Ultra-performance polymer and composites in extrusion based additive manufacturing with space applications

Renewed global interest in space exploration will increase demand for compact spacecrafts such as CubeSats. Launch costs are still expensive, at around $20,000/kg to deliver satellites to low earth orbit (LEO). Strategies to reduce launch costs, such as using lightweight materials and optimization of structural topology are of commercial significance and will assist in opening greater access to space. The structure of a CubeSat is currently manufactured from lightweight metals such as Aluminium. A greater strategic use of lower density polymers will reduce mass and result in cost savings. Topology optimization of structures and reduction of joints and fixtures can also result in mass reduction. Advanced manufacturing techniques such as Additive Manufacturing (AM) can allow increased topology optimization due to reduced design constraints compared to traditional manufacturing methods. Material extrusion technologies such as Fused filament fabrication (FFF) is a type of AM which uses thermoplastics as raw material feedstocks. Currently there are limited materials available suitable for FFF that can withstand space environment including extreme temperature fluctuations, high vacuum, UV radiation, galactic cosmic radiation, etc. Out of the ultra-performance polymers used in FFF process, a certain grade of Polyether ketone ketone (PEKK) was appealing due to its thermal, mechanical, outgassing, and chemical resistance properties which ensures its applicability in space environment while offering processability in FFF. While previous studies have exposed ultra-performance polymers to simulated space environment such as thermal cycles, UV radiation, protons and electrons, limited studies have analysed the effects of heavy ions from galactic cosmic rays (GCR) and solar particle events (SPE) on these polymers. As some ultra-performance polymers are already being used to manufacture end-use components for space, it is crucial to gain an understanding about the durability of these polymers when used in space. Initial experiments involved exposing FFF printed and compression moulded samples of PEKK to heavy ion irradiation using particle accelerators at ANSTO, Lucas Heights. Absorbed dose and fluence of particles were determined from literature and SPENVIS simulations. Chemical, surface, and micro mechanical properties of PEKK was studied before and after irradiation to gain an understanding of durability of PEKK when exposed to radiation in space. Performance of FFF printed parts in space environment can be improved by optimizing FFF process parameters as properties of finished parts are highly dependent on process parameters as observed in previous studies. However, due to costs and printing difficulty associated with ultra-performance polymers, there is a need to use predictive design of experiments (DoE) methods to reduce the experimental runs while optimising process parameters. A comparison between full factorial and Taguchi design of experiments were performed in Chapter 3 where selected process parameters were varied, and mechanical and surface properties were analysed. Due to cost and printing difficulty associated with ultra-performance thermoplastics, an engineering semi-crystalline thermoplastic, Nylon 6/66 was used in Chapter 3. As PEKK and Nylon 6/66 are both semi-crystalline, the idea was to develop fundamental knowledge on relationships between structure-process-property of semi-crystalline polymers in the FFF process that are applicable for PEKK and other ultra-performance semi-crystalline thermoplastics within FFF. Results showed that when responses varied linearly, Taguchi DoE could accurately predict responses while reducing experimental effort by 50%.    Taguchi DoE was thus used in the two subsequent chapters to fully optimize process parameters for PEKK. Tensile properties of PEKK were optimized in Chapter 4 using Taguchi DoE by altering build orientation, number of contours, raster angle, and infill pattern. As number of contours significantly influenced the tensile properties as observed from main effects plots, no. of contours was varied in more levels in the subsequent chapter. Chapter 5 optimized numerous properties such as compressive, flexure, porosity, thermomechanical and dynamic mechanical. Once neat PEKK was fully characterized, properties of PEKK were further improved using additives to achieve multi-functional properties required in space in Chapter 6. Upon reviewing various additives used in the FFF process from literature, Graphene nano platelets, Graphene Oxide and Boron Carbide were used to reinforce the ultra-performance polymer and their thermal, chemical, mechanical, microstructure, and electrical properties were studied. Printability of the composites were confirmed by comparing their rheology with neat PEKK and printable commercial PEKK matrix composites. Small samples were printed using the composite filaments and characterized to identify favourable additives for PEKK in the FFF process. Potential future research opportunities were then identified in the final Chapter.