Influence of Graphite Morphologies on Tribology Properties and Reliability of Copper/Graphite Composites

Copper/graphite composites with various of graphite morphologies containing powder (~5 μm), flake graphite (445~636 μm) and near spherical graphite (200~300 μm) were prepared by vacuum hot pressing sintering process. The influences of the graphite morphologies on the friction-wear properties and mechanisms of copper/graphite composites coupled with Al₂O₃ pin were investigated, and the reliability of the materials under loading was discussed as well. The results showed that the distribution of graphite phase and copper in the material also changed with various of graphite morphologies, which affected the tribological properties and mechanical properties of the material. On the basis of keeping the graphite phase content in the composite unchanged, when the graphite phase morphology was transformed from micron powder to large flake graphite with anisotropic structure, and then to large nearly spherical particle graphite with isotropic structure, the number of the weak interfaces formed by graphite phase and copper in the material decreased gradually, and the copper matrix became more continuous in three-dimensional space. When the graphite phase was near spherical particles, the bending strength, compressive strength, fracture toughness and impact toughness of the material were as high as 155.4±3.6 MPa、353.5±24.7 MPa、5.3±0.6 MPa·m^1/2 and 4.0±0.4 J/cm². In addition, the morphology of graphite phase also had a significant impact on the tribological properties of the material. When the morphology of graphite was powder, the more weak interfaces between the graphite phase and the metal copper were formed, and the continuity of the copper matrix was significantly separated by the graphite. Under the action of friction sliding, the separated copper particles could be stripped into the friction interface easily, and then forming the “three-body” wear within friction counterpart, thus resulting in a large abrasion of the material. When the graphite phase existed as flakes, the aggregation degree of the graphite phase was relatively increased, so that the continuity of the metal copper was relatively improved, which could avoid the occurrence of wear process similar to the composite graphite powder in the material. However, the flake graphite was lamellar with typically anisotropic. With the loss of the flake graphite on the frictional surface of the material or the large number of flake graphite perpendicular to the surface of the material, the friction coefficient between the friction counterpart fluctuated greatly. The composites could have relatively uniform spatial distribution of graphite phase and three-dimensional continuous structure of copper matrix when the graphite phase was nearly spherical, and it presented a soft/hard alternate structure of lubricating phase/matrix. Therefore, the copper/graphite composite had low and stable friction coefficient, excellent wear resistance and mechanical properties. The friction coefficient and wear rate of the material coupled with Al₂O₃ pin could be able to as low as 0.13±0.02 and 5.4×10⁻⁶ mm³/(N·m).