Tensile behaviour of poly(ether-ether-ketone) (PEEK)/Ni foam co-continuous phase composites

In this work, eight types of poly (ether-ether-ketone) (PEEK)/Ni foam co-continuous phase composites (PEEK–Nis) were fabricated by hot pressing. Unfilled PEEK polymer was also prepared in two different methods: P-1 was prepared by the same method with PEEK–Nis, and P-2 was synthesised via extrusion. The tensile properties and fracture processes of PEEK–Nis were investigated and compared with those of unfilled PEEK polymer. Results showed that the stress–strain curves of PEEK–Nis exhibited elastic deformation, yielding and necking. Nickel foam could improve the tensile toughness and elongation-at-break of PEEK–Nis by increasing the number of edges or decreasing the pore quantities of Ni foam. The Young's moduli of PEEK–Nis were related only to metal content. The Young's moduli of PEEK–Nis increased with Ni volume content, and this relationship showed a secondary upward trend. The fracture surfaces of 10 kinds of materials after tensile tests were observed through scanning electron microscopy. Mirror, mist and hackle zones were found on the fracture surfaces of PEEK–Nis and the unfilled PEEK polymer. Morphological analysis showed that a crack could begin in one site and propagate within the material. However, the Ni prism structure could effectively decelerate crack propagation and even stop the crack, causing the crack to restart in the mirror zone. Two main parameters influence the tensile behaviours of PEEK–Nis including the quantity of Ni edge (or PPI value) and edge thickness. The Ni edge quantity of the foam played a positive role in the tensile behaviour of PEEK–Nis. However the effect of edge thickness has two side effects. In Ni foam, intergranular fracture replaced plastic fracture and degraded tensile performance if edge thickness exceeded 70 μm. Amongst all PEEK–Nis studied in this work, PEEK–NiA-1 exhibits the best performance and its tensile toughness was more than 1.6 times that of P-1.