Ultralow wear of polytetrafluoroethylene (PTFE) composites filled with ceramic particles  

Anying Wang¹, Shuai Yan¹, Bin Lin¹

¹ Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, China

INTRODUCTION: PTFE is a very potential self-lubricating matrix material with low friction coefficient and corrosion resistance except high wear rate. Many efforts have been made to increase the wear resistance of the PTFE by modifying with ceramic particles, which own prominent advantages of hardness, chemical stability and economy.

Yu et al. found that micro-size ceramic particles of SiC and Si₃N₄ as the fillers increased the wear resistance of PPS.¹ The fillers accelerated the decomposition of PPS and generated certain compounds which accounted for the improvement in anti-wear properties. Shen et al. reported that the micro-SiO₂ particulates (about 50μm) as the filler increased the friction coefficient of PTFE filled epoxy composites significantly due to the gradual accumulation of the fractured SiO2 particles on the worn composite surface.² Zhang et al. found that the nano-SiO2 enhanced the anti-wear property of the epoxy composites. Nano-SiO₂ particles were deposited onto the interface and generated transfer films with different structures and functionalities, which exhibited excellent lubricating performance.^3,4,5

These studies have verified the effectiveness of these typical ceramic particles on improving the friction and wear properties of the polymer composites. But currently, there was lack of systematic comparison on tribological properties of the PTFE composites filled with these ceramic particles. In order to fill the blank in this area, micro-scale silicon based ceramic particles were selected as fillers to prepare PTFE composites. What’s more, the duplex stainless steel was selected as the counterface to maximize the corrosion resistance of the wear system.

METHODS:  The average particle size of Si₃N₄ and SiO₂ was 1-3μm. Filler proportions were arranged from 5wt. % to 20wt. %. Test was performed between composites pin specimen and duplex stainless steel disk at a speed of 0.25 m/s and nominal contact pressure of 2 MPa. The wear tests lasted for 1 h in the ambient atmosphere with relative humidity of 50–60%. Prior to each test, the duplex stainless steel disk was polished to the roughness of Ra = 0.2 μm. The composites pin was slid against a P2000 diamond paper to a roughness of Ra = 0.3 μm, ensuring a good surface contact between the counterface. An electronic balance with an accuracy of 0.01 mg was used to measure the wear loss of the composite pin. In order to avoid accidental error, two replicate tests were performed for each sample and the average values were measured in this paper.  

Figure 1 - A) Wear rates of the PTFE composites. B) FTIR spectra of the surfaces of PTFE composites pin under the fillers content of 10wt. %

RESULTS:  The wear rates of the PTFE composites filled with different ceramic particles have been showed in the Fig.1A. The wear rate of Si₃N₄-PTFE composite decreased sharply then increased gradually with increase of fillers content. The lowest wear rate was 0.554×10^-6 mm³/(Nm) at 10 wt. % content, which was 1127 times lower than that of pure PTFE. The wear rate of the SiO₂-PTFE composite sharply decreased when SiO₂ content increased. The wear rate reached the lowest value (2.21×10^-7 mm³/(Nm)) near 20 wt. %, which was 2826 times lower than pure PTFE.

Fig.1B shows FTIR spectra of the counterface of PTFE composites. New absorption peaks were observed from the worn surface of the Si₃N₄-PTFE composites at 1651cm^-1 and 1429cm^-1, corresponding to the stretching vibration of C=O and C-O separately, which revealed the production of the perfluorinate carboxylate end groups. The carboxylate end groups could chelate to metals, generating the metal chelate salts of perfluorinated carboxylic acids M₂(R_f-COO).^6,7 Meanwhile, the generation of the broad peak O-H at 3232cm^-1 meant the emergence of the R_f-COOH.⁶ The SiO₂-PTFE composite followed the same phenomena and results.

DISCUSSION: For fillers Si₃N₄ or SiO₂, the worn surface of the composite pins was clearly attached with scaly cover. The scaly cover was enhanced by broken fillers which were peeled off from the matrix. The scaly cover hindered the further transfer of the composites and thus reduced the wear rate tremendously.

The thicker scaly cover indicated that more matrix materials were consumed in the chemical reaction while more broken fillers were gathered there. Plough scratch was observed for both Si₃N₄ and SiO₂. The wear was the abrasive wear typically. It is obvious that thicker scaly cover could protect the worn surface better while the excessive accumulation of particles could break this protection due to abrasive wear. So the key to improve the wear resistance of the PTFE composites was to find the balance between them. Compared with the SiO₂-PTFE composites under same fillers content of 10 wt. %, the protective layer covered on the Si₃N₄-PTFE composites was thicker and reached the balance in advance. This explained why the wear of Si₃N₄-PTFE composites reached the inflection point first.

For both kind of filler, the scaly cover could be a kind of relatively stable compounds derived from surface friction chemical reaction between the composites and the duplex stainless steel.⁸ The tribochemical reactions caused the breaking of C–C bonds of PTFE molecular chain and generated the new resultants. A certain degree of humidity in the air was a necessary condition for the reactions. The metals in the resultant M₂(R_f-COO) including Cr and Fe were from the duplex stainless steel. The element Cr was far easier to get involved into the reactions than the element Fe. The generation of the metal chelate salts of perfluorinated carboxylic acids aided in the bonding of the transfer film and the steel disk. In a word,the ultralow wear was due to the tribochemical reactions. The formation of the scaly cover hindered the further transfer of the composites and the generation of the metal chelate salts of perfluorinated carboxylic acids enhanced the adhesion of transfer film.

REFERENCES: 1. Laigui Yu, Wear (1998), 2. J. T. Shen, Wear (2015), 3.Ga Zhang, Wear (2015), 4. Ligang Zhang, Tribol. Int. (2016), 5. Ligang Zhang, Tribol. Int. (2017), 6. Angela. A. Pitenis, Tribol. Lett. (2015), 7. Brandon. A. Krick, Tribol. Int. (2016), 8. Huimin Qi, Tribol. Int. (2016)