Scratch resistance of polycarbonate containing ZnO nanoparticles: effects of sliding direction.

Abrasive wear resistance of injection molded polycarbonate (PC) and polycarbonate + zinc oxide nanocomposites containing 0.5 wt% ZnO nanoparticles was determined as a function of the sliding direction with respect to injection flow. First we have performed single scratch testing under progressively increasing the load applied. Then sliding wear testing consisting of 15 successive scratches along the same groove was performed. Neat PC shows anisotropic behavior, with instantaneous penetration depth more than 50% higher in the direction parallel to the melt injection flow than in the transverse direction. Viscoelastic recovery after scratching of neat PC is also higher in the longitudinal than in the transverse direction, hence final residual depth values are similar in both directions. The addition of ZnO nanoparticles reduces the instantaneous penetration depth in the longitudinal direction but lowers viscoelastic recovery so that the residual depth is large. In the transverse direction, the scratch resistance is similar for neat PC and the nanocomposite. Dynamic mechanical analysis, SEM/FIB results and wear mechanisms from SEM observations of wear scars are discussed. Below the glass transition region the nanocomposite has distinctly higher storage modulus E' than PC--a clear reinforcement effect. However, the addition of ZnO nanoparticles to the polymer increases the material brittleness at room temperature by a factor of 2.72.