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Velocity-dependent wear behavior of phosphate laser glass
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Velocity-dependent wear behavior of phosphate laser glass Hongtu He, Liang Yang, Jiaxin Yu∗, Yafeng Zhang, Huimin Qi Key Laboratory of Testing Technology for Manufacturing Process in Ministry of Education, State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang, 621010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphate glass Water Velocity Crack Tribochemical wear
Using a reciprocating sliding tribometer, the velocity-dependent wear behaviors of phosphate laser (PL) glass were investigated in dry and humid air. The experimental results show that the velocity dependence of wear in PL glass is very sensitive to the presence of water. In dry air, the velocity-dependent wear of PL glass shows fracture-dominated damage behavior. With increasing velocity, the Hertzian cracks increase first and then tend to saturation. Simultaneously, the material-removal volume also increases first and then keeps almost unchanged. However in humid air, the wear mechanism transforms into tribochemistry-controlled wear process, and almost no crack forms on glass surface for various velocities. With increasing velocity, the stress-enhanced hydrolysis becomes weaker and material-removal volume of PL glass decreases sharply. These results may help understand the surface damage and material removal of phosphate laser glass during machining and serving in various conditions.
1. Introduction Researches on phosphate laser (PL) glass have rapidly grown over the past decades due to the strong interests in its applications in highpeak-power laser system [1]. In many cases, PL glasses are required to obtain high-quality optical surfaces and almost defect-free subsurface via a series of machining processes, such as grinding, abrading and polishing [2]. The origin of material removal of PL glass during these machining processes is the interaction between glass and machining tool (abrasive particles) under the given applied load and shearing velocity [3]. Depending on those machining parameters, the materialremoval rate, residual stress distribution, and (sub)surface damage of PL glass will vary to affect the glass surface quality [4,5]. Tribological experiments provide an access to mimic the interactions between glass and machining tool to understand the material-removal and damage process of PL glass. Previously, it was found that the increased applied load and friction cycles can increase the probability of material-removal and subsurface fracture in PL glass during scratch tests [6,7]. More importantly, both liquid water and adsorbed water from humid air can aggravate the material-removal of PL glass, but suppress the subsurface fracture [6–9]. The enhanced material-removal by water was ascribed to the aggravated hydrolysis of P-O-P glass network under frictional shear stress, while the suppressed subsurface fracture was due to the lubricant role of water [6]. In addition to the applied load and environments that glass is exposed to, the chemical
∗
activity of counter-surface also significantly affects the wear behavior of PL glass. It was found that when rubbing against a silica ball, the wear volume of PL glass was lower in water than that in dry air, while it was higher in water than that in dry air when rubbing against Si3N4, Al2O3, and ZrO2 balls [10]. Furthermore, CeO2 particle was found to make PL glass suffered from tribochemical wear more susceptibly than diamond tip at nanoscale [11]. All these previous investigations didn't consider the role of sliding velocity in the glass wear/damage. Actually, sliding velocity can significantly affect the wear/damage behaviors of some solid materials [12]. For instance, it was reported that with increasing sliding velocity, the interfacial heat can increase significantly, resulting in an increase of the oxide wear of C/C-SiC composites containing Ti3SiC2 [13], and the increase of plastic deformation of multi-wall carbon nanotubes [14]. In contrast, with the increase in sliding velocity, the wear of Si3N4 decreased [15]. It was demonstrated that the increased interfacial heat decreased the amount of adsorbed water at sliding interface, weakening the rates of water-induced tribochemical reaction of Si3N4 and resulting in a lower wear rate. Moreover, the similar velocity-dependent wear behaviors also were observed in the gallium arsenide (GaAs) and monocrystalline silicon at nanoscale [16,17]. It was proposed that, at high velocity conditions, the interfacial water bridge formed more difficultly due to the insufficient contact time; while at low velocity conditions, the formation and rupture of interfacial bonding bridge was more sufficient to cause a serious chemical wear of GaAs and
Corresponding author. E-mail address: [email protected] (J. Yu).
https://doi.org/10.1016/j.ceramint.2019.06.232 Received 5 April 2019; Received in revised form 8 June 2019; Accepted 21 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hongtu He, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.06.232
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monocrystalline silicon. Therefore, sliding velocity can influence the accumulation of interfacial heat and adsorption of water impinging from the gas phase, affecting the material removal of some crystals and ceramics. Oxide glass is a kind of typical brittle material; also, most of them readily react with water molecule under frictional shear stress. As a result, the sliding velocity is expected to affect the friction and wear behaviors of oxide glasses in humid air. However, there are rare investigations to report the velocity dependence of wear behaviors in glass materials. It was reported that the wear volume of soda lime silica glass showed an inverse power law dependence on scratching velocity [18], and they speculated that the time of sliding contact and the shear stress beneath the indenter were the main reason for the velocity-dependent scratch behaviors. The investigation above only considered the physical response of glass under various sliding velocities. In fact, the chemical contribution also needs to be considered in the velocity-dependent wear behavior of oxide glasses. More recently, it was found that the wear volume of soda lime silica glass increased with scratching velocity in dry air, which was contributed from the increased frictioninduced temperature rise with scratching velocity; by contrast, the wear volume of soda lime silica glass decreased with scratching velocity in humid air, which was attributed to the much less mechanochemical reactions at high velocity conditions [19]. Compared to silicate glasses, the water resistances of phosphate glasses are much poorer due to its instinct chemical structure [20]. Therefore, the velocity-dependent material-removal and cracking of phosphate glasses will be different from that of silicate glasses. Nevertheless, how the sliding velocity affects the material-removal and damage of PL glass still need to be addressed. In present paper, the effects of sliding velocity on the friction and wear of PL glass were investigated by rubbing against an alumina ball in dry and humid environments. The friction, material-removal, and surface/subsurface cracking of PL glass were quantitatively studied, the water-related chemical contribution was emphasized, and the corresponding mechanisms were discussed. Research results provide further insights into the role of water and sliding velocity on the wear behaviors in PL glass during friction process.
Fig. 1. Friction coefficient vs. the number of cycles under various velocities in dry air (a) and humid air (b), and the stable friction coefficient as a function of sliding velocity in the two environments (c).
2. Experimental methods of wear cycles was 50, and the sliding velocity, v, was set to vary from 0.2 to 7.5 mm/s. After the wear tests, the images of wear tracks were obtained by optical microscopy (BX51-P, Olympus, Japan). Then, their three-dimensional topographies were analyzed by a white light scanning profilometry (MFT3000, Rtec, USA). To ensure the repeatability of experiment data, each experiment was repeated by at least 5 times. Whereafter, the wear debris and surface damage details were observed using scanning electron microscopy (EVO18, Zeiss, Germany). The chemical composition of the wear debris of PL glass under various sliding velocities was detected by Raman spectra (InVia, Renishaw, England), and the chemical compositions of wear track on PL glass under various sliding velocities were analysed by an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific Inc., USA). AlKα X ray was used as source. All binding energies were referenced to carbon 1s component set to 284.8 eV.
The glass sample used in the current study is polished N31 Nddoped phosphate laser glass (50–60 wt% P2O5, 8–12 wt% Al2O3, 10–14 wt% K2O, 8–12 wt% BaO, 2–3 wt% Li2O, 1–3 wt% Nd2O3), purchased from Shanghai Daheng Optics and Fine Mechanics Co., Ltd., China, which is an important optical glass used in high peak-power laser systems. Alumina ball with 4 mm diameter was used as a countersurface. The physical properties of PL glass and alumina ball were listed in Table 1. All the friction and wear tests were performed via a universal ballon-flat reciprocating sliding tribometer (MFT3000, Rtec, USA). These tests were performed in dry air (relative humidity < 3%) and humid air (relative humidity = 55 ± 2%) at room temperature (22 ± 2 °C). The dry air environments were achieved by only flowing dry air into the environmental chamber, while the humid air was achieved by flowing the water and mixed dry air into the chamber. A RH detector was used to real-time monitor the RH in the chamber. All these preparation and cleaning details can be found elsewhere [6,9]. During the friction and wear experiments, the normal load was a constant of 0.5 N, the number
3. Results 3.1. Velocity-dependent friction coefficient of PL glass
Table 1 Physical properties of PL glass and alumina ball. Materials
Hardness (GPa)
Young's modulus (GPa)
Poisson's ratio
PL glass Alumina ball
3.9 18
67 380
0.27 0.22
Fig. 1(a) and (b) show the friction coefficient as a function of friction cycles in dry and humid air. In dry air, the friction coefficient is initially at ∼0.2, then increases to 0.9–1.1 when the friction cycles attains 5–10 for various sliding velocity, indicating a 5–10 cycles’ running-in period. Afterwards, the friction coefficient become stable as 2
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Fig. 2. Optical microscopy images of wear tracks in PL glass under various sliding velocities in dry and humid air.
the sliding went on, and the stable value in friction coefficient depends on the velocities. In contrast to dry air, the running-in period of friction coefficient in humid air became obvious longer, regardless of sliding velocities (Fig. 1(b)). It implies that adsorbed water molecular impinging from the gas phase may affect both the friction and wear of PL glass in humid air. In order to quantitatively compare the effects of velocity on the friction coefficient of PL glass in dry and humid air, Fig. 1(c) summarizes the stable friction coefficient as a function of sliding velocity based on the results in Fig. 1 (a) and (b). It is clear that, with the increase in velocity from 0.2 to 7.5 mm/s, the corresponding stable friction coefficient increases gradually from ∼0.9 to ∼1.1 in dry air, while it decreases gradually from ∼0.9 to ∼0.6 in humid air. It is worth noting that the stable friction coefficient in humid air is significantly lower than that in dry air, especially for the higher velocity conditions. 3.2. Velocity-dependent wear behaviors of PL glass The velocity-dependent wear behaviors of PL glass are also different in dry and humid air, as shown in Fig. 2. In dry air, the wear debris (worn particles) is very little at low velocity conditions (0.2 mm/s). As the velocity increases, the wear debris begins to scatter over the entire wear track. By contrast, the distribution of wear debris in humid air becomes different. Under a lower velocity of 0.2 mm/s, wear debris accumulates mostly at the tail of wear track by reciprocating friction, and little wear debris adheres inside the wear track. As the sliding velocity increases, wear debris at the tail of wear track become less, and partial wear debris begin to adhere inside the wear track. When the velocity increases to 7.5 mm/s, almost all the wear debris are adhered inside the wear track rather than scattered over the tail of the wear track. To observe the details of the wear debris under various velocities, Fig. 3 selectively shows the SEM images of wear debris at the wear track at the lowest and highest velocities. In dry air, at the lowest velocity (0.2 mm/s), wear debris are chipped into regular flakes during the wear process because of the sufficient time for the sliding contact. At the highest velocity (7.5 mm/s), roller-shaped and chip-shaped wear debris are found on the track, due to the very fast reciprocating sliding. All the results in dry air show the characteristic of typical mechanical wear. In contrast, the wear debris in humid air become larger than in dry air, which might due to that wear debris become more hydrated by adsorption of water molecules impinging from the gas phase. At the lowest velocity of 0.2 mm/s, there are large agglomerates of wear particles adhered outside of the sliding contact area. As the velocity increases to 7.5 mm/s, the wear debris adheres inside the contact area and is cut into some larger and hydrated chips. Fig. 4 (a) shows the profilometry images of PL glass under various velocities without any cleaning treatment. Obvious wear debris adhesion is observed inside the wear track at high velocity (7.5 mm/s) in
Fig. 3. SEM images of wear debris in PL glass under the velocities of 0.2 and 7.5 mm/s in dry and humid air.
humid air, which makes the worn glass surface hillock-like, rather than a typical wear groove. These results are in good agreement with the results in Fig. 2. After cleaning these frictional glass surfaces, the profilometry images of all wear tracks are measured again. Fig. 4 (b) and (c) shows the profilometry images and corresponding cross-sectional profile lines of these wear tracks in PL glass after cleaning surface. It is clear that all the wear tracks become groove-like, confirming the wear debris adhered inside the wear track can be cleaned off [21]. As shown in Fig. 4(c), in dry air, the typical wear width and depth of PL glass are ∼90 μm and ∼0.05 μm under a low velocity of 0.2 mm/s. As the velocity increases, the corresponding wear groove gradually becomes wider and deeper. When the velocity increases to 1.5 mm/s, the wear width and depth increases to ∼150 μm and ∼0.1 μm, respectively. Then, the wear of PL glass becomes almost independent on the sliding velocity under the given velocity conditions. When the velocity increases to 7.5 mm/s, the typical wear width and depth only varies to ∼180 μm and ∼0.09 μm, respectively. In contrast to dry air, the adsorbed water impinging from the gas phase make the velocity 3
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Fig. 5. The material-removal volume of PL glass as a function of sliding velocity in dry and humid air.
4. Discussions 4.1. Velocity-dependent surface cracking in PL glass PL glass is a kind of typical brittle material, and the friction-induced fracture is considered as the important damage mode in the previous scratch researches of some other oxide glasses [6,18]. Due to the coverage of wear debris, cracks cannot be observed in the wear tracks, as shown in Figs. 2 and 3. However, after cleaning the wear debris in the wear tracks, the surface cracking of PL glass can be observed. Fig. 6 shows the SEM images of wear tracks in PL glass under various sliding velocities in dry and humid air after cleaning wear debris. It shows different cracking degree depended on the velocity and environment. In dry air, there are a little Hertzian cracks combined with some mild scratches inside the track at the low velocity of 0.2 mm/s. Two directions of Hertzian cracks are observed due to the reciprocating scratches. As the velocity increases from 0.2 mm/s to 1.5 mm/s, the Hertzian cracks become longer and more intensive. With the further increase in velocity from 1.5 mm/s to 7.5 mm/s, the length and density of Hertzian cracks changes slightly. However in humid air, for all velocity conditions, there are almost no Hertzian cracks to be found on the glass surface, instead, only some mild scratches are observed inside the wear track. Once the Hertzian cracks appear on the glass surface during the friction process, they are expected to propagate towards glass subsurface, causing obvious subsurface damage. Fig. 7 selectively compares the subsurface damage of PL glass beneath the wear tracks at the lowest and highest velocities in the two environments. Some curve-shaped subsurface fracture zones are observed underneath the wear track for dry air condition. As the velocity increases from 0.2 to 7.5 mm/s, the subsurface fracture zone become larger and shear deformation are more serious, combined some secondary cracks. It needs to note that the depth of subsurface fracture zone (tens of micrometers) is significantly deeper than the material-removal depth (≤∼0.1 μm, as shown in Fig. 4), indicating that the cracking is a more dominant damage mode than material-removal in dry air. All these results reveal a typical mechanically-damaged mechanism of PL glass in dry air. However, in humid air, there is almost no discernible subsurface damage, even if the velocity attains 7.5 mm/s. These results confirm that cracking dominates the surface and subsurface damage of PL glass in dry air, but not in humid air. In our previous nanowear research of PL glass, it was found that both the damage mode and wear volume of PL glass in dry air were almost same with those in vacuum condition [11]. It indicates the medium in dry air, such as oxygen and nitrogen, cannot affect the wear of PL glass. As a result, the wear mechanism of PL glass in dry air is same with that in vacuum, which is dominated by mechanical interaction without tribochemical wear. Accordingly, it is believed that, in this work, the velocity-dependent wear behaviors of PL glass are also
Fig. 4. Optical profilometry images of the wear tracks in PL glass without cleaning (a) and after cleaning wear debris (b) in dry air and humid air. (c) shows the cross-sectional profile lines of wear tracks in (b).
dependence of wear in PL glass become completely different. The wear width and depth of PL glass are ∼150 μm and ∼0.45 μm at the low velocity of 0.2 mm/s, which is much more severe than that in dry air under the same velocity condition. As the velocity increases, the corresponding wear groove gradually become narrower and shallower, showing a complete different trend compared to that in dry air. When the sliding velocity increases to 7.5 mm/s, the typical wear width and depth of PL glass decreases to only ∼70 μm and ∼0.08 μm, respectively. Based on the cross-sectional lines in Figs. 4(c), Fig. 5 quantitatively shows the material-removal volume of PL glass as a function of sliding velocity in the two environments. It can be found that, in dry condition, with the increase in velocity from 0.2 mm/s to 1.5 mm/s, the corresponding material-removal volume increases from ∼0.1 × 105 μm3 to ∼0.5 × 105 μm3, then the material-removal volume seems to be independent on velocity with the further increase in velocity from 1.5 mm/s to 7.5 mm/s. However, in humid air, with the increase in sliding velocity from 0.2 mm/s to 7.5 mm/s, the material removal volume decreases from 1.8 × 105 μm3 to 0.15 × 105 μm3.
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Fig. 6. SEM images of wear tracks in PL glass under various sliding velocities in dry and humid air after cleaning the wear debris.
in dry air and vacuum environments under the same load conditions [7,11,28]. The capillary bridge formed from adsorbed water was proposed to help in hydrolyzing P-O-P network under the frictional shear stress, and thus accelerated the material-removal of PL glass in humid air. Therefore in this study, the different material-removal volume at various velocities in humid air is also expected to be related to the stress-enhanced hydrolysis induced by interfacial absorbed water. In order to explore the role of tribochemical reactions in the material-removal of PL glass under various velocity conditions, Fig. 8 selectively compares the high-resolution P2p spectra measured by XPS in the wear track of PL glass at lowest and highest velocity in dry and humid air. Clearly, no significant change of intensity of P for the dry air cases, regardless of the sliding velocities. This implies that no chemical process involves in the wear of PL glass in dry air. In contrast, the intensity of P in the wear track of PL glass after wear tests in humid air is lower than the pristine glass. These results suggest a tribochemically modified surface layer may form at the wear track of PL glass after wear tests in humid air. At lower sliding velocity conditions, the much lower intensity of P suggests a more severe tribochemical wear process occurs for the PL glass, which is consistent with more material removal of PL glass in humid air at lower sliding velocity conditions (Fig. 5). Moreover, Fig. 9 compares the Raman spectra of the wear debris at PL glass surface under various sliding velocities in dry and humid air. It is clear that there is no significant change of P-O-P network after wear tests, observing from the peak at 704 cm−1 and 1204 cm−1 [29,30]. Note that in the typical O-H stretching region (3000-3400 cm−1), the peak at ∼3200 cm−1 is obvious in humid air, but not discernible in dry air case regardless of sliding velocity. The results in Figs. 8 and 9 confirm that PL glass is damaged mechanically in dry air, but tribochemical wear involving water molecules contributes to the material removal in humid air. Additionally, the intensity of the peak at ∼3200 cm−1 at the high velocity of 7.5 mm/s is weaker than that at the
Fig. 7. Optical microscopy images of the subsurface beneath the wear tracks in PL glass at the velocities of 0.2 and 7.5 mm/s in dry and humid air.
dominated by mechanical wear in dry air. In mechanical wear, the material damage would become more severe with increasing sliding velocity due to the stronger impact [22]. For brittle material, stronger impact at higher sliding velocity is expected to cause more serious cracking, which agrees with observed more Hertzian crack and deeper subsurface fracture of PL glass at higher sliding velocity, as shown in Figs. 6 and 7. Therefore, in dry air, the velocity-dependent wear of PL glass is dominated by mechanical interaction where the stronger impact at higher sliding causes a more severe damage.
4.2. Velocity-dependent tribochemical wear in humid air The surface and subsurface cracking are greatly suppressed in humid air, indicating that the damage mode of PL glass during the wear process in humid air is dominated by material-removal rather than brittle fracture. As a result, the water molecules in humid air play an important role to affect the velocity-dependent wear mechanism of PL glass. The results in Fig. 4 indicate that the material-removal is significantly affected by sliding velocity. Different from the dry conditions, the material-removal volume of PL glass is found to decrease sharply with increasing velocity, especially in the relatively low velocity range (v < 3 mm/s). The reduced material-removal volume at higher velocity conditions should be related to the tribochemical effects involving water absorption at the friction interface. Generally, water can corrode PL glass due to its poor chemical durability [23,24]. It can even occur under stress-free conditions by hydrolyzing P-O-P network, and this hydrolysis process can be expressed as Eq. (1) [9,10]: P-O-P + H2O→P-OH + OH-P
(1)
Under tensile and shear stress, this chemical reaction in Eq. (1) can be accelerated, causing a more serious damage on glass surface. This process is also known as stress-enhanced hydrolysis or stress-corrosion for most glass materials [25–27]. In our previous studies, it was found that nanowear of PL glass in humid air was much more severe than that
Fig. 8. High-resolution P2p spectra measured by XPS in the wear track of PL glass at the velocities of 0.2 and 7.5 mm/s in dry and humid air. 5
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subsequent tribochemical reactions (stress-enhanced hydrolysis), therefore, the material removal decreases with increasing velocity. For some other known materials, such as Si3N4 [15], Si(100) [16], GaAs [17], the fact that the wear of these materials decrease with the increasing sliding velocity has also been reported. The velocity-dependent wear behaviors for all these materials are verified to be related to the tribochemical reaction with the environmental medium. The less water absorption at the frictional interface under the higher sliding velocity contributes to the weaker wear of all these materials, which shares the similar wear mechanism of PL glass in humid air at various sliding velocities. It is worth noting that the at lower sliding velocity conditions (v < 2 mm/s), the wear of PL glass in dry air is lower than that in humid air; in contrast, the wear of PL glass in dry air is higher than that in humid air when the sliding velocities exceed 2 mm/s (Fig. 5). Thus, another important aspect can be addressed in present study. Despite the water resistance of PL glass is poor, the presence of water at PL glass interface can not always guarantee a more serious material-removal than the dry air case any more. Depending on the sliding velocity, the wear of PL glass in dry air can be lower or higher than the humid air case, regardless of the dominating roles in the surface and subsurface damage of PL glass. It also should be noted that the velocity-dependent of wear of PL glass is striking different from that of soda lime silica glass, where the wear volume of soda lime silica glass in dry air is always more severe than that in humid air, regardless of the sliding velocities [19]. There are two possible reasons for this. On the one hand, the counter-surface is pyrex glass ball for the wear of soda lime silica glass, and the pyrex glass ball surface adheres lots of wear debris in both dry and humid conditions [19]. In the case of PL glass, the counter-surface is alumina ball, which shows no measurable wear in both dry and humid air. On the other hand, the unit structure of PL glass and soda lime silica glass are different. Generally, the P-O-P phosphate units readily interconnect in linear chain-like structure, rather than the cross-link structure in silicate glasses [20]. Therefore, due to the discrepancies in mechanical and chemical durability, the wear of PL glass in dry air can be lower or higher than that in humid air, depending on the sliding velocity (Fig. 5). In summary, velocity-dependent wear behaviors of PL glass are strongly depended on the environment conditions, and the surface damage and material-removal are very sensitive to the presence of water. In dry environment, the velocity-dependent wear behavior of PL glass is determined by the strength of mechanical interaction between glass and counter-surface at various sliding velocities. In contrast, in humid air, that is controlled by the degree of tribochemical wear (stress-enhanced hydrolysis) at various velocities. As a result, the wear of PL glass is a velocity and humid coupled wear process. The data in this work provide a clue that, to completely understand the PL glass behavior, more relative humidity need be considered during the wear experiment, in addition to considering various velocities. This interesting issue is worth to be investigated in the future works.
Fig. 9. Raman spectra of the wear debris in PL glass at the velocities of 0.2 and 7.5 mm/s in dry and humid air.
low velocity of 0.2 mm/s, implying wear debris contained more OH groups after wear tests at lower velocity conditions. The increased OH groups should come from the hydrolysis of P-O-P network following Eq. (2), suggesting that the stress-enhanced hydrolysis of PL glass becomes more serious with the decrease in sliding velocity. Both the lower intensity of P in XPS spectra at wear track (Fig. 8) and stronger OH stretching of wear debris (Fig. 9) at lower velocity conditions in humid air reveal the stronger hydrolysis of PL glass at lower sliding velocity. The stronger stress-enhanced hydrolysis must come from higher shear stress or more water adsorption. In Fig. 1(c), the friction coefficient at 0.2 mm/s is 1.5 times of that at 7.5 mm/s, indicating the higher shear stress at lower sliding velocity, which contributes the stronger stress-enhanced hydrolysis on the one hand. On the other hand, the friction induced interfacial temperature rise would affect the interfacial water absorption and further affect the hydrolysis intensity. During the wear process, friction heat produced at sliding interface is well-known to increase with increasing sliding velocity [31,32], and the friction-induced temperature rise can be estimated by Eq. (2) according to some previous researches [31,32]:
ΔT =
μPv 4a (k1 + k2)
(2)
Where ΔT is the temperature rise, μ is the friction coefficient, P is the applied normal load, v is the sliding velocity, a is the radius of contact asperity at the contact area, k1 and k2 is the thermal conductivity coefficient of PL glass and alumina ball, respectively. Based on Eq. (2), it is conceivable that, under the same load condition, ΔT is proportional to the product of friction coefficient and velocity. According to the results in Fig. 1(c), the ΔT at the highest velocity (7.5 mm/s) is estimated to be ∼43 times higher than that at the lowest velocity (0.2 mm/ s). The higher temperature rise will significantly obstruct water absorption and reduce the amount of reacted water molecules at friction interface, thus further weaken stress-enhanced hydrolysis. Moreover, the contact time between friction pairs is relatively shorter at the higher velocity, which also reduces the possibility of water absorption at frictional interface, resulting in a weaker hydrolysis. In contrast, at low velocity conditions, the sliding contact time is long enough for water adsorption to allow the more sufficient reaction between absorbed water molecules and glass network under frictional shear stress. Stressenhanced hydrolysis destroys the backbone of PL glass and thus assists the material removal at low velocity. This is why we found the lower intensity of P in XPS spectra (Fig. 8) and stronger OH stretching (Fig. 9) at lower velocity conditions in humid air. In a word, high sliding velocity increases the interfacial temperature, reduces the sliding contact time, further significantly confines interfacial water adsorption and
5. Conclusion The velocity-dependent wear behaviors of PL glass are investigated by a reciprocating ball-on-flat tribometer in dry and humid air. The experimental results show that velocity dependence of wear in PL glass is very sensitive to the presence of water. In dry air, the velocity-dependent wear of PL glass shows fracture-dominated damage behavior. With the increase in sliding velocity, the Hertzian cracks increases first and then tends to saturation. Simultaneously, the material-removal volume of PL glass also increases first and then keeps almost unchanged. In contrast, in humid air, the wear mechanism transforms into tribochemical controlled wear process and almost no cracks forms for all the velocity conditions. With the increase in sliding velocity, the material-removal volume of PL glass decreases in humid air. At the lower velocity condition, the absorbed water film between the contact 6
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asperities forms more readily, and the stress-enhanced hydrolysis occurs more readily due to the relatively low interfacial temperature rise and more abundant sliding contact time, which assists the materialremoval of PL glass.
[15]
[16]
Acknowledgement The authors are grateful for financial support from the National Natural Science Foundation of China [Grant No. 51575462 and No. 51605401], and the Scientific Research Fund of Sichuan Provincial Education Department, China [18ZA0504, 17ZA0408], and Research Fund Supported by Sichuan Science and Technology Program [18YYJC0905].
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Velocity-dependent wear behavior of phosphate laser glass Hongtu He, Liang Yang, Jiaxin Yu∗, Yafeng Zhang, Huimin Qi Key Laboratory of Testing Technology for Manufacturing Process in Ministry of Education, State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and Technology, Mianyang, 621010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphate glass Water Velocity Crack Tribochemical wear
Using a reciprocating sliding tribometer, the velocity-dependent wear behaviors of phosphate laser (PL) glass were investigated in dry and humid air. The experimental results show that the velocity dependence of wear in PL glass is very sensitive to the presence of water. In dry air, the velocity-dependent wear of PL glass shows fracture-dominated damage behavior. With increasing velocity, the Hertzian cracks increase first and then tend to saturation. Simultaneously, the material-removal volume also increases first and then keeps almost unchanged. However in humid air, the wear mechanism transforms into tribochemistry-controlled wear process, and almost no crack forms on glass surface for various velocities. With increasing velocity, the stress-enhanced hydrolysis becomes weaker and material-removal volume of PL glass decreases sharply. These results may help understand the surface damage and material removal of phosphate laser glass during machining and serving in various conditions.
1. Introduction Researches on phosphate laser (PL) glass have rapidly grown over the past decades due to the strong interests in its applications in highpeak-power laser system [1]. In many cases, PL glasses are required to obtain high-quality optical surfaces and almost defect-free subsurface via a series of machining processes, such as grinding, abrading and polishing [2]. The origin of material removal of PL glass during these machining processes is the interaction between glass and machining tool (abrasive particles) under the given applied load and shearing velocity [3]. Depending on those machining parameters, the materialremoval rate, residual stress distribution, and (sub)surface damage of PL glass will vary to affect the glass surface quality [4,5]. Tribological experiments provide an access to mimic the interactions between glass and machining tool to understand the material-removal and damage process of PL glass. Previously, it was found that the increased applied load and friction cycles can increase the probability of material-removal and subsurface fracture in PL glass during scratch tests [6,7]. More importantly, both liquid water and adsorbed water from humid air can aggravate the material-removal of PL glass, but suppress the subsurface fracture [6–9]. The enhanced material-removal by water was ascribed to the aggravated hydrolysis of P-O-P glass network under frictional shear stress, while the suppressed subsurface fracture was due to the lubricant role of water [6]. In addition to the applied load and environments that glass is exposed to, the chemical
∗
activity of counter-surface also significantly affects the wear behavior of PL glass. It was found that when rubbing against a silica ball, the wear volume of PL glass was lower in water than that in dry air, while it was higher in water than that in dry air when rubbing against Si3N4, Al2O3, and ZrO2 balls [10]. Furthermore, CeO2 particle was found to make PL glass suffered from tribochemical wear more susceptibly than diamond tip at nanoscale [11]. All these previous investigations didn't consider the role of sliding velocity in the glass wear/damage. Actually, sliding velocity can significantly affect the wear/damage behaviors of some solid materials [12]. For instance, it was reported that with increasing sliding velocity, the interfacial heat can increase significantly, resulting in an increase of the oxide wear of C/C-SiC composites containing Ti3SiC2 [13], and the increase of plastic deformation of multi-wall carbon nanotubes [14]. In contrast, with the increase in sliding velocity, the wear of Si3N4 decreased [15]. It was demonstrated that the increased interfacial heat decreased the amount of adsorbed water at sliding interface, weakening the rates of water-induced tribochemical reaction of Si3N4 and resulting in a lower wear rate. Moreover, the similar velocity-dependent wear behaviors also were observed in the gallium arsenide (GaAs) and monocrystalline silicon at nanoscale [16,17]. It was proposed that, at high velocity conditions, the interfacial water bridge formed more difficultly due to the insufficient contact time; while at low velocity conditions, the formation and rupture of interfacial bonding bridge was more sufficient to cause a serious chemical wear of GaAs and
Corresponding author. E-mail address: [email protected] (J. Yu).
https://doi.org/10.1016/j.ceramint.2019.06.232 Received 5 April 2019; Received in revised form 8 June 2019; Accepted 21 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hongtu He, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.06.232
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monocrystalline silicon. Therefore, sliding velocity can influence the accumulation of interfacial heat and adsorption of water impinging from the gas phase, affecting the material removal of some crystals and ceramics. Oxide glass is a kind of typical brittle material; also, most of them readily react with water molecule under frictional shear stress. As a result, the sliding velocity is expected to affect the friction and wear behaviors of oxide glasses in humid air. However, there are rare investigations to report the velocity dependence of wear behaviors in glass materials. It was reported that the wear volume of soda lime silica glass showed an inverse power law dependence on scratching velocity [18], and they speculated that the time of sliding contact and the shear stress beneath the indenter were the main reason for the velocity-dependent scratch behaviors. The investigation above only considered the physical response of glass under various sliding velocities. In fact, the chemical contribution also needs to be considered in the velocity-dependent wear behavior of oxide glasses. More recently, it was found that the wear volume of soda lime silica glass increased with scratching velocity in dry air, which was contributed from the increased frictioninduced temperature rise with scratching velocity; by contrast, the wear volume of soda lime silica glass decreased with scratching velocity in humid air, which was attributed to the much less mechanochemical reactions at high velocity conditions [19]. Compared to silicate glasses, the water resistances of phosphate glasses are much poorer due to its instinct chemical structure [20]. Therefore, the velocity-dependent material-removal and cracking of phosphate glasses will be different from that of silicate glasses. Nevertheless, how the sliding velocity affects the material-removal and damage of PL glass still need to be addressed. In present paper, the effects of sliding velocity on the friction and wear of PL glass were investigated by rubbing against an alumina ball in dry and humid environments. The friction, material-removal, and surface/subsurface cracking of PL glass were quantitatively studied, the water-related chemical contribution was emphasized, and the corresponding mechanisms were discussed. Research results provide further insights into the role of water and sliding velocity on the wear behaviors in PL glass during friction process.
Fig. 1. Friction coefficient vs. the number of cycles under various velocities in dry air (a) and humid air (b), and the stable friction coefficient as a function of sliding velocity in the two environments (c).
2. Experimental methods of wear cycles was 50, and the sliding velocity, v, was set to vary from 0.2 to 7.5 mm/s. After the wear tests, the images of wear tracks were obtained by optical microscopy (BX51-P, Olympus, Japan). Then, their three-dimensional topographies were analyzed by a white light scanning profilometry (MFT3000, Rtec, USA). To ensure the repeatability of experiment data, each experiment was repeated by at least 5 times. Whereafter, the wear debris and surface damage details were observed using scanning electron microscopy (EVO18, Zeiss, Germany). The chemical composition of the wear debris of PL glass under various sliding velocities was detected by Raman spectra (InVia, Renishaw, England), and the chemical compositions of wear track on PL glass under various sliding velocities were analysed by an X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific Inc., USA). AlKα X ray was used as source. All binding energies were referenced to carbon 1s component set to 284.8 eV.
The glass sample used in the current study is polished N31 Nddoped phosphate laser glass (50–60 wt% P2O5, 8–12 wt% Al2O3, 10–14 wt% K2O, 8–12 wt% BaO, 2–3 wt% Li2O, 1–3 wt% Nd2O3), purchased from Shanghai Daheng Optics and Fine Mechanics Co., Ltd., China, which is an important optical glass used in high peak-power laser systems. Alumina ball with 4 mm diameter was used as a countersurface. The physical properties of PL glass and alumina ball were listed in Table 1. All the friction and wear tests were performed via a universal ballon-flat reciprocating sliding tribometer (MFT3000, Rtec, USA). These tests were performed in dry air (relative humidity < 3%) and humid air (relative humidity = 55 ± 2%) at room temperature (22 ± 2 °C). The dry air environments were achieved by only flowing dry air into the environmental chamber, while the humid air was achieved by flowing the water and mixed dry air into the chamber. A RH detector was used to real-time monitor the RH in the chamber. All these preparation and cleaning details can be found elsewhere [6,9]. During the friction and wear experiments, the normal load was a constant of 0.5 N, the number
3. Results 3.1. Velocity-dependent friction coefficient of PL glass
Table 1 Physical properties of PL glass and alumina ball. Materials
Hardness (GPa)
Young's modulus (GPa)
Poisson's ratio
PL glass Alumina ball
3.9 18
67 380
0.27 0.22
Fig. 1(a) and (b) show the friction coefficient as a function of friction cycles in dry and humid air. In dry air, the friction coefficient is initially at ∼0.2, then increases to 0.9–1.1 when the friction cycles attains 5–10 for various sliding velocity, indicating a 5–10 cycles’ running-in period. Afterwards, the friction coefficient become stable as 2
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Fig. 2. Optical microscopy images of wear tracks in PL glass under various sliding velocities in dry and humid air.
the sliding went on, and the stable value in friction coefficient depends on the velocities. In contrast to dry air, the running-in period of friction coefficient in humid air became obvious longer, regardless of sliding velocities (Fig. 1(b)). It implies that adsorbed water molecular impinging from the gas phase may affect both the friction and wear of PL glass in humid air. In order to quantitatively compare the effects of velocity on the friction coefficient of PL glass in dry and humid air, Fig. 1(c) summarizes the stable friction coefficient as a function of sliding velocity based on the results in Fig. 1 (a) and (b). It is clear that, with the increase in velocity from 0.2 to 7.5 mm/s, the corresponding stable friction coefficient increases gradually from ∼0.9 to ∼1.1 in dry air, while it decreases gradually from ∼0.9 to ∼0.6 in humid air. It is worth noting that the stable friction coefficient in humid air is significantly lower than that in dry air, especially for the higher velocity conditions. 3.2. Velocity-dependent wear behaviors of PL glass The velocity-dependent wear behaviors of PL glass are also different in dry and humid air, as shown in Fig. 2. In dry air, the wear debris (worn particles) is very little at low velocity conditions (0.2 mm/s). As the velocity increases, the wear debris begins to scatter over the entire wear track. By contrast, the distribution of wear debris in humid air becomes different. Under a lower velocity of 0.2 mm/s, wear debris accumulates mostly at the tail of wear track by reciprocating friction, and little wear debris adheres inside the wear track. As the sliding velocity increases, wear debris at the tail of wear track become less, and partial wear debris begin to adhere inside the wear track. When the velocity increases to 7.5 mm/s, almost all the wear debris are adhered inside the wear track rather than scattered over the tail of the wear track. To observe the details of the wear debris under various velocities, Fig. 3 selectively shows the SEM images of wear debris at the wear track at the lowest and highest velocities. In dry air, at the lowest velocity (0.2 mm/s), wear debris are chipped into regular flakes during the wear process because of the sufficient time for the sliding contact. At the highest velocity (7.5 mm/s), roller-shaped and chip-shaped wear debris are found on the track, due to the very fast reciprocating sliding. All the results in dry air show the characteristic of typical mechanical wear. In contrast, the wear debris in humid air become larger than in dry air, which might due to that wear debris become more hydrated by adsorption of water molecules impinging from the gas phase. At the lowest velocity of 0.2 mm/s, there are large agglomerates of wear particles adhered outside of the sliding contact area. As the velocity increases to 7.5 mm/s, the wear debris adheres inside the contact area and is cut into some larger and hydrated chips. Fig. 4 (a) shows the profilometry images of PL glass under various velocities without any cleaning treatment. Obvious wear debris adhesion is observed inside the wear track at high velocity (7.5 mm/s) in
Fig. 3. SEM images of wear debris in PL glass under the velocities of 0.2 and 7.5 mm/s in dry and humid air.
humid air, which makes the worn glass surface hillock-like, rather than a typical wear groove. These results are in good agreement with the results in Fig. 2. After cleaning these frictional glass surfaces, the profilometry images of all wear tracks are measured again. Fig. 4 (b) and (c) shows the profilometry images and corresponding cross-sectional profile lines of these wear tracks in PL glass after cleaning surface. It is clear that all the wear tracks become groove-like, confirming the wear debris adhered inside the wear track can be cleaned off [21]. As shown in Fig. 4(c), in dry air, the typical wear width and depth of PL glass are ∼90 μm and ∼0.05 μm under a low velocity of 0.2 mm/s. As the velocity increases, the corresponding wear groove gradually becomes wider and deeper. When the velocity increases to 1.5 mm/s, the wear width and depth increases to ∼150 μm and ∼0.1 μm, respectively. Then, the wear of PL glass becomes almost independent on the sliding velocity under the given velocity conditions. When the velocity increases to 7.5 mm/s, the typical wear width and depth only varies to ∼180 μm and ∼0.09 μm, respectively. In contrast to dry air, the adsorbed water impinging from the gas phase make the velocity 3
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Fig. 5. The material-removal volume of PL glass as a function of sliding velocity in dry and humid air.
4. Discussions 4.1. Velocity-dependent surface cracking in PL glass PL glass is a kind of typical brittle material, and the friction-induced fracture is considered as the important damage mode in the previous scratch researches of some other oxide glasses [6,18]. Due to the coverage of wear debris, cracks cannot be observed in the wear tracks, as shown in Figs. 2 and 3. However, after cleaning the wear debris in the wear tracks, the surface cracking of PL glass can be observed. Fig. 6 shows the SEM images of wear tracks in PL glass under various sliding velocities in dry and humid air after cleaning wear debris. It shows different cracking degree depended on the velocity and environment. In dry air, there are a little Hertzian cracks combined with some mild scratches inside the track at the low velocity of 0.2 mm/s. Two directions of Hertzian cracks are observed due to the reciprocating scratches. As the velocity increases from 0.2 mm/s to 1.5 mm/s, the Hertzian cracks become longer and more intensive. With the further increase in velocity from 1.5 mm/s to 7.5 mm/s, the length and density of Hertzian cracks changes slightly. However in humid air, for all velocity conditions, there are almost no Hertzian cracks to be found on the glass surface, instead, only some mild scratches are observed inside the wear track. Once the Hertzian cracks appear on the glass surface during the friction process, they are expected to propagate towards glass subsurface, causing obvious subsurface damage. Fig. 7 selectively compares the subsurface damage of PL glass beneath the wear tracks at the lowest and highest velocities in the two environments. Some curve-shaped subsurface fracture zones are observed underneath the wear track for dry air condition. As the velocity increases from 0.2 to 7.5 mm/s, the subsurface fracture zone become larger and shear deformation are more serious, combined some secondary cracks. It needs to note that the depth of subsurface fracture zone (tens of micrometers) is significantly deeper than the material-removal depth (≤∼0.1 μm, as shown in Fig. 4), indicating that the cracking is a more dominant damage mode than material-removal in dry air. All these results reveal a typical mechanically-damaged mechanism of PL glass in dry air. However, in humid air, there is almost no discernible subsurface damage, even if the velocity attains 7.5 mm/s. These results confirm that cracking dominates the surface and subsurface damage of PL glass in dry air, but not in humid air. In our previous nanowear research of PL glass, it was found that both the damage mode and wear volume of PL glass in dry air were almost same with those in vacuum condition [11]. It indicates the medium in dry air, such as oxygen and nitrogen, cannot affect the wear of PL glass. As a result, the wear mechanism of PL glass in dry air is same with that in vacuum, which is dominated by mechanical interaction without tribochemical wear. Accordingly, it is believed that, in this work, the velocity-dependent wear behaviors of PL glass are also
Fig. 4. Optical profilometry images of the wear tracks in PL glass without cleaning (a) and after cleaning wear debris (b) in dry air and humid air. (c) shows the cross-sectional profile lines of wear tracks in (b).
dependence of wear in PL glass become completely different. The wear width and depth of PL glass are ∼150 μm and ∼0.45 μm at the low velocity of 0.2 mm/s, which is much more severe than that in dry air under the same velocity condition. As the velocity increases, the corresponding wear groove gradually become narrower and shallower, showing a complete different trend compared to that in dry air. When the sliding velocity increases to 7.5 mm/s, the typical wear width and depth of PL glass decreases to only ∼70 μm and ∼0.08 μm, respectively. Based on the cross-sectional lines in Figs. 4(c), Fig. 5 quantitatively shows the material-removal volume of PL glass as a function of sliding velocity in the two environments. It can be found that, in dry condition, with the increase in velocity from 0.2 mm/s to 1.5 mm/s, the corresponding material-removal volume increases from ∼0.1 × 105 μm3 to ∼0.5 × 105 μm3, then the material-removal volume seems to be independent on velocity with the further increase in velocity from 1.5 mm/s to 7.5 mm/s. However, in humid air, with the increase in sliding velocity from 0.2 mm/s to 7.5 mm/s, the material removal volume decreases from 1.8 × 105 μm3 to 0.15 × 105 μm3.
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Fig. 6. SEM images of wear tracks in PL glass under various sliding velocities in dry and humid air after cleaning the wear debris.
in dry air and vacuum environments under the same load conditions [7,11,28]. The capillary bridge formed from adsorbed water was proposed to help in hydrolyzing P-O-P network under the frictional shear stress, and thus accelerated the material-removal of PL glass in humid air. Therefore in this study, the different material-removal volume at various velocities in humid air is also expected to be related to the stress-enhanced hydrolysis induced by interfacial absorbed water. In order to explore the role of tribochemical reactions in the material-removal of PL glass under various velocity conditions, Fig. 8 selectively compares the high-resolution P2p spectra measured by XPS in the wear track of PL glass at lowest and highest velocity in dry and humid air. Clearly, no significant change of intensity of P for the dry air cases, regardless of the sliding velocities. This implies that no chemical process involves in the wear of PL glass in dry air. In contrast, the intensity of P in the wear track of PL glass after wear tests in humid air is lower than the pristine glass. These results suggest a tribochemically modified surface layer may form at the wear track of PL glass after wear tests in humid air. At lower sliding velocity conditions, the much lower intensity of P suggests a more severe tribochemical wear process occurs for the PL glass, which is consistent with more material removal of PL glass in humid air at lower sliding velocity conditions (Fig. 5). Moreover, Fig. 9 compares the Raman spectra of the wear debris at PL glass surface under various sliding velocities in dry and humid air. It is clear that there is no significant change of P-O-P network after wear tests, observing from the peak at 704 cm−1 and 1204 cm−1 [29,30]. Note that in the typical O-H stretching region (3000-3400 cm−1), the peak at ∼3200 cm−1 is obvious in humid air, but not discernible in dry air case regardless of sliding velocity. The results in Figs. 8 and 9 confirm that PL glass is damaged mechanically in dry air, but tribochemical wear involving water molecules contributes to the material removal in humid air. Additionally, the intensity of the peak at ∼3200 cm−1 at the high velocity of 7.5 mm/s is weaker than that at the
Fig. 7. Optical microscopy images of the subsurface beneath the wear tracks in PL glass at the velocities of 0.2 and 7.5 mm/s in dry and humid air.
dominated by mechanical wear in dry air. In mechanical wear, the material damage would become more severe with increasing sliding velocity due to the stronger impact [22]. For brittle material, stronger impact at higher sliding velocity is expected to cause more serious cracking, which agrees with observed more Hertzian crack and deeper subsurface fracture of PL glass at higher sliding velocity, as shown in Figs. 6 and 7. Therefore, in dry air, the velocity-dependent wear of PL glass is dominated by mechanical interaction where the stronger impact at higher sliding causes a more severe damage.
4.2. Velocity-dependent tribochemical wear in humid air The surface and subsurface cracking are greatly suppressed in humid air, indicating that the damage mode of PL glass during the wear process in humid air is dominated by material-removal rather than brittle fracture. As a result, the water molecules in humid air play an important role to affect the velocity-dependent wear mechanism of PL glass. The results in Fig. 4 indicate that the material-removal is significantly affected by sliding velocity. Different from the dry conditions, the material-removal volume of PL glass is found to decrease sharply with increasing velocity, especially in the relatively low velocity range (v < 3 mm/s). The reduced material-removal volume at higher velocity conditions should be related to the tribochemical effects involving water absorption at the friction interface. Generally, water can corrode PL glass due to its poor chemical durability [23,24]. It can even occur under stress-free conditions by hydrolyzing P-O-P network, and this hydrolysis process can be expressed as Eq. (1) [9,10]: P-O-P + H2O→P-OH + OH-P
(1)
Under tensile and shear stress, this chemical reaction in Eq. (1) can be accelerated, causing a more serious damage on glass surface. This process is also known as stress-enhanced hydrolysis or stress-corrosion for most glass materials [25–27]. In our previous studies, it was found that nanowear of PL glass in humid air was much more severe than that
Fig. 8. High-resolution P2p spectra measured by XPS in the wear track of PL glass at the velocities of 0.2 and 7.5 mm/s in dry and humid air. 5
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subsequent tribochemical reactions (stress-enhanced hydrolysis), therefore, the material removal decreases with increasing velocity. For some other known materials, such as Si3N4 [15], Si(100) [16], GaAs [17], the fact that the wear of these materials decrease with the increasing sliding velocity has also been reported. The velocity-dependent wear behaviors for all these materials are verified to be related to the tribochemical reaction with the environmental medium. The less water absorption at the frictional interface under the higher sliding velocity contributes to the weaker wear of all these materials, which shares the similar wear mechanism of PL glass in humid air at various sliding velocities. It is worth noting that the at lower sliding velocity conditions (v < 2 mm/s), the wear of PL glass in dry air is lower than that in humid air; in contrast, the wear of PL glass in dry air is higher than that in humid air when the sliding velocities exceed 2 mm/s (Fig. 5). Thus, another important aspect can be addressed in present study. Despite the water resistance of PL glass is poor, the presence of water at PL glass interface can not always guarantee a more serious material-removal than the dry air case any more. Depending on the sliding velocity, the wear of PL glass in dry air can be lower or higher than the humid air case, regardless of the dominating roles in the surface and subsurface damage of PL glass. It also should be noted that the velocity-dependent of wear of PL glass is striking different from that of soda lime silica glass, where the wear volume of soda lime silica glass in dry air is always more severe than that in humid air, regardless of the sliding velocities [19]. There are two possible reasons for this. On the one hand, the counter-surface is pyrex glass ball for the wear of soda lime silica glass, and the pyrex glass ball surface adheres lots of wear debris in both dry and humid conditions [19]. In the case of PL glass, the counter-surface is alumina ball, which shows no measurable wear in both dry and humid air. On the other hand, the unit structure of PL glass and soda lime silica glass are different. Generally, the P-O-P phosphate units readily interconnect in linear chain-like structure, rather than the cross-link structure in silicate glasses [20]. Therefore, due to the discrepancies in mechanical and chemical durability, the wear of PL glass in dry air can be lower or higher than that in humid air, depending on the sliding velocity (Fig. 5). In summary, velocity-dependent wear behaviors of PL glass are strongly depended on the environment conditions, and the surface damage and material-removal are very sensitive to the presence of water. In dry environment, the velocity-dependent wear behavior of PL glass is determined by the strength of mechanical interaction between glass and counter-surface at various sliding velocities. In contrast, in humid air, that is controlled by the degree of tribochemical wear (stress-enhanced hydrolysis) at various velocities. As a result, the wear of PL glass is a velocity and humid coupled wear process. The data in this work provide a clue that, to completely understand the PL glass behavior, more relative humidity need be considered during the wear experiment, in addition to considering various velocities. This interesting issue is worth to be investigated in the future works.
Fig. 9. Raman spectra of the wear debris in PL glass at the velocities of 0.2 and 7.5 mm/s in dry and humid air.
low velocity of 0.2 mm/s, implying wear debris contained more OH groups after wear tests at lower velocity conditions. The increased OH groups should come from the hydrolysis of P-O-P network following Eq. (2), suggesting that the stress-enhanced hydrolysis of PL glass becomes more serious with the decrease in sliding velocity. Both the lower intensity of P in XPS spectra at wear track (Fig. 8) and stronger OH stretching of wear debris (Fig. 9) at lower velocity conditions in humid air reveal the stronger hydrolysis of PL glass at lower sliding velocity. The stronger stress-enhanced hydrolysis must come from higher shear stress or more water adsorption. In Fig. 1(c), the friction coefficient at 0.2 mm/s is 1.5 times of that at 7.5 mm/s, indicating the higher shear stress at lower sliding velocity, which contributes the stronger stress-enhanced hydrolysis on the one hand. On the other hand, the friction induced interfacial temperature rise would affect the interfacial water absorption and further affect the hydrolysis intensity. During the wear process, friction heat produced at sliding interface is well-known to increase with increasing sliding velocity [31,32], and the friction-induced temperature rise can be estimated by Eq. (2) according to some previous researches [31,32]:
ΔT =
μPv 4a (k1 + k2)
(2)
Where ΔT is the temperature rise, μ is the friction coefficient, P is the applied normal load, v is the sliding velocity, a is the radius of contact asperity at the contact area, k1 and k2 is the thermal conductivity coefficient of PL glass and alumina ball, respectively. Based on Eq. (2), it is conceivable that, under the same load condition, ΔT is proportional to the product of friction coefficient and velocity. According to the results in Fig. 1(c), the ΔT at the highest velocity (7.5 mm/s) is estimated to be ∼43 times higher than that at the lowest velocity (0.2 mm/ s). The higher temperature rise will significantly obstruct water absorption and reduce the amount of reacted water molecules at friction interface, thus further weaken stress-enhanced hydrolysis. Moreover, the contact time between friction pairs is relatively shorter at the higher velocity, which also reduces the possibility of water absorption at frictional interface, resulting in a weaker hydrolysis. In contrast, at low velocity conditions, the sliding contact time is long enough for water adsorption to allow the more sufficient reaction between absorbed water molecules and glass network under frictional shear stress. Stressenhanced hydrolysis destroys the backbone of PL glass and thus assists the material removal at low velocity. This is why we found the lower intensity of P in XPS spectra (Fig. 8) and stronger OH stretching (Fig. 9) at lower velocity conditions in humid air. In a word, high sliding velocity increases the interfacial temperature, reduces the sliding contact time, further significantly confines interfacial water adsorption and
5. Conclusion The velocity-dependent wear behaviors of PL glass are investigated by a reciprocating ball-on-flat tribometer in dry and humid air. The experimental results show that velocity dependence of wear in PL glass is very sensitive to the presence of water. In dry air, the velocity-dependent wear of PL glass shows fracture-dominated damage behavior. With the increase in sliding velocity, the Hertzian cracks increases first and then tends to saturation. Simultaneously, the material-removal volume of PL glass also increases first and then keeps almost unchanged. In contrast, in humid air, the wear mechanism transforms into tribochemical controlled wear process and almost no cracks forms for all the velocity conditions. With the increase in sliding velocity, the material-removal volume of PL glass decreases in humid air. At the lower velocity condition, the absorbed water film between the contact 6
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asperities forms more readily, and the stress-enhanced hydrolysis occurs more readily due to the relatively low interfacial temperature rise and more abundant sliding contact time, which assists the materialremoval of PL glass.
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Acknowledgement The authors are grateful for financial support from the National Natural Science Foundation of China [Grant No. 51575462 and No. 51605401], and the Scientific Research Fund of Sichuan Provincial Education Department, China [18ZA0504, 17ZA0408], and Research Fund Supported by Sichuan Science and Technology Program [18YYJC0905].
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