Inorganic fullerene like Tungsten di-sulfide (IF-WS2) nanoparticles with 80 nm average diameter were prepared by Professor Yanqui Zhu’s group at the University of Exeter. Polyether ether ketone (PEEK) 450 G was purchased from Victrex, (Mw ≈ 44,000 g/mol, Tg ≈ 143°C, Tm ≈ 343°C) and used as the matrix. To produce filaments for 3D printing, the IF-WS₂ nanoparticles were melt compounded with PEEK at 0.5, 1, and 2 wt% loading using a twin-screw extruder with 16 mm diameter, length to diameter ratio (L/D) of 40, and screw speed of 30 rpm. The average diameter of filaments was monitored and kept at 1.75 ± 0.15 mm. The screw configuration was designed in a way to enhance nanoparticle dispersion using screw elements consisting of transporting elements for forward conveying of extrudate, kneading elements with 90°, 60°, and 30° twist angles for dispersion, and reverse screw element. The temperatures indicated in each zone of the extruder were: 365 (feeding zone), 400 (melting and compression zone), 385°C (metering zone and die extrusion). The extruded filaments were cooled at room temperature (∼25°C).

A modified UM2 + FDM 3D printer was used for to print PEEK nanocomposite specimens which enables extrusion temperatures of up to 420°C (using an all-metal hot end), bed temperature of maximum 350°C (via heating elements embedded under the print bed), and heating lamps for regulation of chamber temperature up to 230°C.

Pellets were cut from the filaments for injection moulding. A Haake MiniJet Pro injection moulding machine was used for production of injection moulded tensile samples, operating at melt temperature of 400°C and mould temperature of 280°C. Specimens with similar dimensions to the injection moulded samples were printed from the extruded filaments using a high temperature Ultimaker. The printing speed, building plate temperature, nozzle temperature and nozzle diameter were set at 40 mm/s, 280°C, 400°C and 2 mm, respectively. A raster orientation of ±45° was applied to produce a uniform morphology. A comparison was made between the mechanical properties of the printed and the injection moulded samples.

Pictures of the 3D-printed and injection moulded samples are presented in Figure 1.

The schematic presentation of the sequence of manufacturing process is demonstrated through a flowchart in Figure 2.

A high resolution FEG SEM (HITACHI), operating at 30 kV and an intensity of 9 × 10^–9 was used to observe the gold coated fractured surfaces of the filaments to evaluate the dispersion and distribution of IF-WS2 nanoparticles in PEEK.

The thermal properties of the 3D printed and injection moulded nanocomposite samples were measured using a TA-Instrument Q1000 differential scanning calorimeter (DSC). 5 mg samples were sealed in aluminium pans and a standard mode heating-cooling-heating cycle at 10°C/min between 20 and 400°C was applied. The amount of degree of crystallinity (χc) was calculated from the crystallization peak (Golbang et al., 2020).

Tensile testing was performed on both 3D printed and injection moulded samples using an Instron testing machine (Instron 3,344) and following the ISO standard: ISO 527–2 Type 5A. Five specimens (75 mm × 4 mm × 2 mm) were tested for each sample with a crosshead speed of 2.0 mm/min at 20°C and using a 2 kN load cell.

The SEM images presented in Figure 3 show the morphology of PEEK nanocomposite filaments with 0.5, 1, and 2 wt% IF-WS₂. Overall, a homogenous distribution of particles is observed in all three loadings. However, there is slightly larger sizes of inclusions present in the 2 wt% and to some extent in the 0.5 wt% samples, indicating some degree of agglomeration in these cases. The higher dispersion degree of particles achieved at 1 wt% IF-WS₂ may be due to the lubricant effect of these nanoparticles. Tendency for agglomeration increases with further increase in the number of particles at the higher loading (2 wt%) (Khare and Burris, 2010; Golbang et al., 2017; Fu et al., 2019; Golbang et al., 2020).

There was no obvious change observed in the distribution and dispersion state of the nanoparticles after the injection moulding and 3D printing process.

Dispersion in a nanocomposite is associated with breaking of agglomerates into smaller sizes and ideally into individual nanoparticles, by overcoming the inter-particle interactions (typically van der Waals forces). The “dispersion state” of nanoparticles is linked to the final properties of the nanocomposite due to the long-range effects of nanoparticle-matrix interface on nearby polymer chains. However, achieving good dispersion is considered as a prominent challenge. The “distribution sate” of particles in a nanocomposite is independent of dispersion state (or the size of inclusions). In a good distribution, the inclusions (e.g., particles, agglomerates, aggregates, … ) are homogeneously distributed in the matrix and isotropic properties are achieved within the nanocomposite. In other words, each inclusion is as far as possible from its nearest neighbour, so that the space is homogeneously filled (Khare and Burris, 2010; Fu et al., 2019).

Changes in the crystallinity of the PEEK nanocomposites after extrusion (i.e., filaments), and shaped via 3D printing and injection moulding are shown in Figure 4.

Incorporation of IF-WS₂ nanoparticles increases the crystallinity of PEEK in samples up to a loading of 1 wt% which can be attributed to the nucleation effect of these nanoparticles. Reduction of crystallinity with a further increase in IF-WS₂ concentration to 2 wt%, is likely to be due to a reduced nucleation effect of the larger agglomerated IF-WS₂ or a confining effect of the particles that restricts molecular mobility and ability of polymer chains to orient and crystallize (Golbang et al., 2020; Antony Samy et al., 2021).

From Figure 4, it can be seen that the overall crystallinity of the 3D printed samples is slightly higher at each filler loading compared with the injection moulded counterparts and filaments. This can be explained by the thermal history experienced in each process. During FDM, the deposited layers are reheated due to the deposition of subsequent layers. This multiple reheating effect contributes to further growth of crystals and higher degrees of crystallinity. During injection moulding, the material is cooled from extrusion temperature (400°C) to the mould temperature (280°C) in less than a minute and then ejected into a room temperature environment. The higher cooling rate in injection moulding compared to that in in FDM, results in a lower degree of crystallinity. The filaments have the lowest degree of crystallinity at each filler loading. This is due to the sudden cool down of the polymer melt to room temperature (Brucato et al., 1993; Yang et al., 2017; Golbang et al., 2020; Schiavone et al., 2020; Antony Samy et al., 2021).

The stress-strain curves of the injection moulded and 3D printed samples are illustrated in Figures 3, 4. The average Young’s modulus and ultimate tensile strength are reported in Table 1. As seen in Figures 5, 6, as well as Table 1, the trend of changes in the Young’s modulus and tensile strength with addition of IF-WS2 nanoparticles are similar, with the addition of IF-WS2 nanoparticles generally increasing the Young’s modulus and tensile strength of PEEK up to a loading of 1 wt%. This increase is due to the presence of the reinforcing filler and the increased crystallinity of these samples. Properties drop off at the 2 wt% loading due to the stress concentration effect of, and lower aspect ratio of, agglomerated IF-WS2, combined with a slightly lower crystallinity. The elongation at break deceases in the injection moulded samples for increasing IF-WS2 which is quite typical when reinforcing fillers are added to a polymer matrix (Pinto et al., 2015; Huang et al., 2019; Rostom and Dadmun, 2019; Golbang et al., 2020). However, the reverse is true for the 3D printed materials with added IF-WS2. Two possible explanations for this improvement are an increase in layer-to-layer bonding due to enhanced thermal conductivity of IF-WS2 containing PEEK and, the lubricating effect of these nanoparticles, contributing to higher molecular mobility and polymer chain inter-diffusion during deposition and therefore better bonding between the layers (Coogan, 2019; Srinivas et al., 2020; Candal et al., 2021).

The 3D printed samples have a lower Young’s modulus and tensile strength for each IF-WS₂ loading in comparison to the injection moulded samples, by about 80 and 70%, respectively. There is a very significant reduction in elongation at break for the 3D printed materials compared to those that are injection moulded. These results are expected due to the weaker inter-layer bonding between the layers in 3D printed samples and the presence of voids. Also, the pressure exerted during injection moulding can compress and strengthen the part (Koštial et al., 2016; Verdejo de Toro et al., 2020).

According to the data presented in Table 1, the Young’s modulus of the 3D printed samples containing 1 wt% IF-WS₂ surpasses that of the injection moulded pure PEEK and the tensile strength is significantly increased to just 7% lower than the unfilled PEEK. Hence, it can be concluded that adding small amounts of IF-WS₂ nanoparticles can overcome the reduced stiffness and tensile strength caused by poor layer adhesion and voids in 3D printed parts.

Overall, the increment in the Young’s Modulus and tensile strength of PEEK with addition of IF-WS₂ nanoparticles is higher for the 3D printed samples in respect to the injection moulded ones. This is ascribed to the improved bonding between the deposited PEEK layers in the presence of IF-WS₂ nanoparticles.