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).

Copper-graphite composites were fabricated via vacuum hot pressing sintering, with three types of graphite phase morphologies: powder (particle size ~5 μm), flaky graphite (particle size 445–636 μm), and near-spherical granular graphite (particle size 200–300 μm). The influence of graphite phase morphology on the friction and wear properties and underlying action mechanism of copper-graphite composites was evaluated using Al₂O₃ ceramics as the friction pair, and the reliability of the materials under external loading was also explored.

The results demonstrate that varying the graphite phase morphology alters the distribution patterns of the graphite phase and metallic copper within the material, consequently affecting both the tribological and mechanical performance of the composite. With the graphite phase content held constant across all samples, as the graphite phase morphology transitions from micron-scale powder to anisotropic bulk flaky graphite, and then to large-size near-spherical granular graphite with excellent isotropy, the number of weak interfaces formed between the graphite phase and the copper matrix gradually decreases, while the three-dimensional continuity of the metallic copper matrix improves significantly. When the material undergoes damage under external loads, the crack propagation stress initiated at the graphite-copper matrix interface can be promptly absorbed and passivated by the continuous copper matrix, substantially enhancing the material’s resistance to crack failure. When the graphite phase adopts a near-spherical granular morphology, the flexural strength, compressive strength, fracture toughness, and impact toughness of the composite reach 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, respectively.

Furthermore, graphite phase morphology exerts a critical influence on the tribological performance of the composites. When the graphite phase exists as powder, a higher density of weak interfaces between the graphite phase and metallic copper severely disrupts the continuity of the copper matrix. Under frictional force, the fragmented copper particles are readily peeled off and enter the friction interface, forming "three-body wear" with the friction pair and causing substantial material wear. When the graphite phase is in flaky form, the aggregation of graphite phases relatively increases, which improves the continuity of the copper matrix and avoids the severe wear observed in composites with powder-type graphite. However, flaky graphite has a large sheet-like structure with strong anisotropy; when surface flaky graphite is worn away, or a large number of flaky graphite sheets are oriented perpendicular to the material surface, large fluctuations in the friction coefficient between the friction pair will occur. When the graphite phase is near-spherical granular, the uniformly dispersed graphite phase, three-dimensional continuous copper matrix, and the soft-hard alternating structure formed by the lubricating graphite phase and load-bearing copper matrix endow the copper-graphite composite with a low and stable friction coefficient and excellent friction-reducing and anti-wear properties. In this study, using an Al₂O₃ pin as the friction counterpart, the friction coefficient and wear rate of the composite are as low as 0.13±0.02 and 5.4×10^−6 mm³/(N·m), respectively.