PTFE/carbon composite radial deep groove ball bearings: Part 1–tribological characterization suitable for ball bearing applications

Abstract

Polytetrafluoroethylene (PTFE) is a self-lubricating, highly stable, high-performing polymer that has expanded its usage in bearing tribology. However, its application in ball bearings is still limited. This article explores using PTFE/carbon composites to evaluate their suitability for radial deep groove ball-bearing applications. The proposed work is divided into two parts. The first part focuses on the tribological characterization of the PTFE/carbon through a ball-on-disc test and examines the effect of reinforcement on bearing performance. The second part focuses on fabricating and analysing PTFE/carbon radial deep groove ball bearings for characteristics. In the first part, a ball-on-disc experiment was performed on pure PTFE (PTFE) and 25% carbon-filled PTFE (CR25) to measure the coefficient of friction (COF) and wear rate. These materials were subjected to three loading conditions at a particular speed using silicon nitride balls (Si₃N₄). The friction coefficient was recorded throughout the interaction, and specific wear rates were calculated. After completing the tests, wear tracks were analyzed using a scanning electron microscope and a Contracer.

Keywords

PTFE carbon polymer tribology wear

Introduction

There is increasing interest in polymers and polymer composites as efficient alternatives to pure metals and hybrid materials.¹ Polymeric composites are popular in tribology for their lightweight, low wear and friction properties and better commercial usage.² Among different polymers, PTFE is an excellent tribological polymer for bearing applications.^3,4 It is a high-performance polymer, a well-known engineering material developed for various industrial applications. Due to its self-lubricating properties,^5,6 it is commonly used as a solid lubricant.^7–9 Its long (CF₂-CF₂)_n chains have low shear strength, leading to a low COF during sliding.¹⁰ With excellent self-lubricating properties,¹¹ PTFE and its composites are used in several bearing applications, including seals, bearing cages, and sliding bearings.^12–14 However, the high wear rate and lower mechanical strength are significant concerns in PTFE performance. Reinforcement of the appropriate filler material can improve its tribological properties.¹⁵ The effect of reinforcement is not limited to improving the tribological properties but also enhances its mechanical strength and thermal properties.^16–20 The filler materials are generally comprised of ceramics,^12,21,22 metals,^23–25 polymers,^26–28 fibers,^29–31 fabrics^32,33 etc. Among all these, carbon-based material reinforcement is commonly used as the filler material in the PTFE,³⁴ which are CNT,^35–37 graphene,^38,39 carbon,^40,41 carbon fibre^42,43 etc. Carbon as a filler material has effectively improved the mechanical and tribological properties of the PTFE in the past.⁴⁴ Incorporating hard carbon materials significantly enhances the tribological properties of PTFE, resulting in a remarkable 15-fold increase in wear resistance. This enhancement can be attributed to the improved hardness and mechanical strength of the PTFE matrix.⁴⁵ However, the carbon reinforcement can lead to a slight increment in COF values during interaction.⁴⁶ The carbon-filled composites also show excellent thermal stability and wear performance in high temperatures. Sujuan and Xingrong⁴⁷ conducted a comprehensive study on the tribological performance of various PTFE composites at elevated temperatures, and the results reveal that carbon-filled PTFE exhibits exceptional wear resistance, demonstrating its effectiveness at ambient and elevated temperatures. Carbon-filled PTFE composites have been used in thrust-bearing applications. The findings of McCarthy and Glavatskih⁴⁸ show that carbon reduces PTFE wear in both dry and lubricated contacts, showcasing carbon’s high suitability as a filler material highly suitable for bearing applications.

To assess the suitability of a PTFE composite for ball bearing applications, it is essential to carefully select the most appropriate tribological test along with the relevant testing parameters to achieve optimal results. In our earlier research efforts, the suitability of the PTFE composites was checked for ball-bearing application through a pin-on-disc test.⁴⁹ In this test, five different PTFE composites were tested at load and speed equivalent to the polymer ball bearing of an inner diameter of 20 mm and outer diameter of 32 mm. Results show that 25% carbon-filled composites have excellent wear resistance at various loads and speeds, but COF values were higher than other composites. Since the interaction in the ball bearing is between the balls and the bearing race. So, a ball-on-disc testing procedure would be feasible and reliable for checking the suitability of the composite for ball-bearing applications. The selection of speed and load should be relevant to the ball-bearing application. Researchers have conducted tribological characterizations of PTFE composites utilizing the ball-on-disc method. However, the load and speed parameters used in these studies may not align with the requirements for ball-bearing applications. In many instances, the authors have tested the materials under low-load^43,50–52 and speed⁵³ conditions, as presented in Table 1. Therefore, ensuring that the selected load and speed parameters for tribological testing through the ball-on-disc method are relevant and representative of the conditions encountered in actual ball-bearing applications is essential. In this article, a ball-on-disc test is performed on 25% carbon filled (CR25) and pure PTFE (PTFE) with silicon nitride balls (Si₃N₄) at 800 RPM under 25, 50, and 75 N loading conditions. The reason for selecting Si₃N₄ balls is to fabricate corrosion-free bearings suitable for underwater applications. Further, the selection of speed and load is based on the polymer-based ball bearing available in the market space. The research findings show that carbon reinforcement has significantly improved the bearing performance, which can be considered for ball-bearing applications. In the second part of the proposed work, detailed characterization is carried out to check the suitability of the fabricated bearings.

Table 1.

Testing parameters of PTFE composites through a ball-on-disc test.

Reinforcement	Ball used	Load	Speed	Ref
20% carbon	GCr15 steel	6–15 N	0.1–0.25 m/s	⁴³
PTW (10%)	GCr15 steel	50–200 N	0.04 m/s	⁵³
20% carbon fiber	GCr15 steel	6–15 N	0.1–0.25 m/s	⁵⁰
PEEK (20%–60%), carbon fiber (10%–15%), BP nanosheet (5%)	Silicon nitride	3 N	2 Hz, 5 mm sliding distance	⁵¹
PTWs (6.5%), Nano SiO₂ (2.2%), Cu (1.1%), aramid pulp (2.2%), MoS₂ (10%), g-C₃N₄ (10%)	GCr15 steel	1–5 N	0.04–0.12 m/s	⁵²

Material and testing

A disc with a thickness of 8 mm and a diameter of 40 mm was prepared for a ball-on-disc experiment. The disc was fabricated from a procured commercial PTFE composite rod in a lathe machine. The rod was manufactured through a cold compression moulding process followed by sintering. The composites were compressed at 300 kg/cm² pressure during moulding and then sintered at 370°C. The rods were procured from Hindustan Nylons, Miraj, India. After fabrication, the disc was polished to eliminate machine marks and achieve a smooth, even surface. The surface of the polishing procedure began with sandpaper grades 320, 600, 1000, and 2000. Afterward, the disc was finally polished using a Struers polishing machine. Then, the disc’s surface roughness was measured with the help of Mitutoyo profilometer TR 200. A silicon nitride (Si₃N₄) ball with a diameter of ¼ inch was used and was obtained from N. Gandhi and Co. Mumbai, India. The properties of the PTFE composites and balls are listed in Table 2.

Table 2.

Properties of the materials.

Properties	Pure PTFE	PTFE + Carbon (25%)	Silicon nitride ball
Designation	PTFE	CR25	Si₃N₄
Density (gm/cm³)	2.14	2.08	3.20
Tensile strength (MPa)	21.08	13.53	700
Elongation	260%	114%	—
Young’s modulus (GPa)	0.40	1.30	310
Poisson’s ratio	0.46	0.46	0.29
Hardness	54 Shore D	61 Shore D	2477 HV
Service temperature	260	250	1100
Roughness (µm)	0.775	0.653	—

The Ducom TR-20 Neo series friction and wear monitor was used to perform the ball-on-disc tribological test under dry conditions. Figure 1 shows the schematic arrangement of the test. The disc was rotated at the constant speed of 800 RPM and was subjected to loads of 25, 50, and 75 N. The contact pressure at the point of contact varies from 50 to 72 MPa for PTFE and 109 to 157 MPa for CR25. The ball was positioned at a wear track of 20 mm, and the tests were performed at room temperature with a relative humidity of 35% in dry conditions. After the completion of the test, the wear track formed on the disc was analysed through the FEI-Apreo-S field emission scanning electron microscope (FESEM) and wear track tracing was done through Mitutoyo MV-2100 Contracer. The formula used to calculate the specific wear rate is given in equation (1).

W R = \frac{∆ V}{F . D}

(1)

Where,ΔV = Volume loss (gm/mm³),F = Normal Load (N),D = Sliding distance (m).

Figure 1.

Schematic diagram of the ball-on-disc test.

Results and discussion

Friction and wear results

The friction coefficients obtained at different loads for PTFE and CR25 are shown in Figure 2(a) and (b). In the case of PTFE, the COF values decrease as the load rises. This trend may be due to the effective transfer of the film on the ball as the load increases, which lowers the COF.⁴⁰ The average COF at 25 N is 0.159, which has decreased to 0.137 at 75 N, almost a 14% decrement (Figure 2(c)). However, in the case of CR25, the load does not affect the COF values and nearly remains the same, as seen in Figure 2(b). In the initial stage of interaction, there is a significant difference in the COF values, but later, after 200 m, the COF remains the same irrespective of the loads. The average COF for CR25 is slightly higher than that of PTFE. Furthermore, the results indicate that incorporating carbon reinforcement does not significantly alter the friction value. Notably, the reinforcement contributes to a tenfold improvement in wear resistance. This is due to the harder carbon, which acts as a barrier for PTFE to get fragmented.⁴⁶ The obtained specific wear rate values are plotted in Figure 2(d). The wear rate data indicates that as the load increased from 25 N to 75 N, there was a notable decrease in the specific wear rate. Specifically, the wear rate for PTFE decreased by 49%, while for CR25, the reduction was even more substantial by 89%. Overall, the results show that PTFE has a lower COF, and incorporating carbon has led to a slight increase in the COF value. However, adding carbon to PTFE enhances its wear resistance.

Figure 2.

Friction and wear results at different loading conditions (a) COF of PTFE, (b) COF of CR25, (c) average COF, (d) specific wear rate.

Wear track analysis

After measuring the friction and wear results, the wear track formed on the disc has been analyzed. The surface morphology was analysed through FESEM in all the loading conditions. The SEM images of PTFE and CR25 at different loading conditions are shown in Figures 3 and 4, respectively. Further, the wear track of all loading has been traced with the help of the Contracer. It helps to determine the approximate penetration of the Si₃N₄ ball during the test on the disc. The SEM image shows that PTFE has a smoother wear track. The tendency of PTFE to transfer its film to its counterpart results in a smoother wear track, which is associated with lower COF. In contrast, the analysis of CR25 reveals visible micro-ploughing and delamination of material across all loading conditions (refer to Figure 4(b)–(d)). This observation indicates a slightly rough wear track surface, indicating abrasive wear. This phenomenon may be attributed to the ineffective transfer of the film, potentially due to the presence of carbon.

Figure 3.

Wear track morphology of PTFE through SEM at different loads (a) 25 N, (b) 50 N, (c) 75 N.

Figure 4.

Wear track morphology of CR25 through SEM at different loads (a) 25 N, (b) 50 N, (c) 75 N.

The profile obtained for PTFE through Contracer presents a smooth parabolic trajectory, as illustrated in Figure 5(a)–(c). In contrast, the profile traced for CR25 exhibits some irregularities, as depicted in Figure 5(d)–(f). This discrepancy may be attributed to micro ploughing, as evidenced by the FESEM images. Additionally, this unevenness could significantly contribute to the elevated COF values observed in CR25 compared to PTFE, potentially hindering the transfer of the film onto the counter ball.

Figure 5.

Wear track tracing through Contracer at 800 RPM under different loads. PTFE (a) 25 N, (b) 50 N, (c) 75 N; CR25, (d) 25 N, (e) 50 N, (f) 75 N.

In both materials, it is observed that as the load increases, the width of the wear track increases. Furthermore, a similar trend was observed in wear track depth, as shown in Figure 5. The observed increase in width and depth can be attributed primarily to the high hardness of Si₃N₄ and the elevated contact pressure at the point of interaction. As the load increases, contact stress also rises, resulting in deeper penetration and subsequent deformation of the composites. The measured wear track depth and width are listed in Table 3. As measured by FESEM, the wear track width indicates a substantial increase in track width for PTFE, rising by approximately 36.15% for load ranging from 25 N to 75 N. In contrast, the CR25 material exhibited a nearly 41.30% increase over the same load range. Additionally, the depth measurements obtained through Contracer showed a notable increase in depth for PTFE, with a 31.23% rise from 25 N to 50 N and a further 44.33% increase when the load was raised from 50 N to 75 N. For CR25, depth also increased with load, though at a lesser rate than that observed in PTFE. Overall, the wear track data suggests that the values for CR25 are approximately 40% to 50% lower than those for PTFE.

Table 3.

Wear track data through Contracer and SEM.

Material	Load (N)	Contracer results		SEM results
Material	Load (N)	Wear track width (mm)	Wear track depth (mm)	Wear track width (mm)	Wear track depth (mm)
PTFE	25	2.4642	0.2045	2.3030	—
	50	2.9965	0.2974	3.0390	—
	75	3.9620	0.5362	3.6070	—
CR25	25	1.2450	0.0204	1.2790	—
	50	2.0128	0.0624	1.9870	—
	75	2.3720	0.1240	2.1790	—

Conclusion

This article analysed the suitability of PTFE and CR25 composite for ball bearing application with Si₃N₄ balls through a ball-on disc test. Based on the results obtained, the following conclusions can be drawn:

• Pure PTFE exhibits a low COF when interacting with Si₃N₄ balls with an average COF value of 0.15. However, it experiences significant wear during the test. Additionally, the wear track data indicates that the depth and width observed may not meet acceptable standards for the use in ball bearings.

• CR25 has slightly higher COF in comparison to PTFE. However, it has shown almost 10 times more wear resistance than PTFE. The wear track data shows that depth and width values are nearly 50% less than the PTFE.

• Overall, the results show that the carbon reinforcement in the PTFE has significantly improved tribological characteristics, and CR25 can be considered for ball-bearing applications.

• In the second part of the paper, fabrication and testing of the PTFE/carbon composite radial deep groove ball bearings will be presented.

Footnotes

Acknowledgments

The authors acknowledge the support provided by BITS Pilani, Pilani Campus, and Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal.

Author contributions

Dhruv Deshwal: Investigation, Data curation, Writing-original draft.

Sachin U. Belgamwar: Methodology, Editing, Supervision.

Siddappa I. Bekinal: Conceptualization and Design, Methodology, Resources, Editing, Supervision and Final approval.

All authors have read and approved the final version of the manuscript.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

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

ORCID iD

Siddappa I. Bekinal

Data availability statement

Data to support the findings of this study are available with the corresponding author and made available upon reasonable request.

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