Study Of New Graphite Particle/Al Alloy Matrix Self-lubricating Bearing Material

The loading capacity and reliability of the bearing material in the automobile and the high-speed locomotive are higher required by the developmental characteristics of energy conservation and environmental protection, safety and comfort in modern traffic. The layered composite bearing material is considered to be the most applied potential and extended valuable bearing material. The research focuses on the material preparation of bearing wear resistant layer and the roll cladding technology and theory of bearing. At present, the copper-lead alloy and high tin-aluminum alloy/steel cladding bearing are mainly used in the industrial production field in China. The several problems exist in these bearing materials for example the poor boundary lubrication ability, low strength and environmental pollution and so on. So, it is the important theoretical and practical application value for the high-strength self-lubricating bearing material that the new technology of high-property composite bearing material is developed. Combined with the National 863 high technology research and development plan (2004AA33G060) project named "The rapid transit key metal materials and application", the preparation technology and theory of the graphite particle reinforced Al alloy self-lubricating bearing material was systematically studied in this paper. The achievements are as follows:First, according to the difficulty of the poor wettability between the graphite particle and Al melt, the electroless copper plating on the graphite particle surface was studied. The influence of solution compositions including SnCl₂ and technological parameters including reaction temperature on the coating quality was importantly investigated. The optimal processing parameters are determined. The copper-coated graphite powder contains 34mass% Cu on the optimal conditions. The homogeneous rose pink copper coating bonds compactly with the graphite powder by scanning electron microscopy.The test result shows that the contact angle is 27°between the copper-coated graphite and Al melt. The result displays that the energy which needs to overcome the surface tension in order to immerse entirely the copper-coated graphite particles into Al melt decreases two orders of magnitude. Even if effect of the gravity force and the floating force is considered, it is only twentieth-value of the untreated graphite particle. It is illustrated that the copper coating on the graphite particles obviously improves the interface wettability of the particle surface and Al melt and creates a good condition to fabricate the graphite particle reinforced Al alloy matrix composites.The optimal processing parameters are determined by systemic investigating the preparation process of the graphite particle reinforced Al alloy matrix composites, and the composites in which the graphite powder is more and uniformly distributes is fabricated. The tribological property is greatly improved with increasing of graphite content. The friction coefficient of the composites containing 8mass% graphite powder is 0.32, decreasing by 30%, and its wear rate decreases by 76% in contrary of the Al alloy matrix.The solid lubricating film composed of Fe oxide, graphite and a little Al oxide is formed on the friction surface by the electron microscopy analysis of the friction surface of the composites. Moreover, this solid lubricating film is more smooth, and the scuffing and scratch are fewer on the friction surface with increasing of content of graphite powder. In addition, it is observed the interface between graphite particles and Al alloy matrix compactly bonds. It is illustrated that Cu and Mg can well improve the interface wettability of graphite particle and Al melt for high content of Cu and Mg in the interface.The hot rolling process of the self-lubricating graphite particle reinforced Al alloy matrix composites was studied for the more uniform distribution of graphite particles in the composites and the more excellent properties. The optimal hot-rolling technological parameters and the process control criterion are determined. The result shows that this material is with good plasticity. Vickers hardness of the rolled strips by hot rolling increases by 46%; its tensile strength increases by 46% and elongation increases by 100% compared with the casting composites. The friction coefficient of the rolled strips is much lower than the casting composites in the dry sliding wear, and furthermore the friction stability is better than the latter. In contrast with AlSn20Cu bearing material, Vickers hardness of the rolled strips increases by 145%, its tensile strength increases by 145%, and its dry friction coefficient decreases by 21%, and its oil lubricating friction coefficient decreases by 16%.The hot cladding rolling process was utilized to prepare first successfully the self-lubricating bimetal Al/steel cladding bearing material. The optimal process conditions are that the rolling temperature is 450℃, the reduction per pass is 70% and then the rolled strips are annealed at 350℃for 1h. After the hot rolling, the metallurgical bonding between Al and steel is realized. The formability of the self-lubricating bimetal Al/steel cladding bearing material was detected. The average cupping value of the self-lubricating bimetal Al/steel cladding bearing material is 8.75mm. It is illustrated that the deforming ability of the material is very good. It is first succeeded that the new type self-lubricating bearing part is preproducted in accordance with the main bearing of Shanghai Volkswagen's Santana car. In contrast with the A500 made in American Federal-Mogul Company, Vickers hardness of the new self-lubricating bearing increases by 96%, its tensile strength increases by 125%, and its dry friction coefficient decreases by 21%, and its oil lubricating friction coefficient decreases by 13%. The result shows that the graphite particle/Al alloy self-lubricating bearing has very good market prospect and popularization value by this process.