, 15.6%, and 26.3% of the pristine wear volume for the applied load of 1 N, 2 N, and 3 N, respectively. Recall that the nominal Hertzian contact pressure is 0.3 ~ 0.5 GPa, which is about one order of magnitude lower than the indentation damage threshold of borosilicate glass. The volume recovery upon sub-Tg annealing could be related to the densification ratio [12]. The larger recovery of the damaged volume after sub-Tg annealing could mean the larger degree of subsurface densification [41,42]. Note that the densification of silicate
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Fig. 3. The optical profilometry images of borosilicate glass substrate after wear tests under an applied load of (a) 1 N, (b) 2 N, and (c) 3 N. (d) Characteristic line profiles of wear tracks before (black line) and after (red line) sub-Tg annealing treatment. The inset shows the magnification of the blue dotted lines region in (d). (e) Volume of the pristine and annealed wear tracks formed under various applied load conditions. (f) Volume recovery ratio of the wear track upon the sub-Tg annealing treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
glasses during Vickers indentation occurs typically under a contact pressure higher than 8 GPa [11,12]. For the indentation of soda lime silica glass with a smooth ball, the densification in the subsurface occurs when the contact pressure is higher than 7 GPa [43]. Thus, it is surprising and intriguing to see the densification underneath the wear track of borosilicate glass in the low compressive pressure condition that would induce elastic deformation only under the normal indentation without tangential shear. 3.3. Hardness and reduced modulus of mechanochemical wear track When the densification of silicate glass occurs under normal indentation tests or hydrostatic pressure tests, the atomic packing density of densified glass increases, which is accompanied with an increase in elastic modulus and hardness [44]. We have tested if a similar phenomenon is observed by measuring the modulus and hardness of the wear track produced at the low contact stress condition. Fig. 4a and 4b compare the hardness (H) and reduced modulus (Er) of the damaged and pristine borosilicate glass substrate by nanoindentation tests, respectively. Since the Poisson’s ratio (v) may vary
with the degree of densification and could not be determined independently [45], Fig. 4b shows the value of reduced modulus (Er) of the tested region rather than the Young’s modulus (E). The H and Er of the region outside the wear track are ~5.9 GPa and ~80 GPa, respectively, which are close to the literature value of N-BK7 borosilicate glass [26]. The H and Er of the wear track gradually increase as the applied load for the wear test increases from 1 N to 3 N. This is consistent with the results in Fig. 3f showing the increase of the subsurface densification predicted from the volume recovery after subTg annealing with the applied load [11,46]. 3.4. Dissolution of mechanochemical wear track in basic solution The structural deformation in the densified region of silicate glass formed under normal indentation experiments could make that region more susceptible to aqueous corrosion [13,15,19]. In order to test if the same phenomenon can be observed, the pristine and annealed wear tracks were immersed in 0.1 M NaOH solution for varying times and then analyzed with the optical profilometry (Fig. 5a). Fig. 5b plots the change in volume of the wear track as a
Fig. 4. (a) Nanohardness and (b) reduced modulus of the wear track formed in liquid water. For comparison, the data of the outside the wear track is also shown. The penetration depth during the nanoindentation measurement was 300 nm.
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Fig. 5. (a) Evolution of wear track of borosilicate surface as a function of corrosion time in 0.1 M NaOH for the pristine wear track and annealed wear track. The pristine wear track was made at the applied load of 3 N in DI water. (b) Evolution of the corresponding wear volume as a function of corrosion time. (c) Preferentially dissolved volume of the pristine and annealed wear tracks compared to the outside the wear track. The data for wear tracks made with the applied load of 1 N and 2 N can be found in Supporting Information (Fig. S4).
function of corrosion time for various applied load conditions. At this high pH condition, the as-received surface can also dissolve. The increase in the wear track volume means the enhanced dissolution rate of the subsurface densified region compared to the original surface. The amount of dissolution volume enhanced by the subsurface densification is plotted in Fig. 5c. It can be seen that the pristine wear track exhibits significant enhancement in dissolution in pH 13 aqueous solution compared to the original surface outside the wear track; in contrast, the sub-Tg annealed wear track show only marginal increase in the dissolution compared to the surface outside the wear track. The data in Fig. 5c is congruent with the fact that the sub-Tg annealing can revert the densified subsurface region to nearly back to the original state (although not complete). In the current study, it is difficult to attribute the difference in the dissolution volume between the pristine and annealed wear tracks exclusively to the volume recovery effect of the densified subsurface and rule out any residual stress effect. It is known that the stress can affect the onset and rate of hydrolysis reactions [47,48]. Previously, it was reported that under alcohol vapor phase lubrication conditions, no visually discernible wear track of soda lime silica glass can be detected [49]. However, after the subsequent hydrothermal treatment at 150 °C for 24 h, the friction-tested region was preferentially etched, leaving a ~5 nm deep trench [49]. Because there was no topographic depression of the friction-tested region in the alcohol vapor lubrication condition, there was no reason to suspect the subsurface densification in that study. Thus, the preferential etching by hot steam was attributed to the subsurface residual stress after the friction test. If the residual stress exists in the densified subsurface region of the pristine wear track, it would be relaxed and relieved during the volume recovery process at the temperature near Tg.
bond length distribution are in good agreement with the indentation induced densification on oxide glass surface [8,9,50]. To simulate the sub-Tg annealing treatment, the Tg of sodium silicate glass is determined by ReaxFF-MD simulations, as shown in Fig. 6b. The ReaxFF-Tg of sodium silicate glass can be estimated as ~850 K. Thus, based on the ReaxFF-Tg, the annealing treatment of sodium silicate glass was conducted at 765 K (0.9 £ ReaxFF-Tg). After sub-Tg annealing, the density of the subsurface region decreases to 2.495 g/cm3 (Fig. 6c) and the probability of lower and higher values in the Si-O-Si bond angle distribution decreases (Fig. 6e). The change of Si-O bond length distribution is marginal (Fig. 6d). The decrease of density and the narrowing of the Si-O-Si bond angle distribution suggest the structural relaxation (or recovery) to some extent with the sub-ReaxFF-Tg annealing. Note that the degree of the structural relaxation of sodium silicate glass depends on the annealing condition. For example, based on the MD simulations with the Teter potential, the Teter-Tg of sodium silicate glass is estimated as ~1166 K, and the structural relaxation of the friction-tested subsurface region becomes larger when the glass is annealed at higher annealing temperature (1050 K, 0.9 £ Teter-Tg). More details about the Si-O bond length and Si-O-Si bond angle distribution after annealing at 1050 K can be found in Supporting Information (Fig. S6). Overall, the ReaxFF-MD results in Fig. 6 support that mechanochemical wear even at the contact stress an order of magnitude lower than the indentation damage threshold can induce the subsurface densification of glass and the densification can be reverted to some extent via subsequent annealing treatment.
3.5. Subsurface structure changes of glass by ReaxFF-MD simulations
The striking contrast between the elastic behavior in the normal indentation test (Fig. 2) and the surface damage after the tangential shear at the same normal load (Fig. 3) clearly suggests that the subsurface damage or deformation mode is quite different depending on whether interfacial shear is involved at the contact point. Even though the mechanical cracking during the shear is prevented by running the friction test in DI water (Fig. S1 and S2), the shear process is accompanied by not only the mechanochemical reactions but also the friction at the sliding interface (Fig. S7). In the past, both mechanochemical reactions and friction are thought to involve the atoms or species exposed at the shearing interface if the applied contact pressure is in the elastic deformation regime. But, the increase in nanoindentation hardness and modulus (Fig. 4) as well as enhanced dissolution in the basic solution (Fig. 5), along with the volume recovery of the wear track after sub-Tg annealing (Fig. 3f), clearly indicate that subsurface densification does occur when the contact interface is sheared even at this light load condition. ReaxFF-MD simulations
ReaxFF MD simulations were carried out to test the subsurface structural changes of glass upon mechanochemical wear at the atomistic level. There is no stick-slip behavior discernable beyond random fluctuations in the simulated friction force (Fig. S5 in Supporting Information). Note that for all the structural analysis from the ReaxFF-MD results, we defined the subsurface region as the 15Athickness layer underneath the surface with the periodicity maintained in the xy-plane, as shown in Fig. 6a, where the penetration of proton and water is negligible [37,38]. The simulation results show that after friction tests, the density of the subsurface region increases from 2.472 g/cm3 to 2.544 g/cm3 (Fig. 6c), implying a densification of ~3% under the given condition. Additionally, after friction tests, the Si-O bond length distribution shifts to a higher value (Fig. 6d), while the Si-O-Si bond angle distribution shifts slightly to a lower value (Fig. 6e). The trend of the variation of Si-O-Si bond angle and Si-O
3.6. Friction-induced subsurface densification even in the elastic contact stress condition
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Fig. 6. (a) The schematics illustrates the subsurface region for structural analysis. (b) Determination of Tg of sodium silicate glass based on ReaxFF-MD simulations. The variation of (c) density, (d) Si-O bond distance, and (e) Si-O-Si bond angle distribution of the subsurface region in sodium silicate glass upon wear and subsequent annealing treatment. Note that the bond length analysis was normalized in order to clearly capture the bond elongation behavior with respect to the maximum value.
also support the subsurface structural damages (Fig. 6). This is schematically summarized in Fig. 7. In normal indentation experiments, the subsurface damage (densification and/or pile-up) is observed when the compressive stress applied by the indenter tip is higher than indentation damage threshold (yield strength and hardness) [12,42]. The pile-up is mainly caused by the plastic flow of subsurface materials to the direction parallel to the indenter surface [8,9,12,51]. Such flow of material is facilitated by forming shear bands in the subsurface region under the indenter [8,9]. In our experimental conditions, there is no pile-up at the periphery of the slide tract discernable within the resolution of white light scanning profilometry (see the inset in Fig. 3a). This is another evidence supporting that the subsurface densification observed in the wear track is not due to normal indentation, but it must be due to frictional shear.
When the topmost surface atoms at the interface are sheared by the counter-surface, the atoms beneath that layer will certainly feel the shear stress along the sliding direction because they are covalently bonded to the topmost surface atoms. This shear stress caused by friction might be responsible for the subsurface densification. When the shear stress is increased, the degree of subsurface densification also increases (Figs. 3 6). How deep this friction-induced densification propagates into the subsurface region will depend on the glass network structure (composition) as well as the magnitude of friction and the availability of water molecules. If water molecules diffuse into the subsurface and its rate is enhanced by the applied stress [52], then it could drastically affect the friction-induced densification process. Previously, the effect of bulk shear stress on densification of glass have been reported at extremely high shear stress (>4 GPa) [53]. The
Fig. 7. Schematic illustration of the friction-induced densification of borosilicate glass at the load condition that induces only elastic deformation when no interfacial shear is involved.
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Fig. 8. Snapshots from MD simulations showing the formation, elongation, and dissociation of interfacial Si-O-Si bonds during the friction process of sodium silicate glass. Note that the sodium ions are excluded for brevity of the image.
fact that shear can enhance densification of the glass affected during the compression is the same; but the mechanism proposed here for the friction case is different from the one proposed in the previous literature [53]. In the bulk shear case, the proposed mechanism considers the possibility of chain entanglements that can reduce the reversible recovery after compressive stress is removed; this phenomenon was observed at extremely high shear stress (>4 GPa) [53]. In the interfacial shear (friction) case, it is known that interfacial covalent bonds can be formed by bridging two solid surfaces moving opposite directions (Fig. 8) [54], which facilitates the transfer of mechanical energy at the shear plane into the subsurface region. Our experimental and computational results indicate that the energy transfer mediated by the interfacial bond formation is substantially large to induce significant wear in the absence of any interfacial water [24,25]. The presence of water at the interface appears to be sufficient to mitigate mechanochemical wear, but it is not enough to prevent subsurface damage (which was manifested as ‘densification’). It was recently reported that the nanometer scale structural inhomogeneity can facilitate local deformation of glass [55,56]. Based on the calculation of mean squared displacement (MSD) in MD simulations [57], we have calculated the dynamic propensity of individual atoms before and after shearing. As shown in Fig. S8 in Supporting Information, some degree of lateral inhomogeneity in the dynamic propensity can be seen in the subsurface region before the interfacial shear; after the interfacial friction, the propensity distribution becomes more homogeneous. On the basis of the MSD calculation, the overall propensities of sodium and silicon atoms are decreased after the friction test. This result may suggest that the densification might be coupled with the decrease in subsurface structural inhomogeneity. The finding of this study implicates that classical contact mechanics theories cannot fully explain the tribological behaviors of silicate glass. Based on the Hertzian contact theory, no material loss is expected if the contact deformation is elastic [58]. However, our data clearly show that even if the contact deformation is purely elastic in the absence of frictional shear, the material loss and subsurface densification can still occur under interfacial shear conditions. Previously, it was shown that chemical strengthening through the exchange of Na+ ions with K+ ions can increase the hardness, modulus, indentation fracture toughness, and crack initiation load of the aluminosilicate glass and soda lime silica glass; but the wear resistance at high humidity conditions is deteriorated [59,60]. These previous reports, along with the data presented in the current study, imply that the ability to resist plastic deformation in the normal indentation condition may not be a good descriptor for the resistance against wear and subsurface damage in tribological conditions which involve frictional shear of the interfaces [61]. 4. Conclusions The friction induced subsurface densification of borosilicate glass has been studied by rubbing with a smooth stainless-steel ball in liquid water at a nominal Hertzian contact pressure of 0.5 GPa. The
results show that the deformation of glass is completely elastic under such a low normal stress without tangential shear. When the frictional shear is involved, surface wear of glass materials occurs through mechanochemical reactions, and the wear tracks become shallower compared to the pristine wear track after the subsequent sub-Tg annealing, suggesting the subsurface densification of borosilicate glass can occur. The increase in nanoindentation hardness and reduced modulus, enhanced dissolution in the basic solution, and the change of the subsurface Si-O-Si bond angle and Si-O bond length distributions after friction tests and subsequent annealing treatment revealed by ReaxFF-MD simulations further support the occurrence of subsurface densification of glass. These results suggest that the friction-induced subsurface densification of oxide glass can occur during the mechanochemical wear process even though the contact stress is far below its indentation damage threshold. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 51605401 and 51575462), Scientific Research Fund of Sichuan Provincial Education Department (17ZA0408), and the National Science Foundation of the United States of America (Grant no. DMR-1609107). HH is also grateful for the funding from Southwest University of Science and Technology (18LZX515) and the Chinese Scholarship Council (CSC) program (201809390005). Supplementary materials Supplementary material associated with this article can be found in the online version at doi:10.1016/j.actamat.2020.03.005. References [1] J.R. Varner, H.J. Oel, Surface defects: their origin, characterization and effects on strength, J. Non. Cryst. Solids 19 (1975) 321–333. [2] S.W. Freiman, S.M. Wiederhorn, J.J. Mecholsky Jr., Environmentally enhanced fracture of glass: a historical perspective, J. Am. Ceram. Soc. 92 (2009) 1371– 1382. [3] C.R. Kurkjian, P.K. Gupta, R.K. Brow, The strength of silicate glasses: what do we know, what do we need to know? Int. J. Appl. Glass Sci. 1 (2010) 27–37. [4] W. Wang, P. Yao, J. Wang, C. Huang, T. Kuriyagawa, H. Zhu, B. Zou, H. Liu, Elastic stress field model and micro-crack evolution for isotropic brittle materials during single grit scratching, Ceram. Int. 43 (14) (2017) 10726–10736. [5] B. Taljat, G.M. Pharr, Development of pile-up during spherical indentation of elastic-plastic solids, Int. J. Solids Struct. 41 (14) (2004) 3891–3904. [6] Q. Qiao, H. He, J. Yu, Evolution of HF etching rate of borosilicate glass by frictioninduced damages, Appl. Sur. Sci. (2020) In press, doi: 10.1016/j. apsusc.2019.144789.
H. He et al. / Acta Materialia 189 (2020) 166 173 [7] H. He, S.H. Kim, L. Qian, Effects of contact pressure, counter-surface and humidity on wear of soda-lime-silica glass at nanoscale, Tribol. Int. 94 (2016) 675–681. [8] S. Yoshida, Indentation deformation and cracking in oxide glass-toward understanding of crack nucleation, J. Non-Cryst. Solids X 1 (2019) 100009. [9] K. Januchta, M.M. Smedskjaer, Indentation deformation in oxide glasses: quantification, structural changes, and relation to cracking, J. Non Cryst. Solids X 1 (2019) 100007. [10] K. Peter, Densification and flow phenomena of glass in indentation experiments, J. Non Cryst. Solids 5 (1970) 103–115. [11] T. Rouxel, H. Ji, T. Hammouda, A. Moreac, Poisson’s ratio and the densification of glass under high pressure, Phys. Rev. Lett. 100 (2008) 225501. [12] H. Ji, V. Keryvin, T. Rouxel, T. Hammouda, Densification of window glass under very high pressure and its relevance to Vickers indentation, Scr. Mater. 55 (2006) 1159–1162. [13] F.M. Ernsberger, Role of densification in deformation of glasses under point loading, J. Am. Ceram. Soc. 51 (1968) 545–547. ment, R. Vialla, D. Vandembroucq, B. Ruffle , Micro-Brillouin spec[14] H. Tran, S. Cle troscopy mapping of the residual density field induced by Vickers indentation in a soda-lime silicate glass, Appl. Phys. Lett. 100 (2012) 231901. [15] B. Hehlen, Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra, J. Phys. Condens. Mat. 22 (2010) 025401. [16] A. Koike, M. Tomozawa, IR investigation of density changes of silica glass and soda-lime silicate glass caused by microhardness indentation, J. Non Cryst. Solids 353 (2007) 2318–2327. [17] A. Perriot, D. Vandembroucq, E. Barthel, V. Martinez, L. Grosvalet, C. Martinet, B. Champagnon, Raman microspectroscopic characterization of amorphous silica plastic behavior, J. Am. Ceram. Soc. 89 (2006) 596–601. [18] S. Reibstein, L. Wondraczek, D. Ligny, S. Krolikowski, S. Sirotkin, J.P. Simon, V. Martinez, B. Champagnon, Structural heterogeneity and pressure-relaxation in compressed borosilicate glasses by in situ small angle X-ray scattering, J. Chem. Phys. 134 (2011) 204502. [19] Y.F. Niu, K. Han, J.P. Guin, Locally enhanced dissolution rate as a probe for nanocontactinduced densification in oxide glasses, Langmuir 28 (2012) 10733–10740. [20] Y. Satoshi, J.C. Sangleboeuf, T. Rouxel, Quantitative evaluation of indentationinduced densification in glass, J. Mater. Res. 20 (2005) 3404–3412. [21] N. Shen, T. Suratwala, W. Steele, L. Wong, M.D. Feit, P.E. Miller, R. Dylla-Spears, R. Desjardin, Nanoscratching of optical glass surfaces near the elastic plastic load boundary to mimic the mechanics of polishing particles, J. Am. Ceram. Soc. 99 (5) (2016) 1477–1484. [22] J. Fu, H. He, W. Yuan, Y. Zhang, J. Yu, Towards a deeper understanding of nanoscratch-induced deformation in an optical glass, Appl. Phys. Lett. 113 (2018) 031606. [23] E. Moayedi, S. Sawamura, J. Hennig, E. Gnecco, L. Wondraczek, Relaxation of scratch-induced surface deformation in silicate glasses: role of densification and shear flow in lateral indentation experiments, J. Non Cryst. Solids 500 (2018) 382–387. [24] H. He, L. Qian, C.G. Pantano, S.H. Kim, Mechanochemical wear of soda lime silica glass in humid environments, J. Am. Ceram. Soc. 97 (7) (2014) 2061–2068. [25] H. He, L. Qian, C.G. Pantano, S.H. Kim, Effects of humidity and counter-surface on tribochemical wear of soda-lime-silica glass, Wear 342 (2015) 100–106. [26] J. Ye, J. Yu, H. He, Y. Zhang, Effect of water on wear of phosphate laser glass and BK7 glass, Wear 376 (2017) 393–402. [27] H. He, T. Xiao, Q. Qiao, J. Yu, Y. Zhang, Contrasting roles of speed on wear of soda lime silica glass in dry and humid air, J. Non Cryst. Solids 502 (2018) 236–243. [28] H. He, J. Luo, L. Qian, C.G. Pantano, S.H. Kim, Thermal poling of soda lime silica glass with nonblocking electrodes-Part 2: effects on mechanical and mechanochemical properties, J. Am. Ceram. Soc. 99 (4) (2016) 1231–1238. [29] J. Luo, S. Bae, M. Yuan, E. Schneider, M.T. Lanagan, C.G. Pantano, S.H. Kim, Chemical structure and mechanical properties of soda lime silica glass surfaces treated by thermal poling in inert and reactive ambient gases, J. Am. Ceram. Soc. 101 (7) (2018) 2951–2964. [30] A. Alaziz, A.J. Barthel, N.D. Surdyka, J. Luo, S.H. Kim, Vapors in the ambient-A complication in tribological studies or an engineering solution of tribological problems? Friction 3 (2015) 85–114. [31] C. Xiao, X. Xin, X. He, H. Wang, L. Chen, S.H. Kim, L. Qian, Surface structure dependence of mechanochemical etching: scanning probe-based nanolithography study on Si(100), Si(110), and Si(111), ACS Appl. Mater. Interfaces. 11 (23) (2019) 20583–20588. [32] L. Chen, J. Wen, P. Zhang, B. Yu, C. Chen, T. Ma, X. Lu, S.H. Kim, L. Qian, Nanomanufacturing of silicon surface with a single atomic layer precision via mechanochemical reactions, Nat. Commun. 9 (1) (2018) 1542. [33] H. He, J. Yu, J. Ye, Y Zhang, On the effect of tribo-corrosion on reciprocating scratch behaviors of phosphate laser glass, Int. J. Appl. Glass Sci. 9 (2018) 352–363.
173
[34] A.J. Barthel, J. Luo, K.S. Hwang, J.Y. Lee, S.H. Kim, Boundary lubrication effect of organic residue left on surface after evaporation of organic cleaning solvent, Wear 350 (2016) 21–26. [35] G. Pharr, W. Oliver, Measurement of thin film mechanical properties using nanoindentation, MRS Bull. 17 (7) (2002) 28–33. € ncke, L. Wondraczek, Plas[36] R. Limbach, A. Winterstein-Beckmann, J. Dellith, D. Mo ticity, crack initiation and defect resistance in alkali-borosilicate glasses: from normal to anomalous behavior, J. Non Cryst. Solids 417-418 (2015) 15–27. [37] S.H. Hahn, J.M. Rimsza, L.J. Criscenti, W. Sun, L. Deng, J. Du, T. Liang, S.B. Sinnott, A.C.T. van Duin, Development of a Reaxff reactive force field for Nasiox/water systems and its application to sodium and proton self-diffusion, J. Phys. Chem. C 122 (34) (2018) 19613–19624. [38] S.H. Hahn, H. Liu, S.H. Kim, A.C.T. Van Duin, Atomistic understanding of surface wear process of sodium silicate glass in dry versus humid environments, J. Am. Ceram. Soc. (2020) in press, doi: 10.1111/jace.17008. [39] P. Zhang, S.X. Li, Z.F. Zhang, General relationship between strength and hardness, Mater. Sci. Eng. A 529 (2011) 62–73. [40] J. Yu, W. Yuan, H. Hu, H. Zang, Y. Cai, F. Ji, Nanoscale friction and wear of phosphate laser glass and BK7 glass against single CeO2 particle by AFM, J. Am. Ceram. Soc. 98 (4) (2015) 1111–1120. [41] S. Yoshida, H. Sawasato, T. Sugawara, Y. Miura, J. Matsuoka, Effects of indenter geometry on indentation-induced densification of soda-lime glass, J. Mater. Res. 25 (11) (2010) 2203–2211. [42] Y. Kato, H. Yamazaki, S. Itakura, S. Yoshida, J. Matsuoka, Load dependence of densification in glass during Vickers indentation test, J. Am. Ceram. Soc. Jpn. 119 (2) (2011) 110–115. [43] K.W. Peter, Densification and flow phenomena of glass in indentation experiments, J. Non Cryst. Solids 5 (2) (1970) 103–115. [44] T. Deschamps, J. Margueritat, C. Martinet, A. Mermet, B. Champagnon, Elastic moduli of permanently densified silica glasses, Sci. Rep. 4 (2014) 7193. [45] G.N. Greaves, A.L. Greer, R.S. Lakes, T. Rouxel, Poisson’s ratio and modern materials, Nat. Mater. 10 (2011) 823–837. [46] X. Guo, C. Zhai, R. Kang, Z. Jin, The mechanical properties of the scratched surface for silica glass by molecular dynamics simulation, J. Non Cryst. Solids 420 (2015) 1–6. [47] T.A. Michalske, B.C. Bunker, Slow fracture model based on strained silicate structures, J. Appl. Phys. 56 (1984) 2686–2693. [48] E. Silva, J. Li, D. Liao, S. Subramanian, T. Zhu, S. Yip, Atomic scale chemo-mechanics of silica: nano-rod deformation and water reaction, J. Comput. Aided Mater. Des. 13 (2006) 135–159. [49] J. Luo, H. Huynh, C.G. Pantano, S.H. Kim, Hydrothermal reactions of soda lime silica glass Revealing subsurface damage and alteration of mechanical properties and chemical structure of glass surfaces, J. Non Cryst. Solids 452 (2016) 93–101. [50] B.T. Poe, C. Romano, G. Henderson, Raman and XANES spectroscopy of permanently densified vitreous silica, J. Non Cryst. Solids 341 (2004) 162–169. , Indentation deformation mecha[51] T. Rouxel, H. Ji, J.P. Guin, F. Augereau, B. Ruffle nism in glass: densification versus shear flow, J. Appl. Phys. 107 (2010) 094903. [52] M. Nogami, M. Tomozawa, Effect of stress on water diffusion in silica glass, J. Am. Ceram. Soc. 67 (2) (1984) 151–154. [53] J.D. Mackenzie, High-pressure effects on oxide glasses: I, densification in rigid state, J. Am. Ceram. Soc. 46 (10) (1963) 461–470. [54] J. Yeon, A.C.T. van Duin, S.H. Kim, Effects of water on tribochemical wear of silicon oxide interfaces: molecular dynamics (MD) study with reactive force field (ReaxFF), Langmuir 32 (2016) 1018–1026. [55] S. Sawamura, L. Wondraczek, Scratch hardness of glass, Phys. Rev. Mater. 2 (2018) 092601(R). [56] O. Benzine, S. Bruns, Z. Pan, K. Durst, L. Wondraczek, Local deformation of glasses is mediated by rigidity fluctuation on nanometer scale, Adv. Sci. 5 (2018) 1800916. [57] K.D. Vargheese, A. Tandia, J.C. Mauro, Origin of dynamical heterogeneities in calcium aluminosilicate liquids, J. Chem. Phys. 132 (2010) 194501. [58] G.G. Adams, M. Nosonovsky, Contact modeling-forces, Tribol. Int. 33 (5 6) (2000) 431–442. [59] N.D. Surdyka, C.G. Pantano, S.H. Kim, Environmental effects on initiation and propagation of surface defects on silicate glasses: scratch and fracture toughness study, Appl. Phys. A. 116 (2014) 519–528. [60] J. Luo, W. Grisales, M. Rabii, C.G. Pantano, S.H. Kim, Differences in surface failure modes of soda lime silica glass under normal indentation versus tangential shear: a comparative study on Na+/K+-ion exchange effects, J. Am. Ceram. Soc. 102 (4) (2019) 1665–1676. [61] B.R. Lawn, Partial cone crack formation in a brittle material loaded with a sliding spherical indenter, Proc. Royal. Soc. Lond. A. 299 (1967) 307–316.
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Friction-induced subsurface densification of glass at contact stress far below indentation damage threshold Hongtu Hea,b,*, Seung Ho Hahnc, Jiaxin Yua, Qian Qiaoa, Adri C.T. van Duinc, Seong H. Kimb,** a Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China b Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, PA 16802, USA c Department of Mechanical Engineering, Pennsylvania State University, PA 16802, USA
A R T I C L E
I N F O
Article History: Received 9 October 2019 Revised 5 February 2020 Accepted 4 March 2020 Available online 10 March 2020 Keywords: Borosilicate glass Wear Water Subsurface densification
A B S T R A C T
It is well known that the densification of oxide glass can occur at high contact pressure (typically >5 GPa) under normal indentation conditions. This study reports that when frictional shear is involved, the subsurface densification of glass can occur under low load conditions that would involve completely elastic deformation if the load is applied along the surface normal direction without any interfacial shear. This phenomenon was observed for a borosilicate glass rubbed with a smooth stainless-steel ball in liquid water at a nominal Hertzian contact pressure of 0.5 GPa. Under these frictional conditions, subsurface cracking is completely suppressed, and surface wear occurs through mechanochemical reactions. Since the mechanochemical wear track was sufficiently smooth, it was possible to employ a sub-glass transition temperature (sub-Tg) annealing method to measure the volume recovery of the densified subsurface region. The frictioninduced subsurface densification of the wear track was also confirmed through nanoindentation measurements and dissolution tests in pH 13 aqueous solutions. Molecular dynamics (MD) simulations with a ReaxFF reactive force also suggested that the subsurface structural change can occur readily when friction is involved at low contact pressure conditions. © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
1. Introductions The surface and subsurface damage of silicate glasses have detrimental effects on their mechanical and chemical durability [1 3]. Such damages are often caused by the physical contact with foreign objects. The effects of physical contacts are typically studied in two test modes: (i) indentation or impact along the surface normal direction without tangential shear and (ii) interfacial shear with a counter-surface applying a compressive stress along the surface normal direction and moving along the tangential direction of the surface. The former generally generates a quasi-isostatic stress field in the glass, while the latter produces more complex stress fields due to interfacial friction [4,5]. During their manufacturing and functional operations, glass surfaces are inevitably subject to not only indentations or impacts, but also various frictional contacts [6,7]. Thus, it is critically needed to understand the surface damage mechanisms of silicate glass under interfacial shear conditions. * Corresponding author at: Key Laboratory of Testing Technology for Manufacturing Process, Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China. ** Corresponding author at: Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, PA 16802, USA. E-mail addresses: [email protected] (H. He), [email protected] (S.H. Kim). https://doi.org/10.1016/j.actamat.2020.03.005 1359-6454/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
A majority of mechanical studies in the literature have focused extensively on the surface damage of glasses under normal indentation conditions [8,9]. It is well known that when a high contact load (pressure) is applied with an indenter beyond the elastic deformation regime, glass undergoes two competing plastic mechanisms densification (or structural compaction) and shear flow before eventually cracking or fracture occurs [10]. From the hydrostatic pressure experiments, it was found that the plastic densification of silica glass starts above a threshold value of 10 GPa and saturates at a level of Dr/r0 = 20% (where r is density) above a pressure level of 20 25 GPa; for soda lime silicate glass, the threshold is about 8 GPa and the density change is about 6.3% [11,12]. As a result, the density, refractive index, elastic modulus and hardness of glass also increase [13 15]. Through characterizations of the densified regions with various techniques such as IR [16], Raman [17], and small-angle X-ray scattering (SAXS) [18], it was shown that such changes are accompanied with structural alterations. The local structural change can also substantially increase the dissolution rate of the densified zone compared to that of pristine glass [19]. The densification process can be reverted by thermal annealing at a temperature slightly below the glass transition (Tg) temperature typically, at 0.9 £ Tg for 2 h. The sub-Tg annealing results in volume recovery at the indentation site [20].
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The mechanical scratch test can be performed by moving the glass sample or the indenter laterally while the glass is compressed with the indenter; this introduces tangential shear stress to the indentation region. In a nanoscratch test using a diamond tip on fused silica, borosilicate glass, and phosphate glass, it was found that the scratch mark becomes shallower after the sub-Tg annealing treatment, suggesting the occurrence of the subsurface densification of the scratched region [21]. The extent of densification varies with the applied load, sliding cycles, and humidity conditions [22]. When comparing the nanoindentation and nanoscratch results of silica glass, borosilicate glass, and soda lime silicate glass, it was found that the volume recovery ratios of these glasses are very similar in both test conditions [23]. During the mechanical scratch test, the contact pressure under the nanoindenter tip is extremely high usually >5 GPa; comparable to or higher than its indentation damage threshold (yield strength and/ or hardness) [8,9]. It is questionable if the findings from mechanical tests under such a high contact pressure pertain to numerous frictional contacts made by foreign objects at low contact pressure conditions far below its indentation damage threshold. When the contact pressure is low enough to induce elastic deformation only in the contact region, the glass surface does not get damaged as long as the counter-surface applying the stress moves along the surface normal direction without tangential shear. However, when subjected to interfacial friction, the glass surface can undergo severe mechanical damage in the dry condition, leaving deep and rough wear scars, probably due to interfacial bonding of two surfaces in contact [24,25]. In humid environments, such mechanical damages are suppressed; instead, shear-induced mechanochemical (often called ‘tribochemical’) reactions can occur [26 30]. The chemical reactions occurring in humid conditions involve water molecules adsorbed on the surface and the end result is often the wear of one of the surfaces either the glass itself or the counter-surface or both depending on the applied load, speed, and humidity as well as the glass and counter-surface compositions [26 29]. The mechanochemical wear scar is topographically smooth and closely follows the evolution of the topographic shape of the counter-surface [24,31]. Thus, it was believed that mechanochemical reactions leading to the surface wear take place mostly at the sliding interface where the adsorbed molecules are being sheared by two solid surfaces. Here, an important question that has not been addressed yet is whether subsurface damage can be made while the topmost surface of glass undergoes shear-induced mechanochemical wear reactions in the elastic contact pressure regime. In the case of crystalline silicon surfaces with a nanoscale contact, the subsurface damage is found to be negligible [32]; however, a systematic study has not been done for silicate glass surfaces. This paper focuses on addressing the question of subsurface densification of glass under frictional shear at the contact stress far below its indentation damage threshold of glass. To study this question, a mechanochemical wear track was first generated on a borosilicate glass surface by rubbing with a smooth stainless-steel sphere in liquid water at a nominal Hertzian contact pressure of 0.3 0.5 GPa which is an order of magnitude lower than the indentation hardness and yield strength of the glass. The liquid water environment was chosen because our previous studies showed that it is efficient to prevent any subsurface cracking during the wear tests [26,33]. Then, we investigated the subsurface densification via (i) volume recovery of the wear track upon subTg annealing, (ii) nanoindentation inside the wear track, (iii) chemical etching of the wear track with pH 13 aqueous solution. All these test results indicated that the subsurface densification indeed occurs during the mechanochemical wear process even though the contract stress is in the elastic deformation regime. To support this conclusion from experimental observations, we conducted molecular dynamics (MD) simulations with a ReaxFF
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reactive force field to show the subsurface structural change of a sodium silicate glass during the interfacial shear and thermal annealing. 2. Materials and methods The polished borosilicate glass slide (Schott N-BK7Ò optical glass, purchased from Hefei Kejing Materials Technology Co., Ltd) with a size of 20 mm £ 20 mm £ 2 mm was used as a substrate in the present study. The physical and chemical properties of this borosilicate glass substrate can be found elsewhere [26]. The AISI 316 stainless steel (SS) ball with a diameter of 4 mm was used as a counter-surface for tribological tests. Using a white light scanning profilometry (MFT3000, Rtec, San Jose, CA), the root mean square roughness of the SS ball was measured to be ~11 nm over a 400 mm £ 400 mm area. Using a ball-on-flat reciprocating tribometer (MFT-3000, Rtec, San Jose, CA), friction and wear of the borosilicate glass substrates were tested in deionized (DI) water. Prior to the friction test, both glass and ball surfaces were cleaned by rinsing with ethanol and then water followed by blow-drying with dry nitrogen gas [34]. The sliding speed was 4 mm/s, the length of the reciprocating track was 5 mm, and the total sliding time was 3 min. The normal load during the surface wear test was 1, 2, and 3 N, which corresponds to the nominal Hertzian contact pressure of ~0.35 GPa, ~0.44 GPa, and ~0.52 GPa (on the flat surface before wear), respectively. All wear tests were repeated independently at least 5 times. After wear tests, the wear track of borosilicate glass substrates and the 316-SS ball surfaces were analyzed with optical microscopy (BX51-P, Olympus, Japan) and white light scanning profilometry (MFT-3000, Rtec, San Jose, CA). The nanomechanical properties of the wear-tested surface (inside the wear track) and the original glass surface (outside the wear track) were analyzed using a nanoindenter (G200, Agilent Technologies, Inc., Santa Clara, CA). The reduced modulus and hardness were calculated from force-displacement curves with the Oliver-Pharr model [35]. Then, the wear-tested samples were annealed for 2 h at 520 °C ( 0.9 Tg) [36] and then re-imaged using profilometry. For the simplicity, the wear track before and after the sub-Tg annealing treatment will be referred as ‘pristine’ and ‘annealed’, respectively, hereafter. The pristine and annealed samples were placed in separate Teflon containers filled with 50 mL of 0.1 M NaOH (pH 13) solution at room temperature. Individual samples were retrieved at 0.2, 0.5, 1, 2, and 3 h and rinsed with DI water and blow-dried with nitrogen gas prior to profilometry observations. In order to investigate the subsurface structural changes during the mechanochemical wear, we carried out MD simulations with ReaxFF reactive force fields. Since the ReaxFF-MD model for the sodium silicate glass have been developed sufficiently in recent studies [37,38], we selected the sodium silicate glass (SiO2:Na2O = 70:30 mol%, 3000 atom-system) as a substrate material, and the amorphous silica as the counter surface. Currently, the N-BK7 glass composition cannot be simulated with ReaxFF-MD because the ReaxFF force fields for boron, potassium, and barium are not available. Fig. 1a shows the snapshot of the MD simulation trajectory obtained from the previous mechanochemical wear study [38]. The simulation within the ReaxFF-MD framework were performed through: (i) vertical compression of counter surface into the substrate, (ii) equilibration of the system under normal load, (iii) applying lateral movement to counter surface to induce the shear stress at the interface, and (iv) vertical separation. The contact area of the slab-on-slab geometry system was 11.86 nm2, where the contact pressure of 0.7 GPa was set to applied throughout the equilibration and sliding processes. Note that the uppermost 8 A-thick portion of silica counter surface was defined as the movable rigid part and the constant velocity vector components of v = (vx, vy, vz) = (0, 0, 0.0002), (0.0001, 0, 0), and (0, 0, 0.0002) [A/fs] (£ 105 [m/s]) were applied to this region for the (i), (iii) and (iv) processes, respectively. In this work, we extracted only the
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Fig. 2. (a) Optical profilometry image and (b) line profile of the borosilicate surface after the normal indentation with the 316-SS ball without shear. The applied load was 3 N. In (a), the dashed circle is drawn based on the calculated Hertzian contact diameter at the position where the indentation test was performed.
Fig. 1. (a) Snapshots from the ReaxFF-MD simulations of silica and sodium silicate glass separated after sliding at ~0.7 GPa for 340 ps in the presence on interfacial water molecules at 300 K. Details of simulations can be found in Ref. [38]. Note that the atoms are color-coded based on whether they are originally parts of silica (red and yellow) or sodium silicate (gray and blue) prior to the sliding. (b) Snapshots of the sodium silicate glass substrate of (a) after annealing at 765 K for 250 ps. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
sodium silicate substrate from the MD trajectory after sliding (the bottom slab in Fig. 1a) and employed it as the initial configuration of our MD simulation. The initial system was first equilibrated under NVT (constant number of atoms, system volume, and temperature) ensemble at 300 K for 100 ps. The trajectories obtained during this equilibration process were used to analyze the glass network structures (Si-O bond length and Si-O-Si bond angle), which represent the structural data immediately after the mechanochemical wear test. Following the equilibration step, we gradually ramped up the temperature of the system to investigate the effect of annealing to the substrate material. From 300 K, the system temperature was increased at a rate of 5 K/ps until it reached the 90% of Tg determined through MD simulations. At this temperature, the system was further equilibrated for 250 ps and cooled back to 300 K at the same rate. During the annealing process, the periodicity of the system was maintained within the xy-plane while the z direction was set to be non-periodic to allow any adsorbed water molecules on the sodium silicate surface to evaporate during the annealing process. After the system reached back to 300 K, NVT equilibration for another 100 ps was carried out to extract the structural data of the silicate glasses after annealing (Fig. 1b). For all ReaxFF-MD simulations carried out to produce the trajectories for structural analysis, an integration time step of 0.25 fs was used for the NVT ensemble with a Nose-Hoover thermostat (t t=100 fs). ReaxFF reactive potential for Na/Si/O/H description was used within the LAMMPS package and the details on how the initial configuration was obtained from the mechanochemical wear are further elaborated in the Refs. [37,38]. 3. Results and discussion 3.1. Elastic deformation of glass under normal indentation without frictional shear Fig. 2 shows a typical optical microscopy image and line profile of the borosilicate glass surface after compressing with the 316-SS ball in liquid water for 30 s. The applied load was 3 N, which corresponds to the nominal contact pressure of ~520 MPa according to the Hertzian contact theory. No discernible indentation damage can be found within the resolution limit of the optical profilometry. Based on the general relationship between strength and hardness [39,40], the yield strength of borosilicate glass can be estimated as ~2 GPa, which is
about four times larger than the maximum contact pressure used in this study (~0.52 GPa). The results in Fig. 2 confirms that the borosilicate glass surface indented with the 316-SS ball deforms elastically under the contact pressure of 0.52 GPa and fully recovers when the ball is removed without any shear during the indentation. The wear data described in the following sections were obtained at the same or lower contact pressure condition, except that the frictional shear was introduced by sliding the ball on the surface. 3.2. Volume recovery of mechanochemical wear track by sub-Tg annealing Fig. 3a 3c shows the representative optical profilometry images of borosilicate glass surface after wear tests at an applied load of 1 N, 2 N, and 3 N, respectively, in DI water. Although no discernable surface damage is observed without tangential shear of the 316-SS ball on the borosilicate glass surface (Fig. 2), there are noticeable wear marks (material removal) at the same contact stress if the ball is sheared against the glass surface. The wear depth and volume, shown in Fig. 3d and 3e, respectively, increase with the applied load. When the wear region is cut by cracking from the back side of the sample, the cross-sectional image shows no sign of subsurface cracks during the wear test (Fig. S1 in Supporting Information). The subsurface cracking occurs during the wear test in humid air at the same normal load and sliding speed conditions (Fig. S2 in Supporting Information). This supports that in liquid water, wear occurs mainly via mechanochemical reactions [7,24,26]. There is no periodic ripple discernable beyond random topographic features smaller than 2 nm from peak to peak (Fig S3 in Supporting Information). In order to check if there is any subsurface densification in the mechanochemical wear track of borosilicate glass formed while the surface is compressed and sheared at a load that would cause only elastic deformation in the normal indentation test, the wear track was treated with the sub-Tg annealing process [9,20]. Fig. 3d and 3e compare the wear line profile and volume, respectively, of the annealed wear track of borosilicate glass with those of the pristine wear track. After the subsequent sub-Tg annealing, the volume of wear track is smaller compared to the pristine case. The volume recovery was calculated from the difference of the wear volume between the pristine wear track (Vi) and annealed wear track (Va) and plotted in Fig. 3f. The recovered volume upon sub-Tg annealing is about 9.3%, 15.6%, and 26.3% of the pristine wear volume for the applied load of 1 N, 2 N, and 3 N, respectively. Recall that the nominal Hertzian contact pressure is 0.3 ~ 0.5 GPa, which is about one order of magnitude lower than the indentation damage threshold of borosilicate glass. The volume recovery upon sub-Tg annealing could be related to the densification ratio [12]. The larger recovery of the damaged volume after sub-Tg annealing could mean the larger degree of subsurface densification [41,42]. Note that the densification of silicate
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Fig. 3. The optical profilometry images of borosilicate glass substrate after wear tests under an applied load of (a) 1 N, (b) 2 N, and (c) 3 N. (d) Characteristic line profiles of wear tracks before (black line) and after (red line) sub-Tg annealing treatment. The inset shows the magnification of the blue dotted lines region in (d). (e) Volume of the pristine and annealed wear tracks formed under various applied load conditions. (f) Volume recovery ratio of the wear track upon the sub-Tg annealing treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
glasses during Vickers indentation occurs typically under a contact pressure higher than 8 GPa [11,12]. For the indentation of soda lime silica glass with a smooth ball, the densification in the subsurface occurs when the contact pressure is higher than 7 GPa [43]. Thus, it is surprising and intriguing to see the densification underneath the wear track of borosilicate glass in the low compressive pressure condition that would induce elastic deformation only under the normal indentation without tangential shear. 3.3. Hardness and reduced modulus of mechanochemical wear track When the densification of silicate glass occurs under normal indentation tests or hydrostatic pressure tests, the atomic packing density of densified glass increases, which is accompanied with an increase in elastic modulus and hardness [44]. We have tested if a similar phenomenon is observed by measuring the modulus and hardness of the wear track produced at the low contact stress condition. Fig. 4a and 4b compare the hardness (H) and reduced modulus (Er) of the damaged and pristine borosilicate glass substrate by nanoindentation tests, respectively. Since the Poisson’s ratio (v) may vary
with the degree of densification and could not be determined independently [45], Fig. 4b shows the value of reduced modulus (Er) of the tested region rather than the Young’s modulus (E). The H and Er of the region outside the wear track are ~5.9 GPa and ~80 GPa, respectively, which are close to the literature value of N-BK7 borosilicate glass [26]. The H and Er of the wear track gradually increase as the applied load for the wear test increases from 1 N to 3 N. This is consistent with the results in Fig. 3f showing the increase of the subsurface densification predicted from the volume recovery after subTg annealing with the applied load [11,46]. 3.4. Dissolution of mechanochemical wear track in basic solution The structural deformation in the densified region of silicate glass formed under normal indentation experiments could make that region more susceptible to aqueous corrosion [13,15,19]. In order to test if the same phenomenon can be observed, the pristine and annealed wear tracks were immersed in 0.1 M NaOH solution for varying times and then analyzed with the optical profilometry (Fig. 5a). Fig. 5b plots the change in volume of the wear track as a
Fig. 4. (a) Nanohardness and (b) reduced modulus of the wear track formed in liquid water. For comparison, the data of the outside the wear track is also shown. The penetration depth during the nanoindentation measurement was 300 nm.
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Fig. 5. (a) Evolution of wear track of borosilicate surface as a function of corrosion time in 0.1 M NaOH for the pristine wear track and annealed wear track. The pristine wear track was made at the applied load of 3 N in DI water. (b) Evolution of the corresponding wear volume as a function of corrosion time. (c) Preferentially dissolved volume of the pristine and annealed wear tracks compared to the outside the wear track. The data for wear tracks made with the applied load of 1 N and 2 N can be found in Supporting Information (Fig. S4).
function of corrosion time for various applied load conditions. At this high pH condition, the as-received surface can also dissolve. The increase in the wear track volume means the enhanced dissolution rate of the subsurface densified region compared to the original surface. The amount of dissolution volume enhanced by the subsurface densification is plotted in Fig. 5c. It can be seen that the pristine wear track exhibits significant enhancement in dissolution in pH 13 aqueous solution compared to the original surface outside the wear track; in contrast, the sub-Tg annealed wear track show only marginal increase in the dissolution compared to the surface outside the wear track. The data in Fig. 5c is congruent with the fact that the sub-Tg annealing can revert the densified subsurface region to nearly back to the original state (although not complete). In the current study, it is difficult to attribute the difference in the dissolution volume between the pristine and annealed wear tracks exclusively to the volume recovery effect of the densified subsurface and rule out any residual stress effect. It is known that the stress can affect the onset and rate of hydrolysis reactions [47,48]. Previously, it was reported that under alcohol vapor phase lubrication conditions, no visually discernible wear track of soda lime silica glass can be detected [49]. However, after the subsequent hydrothermal treatment at 150 °C for 24 h, the friction-tested region was preferentially etched, leaving a ~5 nm deep trench [49]. Because there was no topographic depression of the friction-tested region in the alcohol vapor lubrication condition, there was no reason to suspect the subsurface densification in that study. Thus, the preferential etching by hot steam was attributed to the subsurface residual stress after the friction test. If the residual stress exists in the densified subsurface region of the pristine wear track, it would be relaxed and relieved during the volume recovery process at the temperature near Tg.
bond length distribution are in good agreement with the indentation induced densification on oxide glass surface [8,9,50]. To simulate the sub-Tg annealing treatment, the Tg of sodium silicate glass is determined by ReaxFF-MD simulations, as shown in Fig. 6b. The ReaxFF-Tg of sodium silicate glass can be estimated as ~850 K. Thus, based on the ReaxFF-Tg, the annealing treatment of sodium silicate glass was conducted at 765 K (0.9 £ ReaxFF-Tg). After sub-Tg annealing, the density of the subsurface region decreases to 2.495 g/cm3 (Fig. 6c) and the probability of lower and higher values in the Si-O-Si bond angle distribution decreases (Fig. 6e). The change of Si-O bond length distribution is marginal (Fig. 6d). The decrease of density and the narrowing of the Si-O-Si bond angle distribution suggest the structural relaxation (or recovery) to some extent with the sub-ReaxFF-Tg annealing. Note that the degree of the structural relaxation of sodium silicate glass depends on the annealing condition. For example, based on the MD simulations with the Teter potential, the Teter-Tg of sodium silicate glass is estimated as ~1166 K, and the structural relaxation of the friction-tested subsurface region becomes larger when the glass is annealed at higher annealing temperature (1050 K, 0.9 £ Teter-Tg). More details about the Si-O bond length and Si-O-Si bond angle distribution after annealing at 1050 K can be found in Supporting Information (Fig. S6). Overall, the ReaxFF-MD results in Fig. 6 support that mechanochemical wear even at the contact stress an order of magnitude lower than the indentation damage threshold can induce the subsurface densification of glass and the densification can be reverted to some extent via subsequent annealing treatment.
3.5. Subsurface structure changes of glass by ReaxFF-MD simulations
The striking contrast between the elastic behavior in the normal indentation test (Fig. 2) and the surface damage after the tangential shear at the same normal load (Fig. 3) clearly suggests that the subsurface damage or deformation mode is quite different depending on whether interfacial shear is involved at the contact point. Even though the mechanical cracking during the shear is prevented by running the friction test in DI water (Fig. S1 and S2), the shear process is accompanied by not only the mechanochemical reactions but also the friction at the sliding interface (Fig. S7). In the past, both mechanochemical reactions and friction are thought to involve the atoms or species exposed at the shearing interface if the applied contact pressure is in the elastic deformation regime. But, the increase in nanoindentation hardness and modulus (Fig. 4) as well as enhanced dissolution in the basic solution (Fig. 5), along with the volume recovery of the wear track after sub-Tg annealing (Fig. 3f), clearly indicate that subsurface densification does occur when the contact interface is sheared even at this light load condition. ReaxFF-MD simulations
ReaxFF MD simulations were carried out to test the subsurface structural changes of glass upon mechanochemical wear at the atomistic level. There is no stick-slip behavior discernable beyond random fluctuations in the simulated friction force (Fig. S5 in Supporting Information). Note that for all the structural analysis from the ReaxFF-MD results, we defined the subsurface region as the 15Athickness layer underneath the surface with the periodicity maintained in the xy-plane, as shown in Fig. 6a, where the penetration of proton and water is negligible [37,38]. The simulation results show that after friction tests, the density of the subsurface region increases from 2.472 g/cm3 to 2.544 g/cm3 (Fig. 6c), implying a densification of ~3% under the given condition. Additionally, after friction tests, the Si-O bond length distribution shifts to a higher value (Fig. 6d), while the Si-O-Si bond angle distribution shifts slightly to a lower value (Fig. 6e). The trend of the variation of Si-O-Si bond angle and Si-O
3.6. Friction-induced subsurface densification even in the elastic contact stress condition
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Fig. 6. (a) The schematics illustrates the subsurface region for structural analysis. (b) Determination of Tg of sodium silicate glass based on ReaxFF-MD simulations. The variation of (c) density, (d) Si-O bond distance, and (e) Si-O-Si bond angle distribution of the subsurface region in sodium silicate glass upon wear and subsequent annealing treatment. Note that the bond length analysis was normalized in order to clearly capture the bond elongation behavior with respect to the maximum value.
also support the subsurface structural damages (Fig. 6). This is schematically summarized in Fig. 7. In normal indentation experiments, the subsurface damage (densification and/or pile-up) is observed when the compressive stress applied by the indenter tip is higher than indentation damage threshold (yield strength and hardness) [12,42]. The pile-up is mainly caused by the plastic flow of subsurface materials to the direction parallel to the indenter surface [8,9,12,51]. Such flow of material is facilitated by forming shear bands in the subsurface region under the indenter [8,9]. In our experimental conditions, there is no pile-up at the periphery of the slide tract discernable within the resolution of white light scanning profilometry (see the inset in Fig. 3a). This is another evidence supporting that the subsurface densification observed in the wear track is not due to normal indentation, but it must be due to frictional shear.
When the topmost surface atoms at the interface are sheared by the counter-surface, the atoms beneath that layer will certainly feel the shear stress along the sliding direction because they are covalently bonded to the topmost surface atoms. This shear stress caused by friction might be responsible for the subsurface densification. When the shear stress is increased, the degree of subsurface densification also increases (Figs. 3 6). How deep this friction-induced densification propagates into the subsurface region will depend on the glass network structure (composition) as well as the magnitude of friction and the availability of water molecules. If water molecules diffuse into the subsurface and its rate is enhanced by the applied stress [52], then it could drastically affect the friction-induced densification process. Previously, the effect of bulk shear stress on densification of glass have been reported at extremely high shear stress (>4 GPa) [53]. The
Fig. 7. Schematic illustration of the friction-induced densification of borosilicate glass at the load condition that induces only elastic deformation when no interfacial shear is involved.
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Fig. 8. Snapshots from MD simulations showing the formation, elongation, and dissociation of interfacial Si-O-Si bonds during the friction process of sodium silicate glass. Note that the sodium ions are excluded for brevity of the image.
fact that shear can enhance densification of the glass affected during the compression is the same; but the mechanism proposed here for the friction case is different from the one proposed in the previous literature [53]. In the bulk shear case, the proposed mechanism considers the possibility of chain entanglements that can reduce the reversible recovery after compressive stress is removed; this phenomenon was observed at extremely high shear stress (>4 GPa) [53]. In the interfacial shear (friction) case, it is known that interfacial covalent bonds can be formed by bridging two solid surfaces moving opposite directions (Fig. 8) [54], which facilitates the transfer of mechanical energy at the shear plane into the subsurface region. Our experimental and computational results indicate that the energy transfer mediated by the interfacial bond formation is substantially large to induce significant wear in the absence of any interfacial water [24,25]. The presence of water at the interface appears to be sufficient to mitigate mechanochemical wear, but it is not enough to prevent subsurface damage (which was manifested as ‘densification’). It was recently reported that the nanometer scale structural inhomogeneity can facilitate local deformation of glass [55,56]. Based on the calculation of mean squared displacement (MSD) in MD simulations [57], we have calculated the dynamic propensity of individual atoms before and after shearing. As shown in Fig. S8 in Supporting Information, some degree of lateral inhomogeneity in the dynamic propensity can be seen in the subsurface region before the interfacial shear; after the interfacial friction, the propensity distribution becomes more homogeneous. On the basis of the MSD calculation, the overall propensities of sodium and silicon atoms are decreased after the friction test. This result may suggest that the densification might be coupled with the decrease in subsurface structural inhomogeneity. The finding of this study implicates that classical contact mechanics theories cannot fully explain the tribological behaviors of silicate glass. Based on the Hertzian contact theory, no material loss is expected if the contact deformation is elastic [58]. However, our data clearly show that even if the contact deformation is purely elastic in the absence of frictional shear, the material loss and subsurface densification can still occur under interfacial shear conditions. Previously, it was shown that chemical strengthening through the exchange of Na+ ions with K+ ions can increase the hardness, modulus, indentation fracture toughness, and crack initiation load of the aluminosilicate glass and soda lime silica glass; but the wear resistance at high humidity conditions is deteriorated [59,60]. These previous reports, along with the data presented in the current study, imply that the ability to resist plastic deformation in the normal indentation condition may not be a good descriptor for the resistance against wear and subsurface damage in tribological conditions which involve frictional shear of the interfaces [61]. 4. Conclusions The friction induced subsurface densification of borosilicate glass has been studied by rubbing with a smooth stainless-steel ball in liquid water at a nominal Hertzian contact pressure of 0.5 GPa. The
results show that the deformation of glass is completely elastic under such a low normal stress without tangential shear. When the frictional shear is involved, surface wear of glass materials occurs through mechanochemical reactions, and the wear tracks become shallower compared to the pristine wear track after the subsequent sub-Tg annealing, suggesting the subsurface densification of borosilicate glass can occur. The increase in nanoindentation hardness and reduced modulus, enhanced dissolution in the basic solution, and the change of the subsurface Si-O-Si bond angle and Si-O bond length distributions after friction tests and subsequent annealing treatment revealed by ReaxFF-MD simulations further support the occurrence of subsurface densification of glass. These results suggest that the friction-induced subsurface densification of oxide glass can occur during the mechanochemical wear process even though the contact stress is far below its indentation damage threshold. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 51605401 and 51575462), Scientific Research Fund of Sichuan Provincial Education Department (17ZA0408), and the National Science Foundation of the United States of America (Grant no. DMR-1609107). HH is also grateful for the funding from Southwest University of Science and Technology (18LZX515) and the Chinese Scholarship Council (CSC) program (201809390005). Supplementary materials Supplementary material associated with this article can be found in the online version at doi:10.1016/j.actamat.2020.03.005. References [1] J.R. Varner, H.J. Oel, Surface defects: their origin, characterization and effects on strength, J. Non. Cryst. Solids 19 (1975) 321–333. [2] S.W. Freiman, S.M. Wiederhorn, J.J. Mecholsky Jr., Environmentally enhanced fracture of glass: a historical perspective, J. Am. Ceram. Soc. 92 (2009) 1371– 1382. [3] C.R. Kurkjian, P.K. Gupta, R.K. Brow, The strength of silicate glasses: what do we know, what do we need to know? Int. J. Appl. Glass Sci. 1 (2010) 27–37. [4] W. Wang, P. Yao, J. Wang, C. Huang, T. Kuriyagawa, H. Zhu, B. Zou, H. Liu, Elastic stress field model and micro-crack evolution for isotropic brittle materials during single grit scratching, Ceram. Int. 43 (14) (2017) 10726–10736. [5] B. Taljat, G.M. Pharr, Development of pile-up during spherical indentation of elastic-plastic solids, Int. J. Solids Struct. 41 (14) (2004) 3891–3904. [6] Q. Qiao, H. He, J. Yu, Evolution of HF etching rate of borosilicate glass by frictioninduced damages, Appl. Sur. Sci. (2020) In press, doi: 10.1016/j. apsusc.2019.144789.
H. He et al. / Acta Materialia 189 (2020) 166 173 [7] H. He, S.H. Kim, L. Qian, Effects of contact pressure, counter-surface and humidity on wear of soda-lime-silica glass at nanoscale, Tribol. Int. 94 (2016) 675–681. [8] S. Yoshida, Indentation deformation and cracking in oxide glass-toward understanding of crack nucleation, J. Non-Cryst. Solids X 1 (2019) 100009. [9] K. Januchta, M.M. Smedskjaer, Indentation deformation in oxide glasses: quantification, structural changes, and relation to cracking, J. Non Cryst. Solids X 1 (2019) 100007. [10] K. Peter, Densification and flow phenomena of glass in indentation experiments, J. Non Cryst. Solids 5 (1970) 103–115. [11] T. Rouxel, H. Ji, T. Hammouda, A. Moreac, Poisson’s ratio and the densification of glass under high pressure, Phys. Rev. Lett. 100 (2008) 225501. [12] H. Ji, V. Keryvin, T. Rouxel, T. Hammouda, Densification of window glass under very high pressure and its relevance to Vickers indentation, Scr. Mater. 55 (2006) 1159–1162. [13] F.M. Ernsberger, Role of densification in deformation of glasses under point loading, J. Am. Ceram. Soc. 51 (1968) 545–547. ment, R. Vialla, D. Vandembroucq, B. Ruffle , Micro-Brillouin spec[14] H. Tran, S. Cle troscopy mapping of the residual density field induced by Vickers indentation in a soda-lime silicate glass, Appl. Phys. Lett. 100 (2012) 231901. [15] B. Hehlen, Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra, J. Phys. Condens. Mat. 22 (2010) 025401. [16] A. Koike, M. Tomozawa, IR investigation of density changes of silica glass and soda-lime silicate glass caused by microhardness indentation, J. Non Cryst. Solids 353 (2007) 2318–2327. [17] A. Perriot, D. Vandembroucq, E. Barthel, V. Martinez, L. Grosvalet, C. Martinet, B. Champagnon, Raman microspectroscopic characterization of amorphous silica plastic behavior, J. Am. Ceram. Soc. 89 (2006) 596–601. [18] S. Reibstein, L. Wondraczek, D. Ligny, S. Krolikowski, S. Sirotkin, J.P. Simon, V. Martinez, B. Champagnon, Structural heterogeneity and pressure-relaxation in compressed borosilicate glasses by in situ small angle X-ray scattering, J. Chem. Phys. 134 (2011) 204502. [19] Y.F. Niu, K. Han, J.P. Guin, Locally enhanced dissolution rate as a probe for nanocontactinduced densification in oxide glasses, Langmuir 28 (2012) 10733–10740. [20] Y. Satoshi, J.C. Sangleboeuf, T. Rouxel, Quantitative evaluation of indentationinduced densification in glass, J. Mater. Res. 20 (2005) 3404–3412. [21] N. Shen, T. Suratwala, W. Steele, L. Wong, M.D. Feit, P.E. Miller, R. Dylla-Spears, R. Desjardin, Nanoscratching of optical glass surfaces near the elastic plastic load boundary to mimic the mechanics of polishing particles, J. Am. Ceram. Soc. 99 (5) (2016) 1477–1484. [22] J. Fu, H. He, W. Yuan, Y. Zhang, J. Yu, Towards a deeper understanding of nanoscratch-induced deformation in an optical glass, Appl. Phys. Lett. 113 (2018) 031606. [23] E. Moayedi, S. Sawamura, J. Hennig, E. Gnecco, L. Wondraczek, Relaxation of scratch-induced surface deformation in silicate glasses: role of densification and shear flow in lateral indentation experiments, J. Non Cryst. Solids 500 (2018) 382–387. [24] H. He, L. Qian, C.G. Pantano, S.H. Kim, Mechanochemical wear of soda lime silica glass in humid environments, J. Am. Ceram. Soc. 97 (7) (2014) 2061–2068. [25] H. He, L. Qian, C.G. Pantano, S.H. Kim, Effects of humidity and counter-surface on tribochemical wear of soda-lime-silica glass, Wear 342 (2015) 100–106. [26] J. Ye, J. Yu, H. He, Y. Zhang, Effect of water on wear of phosphate laser glass and BK7 glass, Wear 376 (2017) 393–402. [27] H. He, T. Xiao, Q. Qiao, J. Yu, Y. Zhang, Contrasting roles of speed on wear of soda lime silica glass in dry and humid air, J. Non Cryst. Solids 502 (2018) 236–243. [28] H. He, J. Luo, L. Qian, C.G. Pantano, S.H. Kim, Thermal poling of soda lime silica glass with nonblocking electrodes-Part 2: effects on mechanical and mechanochemical properties, J. Am. Ceram. Soc. 99 (4) (2016) 1231–1238. [29] J. Luo, S. Bae, M. Yuan, E. Schneider, M.T. Lanagan, C.G. Pantano, S.H. Kim, Chemical structure and mechanical properties of soda lime silica glass surfaces treated by thermal poling in inert and reactive ambient gases, J. Am. Ceram. Soc. 101 (7) (2018) 2951–2964. [30] A. Alaziz, A.J. Barthel, N.D. Surdyka, J. Luo, S.H. Kim, Vapors in the ambient-A complication in tribological studies or an engineering solution of tribological problems? Friction 3 (2015) 85–114. [31] C. Xiao, X. Xin, X. He, H. Wang, L. Chen, S.H. Kim, L. Qian, Surface structure dependence of mechanochemical etching: scanning probe-based nanolithography study on Si(100), Si(110), and Si(111), ACS Appl. Mater. Interfaces. 11 (23) (2019) 20583–20588. [32] L. Chen, J. Wen, P. Zhang, B. Yu, C. Chen, T. Ma, X. Lu, S.H. Kim, L. Qian, Nanomanufacturing of silicon surface with a single atomic layer precision via mechanochemical reactions, Nat. Commun. 9 (1) (2018) 1542. [33] H. He, J. Yu, J. Ye, Y Zhang, On the effect of tribo-corrosion on reciprocating scratch behaviors of phosphate laser glass, Int. J. Appl. Glass Sci. 9 (2018) 352–363.
173
[34] A.J. Barthel, J. Luo, K.S. Hwang, J.Y. Lee, S.H. Kim, Boundary lubrication effect of organic residue left on surface after evaporation of organic cleaning solvent, Wear 350 (2016) 21–26. [35] G. Pharr, W. Oliver, Measurement of thin film mechanical properties using nanoindentation, MRS Bull. 17 (7) (2002) 28–33. € ncke, L. Wondraczek, Plas[36] R. Limbach, A. Winterstein-Beckmann, J. Dellith, D. Mo ticity, crack initiation and defect resistance in alkali-borosilicate glasses: from normal to anomalous behavior, J. Non Cryst. Solids 417-418 (2015) 15–27. [37] S.H. Hahn, J.M. Rimsza, L.J. Criscenti, W. Sun, L. Deng, J. Du, T. Liang, S.B. Sinnott, A.C.T. van Duin, Development of a Reaxff reactive force field for Nasiox/water systems and its application to sodium and proton self-diffusion, J. Phys. Chem. C 122 (34) (2018) 19613–19624. [38] S.H. Hahn, H. Liu, S.H. Kim, A.C.T. Van Duin, Atomistic understanding of surface wear process of sodium silicate glass in dry versus humid environments, J. Am. Ceram. Soc. (2020) in press, doi: 10.1111/jace.17008. [39] P. Zhang, S.X. Li, Z.F. Zhang, General relationship between strength and hardness, Mater. Sci. Eng. A 529 (2011) 62–73. [40] J. Yu, W. Yuan, H. Hu, H. Zang, Y. Cai, F. Ji, Nanoscale friction and wear of phosphate laser glass and BK7 glass against single CeO2 particle by AFM, J. Am. Ceram. Soc. 98 (4) (2015) 1111–1120. [41] S. Yoshida, H. Sawasato, T. Sugawara, Y. Miura, J. Matsuoka, Effects of indenter geometry on indentation-induced densification of soda-lime glass, J. Mater. Res. 25 (11) (2010) 2203–2211. [42] Y. Kato, H. Yamazaki, S. Itakura, S. Yoshida, J. Matsuoka, Load dependence of densification in glass during Vickers indentation test, J. Am. Ceram. Soc. Jpn. 119 (2) (2011) 110–115. [43] K.W. Peter, Densification and flow phenomena of glass in indentation experiments, J. Non Cryst. Solids 5 (2) (1970) 103–115. [44] T. Deschamps, J. Margueritat, C. Martinet, A. Mermet, B. Champagnon, Elastic moduli of permanently densified silica glasses, Sci. Rep. 4 (2014) 7193. [45] G.N. Greaves, A.L. Greer, R.S. Lakes, T. Rouxel, Poisson’s ratio and modern materials, Nat. Mater. 10 (2011) 823–837. [46] X. Guo, C. Zhai, R. Kang, Z. Jin, The mechanical properties of the scratched surface for silica glass by molecular dynamics simulation, J. Non Cryst. Solids 420 (2015) 1–6. [47] T.A. Michalske, B.C. Bunker, Slow fracture model based on strained silicate structures, J. Appl. Phys. 56 (1984) 2686–2693. [48] E. Silva, J. Li, D. Liao, S. Subramanian, T. Zhu, S. Yip, Atomic scale chemo-mechanics of silica: nano-rod deformation and water reaction, J. Comput. Aided Mater. Des. 13 (2006) 135–159. [49] J. Luo, H. Huynh, C.G. Pantano, S.H. Kim, Hydrothermal reactions of soda lime silica glass Revealing subsurface damage and alteration of mechanical properties and chemical structure of glass surfaces, J. Non Cryst. Solids 452 (2016) 93–101. [50] B.T. Poe, C. Romano, G. Henderson, Raman and XANES spectroscopy of permanently densified vitreous silica, J. Non Cryst. Solids 341 (2004) 162–169. , Indentation deformation mecha[51] T. Rouxel, H. Ji, J.P. Guin, F. Augereau, B. Ruffle nism in glass: densification versus shear flow, J. Appl. Phys. 107 (2010) 094903. [52] M. Nogami, M. Tomozawa, Effect of stress on water diffusion in silica glass, J. Am. Ceram. Soc. 67 (2) (1984) 151–154. [53] J.D. Mackenzie, High-pressure effects on oxide glasses: I, densification in rigid state, J. Am. Ceram. Soc. 46 (10) (1963) 461–470. [54] J. Yeon, A.C.T. van Duin, S.H. Kim, Effects of water on tribochemical wear of silicon oxide interfaces: molecular dynamics (MD) study with reactive force field (ReaxFF), Langmuir 32 (2016) 1018–1026. [55] S. Sawamura, L. Wondraczek, Scratch hardness of glass, Phys. Rev. Mater. 2 (2018) 092601(R). [56] O. Benzine, S. Bruns, Z. Pan, K. Durst, L. Wondraczek, Local deformation of glasses is mediated by rigidity fluctuation on nanometer scale, Adv. Sci. 5 (2018) 1800916. [57] K.D. Vargheese, A. Tandia, J.C. Mauro, Origin of dynamical heterogeneities in calcium aluminosilicate liquids, J. Chem. Phys. 132 (2010) 194501. [58] G.G. Adams, M. Nosonovsky, Contact modeling-forces, Tribol. Int. 33 (5 6) (2000) 431–442. [59] N.D. Surdyka, C.G. Pantano, S.H. Kim, Environmental effects on initiation and propagation of surface defects on silicate glasses: scratch and fracture toughness study, Appl. Phys. A. 116 (2014) 519–528. [60] J. Luo, W. Grisales, M. Rabii, C.G. Pantano, S.H. Kim, Differences in surface failure modes of soda lime silica glass under normal indentation versus tangential shear: a comparative study on Na+/K+-ion exchange effects, J. Am. Ceram. Soc. 102 (4) (2019) 1665–1676. [61] B.R. Lawn, Partial cone crack formation in a brittle material loaded with a sliding spherical indenter, Proc. Royal. Soc. Lond. A. 299 (1967) 307–316.