Novel multiscale polymer composites for tribological applications in space

The space environment places many demands on polymeric materials. While polymers have low weight and good mechanical properties, they often require reinforcement to improve their tribological properties. Multiscale composites, combining micro- and nano-sized reinforcements, are a potential solution for complex environmental conditions. This work aimed to assess the tribological behaviour of multiscale polymer composites in environmental conditions similar to space.
Tribological behaviour of a commercial polymer was evaluated to identify key wear mechanisms in dry sliding. Commercial PEEK and a PEEK-carbon-fibre composite were tested with variations in load, sliding time and at ambient and sub-zero temperatures. Following this, experimental multiscale polymer composites containing both short carbon fibres and carbon nanoparticles were synthesised and characterised. They were then subjected to dry sliding in ambient, vacuum and cold vacuum conditions. Friction and wear data were established, and the worn surfaces and transfer films were analysed with optical and indentation techniques.
Both commercial and experimental materials showed an improvement in friction and wear at low temperatures, attributed to a more stable transfer film. The presence of nanoparticles improved fibre-matrix bonding in multiscale composites, which in turn improved the performance in ambient conditions. In vacuum, overall wear was reduced and attributed to the presence of a stable transfer film, as well as the formation of load-bearing plateaux on the sample surface which contain redistributed polymer and carbon fibres. It was proposed that a cyclical wear process was occurring, which slowed the overall wear rate.
The work showed that certain conditions can promote transfer film formation in multiscale polymer composites, which is a key factor for optimising friction and wear performance. Lower environmental temperatures reduce the system energy, which keep the polymer below its pv limit. Multiscale reinforcement can result in better interfacial bonding between the different phases, as well as improving the material properties through increased load-bearing and thermal conductivity. A vacuum environment can allow nanoparticles to function more effectively and promote stable transfer films. This work shows that improvements in tribological performance can be made through material development and by optimising the operating environment.