石墨相形态对铜/石墨复合材料摩擦学性能和可靠性的影响

采用真空热压烧结工艺制备了石墨相形态为粉体(粒径约5 μm)、鳞片状(粒径445~636 μm)和近球形颗粒状(粒径200~300 μm)的铜/石墨复合材料，考察了以Al₂O₃陶瓷为摩擦副条件下石墨相形态对铜/石墨复合材料摩擦磨损性能及作用机制的影响，并探讨了材料在外载作用下的可靠性. 结果表明:石墨相形态不同时，石墨相和金属铜在材料中的分布方式也随之改变，进而影响到材料的摩擦学性能和力学性能. 在保持复合材料中石墨相含量不变的基础上，将石墨相形态从微米级粉体转变为各向异性的大块鳞片状石墨，再转变为各向同性较好的大尺寸近球形颗粒状石墨时，石墨相在材料中与金属铜形成的弱界面含量逐渐减小，金属铜的三维连续性变得更好. 材料在受到外载破坏时，从石墨相与铜基体界面萌生裂纹的扩展应力可被连续金属铜及时吸收钝化，使材料抵抗裂纹破坏的能力明显提高. 当石墨相为近球形颗粒状时，材料的抗弯强度、抗压强度、断裂韧性和冲击韧性分别高达155.4±3.6 MPa、353.5±24.7 MPa、5.3±0.6 MPa·m1/2和4.0±0.4 J/cm2. 此外，石墨相形态对材料的摩擦学性能也有重要影响，当石墨相以粉体形态存在时，石墨相与金属铜间形成的弱界面越多，铜基体的连续程度被石墨显著割裂，在摩擦力作用下割裂的铜颗粒易被剥离进入摩擦界面，与摩擦副形成“三体”磨损，导致材料的大量磨损. 当石墨相以鳞片状形态存在时，石墨相的聚集程度相对增加，使得金属铜的连续程度相对提高，可避免发生类似复合粉体形态石墨材料的磨损. 但是，鳞片状石墨呈大块片层状，形状各向异性，随着材料表面鳞片石墨的摩擦损耗，或者垂直于材料表面的鳞片石墨较多时，将造成摩擦副间摩擦系数较大的波动. 当石墨相为近球形颗粒状时，较为均匀的石墨相空间分布状态、三维连续结构的铜基体和润滑相/承载基体呈现的软/硬交替结构使得铜/石墨复合材料具有低且平稳的摩擦系数以及优异的减摩抗磨性能. 本文中以Al₂O₃栓为摩擦对偶时，复合材料的摩擦系数和磨损率分别低至0.13±0.02和5.4×10?6 mm3/(N·m). 

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·m1/2 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).