Abstract
Research related to the effect of oil fouling on the mechanical and tribological properties of the fabric composite are rare. Herein, Nomex fabric/phenolic composites were soaked in aviation hydraulic fluid or jet fuel oil for 24 h, and their corresponding mechanical and tribological properties were investigated. It was found that the mechanical property of the oil-fouled Nomex fabric/phenolic composites decreased compared to that of untreated fabric composite. Moreover, sliding wear tests indicated that the lubricating effect of aviation hydraulic fluid and jet fuel oil improved the tribological property of the oil-fouled fabric composites obviously, allowing them to exhibit lower friction coefficients and wear rates, compared to the original fabric composite. We also found that the aviation hydraulic fluid-fouled fabric composite and the jet fuel oil-fouled fabric composite displayed varied tribological behaviors and wear mechanisms under the same sliding condition.
Introduction
For the driving system and the internal combustion engine, mechanical friction losses of moving members and wastage of energy and fuel caused by friction are significant.1–3 High-performance fabric composites, which exhibit excellent self-lubricating and mechanical properties, were employed as liner adhered onto the body of bearings, pistons and seals or as construction components to lessen friction between the rubbing pairs and protect them from scratch.4–6 In recent years, the tribological properties of fabric composites working under dry conditions have been extensively studied for obtaining better application performance.7–9 However, the mechanical and tribological performances of oil-fouled fabric composites have scarcely been evaluated.
In practice, bearings, pistons and seals usually work under the conditions oil existed or prone to be fouled by oil. For instance, working in aviation hydraulic fluid is inevitable for fabric composites applied in hydraulic pressure system. Besides, the tribological components in internal combustion engines will frequently work in fuel-existed conditions. According to previous reports, the absorption and adsorption of fluids affect the mechanical and tribological properties of polymers composites.10–14 It is indubitable that, when applied as tribological components of aircrafts, the mechanical and tribological performances of fabric composites are crucial to the security of the aviation. Research is thus urgently needed to evaluate the impact of oil fouling on the mechanical and tribological performances of fabric composites.
Herein, we studied the impact of aviation hydraulic fluid and jet fuel oil on the mechanical and tribological properties of Nomex fabric/phenolic composite. Tensile and bonding strength were tested for original Nomex fabric/phenolic composite and fabric composites soaked in aviation hydraulic fluid or jet fuel oil for 24 h. Besides, sliding wear tests were carried out under different applied loads to determine the tribological properties of these composites. The measured favorable tribological properties of oil-fouled fabric composites indicated the great potential of application of fabric composites under oil-existed conditions. This study was supposed to provide the application of fabric composites as tribo-materials in oil-existed conditions with some basic guidance.
Experimental
Materials
The plain weave Nomex fabric with the weight/area of 1.37 g/cm2 was knitted with the Nomex fibers purchased from DuPont Plant. The adhesive resin (204 phenolic resin: resol) was provided by Shanghai Xing-guang Chemical Plant, China. The rest chemicals were all of analytical grade and used as received.
Specimen preparation
The Nomex fabric was cleaned with petroleum ether and ethanol sequentially in Soxhlet extractor and then dried in an oven at 50℃. Subsequently, the fabric was immersed in the adhesive solution. After several cycles of immersion processes in adhesive solution, the mass fraction of the Nomex fabric in the fabric-resin composite reached to about 65 ± 5%. Finally, the resin-coated fabric was cut into pieces and adhered onto the AISI-1045 steel (size of Φ 45 mm × 8 mm, surface roughness of 0.45 µm) with 204 phenolic resin and then cured at 180℃ for 2 h.
Quantity of absorbed and adsorbed oils.
Tensile and bonding strength test
The tensile and bonding strength of original fabric composite and composite soaked in aviation hydraulic fluid or jet fuel oil for 24 h were determined by a DY35 universal materials test machine at a constant speed of 50 mm/min. The dimensions of the sample used for tensile strength test were 100 mm in length, 20 mm in width, and 800 ± 30 µm in thickness. The tensile strength σb (MPa)
Before carrying out the bonding property tests, the impregnated fabric was cut into pieces (20 mm in length and 14 mm in width). After that, the test pieces were affixed between two steel plates with the adhesive resin and then cured at 180℃ for 2 h under a certain pressure. The bonding strength τ (MPa) was calculated as
Friction and wear test
Sliding wear tests were performed in a Xuanwu-III pin-on-disk tribometer (see Figure 1). In the pin-on-disk tester, a stationary steel pin slid against the Nomex fabric composite specimen in a rotating steel disk. The flat-ended AISI-1045 pin (diameter 2 mm) was secured to the load arm with a chuck. The distance between the center of the pin and the axis was 12.5 mm.
Schematic diagram of pin-on-disc friction and wear tester.
The pin was polished with 350 -, 700 -, and 900-grade water-proof abrasive papers in sequence to a roughness of 999 ± 73 nm before each test and then cleaned with acetone. The sliding wear tests were performed on the Xuanwu-III pin-on-disk tribometer at room temperature for 2 h, with the loads in the range of 60–100 MPa and speed fixed at 0.13 m/s. After each test, the corresponding wear volume loss (V) of the composite was acquired by measuring the depth of the wear scar on a micrometer with the resolution of 0.001 mm. Eight measurements distributed evenly and symmetrically on the wear scar were made to determine the wear depth of each specimen. The wear performance was expressed by wear rate (ω, m3 (N m)−1) as follows: ω = V*(PL)−1, where V is the wear volume loss in m3, P is the load in Newton, and L is the sliding distance in meter.
The friction coefficient which was measured from the frictional torque gained by a load cell sensor could be obtained from the computer running the friction-measure software. Each experiment was carried out three times and the average value was used. Scanning electron microscope (SEM, JSM-5600LV) was used to observe the worn surface morphology of the fabric composites and the pins.
Results and discussion
Mechanical properties of original and oil-fouled fabric composites
To evaluate the influence of oil soaking on the mechanical properties of Nomex fabric composite, tensile and bonding strength of composite A, composite B, and composite C were tested. It was found that composite A exhibited the optimal tensile and bonding strength, followed by composite B and then composite C (see Figure 2). It was indicated that the absorption of both aviation hydraulic fluid and jet fuel oil, especially jet fuel oil, induced an obvious decrease of tensile and bonding strength of Nomex fabric/phenolic composite.
Tensile and bonding strength of composite A, composite B, and composite C.
Figure 3 shows the fracture surfaces of composite A, composite B, and composite C. It can be seen that, on the fracture surface of composite A, most fiber bonded together and integrated well with adhesive resin (see Figure 3a). After being soaked in aviation hydraulic fluid or jet fuel oil for 24 h, fibers on the fracture surfaces debonded from each other and the adhesive resin (see Figure 3b and c), indicating the decrease of adhesive force between the fiber and resin to a great extent. It was logically deduced that absorption of oils, especially jet fuel oil, caused the decrease of adhesion property of phenolic adhesive, and the adhesion failure of adhesive induced the detachment between resin and fibers.
SEM images of the fracture surfaces of (a) composite A, (b) composite B, and (c) composite C.
Tribological properties of original and oil-fouled fabric composites
It is widely recognized that the lubricating effect of oils contributes to the friction reduction and antiwear improvement of composites.14,15–17 Herein, to evaluate the impact of aviation hydraulic fluid or jet fuel oil on the tribological property of Nomex fabric composite, sliding wear tests under differed applied loads were performed for composite A, composite B, and composite C. Friction coefficients and wear rates of the three composites under differed applied loads are shown in Figure 4. As illustrated in Figure 4(a), friction coefficients of the three composites followed the order of composite A > composite B > composite C under all the investigated loads. It was observed in the testes that the residual of aviation hydraulic fluid was more than that of jet fuel oil owing to the more adsorption and less volatilization of aviation hydraulic fluid during the tests. Hence, it was reasonably deduced that the lubricating effect of aviation hydraulic fluid contributed greatly to the lowest friction coefficient of composite C.
Friction coefficient (a) and wear rate (b) of composite A, composite B, and composite C as a function of applied load. The sliding speed in the tests was 0.13 m/s.
Figure 4(b) shows the wear rates of composite A, composite B, and composite C under differed applied loads. It can be seen that under lower loads the wear rates of composite B and composite C were lower than that of composite A. Interestingly, it was found that the wear rate of composite B was lower than that of composite C, when the applied load was 60 MPa. However, with further increasing of applied load, wear rate of composite B became higher than that of composite C. When a load of 100 MPa was applied, the wear rates of the three composites were almost equal.
It was deduced that it mainly the resin matrix coated on the fabric surface underwent abrasion under lower loads. Based on the mechanical test results, damage caused by absorption of jet fuel oil to the resin matrix was more severe than that caused by absorption of aviation hydraulic fluid. Resultantly, the wear rate of composite B was lower than that of composite C under the applied load of 60 MPa. As the applied load increased, both fabric and resin matrix participated in the abrasion and more microcracks came into being under high pressure. The amount of aviation hydraulic fluid adsorbed and retained on the surface of the fabric composite was more than that of jet fuel oil. During the wear process, aviation hydraulic fluid would inevitably penetrate into the microcracks on the worn surface and softening could take place in the cured adhesive, so that the adhesive softened and the firm bond between resin and fiber became loosened. 18 Accordingly, wear rate of composite B came out higher than that of composite C under higher applied loads. When the shearing force kept increasing, influence of fouling on the abrasion of the fabric composite diminished as the oils were squeezed out of the worn surfaces, characterized by the nearly equal wear rates of untreated and oil-fouled fabric composites. Based on the above results, it was concluded that the applied oil, the quantity of the adsorbed oil, and the applied load were all critical factors determining the antiwear property of the fabric composites.
Worn surfaces of original and oil-fouled fabric composites and their counterpart pins
Worn surfaces of composite A, composite B, and composite C under the applied load of 80 MPa, speed of 0.13 m/s are shown in Figure 5. Figure 5(a) shows that adhesive wear dominated the abrasion of composite A. Broken fibers in the magnified image indicated that the exposed fibers were cut down from the composite (see Figure 5d). Comparatively, the worn surfaces of composite B and composite C were smoother without evidence of any pits (see Figure 5b and c), which was attributed to the lubricating effect of oils. Besides, the damage of fibers on worn surfaces of composite B and composite C was also milder than that of composite A (see Figure 5e and f).
SEM images of the worn surfaces of composite A, composite B, and composite C. (d), (e), and (f) are their corresponding images with a higher magnification. The applied load and sliding speed in the tests were 80 MPa and 0.13 m/s, respectively.
It was worth noticing that, for fabric composites soaked in oil, more broken fiber bundles were detected on the worn surface of composite B (see Figure 5b). Whereas, exposed and broken fibers were much less on the worn surface of composite C (see Figure 5c), in agreement with the lower wear rate of composite C than that of composite B under the applied load of 80 MPa. It was supposed that aviation hydraulic fluid on the worn surface of composite B accelerate the abrasion of adhesive and fabric by way of penetrating into the composite along micro cracks.
Figure 6 shows the morphology of counterpart pins the fabric composites slid against. As shown in Figure 6(a), on the pin composite A slid against, masses of parallel furrows and a discontinuous transfer film came into being. With respect to pins composite B and composite C slid against, less scratches and more continuous transfer films were detected (see Figure 6d and c), signifying the lubricating effect of oils. In conclusion, though deteriorated mechanical properties of oil-fouled Nomex fabric/phenolic composites, outstanding tribological properties characterized by decreased friction coefficients and wear rates were monitored under the investigated sliding conditions, owing to the lubricating effect of the oils. Logically, Nomex fabric/phenolic composite can still serve as outstanding candidate for tribo-materials applied under oil-existed conditions.
SEM images of the counterpart pins slid against composite A (a), composite B (b), and composite C (c). The applied load and sliding speed in the tests were 80 MPa and 0.13 m/s, respectively.
Conclusion
Nomex fabric composites were soaked in aviation hydraulic fluid or jet fuel oil for 24 h, and their mechanical and tribological properties were investigated and compared to that of original composite. Mechanical tests indicated that the absorption of aviation hydraulic fluid and jet fuel oil induced a decline in mechanical property of the fabric composite by influencing the mechanical property of phenolic resin matrix. Sliding wear tests indicated that the antiwear properties of the three composites under low applied loads were closely related with their mechanical properties. Nevertheless, with the applied load increasing, the wear rate of oil-fouled composite was determined by the quantity of the oil adsorbed on the composite surface. When the applied load was higher, the influence of adsorbed and absorbed oil on the abrasion of fabric composite diminished.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The financial support of the National Science Foundation of China (grant no. 51375472 and 51305429) and the “Western Action Program”.