Tribological behavior of shape memory cyanate ester materials and their tunable friction mechanism

High-performance polymer friction materials with tunable tribological behavior to fit varied work conditions remain a challenge of widespread interest for a variety of applications. Shape memory polymer exhibits morphing and modulus changing over temperature changing provides a promising material to adjust the friction process. Herein, we investigated the tribological properties of shape memory cyanate ester (SMCE) under different conditions. The SMCE exhibits the tribological behavior of good friction material with stable high coefficient of friction (COF) and a low wear rate. Besides, the COF increases and wear rate decreases with the temperature increasing show the tunable friction property of the SMCE. We propose a new model of wear-compensation through shape recovery to explain the adjustable friction behavior of thermal-responsive polymer from the aspect of shape recovery and energy conversion. This study provides a high-performance friction material and paves the route for the application of shape memory polymer (SMP) in tribology field with tunable property.


Introduction
Attributed to the variety of monomers, tunable molecular configurations, and various aggregate structures, polymer could be tailored with diversity tribological properties, including the self-lubricating materials with low coefficient of friction (COF) and the friction material with high COF but low wear rate [1,2]. Polymer has been widely used in tribological field to play the role of saving energy in the form of lubrication, or transmitting kinetic energy [3]. Currently, the rapid development of high technology industry requires high-performance polymer friction materials, such as wind turbines and brake pads fields. However, the serious thermal decay of polymer friction materials, the severe wear, vibration, and noise with unstable friction coefficient limited the application of polymer friction material in the severe environmental conditions [2,4]. Especially, the low and unstable high temperature instantaneous friction coefficient, high wear with short service life of the polymer friction material for the high temperature condition is an urgent issue that requires to be solved. In order to adapt to severe conditions, it is necessary to develop a high-performance polymer friction material with low wear rate and stable COF.
Cyanate ester (CE) with excellent temperature resistance, high strength, low dielectric constant, chemical corrosion resistance, and space radiation resistance [5,6] that has been widely used in aerospace, www.Springer.com/journal/40544 | Friction flame retardant, and other fields [7,8]. With moderate toughness between epoxy resin and polyimide, which are well-studied in tribological field as highperformance polymer, CE is expected to perform a wear resistance polymer. However, the high density of triazine caused brittleness of CE cannot be excluded to consider the friction material, that fewer work has been reported on the tribological behavior of CE. Wu et al. [9] demonstrated the high COF (about 0.54) and quite low wear rate (~2.965 × 10 -7 mm 3 /(N·m)) of CE material. Besides, they found adding certain content of zirconium boride (ZrB 2 ) to CE matrix could significantly reduce the COF to obtain the self-lubrication polymer composites. Nevertheless, Li et al. [10] reported the CE could play the role of toughness modifier for bismaleimide (BMI) thermoset to obtain the high wear resistance friction material BMI/CE. In addition, the COF and wear rate of the BMI/CE can be further tuned by adding carbon nanotubes. Previously, we synthesized a CE polymer network with excellent properties by introducing epoxy resin and liquid nitrile rubber [11]. Due to the diluted triazine density and rather flexible rubber segment, the CE polymer network exhibited a shape memory effect (SME), which allows the material to morph and capable of fixing the deformation and followed to recover the original shape under external stimuli [11,12]. The shape memory cycle involves the morphing and modulus changing, which also determines the tribological behavior of the polymer in friction process. It has been reported that adding fillers to shape memory polyimide can reduce the COF and wear rate of the material [13,14]. On the other hand, the SME can program the surface roughness and adhesion and adjust the COF significantly [15]. Overall, the SME enables the SMP with integrated modulus and surface roughness/adhesion varying in response to external thermal stimulation. It is expected to remotely/actively tune friction properties and broaden the application of such smart friction material in wide fields. Such as the aviation engine fuel system accessories sealing parts, the tunable friction property allows it to adaptive seal the system with the operation conditions. Although the pioneer works pointed out the applicability of the SMP in the tribological field, depth exploring of the mechanism remains a contemporary challenge.
Herein, we explored the tribological behavior of the shape memory cyanate ester. To verify the effect of shape memory effect on friction properties, different polymer matrix, temperature/loads conditions were comparably applied for the friction test. We anticipated that shape memory effect would lead to tunable tribological behavior, as the polymer mechanical properties and friction interface would be adjusted by shape memory process.

Synthesis of shape memory cyanate ester (SMCE)
The SMCE was synthesized according to our previous report [11]. A flask was charged with certain amount of cyanate ester, epoxy, and carboxyl-terminated liquid nitrile rubber, and then it was heated till the cyanate was molten and mixed uniformly with GE36 and CTBN. Next, the mixture was vacuum degassed and thermal cured with the programmed temperature of 120 °C, 3 h, 150 °C, 2 h, and 180 °C, 2 h. The curing procedure of pure cyanate ester material was set to 120 °C, 3 h, 150 °C, 2 h, 180 °C, 2 h, and 240 °C, 3 h. The mass ratios of cyanate ester, epoxy, and CTBN = 10/3/3, 10/3/4, and 10/3/5, are respectively named AGC1, AGC2, and AGC3, while CE0 refers to the pure cyanate ester.

Characterizations
The friction experiment was carried out on the ball-on-disk mode of rotation friction tester (CSEM-THT07-135, Switzerland), which is schematic described in Fig. 1(a). The friction tester consists of a sensor, counterweight, fixed bar, and sample stand. A hightemperature heater was installed under the sample stand, which was surrounded by an insulating sleeve. The GCr15 steel balls (φ = 3 mm) were loaded on the sample as friction pair, and samples with a thickness of 1.5 mm were used as-prepared for the tribology test. The friction test was conducted with the normal load of 1, 2, and 3 N, respectively, at a friction speed of 5 cm/s, the steel ball slide on the film with radius of 5 mm, and the tribological behavior within friction distance of 500 m was collected. Different temperature that including room temperature (RT), 70, and 110 °C were applied for the test to explore the effect of temperature on tribological behavior. More than three samples for each entry were applied for friction test to obtain the reliable results. In addition to the experimental data described in the main text, other frictional experimental data demonstrating the reliability and repeatability of the experimental data are placed in Figs. S1-S4 in the Electronic Supplementary Material (ESM). A dynamic mechanical analysis (DMA, Netzsch 242C) was used to measure storage modulus of these A field emission scanning electron microscope (FESEM, Thermofisher Scientific firm, FEI Apreo S) was used to investigate the morphology of wear scars and debris. The Shore hardness tester (LX-A) was used to measure the hardness of the material at different temperature. In addition, an Olympus optical microscope (STM6) was used to characterize the transfer film on the steel ball. A 3D profilometer (Laser confocal three-dimensional profiler, GW-3) was applied to analyze the wear scar and the calculation of wear rate. The calculation formula of the wear rate (K, mm 3 /(N·m)) is shown as Eq. (1): where ΔV refers the wear volume (mm 3 ), which was calculated by the 3D profilometer, P refers the normal load (N) applied on the sample during friction, and L refers the total friction distance (m).
To obtain the reliable results, more than five areas dispersed on the wear scar were chosen to calculate the average wear volume through 3D profilometer images.

Results and discussion
As described in our previous work [11], the cyanate ester polymer could be tailored with target property through introducing other polymer segments. The incorporating of epoxy resin and carboxyl-terminated liquid nitrile rubber to CE enable the polymer to perform a varied crosslinking density, transition temperature (T r ), and shape memory effect. The T r of prepared AGC1, AGC2, and AGC3 are 133.5, 107.6, and 91.5 °C, respectively. The T r decreases with the increasing amount of CTBN due to the increasing of flexibility segments. As a high-performance polymer, the cyanate ester with large amount of triazine exhibits high modulus and strength, adapted toughness, which is between epoxy resin and polyimide [1,16], that the cyanate ester is expected to demonstrate a favorable tribological behavior. As shown in Fig. 1(a), the tribological behavior of prepared cyanate ester polymers were investigated through a CSM tribometer. First, the coefficient of friction (COF) and wear rate (K) of the AGC and CE0 under 1 N load at room temperature were comparatively studied. No discernible difference between the average COFs of AGC and CE0 was observed, they are all ranged in 0.46-0.55. The COF is relatively stable in the following steady stage friction period, which is beneficial to reduce vibration and noise generated by friction, while the machine is running [16,17]. The friction process involves surface interaction and energy conversion. The COF reflects the resistance force of the polymer matrix toward counterpart sliding. When a counterpart was loaded with certain force and pushed forward, the yielded normal force facilitates the interface engaging and molecules bonding between the sample and the counterpart, while the shear force tends to overcome the resistance caused by the roughness. When the shear force is less than the cohesive energy of the polymer material, it caused the polymer chain motion, however the polymer chains usually restrained in the crosslinked network, that the kinetic energy was dissipated in the form of heat generation. In contrast, when the shear force is higher than the cohesive energy, a large amount of chemical bonds breaking will be caused, in this case a wear emerged [18]. The cyanate ester polymer network with the high density of triazine rings and polar nitrile and hydroxyl groups formed the 3D structure (Figs. S5(a) and S5(b)) in the ESM), the rather high adhesive to the counterpart of GCr15 caused a higher COF in the range of 0.46-0.55.
The wear rate was obtained through the calculation volume of the wear scar, as shown in Figs. 1(d)-1(f), the significant arc grooves with the depth around 2.5 μm were the results of the polymer matrix being worn. The wear rate ranges between 1.08 × 10 -5 -3.18 × 10 -5 mm 3 /(N·m) (Fig. 1(c)), which could be attributed to the adhesive wear mechanism and high COF of these SMCEs. When these SMCEs and CE0 were subjected to friction experiments at room temperature, 1 N load resulted in high pressure due to the small area of point contact, and followed with a large wear rate ( Fig. 1(c)). In comparison with SMCE, the wear rate of CE0 decreased to 1.08 × 10 -5 mm 3 /(N·m), it could be attributed to the enhanced hardness (~85). Since the high hardness increased the anti-adhesion and transfer resistance of the sample to the counterpart. The stable COF with a relatively higher value but rather acceptable wear rate of this SMCE indicates the applicability of this SMCE as friction materials in the form of actuating (transmitting kinetic energy) or brake machine.
The morphology of the wear scar was further analyzed through the SEM. Figures 1(g)-1(i) show characteristic adhesive wear morphology of the wear scar [19,20]. The wear scar of these samples (Figs. 1(d)-1(i), S6 and S7 in the ESM) are relatively narrow, about 150-200 μm. However, a large amount of wear debris accumulated along the sliding trail and the friction counterpart was caused by the high adhesion between the junctions, the relative softer SMCE in compare with CGr15 was teared and adhered to CGr15 under the shear force. When the two friction surfaces were in contact, the relative sliding with certain load resulted in plastic deformation or surface rupture. Besides, the friction surface temperature rising could cause the surface soften or molten in severe cases.
Besides, the optical microscope images (Figs. 1(j)-1(l)) of the upper ball counterpart show the small scratched area with a diameter of around 100 μm, the worn of the subsurface layer of the hard metal further confirm the abrasion in adhesive wear mode. Furthermore, barely transfer film was observed in the wear scar on upper ball, which is typical for thermosets, that all the SMCE samples remained a stable higher COF at RT [1,21]. Besides, the required running-in time to reach stable COF was varied, all samples entered their steady stage after 150 m ( Fig. 1(b)). The progressively plastic deformation and accumulation of wear debris at the initial phase of friction caused the COF increasing, high hardness is conducive to reducing the plastic deformation and wear rate,, as shown in Fig. 1(c), thus the running-in time decreased with the increasing of hardness.
Due to the temperature dependence of the SMCE, which transfers from high modulus state to elastic state with the temperature increasing through the transition temperature ( Fig. 3(a)), the SMCE may perform a varied tribological behavior at different temperature. AGC3 with a transition temperature of 93 °C and good toughness [11] was chosen for the further friction experiments to verify the effect of the temperature on tribological behavior. Then the detail friction of AGC3 at room temperature, T g -20 °C and T g + 20 °C were comparatively studied. As shown in Figs. 2(a) and 2(c), the average COF of AGC3 increases from 0.54 at RT to 0.67 at 70 o C, and then to 0.89 at 110 °C. The cyanate ester shape memory polymer (AGC3) shows a wide transition temperature (T r = 93 °C) with the width of the T r about 60 °C [11]. Increasing temperature to 70 °C and then to 110 °C caused the storage modulus of SMCE decreasing to 377 and 34 MPa, respectively, from 983 MPa at room temperature ( Fig. 3(a)), that an elastic deformation dominated the initial phase of friction at the temperature close to T r . While applied the same load for the friction, lower modulus resulted in a larger elastic deformation followed with the increasing roughness of the contact surface, thus an increasing COF with temperature increasing was obtained. Relatively, increasing the normal load for the friction test at 70 °C also caused the COF slightly increasing, the COF of the friction under 2 and 3 N at 70 o C are about 0.77 and 0.73, respectively (Figs. 2(b) and 2(d)). The increased normal force could raise the deformation and contact area, the contradictory effect led a similar COF evolution curve for AGC3 friction under 2 and 3 N. Whereas, the effect of temperature on the COF is significant in compare to the increasing load. The COF evolution curves of AGC3 under different load at RT show the similar results, the COF slightly  increases with the increasing load but not significant, as shown in Fig. S8 in the ESM. Since the material is softer at 110 °C, increasing the load at this temperature results in a slight decrease in COF of the material, but the COF is still high (Fig. S8 in the ESM).
The hardness of AGC3 decreases significantly from 98.78 to 83.14 with the temperature increasing ( Fig. 3(b)), while the toughness and the mobility of the polymer chain are increasing. The increased softness resulted in the larger contact surface. Then the larger soft surface provides higher resistance against the counterpart sliding, which caused an increased COF. Furthermore, due to the Poisson's ratio of the cyanate equals of about 0.4, the bulge will be generated surround the contact as consequence of the surface deformation caused by compression (Fig. 3(c)). Figure 3(c) also shows the trace after a tip sliding on the AGC3 surface at 110 °C, the protrusion accumulated in front of the sliding direction clearly confirms the bulge generation in the wear trail during friction, which increases the frictional resistance. Increasing load or temperature for the friction test would both cause larger deformation, that larger wrinkles appeared on the wear scar. The resulted increasing roughness of the contact surface plays an important role in COF increasing (Fig. 3(d)).
Although the COF of AGC3 at 110 °C was not steady, the low wear rate (4.24 × 10 -6 mm 3 /(N·m)) at 110 °C was nearly an order of magnitude lower in comparison to that at RT (Fig. 4(a)). At 70 °C, the wear rate increases (Fig. 4(b)) with the load increasing, it increased from 3.24 × 10 -6 mm 3 /(N·m) under 1 N to the maximum wear rate up to 7.39 × 10 -6 mm 3 /(N·m) under 3 N, which is still much lower than the wear rate at RT under 1 N. The two-dimensional sectional contour image of the wear at RT (Figs. 1(d) and 1(f) and S9 in the ESM) and high temperature (Figs. S10 and S11 in the ESM) clearly show the different wear conditions. The wear rate of AGC3 under different loads at RT and 110 °C were also comparatively studied. Figure S5 in the ESM shows the wear rate at RT increases slightly from 2.84 × 10 -5 mm 3 /(N·m) to around 3.5 × 10 -5 mm 3 /(N·m) with the load increased from 1 to 2 and 3 N. And in Fig. S12 in the ESM, the wear rate at 110 °C increases from 4.2×10 -6 mm 3 /(N·m) (Fig. 4(a)) to around 9.1×10 -6 mm 3 /(N·m) with the load increased from 1 to 2 and 3 N. Overall, the wear rate of SMCE at | https://mc03.manuscriptcentral.com/friction temperature close to transition temperature is much lower than that at RT. 3D profilers (Figs. 4(c)-4(f)), SEM and optical pictures (Fig. 5) were applied to further analyze the morphology of wear scars at high temperature. Figures 4(c)-4(f) show the wear scar at different temperature/load, a shallow wear trail with small amount of debris along the periphery of the track were observed. The width of the wear scar increasing from 200 to 300 μm with the load increasing from 1 to 3 N, it is consistent with the diameter of the contact area on the upper ball increased from 200 to 300 μm (Figs. 5(e)-5(g)). The 3D profile and 2D sectional contour image of wear scar in Figs. S6 and S7 in the ESM show the similar result that high load caused higher wear at both of RT and 110 °C, but the wear at RT is relatively larger than that at high temperature. It can also be seen in the SEM image of Fig. 5(c) that a large number of wrinkles appear on the wear scar under 3 N at 70 °C. Because the high shear force led to greater plastic deformation of the material on wear scar in comparison with the situation under lower load. As a result of the increased unrecoverable greater plastic deformation with increasing load, the increase in wear volume with increasing the load was observed (Fig. 4(b)). When the steel ball slides on the soft surface under same load, the one with lower hardness and modulus contributed higher resistance due to larger deformation. The above results indicate that this SMCE could be used for the high temperature as the friction tunable material. In summary, the SMCE exhibits a characteristic temperature dependence tribological behavior, and it shows high COF but low wear at a wide range of temperature friction. Since the good radiation resistance and high temperature resistance of cyanate ester [22], it could be used as high temperature friction material in harsh conditions, such as the application in aerospace field. More importantly, the shape memory polymer can perceive changes in external light, heat, electricity, magnetism, and other factors by incorporation with the photothermal, conductor, or magnetic fillers [7,23]. Thus, this type of SMCE could realize smart tribological property, through the remote control of temperature that allows the SMCE to adjusting the tribological behavior with stable COF and proper wear rate to fit the application requirement.
In general, higher temperature facilitates the polymer chains to rearrange with additional high mobility, thus, decreasing cohesive energy and the elastic deformation yielded a larger contact area during friction would be obtained. When the same weight was loaded for the friction test, the wear rate should be increased at high temperature in compare with that at low temperature, especially the friction experiment at temperature above T r , a serve wear or wear failure may usually observed [24]. However, the wear rate at 110 °C is an order of magnitude lower than the wear rate at room temperature. The SME may play a critical role on the wear rate reducing at high temperature.
Due to the intrinsic SME of SMP, which could be easily deformed and fixed to a new temporary shape by programming temperature, and the original shape could be restored when heating it to above T r [25]. As illustrated in Fig. 6(a), the AGC3 surface was imprinted with the letters of "LUT" (Lanzhou University of Technology) and "LICP" (Lanzhou Institute of Chemical Physics) by a seal at 110 °C, the letters could be recorded at RT Right the heating, the smooth surface could be recovered when heating at 110 °C. Shape memory cycle consists of energy storing through fixing the deformation and releasing the stored www.Springer.com/journal/40544 | Friction energy in the form of shape recovery. When conducting high-temperature friction experiments, the weightbearing upper ball sliding on the surface of SMCE caused the deformation, which will form the wear trail under reciprocating rotation. However, the deformation tended to recover at temperature close to T r . To achieve the indention recovery an additional energy is required. Thus, the shape recovery consumes the energy to alleviate the temperature raising caused by the friction, that it is beneficial to reduce the wear rate in the form of wear-compensation through shape recovery. The recovery hysteresis during the rotation friction and the repeatedly compression under normal force resulted in the deeper/widened wear trail. However, SMP performs fast recovery at higher temperature in compare of the temperature lower than T r , that the width/depth of wear trail of the friction at 110 and 70 °C is much lower than that at room temperature. The model of wear-compensation through shape recovery is schemed as shown in Fig. 6(b). High temperature is favored for the shape recovery due to the sufficient thermal energy for triggering the polymer chain moving to the free-state. Larger deformation can be caused with the increasing temperature during friction, but the fast and high shape recovery ratio allows the deformed wear surface recovery to reduce the wear, the energy dissipation occurs meanwhile to alleviate the kinetic energy of the polymer. In brief, the shape memory cyanate ester can be used as friction material to intelligently regulate both of the COF and wear rate.

Conclusions
In summary, we studied the tribological behavior of shape memory cyanate ester (SMCE). The SMCE performed a high but rather stable coefficient of friction (COF) in the range of 0.46-0.89 and relatively low wear rate 3.18 × 10 -5 -3.24 × 10 -6 mm 3 /(N·m), it provides a reliable friction material for the application in the fields of transmitting kinetic energy or brake machine. Furthermore, SMCE showed a tunable friction due to the temperature dependence, the COF increases but wear rate decreases with temperature increasing. Through the analysis of wear scar, counterpart surface, combining with the COF evolution and wear rate results under different temperature and different loads, the friction and wear mechanism of this shape memory polymer was explored. Due to the high modulus, hardness, and polar polymer segments, SMCE performed an adhesive wear at room temperature with high COF and relative high wear rate. Increasing temperature enables the shape recovery of contact surface deformation caused by friction normal force that yields an increasing COF but wear rate is reduced in a large scale. Besides, shape recovery requires heating, it could attenuate the cohesive energy decreasing and enhance the wear resistance. We propose a new model of wearcompensation through shape recovery to explain the adjustable friction behavior of thermal-responsive polymer from the aspect of shape recovery and energy conversion. The tunable tribological behavior offers the SMCE new application as smart friction materials in broad fields, especially for the parts which are hard to assemble. Such as the aviation engine fuel system accessories sealing parts, the tunable friction property allows it to adaptive seal the system with the varied operation conditions. The tunable tribological behavior offers the SMCE new application as smart friction materials in broad fields, especially for the parts which are hard to assemble. She is now working in LICP. Her current research interests concern the shape memory polymers, stimuli-responsive polymers, and smart lubrication regulation.