Influence of orientation of long fibers in carbon fiber–polyetherimide composites on mechanical and tribological properties

A composite with unidirectional carbon fibers (CF) (≈80 vol.%) and high performance thermoplastic polyetherimide matrix (PEI) was developed and evaluated for various mechanical properties as a function of fiber orientation angle (0°, 15°, 30°, 45°, 60°, 75° and 90°) with respect to loading direction. It was observed that in general, these properties deteriorated with the increase in fiber orientation angle, though composite with 90° fibers was not poorest in mechanical properties. Overall, CF reinforcement in 0° (parallel to loading direction) enhanced all strength and modulus properties of PEI significantly. Tribo-evaluation in adhesive wear mode on pin on disc machine against mild steel under different loads indicated that the coefficient of friction (μ) increased with fiber angle with respect to sliding direction and decreased with load, in general. Very low specific wear rate (K0), in the order of 4–7 × 10−16 m3/Nm was observed for 0° orientation while fibers in 90° showed wear rate in the range of 14–25 × 10−16 m3/Nm, which was approximately four times higher than the earlier case. Overall fiber reinforcement in 0° orientation proved beneficial from both strength and tribo-performance point of view. SEM proved useful to correlate wear rate with wear mechanisms. Gradual decrease in extent of wear thinning of long fibers and increase in brittle fracture of fibers and their peeling off tendency from the matrix with increase in angle of fiber orientation were observed to be the main reasons for the deterioration of tribo-properties of composites with increase in fiber orientation in composites from 0° to 90°.