Local physicochemical changes in CF-PEEK and mechanical performance

Temperature and moisture have been shown to have an important effect on the crystallinity and transition temperatures of semi-crystalline thermoplastics, which in turn locally and globally affect the mechanical performance of the composite [21,182,185]. This is particularly important in

the case of a friction riveted joint, because its final failure under quasi-static and cyclic loadings is mainly triggered by damage accumulation in the composite, as shown in Section 7.1.2. Therefore, DSC coupled with nanohardness testing were performed, to investigate the physicochemical change of the composite matrix after 28 days of aging and its influence on the composite’s local mechanical properties. For the DSC analysis the material was extracted following the procedure described in Section 5.2.7, and for nanohardness the procedure for measurements described in Section 5.2.8.2 was adopted. It is believed that the material that consolidated in the composite against the rivet shaft (CTMAZ) was exposed to a similar thermal cycle as the flash material, due to the composite’s low thermoconductivity (see Table 5.2). Therefore, the nanohardness of the CTMAZ material was further correlated with the DSC results for the flash material. The DSC results are summarized in Table 9.1.

The initial heating curves of the unaged and aged samples are compiled in Appendix L.

Table 9.1 Main parameters obtained from DSC analysis (Tcc – cold crystallization temperature, Tm – melting temperature, Tc – crystallization temperature, Xc – degree of crystallization) for unaged and hydrothermally aged BM and TMAM materials.

The aging process had little effect on the transition temperatures of CF-PEEK compared to its effect on the degree of crystallinity, which exhibited an increase of 7 % derived from Xc, aged BM = (31.0 ± 3.0) % and 42 % from Xc, aged TMAM = 34 % when compared to unaged BM derived from Xc, unaged BM = (29.0 ± 2.0) % and the TMAM from Xc, unaged TMAM = (24.0 ± 2.0) %, respectively. Long term exposure of the joint to below Tg (71 °C) and humidity seems to induce additional mobility into the amorphous phase of PEEK, leading to macromolecular ordering and nucleation of crystals.

Although PEEK has high thermal and chemical stability [21,258], Cornélis, Kander, and Martin [135]

showed that the material can be swollen by solvents including acetone, which caused its plasticization and a solvent-induced crystallization (SIC) process. Even an amorphous phase can undergo a degree of reordering during annealing with a reduction of the free volume content, hence increasing the local mechanical properties of the polymer [259]. Despite the fact that water acts as a plasticizer when diffused within the polymer chains [260], it is believed that the aging temperature plays a more important role in increasing the mobility of the amorphous phase, because PEEK has a low water saturation level [185]. Moreover, post-crystallization of the exposed polymer can be maximized by the presence of fibers. Vieille, Albouy, and Taleb [261] reported the strong nucleating effect of the

Tcc [°C] Tm [°C] Tc [°C] Xc [%]

fibers over a long aging period at temperatures below Tg for CF-PEEK, resulting in an increase of 3 % in the degree of crystallinity by a transcrystallinity mechanism. As reported by Gao et al. [128], the polar carbonyl groups of the PEEK matrix, when exposed to such environmental conditions, tend to lose conformations near the fiber surface, approximating to each other and being absorbed by functional groups of the fiber surface. This enhances fiber-matrix compatibilization by epitaxial nucleation of crystals on the fiber surface. Thus, the adhesion of PEEK to the fiber surfaces increases and this in turn leads to better local mechanical properties of the composite [134,262].

The increase in crystallinity was more pronounced in the TMAM than in the BM. One may assume that the broken fibers in the TMAM created more nucleation sites for transcrystallinity as well as presented a larger surface area that was exposed to the temperature and humidity, which in turn could accelerate crystallization kinetics [126,263].

Figure 9.5 summarizes the dynamic indentation modulus and nanohardness of BM and material in the CTMAZ before and after 28 days of aging (the detailed results of this are shown in Appendix E). The BM showed it was less sensitive to hydrothermal aging, as no significant changes in the modulus or hardness were observed. However, comparing the unaged and aged specimens of material from the CTMAZ the hardness increased by 20 % to (0.200 ± 0.009) GPa, while the dynamic modulus increased by 10 % to (3.9 ± 0.2) GPa. The surface mechanical properties of semicrystalline polymers are a function of the degree of crystallinity. The higher the crystalline content the more ordered are the molecular chains, restricting the mobility of the amorphous phase, resulting in stronger mechanical properties of the polymeric surface [216].

Figure 9.5 Average and standard deviation of indentation modulus and hardness of PEEK as joined and after 28 days of hydrothermal aging, obtained by CSM technique.

Contrary to the expected negative effects of corrosion-driven damage on the CF-PEEK surface after aging (Figure 9.4), the extent of galvanic corrosion seems to be restricted to the composite surface, not affecting the mechanical properties of the bulk composite material. Therefore, the local mechanical integrity of aged CF-PEEK seems more influenced by increasing transcrystallinity than the galvanic coupling effect.

A hardness map of the Ti6Al4V rivet after four weeks of aging is presented in Appendix M.

The effects of hydrothermal aging on the microstructure and hence on local mechanical properties of the Ti6Al4V rivet were neglected, once no changes were detected from the typical microhardness profile of the rivet’s mid-plane. Although Ti6Al4V is renowned for its outstanding resistance to corrosive media, due to formation of a stable passive oxide surface layer [84,264], environmentally assisted crack growth in titanium alloy was reported by Bache and Evans [265]. According to the authors, a fully lamellar microstructure was prone to hydrogen embrittlement of the elongated and aligned α-β interface owing to the β stabilizing effect of hydrogen, which was seen to accelerate crack growth under fatigue testing. However, mill annealed and bimodal microstructural variants of Ti6Al4V were shown largely insensitive to such effects [265]. In the case of the friction riveted joints, although the rivet tip presented lamellar structures induced by the joining process (see Figure 6.9), the entire rivet shaft in the joining area was protected by reconsolidated polymer (see Figure 6.10), which inhibited direct contact between the metal and the corrosive media, and therefore the local mechanical properties were not compromised.