RHEOLOGICAL , MECHANICAL AND TRIBOLOGICAL PROPERTIES OF CARBON-NANOFIBRE REINFORCED POLY ( ETHER ETHER KETONE ) COMPOSITES

Poly(ether ether ketone) nanocomposites containing vapour-grown carbon nanofibres (CNF) were produced using standard polymer processing techniques. At high shear rates no significant increase in resin viscosity was observed. Nevertheless, the addition of the CNFs results in a higher melt strength at 360°C. Electron microscopy confirmed the homogeneous dispersion and alignment of nanofibres in the polymer matrix. Evaluation of the mechanical composite properties revealed a linear increase in tensile stiffness and strength with nanofibre loading fractions up to 15 wt% whilst matrix ductility was maintained up to 10 wt%. An interpretation of the composite performance by short-fibre theory resulted in rather low intrinsic stiffness properties of the vapour-grown CNF. Differential scanning calorimetry showed that an interaction between matrix and the nanoscale filler can occur during processing which needs to be taken into account when evaluating the filler properties. Furthermore, unidirectional sliding tests against two different counterpart materials (100Cr6 martensitic bearing steel, X5CrNi18-10 austenitic stainless steel) were performed. The carbon nanofibres were found to reduce the wear rate of PEEK significantly.


Introduction
Ten years after the discovery of carbon nanotubes [1] we still lack a fundamental understanding of the structure-property relationships for the vast number of different nanostructures available today.Polymer nanocomposites enable both an evaluation of the fundamental properties of the reinforcing nanophase and the development of novel composites with improved physical, mechanical and tribological properties.Potential changes in the polymer morphology due to the presence of a nanophase are important to understand and correctly interpret nanocomposite properties.The presented work was aimed at using standard polymer processing techniques to produce nanocomposites of vapour-grown carbon nanofibres (CNF) and a semi-crystalline poly(ether ether ketone) (PEEK) matrix, with regard to the resulting mechanical and tribological properties.Particular emphasis was placed on the nanofibre-matrix interaction during crystallisation to allow a fuller understanding of the composite properties.

Experimental
PEEK powder grade Victrex 450G was purchased from ICI.The vapour-grown CNF were obtained from Applied Sciences Inc, USA.Macroscopic PEEK nanocomposite masterbatches containing 0, 5, 10 and 15 wt% CNF were prepared using a Berstorff corotating twin-screw extruder with a length-to-diameter ratio of 33.The processing temperatures were set at about 380 °C.Tensile bars according to the ISO 179A standard were manufactured and heat-treated for 30min at 200 °C and 2 hours at 220 °C prior to testing.To further increase the orientation of the CNFs, the melt was streched to form fibres, using a Rheometrics Scientific capillary rheometer at 370 °C.For standard shear viscosity measurements a melt pump with attached capillary die is fed with a single screw extruder to ensure the same thermal history throughout the whole measurement.The rheological data was obtained at 360 °C.Differential scanning calorimetry (DSC) analysis was performed using Perkin Elmer equipment, on samples taken from the core of the heat-treated injection moulded specimens.Crystallisation and melting patterns were recorded at 10 °C/min between 50 and 400 °C.In addition, isothermal thermograms were recorded at 303 °C, after the samples were quenched from the molten state at 500 °C/min to the chosen temperature.Field emission gun scanning electron microscopy (FEGSEM) was carried out on fracture surfaces of the injection moulded specimens after tensile testing.For imaging the samples were coated with a 20nm layer of gold.In addition, transmission electron microscopy (TEM) analysis was performed on samples microtomed at room temperature.The wear tests were performed according to ISO 7148-2/section 5.2 with a ball-on-prism test system.Each material combination was tested three times for 60 hours.Light microscopic images were taken from the wear surfaces after the completion of the test.

Results and Discussion
Rheological investigations revealed a modest influence of the composite composition on the shear viscosity at high shear rates [fig 1].Thus, most equipment suitable for PEEK processing can be used without modifications to handle PEEK-CNF composites.As can be seen in figure 3 the addition of CNF significantly reduces the specific wear rate of PEEK.However, the wear rate of the CNF composites appears not to be strongly dependent on the CNF content within the range of 5 to 15 wt%.There is a small optimum in the specific wear rate for a loading of 10 wt% of nanofibres.Such an optimum at about 10 wt% is also described in the literature for glass and carbon fibre reinforced PEEK

Conclusion
PEEK-CNF composites can be successfully manufactured using standard processing technology.An increase in the melt strength without a significant increase of the shear viscosity at high shear rates can be observed.Due to a good dispersion and alignment of the CNFs within the injection-moulded parts, coupled with a good interfacial bonding,the nanofibres act as a conventional reinforcement in the PEEK matrix affecting the matrix ductility at loadings up to 10 wt%.During the spinning of fine nanocomposite filaments, the orientation of the PEEK molecular chains is reduced in the vicinity of the nanofibres which results in a higher degree of crystallisation.As a result, the mechanical properties of such nanocomposite fibres determined from tensile tests show a nonlinear increase with increasing CNF content.Here, the changes in the degree of crystallization of the PEEK matrix need to be taken into account. .The addition of low CNF contents leads to a linear increase in tensile and bending stiffness and tensile yield stress and strength without negatively.The wear rate of the macroscopic PEEK-CNF compounds is significantly reduced due to a lubricating effect of nanofibre wear debris.The small size of the CNFs and the good processability of such compounds suggest new applications in the field of microstructured parts under tribological loading conditions, which cannot be achieved with other presently existing compounds with macroscopic fillers.

Figure 1 :
Figure 1: shear viscosity of PEEK-CNF at 360 °C Tensile tests of the injection moulded samples were performed at room temperature with a Zwick universal testing machine.An increasing CNF content results in a linear increase of the Young's modulus [fig 2].Up to a 10 wt% loading of CNFs the ductility of the compound is not significantly decreased.For these injection-moulded and subsequently heat-treated samples no variations in crystal structure and the overall degree of crystallinity could be observed.