Regulation Mechanism of Salt Ions for Superlubricity of Hydrophilic

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Regulation mechanism of salt ions for superlubricity of hydrophilic polymer cross-linked networks on Ti6Al4V Caixia Zhang, Yuhong Liu, Zhifeng Liu, Hongyu Zhang, Qiang Cheng, and Congbin Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04429 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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Regulation mechanism of salt ions for superlubricity of hydrophilic polymer cross-linked networks on Ti6Al4V Caixia Zhang1, Yuhong Liu2*, Zhifeng Liu1*, Hongyu Zhang2, Qiang Cheng1, Congbin Yang1 1

Beijing Key Laboratory of Advanced Manufacturing Technology, Beijing University of

Technology, Beijing 100124, China 2

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, PR China

KEYWORDS: superlubricity, salt ions, poly(vinylphosphonic acid), cross-linked networks, Ti6Al4V

ABSTRACT: Poly (vinylphosphonic acid)(PVPA) cross-linked networks on Ti6Al4V show superlubricity behavior when sliding against polytetrafluoroethylene (PTFE) in water-based lubricants. The superlubricity can occur, but only with the existence of salt ions in the polymer cross-linked networks. This is different from the phenomenon in most polymer brushes. An investigation into the mechanism revealed that cations and anions in the lubricants worked together to yield the superlubricity even in harsh conditions. It is proposed that the preferential interactions of cations with PVPA molecules rather than water molecules are the main reason for the superlubricity in water-based lubricants. Anions’ interaction with water molecules regulates

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the properties of the tribological interfaces, which influences the magnitude of the friction coefficient. Due to the novel cross-linked networks and the interactions between cations and polymer molecules, their superlubricity can be maintained even at a high salt ion concentration of 5 M. The excellent properties make the PVPA-modified Ti6Al4V a potential candidate for application in artificial implants.

INTRODUCTION Extremely low friction is a common phenomenon observed at tribological interfaces of living organisms, such as hip, knees, and shoulder joints. It has been shown that the mechanisms of the highly efficient lubrication of natural joints are related to the complex structure of cartilage and hydrophilic biomacromolecules1. These have inspired researchers, and many novel biomimetic designs have been developed2,3 for the artificial implants. Polymer based surface modification is a very promising method to reduce friction for the application of artificial implants, accompanied with the widely reported superlubricity phenomenon4-6. Polymer brushes, such as Poly (ethylene glycol) (PEG) brushes, polyacrylamide (PAAm) brushes7-9 and 2-methacryloxyethyl phosphorylcholine (pMPC) brushes10-12, were widely reported to exhibit a low friction coefficient due to the highly-hydrated monomers of chains and the resistance to mutual interpenetration. Cross-linked structure, e.g. hydrogel13, is another useful design to improve lubrication. However, the degree of the cross-linking has opposite effect on lubrication and bearing capacity. This problem can be improved by introducing zwitterionic blends and novel soft/hard composite. Zwitterionic blends were reported to serve as a boundary lubricant and promoted reduction in friction through hydration lubrication14. Soft/hard composite displayed very low friction coefficient under heavy load15.


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The structure and properties of polymers can be significantly affected by external environment, such as pH, temperature and ions16-19. Further, the tribological properties of the polymermodified surfaces can be regulated20. The effects of salt ions have been widely investigated. The polymer chains display conformational change from fully extended state in water to collapsed state in salt solutions, due to electrostatic screening effect and osmotic pressure in chains21,22. In addition, interaction between ions and charged monomers of polymer chains is usually another driven force to induce the conformation change23,24. Therefore, the characteristics of ion-paired state are critical for the properties of the polymer brushes. The friction of the polyelectrolyte brush-covered surfaces was tuned to vary between superior lubrication and ultrahigh friction based on counterion-driven interaction25, 26. The energy dissipation mechanism resulting from the stronger interaction between the calcium ions and carboxylate groups of polyacrylic chains were reported to account for the high friction27. The influence of ions on the hydration properties of interface also regulates their lubrication. There was a similar amount of water molecules on polymer surfaces in different salt solutions with the same concentration. But the mobility of interfacial water molecules promoted various polymer lubrication at the interfaces28. The concentration of the salt solution also affects the lubrication property of polymer brushes. Yu found that increasing the concentration of NaNO3 caused the polymer chains to collapse, which was associated with the increasing friction forces29. Spencer stated similar phenomenon that high concentration of NaCl was deleterious to the lubrication of PEG due to more collapsed conformation30. For the cross-linked polymer structures, salt ions were related to their swelling capacity31,

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In this study, the regulation mechanisms of salt ions in lubricants for their superlubricity were investigated based on the tribological properties of the PVPA-modified Ti6Al4V in phosphate buffer solution33. The key functions of the cations and anions for polymer cross-linked networks are different from polymer brushes. In addition, with the cross-linked networks, the superlubricity of the PVPA-modified Ti6Al4V can be maintained in an environment of high salt ion concentration. MATERIALS AND METHODS Materials. PVPA (97%), a type of polymer with good biocompatibility, was supplied by Sigma Aldrich. Ti6Al4V (100 mm × 100 mm, 1 mm thickness) foils were purchased from Goodfellow, Inc. These foils were cut into small squares of 10 mm × 10 mm. In order to achieve uniform substrates, these square pieces were polished using a polishing slurry (Research Institute of Tsinghua University, Shenzhen) to achieve mirror surfaces (Ra ≈ 2 nm). Phosphate-buffered saline (PBS, pH = 7.2), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solution, acetate buffer solution, NaCl, NaH2PO4, KH2PO4, and so on, were all supplied by Sigma Aldrich. Polytetrafluoroethylene (PTFE) balls (Ra ≈ 280 nm) with a diameter of about 6.35 mm were obtained from Quanying, Inc. All the reagents mentioned above were used without purification. Preparation of PVPA cross-linked networks on Ti6Al4V. PVPA cross-linked networks were prepared on the Ti6Al4V substrates based on the method of evaporating self-assembly horizontally29. Briefly, the Ti6Al4V foils with oxide layer were obtained by heating in air at 140 ºC for 8 h firstly. The pretreated foils were then placed into a PTFE mold horizontally. Appropriate PVPA aqueous solution was injected into the mold and the mold was heated at low


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temperature to accelerate the physically adsorption of PVPA molecules on Ti6Al4V. Lastly, the cross-linked networks was formed on the Ti6Al4V after heating samples at 200 °C for 8 h. Focused ion beam/scanning electron microscopy for determining the cross section of the PVPA-modified Ti6Al4V. To determine the stabilization function of salt ions for the PVPA crosslinked networks on Ti6Al4V, an etching process was performed using a dual column FIB/SEM (TESCAN Company) with a Ga ion beam. Before etching, an Au ﬁlm with a thickness of about 15 nm was deposited on the test samples to increase their electrical conductivity. A Pt coating was also deposited on the samples to protect the PVPA cross-linked networks during etching owing to the soft characteristic of polymer. The acceleration voltage and current were set to be 30 kV and 1 nA at a normal incident angle, respectively. The etching section was polished with 100 pA current to achieve a clear cross section. Universal micro-tribometer for the evaluation of tribological properties. The ball-on-disc machine called a universal micro-tribometer (UMT-3, CETR) was used to characterize the tribological properties of the PVPA-modified Ti6Al4V/PTFE interfaces in different lubricants. Briefly, a motor underneath the disc controlled the motor pattern of reciprocation and sliding speed. A precise two-dimensional sensor can measure the normal load and frictional force generated during sliding contact simultaneously. All the experiments were performed at 37 °C with a motion frequency of 2 Hz (ISO 18192-1 and ASTM F2423) according to the characteristics of the human body. The normal load used was 2.5 N, yielding a maximum initial Hertzian pressure of 25.19 MPa. This was calculated based on the characteristics of the PTFE ball and the PVPA-modified Ti6Al4V used in these experiments. To minimize error, the downholder was regulated carefully to ensure the same friction coefficient during the back and forth


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movements. All experimental results were obtained by averaging the values of at least four repetitions. X-ray photoelectron spectroscopy for evaluating the surface elements. ESCALAB 250 XI (Thermo Scientific Instrument, USA) equipped with a monochromatized Al K R X-ray source was used to measure surface properties of the PVPA-modified Ti6Al4V. The sputter depth profiles module was mainly performed to detect the variation trends of elements in the PVPA cross-linked networks with increasing etching depth. The spot size was a small square with dimensions of 500 µm × 500 µm. The applied acceleration voltage of the Ar ion beam was 1000 eV, while the ion beam current was 10 µA. During the etching process, the PVPA cross-linked networks on the Ti6Al4V were peeled away layer by layer. Elemental data were collected every 300 s. The results were analyzed using the built-in software after calibrating the binding energy values against a C1s peak at 285 eV. RESULTS AND DISCUSSION The superlubricity of PVPA cross-linked networks assisted by salt ions. PVPA crosslinked networks were combined to the Ti6Al4V substrates through covalent bonds based on the method of evaporating self-assembly horizontally33,34. The PVPA molecules have a layer-bylayer gradient arrangement within the networks, as shown in Figure S1 (Supporting Information). This kind of networks configuration ensures the robustness of the PVPA molecules on Ti6Al4V. However, these PVPA cross-linked networks were stable in PBS and other salt solutions, but not in deionized water33. With the aid of FIB etching, a SEM image displayed the cross section of the PVPA-modified Ti6Al4V after immersed into deionized water and PBS respectively. The dark gray layer in Figure. 1a is the PVPA cross-linked networks. Obviously, after immersed into


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deionized water, the PVPA cross-linked networks disappeared (Figure. 1b). These phenomena inspired us that salt ions in lubricants played a part in stabilizing the PVPA cross-linked networks on Ti6Al4V.

Figure 1. The cross section of the PVPA-modified measured by FIB/SEM Ti6Al4V after immersing into: (a) PBS, (b) deionized water. The interaction between salt ions and PVPA molecules was then proved by a series of novel experiments. The PVPA-modified Ti6Al4V was first immersed in PBS for 12 h. Then, the samples were taken out and dried to remove the moisture. The pretreated PVPA-modified Ti6Al4V displayed superlubricity in deionized water when sliding against PTFE balls at an average sliding speed of 12 mm/s, with a normal load of 2.5 N. Figure. 2a shows the friction coefficient versus time. The value of the pretreated PVPA-modified Ti6Al4V in deionized water is approximately 0.005 (red star line), which is nearly the same as the result of the non-pretreated samples in PBS (black dot line). The surface topographies of the pretreated sample in the tribological contact areas after sliding for 0.5 h are shown in Figure. 2b. A slight scratch was found on the pretreated PVPA-modified Ti6Al4V. No wear scar was observed on the PTFE ball. These results support the excellent tribological properties of the pretreated PVPA cross-linked networks in deionized water.


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Figure. 2 Tribological properties of the PVPA cross-linked networks: (a) Variations in the friction coefficients of the pretreated PVPA-modified Ti6Al4V in deionized water and nonpretreated PVPA-modified Ti6Al4V in PBS sliding against PTFE balls, (b) wear patterns of the two tribo-pairs after sliding experiments. Salt ions are the main reasons for this phenomenon of the immersed pretreated PVPAmodified Ti6Al4V sliding in deionized water. Figure. 3a shows the XPS spectrum (black solid line) collected from the wear trace of the pretreated PVPA-modified Ti6Al4V. A high amount of P2s peak and P2p peak were observed, which is similar to the results of the non-pretreated PVPA-modified Ti6Al4V lubricated by PBS (red dash line), indicating the stability of PVPA cross-linked networks on the pretreated samples. In addition, peaks of Na1s and K1s were observed, which are the elements in PBS, on the spectrum of the pretreated PVPA-modified Ti6Al4V. These results confirm the potential behaviors of salt ions during the soaking treatment. XPS depth etching was then performed on the wear trace area. The results are displayed in


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Figure. 3b. The variation trends of elemental K and Na are the same as C, which is present only in the PVPA molecules. This makes it clear that the salt ions in PBS slipped into the PVPA cross-linked networks during the immersion treatment. The salt ions in the networks do not slip out when the pretreated PVPA-modified Ti6Al4V slide against the PTFE balls in deionized water. Further, detailed high resolution XPS scans were recorded at the etching point 14 (Ti element of Ti6Al4V substrates appeared from this point) to investigate the link between PVPA layers and Ti6Al4V substrate after tribological experiments. Figure 3c shows the detailed highenergy O1s spectrum. The peak at 531.6 eV is pertained to P-O-Ti35,36, ensuring a stable covalent link between the bottom layer of PVPA cross-linked networks and the Ti6Al4V substrates. Peak at 532.5 eV corresponds to P-O-P37 of phosphonate anhydrides. Therefore, all PVPA molecules are still linked to Ti6Al4V substrates through covalent bonds even after tribological experiments. These results not only ensured the robustness of PVPA cross-linked networks, but also showed that the slipping in this tribological system only occurred between the surface of PTFE ball and the surface of PVPA cross-linked networks. And, the binding energy of 530.9 eV pertains to PO- Na+/K+

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networks and replaced the hydrogen ions of hydroxyls in phosphate anhydrides. In a word, the stable experimental results of the pretreated PVPA-modified Ti6Al4V in deionized water above highlight the critical function of the salt ions for the superlubricity of the PVPA cross-linked networks.


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Figure. 3 Elements analysis of the pretreated PVPA-modified Ti6Al4V after sliding in deionized water for 0.5 h: (a) XPS survey scans of the wear track, (b) XPS depth analysis of the wear track, (c) XPS-determined O1s spectrum at etching point 14.


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Regulation mechanisms of cations and anions for superlubricity of PVPA cross-linked networks on Ti6Al4V respectively. The interaction between salt ions and polymer brushes on modified surfaces is an interesting phenomenon, which has been widely researched. Based on the excellent tribological properties of PVPA-modified Ti6Al4V, the movement behaviors of salt ions, further cations and anions respectively, on the PVPA cross-linked networks were investigated in our study. From the experimental results above, it is proposed that salt ions are closely related to the superlubricity of the PVPA cross-linked networks on Ti6Al4V. However, because the elements of the anions in PBS are the same as some of those in the PVPA molecules, the movements of anions to the PVPA cross-linked networks cannot be determined from the results in Figure. 3b. Therefore, the experiments of PVPA-modified Ti6Al4V in other lubricants, whose anions contain elements different from that of PVPA molecules, were performed to confirm the action of anions. Solutions of HEPES (a zwitterionic organic buffering agent) and acetate buffer with the concentrations of 1 M were prepared. Their pH values were regulated to 7.2 using sodium hydroxide successively. The value is in accordance with the environment of human body. NaCl solution with the concentrations of 0.1 M was also prepared to analyze the case of inorganic anions at the same time. The PVPA-modified Ti6Al4V was then immersed in the three solutions successively for 12 h. After rinsing the pretreated samples with deionized water, XPS depth profiling was performed on the three pretreated PVPA-modified Ti6Al4V. The results are listed in Figure. 4. The relative content of elemental S, which is one of the elements in the anion of HEPES, is always approximately 0, as shown in Figure. 4a. Simultaneously, a similar phenomenon occurs, as shown in Figure. 4b. The relative content of Cl is zero (the starshaped points). The elements of acetate group are the same as that of PVPA. There are no elements whose relative content are always zero during the etching process (Figure. 4c). These


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results ensure that the anions in lubricants cannot slip into the PVPA cross-linked networks. That is, there is no interaction between anions and PVPA molecules within the networks. On basis of the fact that PVPA cross-linked networks on Ti6Al4V is stable in PBS, but not in deionized water, it can be concluded that cations are the factors for stabilizing the PVPA crosslinked networks. It is well known that the covalent bond of P-O-P in phosphate anhydrides, which is the basic for the cross-linked networks, can be hydrolyzed when attacked by water molecules39. This induces the damage of the covalent bonds between the different PVPA molecular layers and further the PVPA cross-linked networks. In the meanwhile, the hydroxyls of phosphate anhydrides will be ionized associated with the appearance of hydrogen ions in water-based solutions40. Therefore, cations tend to adsorb onto the residual oxygen atoms of phosphate anhydrides, which in fact replace the hydrogen ion of hydroxyls. The weaker electronwithdrawing property of cations (such as Na+) than H+ towards to oxygen atom of the former hydroxyls is beneficial to strengthening the P-O-P covalent bond in phosphate anhydrides. It can be assumed that there is a competition between the cations and water molecules. when the cations, whose electron-withdrawing ability are weaker than hydrogen ion, adsorb onto the phosphate anhydrides firstly, hydrolysis of phosphate anhydrides can be avoided. The behaviors of cations increase the stability of the PVPA cross-linked networks on Ti6Al4V.


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Figure. 4 Variations in relative contents for different elements during XPS depth profiling of the PVPA-modified Ti6Al4V after immersing into: (a) HEPES solution, (b) NaCl solution, (c) acetate buffer solution.


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The key behavior of the anions was then investigated without the stabilization function. Using these solutions as lubricants, the tribological experiments were performed under the same conditions as that in deionized water. Figure. 5a shows the average friction coefficients. The friction coefficient of the PVPA-modified Ti6Al4V lubricated by acetate buffer is approximately 0.015, which is higher than those of NaCl and HEPES solutions, approximately 0.005. Nonetheless, the wear is slight after sliding for 0.5 h in all the three cases, as shown in Figure. 5b, c, and d. The negligible wear on the two tribo-pairs in Figure. 5c indicates that the PVPA crosslinked networks are stable in acetate buffer. Therefore, the increased value of the friction coefficient in the acetate buffer is not due to the damage of the networks. According to Hamrock−Dowson theory of elastohydrodynamic lubrication (EHL) of point contacts41, 42, the minimum film thickness (hmin) for the PVPA-modified Ti6Al4V lubrication system is approximately 0.82 nm. Considering the surface roughness of the two tribo-pairs, Ratio λ is about 0.0041, meaning that the lubrication is in the regime of boundary lubrication (Supporting Information). It is reasonable to propose that the PVPA molecules and lubricants on the interfaces are the main factors that influence the friction coefficient. In addition, when the anions in lubricants are the same, the lubrication behaviors of the PVPA cross-linked networks are not affected by different cations, as shown in Figure S2 (Supporting Information). Reason is that, on the friction interfaces, anions (H2PO4-) play a greater role than cations (K+, Na+) in despite of the critical role of cations within the networks (Figure S3 in Supporting Information). Cations in lubricants influence the lubrication properties little, which simultaneously confirms the regulation function of anions for the friction coefficient. Hydration characteristics of anions regulate their lubrication performance on interfaces. HEPES is a kind of zwitterionic organic buffering agent. The anion of HEPES with sulfonate


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zwitterionic characteristics shows strong interaction with water molecules43, just like the phosphate groups44. Although betaines with carboxyl moieties also show perfect hydration because of the zwitterionic characteristic45, but the hydroscopicity of the carboxyl moieties in acetate is weakened. This is the main reason for the high friction coefficient in acetate buffer solution. Chloride ion is the anion of NaCl. The strong adsorption of chloride ion with water molecules makes it display good lubrication characteristics46. Therefore, the hydrophilic character on the PVPA interfaces is not changed by the chloride ion, explaining its ultralow friction coefficient as that in HEPES. In a word, the phosphate groups on the PVPA molecules, accompanied with the hydrophilic lubricants, such as HEPES with zwitterionic groups and chloride ion with strong hydration, constitute the frictional interfaces with perfect hydratability. The fluid-like manner of water molecules on the hydration layer benefits the ultralow friction coefficient.


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Figure. 5 The tribological properties of the PVPA-modified Ti6Al4V in different lubricants: (a) average friction coefficients sliding against PTFE balls, (b) wear patterns of two tribo-pairs after sliding in HEPES solution, (c) wear patterns of two tribo-pairs after sliding in acetate buffer solution, (d) wear patterns of two tribo-pairs after sliding in NaCl solution. According to the analysis above, the tribological properties of the PVPA-modified Ti6Al4V are regulated by the salt ions in the lubricants. The cations are beneficial to the stability of the PVPA cross-linked networks because of their interaction with the phosphate anhydrides to prevent their hydrolysis reactions. The strength of interactions between anions and water molecules lead to a change in the frictional interfaces, which control the friction coefficients of the tribological interfaces. In a word, with stable PVPA cross-linked networks, anions with high hydrations maintain the ultralow friction coefficients of the PVPA networks. Figure. 6 displays the schematic of salt ions behaviors in the PVPA cross-linked networks on Ti6Al4V in salt solutions.


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Figure. 6 schematic of salt ions behaviors in the PVPA cross-linked networks in salt solutions Salt concentration-independent superlubricity of the PVPA cross-linked networks. Salt ions are a critical component in the synovial fluid in the human body. Some studies have shown that salt ions, especially at high concentration, can destroy the hydration structure around the chains of polymers on the modified surfaces, which may lead to a change in tribological results30. Inspired by the phenomenon reported above, the impacts of the salt ion concentration on the tribological properties of the PVPA cross-linked networks were explored in our study. Because the main constituent of saline is NaCl, it was chosen for preparing the salt solutions with different concentrations. The concentration of chloride ion in blood plasma is 0.104 M–0.117 M. Therefore, the chloride ion concentrations in the salt solutions were regulated as 0.0125 M, 0.025 M, 0.05 M, and 0.1 M to study the effect of salt ions on the superlubricity of the PVPA crosslinked networks. The experiments of the PVPA-modified Ti6Al4V were performed at an average sliding speed of 12 mm/s with a normal load of 2.5 N. Figure. 7a displays the average friction coefficients. The values are all approximately 0.006. The little difference among the four concentrations indicates that the concentration of salt ions over the range of that in the human body has no influence on the lubrication behaviors of the PVPA cross-linked networks on


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Ti6Al4V. The variation of the friction coefficients with time is very little, as shown in Figure. 7b, confirming a stable tribology. These phenomena ensure that when the concentration of salt ions is less than 0.1 M, the perfect lubrication of PVPA cross-linked networks cannot be destroyed. The superlubricity of the PVPA-modified Ti6Al4V is stable in the environment in vivo, indicating their potential use in implants.

Figure. 7 Tribological properties of the PVPA-modified Ti6Al4V in salt solutions with NaCl concentrations of 0.0125 M, 0.025 M, 0.05 M and 0.1 M: (a) average friction coefficients versus NaCl concentration, (b) variations of the friction coefficient versus time in different NaCl concentrations.


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The special PVPA cross-linked networks are the main reason for the novel tribological properties of the PVPA-modified Ti6Al4V. Polymer brushes-modified surfaces were widely reported. Usually, the polymer brushes, despite few zwitterionic polymers with “antipolyelectrolyte effect” salt responsive behaviors,17 will be damaged by the salt ions in a high concentration10, as shown in Figure. 8a. Because the electrostatic repulsion and steric hindrance are the main mechanism for reducing the friction coefficient. When salt ions were added into the lubricants, the interactions between molecular chains were destroyed, followed by a collapse of the brushes. This strongly affected the tribological properties. However, in this study, the basis of the superlubricity of PVPA-modified Ti6Al4V is the stability of the PVPA cross-linked networks in water-based lubricants. Cations in lubricants are the critical factor for the stability. When there is a large amount of salt ions in the lubricants, more cations tend to slip into the PVPA crosslinked networks. This is beneficial for stabilizing and then strengthening the robustness of the network structures, as shown in Figure. 8b. In addition, the hydrous chloride ion will not damage the hydratability of the interfaces. Therefore, NaCl not only has no influence on the ultralow friction when the concentration is higher than 0.0125M, but also is beneficial to the stability of superlubricity. That is, the lubrication behaviors of PVPA cross-linked networks are not sensitive to the salt ion solutions, whose anions are hydrophilic enough.


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Figure. 8 Schematic diagram of the change for polymer-modified surfaces in salt solutions: (a) brushes, (b) cross-linked networks. Based on the PVPA cross-linked networks, the PVPA-modified Ti6Al4V displays excellent tribological responses at a high concentration of salt ions. NaCl was added into PBS to regulate chlorid ion concentrations as 0.5 M, 1 M, 2 M and 5 M. Some of these concentrations are much harsher than the seawater environment47. Then, the tribological tests were performed in these lubricants. The average values of the friction coefficients are nearly the same, approximately 0.006, as shown in Figure. 9a. However, the fluctuations of the friction coefficient lines (Figure. 9b) are a little larger compared to those in Figure. 7b. The reason may be that the dynamic balance of the crystallization and dissolution for NaCl is fast and violent at high concentrations, which has little change on the interface state. This leads to the small fluctuation in the friction coefficient. Whereas, in spite of the fluctuation, wear traces on the two tribo-pairs are negligible at the high concentration of 5 M, as shown in Figure. 9c. The ultralow wear and friction coefficient in high salt concentrations declare that the PVPA cross-linked networks are independent to salt concentration, which is an important characteristic to guarantee their wide use in other fields. The special cross-linked networks and the stability of cations for this structure work together to achieve the efficient tribological properties.


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Figure. 9 Tribological properties of the PVPA-modified Ti6Al4V in salt solutions with NaCl concentrations of 0.5 M, 1 M, 2 M and 5 M: (a) average friction coefficients versus NaCl concentrations, (b) variations of the friction coefficient versus time at different NaCl concentrations, (c) wear patterns of the two tribo-pairs after sliding in 5 M NaCl solution for 0.5 h.


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CONCLUSIONS The PVPA-modified Ti6Al4V display excellent tribological properties in a wide range of water-based lubricants, such as HEPES, NaCl, NaH2PO4, and KH2PO4 solutions, with an ultralow friction coefficient of approximately 0.005 and negligible wear. Salt ions in the lubricants play a critical part for superlubricity owing to their interactions with PVPA crosslinked networks. The pivotal role of cations is reinforcing the robustness of the networks by interacting with the PVPA molecules through phosphate anhydrides to prevent their hydrolysis. Anions can regulate the friction coefficient by changing the hydratability of the PVPA networks interfaces. The stronger interaction between anions and water molecules, the less is the influence of anions on the hydrous PVPA interfaces, accompanied with a friction coefficient of less than 0.01. In addition, the tribological properties of the PVPA cross-linked networks are independent of the NaCl concentrations over the range of 0.0125 M to 5 M because of the cross-linked networks and the strong interactions between cations and phosphate anhydrides in the PVPA molecules. This indicates the salt concentrations-independent tribological properties of the PVPA-modified Ti6Al4V. Taking advantage of a novel designed cross-linked networks, PVPAmodified Ti6Al4V can be a potential candidate use in implants.

ASSOCIATED CONTENT Supporting Information: The layer-by-layer configuration of PVPA cross-linked networks, the investigation of lubrication regime for the PVPA-modified Ti6Al4V lubrication system and the influence of cations to the friction coefficient were described. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author: Email address: [email protected]. Email address: [email protected].

ACKNOWLEDGMENT The authors would like to thank professor Guoshun Pan from Tsinghua University for providing the polishing solution of titanium. We also acknowledge the funding supported by the Beijing Natural Science Foundation (3164040), the National Natural Science Foundation of China (51522504) and (51575009), the China Postdoctoral Science Foundation (2016M591032), the Beijing Postdoctoral Research Foundation (2016ZZ-14) and the Postdoctoral Research Foundation of Chaoyang District (2016ZZ-01-03).

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Regulation Mechanism of Salt Ions for Superlubricity of Hydrophilic

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