4.1 Chemical and physical characterizations of the worn polyimide composites
The possible change of chemical components inside the worn tracks formed under the different experimental conditions were characterized using XPS. Figure 5 compares the C1s XPS spectra. On a pristine polyimide composites surface, we can observed four C1s peaks in the range of bond energy from 282 eV to 295 eV, which are respectively assigned to C-C, C-H at 284.8 eV, imide carboxyl-type bonding (C-O, C-N) at 286.1 eV, C = O at 288.5 eV and C-F at 292.2 eV [37–40]. After the friction tests, the C-F peaks almost completely disappear on the worn surfaces under all experimental conditions. Table 3 display the area fractions of all components estimated from the XPS spectra in Fig. 5. Besides C-F component, both the peaks of C-O (C-N) bonds and C = O bonds weaken correspondingly compared to the pristine polyimide composites surface. In general, the decrease amplitude of these two peak areas in rolling-sliding condition is larger than that in pure sliding motion. The addition of PAO4 oil can further reduces the area fractions of these two peaks under pure sliding and rolling-sliding motions. It is consistent with the wear behaviors shown in Fig. 4 that the lubricating oil can facilitate tribochemical reaction and then induce more severe wear of polyimide composites.
Table 3
Parameters of C 1s components of the pristine polyimide composites surface and inside the worn tracks formed under the different experimental conditions.
C 1s component
|
|
|
C-C, C-H
|
C-O, C-N
|
C = O
|
C-F
|
Binding Energy (eV)
|
|
284.8
|
286.1
|
288.9
|
292.2
|
Area Fraction (%)
|
Pristine polyimide
|
40.2
|
7.5
|
6.2
|
46.1
|
Rolling-sliding without oil
|
88.4
|
6.1
|
5.5
|
0
|
Sliding without oil
|
84.5
|
7.1
|
6.1
|
2.3
|
Rolling-sliding with oil
|
93.4
|
4.5
|
2.1
|
0
|
Sliding with oil
|
88.8
|
6.3
|
4.9
|
0
|
Fig. 6 shows the O1s XPS spectra of the worn regions compared to that of a pristine polyimide composites surface. On a pristine surface, the O1s peaks locating at 531.9 eV and 533.7 eV represent C=O bond and C-O bond, respectively [41-43]. Besides that, a new O1s peak at 529.9 eV that highly corresponds to metal oxide [43, 44] can be observed inside the wear tracks formed in rolling-sliding motions but is absent in pure sliding. It is impossible that the oxidation of metal is easier to be activated by rolling motion than sliding motion since the former should corresponds to a weaker shear interaction between the contact interfaces. In rolling-sliding motion, the relative sliding between ball/holder contact interfaces and ball/screw contact interfaces can induce high frictional temperature, which may result in the metal oxidation. This mechanism can also explain the much lower area fraction of C-O peaks under rolling-sliding conditions (Table 4). The high frictional temperature aggravates the rupture of C-O bond and the decomposition of the imide carboxyl bond [39]. The decrease of IC-O/IC=O ratio (area of C-O peak divides by area of C=O peak) estimated from O1s XPS spectra (Table 4) indicates that the C-O bonds in polyimide composites are preferentially broken in the tribochemical reactions.
Table 4
Parameters of O 1s components on a pristine polyimide surface and in the worn regions formed under different experimental conditions.
O 1s component
|
|
|
Metallic oxide
|
C = O, C-OH*
|
C-O
|
Binding Energy (eV)
|
529.9
|
531.9
|
533.7
|
Area Fraction (%)
|
Pristine polyimide
|
0
|
59.5
|
40.5
|
Sliding-rolling without oil
|
0
|
74.2
|
18.5
|
Sliding without oil
|
7.3
|
62.3
|
37.7
|
Rolling-sliding with oil
|
13.8
|
76.7
|
9.5
|
Sliding with oil
|
0
|
66.8
|
33.2
|
* This binding energy may be affected by C-OH bond [45, 46].
Fig. 6 compares topographies of the wear tracks on polyimide composites surfaces formed in pure sliding and rolling-sliding motions under oil lubrication. In pure sliding test, the furrow-like scratches in the wear track are characterized using SEM (Fig. 6a), consistent with the WLI image shown in Fig. 4. In rolling-sliding test, the worn surface is relatively smooth and melting products can be observed at the end of the wear track. This result also supports that the high frictional temperature should play a significant role in the serious tribochemical wear of polyimide composites during the rolling-sliding motion. [38]
4.2 Generation of black products in pure sliding at high temperature
The optical images of the wear tracks on polyimide surfaces (Fig. 3) show that the black products are only formed in rolling-sliding motion under room temperature. The results in Figs. 6 and 7 imply that this process may be strongly related to the high friction temperature induced by the compound motion. To prove this hypothesis, a pure sliding test of polyimide composites against steel ball was conducted under a heating environment. In this experiment, the sample platform was firstly heated to 220 degrees and the temperature transferred to the polyimide composites surface decreased to around 120 degrees (estimated by finite element analysis). After that, the pure sliding tests started after addition of the PAO4 oil on the heated sample surface (upper-left inset in Fig. 8a). Different from the pure sliding wear in room temperature (upper-left inset in Fig. 3b), the black products can be observed in the relatively smooth worn region under high temperature (upper-right inset in Fig. 8a). This behavior combining with the results in Fig. 3 confirm that the black products can be formed at high temperature (heating or friction-induced).
Under the heating condition, the friction coefficient of polyimide composites sliding against steel ball under PAO4 oil lubrication is around 0.07, which is larger than that at room temperature (~ 0.05 in Fig. 3b). There are two possible reasons. One is due to the degeneration of the lubricating property along the black products forming. Another is due to the reduction of the elastic modulus of substrate at high temperature, which corresponds to a larger contact area [12]. Figure 8b shows the O 1s XPS spectrum measured in the worn region formed in pure sliding at high temperature, which is more close to that formed in rolling-sliding at room temperature (Fig. 6). Besides the C = O bond at 533.7 eV and the C-O bond at 531.9 eV, the peak at 529.9 eV corresponding to metal oxide can be observed in the O 1s XPS spectrum, indicating that the steel ball should participate the tribochemical reaction at high temperature.
4.3 Degradation of polyimide composites induced by thermal treatment
The direct thermal treatments were operated to further detect the degradation of polyimide composites at high temperature. Using a tube furnace, the samples were heated at a high temperature of 220 degrees in room air. After heating 5 minutes, no obvious change of the topography was observed for the polyimide composites surface covered without PAO4 oil (upper inset in Fig. 9). Differently, the color of the polyimide composites covered with oil turned dark brown after the thermal treatment (bottom optical image). Figure 9 compares the C 1s XPS spectra measured on these two surfaces. It is obvious that, compared to a pristine polyimide composites surface, the relative intensities of the C-F peaks (292.2 eV) decrease significantly after thermal treatments under these two conditions. Besides that, the values of IC−O/C−N/IC−C/C−H (area of C-O, C-N peak divides by area of C-C, C-H peak) and IC=O/IC−C/C−N (area of C = O peak divides by area of C-C, C-H peak) after thermal treatments (Table 5) are much smaller than that of the pristine surface (Table 3). The results indicate that both the polyimide substrate and PTFE degrade under the high temperature of 220 degrees.
Here, we also observed that the area fractions of C-F peaks are very close after these two thermal treatments, but that of C-O, C-N peak and C = O peak are smaller under the condition with PAO4 oil. Therefore, the formation of dark brown products only under the thermal treatment with oil indicates that this process should not relate to the decomposition of PTFE, but relate to the degradation of polyimide substrate. Furthermore, these results also imply that the PAO4 oil may react with polyimide substrate that facilitates the rupture of C-O and C = O bonds. It is consistent with the more severe surface wear of polyimide composites under PAO4 oil lubrication in rolling-slilding motion (Fig. 4). Comparing the XPS results in Table 3 and Table 5, we can find that IC−F/IC−C/C−H, IC−O/C−N/IC−C/C−H and IC=O/IC−C/C−N after thermal treatments decrease a lot compared to the pristine surface but are still larger than that measured in the formed wear tracks. During pure sliding and rolling-sliding, the mechanical interaction (i.e., shear stress, compress stress or impact stress) is capable of facilitating the bond break. Alternatively, the friction temperature in pure sliding or rolling-sliding tests may exceed the temperature in heating tests, resulting in a more severe degradation of polyimide composites.
Table 5
Parameters of C 1s components on the heated polyimide composites surfaces covered without and with PAO4 oil.
C 1s component
|
|
|
C-C, C-H
|
C-O, C-N
|
C = O
|
C-F
|
Binding Energy (eV)
|
284.8
|
286.1
|
288.3
|
292.2
|
Area Fraction (%)
|
Without oil
|
71.1
|
8.8
|
7.7
|
12.4
|
With oil
|
74.5
|
8.2
|
5.2
|
12.1
|
4.4 Traction curves without and with oil lubrications
The complex movements of the ball in the ball holder would affect the lubrication performance of the system, so we analyze the motion of the ball through an in situ optical observation system. To observe the contact region of the steel ball surface, the transparent glass substrate instead of polyimide composites was applied to match the requirement of the in situ observation system which imaged the ball surface through the glass sample. Here, we propose that the motions would be similar between the steel/glass pairs and steel/polyimide pairs due to the close friction coefficients of these two systems under the same experimental conditions (i.e., around 0.1 and 0.05 at the load of 4 N and the sliding speed of 8.5 cm/s without and with oil lubrications).
Figures 10a and b respectively show the optical images of the steel balls at various moments under the conditions without and with oil lubrications. A mark was made at the steel ball surface and its boundary was used to identify the ball motion. When the glass substrates were moving under a given load of 4 N and a sliding speed of 7 mm/s, the mark boundaries at the steel ball surfaces were found to move around 2 mm after 0.96 s without oil lubrication (Fig. 10a) and after 0.72 s with oil lubrication (Fig. 10b) due to the rolling of the steel balls during sliding process. Based on these observations, we can define the slide-to-roll-ratio of the experiment system and estimate the related traction curve based on the friction coefficients obtained at different sliding speeds. As the traction curves shown in Figs. 10c and d, the friction coefficients decrease with the increase of the slip-to-roll-ratio either without or with oil lubrication. One accepted reason is that local temperature rise at higher slip-to-roll-ratio would reduce the mechanical properties of the contacted materials and then lower the mechanical interaction between sliding/rolling interface [47, 48]. Furthermore, higher temperature might increase the viscosity of the lubricated oil that thicken the interfacial oil film [49–51]. Then, the interfacial shear stress would decrease, resulting lower friction coefficients. The results are consistent with the phenomenon of black products forming at steel/polyimide sliding/rolling interface due to the high flash temperature.
4.5 Mechanism of black products formation depending on motion modes
Different from the pure sliding, the rolling-sliding motion of polyimide composites against steel ball can promote the generation of black products (Fig. 3), which weakens the tribological performance of polyimide composites especially under PAO4 oil lubrication (Figs. 3b and 4). First, the additional sliding of ball/holder contact interfaces and ball/screw contact interfaces in rolling-sliding motion cause high frictional temperature, which facilitates the rupture of C-O and C = O bonds in polyimide substrate. This hypothesis can be clarified by the pure sliding results at high temperature where the black products are formed (Fig. 8a). As the surrounding temperature increasing, the aromatic structure of the polyimide material thermally decomposes into amide monomers, then resulting in the lower mechanical strength and more severe wear [52]. The results in heating tests (Fig. 9) prove that the black products are generated along with the C-O and C = O bonds break in polyimide material although the change of molecular structure during this reaction needs to be further detected using, for example, computational simulations.
Second, PAO4 oil should participate the tribochemical reaction and facilitate the degradation of polyimide composites. It can be proved by the XPS results in Tables 3 and 5 where the relative area fractions of C-O, C-N peak and C = O peak further decrease when the PAO4 oil is added on the polyimide composites surface in the friction tests and after the thermal treatments. At last, metal oxide may also contribute to the formation of black products in rolling-sliding motion, as predicted by the XPS results in Fig. 6. The high frictional temperature may induce not only the reaction of steel with air but also the reaction of iron with the pendent oxygen of polyimide composites during the wear process [53].
To prove this mechanism, another friction test of the polyimide composites against Al2O3 ball was conducted under PAO4 oil lubrication. As shown in Fig. 10, the friction coefficients are 0.06 in rolling-sliding motion and 0.008 in pure sliding motion, both of which are larger than that of polyimide composites and steel pairs under the same experimental conditions (Fig. 3b). Although the black products are formed after the friction test against Al2O3 ball (upper-right inset in Fig. 10), its amount is far less than that against the steel ball (upper-right inset in Fig. 3b). It is normally that the thermal conductivity of Al2O3 (33 W/m·K) is higher than that of steel (80 W/m·K). The higher friction force of the polyimide composites against steel ball should correspond to a larger frictional temperature [54]. Therefore, the formation of more black products under this condition should be related to the metal oxidation.