Various industrial and societal developments in recent times are connected to climate change, sustainability, and green technology. Among these are legislations and directives from governing bodies and institutions, as well as research efforts and investments from academia and industry. One major example of an institution getting involved is that of the United Nations (UN) who published their UN Sustainable Development Goals (SDG) in 2015, in which they define pathways to a more sustainable future. A number of these goals are directly associated with the adaptation and development of sustainable engineering solutions, including enhanced resource and energy efficiency. The selection of materials in a tribological context (i.e. the study of surfaces in contact and motion) plays a major role in achieving these objectives, directly impacting resource efficiency. Frictional losses in machines and equipment and the wear of their constituents have a direct impact on the energy efficiency and sustainability of systems. An important class of tribo-materials are polymeric components. Especially thermoplastic polymers and their composites have gained significant importance in sectors like green energy production or transportation over the last few decades due to their favourable strength-to-weight ratio, corrosion resistance and, in many cases, self-lubricity, as well as their overall tailorable properties. However, they are mostly produced from ecologically undesirable fossil-based resources. Bio-based options both for reinforcements and matrix polymers to this day often lack strength or consistency of properties. In the spirit of these aspects, this thesis explores the development and characterisation of high-performing thermoplastic composites containing or consisting of bio-based materials, focusing on their thermal and mechanical properties, morphology, and tribological performance.

Polyoxymethylene (POM) and polyamide 11 (PA11) were selected as matrix materials in this work as they are already widely employed in tribological applications, but often in combination with fossil-based reinforcements. They possess appreciable mechanical strength, temperature stability, and chemical resistance. Their higher hydrophilicity compared to other common engineering thermoplastics, furthermore, makes them favourable choices for combining with natural-based materials. PA11 moreover contributes to the aspect of sustainability by virtue of being fully bio-based. Short regenerated cellulose fibres and cellulose nanocrystals (CNC) were selected as reinforcements based on their outstanding mechanical strength and higher thermal stability compared to other bio-based fillers. The short cellulose fibres were added to POM, while the CNCs were incorporated into PA11, both via melt-mixing processes leading to homogeneously dispersed systems in both cases.

The gathered results on the POM composites showed a significant increase in strength under tension and flexion at the highest fibre content as well as a rise in crystallinity upon introduction of the reinforcements. Tribological evaluations have shown that the fibre addition led to an in individual cases substantial and in general noticeable reduction of the wear coefficient at a wide range of conditions, both in preliminary tests and in the subsequent p · v range assessment. The effect on the coefficient of friction, however, was on average detrimental, especially at the lower tested speed of 0.5 m · s−1. Nevertheless, the short cellulose fibres stabilised the friction behaviour of POM at harsher p · v conditions. A main reason for these improvements was the change in tribofilm formation towards a higher coverage of the wear track, protecting the polymer samples from the asperities of the countersurface discs.

Using CNC, notable improvements of the crystallinity, compressive strength and thermal stability of the polymer and its properties were achieved. The CNC also reduced the wear coefficient of PA11 by close to 90 % and was instrumental for the tribofilm formation on the countersurfaces. Additionally, the coefficient of friction decreased as well, most likely explained through the increased presence of polymeric material on the countersurface discs, while also crystallinity further increased through possible strain-induced crystallisation. Raman spectroscopy, moreover, was proven to be a capable non-destructive evaluation method for tribofilm morphology, providing highly valuable insights for understanding wear and friction mechanisms that are otherwise often difficult to obtain. The effect of annealing at different parameters on the PA11-based composites was evaluated as well and found to be an important influence factor for crystallinity, crystal structure and tribological properties. Further improvements of especially the wear coefficient were obtained, leading to a total reduction of more than an order of magnitude when compared to the as-processed neat PA11. Reducing the coefficient of friction by thermal post-processing was successful as well, which again is assumed to be rooted in the aforementioned change in tribofilm formation and appearance as well as adjustments of the crystal structure, as proven by X-ray diffraction (XRD) experiments.

Overall, the cellulosic materials at both size scales improved the wear resistance of either matrix polymer significantly, while also in certain circumstances providing a lower coefficient of friction. In conclusion, this work shows the potential of bio-based reinforcements and composites to be successfully employed as engineering composite materials for load-bearing applications.