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Tribological behaviors of self‒mated Cu36Zr48Ag8Al8 bulk metallic glass under H2SO4 conditions with different concentrations
Tribology International 136 (2019) 395–403
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Tribological behaviors of self‒mated Cu36Zr48Ag8Al8 bulk metallic glass under H2SO4 conditions with different concentrations
T
Xiaofang Jianga,b, Junjie Songa,∗, Yunfeng Sua, Hengzhong Fana, Yongsheng Zhanga,∗∗, Litian Hua a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China
ARTICLE INFO
ABSTRACT
Keywords: Bulk metallic glass Wear mechanism Corrosion Mechanical seal
This study investigated the tribological properties of Cu36Zr48Ag8Al8 BMG against Cu36Zr48Ag8Al8 BMG under H2SO4 corrosive conditions with different concentrations. Results show that Cu36Zr48Ag8Al8 BMG exhibits good tribological behaviors under H2SO4 conditions. Especially, when the material was soaked in 15 wt% H2SO4 solution, the friction coefficient and wear rate can be as low as 0.10 ± 0.01 and (0.28 ± 0.23) × 10−6 mm3 N−1m−1. It is mainly attributed to the existence of metal oxide films formed by surface oxidation and passivation actions as well as the fluid lubricating films formed by H2SO4 solution and dissolution reaction products ( Al2 (SO4 )3 and CuSO4 ) on the worn surfaces. These perfect properties promote Cu36Zr48Ag8Al8 BMG to have the promise to replace the stainless steel under H2SO4 corrosive condition.
1. Introduction High‒performance bulk metallic glass (BMG) are potential candidates to replace the conventional alloys for the structural application of moving components in corrosion environment due to their excellent mechanical properties and corrosion resistance [1‒3]. For the conventional alloys (such as stainless steel and copper alloy) with crystal structures, the grain boundaries and dislocations induce them sensitive to the corrosive environment and make them unable to meet the application requirements of some extreme conditions [4]. Therefore, it is significant to reveal the tribological behaviors of the BMG in some extreme conditions for guiding their application as the structural components. Over the past decades, quite a large number of BMG systems have been developed, such as Cue, Nie, Zre, Fee and Ti–based BMG [5‒9]. Studies have shown that different types of bulk metallic glass have large differences in glass forming ability (GFA), which is mainly related to the thermodynamic driving forces, kinetic factors and configuration structures during the forming process [10,11]. Cu–based bulk metallic glass is a rare kind of BMG system with high glass‒forming ability, large supercooled liquid region, superior wear resistance [12,13] and mechanical properties (high strength, high fracture toughness, obvious plasticity) [14,15]. Specially, the critical dimension of Cu36Zr48Ag8Al8 BMG is 25 mm [16], the fracture toughness is 16.9 MPa·m1/2, Young's modulus is 115 GPa and the tensile strength is as high as 1850 MPa ∗
[17,18]. These high glass‒forming ability and mechanical properties promote Cu36Zr48Ag8Al8 BMG to have great potential application as moving components under corrosive condition. Compared to crystalline alloys, BMG have excellent corrosion resistance due to their chemically uniform single‒phase nature and no crystalline defects. Rios CT et al. found that the corrosion resistance of Fe43·2Co28·8B19·2Si4·8Nb4 BMG is higher than that of crystalline alloy with the same compositions, and the corrosion current density of Fe‒based BMG is just 0.63 times than that of crystalline alloy [19]. Liu Y et al. studied the potentiodynamic polarization curves of Ti47Cu38Zr7·5Fe2·5Sn2Si1Ag2 BMG, Tie6Ale4V alloy and Coe28Cre6Mo alloy in PBS solution. Compared to the other two alloys, BMG could spontaneously passivate before pitting corrosion occurs, and it has a wider passivation zone and the lower corrosion current density [20]. Although bulk metallic glass have excellent corrosion resistance, they exhibit different service behaviors in different environments and they are also not suitable for all corrosive environment [21‒23]. Aditya A et al. evaluated the corrosion and wear behavior of Zr41·2Cu12·5Ni10Ti13·8Be22.5 and Zr57Cu15·4Ni12·6Al10Nb5 BMG in NaCl solution. Results show that the presence of Cl− ion in solution could accelerate the pitting corrosion of Zr‒based glasses [24]. Lee PY et al. investigated the chemical stability of the surface films produced in HCl and H2SO4 solutions for the BMG contained with different content of Cu element. It is found the passivation films that with more Ti and Zr ions and less Cu ion exhibited a higher corrosion resistance, especially when they served in oxidizing acids [25]. Therefore, the chemical properties of the BMG materials in which
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Song), [email protected] (Y. Zhang).
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https://doi.org/10.1016/j.triboint.2019.04.006 Received 25 January 2019; Received in revised form 2 April 2019; Accepted 2 April 2019 Available online 04 April 2019 0301-679X/ © 2019 Elsevier Ltd. All rights reserved.
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roughness less than 0.1 μm) except for the surface of 0.5 cm2. Potentiodynamic polarization curves of the materials were measured at a potential sweep rate of 50 mV/min. To ensure a stable conditions for potentiodynamic polarization tests, the variation of open circuit potential (OCP) with time was monitored for 60 min. As comparison, the electrochemical performance of 316 stainless steel (316 SS) was also measured in the same method. All experiments were conducted at least three times to ensure the repeatability of experimental results. After potentiodynamic polarization tests, the corrosive morphologies of samples were measured by SEM. The friction and wear tests were performed on a standard UMT friction and wear tester (UMT–3MT, USA) in a reciprocating motion at room temperature. The test conditions were selected according to a specific application of valves to be applied in H2SO4 solution. In the working, the friction pairs of valves are sliding against with each other in a reciprocating motion at a certain frequency to achieve the switch effect. The contact pressure between friction pairs is larger than 1.0 MPa to ensure a good sealing effect. In that case, we set the simulated contact pressure was 2.0 MPa (it is the applied load divided by ideal contact area, and the applied load was 6.28 N), the reciprocating frequency was 8 Hz (corresponding to 0.08 m/s), the linear stroke was 5.0 mm (the maximum amplitude of the testing machine) and sliding time was 60 min (the total sliding distance is 288 m). Meanwhile, the self‒mated mode was used for the tribological tests of Cu36Zr48Ag8Al8 BMG, and the dimensions of the frictional pairs were ø2 mm × 15 mm (pin) and 3 mm × 4 mm × 15 mm (block). The experimental configure is shown in Fig. 1b. In order to reveal the tribological properties of self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 solution with different concentrations, the friction and wear tests for them under dry‒sliding condition and 0 wt%, 5 wt%, 8 wt%, 10 wt%, 15 wt%, 30 wt%, 50 wt%, 60 wt% and 70 wt% H2SO4 conditions were performed. It is worth noting that the frictional contact surface of the counterpart materials were completely immersed in the liquid media during the sliding process by using a PTFE mold as solution container [29]. Before the experiments, the surface roughness of each sample was carefully polished to below 0.1 μm (Ra). The friction coefficients and wear rates presented in this study are the average values obtained by measuring at least three times under the same condition. For comparison, the tribological tests of self‒mated 316 SS (the dimensions of ø2 mm × 15 mm for pin and 3 mm × 4 mm × 15 mm for block) were also performed under the same conditions. After tribological tests, the samples were washed with deionized water under the small force, then the morphologies of worn surfaces were investigated by SEM and non–contact 3D surface profilometer (MicroXAM–800), and the chemical composition of the worn surface were analyzed by EDS and XPS (ESCALAB 250Xi), and the morphologies of the section surface were observed by FE‒SEM (SU8020). The wear rates (w) of the samples were calculated from the formula w = dV /(dL × dF ) (V is the volume loss which is determined by MicroXAM–800 3D surface profilometer, L is the sliding distance and F is the applied load) [30]. Moreover, the compositions of the dissolved
environment it is served is great important, that may further determine the stability of the system and the reliability of the operation. Thus, it is important to investigate the tribological and corrosive properties of the materials in the corresponding service environment which will be used in. At present, there are many researches on the corrosion resistance of Cu‒based BMG with different systems in H2SO4 solution, including (Cu47Zr11Ti34Ni8)99.5M0.5 (M = 0, Cr, Mo and W) [26], (Cu47Zr11Ti34Ni8)100−xMox (x = 0, 1 and 2 at.%) [27], Cu60Zr30Ti10 [28]. However, as a moving part, there are no systematic researches about the influence of the concentrations of H2SO4 corrosive solutions on the tribological behaviors of BMG. In order to expand the applications of BMG and satisfy the higher demand of harsh work conditions, such as the application of dynamic seals in chemical engineering, it is need to gain abundant theoretical achievements and experimental researches on the tribological behaviors of Cu36Zr48Ag8Al8 BMG. Consequently, the electrochemical, friction and wear properties of self‒mated Cu36Zr48Ag8Al8 BMG in H2SO4 solutions with different concentrations were investigated. Moreover, the friction‒reducing, anti‒wear and anti‒corrosion mechanisms of the materials were revealed. 2. Materials and methods The Cu36Zr48Ag8Al8 bulk metallic glass (BMG) was prepared by melt‒casting method in a vacuum arc melting furnace equipped with a water–cooling system under Ti–gettered argon atmosphere. Commercially available Cu ingot (99.999%), Zr ingot (99.95%), Ag ingot (99.99%) and Al ingot (99.99%) were used in this study. The alloy ingots were obtained by melting these four kinds of metals with a define weight according to the atomic ratio of Cu36Zr48Ag8Al8 BMG. All the alloy ingots were melted at least four times under electromagnetic stirring to obtain good homogeneity. Two kinds of bars with different dimensions (ø2 mm × 65 mm and 3 mm × 4 mm × 65 mm) were fabricated by remelting the alloy ingots and casting them into the copper mold with a certain inside dimension. The microstructure and chemical structure of BMG bars were characterized by X–ray diffraction (XRD, EMPYREAN) and scanning electron microscope (SEM, JSM–5600LV). The thermodynamic behaviors of the BMG bars were measured by differential scanning calorimetry (STA 449C), the temperature range is from 298 K to 900 K with a heating rate of 20 K/s under pure nitrogen flowing. The microhardness tests were performed on the surface (Ra≈0.1 μm) of alloy by using a Vickers microhardness tester (MH‒5‒VM) with a load of 200 g (1.96 N) and a dwell time of 5 s. The electrochemical performance of Cu36Zr48Ag8Al8 BMG in H2SO4 electrolyte was investigated by a three–electrode cell system with a platinum counter electrode and an Hg/Hg2SO4 reference electrode, and the experimental configuration as show in Fig. 1a. Taking into account the service stability of Hg/Hg2SO4 reference electrode, a relatively low concentration of H2SO4 solution (10 wt%) was selected as electrolyte. The working electrode was prepared by coating with insulating varnish on the remaining surface of bar (3 mm × 4 mm × 65 mm, surface
Fig. 1. Schematic diagrams of electrochemical measurement (a) and friction and wear test (b). 396
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Fig. 3. Electrochemical experiment results of the Cu36Zr48Ag8Al8 BMG and 316 SS in 10 wt% H2SO4 solution at room temperature: open circuit potential variation with time (a) and potentiodynamic polarization curves (b).
Fig. 2. XRD patterns (a) and DSC curves (b) of Cu36Zr48Ag8Al8 BMG.
surface is more stable and less susceptible to corrosion reactions [32,33]. Moreover, from Fig. 3b can been seen that the Cu36Zr48Ag8Al8 BMG has a higher corrosion potential (Ecorr) and lower corrosion current density (icorr) than those of 316 SS, that is, Cu36Zr48Ag8Al8 BMG has a higher corrosion resistance in H2SO4 solution [19]. In here, the corrosion current density is the intersection of the tangent to the region of the strong polarization region in the cathodic polarization curve ( ± 50 mV is taken as the limit) and the line perpendicular to the self‒corrosion potential [34]. In addition, the anodic polarization curve of Cu36Zr48Ag8Al8 BMG presents a wider spontaneous passivation area at the low current density, whereas the polarization curve of 316 SS has a clear anodic peak. It also demonstrates that passivation films are easily to be formed on the surfaces of Cu36Zr48Ag8Al8 BMG, and prevent the materials from further corrosion under H2SO4 condition [35]. Meanwhile, the formed passivation films are very stable in a wide range of potential, which also induce Cu36Zr48Ag8Al8 BMG to have a higher corrosion resistance than 316 SS. In addition, with the increase of the applied potential above 0.0 V, the current density of the 316 SS is rapidly increased. In this case, the formed passivation films on the surface of 316 SS are easily to fail, and thereby accelerating the corrosion of 316 SS. However, the current density of Cu36Zr48Ag8Al8 BMG increases gradually with the increase of applied potential and never occurs the phenomenon of rapid increase. Moreover, from the SEM images of the surface of BMG and 316 SS after potentiodynamic polarization tests can be seen that there are no obvious corrosion marks on the surface of BMG, whereas 316 SS presents noticeable corrosion pits on its surface (Fig. 4). Thus, the formed passivation films on the surfaces of Cu36Zr48Ag8Al8 BMG are extremely insensitive to the H2SO4 condition, which makes Cu36Zr48Ag8Al8 BMG to have a higher corrosion resistance than 316 stainless steel.
corrosion products in solution after frictional tests were analyzed by investigating the crystal structures and chemical elements of products on the friction surface of BMG after its surface containing with fluid lubrication films was dried in a vacuum drying oven at room temperature. 3. Results and discussion 3.1. Structure characterization and electrochemical property of Cu36Zr48Ag8Al8 bulk metallic glass Fig. 2a presents the XRD patterns of Cu36Zr48Ag8Al8 BMG with different dimensions. It can be seen that both the XRD patterns of Cu36Zr48Ag8Al8 BMG pins and blocks are similar. There is only a broad halo peak at the position around 2θ = 38°, that is, the prepared BMG possess fully amorphous structure. In addition, it can be seen from Fig. 2b that the glass transition temperature (Tg) and the onset crystallization temperature (Tx) is about 684 K and 790 K, respectively. This result is consistent with the report in the previous research [31]. The presence of a wide supercooled liquid region (about 106 K) promotes the as‒cast Cu36Zr48Ag8Al8 BMG to have a high glass–forming ability in these two dimensions. Fig. 3 presents the electrochemical properties of Cu36Zr48Ag8Al8 BMG and 316 SS in 10% H2SO4 solution. From Fig. 3a can be seen that the OCP values of the two alloys could reach a relatively stable state after being immersed about 500 s, but the value of OCP for BMG is −0.428 V/SCE and for 316 SS is −0.845 V/SCE. The small negative value of BMG indicates the spontaneously formed protective passivation films on the 397
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Fig. 4. SEM images of the surface of Cu36Zr48Ag8Al8 BMG (a) and 316 SS (b) after electrochemical experiment. The inset on the right top is the enlarged image.
3.2. Tribological properties of Cu36Zr48Ag8Al8 bulk metallic glass under H2SO4 conditions
self‒mated Cu36Zr48Ag8Al8 BMG has the smallest friction coefficient (0.10 ± 0.01) and wear rate ((0.28 ± 0.23) × 10−6 mm3 N−1m−1), which just is 29.5% and 1.9% of those in 0% H2SO4 solution and is 33.4% and 2.9% of those in air (dry‒sliding condition), respectively. However, the 316 SS has obvious large friction coefficient (0.44 ± 0.02) and wear rate ((10.71 ± 0.64) × 10−6 mm3 N−1m−1). Besides, the concentrations of the H2SO4 solutions have a significant influence on the friction coefficient curves of the self‒mated Cu36Zr48Ag8Al8 BMG (Fig. 6). The phenomenon can be concluded as following situations. In all H2SO4 solutions, the friction coefficient of the self‒mated Cu36Zr48Ag8Al8 BMG shows a decreasing trend in the running‒in stage. Meanwhile, when the concentrations of the H2SO4 solution is below 15%, the self‒mated Cu36Zr48Ag8Al8 BMG has a relative stable friction coefficient with minor fluctuations in the running‒in stage; but with the increase of the sliding time, the friction coefficient presents large fluctuations (Fig. 6b‒d). However, the frictional state of the materials shows a completely different phenomenon when the concentrations of the H2SO4 solution is above 15%. The friction coefficient of self‒mated Cu36Zr48Ag8Al8 BMG shows large fluctuations in the running‒in stage while the materials have a friction coefficient with small fluctuations after that stage. It is worth noting that when the concetrations of the H2SO4 solution is among 8% to approximately 15%, the friction coefficient of the material rapidly decreases to an ultra‒low value of 0.02–0.03 in the stable stage of sliding. As the further increase of the H2SO4 solution concentration, the large fluctuation stage brings forward, and the stable stage becomes earlier to be reached and more stable in whole friction process (Fig. 6f‒i).
Fig. 5 illustrates the friction coefficients and wear rates of the self‒mated Cu36Zr48Ag8Al8 BMG and 316 SS under dry‒sliding condition and H2SO4 corrosive conditions with different concentrations. Cu36Zr48Ag8Al8 BMG exhibits better tribological behaviors under all H2SO4 conditions than those of self‒mated 316 SS, except under 0% H2SO4 solution (deionized water) condition. The friction coefficients and wear rates of BMG in ˃ 0% H2SO4 solutions can lower than 0.29 ± 0.01 and (2.45 ± 0.42) × 10−6 mm3 N−1m−1. In addition, the tribological behaviors of Cu36Zr48Ag8Al8 BMG are changed with the variation of the concentrations of the H2SO4 solutions. With the increase of the concentrations of H2SO4 solutions, both the friction coefficient and wear rate rapid decrease firstly and then increase gradually. When the concentrations of H2SO4 solution is around 15%, the
3.3. Relationship among tribological behaviors of self‒mated Cu36Zr48Ag8Al8 BMG and H2SO4 solution concentrations From above results, it can be concluded that the concentration of H2SO4 solution has a significant influence on the friction and wear properties of Cu36Zr48Ag8Al8 BMG. To in‒depth reveal the friction‒reducing and anti‒wear mechanisms of the material, the schematic diagrams of the frictional process for the self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with different concentrations were developed in Fig. 7. Firstly, the presence of fluid media between the contact surfaces of friction pairs not only plays a role to flush the wear debris, but also acts as a fluid lubricant to reduce and even avoid the direct contact between friction pairs, and thus reducing the friction and wear of the material [36,37]. The viscosity of the fluid media is an important factor to control the load–carrying ability of the formed fluid lubricating films on the friction surfaces. At room temperature, the viscosity of the H2SO4 solution increases significantly with the increase of the concentration of H2SO4 solution, and which is from ∼1 g m−1s−1 at 0% H2SO4 to ∼11 g m−1s−1 at 70% H2SO4 solution [38]. The large viscosity of the fluid will greatly increase the load–carrying ability and continuity of the fluid lubricating films and even increase the thickness of them. The formation of fluid lubricating film
Fig. 5. Friction coefficients (a) and wear rates (b) of self‒mated Cu36Zr48Ag8Al8 BMG (column graph) and self‒mated 316 SS (scatter graph) under dry‒sliding condition and H2SO4 conditions with different concentrations. 398
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Fig. 6. Friction coefficient curves of self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with different concentrations: 0% (a), 5% (b), 8% (c), 10% (d), 15% (e), 30% (f), 50% (g), 60% (h) and 70% (i).
with certain thickness and load–carrying ability could significantly reduce the direct contact area between friction pairs, and thereby converting the friction sliding between metal and metal to the friction sliding between metal and liquid media (Fig. 7) [39]. Thus, the self‒mated Cu36Zr48Ag8Al8 BMG exhibits an obvious lower friction coefficient in solution containing H2SO4, and also avoiding the severe abrasive wear like as that under deionized water condition. For instance, the depth of wear track of BMG block in 15% H2SO4 solution can reduce to 35.6% of that in 0% H2SO4 solution and 51.3% of that in dry‒sliding condition (Fig. 8). Moreover, the formed metal oxide films on the friction surfaces have a great impact on the tribological behaviors of Cu36Zr48Ag8Al8 BMG. Fig. 9 presents the XPS survey spectra (analyzing the positions on the worn track) and the narrow scans of Cu 2p, Ag 3d, Zr 3d and Al 2p for the worn track of BMG block washed with deionized water and dried after friction tests under dry‒sliding, 0% and 15% H2SO4 conditions. The results demonstrate that the metal oxide films containing with ZrO2, Al2O3 and CuO are formed on the frictional surfaces of materials. Moreover, the thickness of the oxidation films is improving with the increase of the concentration of H2SO4 solution (Fig. 10). The thickness of the oxidation films for the sample to be immersed in 0% H2SO4 solution is about 1.82 μm, and the thickness of the oxidation films for the sample to be immersed in 70% H2SO4 solution could reach about 14.71 μm. Usually, the metal oxides have a relative higher hardness than the BMG, that is, the larger the thickness of the oxidation films could cause a higher surface hardness of BMG. Microhardness tests also demonstrate the surface hardness of the BMG could improve from 536 ± 12 Hv (testing immediately after carefully polished) to
567 ± 14 Hv (testing after the material was soaked in 30% H2SO4 solution for 60 min). The formation of oxides films with a certain thickness and a better corrosion resistance can hinder the BMG materials from corrosion solution, thus preventing further corrosion of materials. Meanwhile, the formation of oxides films with a relatively higher hardness can further improve the wear resistance of the BMG [40,41]. The metal oxide films are mainly formed from the oxidation reactions of the active metals which are exposed on the surface of BMG alloy. For > 0% H2SO4 solution condition, although the Al2O3 and CuO are easily to be dissolved in the H2SO4 solution with low concentration according to Al2 O3 + 3H2 SO4 = Al2 (SO4 )3 + 3H2 O and CuO + H2 SO4 = CuSO4 + H2 O , it gives element Zr more chances to be exposed on the surface of the material and formed more ZrO2 (Figs. 9 and 11). The formed ZrO2 could protect the material from the corrosion of H2SO4 and improve the wear resistance of Cu36Zr48Ag8Al8 BMG. However, the dissolution of metal oxides and exposure of internal metals would cause the occurrence of fluctuation of friction coefficient. Therefore, there are some abrupt changes at the friction coefficient curves of self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with low concentrations (Fig. 6c‒e). Nevertheless, the formed Al2 (SO4 )3 and CuSO4 that have significantly higher molecular mass than that of H2SO4 could significantly improve the viscosity of the fluid on the friction surfaces of the material and enhance the load‒carrying ability and continuity of the fluid lubricating films [42,43], thereby reducing the friction and wear of the Cu36Zr48Ag8Al8 BMG (Figs. 7 and 8). Thus, the material has ultra‒low friction coefficients and wear rates in the 10% and 15% H2SO4 solutions after running‒in stage of friction sliding.
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Fig. 7. Schematic diagrams of the frictional process for the self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with different concentrations: 0% (a), 15% (b) and 70% (c).
Furthermore, with the increase of the concentrations of H2SO4 solutions, the amount of formed metal oxides on the surfaces of BMG also is increased (Figs. 9f and 10d). When the friction pairs are sliding in the H2SO4 solution with high concentration, more amount metal oxides could be formed and accumulated on the friction surface of BMG (Fig. 7c). Under high concetration H2SO4 conditions, both the oxidation by O2 and passivation by H2SO4 could promote the metals to form metal oxides. The passivation reactions are mainly included 2Al + 3H2 SO4 = Al2 O3 + 3SO2 + 3H2 O and Zr + 2H2 SO4 = ZrO2 + 2SO2 + 2H2 O , and these reactions mainly occurred in the initial stage of sliding. Thus, the friction coefficient of the self‒mated Cu36Zr48Ag8Al8 BMG presents many large fluctuations at the beginning of friction sliding. With the sliding in a high concentration H2SO4 solution, the friction coefficient curve of the material could become more stable (Fig. 6g‒i). That is mainly because that metal oxide films with a certain thickness (14.71 μm) can be formed on the surface of the friction pairs and improve the surface chemical stability. Specially, the oxides content on the worn surface of BMG immersed in 70% H2SO4 solution could be as high as 85% (Fig. 9f). However, the formation of the stable surface oxide films on the worn surfaces of BMG in high concentration H2SO4 solution could convert the friction pairs from BMG/BMG to ceramic (metal oxides)/ceramic (metal oxides) [44]. Meanwhile, there are few and even no soluble corrosion products to be formed from oxides films like as the phenomenon to be occurred in low concentration H2SO4 solution (Fig. 11). Those may caused self‒mated
Cu36Zr48Ag8Al8 BMG to have a relative higher friction coefficient and wear rate in the high concentration H2SO4 solution (˃30% H2SO4 solution) than that under 15% H2SO4 condition. 4. Conclusion In this study, the tribological behaviors of Cu36Zr48Ag8Al8 bulk metallic glass under H2SO4 condition with different concentrations were investigated. Cu36Zr48Ag8Al8 BMG exhibits good friction‒reducing, wear resistance and anticorrosion properties under different concentrations H2SO4 conditions. These excellent properties are mainly attributed to the positive effects from the formation of metal oxide films and fluid lubricating films on the frictional surfaces during sliding process. The fluid lubricating films were formed by H2SO4 solution and reaction products ( Al2 (SO4 )3 and CuSO4 ) could convert the friction between BMG and BMG to the friction between BMG and fluid, and thus reducing the friction and wear of materials. Meanwhile, the metal oxide films were formed by surface oxidation and passivation actions could significantly improve the wear resistance of BMG and protect it from the corrosion of H2SO4 solutions. In addition, with the variations of the concentrations of H2SO4 solutions, the chemical components, load‒carrying ability and continuity of the formed metal oxide films and fluid lubricating films will also be changed, thereby affecting the tribological behaviors of Cu36Zr48Ag8Al8 bulk metallic glass. When the concentrations of H2SO4 solution is around 15%, the self‒mated Cu36Zr48Ag8Al8 BMG has the smallest friction
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Fig. 8. Surface characteristic of Cu36Zr48Ag8Al8 BMG after wear tests under different conditions: 3D morphologies (a, d, g), sectional morphologies (b, e, h) and SEM images (c, f, i) of worn tracks of BMG block under dry‒sliding condition (a–c), 0% H2SO4 condition (d–f) and 15% H2SO4 condition (g–i). The inset on right side of Fig. 8 (c, f and i) shows the SEM images of BMG pin.
Fig. 9. XPS spectra and quantitative calculation on the worn tracks of BMG block under dry‒sliding condition and under 0% and 15% H2SO4 conditions: survey spectra (a) and narrow scans for spectra of Cu 2p (b), Ag 3d (c), Zr 3d (d), Al 2p (e) and chemical compositions (f) of tribolayer on worn tracks of BMG block.
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Fig. 10. SEM‒BEI images of the cross section of BMG block: after friction tests in the air (a) and in 0% (b), 15% (c) and 70% H2SO4 (d) solutions, respectively.
Fig. 11. Morphologies and EDS maps of the worn tracks of BMG block to be immersed in 15% (a–g) and 70% (h–n) H2SO4 solutions.
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coefficient and wear rate with the respective value is 0.10 ± 0.01 and (0.28 ± 0.23) × 10−6 mm3 N−1m−1, which just is 29.5% and 1.9% of those in 0% H2SO4 solution and is 33.4% and 2.9% of those under dry‒sliding condition.
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Tribological behaviors of self‒mated Cu36Zr48Ag8Al8 bulk metallic glass under H2SO4 conditions with different concentrations
T
Xiaofang Jianga,b, Junjie Songa,∗, Yunfeng Sua, Hengzhong Fana, Yongsheng Zhanga,∗∗, Litian Hua a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China
ARTICLE INFO
ABSTRACT
Keywords: Bulk metallic glass Wear mechanism Corrosion Mechanical seal
This study investigated the tribological properties of Cu36Zr48Ag8Al8 BMG against Cu36Zr48Ag8Al8 BMG under H2SO4 corrosive conditions with different concentrations. Results show that Cu36Zr48Ag8Al8 BMG exhibits good tribological behaviors under H2SO4 conditions. Especially, when the material was soaked in 15 wt% H2SO4 solution, the friction coefficient and wear rate can be as low as 0.10 ± 0.01 and (0.28 ± 0.23) × 10−6 mm3 N−1m−1. It is mainly attributed to the existence of metal oxide films formed by surface oxidation and passivation actions as well as the fluid lubricating films formed by H2SO4 solution and dissolution reaction products ( Al2 (SO4 )3 and CuSO4 ) on the worn surfaces. These perfect properties promote Cu36Zr48Ag8Al8 BMG to have the promise to replace the stainless steel under H2SO4 corrosive condition.
1. Introduction High‒performance bulk metallic glass (BMG) are potential candidates to replace the conventional alloys for the structural application of moving components in corrosion environment due to their excellent mechanical properties and corrosion resistance [1‒3]. For the conventional alloys (such as stainless steel and copper alloy) with crystal structures, the grain boundaries and dislocations induce them sensitive to the corrosive environment and make them unable to meet the application requirements of some extreme conditions [4]. Therefore, it is significant to reveal the tribological behaviors of the BMG in some extreme conditions for guiding their application as the structural components. Over the past decades, quite a large number of BMG systems have been developed, such as Cue, Nie, Zre, Fee and Ti–based BMG [5‒9]. Studies have shown that different types of bulk metallic glass have large differences in glass forming ability (GFA), which is mainly related to the thermodynamic driving forces, kinetic factors and configuration structures during the forming process [10,11]. Cu–based bulk metallic glass is a rare kind of BMG system with high glass‒forming ability, large supercooled liquid region, superior wear resistance [12,13] and mechanical properties (high strength, high fracture toughness, obvious plasticity) [14,15]. Specially, the critical dimension of Cu36Zr48Ag8Al8 BMG is 25 mm [16], the fracture toughness is 16.9 MPa·m1/2, Young's modulus is 115 GPa and the tensile strength is as high as 1850 MPa ∗
[17,18]. These high glass‒forming ability and mechanical properties promote Cu36Zr48Ag8Al8 BMG to have great potential application as moving components under corrosive condition. Compared to crystalline alloys, BMG have excellent corrosion resistance due to their chemically uniform single‒phase nature and no crystalline defects. Rios CT et al. found that the corrosion resistance of Fe43·2Co28·8B19·2Si4·8Nb4 BMG is higher than that of crystalline alloy with the same compositions, and the corrosion current density of Fe‒based BMG is just 0.63 times than that of crystalline alloy [19]. Liu Y et al. studied the potentiodynamic polarization curves of Ti47Cu38Zr7·5Fe2·5Sn2Si1Ag2 BMG, Tie6Ale4V alloy and Coe28Cre6Mo alloy in PBS solution. Compared to the other two alloys, BMG could spontaneously passivate before pitting corrosion occurs, and it has a wider passivation zone and the lower corrosion current density [20]. Although bulk metallic glass have excellent corrosion resistance, they exhibit different service behaviors in different environments and they are also not suitable for all corrosive environment [21‒23]. Aditya A et al. evaluated the corrosion and wear behavior of Zr41·2Cu12·5Ni10Ti13·8Be22.5 and Zr57Cu15·4Ni12·6Al10Nb5 BMG in NaCl solution. Results show that the presence of Cl− ion in solution could accelerate the pitting corrosion of Zr‒based glasses [24]. Lee PY et al. investigated the chemical stability of the surface films produced in HCl and H2SO4 solutions for the BMG contained with different content of Cu element. It is found the passivation films that with more Ti and Zr ions and less Cu ion exhibited a higher corrosion resistance, especially when they served in oxidizing acids [25]. Therefore, the chemical properties of the BMG materials in which
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Song), [email protected] (Y. Zhang).
∗∗
https://doi.org/10.1016/j.triboint.2019.04.006 Received 25 January 2019; Received in revised form 2 April 2019; Accepted 2 April 2019 Available online 04 April 2019 0301-679X/ © 2019 Elsevier Ltd. All rights reserved.
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roughness less than 0.1 μm) except for the surface of 0.5 cm2. Potentiodynamic polarization curves of the materials were measured at a potential sweep rate of 50 mV/min. To ensure a stable conditions for potentiodynamic polarization tests, the variation of open circuit potential (OCP) with time was monitored for 60 min. As comparison, the electrochemical performance of 316 stainless steel (316 SS) was also measured in the same method. All experiments were conducted at least three times to ensure the repeatability of experimental results. After potentiodynamic polarization tests, the corrosive morphologies of samples were measured by SEM. The friction and wear tests were performed on a standard UMT friction and wear tester (UMT–3MT, USA) in a reciprocating motion at room temperature. The test conditions were selected according to a specific application of valves to be applied in H2SO4 solution. In the working, the friction pairs of valves are sliding against with each other in a reciprocating motion at a certain frequency to achieve the switch effect. The contact pressure between friction pairs is larger than 1.0 MPa to ensure a good sealing effect. In that case, we set the simulated contact pressure was 2.0 MPa (it is the applied load divided by ideal contact area, and the applied load was 6.28 N), the reciprocating frequency was 8 Hz (corresponding to 0.08 m/s), the linear stroke was 5.0 mm (the maximum amplitude of the testing machine) and sliding time was 60 min (the total sliding distance is 288 m). Meanwhile, the self‒mated mode was used for the tribological tests of Cu36Zr48Ag8Al8 BMG, and the dimensions of the frictional pairs were ø2 mm × 15 mm (pin) and 3 mm × 4 mm × 15 mm (block). The experimental configure is shown in Fig. 1b. In order to reveal the tribological properties of self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 solution with different concentrations, the friction and wear tests for them under dry‒sliding condition and 0 wt%, 5 wt%, 8 wt%, 10 wt%, 15 wt%, 30 wt%, 50 wt%, 60 wt% and 70 wt% H2SO4 conditions were performed. It is worth noting that the frictional contact surface of the counterpart materials were completely immersed in the liquid media during the sliding process by using a PTFE mold as solution container [29]. Before the experiments, the surface roughness of each sample was carefully polished to below 0.1 μm (Ra). The friction coefficients and wear rates presented in this study are the average values obtained by measuring at least three times under the same condition. For comparison, the tribological tests of self‒mated 316 SS (the dimensions of ø2 mm × 15 mm for pin and 3 mm × 4 mm × 15 mm for block) were also performed under the same conditions. After tribological tests, the samples were washed with deionized water under the small force, then the morphologies of worn surfaces were investigated by SEM and non–contact 3D surface profilometer (MicroXAM–800), and the chemical composition of the worn surface were analyzed by EDS and XPS (ESCALAB 250Xi), and the morphologies of the section surface were observed by FE‒SEM (SU8020). The wear rates (w) of the samples were calculated from the formula w = dV /(dL × dF ) (V is the volume loss which is determined by MicroXAM–800 3D surface profilometer, L is the sliding distance and F is the applied load) [30]. Moreover, the compositions of the dissolved
environment it is served is great important, that may further determine the stability of the system and the reliability of the operation. Thus, it is important to investigate the tribological and corrosive properties of the materials in the corresponding service environment which will be used in. At present, there are many researches on the corrosion resistance of Cu‒based BMG with different systems in H2SO4 solution, including (Cu47Zr11Ti34Ni8)99.5M0.5 (M = 0, Cr, Mo and W) [26], (Cu47Zr11Ti34Ni8)100−xMox (x = 0, 1 and 2 at.%) [27], Cu60Zr30Ti10 [28]. However, as a moving part, there are no systematic researches about the influence of the concentrations of H2SO4 corrosive solutions on the tribological behaviors of BMG. In order to expand the applications of BMG and satisfy the higher demand of harsh work conditions, such as the application of dynamic seals in chemical engineering, it is need to gain abundant theoretical achievements and experimental researches on the tribological behaviors of Cu36Zr48Ag8Al8 BMG. Consequently, the electrochemical, friction and wear properties of self‒mated Cu36Zr48Ag8Al8 BMG in H2SO4 solutions with different concentrations were investigated. Moreover, the friction‒reducing, anti‒wear and anti‒corrosion mechanisms of the materials were revealed. 2. Materials and methods The Cu36Zr48Ag8Al8 bulk metallic glass (BMG) was prepared by melt‒casting method in a vacuum arc melting furnace equipped with a water–cooling system under Ti–gettered argon atmosphere. Commercially available Cu ingot (99.999%), Zr ingot (99.95%), Ag ingot (99.99%) and Al ingot (99.99%) were used in this study. The alloy ingots were obtained by melting these four kinds of metals with a define weight according to the atomic ratio of Cu36Zr48Ag8Al8 BMG. All the alloy ingots were melted at least four times under electromagnetic stirring to obtain good homogeneity. Two kinds of bars with different dimensions (ø2 mm × 65 mm and 3 mm × 4 mm × 65 mm) were fabricated by remelting the alloy ingots and casting them into the copper mold with a certain inside dimension. The microstructure and chemical structure of BMG bars were characterized by X–ray diffraction (XRD, EMPYREAN) and scanning electron microscope (SEM, JSM–5600LV). The thermodynamic behaviors of the BMG bars were measured by differential scanning calorimetry (STA 449C), the temperature range is from 298 K to 900 K with a heating rate of 20 K/s under pure nitrogen flowing. The microhardness tests were performed on the surface (Ra≈0.1 μm) of alloy by using a Vickers microhardness tester (MH‒5‒VM) with a load of 200 g (1.96 N) and a dwell time of 5 s. The electrochemical performance of Cu36Zr48Ag8Al8 BMG in H2SO4 electrolyte was investigated by a three–electrode cell system with a platinum counter electrode and an Hg/Hg2SO4 reference electrode, and the experimental configuration as show in Fig. 1a. Taking into account the service stability of Hg/Hg2SO4 reference electrode, a relatively low concentration of H2SO4 solution (10 wt%) was selected as electrolyte. The working electrode was prepared by coating with insulating varnish on the remaining surface of bar (3 mm × 4 mm × 65 mm, surface
Fig. 1. Schematic diagrams of electrochemical measurement (a) and friction and wear test (b). 396
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Fig. 3. Electrochemical experiment results of the Cu36Zr48Ag8Al8 BMG and 316 SS in 10 wt% H2SO4 solution at room temperature: open circuit potential variation with time (a) and potentiodynamic polarization curves (b).
Fig. 2. XRD patterns (a) and DSC curves (b) of Cu36Zr48Ag8Al8 BMG.
surface is more stable and less susceptible to corrosion reactions [32,33]. Moreover, from Fig. 3b can been seen that the Cu36Zr48Ag8Al8 BMG has a higher corrosion potential (Ecorr) and lower corrosion current density (icorr) than those of 316 SS, that is, Cu36Zr48Ag8Al8 BMG has a higher corrosion resistance in H2SO4 solution [19]. In here, the corrosion current density is the intersection of the tangent to the region of the strong polarization region in the cathodic polarization curve ( ± 50 mV is taken as the limit) and the line perpendicular to the self‒corrosion potential [34]. In addition, the anodic polarization curve of Cu36Zr48Ag8Al8 BMG presents a wider spontaneous passivation area at the low current density, whereas the polarization curve of 316 SS has a clear anodic peak. It also demonstrates that passivation films are easily to be formed on the surfaces of Cu36Zr48Ag8Al8 BMG, and prevent the materials from further corrosion under H2SO4 condition [35]. Meanwhile, the formed passivation films are very stable in a wide range of potential, which also induce Cu36Zr48Ag8Al8 BMG to have a higher corrosion resistance than 316 SS. In addition, with the increase of the applied potential above 0.0 V, the current density of the 316 SS is rapidly increased. In this case, the formed passivation films on the surface of 316 SS are easily to fail, and thereby accelerating the corrosion of 316 SS. However, the current density of Cu36Zr48Ag8Al8 BMG increases gradually with the increase of applied potential and never occurs the phenomenon of rapid increase. Moreover, from the SEM images of the surface of BMG and 316 SS after potentiodynamic polarization tests can be seen that there are no obvious corrosion marks on the surface of BMG, whereas 316 SS presents noticeable corrosion pits on its surface (Fig. 4). Thus, the formed passivation films on the surfaces of Cu36Zr48Ag8Al8 BMG are extremely insensitive to the H2SO4 condition, which makes Cu36Zr48Ag8Al8 BMG to have a higher corrosion resistance than 316 stainless steel.
corrosion products in solution after frictional tests were analyzed by investigating the crystal structures and chemical elements of products on the friction surface of BMG after its surface containing with fluid lubrication films was dried in a vacuum drying oven at room temperature. 3. Results and discussion 3.1. Structure characterization and electrochemical property of Cu36Zr48Ag8Al8 bulk metallic glass Fig. 2a presents the XRD patterns of Cu36Zr48Ag8Al8 BMG with different dimensions. It can be seen that both the XRD patterns of Cu36Zr48Ag8Al8 BMG pins and blocks are similar. There is only a broad halo peak at the position around 2θ = 38°, that is, the prepared BMG possess fully amorphous structure. In addition, it can be seen from Fig. 2b that the glass transition temperature (Tg) and the onset crystallization temperature (Tx) is about 684 K and 790 K, respectively. This result is consistent with the report in the previous research [31]. The presence of a wide supercooled liquid region (about 106 K) promotes the as‒cast Cu36Zr48Ag8Al8 BMG to have a high glass–forming ability in these two dimensions. Fig. 3 presents the electrochemical properties of Cu36Zr48Ag8Al8 BMG and 316 SS in 10% H2SO4 solution. From Fig. 3a can be seen that the OCP values of the two alloys could reach a relatively stable state after being immersed about 500 s, but the value of OCP for BMG is −0.428 V/SCE and for 316 SS is −0.845 V/SCE. The small negative value of BMG indicates the spontaneously formed protective passivation films on the 397
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Fig. 4. SEM images of the surface of Cu36Zr48Ag8Al8 BMG (a) and 316 SS (b) after electrochemical experiment. The inset on the right top is the enlarged image.
3.2. Tribological properties of Cu36Zr48Ag8Al8 bulk metallic glass under H2SO4 conditions
self‒mated Cu36Zr48Ag8Al8 BMG has the smallest friction coefficient (0.10 ± 0.01) and wear rate ((0.28 ± 0.23) × 10−6 mm3 N−1m−1), which just is 29.5% and 1.9% of those in 0% H2SO4 solution and is 33.4% and 2.9% of those in air (dry‒sliding condition), respectively. However, the 316 SS has obvious large friction coefficient (0.44 ± 0.02) and wear rate ((10.71 ± 0.64) × 10−6 mm3 N−1m−1). Besides, the concentrations of the H2SO4 solutions have a significant influence on the friction coefficient curves of the self‒mated Cu36Zr48Ag8Al8 BMG (Fig. 6). The phenomenon can be concluded as following situations. In all H2SO4 solutions, the friction coefficient of the self‒mated Cu36Zr48Ag8Al8 BMG shows a decreasing trend in the running‒in stage. Meanwhile, when the concentrations of the H2SO4 solution is below 15%, the self‒mated Cu36Zr48Ag8Al8 BMG has a relative stable friction coefficient with minor fluctuations in the running‒in stage; but with the increase of the sliding time, the friction coefficient presents large fluctuations (Fig. 6b‒d). However, the frictional state of the materials shows a completely different phenomenon when the concentrations of the H2SO4 solution is above 15%. The friction coefficient of self‒mated Cu36Zr48Ag8Al8 BMG shows large fluctuations in the running‒in stage while the materials have a friction coefficient with small fluctuations after that stage. It is worth noting that when the concetrations of the H2SO4 solution is among 8% to approximately 15%, the friction coefficient of the material rapidly decreases to an ultra‒low value of 0.02–0.03 in the stable stage of sliding. As the further increase of the H2SO4 solution concentration, the large fluctuation stage brings forward, and the stable stage becomes earlier to be reached and more stable in whole friction process (Fig. 6f‒i).
Fig. 5 illustrates the friction coefficients and wear rates of the self‒mated Cu36Zr48Ag8Al8 BMG and 316 SS under dry‒sliding condition and H2SO4 corrosive conditions with different concentrations. Cu36Zr48Ag8Al8 BMG exhibits better tribological behaviors under all H2SO4 conditions than those of self‒mated 316 SS, except under 0% H2SO4 solution (deionized water) condition. The friction coefficients and wear rates of BMG in ˃ 0% H2SO4 solutions can lower than 0.29 ± 0.01 and (2.45 ± 0.42) × 10−6 mm3 N−1m−1. In addition, the tribological behaviors of Cu36Zr48Ag8Al8 BMG are changed with the variation of the concentrations of the H2SO4 solutions. With the increase of the concentrations of H2SO4 solutions, both the friction coefficient and wear rate rapid decrease firstly and then increase gradually. When the concentrations of H2SO4 solution is around 15%, the
3.3. Relationship among tribological behaviors of self‒mated Cu36Zr48Ag8Al8 BMG and H2SO4 solution concentrations From above results, it can be concluded that the concentration of H2SO4 solution has a significant influence on the friction and wear properties of Cu36Zr48Ag8Al8 BMG. To in‒depth reveal the friction‒reducing and anti‒wear mechanisms of the material, the schematic diagrams of the frictional process for the self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with different concentrations were developed in Fig. 7. Firstly, the presence of fluid media between the contact surfaces of friction pairs not only plays a role to flush the wear debris, but also acts as a fluid lubricant to reduce and even avoid the direct contact between friction pairs, and thus reducing the friction and wear of the material [36,37]. The viscosity of the fluid media is an important factor to control the load–carrying ability of the formed fluid lubricating films on the friction surfaces. At room temperature, the viscosity of the H2SO4 solution increases significantly with the increase of the concentration of H2SO4 solution, and which is from ∼1 g m−1s−1 at 0% H2SO4 to ∼11 g m−1s−1 at 70% H2SO4 solution [38]. The large viscosity of the fluid will greatly increase the load–carrying ability and continuity of the fluid lubricating films and even increase the thickness of them. The formation of fluid lubricating film
Fig. 5. Friction coefficients (a) and wear rates (b) of self‒mated Cu36Zr48Ag8Al8 BMG (column graph) and self‒mated 316 SS (scatter graph) under dry‒sliding condition and H2SO4 conditions with different concentrations. 398
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Fig. 6. Friction coefficient curves of self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with different concentrations: 0% (a), 5% (b), 8% (c), 10% (d), 15% (e), 30% (f), 50% (g), 60% (h) and 70% (i).
with certain thickness and load–carrying ability could significantly reduce the direct contact area between friction pairs, and thereby converting the friction sliding between metal and metal to the friction sliding between metal and liquid media (Fig. 7) [39]. Thus, the self‒mated Cu36Zr48Ag8Al8 BMG exhibits an obvious lower friction coefficient in solution containing H2SO4, and also avoiding the severe abrasive wear like as that under deionized water condition. For instance, the depth of wear track of BMG block in 15% H2SO4 solution can reduce to 35.6% of that in 0% H2SO4 solution and 51.3% of that in dry‒sliding condition (Fig. 8). Moreover, the formed metal oxide films on the friction surfaces have a great impact on the tribological behaviors of Cu36Zr48Ag8Al8 BMG. Fig. 9 presents the XPS survey spectra (analyzing the positions on the worn track) and the narrow scans of Cu 2p, Ag 3d, Zr 3d and Al 2p for the worn track of BMG block washed with deionized water and dried after friction tests under dry‒sliding, 0% and 15% H2SO4 conditions. The results demonstrate that the metal oxide films containing with ZrO2, Al2O3 and CuO are formed on the frictional surfaces of materials. Moreover, the thickness of the oxidation films is improving with the increase of the concentration of H2SO4 solution (Fig. 10). The thickness of the oxidation films for the sample to be immersed in 0% H2SO4 solution is about 1.82 μm, and the thickness of the oxidation films for the sample to be immersed in 70% H2SO4 solution could reach about 14.71 μm. Usually, the metal oxides have a relative higher hardness than the BMG, that is, the larger the thickness of the oxidation films could cause a higher surface hardness of BMG. Microhardness tests also demonstrate the surface hardness of the BMG could improve from 536 ± 12 Hv (testing immediately after carefully polished) to
567 ± 14 Hv (testing after the material was soaked in 30% H2SO4 solution for 60 min). The formation of oxides films with a certain thickness and a better corrosion resistance can hinder the BMG materials from corrosion solution, thus preventing further corrosion of materials. Meanwhile, the formation of oxides films with a relatively higher hardness can further improve the wear resistance of the BMG [40,41]. The metal oxide films are mainly formed from the oxidation reactions of the active metals which are exposed on the surface of BMG alloy. For > 0% H2SO4 solution condition, although the Al2O3 and CuO are easily to be dissolved in the H2SO4 solution with low concentration according to Al2 O3 + 3H2 SO4 = Al2 (SO4 )3 + 3H2 O and CuO + H2 SO4 = CuSO4 + H2 O , it gives element Zr more chances to be exposed on the surface of the material and formed more ZrO2 (Figs. 9 and 11). The formed ZrO2 could protect the material from the corrosion of H2SO4 and improve the wear resistance of Cu36Zr48Ag8Al8 BMG. However, the dissolution of metal oxides and exposure of internal metals would cause the occurrence of fluctuation of friction coefficient. Therefore, there are some abrupt changes at the friction coefficient curves of self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with low concentrations (Fig. 6c‒e). Nevertheless, the formed Al2 (SO4 )3 and CuSO4 that have significantly higher molecular mass than that of H2SO4 could significantly improve the viscosity of the fluid on the friction surfaces of the material and enhance the load‒carrying ability and continuity of the fluid lubricating films [42,43], thereby reducing the friction and wear of the Cu36Zr48Ag8Al8 BMG (Figs. 7 and 8). Thus, the material has ultra‒low friction coefficients and wear rates in the 10% and 15% H2SO4 solutions after running‒in stage of friction sliding.
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Fig. 7. Schematic diagrams of the frictional process for the self‒mated Cu36Zr48Ag8Al8 BMG under H2SO4 conditions with different concentrations: 0% (a), 15% (b) and 70% (c).
Furthermore, with the increase of the concentrations of H2SO4 solutions, the amount of formed metal oxides on the surfaces of BMG also is increased (Figs. 9f and 10d). When the friction pairs are sliding in the H2SO4 solution with high concentration, more amount metal oxides could be formed and accumulated on the friction surface of BMG (Fig. 7c). Under high concetration H2SO4 conditions, both the oxidation by O2 and passivation by H2SO4 could promote the metals to form metal oxides. The passivation reactions are mainly included 2Al + 3H2 SO4 = Al2 O3 + 3SO2 + 3H2 O and Zr + 2H2 SO4 = ZrO2 + 2SO2 + 2H2 O , and these reactions mainly occurred in the initial stage of sliding. Thus, the friction coefficient of the self‒mated Cu36Zr48Ag8Al8 BMG presents many large fluctuations at the beginning of friction sliding. With the sliding in a high concentration H2SO4 solution, the friction coefficient curve of the material could become more stable (Fig. 6g‒i). That is mainly because that metal oxide films with a certain thickness (14.71 μm) can be formed on the surface of the friction pairs and improve the surface chemical stability. Specially, the oxides content on the worn surface of BMG immersed in 70% H2SO4 solution could be as high as 85% (Fig. 9f). However, the formation of the stable surface oxide films on the worn surfaces of BMG in high concentration H2SO4 solution could convert the friction pairs from BMG/BMG to ceramic (metal oxides)/ceramic (metal oxides) [44]. Meanwhile, there are few and even no soluble corrosion products to be formed from oxides films like as the phenomenon to be occurred in low concentration H2SO4 solution (Fig. 11). Those may caused self‒mated
Cu36Zr48Ag8Al8 BMG to have a relative higher friction coefficient and wear rate in the high concentration H2SO4 solution (˃30% H2SO4 solution) than that under 15% H2SO4 condition. 4. Conclusion In this study, the tribological behaviors of Cu36Zr48Ag8Al8 bulk metallic glass under H2SO4 condition with different concentrations were investigated. Cu36Zr48Ag8Al8 BMG exhibits good friction‒reducing, wear resistance and anticorrosion properties under different concentrations H2SO4 conditions. These excellent properties are mainly attributed to the positive effects from the formation of metal oxide films and fluid lubricating films on the frictional surfaces during sliding process. The fluid lubricating films were formed by H2SO4 solution and reaction products ( Al2 (SO4 )3 and CuSO4 ) could convert the friction between BMG and BMG to the friction between BMG and fluid, and thus reducing the friction and wear of materials. Meanwhile, the metal oxide films were formed by surface oxidation and passivation actions could significantly improve the wear resistance of BMG and protect it from the corrosion of H2SO4 solutions. In addition, with the variations of the concentrations of H2SO4 solutions, the chemical components, load‒carrying ability and continuity of the formed metal oxide films and fluid lubricating films will also be changed, thereby affecting the tribological behaviors of Cu36Zr48Ag8Al8 bulk metallic glass. When the concentrations of H2SO4 solution is around 15%, the self‒mated Cu36Zr48Ag8Al8 BMG has the smallest friction
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Fig. 8. Surface characteristic of Cu36Zr48Ag8Al8 BMG after wear tests under different conditions: 3D morphologies (a, d, g), sectional morphologies (b, e, h) and SEM images (c, f, i) of worn tracks of BMG block under dry‒sliding condition (a–c), 0% H2SO4 condition (d–f) and 15% H2SO4 condition (g–i). The inset on right side of Fig. 8 (c, f and i) shows the SEM images of BMG pin.
Fig. 9. XPS spectra and quantitative calculation on the worn tracks of BMG block under dry‒sliding condition and under 0% and 15% H2SO4 conditions: survey spectra (a) and narrow scans for spectra of Cu 2p (b), Ag 3d (c), Zr 3d (d), Al 2p (e) and chemical compositions (f) of tribolayer on worn tracks of BMG block.
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Fig. 10. SEM‒BEI images of the cross section of BMG block: after friction tests in the air (a) and in 0% (b), 15% (c) and 70% H2SO4 (d) solutions, respectively.
Fig. 11. Morphologies and EDS maps of the worn tracks of BMG block to be immersed in 15% (a–g) and 70% (h–n) H2SO4 solutions.
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coefficient and wear rate with the respective value is 0.10 ± 0.01 and (0.28 ± 0.23) × 10−6 mm3 N−1m−1, which just is 29.5% and 1.9% of those in 0% H2SO4 solution and is 33.4% and 2.9% of those under dry‒sliding condition.
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