Low friction of PAN-based carbon fiber filled PTFE in high-purity hydrogen

R. Taninokuchi¹, Y. Sawae ^{2, 3}, R. Umei¹, K. Sakaki¹, T. Morita^{2, 3}, J. Sugimura^{2, 3, 4}

¹Department of Mechanical Engineering, Graduate School of Engineering, Kyushu University,

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

²Department of Mechanical Engineering, Faculty of Engineering, Kyushu University,

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

                              ³International Institute for Carbon-Neutral Energy Research, Kyushu University,

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

⁴Research Center for Hydrogen Industrial Use and Storage, Kyushu University,

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

INTRODUCTION: PTFE has a self-lubricating property and its composites with filler materials are widely used as polymer seals for gas compressors and valves¹. In our previous study, a PTFE composite showed the characteristic low friction in hydrogen gas environment by using PAN-based carbon fiber (CF) as its filler material. At that time, the large area of composite surface was covered with a self-formed Carbon film² after the sliding test. However, the mechanism of self-forming carbon film is not clear. Moreover, frictional behavior of PAN-based CF filled PTFE is affected by the purity of atmospheric gas, especially the amount of moisture content of the gas environment³.

This study aims to explore the low friction mechanism of PAN-based CF filled PTFE with the self-formed carbon film on the sliding surfaces. We evaluated the effect of the amount of moisture content in the high purity hydrogen gas by using a pin on disk friction tester, laser scanning microscopy, Raman microscopy, quantitative mechanical property mapping, and lateral force microscopy.

METHODS: Sliding tests were conducted in high-purity hydrogen gas using a pin-on-disk type tribometer installed in an environmental control chamber equipped with a scroll vacuum pump, a turbo molecular vacuum pump and gas filters. In this apparatus, it is possible to perform a sliding test while controlling the moisture content at the ppm level in a high purity gas atmosphere.

PTFE filled with 20 vol. % PAN-based CF was used for the pin specimen. The steel disk specimen was made of SUS440C (JIS). The disk surface was polished to a surface roughness Ra = 0.05 μm. After attaching the specimen, the inside of the chamber was evacuated to 5.0×10^-4 Pa, then the hydrogen gas was introduced to the chamber. The moisture content of the gas was controlled by using a hygroscopic filter and a humidifier. The contact pressure was 1 MPa, the sliding speed was 2 m/s, and the sliding distance was 40000 m. The moisture content of the hydrogen gas was controlled at the four different levels: A (0.6 ppm), B (7.8 ppm), C (18.8 ppm), and D (44.7 ppm), to evaluate the effect of trace water content.

RESULTS: Figure 1 illustrates the transition of friction coefficient with the sliding distance. Under the condition A (0.6 ppm), the friction coefficient sharply decreased after the start of the test, showing the stable low friction of about 0.05 after 2000 m sliding. On the other hand, the friction coefficient was unstable and was not as low as the condition A under the condition B. Furthermore, under the condition C and D, the coefficients of friction became about 0.15 after sliding 15000 m, and then they increased with the increment of the sliding distance.

Figure 1. Friction coefficient curves under different moisture content of the gas

DISCUSSION: The surface topography was investigated by using laser scanning microscopy. It showed that the carbon fibers protruded from the worn surface under the condition of extremely low moisture content (condition A). On the other hand, the protrusion of the carbon fibers was suppressed under the condition of not-extremely low moisture content (condition D). This indicates that the amount of moisture content inside the gas influences to the wear of carbon fiber.

Next, quantitative mechanical property mapping and lateral force microscopy were conducted. They showed two suggestions; first PTFE had slightly lower sheer force than PAN-based carbon fiber, but was much more adhesive than carbon fiber; second adhesion of PTFE was inhibited under the condition of extremely low moisture content due to the carbon film formed on PTFE surface. This carbon film on PTFE surface seems to be the cause of the low and stable friction coefficient. Under the condition of not-extremely low moisture content, this carbon film was destroyed due to the wear of the carbon fiber indicated by the topography, therefore the coefficient of friction was not as low as under extremely low moisture content.

REFERENCES:  1. Y. Sawae, J. Sugimura, “Tribology of Polymer Sealing Materials in Hydrogen”, Journal of the Japan Society of Polymer Processing, vol. 25, no.2, pp.77-82 (2013), 2. Y.Sawae et al., “Friction and wear mechanism of carbon fiber-filled PTFE in high purity hydrogen gas (in Japanese)”, Proceedings of Tribology Conference 2017 spring in Japan (2017), 3. H.kojima, Y.sawae, “Effect of trace moisture content on friction of carbon fiber filled PTFE in high purity gas”, Proceedings of Malaysian International Tribology Conference 2015, pp. 125-126 (2015)