Tribological and Mechanical Performance of Thermoplastic Composites : Effect of Cryogenic Aging and Testing-Environment

• Recent advancements in energy, space, medicine, and transportation sectors have raised the demand for high-performing materials capable of functioning under demanding conditions such as low temperatures, vacuum, and cryogenic liquids like liquid nitrogen, natural gas, and hydrogen. Besides exposure to cryogenic environments, materials must withstand extensive loads and temperature gradients, inevitably leading to decreases in material properties, premature wear, reduced service life, and early failure due to aging. The performance of various components, including valves, bearings, seals, and gaskets, often determines the overall system performance.High-performance thermoplastic polymers, particularly polyetheretherketone (PEEK) and polyimide (PI), are favourable candidates for demanding tribological applications due to their exceptional mechanical, chemical, and thermal properties. Although PEEK and PI-based materials have been utilised for cryogenic applications, they still do not fully meet all the requirements. To improve their mechanical properties and tribological performance, various modifications and fillers, including carbon fibres and solid lubricants such as graphite, molybdenum disulphide (MoS2), and polytetrafluoroethylene (PTFE), are introduced. While most studies focus on their behaviour in ambient conditions, ambiguities remain regarding their performance and failure mechanisms in demanding environments. Moreover, there has been limited investigation on the impact of long-term or cyclic exposure to cryogenic temperatures and liquids on the properties of polymers and polymer composites. This thesis investigates thermal, thermo-mechanical, and mechanical properties, as well as the tribological performance, of selected commercially available high-performance thermoplastic composites based on PEEK and PI matrices under varying test conditions. Friction behaviour and wear mechanisms of the materials were studied in detail in air and vacuum at 25 °C, as well as in vacuum at –100 °C. Thermo-mechanical and mechanical properties, as well as failure mechanisms, were examined in a temperature range from –195 °C to 25 °C. Furthermore, the thermal, mechanical, and tribological performance of PEEK and PIbased materials were further evaluated at 25 °C after the materials underwent two types of cryogenic aging: 1) long-term cryogenic aging in liquid nitrogen at –196 °C for 5 months, and 2) cyclic-aging for 12 weeks; each cycle was 7 days and included two steps: 6 days in liquid nitrogen at –196 °C and 1 day in an oven at 40 °C.The results show that material performance and wear mechanisms are significantly influenced by the test environment, temperature, and cryogenic aging, depending on polymer structure and composition. Variations in polymer structure and the addition of fillers improved mechanical properties at room and low temperatures without significantly affecting thermal properties. Polymer matrix physical shrinkage at low temperatures enhanced the filler/matrix interface, resulting in fracture toughness improvements of up to 40% for PEEK and 150% for PI. However, thermal stresses at the interface with carbon fibres and graphite competed with matrix toughening.In vacuum environment, PEEK exhibited a lower coefficient of friction, but a higher wear rate compared to its performance in air, mainly due to desorption of water molecules. The tribological performance of PEEK improved with the addition of carbon fibres, graphite, and PTFE. PI1 maintained consistent performance across all environments, while PI5 displayed notable variations, including a very low coefficient of friction in vacuum. The addition of MoS2 or graphite to PI1 reduced the coefficient of friction and wear rate enhancing transfer film formation, and PTFE mitigated significant changes.At low temperature in vacuum, improved toughness allowed PEEK to withstand increased contact pressure, reducing the wear rate. PEEK composites with carbon fibres, graphite, and PTFE showed a very low coefficient of friction (0.02). In contrast, PI experienced increased coefficient of friction and wear rate due to high contact stresses and abrasive wear. The formation of a thin, homogeneous transfer film on the countersurface and polymer disk was advantageous in reducing friction and wear.Cryogenic aging caused embrittlement in PEEK-based materials and neat PI due to incomplete chain relaxation, but it improved the filler/matrix interface in PI composites, enhancing fracture toughness by 58% for the PI/MoS2 composite. While neat polymers experienced minimal effects, some composites showed up to a 220% increase in wear rate. Cryogenic cyclic aging demonstrated that PEEK was more sensitive to aging than PI, with a 22% reduction in fracture toughness for PEEK and up to a 93% increase for PI. Tribological tests indicated increased abrasive wear for unreinforced polymers and pronounced adhesive wear for composites, leading to more homogeneous transfer film generation. In vacuum, the wear rate of aged graphite-reinforced PI increased dramatically by 10920%.The outcome of the presented research demonstrates the need for custom-designed polymer-based materials to fit specific demanding tribological applications, and consider cryogenic aging effect on polymer properties, which is not currently the case.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Tribologi (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Tribology (hsv//eng)

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