Experimental Analysis of Crystallinity and Mechanical Properties for Fused Filament Printed Polyetherketone Composites

Te objective of this article is to examine the impacts of molybdenum disulphide (MoS 2 ) and graphite-flled (Gr) poly-etheretherketone (PEEK) composites that have been fabricated through 3D printing on their mechanical properties and crystallinity. Seven samples and thirty-fve dog bones were produced using diferent flament strands to conduct the analysis. Before extrusion into flaments, the solid lubricants, MoS 2 , and graphite were uniformly dispersed within the PEEK through mechanical blending. At a concentration of 10 wt.%, the PEEK/MoS 2 composites exhibited the highest tensile strength, measuring approximately 104MPa, while the PEEK/Gr composites displayed the lowest tensile strength at the same concentration, approximately 36MPa. In addition, the PEEK/MoS 2 composites demonstrated better elongation, approximately 4.7%, compared to the PEEK/Gr composites, which exhibited approximately 2.3% elongation. X-ray difraction (XRD) data revealed that neither MoS 2 nor graphite signifcantly interacted with the PEEK matrix. Te degree of crystallinity, as determined by density matrices, indicated that the printed PEEK composites possessed a higher level of crystallinity, approximately 62% at a concentration of 5 wt.%, than the calculated values. Tis suggests that the flament-making and 3D printing processes had an annealing efect. Te signifcance of solid lubricant content and dispersion in shaping the mechanical properties and crystal formation of 3D-printed PEEK composites is emphasized in this study. Furthermore, this research provides valuable insights for optimizing PEEK-based materials for various applications.


Introduction
In the era of Industry 4.0, signifcant strides have been made in innovative manufacturing methods, unlocking novel avenues for product design and unprecedented efciencies in production.Traditional manufacturing techniques, renowned for achieving high production rates, precise control over material properties, and cost competitiveness, are being complemented by the emergence of additive manufacturing (AM), also known as 3D printing.AM ofers unique advantages, including the capacity to fabricate intricate geometries and customized designs with minimal reliance on specialized tooling or the generation of material waste.Tis renders AM particularly well suited for applications in prototyping, small-scale production, and the accelerated development of new products.[1,2].
AM has revolutionized the manufacturing industry by ofering innovative and efcient ways to create complex parts in various industries.By integrating AM with conventional methods, manufacturers can harness the synergistic strengths of both approaches, resulting in heightened fexibility, enhanced operational efciency, and a deeper wellspring of innovation in the domain of polymers and composites.Te recent advancements in AM technologies have ushered in shorter design and prototyping phases during new product development, giving rise to advanced product designs and tooling concepts.Furthermore, the evolving market dynamics have made 3D printers increasingly accessible across various sectors, encompassing research, industrial, and consumer applications [1].
One of the cheapest AM techniques is material extrusion (ME) of fused flament fabrication (FFF), also known as fused deposition modeling (FDM).FFF has gained popularity due to its advantages over traditional manufacturing processes [3][4][5].
Te Stratasys corporation patented ME and FFF.FFF is a method that deposits layers of molten thermoplastic material to produce 3D objects.Layer height, nozzle diameter, printing speed, and extrusion temperature are the main factors that afect the process [6].
FFF ofers manufacturers the freedom of accessible design, enabling the creation of complex parts that were previously challenging or impossible to manufacture.With FFF, there is no need for moulds or tools, reducing the cost and time associated with traditional manufacturing processes [7][8][9].In addition, FFF allows control of some mechanical properties such as porosity, structure, and crystallinity by manipulating contents and components such as fbre volume and orientation within its manufacturing limits [10][11][12].
FFF, which uses a thermoplastic flament as a raw material, is one of the most appreciated additive manufacturing techniques [13,14].Te FFF technique is an appealing approach for producing high-performance components utilising plastic composite materials because of its inherent benefts, including price, speed, and ease of use [15][16][17].
A trend in FFF is the improvement of printer capabilities, such as increased speed and resolution, by continuous advancements in technology and decreasing costs.Tis allows faster and more precise parts production, reducing lead times and increasing efciency [18,19].Tere are certain drawbacks to consider with FFF techniques, though.Lamination, a term for visible layer lines, may be present, necessitating further postprocessing for smoother fnishes.When compared to other approaches, layer-by-layer construction may have inferior strength.Large-scale manufacturing is constrained by the ME method's slower speed and size limits.Intricate designs can make it difcult to remove support structures, possibly necessitating physical labour and equipment [20].
ME represents the most readily accessible 3D printing technology currently available, and it is particularly well suited for the fabrication of functional components from functionally graded materials (FGMs).Tis category of components, characterized by intentional variations in structure, chemistry, microstructure, and properties within a single part, represents the forefront of manufacturing technology.It holds great promise for diverse applications spanning multiple industries, including but not limited to medical devices and aerospace [1,21,22].
In the context of polymer in AM trends, creating functional parts necessitates using a semicrystalline polymer to deliberately alter material properties across a spectrum ranging from maximum to minimum achievable crystallinity.Tis requirement eliminates the most common ME printing materials, such as ABS and nylon, as they are classifed as amorphous polymers.PLA, another frequently used ME material, is a semicrystalline polymer; however, its crystallinity level is relatively low, and the temperature range within which its crystalline properties can be modifed is limited, making it unsuitable for creating a demonstrable ME FGM [1,23].Polyetheretherketone (PEEK) demonstrates signifcant crystalline characteristics and boasts a broad temperature range within which its material attributes can be adjusted.When a semicrystalline polymer like PEEK is subjected to the cooling process from a molten state or heating from an amorphous solid state, it undergoes crystallization.
PEEK is a high-performance thermoplastic material widely used as a matrix within the burgeoning thermoplastic composite industry.Originating in the 1980s through the eforts of Imperial Chemical Industries, PEEK continues to exhibit considerable promise due to its nonreactive nature towards chemical reagents, remarkable heat resistance, exceptional elastic modulus, and outstanding durability in thermally oxidative environments.PEEK is synthesized through a biphenyl (hydroquinone) reaction with a fuorinated aromatic compound in a polar aprotic solvent, diphenyl sulfone.Tis synthesis's preference for fuorinated derivatives arises from their superior reactivity and heightened electronegativity relative to chlorinated derivatives [2,24].
Furthermore, studies have indicated that components manufactured through the FFF process using flled polymers exhibit enhanced mechanical performance and have the potential to acquire novel and improved attributes that are not observed in unflled polymers or conventional production methods [1,[24][25][26][27][28][29][30].
Due to its superior mechanical and chemical qualities, PEEK composite is currently a preferred option in today's 3D manufacturing tasks.Due to the flling process of impregnating polymers, reheating during the FFF techniques changes the crystal formation of semicrystalline polymers like PEEK.
However, it is noted that 3D printing can be difcult due to the high heat temperature requirement of PEEK composites [4,[31][32][33].
Most recent research carried out by other researchers focused on the traditional way of manufacturing, and little attention or no attention is given to the efect of solid lubricants on the FFF-printed polymer's crystal level or content.
Multiple variables, including the composition ratio, fller type, printing temperature, and manufacturing methodologies, exert an impact on the interplay between crystallinity levels and mechanical attributes in polymers.Furthermore, it is worth emphasizing that as crystallinity levels increase, tensile properties tend to exhibit enhancement.However, it is important to note that the specifc infuence of crystal content on tensile properties may vary due to factors such as the nature of the polymer, mixture proportions, distribution, and the conditions associated with thermal processing [6,[34][35][36][37].
In recent times, comprehensive investigations have been scarce into FFF's mechanical and tribological characteristics utilising PEEK composites.Notably, these studies have often 2 Journal of Engineering overlooked the infuence of fller materials, particularly solid lubricants, on the percentage of crystalline structure within the composite produced via the FFF technique [4,38,39].
Hence, this research endeavour aims to investigate and elucidate the infuence of solid lubricant variants and their respective quantities on the formation of crystalline content within FFF-fabricated PEEK composites.Furthermore, this study seeks to comprehensively analyze the impact of crystalline content on the mechanical properties of the FFFprinted PEEK composite.

Sample Preparation.
Inorganic graphite (Gr.), molybdenum disulphide (MoS 2 ), and PEEK 45 nanoparticles with average diameters of 45 μm, 100 μm, and 150 μm, respectively, were provided by Kayla Africa, a supplier and distributor at the Vaal University of Technology.Te matrix (PEEK) 450 delivered by the Kayla Africa supplier and distributor has a data sheet containing this information: average molecular weight (MW) of 44,000 g/mol l, glass temperature (Tg) of 143 °C, and melting temperature (Tm) of 343 °C.A particle size distribution was carried out for each powder to ascertain that the powder supplied was below 300 microns to avoid agglomeration.Each solid lubricant, namely, molybdenum disulphide (MoS 2 ) and graphite (Gr) of volume 3 wt.%, 5 wt.%, and 10 wt.%, was mixed diferently with the PEEK using a mechanical blender shown in Figure 1.Te mechanical mixing frst dispersed the lubricants uniformly using a mechanical blender before extruding them into flaments.
Te powder generated from the mixing and blending was further made into flament using a Filastruder flament-making machine.Te Filastruder was initially modifed to sustain a higher temperature than its manufactured limit by replacing the default PTFE-insulated Type K thermocouple with an E3D glass fbre-insulated Type K cartridge thermocouple and increasing the power supply input from 12 volts to 20 volts to increase the heating capacity of the flament maker extruder.Te Filastruder was PID-tuned to handle the temperature and torque (speed of screw rotation) during extrusion to reduce the efect of die swelling.A top table set-up called Filawinder was used to reel in the flament produced on a spool.Te Filawinder was employed to mitigate the die swelling by using an optical laser to monitor and correct the flament diameter by the variance of tensioning or compressing the flament by the speed of motors.Te Filastruder was confgured to the highest possible temperature of about 420 °C with a single screw rotating and a nozzle diameter of 1.75 mm.
A control flament of pure PEEK was also extruded.Ten, 7 groups of the extruded flament strands were printed using a retroftted Fabbster, an FFF technology printer.Te printing parameter was set using open-sourced 3D Printer Slicr software, as tabulated in Table 1.Te printing bed was heated to a temperature of 120 °C, and the room temperature was 23 °C.
5 tensile dog bones from each of the 7 groups of flaments produced were printed to make 35 pieces per Type 1 of the ASTM D638 standard for plastic tensile testing using the build orientation and printing direction as shown in Figure 2.

Printer Modifcations.
Te Fabbster 3D printer was modifed for printing PEEK samples.Te printer was initially designed for polymer printing and had a temperature limit of 280 °C for ABS.Te hot end temperature was increased to 470 °C to enable PEEK printing, and the bed temperature was set to 150 °C.Te modifcations included replacing the stock hot end with a micro-Swiss direct drive mounted on aluminium as shown in Figure 3. Te printer's software was modifed using VS-code with PlatformIO, and the frmware was edited from the Marlin 2 repository.Te control board was upgraded to the MKS Rumba 32-bit controller, enabling heat bed confguration, high-temperature hot end, and sensor-less homing, eliminating the need for optical or mechanical end stops.Te microstepping of the motors was set to 1/32 for each axis, and the motor current was limited to 750 mA to prevent overheating of the stepper motors.Te printer's axes were equipped with dual stepper motors, and sensor-less homing was activated using the TMC2226 driver.
Mechanical modifcations involved changing the extrusion system from Bowden tube style (shown in Figure 4(a)) to direct drive (shown in Figure 3) using custom brackets.Figure 4(b) shows the custom 3D-designed bracket used for hanging the micro-Swiss drive.Te printer was successfully retroftted to handle high-temperature PEEK printing with improved control and safety features.

Characterisation.
Te mechanical tests were conducted on plastic samples using an Inston 3369 universal testing machine (UTM) equipped with an optical extensometer.Te UTM was loaded with a 30 kN cell, following the ASTM D 638-14 test standards.Te tests were performed at room temperature (23 °C) and atmospheric pressure (101.33 kPa), with a constant crosshead speed of 5 mm/min, per ASTM standards for plastics.Instron software determined tensile strength and elongation values and saved them as a CSV fle.
X-ray difraction analysis (XRD) was performed on both the original PEEK polymer and the manufactured sample to examine the impact of solid lubricant on the sample's crystallinity.Te Empyrean difractometer was used, employing Cu Kα radiation with 45 KV and 40 mA electrical charge, measuring within 5 °to 35 °.Te degree of crystallinity (X c ) was calculated using the Hermans-Weidinger diagrammatic method, as described by Gusev (1978) [40] in the following equation: X c represents the degree of crystallinity, A C is the Area of crystal peaks in the XRD, and A a is the Area of amorphous peaks in the XRD.Te calculated X c value was then compared to the theoretical crystallinity level obtained from polymer density calculations, as given by the following equation: where ρ th is the theoretical density of flled PEEK, ρ p is the density of PEEK (1.32 × 10 − 3 kg/cm 3 ), W p is the weight ratio of PEEK, ρ s is the density of the solid lubricant, and W s is the weight ratio of the solid lubricant.Te density matrix (ρ m ) was derived from Agarwal et al.'s composite analysis textbook (2006) by using the following equation [41]: To determine the level of crystallinity in the printed PEEK, (4) was employed, using the density matrix (ρ m ) from Bhuiyan's report (2020) on crystallinity and amorphous measuring techniques [42].
In this equation, X c represents the degree of crystallinity, ρ c is the density of PEEK in its crystalline phase (1.4 g/cm 3 ), ρ a is the density of PEEK in its amorphous phase (1.263 g/cm 3 ), and ρ m is the density matrix.Based on supplier information, the densities of graphite and MoS 2 used were 1.226 g/cm 3 and 5.06 g/cm 3 , respectively.

Results and Discussion
3.1.Mechanical Properties.Te addition of graphite to PEEK (Gr./PEEK) led to a nonlinear decrease in tensile strength within the weight fraction range of 3% to 10%.A similar downward trend was observed for elongation at break as shown in Figures 5 and 6.On the other hand, Young's modulus exhibited a nonlinear increase.Te decrease in elongation at break can be attributed to the brittleness and  Moreover, the reduction in tensile strength could be infuenced by decreased adhesion forces between PEEK molecules and the further oxidation of graphite during flament production and printing.Both PEEK composites showed a sharp drop in elongation between 0 wt.% and 3 wt.%,followed by a gradual reduction.Te increased presence of solid lubricants resulted in higher brittleness and weaker interfacial bonds within the PEEK matrix.In addition, incorporating solids in PEEK could reduce elongation or crack initiation due to agglomeration.
Conversely, the incorporation of MoS 2 into PEEK (MoS 2 /PEEK) showed a nonlinear rise in tensile strength and Young's modulus, reaching a maximum increase of 61%.Tis improvement can be attributed to the even distribution of MoS 2 within the PEEK matrix, which aligns with fndings from a study by Yan et al. on MoS 2 /PEEK [44].
MoS 2 /PEEK exhibited superior mechanical properties compared to PEEK/Gr.Te highest recorded tensile strength was 101 MPa for a 10 wt.% MoS 2 content.Te addition of both solid lubricants in 3D-printed PEEK had distinct efects on tensile strength.However, it should be noted that the experimental tensile strength showed a slight reduction compared to the known theoretical values, which is consistent with trends observed in other studies on 3D-printed polymers.Such a reduction in tensile strength is attributed to the inherent characteristics of 3D-printed objects, which typically exhibit 70% to 80% of the tensile strength of their traditionally fabricated counterparts [30,45].
Regarding Young's modulus, representing the ratio of stress to strain with an elastic limit, there were diferent responses to the two solid lubricants.Figure 7 illustrates the behaviour of PEEK with the two solids.Te control 3Dprinted PEEK had a modulus of elasticity of 3.89 GPa.Gr/ PEEK showed an almost linear increase in Young's modulus with increasing additive fractions.In contrast, Young's modulus for MoS 2 /PEEK reached a plateau between 3 wt.%and 5 wt.%.Te addition of solid lubricants increased the stifness of the PEEK matrix, resulting in a consistent rise in the elastic modulus of the printed PEEK composite.Te highest recorded modulus values were 4.49 GPa for Gr./PEEK and 4.48 GPa for MoS 2 /PEEK at a 10 wt.% fller content, surpassing the control sample.Tis increase is attributed to the enhanced stifness of the solids as their content increases.
Adding MoS 2 to PEEK enhances stress strength, particularly when the loading surpasses 3 wt.%.Tis improvement is likely because of MoS 2 's higher matrix density and better dispersion within PEEK.On the other hand, the inclusion of graphite fllers shows a consistent decline in strength, indicating agglomeration within the PEEK matrix.Studies by Fu et al. (2008) and Chen et al. ( 2018) have shown that adding graphite beyond 0.3 wt.% in PEEK leads to agglomeration, resulting in weaker bonds and reduced fatigue resistance [46,47].Fu et al. [47] further emphasized the signifcance of matrix interfacial adhesion in determining the tensile strength of polymer composites, as it infuences the efectiveness of stress transfer between fllers and the polymer.MoS 2 /PEEK exhibits improved stress strength, likely due to its higher matrix density and dispersion, while graphite fllers tend to agglomerate, weakening the material.Te interfacial adhesion between the matrix and fllers plays a crucial role in determining the overall tensile strength of composite materials.[43,44,48].
Adding MoS 2 to PEEK resulted in XRD curves plotted in Figure 9 for 3 wt.%, 5 wt.%, and 10 wt.% MoS 2 .PEEK typically exhibits semicrystalline properties with intensity defections in the 2θ range of 18 °to 30 °. Te addition of MoS 2 produced a defection at around 13 °on plane 200, indicating lower crystallinity levels in the flled PEEK.Te peaks of the MoS 2 -flled PEEK shifted slightly to the right, indicating the presence of compressive strain.Te appearance of a new peak around 13 °on plane 200 in the PEEK/MoS 2 XRD graph suggests no signifcant chemical reaction between MoS 2 and PEEK during the flamentation extrusion and printing process.Te trend observed is like fndings in other studies on MoS 2 XRD patterns.
Adding Gr. to the PEEK followed a similar trend to the PEEK/MoS 2 , as depicted in Figure 10.Te graphite-flled PEEK displayed an additional peak at around 26 °on plane 002, indicating no signifcant chemical reaction between PEEK and graphite.Te elongation of the graphite peak in PEEK increased with higher graphite ratios, like in previous studies on graphene and graphite oxides.
Te Hermans-Weidinger diagrammatic method was employed to determine the degree of crystallinity.Te peaks' width, height, and regions under the curve were analysed using the peaks analyser function in the Origin plotter  2 and Figure 11 present the experimental data for the degree of crystallinity and the overall graph area obtained from the XRD analysis or graphs.
In theory, incorporating fllers into any polymer typically results in a linear increase in the material's density.When MoS 2 is added, the composite's density is signifcantly higher than that of the graphite-flled counterpart, leading to an improvement in density of approximately 50% compared to the MoS 2 alone as shown in Table 3 and Figure 12.
Te density matrix of both solids within the PEEK matrix decreases nonlinearly and is inversely related to the composite density.Teoretically, the degree of crystallinity depends on the density matrix; as the solids in PEEK increase, the amorphous content in the composite polymer also increases.Te PEEK/Gr.exhibited the lowest degree of    crystallinity, approximately 11%.Tis decrease directly infuenced the tensile strength and elongation at the break of the composite material.which is the indication that the printed PEEK is in a semicrystalline phrase.New crystalline peaks were created at plane 200 at 13 °and 002 at 26 °for MoS 2 and Gr., respectively, with no existing PEEK peak destroyed, indicating no signifcant reaction with the solid lubrication.(vi) Te crystallinity level is higher than the calculated theoretical values, with an average diferential of 25% for every weight ratio.Tese higher values suggest the annealing efect from reheating during flament-making and 3D printing processes.Te highest value recorded was at 5 wt.%MoS 2 flling valued at 71% compared to the plain-controlled pure PEEK at 55%.

Conclusion
In summary, the research emphasized the importance of the solid lubricant content and its dispersion in shaping the mechanical properties and level of crystal formation in 3Dprinted PEEK composites.

Figure 2 :
Figure 2: (a) Te computer 3D representation of dog bone; (b) the printing and building direction of the tensile dog bone.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: (a) Te front and (b) the side view of the assembled and modifed printer with micro-Swiss direct drive.

Figure 11 :
Figure 11: Te experimental degree of crystallinity of both flled PEEK and unflled PEEK.

Table 1 :
Information incorporated in slicer fle for 3D printer.

Table 2 :
Te experimental data for the degree of crystallinity and overall graph area derived from the XRD area.