Structural, thermal, and mechanical characterisation of PEEK-based composites in cryogenic temperature

Thermal, thermo-mechanical and mechanical properties of four different commercially available polyetheretherketones (PEEK) based materials were investigated. PEEK matrix was either modified and/or reinforced with carbon fibres, graphite and/or PTFE. Impact strength was measured at three different temperatures: 25 °C, −100 °C, and −195 °C. At 25 °C, thermal stability and mechanical properties, including the elastic modulus, compression, and impact strength, were enhanced with the addition of carbon fibres. Matrix modification had a minor impact on thermal stability, while the mechanical properties decreased, except for impact strength. At −100 °C, the mechanical properties of the neat polymers were improved, including increased impact strength by 20% compared to values at 25 °C. Addition of fillers hindered the rise of impact strength due to complex mechanisms caused by different coefficients of thermal expansion of reinforcements and matrix. At −195 °C, the significant increase of impact strength was revealed for unmodified PEEK reaching 30 times higher values than at 25 °C, while matrix modification suppressed the rise of impact strength. The scratch test indicated the superior behaviour of unfilled PEEK during the tested load range (up to 15 N), while the effect of the fillers was observed at much lower load threshold of 7 N.