Tribological characteristics of graphene nanoplatelets-filled PTFE under dry sliding and sea water environmental conditions

Abstract

This study focuses on enhancing the tribological performance of polytetrafluoroethylene (PTFE) composites by incorporating graphene nanoplatelets (GNPs) through compression moulding technique. The main aim of the research was to investigate how varying weight percentages of GNP (1 wt %, 3 wt % and 5 wt %) influence the friction and wear properties of PTFE composites under both dry sliding and saline water environment. A significant reduction in the coefficient of friction (COF) and wear rate was observed with the addition of GNP, particularly in seawater. Specifically, the reduction of COF in sea water reached up to 58.765% with 3% of GNP compared to dry sliding. For wear reduction, a reduction of up to 90.247% was observed with 5% GNP in sea water condition. These findings reveal the potential of GNP-filled PTFE composites in applications requiring enhanced tribological properties in both dry and aqueous environments. The originality of this work lies in the comprehensive analysis of the environmental influence on GNP-filled PTFE composites, providing understandings of their applicability in marine environments.

Keywords

Tribology polytetrafluoroethylene graphene composites friction wear

Introduction

Polymers are regarded to be the advancing metal replacement materials for aqueous environments, thanks to their above par resistance to corrosion and extremely low absorption of water.¹ Engineering polymers, however, fall short of matching the strength of metals and alloys, have lower working temperatures and show low resistance to wear. Ceramic, metal or non-metal elements or fibre inclusion enhances the mechanical characteristics and wear resistance, thus producing high-performance polymer composites, which are special high-performance materials with comparable strength to weight ratio and very cost effective to synthesize. However, the choice of matrix (polymer) is significant in the production of components made of reinforced polymers. PTFE is an engineering polymer that is currently being used extensively as replacement of metals in applications requiring good mechanical properties. This non-metal does not corrode, is self-lubricating and has a very low coefficient of friction, very less weight relatively and can be moulded into large objects with much less effort and cost.^1–3 Carbon, glass, bronze etc., are a few of these additives which have improved the wear resistance of PTFE.^4–6

PTFE characterization for tribology under dry sliding conditions has been extensively researched.^4–30 But only a few handful research data has been published about tribological studies of PTFE in aqueous mediums.^2,31–40 Lancaster first investigated polymers, filled with carbon fibre, tribological behaviour in sea water and other aqueous media³¹ and concluded that counterface type and its topography affect wear strongly and that counterface corrosion is an important factor in the process of wear. Wang et al. in their studies experimented on PTFE composites filled with graphite and carbon fibre and Ekonol against steel (GCr15) and Ni-Cr-WC alloy in various aqueous environments.^2,32 They found out that corrosive effect of the sea water on the counter body and the lubricating effects of the polymer are main factors which regulate its wear and friction behaviour. Tribological behaviour of PTFE and its composites (PTFE with PEEK [polyether ether ketone], FEP [perfluoroethylene propylene], PI [polyimide] and PHBA [poly phenyl p-hydroxybenzoate]) was studied by Chen et al.³³ Chen et al. in this study concluded that the deposition of calcium carbonate and magnesium hydroxide on the surface resulted in the observed lower wear rate in sea water lubrication than in pure water. Yang and Huang varied the percentage of glass fibre (10, 20 and 30%) in. PTFE and investigated into its tribological behaviour against aluminium oxide counterface in sea water.³⁴ Their results showed the best wear and friction performance at 20% glass fibre-filled PTFE. Mohammad Jebran et al.^35–40 in various investigations studied the tribological behaviour of different PTFE composites under dry sliding conditions and aqueous environments and concluded that the coefficient of friction in sea water environment is significantly less than in dry sliding due to the development of chromium and iron oxides on the composite surface. They also observed that the wear rate is higher when sliding in distilled water environment. This variation was attributed to non-adherence of polymer film on the surface.

Being two-dimensional, graphene has special wear and friction characteristics not found in most other materials. In addition to its well-known mechanical, electrical, optical and thermal capabilities, graphene has lubricating properties that can be used as a solid or colloidal liquid. The main factors contributing to its remarkable tribological performance are its excellent chemical inertness, extraordinary strength and easy shear capability on its highly compact and fundamentally flat surface. Graphene has recently been identified as non-toxic and benign to the environment having exceptional mechanical characteristics such as hardness, high strength and fracture toughness. In its many different uses, graphene has drawn interest from scientists and engineers worldwide.^41,42 Recently, the discipline of tribology has been utilising graphene due to its outstanding optical, physical, electrical and mechanical capabilities.^43,44 Wang et al.⁴⁵ studied the effect of reinforcements (graphene platelets) in poly vinyl chloride. It was reported that composites of graphene/PVC possess low wear and friction in comparison to virgin PVC material. These results indicate that graphene enhances wear resistance and frictional coefficient of polymers. Xu et al.⁴⁶ fabricated composites of graphene/ultra-high molecular weight polyethylene (UHMWPE) and studied their mechanical and tribological properties using micro-hardness tester and high-speed reciprocating friction tester. The results indicate that graphene addition to UHMWPE not only increases the composite micro-hardness, but it also significantly decreases the composite wear volume. Friction coefficients also lowered with the increased graphene concentration. Chih et al.⁴⁷ prepared composite coatings of UHMWPE and 0–4.6 wt % of GNP and assessed the frictional characteristics of coatings based on sprayed graphene and graphene/UHMWPE composites. Friction coefficients were measured with a ball-on-disk tribometer, and mechanical properties were calculated by nano-indentation. The findings revealed that GNP/UHMWPE present improved tribological behaviour and mechanical properties than neat UHMWPE. These studies confirm that graphene has enormous potential as a solid lubricant reinforcement substance that will counteract wear and energy from friction and loss of material in polymers.

Sashi et al.²⁰ reported the wear suppression of PTFE by inclusion of graphene nanoplatelets and micro-graphite fillers. They found out that even less than 1 wt % incorporation of GNP in PTFE reduced the wear rate 50 times. They also observed a 4000 times decrease in the wear rate of PTFE when incorporated with 10 wt % GNP. Reduction in wear rate with addition of micro-graphite in PTFE was also reported, and it was 10–30 times less than the wear rate reduction of GNP/PTFE nanocomposites as compared to unfilled PTFE. However, these tests were carried out in dry sliding conditions.

From the review of literature above, it is evident that extensive research has been carried out to evaluate the tribological behaviour of virgin and composite PTFE in dry and aqueous media. Still, no studies are reported on the graphene-filled PTFE behaviour for friction and wear against stainless steel in sea water medium. Thus, this study presents investigations into the tribological performance of varying weight percentages of GNP (graphene nanoparticles)-filled PTFE in sea water environment in comparison with dry sliding conditions. Characterization studies of the worn surfaces with respective friction and wear mechanisms and the surface morphological examinations have been interpreted.

Materials and methods

Materials

Graphene nanoplatelets (GNPs) of thickness 2–4 nm and virgin PTFE powder (size 60–70 μm) were acquired through reliable sources in India. The GNP-filled PTFE composites (1 wt % GNP + PTFE, 3 wt % GNP + PTFE and 5 wt % GNP + PTFE) were fabricated in the form of cylindrical discs (25 mm diameter × 10 mm height) via compression moulding route at a compaction pressure of 40 MPa for 15 min. Green samples produced were sintered to the temperature of ∼360°C at a rate of ∼100°C/hr followed by cooling to room temperature.⁴⁸ Before compression moulding, wet-mixing technique was employed to produce a homogenous mixture of GNP and PTFE powder using the solvent, perfluoroheptane, as described by Rooyen et al.⁴⁹ This process is illustrated in Figure 1.

Cylindrical pins with a circular contact area of diameter 4 mm were fabricated from AISI 316 Stainless steel rods using a lathe machine. Both the composite discs and the pins were polished to obtain good surface finish before testing using SiC papers with different grit sizes (600, 800, 1000, 1200, 1500 and 2000 μm). The samples were polished further on a polishing cloth using diamond pastes and then cleaned with ethanol. Using a non-contact 3D surface profilometer supplied by Rtec Instruments, USA, the average surface roughness was also measured (Figure 2).

Artificial seawater was formulated in the lab using ASTM D1141-98. Table 1 gives the concentration of different metal salts and trace elements used to formulate the seawater. The obtained values of salinity, Cl⁻ and pH are given in Table 2, which are found to comply with the literature.^50,51

Table 1.

Chemical composition of the prepared seawater.

Compound	NaCl	Na²SO⁴	MgCl²	CaCl²	SrCl²	KCl	NaHCO³	KBr	H³BO³	NaF
Concentration (g/l)	24.53	4.09	5.20	1.16	0.025	0.695	0.201	0.101	0.027	0.003

Table 2.

Properties of artificial seawater solutions prepared.

Medium	[Cl⁻] (mol/l)	Salinity (%)	pH
Sea water	0.558	35	8.2

Methods

Tribo-testing

The tribological testing was carried out on a pin on disc Rtech tribo-tester (supplied by Rtec Instruments) in seawater and dry sliding (ambient air) under reciprocal sliding settings. The tribo-tester and schematics of the tribo-pair are shown in Figure 3. Tribological experiments were carried out at 5 N normal load while limiting the frequency of reciprocation at 10 Hz and 2 mm reciprocating stroke. The test duration was kept constant at 16 min. All the tests were conducted at ambient room temperature of 24°C ± 1. Using a computer connected to the tribometer with DAQ System, the COF was determined. The test conditions are reported in Table 3.

Table 3.

Test conditions.

S. No.	GNP percentage	Frequency (Hz)	Load (N)	Environment
1	0	10	5	Ambient air (dry sliding)
2	1	10	5	Ambient air (dry sliding)
3	3	10	5	Ambient air (dry sliding)
4	5	10	5	Ambient air (dry sliding)
5	0	10	5	Sea water
6	1	10	5	Sea water
7	3	10	5	Sea water
8	5	10	5	Sea water

The wear volume or material volume loss was acquired using optical profilometer. Further, this formula was used to calculate the specific rate of wear.

S p e c i f i c w e a r r a t e = \frac{v o l u m e l o s s ({m m}^{3})}{n o r m a l l o a d (N) \times s l i d i n g d i s t a n c e (m m)}

(1)

Each of the experiments was carried out in a set of three tests. The resultant mean was calculated and is reported.

The worn surface morphology was studied using Field Emission Scanning Electron Microscopy (FESEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS). Gold coating was done on polymer samples for making them conductive.

Results and discussion

Characterization of samples

The fresh sample discs of 1, 3 and 5 wt % of GNP-filled PTFE composites were polished to an average surface roughness of R_a = 0.02, 0.019 and 0.02 μm, respectively (Figure 2). Figure 4 shows the XRD spectra of virgin PTFE and 1, 3 and 5 wt % of GNP-filled PTFE. The intense peak at 2θ = 18.2° aligns with the crystalline phase of PTFE between the (1 0 0) planes of hexagonal unit cell of PTFE. The peaks at 31.8 and 36.9 are also strong PTFE peaks in accordance to those previously found in References 52 and 53. There are characteristic C (0 0 2) reflections of graphene around 26.6°. Another less prominent peak at 41° (1 0 1) is also observed. Moreover, the intensities of the peaks corresponding to GNPs are found to increase when the GNP content increases.

Hardness tests

The hardness tests were conducted on GNP-filled PTFE according to ASTM D-2240 on Shore-D scale. The hardness of GNP-filled PTFE increased with an increase in the GNP weight percentage in the composite. The values are given in Table 4.

Table 4.

Hardness (Shore D) of GNP-filled PTFE composites.

Sample	Hardness (average)
Virgin	56.5
1 wt %	57.3
3 wt %	58.1
5 wt %	58.9

Friction analysis

Figure 5(a)–(d) show the friction coefficient versus sliding time for 0, 1, 3 and 5 wt percentages of GNP-filled PTFE nanocomposites, respectively, sliding against SS in dry sliding and seawater environments. From Figure 3, it can be noted that the COF for all the weight percentages of graphene nanoplatelets under dry sliding fluctuates above the coefficient of friction in sea water.

Further, 5 wt % GNP/PTFE nanocomposite shows the lowest COF at ambient temperature (dry sliding conditions). The COF fluctuates between 0.043 and 0.086 with a downward trend shown between 200 and 600 s. The COF the increases further till the end of the test conducted. Additionally, the lowest friction coefficient is attained for 5 wt % GNP/polytetrafluoroethylene in sea water. The COF initially increases with advancement in sliding time till 500 s and then starts to value down. The average friction coefficients for 1, 3 and 5 wt % GNP/polytetrafluoroethylene in dry sliding test settings are given in Figure 6(a). It is clear from Figure 6(a) that in dry sliding conditions (at ambient temperature), average COF for 5 wt % GNP/polytetrafluoroethylene is the lowest (0.632) followed by 3 wt % (average COF 0.754) and 1 wt % (average COF 0.109). Also, in sea water environment, for the 5 wt % GNP/polytetrafluoroethylene, average COF is the lowest (0.0303) followed by 3 wt % (average COF 0.311) and 1 wt % (average COF 0.765). The obtained data clearly indicate that the friction performance of 5 wt % GNP/PTFE is better than 3 wt % and 1 wt % GNP/PTFE under every combination of conditions. This is consistent with the findings from Reference 54, according to which the frictional co-efficient rises with increase in wt % of graphite in polytetrafluoroethylene. But, in Reference 54 the friction testing was limited to the dry state only and the percentages of graphite concentration chosen were 2, 5 and 10. Additionally, in the current study, the outcomes make this clear that for all the graphene/polytetrafluoroethylene composites, minimum value for the average co-efficient of friction is observed in sea water as opposed to sliding under dry conditions. So, superior friction performance is exhibited in sea water in comparison to dry sliding environment. This thus shows consistency with the obtained results in Reference 55 and 56 wherein it is reported by the authors that the introduction of water decreases the COF at the interface of polymer/metal friction pair.

In the current investigation, the highest frictional coefficient of GNP/PTFE composites is exhibited under dry sliding. This can be co-related with the presence of the chunk of PTFE present on the surface of the counterface pin as revealed in the SEM image of the counterface stainless steel pins (Figure 7(a)). It signifies that the transfer of PTFE to stainless steel counter surface has taken place due to wear of GNP/PTFE composite. On the other hand, lesser friction values were seen in seawater for all composites of GNP/polytetrafluoroethylene compared to alternative testing circumstances. Scanning electron microscopic analysis aided with EDS of the worn surfaces of the counterface SS 316 pins after tribo-tests in sea water showed the deposition of oxide and chloride films on the surface illustrated in Figure 7(b), (d) and (f). Since these oxides are lubricious, they aid in lowering wear and friction.^57,58 The corrosion of stainless steel is the cause of the development of these oxides. Chromium, while being in contact with oxygen, aids in the formation of a thin oxide layer of corrosion products on stainless steel (passivation). This effect is also exhibited by chromium under water submersion.⁵⁹ Stainless steel’s ability to withstand corrosion is provided from this oxide film (Cr₂O₃), which prevents further corrosion of the metal underlying this film by acting as a barrier and preventing ion migration from the alloy into the surrounding phase.^60,61 Seawater, on the other hand, corrodes metals, including stainless steel, due to the chlorine it contains which corrodes metals. Iron and chromium oxides are the products of this corrosion.⁶² These oxides aid in lowering wear and friction. This is further confirmed by the EDS of these surfaces (Figure 8) wherein the findings of iron, sodium, chlorine and oxygen correlate to the analysis of the SEM images of these surfaces. These findings are in conformity with the results published in Reference 36, wherein it has been stated that the formation of corrosion product films on the surfaces in sea water environment leads to the enhanced tribological behaviour. Hence, the reason for the reduced friction of composites of GNP and PTFE in sea water environment is the formation of these films, which are lubricious, on the worn surfaces of GNP/PTFE composites.

Wear analysis

Figure 6(b) expresses the rate of wear of 0 wt %, 1 wt %, 3 wt % and 5 wt % GNP/PTFE, sliding against SS pins sliding in dry and in sea water surroundings. From Figure 6(b), it can be observed that 5 wt % GNP/polytetrafluoroethylene exhibits improved outcomes for wear than 3 wt % and 1 wt % GNP/polytetrafluoroethylene throughout every set of test circumstances. Comparable outcomes were obtained in Reference 54 wherein wear resistance is reported to improve with an increase in the weight percent concentration of graphite. However, as previously stated earlier, in Reference 54, only dry sliding conditions were employed for the tribo-tests and 2 wt %, 5 wt % and 10 wt % of graphite were selected. In the current scenario, 5 wt % GNP/PTFE exhibits that the least amount of specific wear rate of 7.9 × 10⁻⁵ mm³/Nm is obtained with sliding under sea water, followed by dry sliding (8.1 × 10⁻⁴ mm³/Nm). Further, 3 wt % GNP/PTFE exhibits the minimum wear rate of 4.5 × 10⁻⁴ mm³/Nm with sliding in sea water followed by sliding in dry conditions (1.73 × 10⁻³ mm³/Nm). Furthermore, with 1 wt % GNP/PTFE the minimum wear rate of 9.7 × 10⁻⁴ mm³/Nm is achieved under sea water environment followed by dry sliding (3.66 × 10⁻³ mm³/Nm).

Figure 9(a) and (b) display the SEM micrographs of worn surfaces of 1 wt % GNP/PTFE under dry sliding and sea water environments, respectively. The predominant factor influencing the wear behaviour of PTFE composites is the abrasive wear mechanism. This concurs with the findings of Blanchet et al.,⁶³ who noted that as a result of the repeated sliding action, larger plates, lumps and slabs of polymer formed on the counterface, disrupting nanoscale transfer sheets, wear debris was produced. Small debris was present on the surface of the composites in this instance. Lump formation is inhibited by GNP fillers in the PTFE, which result in thinner transfer films with better adherence to the counterface.⁶⁴ Figure 9(a) shows very prominent abrasive marks indicating severe wear. In sea water, however, the abrasion marks are less pronounced comparatively with slight delamination, thus implying improvement in wear due to the change in environment (Figure 9(b)). The SEM micrographs of worn surfaces of 3 wt % GNP/PTFE are showcased in Figure 9(c) and (d) in ambient-air-dry sliding and under sea water conditions, respectively. The worn surface smooths out as GNP loading increases, as Figure 9(f) illustrates. The absence of fissures and extensive peeling caused by transfer film production means that GNPs cannot obstruct the PTFE crystals from drawing out, greatly enhancing the wear resistance of PTFE composites.⁶⁵ In Figure 9(c), abrasion marks along with some adhesion are visible. In Figure 9(d), SEM of worn surface in sea water shows fewer abrasive marks comparatively. Also, Figure 9(e) and (f) show the SEM micrographs of worn surfaces of 5 wt % GNP/PTFE under dry sliding and natural sea water environments, respectively. Adhesion and low delamination are observed in dry sliding as shown in Figure 9(d) while slight delamination is observed in Figure 9(e) corresponding to testing in sea water. Paralleling the SEM micrographs (Figure 9(a), (c) and (d)), it is distinctly noted that 5 wt % GNP/PTFE exhibits better resistance to wear than 3 and 1 wt % GNP/PTFE under dry sliding. According to a widely accepted model for PTFE wear, wear results from subsurface fissures propagating parallel to the sliding surface at a depth of a few micrometres, and debris generation happens when these cracks eventually meet the surface. Because of their small size, micro-scale fillers can disrupt debris formation processes like these.¹³ This is because matrix wear exposes them to the sliding surface, where they accumulate and still extend many micrometres back into the subsurface. As a result, wear can be reduced by two to three orders of magnitude.⁶⁶ Our findings suggest that the substantial aspect ratio and micro-scale in-plane dimensions of graphene platelets make them ideal for interfering with debris buildup in PTFE. This is seen in Figure 9(f), which shows a substantially lower wear debris size. Also, the thickness of these sheets at the nanoscale suggests that the number density of graphene platelets in the matrix is significantly higher than that of traditional micro-fillers at a given weight percent of additives.²⁰ Furthermore, the analogous Scanning Electron micrographs of SS AISI 316 pins under dry sliding illustrate non-tenacious and loose films transferred on the surfaces (Figure 7(a), (c) and (e)). This is suggestive of elevated wear of the GNP/PTFE composites during dry sliding in comparison to alternative conditions of testing. In sea water environment, comparing SEM micrographs (Figure 9(b), (d) and (f)), it is evident that 5 wt % is superior in resisting wear than 3 wt % and 1 wt %. Additionally, the related Scanning Electron Micrographs of the SS pins in sea water (Figure 7(b), Figure 8(b) and Figure 7(f)) also show the formation of lubricious films. Thus, it is evident that the enhanced resistance to wear of GNP/PTFE composites in the artificial sea water is due to the developing of lubricating films while sliding in an environment of sea water. The corrosive nature of seawater causes these coatings to form. These films comprise oxides of chromium and iron. This has been documented elsewhere in Reference 67.

This will cause sub-surface matrix cracks to be deflected through a tortuous path by the high aspect ratio, two-dimensional graphene platelets.

Conclusions

All the GNP/polytetrafluoroethylene composites showcase a higher frictional coefficient and specific rate of wear under dry sliding conditions relative to sliding in sea water. This is attributed to the creation of discontinuous transfer films which are non-tenacious on the counter surface SS pins under dry sliding. The superior tribological behaviour of GNP/polytetrafluoroethylene composites in seawater compared to that of dry running is attributed to generation and settling of lubricious films on the sliding worn surfaces of stainless-steel pins in sea water as revealed by SEM and EDS analyses of these surfaces. Some important inferences are drawn below:

1. The 5 wt % GNP/PTFE composite has superior performance when it comes to friction than 3 and 1 wt % GNP/polytetrafluoroethylene composites under each arrangement of parameters. However, the 1 wt % GNP/polytetrafluoroethylene composite exhibits higher wear efficiency than 3 and 5 wt % GNP/polytetrafluoroethylene composites under each set of factors.

2. Under dry sliding conditions, all the GNP/PTFE composites exhibit higher specific wear rate and COF than sliding in sea water. Non-tenacious and disconnected transfer films observed on the counter surface SS pins under dry sliding can be ascribed to these measurements.

3. The GNP/polytetrafluoroethylene composites exhibit enhanced tribological behaviour in sea water compared to dry running. This is credited to developed lubricious films on the counter surface SS pins sliding in sea water.

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) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Ahmad Hafeeyi Mir

Data availability statement

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.*

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