Small friction coefficient can be obtained in the sliding surface with the dihedral angles between 30° and 60° due to mild contact conditions. Anisotropic friction behavior of aligned and oriented graphite flakes/copper composite

Abstract Graphite flakes as lubricant are widely used for friction materials, but little is known about the friction behavior at sliding surface oriented at some angle to the basal plane of graphite. In this paper, oriented graphite flakes/copper (volume ratio of 7/3) composite was prepared by vacuum hot-pressing sintering. The friction coefficient evolution of the composite with the dihedral angle (φ) between sliding surface and graphite basal plane was estimated based on the numerical solutions of the anisotropic stiffness at the contact plane, and measured experimentally. Theoretical estimates suggested an increase of the friction coefficient induced by adhesion with raising the dihedral angle, and a decreased one caused by ploughing. A pronounced trend of the former happened with the dihedral angles of more than 75°, while for the latter it was conspicuous with the dihedral angles of less than 15°. Through ball-on-disk friction experiments, the friction coefficients were fairly low with the dihedral angles from 30° to 60°, which was consistent with the theoretical prediction, and a monotonous decrease of the wear rate was exhibited with increasing the dihedral angle due to increased shear modulus and hardness. This anisotropic friction behavior may provide new ideas for the designs of friction materials.