Study on the reinforcing effect of nano-SiO₂ filled PTFE composites

Frictional components made from various metals and alloys are gradually being replaced by polymers and polymer composites. Several of these composites are based on polytetrafluoroethylene (PTFE).¹ These materials reduce costs and improve reliability and durability. In despite of all the superior performance characteristics of PTFE composites, some of its characteristics (e.g. poor wear resistance) diminish its utility. In tribological applications, using reinforcing particles as fillers plays an important role in reducing the friction coefficient and the wear rate.

In order to improve the anti-wear and anti-friction ability of pure PTFE, PTFE has been reinforced with several materials such as bronze, glass, carbon fibers, h-BN, SiO₂, graphite flakes and other inorganic fillers.^2–7 The wear rate and coefficient of friction for polytetrafluoroethylene (PTFE) composites have been evaluated under dry and lubricated conditions.^8–10 They have shown a better performance than pure PTFE. Polytetrafluoroethylene (PTFE) fluoropolymer is a well-known choice for coatings, insulation, thermal sealing, lubrication, bearings, and clinical applications.¹¹ 

In this paper, the mechanical properties of Nano-SiO₂ filled PTFE composites are evaluated. The wear and friction behaviour of PTFE composites operating under variable loading and dry sliding were studied in detail.

The base polytetrafluoroethylene (PTFE) material was obtained from Anyang Ultrahigh Industrial Technical Co. Ltd. The SiO₂ had an average particle size below 100 nm. A ZETA potential analyzer (SZ-100, HORIBA, Japan) was used to analyze the particle size of the silica particles. The silica particles were mixed with pure alcohol in order to generate a good dispersion of such particles. XRD analysis of the nano-SiO₂ particles was performed with a powder X-ray diffractometer (D8 Advance, BRUKER, Germany).

Figure 1(a) and Figure 1(b) present the grain size distribution and the X-ray diffraction pattern of nano-SiO₂, respectively. The grain size of 90% nano-SiO₂ was no more than 100 nm. The size was measured by means of a laser particle size analyzer. The troughs and characteristic peaks in the XRD spectra were not obvious, indicating that the nano-SiO₂ was amorphous. The SiO₂ nanoparticles were dissolved with pure alcohol and shocked by the ultrasonic machine for 15 min in order to make the mixture between SiO₂ nanoparticles and PTFE.

The next tests were performed in the SiO₂ filled PTFE composites: ball indentation hardness test, tensile strength and elongation at break test, friction and wear test. The characteristics of each test are as follows:

1. Ball indentation hardness test. This experiment was conducted by means of a Rockwell hardness tester (XHR-150, PLASKING, Changzhou, China). The schematic diagram of the shape of the hardness test samples is shown in Figure 2(a). This experiment was based on the GB/T3398 hardness measurement standard. A ball head with a 6.35 mm diameter was selected as the contact pair. The experiment was performed as follows: a 98 N primary test force and a subsequent 980 N main test force were applied to the sample for 30 s. The Rockwell hardness (HR) of the PTFE composite samples was obtained based on the different indentations caused by the primary and main forces. Six measurements were made on one sample in order to improve the accuracy of the measurement data. The Rockwell hardness was calculated by using the next equation:

   $HR=K−h1−h0C$

   (1)

   where C is a constant with a value of 0.002 mm, K is a constant with a value of 130, h₁ is the main test force dent depth, and h₀ is the primary test force dent depth.

2. Tensile strength and elongation at break test. This experiment were performed with an electronic universal testing machine (SMT-5000, SAYSI, Yangzhou, China). The methodology was based on the GB/T 1040-2006 standard for the determination of tensile properties. The tensile rate was 500 mm/min. The sample geometry is shown in Figure 2(b). The specimen was vertically fixed on the fixture of the testing machine and calibrated with a length calibration device, in order to measure the percentage of elongation at break of the sample. Six tests of tensile strength and elongation at break were carried out.

3. Friction and wear test. Before and after the friction and wear test, the weight of the samples was measured by means of an electronic balance (AUW220D, SHIMADZU, Japan). A material surface performance comprehensive tester (CFT-I, ZHONGKEKAIHUA, Lanzhou, China) was used for the friction and wear test. The sample geometry is shown in Figure 2(c). The experimental setup for this test is shown in Figure 3. Nanoparticles filled PTFE composites and regular size particles filled PTFE composites were tested. For 30 min, the samples were rotated while a force load was applied. Two different rotational speeds (200 RPM and 400 RPM) were used. The applied forces were: 20 N, 40 N, and 60 N. The friction coefficient data was fitted to a curve in order to analyze the effects of the size of the filling particles in the friction coefficient of the PTFE composites.

4. Friction and wear surfaces analysis. A scanning electron microscope (FEI Quanta 650 FEG) was used to observe the wear surface microstructure and analyze the friction mechanism of the composites. A three-dimensional surface profilometer (ST400, NANOVEA, USA) was used to observe the surface roughness and maximum dent height of the wear surface. This made possible to analyze the differences in the wear surfaces of the PTFE composites under the different experimental conditions applied.

The ball indentation hardness for the PTFE composite samples are shown in Figure 4. The average hardness values for the regular SiO₂ and nano-SiO₂ filled PTFE composites were 31.48 and 35.38, respectively. According to the hardness measurement of the samples, it can be inferred that the addition of Nano-particles can improve the hardness of the composite material when compared to the regular particles. In comparison with the regular particles filled composite, the Nano-SiO₂ filled composite had a 12.4% increase in the ball indentation hardness. The increase of hardness can prolong the service life of the composite material.

The test results of tensile strength and elongation at break are shown in Figure 5. The average values of the tensile strength and elongation at break for the regular SiO₂ filled PTFE composites were 30.45 MPa and 302.62%, respectively. The average values of the tensile strength and elongation at break for the nano SiO₂ filled PTFE composites were 31.02 MPa and 321.66%, respectively. In comparison with the regular SiO₂ filled PTFE composites, the PTFE composite with Nano-SiO₂ particles as fillers had an improvement in the properties of tensile strength and elongation at break; the tensile strength increased by 1.8% and the elongation at break increased by 5.9%. Although the nano-sized SiO₂ as PTFE composites filler can improve the tensile strength and elongation at break performance of the base material, the room for performance improvement is limited due to the excellent performance of the material itself in these two aspects.

Figure 6(a) shows the friction coefficient data and fitting curves for the PTFE composites tested at 200rpm. A Lorentz function was used for the fitting process, and the results were consistent with the trend of the original data. Figure 6(b) shows the average friction coefficient of the PTFE composites.

According to Figure 6, there were several micro convex bodies on the material surface during the first 5 minutes of the test leading to a relatively large variation in the friction coefficient. In the range from 5 to 10 minutes, a stable friction coefficient caused by smoother micro convex bodies in the nano particles composite under loads of 40 N and 60 N can be observed; in the case of the regular SiO₂ particle filled sample, the friction coefficient steadily increased as a consequence of the abrasive dust during the friction experiment. In the range from 10 to 30 minutes of the experiment, the friction coefficient of the sample achieved steady state for most load conditions. It can be inferred that the samples under these loads already reached a cyclical equilibrium state. By inspecting, both the friction coefficient curve (Figure 6(a)) and average friction coefficient (Figure 6(b)), it can be found that the friction coefficient of the Nano-SiO₂ filled samples under 20 N and 60 N loads decreased when compared with that of the samples filled with regular particles, and that the Nano-SiO₂ filled PTFE composites under a 20 N load had the lowest friction coefficient. For a load of 40 N, the friction coefficient of the conventional particle filler sample continuously rise and become higher than that of the Nano-SiO₂ filled sample for times higher than 20 min. For times above 20 minutes, the friction curve of the Nano-SiO₂ filled samples is stable without obvious upward trends, indicating that the addition of Nano-SiO₂ particles can effectively reduce the friction in the samples and make the friction process more stable.

SEM was used to study the microscopic characteristics of the samples surface after the wear and friction tests. Figure 7 shows the wear surface micrograph for the PTFE composite samples. It can be seen that the regular particles filled sample surface has obvious micro convex bodies or wear debris and agglomeration. Agglomeration was probably caused by the wear debris, which moves along with the rotational body. The debris and its agglomeration are related to the applied loads. In accordance to the friction coefficient curves, higher loads promote wear debris leading to higher friction coefficients. On the friction surface of the Nano-SiO₂ filled samples, the amount of micro convex body and wear debris was clearly lower. The wear debris agglomeration caused by the test followed a cyclic process from agglomeration to falling off or grinding, then the friction and wear of the samples entered achieved steady state. Therefore, it can be concluded that the addition of nano SiO₂ to the PTFE composite can effectively improve its anti-wear and anti-friction capabilities.

Figure 8 shows the three-dimensional morphology of the wear surface in the regular SiO₂ PTFE composites and Nano-SiO₂ PTFE composites for various loads. By observation of the three-dimensional morphology diagrams, it can be seen that the friction between the sample and contact pair generated u-shaped circular grooves. With the increase of load, the width and depth of the groove increased. Because of the low hardness of the sample when compared with the contact pair, the friction produces wear accompanied with wear debris. The increase of the load leads to the increase of the contact pressure, and an expansion of the wear surface. Figures 9(a) and 9(b) show examples of the wear width and depth results obtained by means of the three-dimensional surface profilometer. Figure 9(c) shows the accurate numerical values of these two scar parameters in function of the filler and the loads applied. The weight loss of the samples is shown in Figure 9(d). The surface roughness and dent depth show a similar behavior as the friction coefficient, the Nano-SiO₂ PTFE composite material conducted to the reduction of friction coefficient, surface roughness, wear depth, and loss of material.

The temporal curves for the friction coefficient are shown in Figure 10(a), and the average friction coefficient of the samples is shown in Figure 10(b). In general, it can be observed that the Nano-SiO₂ filled PTFE composites samples show lower average friction coefficients than those of the regular SiO₂ particles for any applied load. Therefore, it can be inferred that Nano-SiO₂ filler has a more effective anti-friction ability. With the increase of the load, the friction coefficient of the composite material decreases gradually, because the micro convex body on the surface of the material is easier to grind under relatively large loads. In consequence, the improvement of the anti-wear performance of the samples reduces the friction coefficient.

SEM was used to observe the morphology of the worn surface. As shown in Figure 11, furrows and a small amount of wear debris can be observed on the surface of the samples. The micro convex bodies on the surface of the composites were difficult to remove under relatively small loads. Consequently, it was difficult for the wear debris to move and filled the furrows leading to the accumulation of wear debris and the increment of the friction coefficient. Nano particles filled composites showed relative low friction coefficient, but the hardness and shear strength of the composite material are much lower than that of the Gcr15 steel contact pair. Therefore, furrows appeared on the surface of the composite material after the experiment too. For a load of 60N, the high load significantly increased the contact pressure between the friction pairs and the formation of wear debris was easier to remove. This led to the reduction of the size and amount of wear debris accumulation. Additionally, the wear debris can fill the furrows facilitating a dynamic balance achievement by the samples. Therefore, the high load of 60 N accomplished lower friction coefficients, as well as number and depth of furrows. In comparison with regular particles filled samples, the surface of the nano-SiO₂ filled samples presented small debris, as well as few and small furrows. This was likely caused by the more uniform nano-SiO₂ particle distribution in the frictional contact surface. The smoother contact between friction pairs caused a better friction coefficient for the nano-SiO₂ filled samples. The addition of Nano-SiO₂ particles can effectively reduce the accumulation of wear debris and furrows, effectively reduce the friction coefficient, and improve the anti-wear and anti-friction ability of the composite materials.

Figure 12 shows the wear surface three-dimensional morphology of the regular SiO₂ PTFE composites and nano-SiO₂ PTFE composites for several applied loads. Similarly to the 200 rpm results, the u-shaped circular groove width increases with the increase of load in both kind of composites (see Figure 13(a)). Contrary to the friction coefficient, the wear surface depth and surface roughness in the samples increased. The reason for this phenomenon is that the sample surface shear stress increases with the increase of the load causing a higher wear width. Wider contact area between the contact pairs reduces the friction coefficient. The weight loss of the samples was measured before and after the friction and wear test (see Figure 13(b)). The surface roughness, maximum dent depth, weight loss, and wear width increase with the rotational speed increase. The reason is that the material is more prone to deformation for higher rotational speeds and the debris can be removed under the influence of friction aggravating the material wear. At the same time, nano-SiO₂ particles inside the PTFE composite material reduce the friction and wear width of the sample. Therefore, it can be concluded that the addition of nano-SiO₂ to the PTFE composite material has an effective anti-wear and anti-friction effect.

Figure 14 show the wear surface energy spectrum analysis diagram for the composites. Figure 15 shows the elemental distribution of O and Si. In comparison to the regular particles filled samples, increments of 3% and 1% in the contents of Si and O were determined for the nano-SiO₂ composite (observe Figure 15). The existence of SiO₂ on the surface of the samples and a more uniform distribution of the nanoparticles were corroborated. The Nano-scale particles reduce the surface roughness of the PTFE composites, which can reduce the accumulation of wear debris on the friction surface, the appearance of larger agglomeration, and the friction coefficient of the samples during the friction and wear test.

1. The ball indentation hardness and tensile strength test were performed for the Nano-SiO₂ filled PTFE samples and the regular particles filled materials. In comparison to the regular particles composite, the hardness increased 12.4%, elongation at break percentage increased 5.9%, and the tensile strength slightly increased 1.8% for the nano-SiO₂ filled PTFE composite. The addition of nano-SiO₂ clearly strengthened the composite.

2. From the comparison of wear width, wear depth, weight loss and coefficient of friction between the nano-SiO₂ filled PTFE composite and the regular particles filled composite, it can be concluded that more effective anti-wear and anti-friction effects were provoked by the nano-SiO₂ filler.

3. The wear mechanism of Nano-SiO₂ filled PTFE composites was mainly abrasive wear. Obvious furrow morphology could be observed on the worn surface, and a certain amount of wear debris was generated. Therefore, fatigue wear was present during the experiments.

This research project was supported by Science and Technology Department, Henan Province, China (182300410169 and 182102210201), and Education Department of Henan Province, China (19HASTIT023).

No potential conflict of interest was reported by the authors.