3.1 MFI and Rheology of sPEEK/HAP-rGO
The flowability of PEEK and its composite is determined using melt flow index (MFI), which was measured at 400°C, in context of filament manufacture by extrusion and subsequent 3D printing of parts. It is observed from Table 1, MFI decreases as rGO concentration rises due to filler resistance to polymer flow [27]. PEEK has the highest MFI (5.204% g/10 min) when compared to its composites, and MFI decreases as rGO increases, which might cause printing concerns. MFI was decreased by 24.1%, 28.3%, and 33.1% when compared to neat PEEK at 2%, 4%, and 6%, respectively [28]. Reduced MFI must be studied carefully and accounted for by raising the printing temperature, particularly for composite with a higher rGO%. To reduce warpage, the bed temperature is maintained constant, and the multiplier factor was modified to account for the greater rGO%. The MFI of polymer rises as the filler infusion increases, as seen throughout the frequency sweep [29]. PEEK has a shear-thinning area at higher frequencies. The limitation of polymer chain motions by different concentration of rGo (2%, 4% and 6%) causes PEEK/HAP to behave similarly, with a little increase in η′.
Table 1. MFI comparison
Composition
|
MFI (g/10 min)
Mean value
|
Standard Deviation
|
sPEEK
|
5.2
|
0.4
|
sPEEK-HAP
|
3.4
|
0.3
|
sPEEK-HAP/rGO(0.5)
|
2.7
|
0.2
|
sPEEK-HAP/rGO(1)
|
1.9
|
0.2
|
sPEEK-HAP/rGO(1.5)
|
3.1
|
0.2
|
3.2 Compression properties
Figure 3a shows the compressive strength of sPEEK, sPEEK/HAP and sPEEK/HAP-rGO composites containing different Mf of rGO at low strain rates. The compressive strength has been found to be strain rate sensitive, increasing as the strain rate increases. As the strain rate increased, the compression strength of the PEEK/HAP composite reduced by 10% compared to the sPEEK sample. PEEK material used is brittle, became amorphous after sulphonation, with a yield point processed by the formation of a neck. Adding HAP to the sPEEK matrix reduces the volume fraction of composite, weakening the overall strength of the material because composite is the principal stressed component. The addition of rGO to the sPEEK/HAP composite boosts compression strength even further. The compression strength of sPEEK/HAP-rGO composites improves as the Mf of rGO increases, compared to sPEEK and sPEEK/HAP composites. With a rise in rGO concentration from 0.5–1.5%, compression strength increases by 3.4%, 5.2%, and 3.8% for 0.001, 0.01, and 0.1 s-1 strain rates, respectively. The maximum compressive strength was observed for 1.5% concentration at 0.1 s−1 strain rate, which is identical to sPEEK but 1.3% less. The reason for this is because GO possesses a hexagonal carbon network with hydroxyl and epoxide functional groups, which improves compression strength by establishing strong interfacial bonding between PEEK and HAP, as well as increasing the composite's mechanical properties.
Figure 3b shows the compressive strength of sPEEK, sPEEK/HAP and sPEEK/HAP-rGO composites containing different Mf of rGO at high strain rates such as 800 and 1000 s−1. It was found that sPEEK and its composites had stronger compression strength at higher strain rates than they do at lower strain rates. As strain rate increased, the compression strength of the PEEK/HAP composite decreased by 15% compared to the sPEEK sample. At high strain rates, however, the material's molecular structure does not have enough time to absorb the deformation, resulting in a decreased fracture rate. When rGO is added to the PEEK/HAP composite, its compression strength improves. The maximum compressive strength was found at a concentration of 1.5% rGO at 800 s−1 strain rate.
This idea also applies to the increase in the density of composites as shown in Fig. 4. It shows that Mf and ρ have a linear relation. This shows that the composites' strength is proportionate to their densities. When comparing identical HAP and rGO filled composites under similar strain rate loading, this occurrence allows for the examination of HAP and rGO filled polymer strength considering specimen density as one factor.
3.3 Strain rate effect
Compression test stress-strain curves of sPEEK and sPEEK/HAP samples at 0.001, 0.01 and 0.1 s−1 strain rates are shown in Fig. 5. The sPEEK exhibits abrupt and linear elastic behaviour until it reaches the apex, after which it yields and the fracture propagates linearly until it reaches fracture failure (Fig. 5a). For all strain rates, the curve had a same pattern. The results reveal that sPEEK exhibits positive strain rate sensitivity with increase in strain rate and the fracture strain increases, from 0.01 to 0.3 for strain rates of 0.001 s−1 to 0.1 s−1. A loss of ductility was observed with increasing strain rates. The inclusion of HAP makes the sPPEK composite amorphous, and the elastic behaviour of the composites decreases as the strain rates increase. sPEEK/HAP composite also showed a sharp and linear until it reaches the peak. The curve gradually declines from the top, resulting in increased energy loss, and continues to fall until it reaches fracture failure (Fig. 5b). Fig. 6(a-c) stress-strain curve of compressive test specimens of sPEEK/HAP-rGO with different Mf of rGO carried under different low strain rates. The curve is sharp and linear until it reaches the peak. After yielding, the curve drops down for a short period and continued to be linear till to the failure. The compressive strength of sPEEK/HAP-rGO composites are enhanced by the carbon content of rGo, which rises with increasing rGo concentration. rGo exhibits good interfacial bonding between HAP and sPEEK
The specimens are subjected to hydrostatic pressure and shear stress during low strain rate loading, causing localised damage. Dispersibility and compatibility can be used to measure the improvement in interfacial bonding strength. Fig. 7 shows the SEM micrographs of sPEEK-HAP/rGO samples with varying rGO concentrations, with sPEEK-HAP sample serving as a control. For the sPEEK-HAP samples, HAP particles were disseminated and agglomerated in a non-uniform manner in the PEEK matrix (Fig. 7a). Similar findings on the biopolymer-bioceramic composites were found to indicate poor compatibility due to differences in chemical and physical characteristics [30]. The degree of agglomeration in the sPEEK-HAP/rGO samples reduced as the rGO concentration increases (Fig. 7b–d). In the fracture surface of sPEEK-HAP/rGO samples with 1.5% rGO, rGO was uniformly distributed in the matrix, which minimises the HAP particle crushing and resulting in lower stress concentration. This helps to avoid local damage within the specimen which enhances the mechanical properties of the composites.
Compressive tests for sPEEK, sPEEK/HAP, and sPEEK/HAP-rGO composites were also performed at high strain rates (800 and 1000 s−1) to validate the strain rate sensitivity properties of sPEEK and its composites for the defined objectives. The stress–strain curve's distinctive form is not the same at both rates. The sPEEK/HAP displays much higher post-yield softening than that of sPEEK (Fig. 5b). The softening effect of sPEEK/HAP composite increases as the strain rate increases, and it is linked to non-isothermal tests. Thermal softening of polymers occurs as a result of insufficient heat transfer out of the material, resulting in increased plastic straining.
The composite's rate sensitivity determines the rate sensitivity of the HAP and rGO loaded sPEEK polymer. At high strain rate, the fracture is induced to propagate through the HAP. Energy is squandered during crack growth, which may be regarded as one factor impacting the energy absorption capacity of HAP loaded polymer. To put it another way, a rapid strain rate allows the fracture to spend less time exploring low-resistance routes, resulting in less energy use. In contrast, with low strain rate loading, the fracture prefers to avoid passing through HAP particles, resulting in HAP particle debonding, as seen in Fig. 7e.
The stress-strain curves of PEEK/HAP-rGO(0.5), PEEK/HAP-rGO(1), and PEEK/HAP-rGO(1.5) specimens conducted at high strain rates are comparable to those made at low strain rates; the curve is broken into two halves here (Fig. 6a-c). The steep and linear elastic curve till to the apex and sudden drop from the apex for a short period and continued to be linear till to fracture. The PEEK/HAP composite's compression strength was found to be lower than that of sPEEK and to increases with increasing rGO concentration. The PEEK/HAP composite, on the other hand, shows a weaker trend than neat sPEEK specimen. Meanwhile, the compressive strength increases as the Mf of rGO increase at the same strain rate. rGo exhibits good interfacial bonding between HAP and sPEEK. The stress-strain curves have similar structure for both strain rates, and the fracture strain is lower at 1000 than at 800 s−1, as expected.
3.4 Tensile testing
The stiffness of the curve was calculated by fitting the first linear segment with a linear curve. Fig. 8 shows the tensile strength of sPEEK, sPEEK/HAP and sPEEK/HAP composites containing different Mf of rGO at low strain rates. The tensile strength has been found to be strain rate sensitive, increasing as the strain rate increases. In comparison to PEEK, the tensile strength of sPEEK/HAP decreased with increasing strain rates. The tensile strength of sPEEK/HAP declined by an average of 11% as strain rates increased, indicating that the addition of HAP significantly reduces the material's tensile strength. The rate sensitivity of sPEEK/HAP composite has been shown to be influenced by its viscoelastic behaviour, which tends to reduce absolute strength due to microscopic local damage. However, the rGO filler, may have little influence on the overall sensitivity of PEEK/HAP composites. The tensile strength of sPEEK/HAP-rGO composites increases as rGO Mf increases as compared to sPEEK and sPEEK/HAP. An average of 3%, 2.5% and 3% increase in tensile strength was observed for 0.001, 0.01 and 0.1 s−1 strain rates respectively, with in increased concentration of rGO from 0.5–1.5%. The oxygen groups and negative ions on the surface of rGO particles are thought to be able to interact efficiently with sPEEK/HAP via H-bonding and electrostatic force. The strong interfacial bonding between PEEK and HAP particles resulting in dramatically improved mechanical characteristics of PEEK composite and also, it improves the dispersion of HAP particles in the sPEEK matrix.
3.5 Strain rate sensitivity
The properties of the elements must be studied in order to comprehend the failure causes of a composite material. Fig. 9 shows the tensile stress-strain curves of sPEEK and sPEEK/HAP samples obtained at strain rates of 0.001, 0.01 and 0.1 s−1. The strain distributions of the gauge section for tensile testing of sPEEK samples at various low strain rates are shown in Fig. 9(a). The curve is separated into two parts, the first of which demonstrates the elastic behaviour of the composites by observing a linear relationship till an apex. After yielding, necking initiated and propagated. For the strain rates of 0.001 and 0.01 s−1, necking progressed slowly to the next section and continued to be linear until the maximum strain reached 110.4%, resulting in final crush failure. At a 0.1 s−1 strain rate, however, necking is focused rather than broadly dispersed, leading in a substantially faster specimen fracture at strain level of 0.2. These findings highlight to PEEK's high sensitivity to strain rate, which is consistent with these reference [31, 32], which reveals that when the strain rate crossed 0.001 s−1, the yield stress of PEEK increased fast due to the greater crystallite perfection.
Figure 9(b) shows strain distribution for tensile strength for sPEEK/HAP samples under different low strain rates. The curve initially shows progressive elastic behaviour till to the apex. After yielding, the curve drops linearly, which adds to energy loss and continued to be drop to attain eventual crush failure for 0.001 and 0.01 s−1 strain rates. At 0.1 s−1 strain rate, the curve drops sharply after the yield point at the strain levels of 0.03, 0.025 and 0.02 for 0.001, 0.01 and 0.1 s−1 strain rates respectively. As the temperature rises, the addition of HAP lowers the crystallinity of PEEK, and a higher strain rate generates more heating inside the specimen. While comparing the stress-wave propagation rates for 0.001, 0.01 and 0.1 s−1, the latter case's rate is substantially lower than the loading rate, limiting necking propagation and resulting in an early fracture.
Figure 10(a,b&c) shows the tensile test stress-strain curves of sPEEK-HAP/rGO(0.5%), sPEEK-HAP/rGO(1%) and sPEEK-HAP/rGO(1.5%) under different low strain rates. All these composites showed similar stress-staring curve nature. The curves showed progressive elastic behaviour till to the apex. Necking was prolonged for a short time after yielding and then dropped linearly at the strain levels of 0.04, 0.055 and 0.06 for 0.001, 0.01 and 0.1 s−1 strain rates respectively for sPEEK-HAP/rGO(0.5%) composite (Fig. 9a), 0.035, 0.05 and 0.065 for 0.001, 0.01 and 0.1 s−1 strain rates respectively for sPEEK-HAP/rGO(1%) composite (Fig. 9b) and 0.045, 0.065 and 0.08 for 0.001, 0.01 and 0.1 s−1 strain rates respectively for sPEEK-HAP/rGO(1.5%) composite (Fig. 9c). These results demonstrated that maximum tensile stresses are sensitive to low strain rate loading for all composites.
3.6 Thermal analysis
The thermal properties of sPEEK loaded with HAP and rGO with variable concentrations were evaluated using the TGA method. Samples of sPEEK, sPEEK/HAP, and sPEEK/HAP-rGO composites were analysed in an inert environment, and the average results of the triplicate tests are given in Fig. 11(a), revealing that the incorporation of HAP affected the thermal degrading behaviour of sPEEK significantly. PEEK and its composites deteriorate in two stages, with sPEEK decomposing at roughly 550°C, which is much higher than the temperature at which most polymers decompose. In first stage, decomposition occurs around 580°C and is related to arbitrary chain scission of the ether and ketone bonds [33]. After 600°C, the second stage of breakdown starts, which is attributed to the development of residue as a result of crosslink breaking and dehydrogenation, resulting in a thermally stable carbonaceous char. Within 30°C of time, sPEEK loses 30% of its weight in the initial stage, and continues to drop 10% of its weight between 580 and 600°C. As a result, the first stage resulted in a total weight decrease of 40%. There is no foreign substance in the sPEEKs media that is associated with a 40% weight reduction. In the second step, just 10% of the weight is lost up to 800°C, resulting in a 50% yield. Beyond 800°C, there is minimal further loss, and the material appears to be stable at 850°C. A 40% weight loss shows that formation of aromatic molecules as products, presumably in the form of phenol, a significant sPEEK breakdown product [34].
sPEEK/HAP-rGO samples had a slightly higher beginning temperature than sPEEK and sPEEK/HAP sample. The initial stage of decomposition of sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples is comparable, with weight loss of roughly 28% and 27%, respectively. At 580°C, the decomposition process begins, and both materials progress to the second stage of decomposition. Further, after 750°C, small loses in weight occurs and the final remaining weight of the materials for sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples is 65% and 68% respectively. For sPEEK/HAP-rGO(1.5) sample, the initial decomposition begins at 600°C, with the weight loss of 21% and continuing to decompose at a faster rate than sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples. After 800°C, the weight of the sPEEK/HAP-rGO(1.5) sample decreased significantly, and the final residual weight was 71%, slightly higher than others, as well as the char fraction.
Similarly, the thermal properties of sPEEK loaded with HAP and rGO with variable concentrations were evaluated in an oxidative environment as shown in Fig. 11(b). Thermal oxidative degradation begins between 510°C and 530°C for sPEEK/HAP, sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples. The temperature at which thermal oxidative decomposition begins in sPEEK/HAP-rGO(1.5) sample is somewhat greater, at around 560°C. sPEEK/HAP loses 35% of its weight in the first stage, with 30% of the weight loss occurring within a 30°C time frame, which correlates to thermal disintegration under inert atmosphere, suggesting that scission of ether and ketone bonds does not enhance by an oxidative environment. The rest of the sPEEK/HAP weight is lost during the second stage, which involves thermally oxidising the whole material at 720°C. sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples lose weight at a slower pace than sPEEK/HAP, although having a same commencement of thermal oxidative decomposition temperature. The first decomposition stage is responsible for a 3% weight loss difference between sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples at roughly 580°C, with weight losses of 25% and 22%, respectively. In the second stage, sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples continue to lose 35% and 32% weight, respectively, at a comparable pace as sPEEK/HAP and there is no further decomposition of materials. For sPEEK/HAP-rGO(1.5) samples, the initial decomposition begins at 600°C, with the weight loss of 20% and continuing to decompose at a faster rate than sPEEK/HAP-rGO(0.5) and sPEEK/HAP-rGO(1) samples. After 700°C there in no further decomposition. The addition of microspheres enhances the thermal stability of sPEEK/HAP-rGO(0.5), sPEEK/HAP-rGO(1) and sPEEK/HAP-rGO(1.5) samples in an inert environment, with sPEEK/HAP-rGO(1.5) samples being the most thermally stable.
The glass transition (Tg) temperatures from DSC analysis of sPEEK loaded with HAP and rGO with differetn concentrations of rGO samples are shown in Table 2. The DSC analysis reveals that the Tg of the neat sPEEK/HAP, sPEEK/HAP-rGO(0.5), and sPEEK/HAP-rGO(1) samples are not considerably different. When the Tg values of sPEEK/HAP-rGO(1.5) samples and sPEEK/HAP are compared, the Tg values of sPEEK/HAP-rGO(1.5) samples is greater. This reveals that while 1.5% Mf of rGo increase the physical interactions between the HAP and sPEEK, improving the mechanical characteristics of the composites, they have no effect on the composite's Tg.