| Graphite-copper composites has been widely used in brake device due to its better braking effect and compressive performance.As the density of copper and graphite are extremely different and incompatible.It is difficult to solve the problem of tissue non-uniformity in conventional casting.Therefore,graphite?flake graphite,copper-coated graphite?-copper composites were fabricated via powder metallurgy technique.However,composite materials of copper and graphite interface wettability problems are particularly important,interface bonding basically determine the performance of composite material.Thus,application and development of graphite are limited in copper composites.In this paper,the influence of graphite particle size and graphite surface metallization on microstructure morphology,density,hardness and frictional wear properties of composites are studied,and the friction and wear mechanism is discussed.The results show that the graphite particle size has significant influence on the microstructure morphology and properties of copper composites.The continuity of copper matrix is increased with the increase of the graphite particle size.Graphite shows strips distribution in the section that is parallel to the pressing direction,and graphite shows blocks distribution in the section that is perpendicular to the pressing direction.And the density and hardness of the composites show the trend of increasing first and then decreasing,while the friction coefficient and wear amount show the trend of decreasing first and then increasing.The composites have the best performance when the particle size of graphite is 270?m,the density is 5.76g/cm3,the microhardness is 107.7HV,and the brinell hardness is 37.5HBS,the average friction coefficient is 0.35,and the wear amount is 7.37×10-6mm3/m under the dry friction condition with the friction velocity of 800r/min.It is found that the wear forms of composites mainly include abrasive wear,peeling wear and oxidation wear by using scanning electron microscope?SEM?to analyze the friction surface of composites.In order to further improve the performance of graphite-copper composites,the above-mentioned graphite with a particle size of 270?m is subjected to surface electroless copper,the influence of the addition of 3wt.%12wt.%graphite/copper-coated graphite on the microstructure and properties of copper composites are investigated.The results show that the copper coating layer forms a spatially three-dimensional network structure,which improves the overall strength of the composites.With the gradually increase of graphite/copper-coated graphite content,the density and hardness of the composites are decreased,but the density and hardness of the copper-coated graphite samples are better than those of the graphite samples.The addition of copper-coated graphite improves the average friction coefficient of copper composites and reduces the amount of wear to a certain extent.At the friction speed of 800r/min,the average friction coefficient of the composites is 0.314 and 0.335 when the graphite/copper-coated graphite content are 9wt.%,and the lowest total wear amount is 8.05×10-6mm3/m and 7.83×10-6mm3/m respectively.In the process of friction and wear,the main wear defects of graphite-copper composites are pits and furrows,while the wear forms are mainly the stripping,abrasive wear and oxidation wear.The main wear defects of copper-coated graphite composites are shallow furrows,while the wear forms are mainly abrasive wear and oxidation wear. |
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