ZnO nanoparticle reinforced polytetrafluoroethylene/polyimide (PTFE/PI) nanocomposites were prepared and their corresponding tribological and mechanical properties were studied in this work. The influences of ZnO loading, sliding load, and velocity on the tribological properties of ZnO/PTFE/PI nanocomposites were systematically investigated. Results reveal that nanocomposites reinforced with 3 wt% ZnO exhibit the optimal tribological and mechanical properties. Specifically, the wear loss decreased by 20% after incorporating 3 wt% ZnO compared to unfilled PTFE/PI. Meanwhile, the impact strength, tensile strength, and elongation-at-break of 3 wt% ZnO/PTFE/PI nanocomposite are enhanced by 85, 5, and 10% compared to pure PTFE/PI blend. Microstructure investigation reveals that ZnO nanoparticles facilitate the formation of continuous, uniform, and smooth transfer film and thus reduce the adhesive wear of PTFE/PI.
Lubrication is critical to the operational safety and reliability of industrial manufacturing and processing. Lubrication technology has been widely used in industrial applications, including roller bearings, journal bearings, and gears. Efficient lubrication is valuable to dissipate frictional heat, extend fatigue life, and reduce friction and wear [
Polymers are extensively used as solid lubricants in dynamic mechanical parts due to their unique properties such as high strength, light weight and excellent wear, and solvent resistance [
Polyimide (PI), a class of high performance engineering plastics, is well known for its excellent mechanical properties and stability at high temperature, as well as superior dielectric properties and good chemical resistance, which have found wide applications in aerospace, automobile, and microelectronics industry [
Zinc oxide (ZnO), with outstanding mechanical properties [
To the best of our knowledge, the effect of ZnO nanoparticles on the tribological and mechanical properties of PI based nanocomposites has rarely been studied. In this work, PTFE/PI blend polymer was reinforced by different loadings of ZnO nanoparticles. The optimal loading was explored in association with greatest tribological and mechanical properties. The microstructures of the worn surface, transfer film, and impact-fractured surface were also examined to understand the reinforcing effect of ZnO in the nanocomposites.
Polyimide powder (YS-20, 1–10
In this work, the mass fraction of PTFE in polymer blend is fixed at 15 wt%. ZnO nanoparticles were added into the PTFE/PI blend with different mass ratios: 1, 2, 3, 5, 8, and 12 wt%, respectively. The mixture is weighted accordingly and blended mechanically. Then, the powder mixture was compressed under the pressure of 20.0 MPa and heated to 365°C in a mold with heating rate of 8°C/min. The compressed composite was held at 365°C for 45 min and then cooled down to ambient temperature in the mold while keeping the pressure unchanged. For friction and wear tests, the block was cut into a ring-shaped sample with 26.0 mm outer diameter, 22.0 mm inner diameter, and 2.5~3.0 mm in shoulder height, as seen in Figure
The schematic diagram of tribological testing configuration.
The friction and wear tests were conducted with a ring-on-ring friction configuration, Figure
The morphology of worn surface and impact-fractured surfaces of the composites was characterized by scanning electron microscope (SEM, QUANTA-200). The transfer films on the steel ring were examined by optical microscope.
The tensile tests were carried out on a Universal Tester (Model CMT4254) at room temperature. The deformation rate was 5 mm/min. The impact tests were performed on an impact test machine (Model XJJ-5). Impact and tensile tests were conducted according to Chinese National Standard GB/T16420-1996 and GB/T16421-1996, respectively. For tensile test sample preparation, each composition was molded into a narrow-waisted dumbbell-shaped specimen, and the size of the narrow part is
Figure
Effect of ZnO loading on wear volume loss and friction coefficient. Load: 100 N; sliding speed: 1.4 m/s.
The effects of sliding speed on wear volume loss and friction coefficient of various specimens at a load of 200 N are shown in Figures
Wear volume loss with ZnO loading at 1.4 and 0.69 m/s. Load: 200 N.
Friction coefficient with ZnO loading at 1.4 and 0.69 m/s. Load: 200 N.
Generally, the friction coefficient decreases with increasing ZnO loading, Figure
To explore the role of ZnO on friction and wear behaviors of ZnO/PTFE/PI nanocomposites, the worn surfaces and transfer films of PTFE/PI, 3 wt% ZnO/PTFE/PI, and 12 wt% ZnO/PTFE/PI composites were comparatively investigated by SEM and optical microscope, Figures
SEM micrographs of worn surfaces (×400, 100 N, 1.4 m/s). (a) PTFE/PI, (b) 3 wt% ZnO/PTFE/PI, and (c) 12 wt% ZnO/PTFE/PI.
Optical micrographs of transfer films (×200, 100 N, 1.4 m/s). (a) PTFE/PI, (b) 3 wt% ZnO/PTFE/PI, and (c) 12 wt% ZnO/PTFE/PI. Arrow indicates the sliding direction.
In Figure
Figure
Figure
Schematic illustration of the role of ZnO at (a) low and (b) high loadings.
The mechanical properties of pure PTFE/PI and ZnO/PTFE/PI composites are shown in Table
Mechanical properties of ZnO reinforced PTFE/PI nanocomposites.
Sample | Unnotched impact strength, kJ/m2 | Tensile strength, MPa | Elongation-at-break, % |
---|---|---|---|
15% PTFE/PI | 27.1 ± 0.3 | 81.7 ± 0.5 | 27.5 ± 1.3 |
1% ZnO/15% PTFE/PI | 35.6 ± 0.6 | 82.8 ± 0.6 | 28.2 ± 0.5 |
3% ZnO/15% PTFE/PI | 50.3 ± 0.7 | 85.3 ± 0.7 | 30.1 ± 1.0 |
8% ZnO/15% PTFE/PI | 29.3 ± 0.7 | 74.0 ± 0.8 | 20.1 ± 1.0 |
Due to its high specific surface area of ZnO, a strong interfacial interaction between ZnO and PTFE/PI matrix would be expected. ZnO is distributed uniformly in the matrix and covered with polymer chains completely when the filler content is relatively low, for example, 3 wt% in this work. It is well known that polymer containing a preexisting crack is a result of external stress and a small craze is often formed at the tip of the crack. ZnO could serve as binder at the craze region to delay the craze growth rate. In this regard, extra energy would be required to debond ZnO from polymer matrix before crack further develops, which contributes to a significant improvement in tensile strength [
SEM micrographs of the impact-fractured surfaces of pure PTFE/PI and 3 wt% and 8 wt% ZnO/PTFE/PI are shown in Figures
SEM impact-fracture surfaces. (a) PTFE/PI, (b) 3 wt% ZnO/PTFE/PI, and (c) 8 wt% ZnO/PTFE/PI. Deboned ZnO nanoparticles are marked by arrows.
To sum up, the tribological and mechanical properties ZnO/PTFE/PI composites have been investigated in this work. In comparison with unfilled PTFE/PI polymer blend, the antiwear property of 3 wt% ZnO/PTFE/PI composite is increased by 20%, which could be attributed to the formation of coherent, uniform transfer film that reduces the adhesive wear of PTFE/PI composites. Meanwhile, the impact strength, tensile strength, and elongation-at-break of the 3 wt% ZnO/PTFE/PI nanocomposite are observed to increase by 85, 5, and 10%, respectively, due to the excellent interfacial interaction between ZnO and PTFE/PI matrix. The achieved superior tribological and mechanical properties of the ZnO/PTFE/PI nanocomposite allow its promising tribological and mechanical applications in bearing, compressor piston rings, impeller, and so forth.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to acknowledge the support from National Basic Research Program of China (no. 2013CB733501), the Project of National Natural Science Foundation of China (no. 21176113), the Scientific Research Foundation of Nanjing Institute of Technology for the talent introduction (YKJ201309), the Scientific Research Foundation of Nanjing Institute of Technology for the innovation funds (CKJA201405), and Nanjing Institute of Technology science and technology innovation fund projects for the college students (N20150232). Partial support from the start-up fund of The University of Akron is also acknowledged.