Mechanism of thermoviscoelasticity driven solid–liquid interface reducing friction for polymer alloy coating

: High-temperature ablation is a common failure phenomenon that limits the service life of the transmission parts on heavy-duty machines used in heavy load, high temperature, high shock conditions due to in-sufficient supply of lubricating oil and grease. Traditional self-lubricating coatings prepared by inorganic, organic or organic-inorganic hybrid methods are prone to be oxidated at high temperatures to lose their friction reducing function, so that it is difficult to meet the engineering requirements of high-temperature lubrication. We design viscoelastic polymer coatings by a high-temperature self-lubricating and wear-resistant strategy. Polytetrafluoroethylene (PTFE, T m = 329 °C ) and polyphenylene sulfide (PPS, T g = 84 °C , T m = 283 °C ) are used to prepare a PTFE/PPS polymer alloy coating. As the temperature increases from 25 to 300 °C , the PTFE/PPS coating softens from glass state to viscoelastic state and viscous flow state, which is owing to the thermodynamic transformation characteristic of the PPS component. Additionally the friction coefficient ( μ ) decreased from 0.096 to 0.042 with the increasing of temperature from 25 to 300 °C . The mechanism of mechanical deformation and surface morphology evolution for the PTFE/PPS coating under the multi-field coupling action of temperature ( T ), temperature–centrifugal force ( T – F ω ), temperature–centrifugal force–shearing force ( T – F ω – F τ ) were investigated. The physical model of “thermoviscoelasticity driven solid–liquid interface reducing friction” is proposed to clarify the self-lubricating mechanism determined by the high-temperature viscoelastic properties of polymers. The high-temperature adjusts the viscosity ( η ) of the coating, increases interface slipping and intensifies shear deformation ( τ ), reducing the friction coefficient. The result is expected to provide a new idea for designing anti-ablation coatings served in high temperature friction and wear conditions.


Introduction
Lubrication failures frequently occurs for the transmission parts in heavy-duty machines in heavy load and high impact conditions, due to the loss of lubricating oil/grease, causing high friction severe wear and high temperature in contact surfaces. Tracks, slide rails, transmission gearboxes and other components used in the field of coal power generation heavy equipment, the working conditions are at high temperature (200-300 °C), and the limit temperature can reach 400 °C. In particular, when being subjected to harsh service conditions, e.g. high temperature and high load, oil film tends to fail and thus the hydrodynamic lubrication effect does not play a role. This situation has been demonstrated to affect the strength, hardness, elastic modulus and plastic deformation ability of the material [1,2] and even cause the surface or the subsurface of the material to melt and soften. Simultaneously, under the action of stronge frictional shearing, loss of the material occurs on its surface, resulting in the "high temperature ablation" phenomenon. Finally, the load capacity of the transmission components deteriorates or even damages. It causes the huge waste of resources, economic losses, and even production accidents in the heavy equipment industry. The use of solid lubricating coatings/film surface technologies is one of the main ways to solve the above problems in heavy load wear and elevated temperature ablation [3][4][5][6].
The traditional solid lubricating coatings and films utilize the lubricating property of the matrix, or add a solid lubricant to the matrix material to achieve self-lubrication, wear resistance and high load capacity. The most widely used solid lubricants/lubricating films are MoS 2 , graphite, Polytetrafluoroethylene (PTFE) and Diamond-like Carbon (DLC), etc., which possessing excellent self-lubricating and wear-resistant performances. With the development of industry the application of solid lubricating coating for heavy load, high temperature and harsh working conditions has attracted the attention of researchers; MoS 2 has a special layered structure that shears easily under sliding contact, giving rise to low coefficients of friction at room temperature [7]. Due to high temperature oxidation, its self-lubricating and wear-resistant performance is weaken, and its application is limited. MoS 2 has been demonstrated to start oxidation at 200 °C, and the oxidation rate increases as the temperature rises [8][9][10]. It is the oxidation products (MoO 3 and MoO 2 ) that causes the COF raised from 0.04 (at 25 °C) to 0.10 (at 250 °C) by affecting the shear properties of the self-lubricating film [11]. Graphite is easily oxidized to CO at 400 °C, and violently degraded to CO 2 at 700 °C. The oxidation and degradation reaction reduces the shear strength of graphite, so that it is difficult to meet the requirements of self-lubricating wear-resistant technologies for high tempretures [12,13]. DLC films have the characteristics of high hardness and chemical inertness. They can be deposited not only on the traditional metal matrix, but also on the rubber matrix to improve the self-lubricating properties of the matrix [14]. However, the thermal hybridization of C-C bonds, relaxation of the structure, and graphitization transition have been demonsctrated to occur once the working temperature exceeds 200 °C, which increases the friction coefficient of DLC films and drastically reduce their service life. Doping Si, W, and other elements is one of the common methods to improve the high temperature resistance of these films. However as the temperature increases to 500 °C, these elements have been demonstrated to be oxidized, aggravating the wear instead [15][16][17][18][19][20]. Cao et al. [21] reported a Ti/Ti-DLC multilayer coating, with COF as 0.12 and wear rate as 2.69 × 10 -7 mm 3 /(N·m). Nevertheless, owing to high temperature oxidation at 400 °C, the average COF and wear rate increased to 0.41 and 79.2 × 10 -7 mm 3 /(N·m), respectively. Thus, the traditional MoS 2 , graphite, DLC, and other self-lubricating coatings/films are not available at high temperatures due to chemical reactions, causing to high friction coefficient and severe wear.
In recent years, because polymer coatings have self-lubricating, wear resistance, impact resistance, anticorrosion protection, super-hydrophobic/hydrophobic, self-cleaning and other functionalization, simple preparation [22][23][24], and thermodynamic stability in a certain temperature range [25], they are expected to be applied in practical engineerings to solve the surface wear, ablation, impact, corrosion and other surfaces failure. Self-lubricating and wear-resistant polymer coatings are one of the research hotspots in the field of surface functional protection technologies. PTFE has excellent self-lubricating properties and is a common component of self-lubricating wear-resistant polymer coatings. The self-lubricating properties of PTFE are related to its molecular structure. The F atoms are helically distributed around the C atoms in the main chain, the C-C bonds and/or C-F bonds in the polymer structure are broken during the friction process, and the active PTFE radicals and F ions react or bond with the counterpart metal ions to form a transfer film with strong adhesion. Therefore, PTFE has a lower coefficient of friction than other polymers [26]. High-performance polymer materials, such as polyimide (PI), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK), have been used to prepare high-temperature self-lubricating and wear-resistant coatings. However, a single-component polymer can only achieve a single performance. It is difficult to meet the actual high-temperature friction conditions. It has been found that PTFE coatings were easy to creep and worn severely under high load conditions [26,27], PI coatings exhibited high friction and severe wear [28], PPS coatings were fragile and lack of impact resistance [29]. Generally, inorganic lubricants, such as h-BN, carbon nano tube (CNT), and graphene [30][31][32][33], are used as fillers, which are added into polymers (PEEK, UHMWPE, PI, and PTFE, etc.) to obtain self-lubricating and wear-resistant. The addition of multi-walled carbon nanotubes (MWCNT) and graphene help to form organic-inorganic structure of PI coatings to improve the mechanical properties and tribological performance under high temperature of 200 °C. The friction coefficient reduced from 0.874 (PI) to 0.677 (PI/3MWCNT) and 0.662 (PI/3GP) separately, with weak self-lubricating performance (μ > 0.2), and the wear rate was less than 1/2 and 1/3 comparing with PI (4.82 × 10 -3 mm 3 ) separately [30,31].
Nemati et al. [34] studied the high-temperature tribological behavior of graphene oxide/PTFE composite coating. It was discovered that the wear rate of the composite coating was 34% of that of the PTFE coating and the friction coefficient was reduced from 0.16 to 0.045. It was proposed that the addition of graphene oxide (15 vol%) promoted the synergistic lubrication effect of PTFE and graphene oxide in the composite coating and the mechanical strength of the coating was improved at high temperature. The wear rate of the composite coating at 400 °C was reduced by an order of magnitude compared to that of the pure PTFE coating and the corresponding coefficient of friction was as low as 0.025. Inorganic fillers improved mechanical properties of the PTFE matrix, however the binding force between inorganic fillers and PTFE was weak, limiting the strength and wear resistance of the composite coating to a certain extent.
So Yan et al. [35] capped poly(p-phenylene benzobisoxazole) fiber with antiwear Nano-MoS 2 to reinforce thermoplastic polyimide (TPI)/PEEK matrix at high temperature, which was benifical to transfer stress from the matrix to fiber during friction. At 200 °C, the COF and the wear rate reduced by 22.9% and 61.1% respectively. For the two aromatic thermosetting co-polyesters (ATSP)-based tribo-pairs, PEEK/PTFE coating, the friction coefficient decreased by 47% from 0.163 at room temperature (RT) to 0.087 at 300 °C. Compared to experiments with uncoated aluminum at RT, the friction coefficient reduced by 35%-40% for ATSP coating, indicating the beneficial effect of pre-deposited transfer layer on the countersurface (pin) [36].
Studies have shown that blend modification of PTFE with other organic polymers can make the blended system not only have the wear resistance and creep resistance properties, but also realize friction reducing properties [37]. The friction coefficient of polyurethane (PU) is 0.34, and that of the composite coating is reduced to 0.15 after 7% PTFE blending modification [38]. The research team used PTFE modified PPS to fabricate the polymer alloy coating with the micro-nano-scale binary structure through conventional coating-curing process. The PTFE/PPS coating showed engineering practicability and excellent properties such as superhydrophobicity, self-cleaning, and wearresistance, etc. [39][40][41][42]. We reported the PTFE/PPS composite coating (thickness of 40 μm) filled with 40 vol% PTFE and cured at high temperature presented longer wear life of 234 m/μm at 320 N and 1.25 m/s and reduced friction coefficient of 0.18, under dry friction conditions at 25 °C [24]. It can be seen that the friction coefficient of the coating obviously decreases with volume concentration of PTFE increasing, and proper volume fraction of PTFE can reduce the friction coefficient and enhance the wear life of the PPS coating. When volume concentration of the PTFE in composite coating is 40 vol%, it gives a suitable lubricant (PTFE) and binder (PPS) for better wear resistance at room temperature [24]. Temperature has a significant impact on the properties of polymer materials. The increase in temperature causes the polymer material to transform from glassy state to viscoelastic state and molten state. During the transition process, the mechanical properties of polymer materials such as elastic modulus and yield strength are weakened. It was found that the elastic modulus of the polymer in the glass transition zone decreases from 10 9 to 10 6 Pa [43][44][45]. Based on the above analyses and previous research results, using the PTFE/PPS polymer alloy coating preparing method, the high temperature viscoelastic properties of the polymer coating could be controlled by adjusting the temperature.
www.Springer.com/journal/40544 | Friction In order to establish the relationship between the high temperature viscoelasticity of polymer coatings and friction reduction at the solid-liquid interface, the main factors affecting the mechanical deformation of polymer coatings and the promotion of the mechanical deformation on friction reduction performance of the coatings are studied.

Materials
The density of commercial PTFE powder is 2.2 g/cm 3 , and the average diameter of the powder is 5 μm, as a polymer lubricant. Water-dispersed PPS resin (with a density of 1.34 g/cm 3 , average diameter of 4 μm), with a solid content of 26-28 wt%, and the deionized water and ethanol as mixed solvents, was used as a binder. The curing temperature and melting temperature of the binder was 380 and 283 °C, respectively. Phenolic resin (PF, density of 1.10 g/cm 3 ) with a solid content of 25 wt% and mixed solvent of ethanol, acetone and ethyl acetate in a volume fraction of 1:1:1, was used as a binding agent, of which the curing temperature was 180 °C.

Coatings preparation
The PTFE/PPS polymer alloy coating was prepared by the following steps: Firstly, water-dispersed PPS was ground and dispersed in a ball mill for 72 h, and PTFE was dispersed in mixed solvents of deionized water and ethanol, and was ground 4-5 times with a tapered mill. The solid content of PTFE was 20-25 wt%. The PPS and PTFE dispersions were mixed evenly in proportion and coating precursors were prepared. The research showed when the volume fraction (VF) of the PTFE in composite coating is 40 vol%, it gives a suitable lubricant (PTFE) and binder (PPS) for better wear resistance [24]. PF was dissolved in mixed solvent of ethanol, acetone and ethyl acetate, and was stirred, and mixed evenly. Then the PTFE mixed solvent dispersion was added in, and mixed uniformly with the phenolic resin, the volume fraction (VF) of PTFE in the PTFE/PF composite coatings was also 40 vol%, and the PTFE/PF coating precursors was prepared.
The 45 # steel test blocks (Φ 45×8 mm) were polished with 320 # sandpaper, and then ultrasonically cleaned with acetone for 5-10 min. The wet coatings on the blocks were prepared by spraying the coating with 0.2 MPa compressed air. After the PTFE/PPS coating was prepared, it was heated and kept at 150 °C for 30 min. The temperature rise to 380 °C within 120 min, and held for 90 min. Then it cooled to room temperature with the furnace, to complete the curing. The PTFE/PF coating was heated and kept at 180 °C for 2 h, and then cooled to room temperature with the furnace to complete the curing. The thickness of the cured coatings was messured with a Qnix 4500 Coating Thickness Gauge (accuracy ±0.1 μm), which works on the principle that the attractive force between the coating and a magnetic metal is inversely proportional to the distance between them.The cured coatings were measured to be 20-30 μm.

High temperature friction test of coating
The heavy-load high-temperature pin-disc wear tester (MS-W6000, Lanzhou Huahui Instrument Equipment Co., Ltd.) was used to evaluate the friction and wear behavior of the coatings. The counterpart pin was 45 # steel (Φ 3 × 20 mm). The tribo-counterpart used the mode of pin-disk contact (disk: Φ 45 × 8 mm), and slided against with each other in a circular unidirectional friction movement (Φ 25 mm). The sliding was performed from room temperature to 300 °C at a sliding velocity of 800 r/min (1.05 m/s), according to the application conditions of heavy-load machines at 10-20 MPa, the tribological properties of the coating were evaluated under the normal load of 17 MPa (normal pressure 118 N, counterpart pin Φ 3 mm). The frictional environment temperature was controlled by furnace and measured through the thermocouple located in the furnace, the furnace (Φ 220×170 mm) and heating chamber (Φ 85×80 mm), the temperature range is 25-600 °C, the heating rate is 0.1-15 °C/min, and the temperature control accuracy is ± 1% (Fig. 1).
The wear life (km/μm) of the coatings were obtained according to the coatings thickness and the time of coatings wear out, as Eq. (1) [46]: where, v is the sliding velocity (800 r/min), r is the wear scar radius (12.5 mm), t is the time of coatings wear out (min), h is the coatings thickness (μm). In this work, the average results of three repeated tests were reported to minimize data scatter. The effect of a single high-temperature effect on the coatings surface morphology were investigated under unload (F N = 0) and static conditions. The synergy effect of centrifugal force and high temperature on the coatings morphology were studied, under the conditions of unload and centrifugal velocity of 800 r/min. Figure 1 shows the schematic diagram of the test rig.

Characterization
The microstructures of the coating before and after the friction test were observed by field emission scanning electron microscope (TESCAN MIRA3 FE-SEM, TESCAN, Czech). The apparent element distribution of the coatings were investigated by energy disperse spectroscopy (EDS) (BRUKER, XFlash6130). The thermal stability of the coating was tested with a synchronous thermal analyzer (STA449F5, NETZSCH), at a heating rate of 10 °C/min from room temperature to 900 °C in air atmosphere. The functional groups on the surface of the coating samples were analyzed by the attenuated total reflectance method using a Fourier infrared tester (IR, Nicolet, Nexus 870). The crystal phases were tested with an X-ray diffraction (DX-2700BX-ray Diffractometer, HAOYUAN). The type and valence state of elements on the sample surfaces were quantitatively tested by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, America). The 3D morphology of coating surfaces were detected by a 3D measuring laser microscope OLS5000, with arithmetic mean height (Sa) calculated.

High-temperature friction reducing behavior of the PTFE/PPS polymer alloy coating
As shown in Fig. 2(a), the PTFE/PPS polymer alloy coating exhibited a dense surface, with the micron-scale rough structure. From XPS spectrum, F 1s and C 1s peaks of the PTFE component show high intensity, while the S 1s and O 1s peaks related to PPS are not obvious ( Fig. 2(b)). This situation indicated that the PTFE with low surface energy had migrated to the coating surface, while the PPS had migrated to the substrate-coating interface. The structure exhibited gradient distribution feature, not only enhancing the adhesion strength between the coating and substrate but also endowing excellent self-lubricating and wearresistant properties of the PTFE/PPS coating [23,24]. The thermodynamic analysis of the PTFE/PPS coating showed ( Fig. 2(c)) that typical glass-transition step of PPS occurred at 84°C (T g ), and its obvious cold crystallization exothermic peak occurred at 124°C (T c ). The endothermic peaks at 283 and 329 °C were corresponded to the melting points of PPS and PTFE, respectively [47,48]. Because the melting points of the PPS and the PTFE were close, it was easy for the two components to cross-link and entangle with each other during the process of high-temperature curing, forming a continuous and dense polymer alloy coating without internal stress. The thermogravimetric (TG) curve showed that the coating began to thermally decompose at 493 °C and decompose completely at 768 °C ( Fig. 2(d)). The result demonstrated that the PTFE/PPS polymer alloy coating system exhibited better thermal stability compared to other adhesive based coatings, and it could be applied to high-temperature friction conditions. Considering that the polymers own different phases with different viscoelastic properties near its glass transition (T g ) and melting point (T m ), the researchers designed five temperature conditions of 25, 100, 150, 200, and 300 °C, and investigated the friction coefficient and wear life of the coating in its glass state (25 °C), viscoelastic state (100, 150, and 200 °C), and viscous flow state (300 °C). The results showed (Fig. 2(e)) that the μ of the coating at 25 °C (glass state) is the highest (μ 25 °C = 0.096), and it kept decreasing as the www.Springer.com/journal/40544 | Friction temperature increased. The μ 100 °C , μ 150 °C , and μ 200 °C of the coating in the viscoelastic state were reduced by 22%, 30%, and 32%, respectively, compared with μ 25 °C . It was found that although the coatings were in the viscoelastic state both at 100 and 200 °C, there was a significant difference between the μ 100 °C and the μ 250 °C . This was because when the friction temperature was close to the T g of the coating, the coating appeared as a highly elastic state with a higher friction coefficient, while when it was close to the T m , the coating appeared as a viscoelastic state with its friction coefficient further reduced. When the coating was in the viscous flow state, the friction coefficient (μ 300 °C ) dropped sharply to (0.042), which was 56% lower than μ 25 °C and 32% lower than μ 200 °C . PPS melts and becomes fluid at 300 °C, and the friction coefficient decreases sharply. It was demonstrated from the change of the friction coefficient that 100 °C (near T g ) and 200 °C (near T m ) are two inflection points. The former presents a trend of changing from a sharp decrease to a slow decrease, while the latter was just the opposite. This might be related to the special phase transition and the sudden change of mechanics of the polymers near their T g and T m , a key issue for the PTFE/PPS polymer alloy coating to achieve outstanding selflubricating performance. The wear life of the PTFE/PPS polymer alloy coating decreased with temperature increasing (Fig. 2(f)). The wear life of the glassy coating tested was 23.9 km/μm, and that of the viscoelastic coating at 200 °C was 0.7 km/μm. The viscous flow coating (300 °C) still had a wear life of 40 m/μm, for its load-bearing capacity losing as the molten state. This was because heating reduced the viscosity of the polymer materials and increased the molecular flow, leading the loss of the polymer coating under the synergistic effect of the pressure

Mechanism of "thermoviscoelasticity driven solid-liquid interface reducing friction"
Applying rheology and glass transition theory of polymer materials to analyze the high-temperature friction reducing behaviors of the PTFE/PPS polymer alloy coating, a physical model of "thermoviscoelasticity driven solid-liquid interface reducing friction" was innovatively proposed (Fig. 3). According to the polymer glass transition theory, the polymer alloy coating would undergo a transition from the glassy state (T where, T is the temperature (°C), between 100 and 300 °C, T g is the glass transition temperature (°C), η (T) is the coating viscosity with the temperature T (Pa·s), and η (T g ) is the coating at the glass transition temperature viscosity (Pa·s). Therefore, if T g and η (T g ) are constants, it could be inferred that η (T) and T are negatively correlated, that is, η 1 > η 2 > η 3 > η 4 > η 5 .
If the substrate, substrate based polymer coating, and the steel pin as the counterpart are regarded as a closed system, there is only energy exchange but no substance exchange. There are 3 interfaces in the system (Fig. 3), including the "substrate-coating" interface (Interface I), the "coating-air" interface (Interface II), and the "coating-pin" interface (Interface III). When the system temperature is T1 (25 °C), T2 (100 °C), and T3 (200 °C), Interface I and Interface III are solid-solid interfaces (γ s-s ) and Interface II is solid-gas interface (γ s-g ). When the temperature rises to T4 (300 °C) or T5(400 °C), Interface I and Interface III transmit to the solid-liquid interfaces (γ s-l ), and Interface II transmits to the liquid-gas interface (γ l-g ).
Taking the polymer alloy coating as the research object (assuming that the internal stress and surface tension of the coating are negligible), for static working conditions without friction and wear, the coating at Interface I is only affected by the adhesion of the substrate (F ad ) and Interface II and Interface III are not affected by any force. For the dynamic friction and wear condition, the coating at Interface I is subjected to centrifugal force (F ω , Eq. (3)) and the F ad , and the coating at Interface II is only subjected to the F ω . In order to establish the physical model, k is introduced as the deformation coefficient to describe the viscoelastic state of the polymer coating at a  4)). The coating at Interface III is subjected to the synergistic effect of the F ω , a normal pressure (F N ), and a frictional shear force (F τ , Eq. (5)). Similarly, τ is introduced as the shear deformation coefficient (Eq. (6)). Since F ω and F N are artificially input and known as constants, F τ is proportional to μ, and τ is a function of η.
where, F ω is the centrifugal force that opposite to the center of the circle (N), m is the mass of the coating micro-area (g), r is the centrifugal radius (m), ω is the angular velocity (rad/s), F τ is the friction shear force (N), the direction is opposite to the tangent direction of circular motion, μ is the friction coefficient of the coating (nondimensional), and F N is the normal pressure perpendicular to the surface of the coating (N). Therefore, establishing the corresponding relationship between the "chemical composition-rheological behavior-friction coefficient" of the polymer coating is the key to revealing the mechanism of "thermoviscoelasticity driven solid-liquid interface reducing friction". It could be realized by studying the transition law of viscoelastic state/physical state of the polymer coating at Interface II and Interface III under different temperatures, as well as how the T, the T-F ω , and the T-F ω -F N effect on the η, surface morphology, and surface composition of the polymer coating. This was because the cavitation gas sealed in the original binary structure of the coating was discharged as heated [41]. The fiber entanglement morphology of the original coating could still be observed on the surface of the 200 °C-heated coating, indicating that the coating was partially softened without flowing. However, it was observed that the surface of the 400 °C-heated coating got much flatter than that of the 200 °C-heated coating, the micro-nano fiber entanglement morphology almost disappeared, and evenly distributed pores formed with the Φ increased by 2 times (about 47 μm) (Figs. 4(c) and 4(f)). Since 400 °C was higher than the T m of PPS and PTFE, the surface morphology of the 400 °C-heated coating was formed after the polymer alloy coating had undergone the steps of softening, melting, cooling, and secondary crystallization. From IR analysis of original coating ( Fig. 4(g)), the characteristic peaks at 1,567, 1,467, and 1,391 cm -1 were stretching vibration absorption peaks of the benzene ring belonged to PPS. The characteristic peak at 802 cm -1 was corresponded to para-stretching vibration of the benzene ring. The C-O absorption peak at 1,014 cm -1 was demonstrated that the PPS component of the polymer alloy coating and O 2 had undergone thermal cross-linking oxidation during the curing process [50]. The peak intensity of the characteristic peaks at 1,203 and 1,146 cm -1 , corresponded to the stretching vibration of C-F groups belonging to the PTFE component [51], was significantly higher than that of the PPS component, indicating that the low surface energy C-F functional groups had migrated onto the surface of coating during the curing process of the coating, leading to the structure of micro-nano fiber entanglement. In addition, it was found that the C-F peak intensity of the coating heated at 200 °C was stronger than that of the original coating, showing that heating aggravated the thermal movement of the PTFE polymer segments within the viscoelastic coating and their migration to the coating surface. Meanwhile, the C-F peak intensity of 400 °C-heated coating presented lower than that of the 200 °C-heated coating, attributing to the thermal decomposition of the PTFE at 400 °C [51]. Figure 4(h) showed that the intensity of the diffraction peak at 18.1°, associated with longrange ordering along the (100) lattice planes in PTFE [52], presented to be stronger on the surface of the 200 °C-heated coating, indicating that heating aggravated the PTFE polymer segments to migrate to the surface. The 20.6° diffraction peak belonged to the composite peak of the PPS component presenting the weakest intensity, which was attributed to a composite peak of (102), (200), and (211) planes (Fig. 4(h)) [50]. The characteristic diffraction peaks attributed to 45 # steel, were characterized on the 45 # steel based coating after heating at 200 °C for 2 h, an excellent evidence of the holes formed in the coating for heating (Fig. 4(e)). The XPS survey scan spectra analysis shows that the C 1s and F 1s spectral peaks of the 200 °C-heated coating were both enhanced compared to those of the original coating (Figs. 4(i) and 4(k)). From the core level spectra C1s, the intensity of C-F 2 peak at 292.2 eV enhanced after heating, while the intensity of Ph-S peak at 284.8 eV weakened. The XPS analysis results are consistent with those of the IR spectra, also due to the migration of the PTFE to the surface. The above results could be explained as that, the viscosity of the polymer coating decreased, free volume increased and the tendency of chain segments movement strengthened during the heating process, promoting the stress concentration points inside the coating develop into holes. As a result, the surface of the coating at Interface II was reconstructed with the fiber morphology disappeared and the surface flatted. The SEM showed that higher the temperature, more obvious surface reconstruction phenomenon tended to occur.

Coupling influence of the high-temperature and centrifuge force
The SEM images showed (Figs. 5(a) and 5(d)) that the impurities, originally attached and embedded in the micro-nano pores of the PTFE/PPS polymer alloy coating, were removed by the centrifugal force at room temperature (Sa = 8.3 μm, Fig. S1(a) in the Electronic Supplementary Material (ESM)), so as to restore the local micropores. The deformation degree of the 200 °C-heated and centrifuged coating was significantly more surpass than the single 200 °C-heated coating and the number of surface micropores were obviously reduced (Figs. 5(b) and 5(e)), attributing to the dual effect of centrifugation and heating that caused the original micro-nano texture to collapse and the macromolecules to flow and fill the original surface micropores (Sa = 6.9 μm, Fig. S1(c) in the ESM). The deformation (k) of the 400 °C-heated and centrifuged coating increased furtherly, as the adhesion of the molten coating and the substrate greatly weakened at Interface I, not enough to overcome the centrifugal force.  As a result, most of the coating was thrown off the surface of the substrate, causing it to become thinner or even expose the substrate (Figs. 5(c) and 5(f)). XPS survey scan spectra showed that the peak position of the 200 °C-heated and centrifugal coating was basically the same as the room temperature centrifugal coating, and F 1s peak intensity was slightly enhanced, which was basically consistent with the XRD results. The O 1s peak of the 400 °C-heated and centrifugal coating surface was significantly enhanced, which was attributed to the high-temperature oxidation products of the metal substrate (Fig. 5(g)). It was proved that the 45 # steel based 400 °C-heated and centrifuged coating was also thrown off under the synergistic effect of 400 °C and centrifugation for the characteristic diffraction peaks of the coating components disappeared by the XRD results ( Fig. 5(h)). However, the diffraction peaks attributed to 45 # steel weakened, due to the high-temperature oxidation produced on the 45 # steel substrate exposed after the surface coating was removed.
As shown in Fig. 6(a), compared with that of the 200 °C-heated coating (t = 2 h), the surface micro-nano fiber entanglement effect of the 200 °C-heated and centrifuged coating (t = 2 h) was enhanced, with the loose and ordered PTFE micro-nano fibers entangled into clusters of fibers ( Fig. 6(b)). The IR and XRD characterization results showed that the content of the PTFE component increased, as the C-F characteristic absorption peaks of PTFE at 1,203 and 1,146 cm -1 (Fig. 6(e)), and the diffraction peaks of PTFE at 18.1°enhanced (Fig. 6(f)), indicating that the migration of PTFE from inside of the viscoelastic PTFE/PPS coating to its surface was intensified by the centrifugal www.Springer.com/journal/40544 | Friction effect. With the extension of the heating-centrifugation effect (8 h), a pit with its diameter (Φ) of about 2 μm was formed on the surface of the coating (Fig. 6(c)). This was because the T m of PPS was lower than that of PTFE. At the same temperature (200 °C), the PPS component in the polymer alloy coating was softer than the PTFE component, and the two were prone to relative flow, leading to phase separation phenomenon observed on the coating surface. Furthermore, with the long-term effect of "heating-centrifugation" (t = 20 h), the diameter of the pit on the coating surface got doubled to about 4 μm, and uniform PPS papillaes with Φ of 300-500 nm could be observed on the surface of the PTFE fibrous cluster, precipitated from the coating inside ( Fig. 6(d)). This result should be owing to the continuous increasing entanglement effect of PTFE fibers on the surface of the coating, and the intensifying relative flow between the PPS component and the PTFE component. The above results showed that the deformation coefficient (k) of the polymer alloy coating was positively correlated with T, F ω , and their action time (t). The longer t was, the more obvious the surface texture reconstruction phenomenon of the coating caused by the T-F ω effect.

Coupling influence of the high-temperature, centrifugal force, and shear force
Through the foregoing studies, researchers had clarified the influence of single T-effect and T-F ω dual effect on the surface elements/functional groups, polymer crystal form and surface morphology of the PTFE/PPS coating. By establishing the relationship between the evolution of the elements/structure and the deformation characteristics (k) of the viscoelastic polymer, the unique "high-temperature viscoelastic" mechanism of the polymer alloy coating was clearly revealed. The high-temperature friction condition could be regarded as the combined effect of T-F ω and F N -F τ represented by T-F ω -F N -F τ . The laws of the combined effect of T-F ω -F N -F τ on the surface elements, functional groups, polymer crystal form, and surface wear scar morphology (Interface III) of the PTFE/PPS coating were furtherly studied. The relationship between the coating structure evolution and the shear deformation (τ) of the viscoelastic polymer could be established by comparing the difference between the research results under T-F ω -F N -F τ and T-F ω .
The tribochemical reaction of the PTFE/PPS polymer alloy coating during the friction process were not considered, and only the effect of temperature on the coating physical states was investigated. The wear scar of the original coating (T = 25 °C) was smooth (Sa = 2.0 μm, Fig. S1(b) in the ESM), continuous and dense while that of highly elastic coating (T = 100 °C > T g ) presented a "fish scales"-like structure and microcracks with a width of about 10 μm. Since 200 °C was lower than the melting points of PTFE and PPS, circular or elliptical morphology proved to be the PPS component was observed on the wear scar surface (Interface III) of the coating after the comprehensive action of T-F ω -F N -F τ . While the PTFE component was proved to be distributed surround the PPS component. The crack width was 40% larger than that of 100 °C, indicating that the width of the wear scar crack increased with the increased temperature (Figs. 7(a) and 7(b)), and the surface of wear scar became rough (Sa=2.5 μm, Fig. S1(d) in the ESM). In addition, EDS element analysis of wear scar at different temperatures (Fig. S2 in the ESM) showed that the flat PTFE was observed to be inlaid in the undulating PPS on the surface of the wear scar after friction at 300 °C and cooling. The surface roughness of the PPS phase increased significantly with the increase of temperature while the PTFE phase kept flat. The former was caused by repeated shearing and tearing of the PPS, and the latter was related to the slippage of linear segments in the PTFE polymer. However, the transfer film of the PTFE/PPS coating formed on the counterpart surface is thick and continuous, which is hardly scraped off during friction process at 25 °C (Fig. 7(c)). Whether it was a highly elastic coating system at 100 °C (Figs. 7(d) and 7(e)) or a viscoelastic coating system at 200 °C (Figs. 7(g) and 7(h)), there was a gap in deformability (η PTFE -η PPS ) between the PTFE component and the PPS component of the PTFE/PPS polymer coating. The transfer film is relatively thinner and discontinuous at 100 and 200 °C (Figs. 7(f) and 7(i)). Furthermore, it was easy to know that the latter is larger than the former. Therefore, in the cooling process after heating, the relative thermodynamic motions of the PTFE polymer chains and the PPS polymer chains in the two viscoelastic coating systems were also different, the essential reason why the cracks got widened by www.Springer.com/journal/40544 | Friction direction. During the actual friction process, the transfer film is molten state (300 °C), (Fig. 7(l)). As shown in Fig. S3 in the ESM, compared with the spectrum of the original tribo-pair surface, the peaks attributed to C and F elements are detected on the tribo-pair surface of the PTFE/PPS coating under same friction and wear condition at 25, 100, and 200 °C, which are consistent with that of the original PTFE/PPS coating in Fig. 4(i). The XPS and SEM (Figs. 7(c), 7(f), 7(i), and 7(l)) results demonstrate the formation of the transfer film. Therefore, the PPS in the coating acted as a liquid film-forming phase (oil/grease) and the viscoelastic PTFE (solid lubricating phase) formed a "polymer-liquid-lubrication" system under the F N , similar to traditional lubricating oil/grease. The viscous flow/molten coating and the 45 # steel counterpart were in "solid-liquid" contact mode. Interface III changed from the original solid-solid interface (γ s-s ) to a solid-liquid interface (γ s-l ). The molten PPS could be completely spread and adhered on the surface of the counterpart, beneficial to formation and self-healing of the transfer film. As the temperature increased, the polymer alloy coating changed from adhesive wear with furrows existing to fatigue wear with cracks forming, which was attributed to the mechanical state change of the coating with temperature increasing. The IR spectrum (Fig. 7(m)) showed that the characteristic absorption peaks intensity of the PTFE at 1,203 and 1,146 cm -1 on the glassy polymer alloy coating wear scar surface decreased sharply. The peak intensity of the C-O vibration absorption peak (1,014 cm -1 ) corresponding to PPS increased significantly, as well as the benzene ring peaks (1,567, 1,467, 1,391, and 802 cm -1 ) and the C-S vibration absorption peak (1,078 cm -1 ). The above results proved that the PTFE originally enriched at Interface Ⅲ separated from the PPS in the glassy coating and got wore under the F N -F τ effect, with the relative content of PPS on the wear scar surface increased. By comparing the peak intensity of the characteristic absorption peaks of the PTFE and the PPS in the IR spectrum, it could be seen that the PTFE component at the wear scar surface of the viscoelastic coating was significantly less than that of the glass coating, while the PPS component increased. This was because the mechanical loadbearing capacity of the polymer alloy coating in the viscoelastic state was lower than that of the glass state, resulting in deeper wear scars and wear of PTFE component by T-F ω -F N -F τ effect. It was also found that the vibration absorption peak at 1,014 cm -1 attributed to the C-O functional group was enhanced, owing to the high temperature intensifying the thermal cross-linking of PPS and O 2 . The softening rate of the PPS component had been delayed to a certain extent by the thermal crosslinking effect, as well as the deformation difference and relative flow between the PPS component and the PTFE component. The XRD spectrum (Fig. 7(n)) showed that the intensity of diffraction peaks of PTFE crystals (18.1°) and PPS crystalline (20.6°) on the wear scar surface of the glassy coating were significantly enhanced. However, the IR results showed that the PTFE was abraded, due to the fact that the PTFE polymer segments were mainly oriented along the direction of F τ in the action of F N -F τ , resulting in transformation from amorphous structure to crystalline structure with the crystallinity degree of PTFE increased [53]. This could explain why the PTFE characteristic diffraction peak intensity detected by XRD method on the wear scar surface was enhanced, but the PTFE characteristic absorption peak intensity detected by IR method was weakened. The diffraction peak of PTFE crystals (18.1°) on the viscoelastic coating wear scar surface was weakened, which was attributed to the increased wear at high temperature.
The research results shown in Figs. 3-6 have proved that the PTFE/PPS polymer alloy coating could be transformed between the glass state, the viscoelastic state, and the viscous flow state (molten state) by adjusting the temperature. In addition, the coatings in these three thermodynamic states exhibit different physical and mechanical properties, including η, elastic modulus, tensile strength, plastic deformation, elongation at break, etc. It is precisely owing to this characteristic of the polymer alloy coating, solid-solid or solid-liquid contact states, and different shear behaviors will be formed between the friction counterpart when it is applied to friction and wear conditions, resulting in different anti-friction and wear-resistant mechanism. The PTFE/PPS polymer alloy coating is glassy state at room temperature, so when it is in contact with the 45 # steel friction counterpart, Interface III is a solid-solid interface 1620 Friction 11(9): 1606-1623 (2023) | https://mc03.manuscriptcentral.com/friction (γ s-s ). Since the polymer segments in the glassy coating are in a frozen state, with η that tends to infinity, strong secondary bond between the polymer segments, large elastic modulus and tensile strength, small shear deformation (τ) will be produced on the coating when affected by F τ . Therefore, it can be considered that the sliding friction reducing effect of PTFE polymer segments is limited due to the hindering effect of the glassy PPS. On the other hand, due to the enhancement of the mechanical strength of the coating system owing to the glassy PPS, the alternating friction shear effect causes PTFE lubricating phase to wear less, leading to a long wear life of the glassy coating.
When the coating is in the temperature range of T g -T m , it behaves viscoelastic state, and Interface III is still a solid-solid interface (γ s-s ). Heating increases the kinetic energy, the amount of free volume, and the degree of free rotation of the chain segment, causing the elastic modulus and tensile strength of the viscoelastic coating to decrease and the plastic deformation and elongation at break to increase. Therefore, the shear deformation coefficient (τ) of the coating increases with the increase of temperature, resulting in the enhancement of the slip and friction reduction effect of the PTFE polymer segments under the T-F ω -F N -F τ effect, with a macroscopic decrease in μ. On the contrary, the slippage effect of polymer segments will weaken the load-bearing capacity of the coating and thus affect the wear resistance of the coating. When the temperature exceeds T m , the coating is melted and becomes viscous fluid state, and Interface III transforms into a solid-liquid interface (γ s-l ). The polymer segments in the viscous fluid coating move violently, and the elastic modulus of the coating decreases sharply, making the viscous fluid coating undergo intermolecular slippage with relatively movement on the mass center of the polymer chain as subjected to the F τ , and tend to flow along the direction of the F τ . It can be seen from Eq. (5) that with both F ω and F N applied to the PTFE/PPS polymer alloy coating for a certain period of time, the shear deformation coefficient (τ) of the viscous fluid coating is much larger than that of the viscoelastic coating and the glassy coating for the viscosity of the liquid is much smaller than that of the solid. The lubrication principle of viscous fluid coating is similar to that of lubricating oil. The molten PPS is like the base oil component in lubricating oil while the viscoelastic PTFE is like the lubricating additive component. The molten PPS carries the viscoelastic PTFE and wets, spreads, adheres and forms a continuous transfer film on the surface of the bare metal friction countpart under the F τ -F N effect. Interface III in the friction and wear system of viscous fluid coating is transformed into a liquid-liquid interface (γ l-l ). The sharply increased fluidity and shear deformability (τ) of the viscous coating are the essential reason for the sudden drop in macroscopic μ, but its wear resistance is also greatly affected by this. The latter is due to the fact that the liquid almost loses its load-bearing capacity, and the molten/viscous flow coating is easily squeezed into the unstressed area after being loaded and continuously loss.
In order to verify the applicability of the proposed physical model to other polymer coatings, we selected PF (T g = 148 °C) and PTFE to prepare the PTFE/PF coating through curing process at 180 °C, which maintains the same tribological behavior change trend as the PTFE/PPS polymer alloy coating (Fig. S4 in the ESM). In summary, "thermoviscoelasticity driven solid-liquid interface reducing friction" physical model is innovatively proposed to reveal the unique high-temperature lubrication mechanism of polymer coatings. The model states that, firstly, high temperature (T) drives the polymer alloy coating to undergo a solid-liquid phase change; then, it is controlled to adjust the viscosity (η) and shear deformation coefficient (τ) of the coating by changing the molecular slip in the coating and the relative flow between the two-phase polymer components; finally, through establishing the corresponding relationship between η, τ, and μ, wear-resistant life, effective friction reduction, and wear resistance can be achieved at the high temperature solid-liquid interface. By establishing a series of corresponding laws of "coating chemical composition-mechanical deformation-friction coefficient-wear-resistant life", a "thermoviscoelasticity driven solid-liquid interface reducing friction" database can be realized, to provide data support for high-temperature lubrication protection technologies applied on the transmission component surface of the heavy-duty engineering equipment.