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Study on the Preparation and Performance of Graphite/Al Composite Material

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Tutor: YaoGuangChun
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: Graphite,Aluminum alloy ZL111,composite materials,Mixing casting,Coating,CeO2,Zr
CLC: TB333
Type: PhD thesis
Year:  2008
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Graphite-particle-reinforced aluminum matrix composite was a good self-lubricating material compared with the Sn-Pb, Cul-Sn, and Al-Pb alloys. It is an environment friendly material with excellent ntifrictional performance and high specific strength, so it can be applied in many fields in the future. But the low density of the graphite particles and the poor wettability with aluminum lead to a main study of fabricated technique focused on the powder metallurgic method and extrusion-casting method. But the obvious shortcomings of these two methods were higher cost and relatively small-scale production. The liquid mechanical stirring technique was paid more and more attention due to its simpler process and lower cost. In this paper, a new graphite-particle-reinforced aluminum matrix composite was fabricated by liquid mechanical stirring technique, in which the graphite was coated by a layer of nano-oxide film, and the basic performance of the composite was systemic studied.To improve the wettability of graphite and molten aluminum, the coating method and mechanism of oxide/graphite were studied firstly. Based on a detailed analysis of the mechanism for oxide coated reaction process and the hydrolyzation process of the inorganic salts, the preparation condition was studied by using the Na2Si03, Ce (NO3) 3 and ZrOCl2 as precursors. The influence of several factors on the characters of oxide/graphite was analyzed by study of the coating ratio and oxidation ratio.By the analysis of oxidation ratio, micrograph of composite particle, it was found out that factors such as pH value, coating temperature, concentration of premonitory matter solution mainly were the dominate factors that affect the coating layer forming. With the optimum parameters, the obtained composite particles had many changes compared with uncoated graphite, such as the decrease oxidation ratio. And a C-O-M bond was confirmed between oxide and the graphite.The preparation process of coated graphite with CeNO3 and ZrOCl2 as precursor was studied with orthogonal experiment. The suitable condition was:pH=6, reaction temperature was 80¡æ, aging time was 1h, concentration of precursor was 0.2M. particle size of graphite was 50¦Ìm.In order to decrease the porosity of the composite material, the vacuum condition was used during the stirring technique process. So material with lower porosity and better mechanical performance could be fabricated. The optimistic processing parameters were:stirring temperature was 560¡æ-600¡æ, stirring time was 5-8min, stirring speed was 1000-1200r/min, temperature of the vacuum furnace was 600¡æ-630¡æ, degree of vacuum was -0.02 MPa--0.04MPa, time for vacuum was¡Ý3min.XRD was used to investigate the interface of oxide/graphite and the Al-matrix. The results showed a weak reactive wettability between SiO2 and Al, a strong reactive wettability between CeO2 and Al and also a weak reactive wettability between ZrO2 and Al, which proved that two oxides of CeO2 and ZrO2 could improve the wettability between graphite and molten aluminum and during fabrication process without corrosion of graphite by molten aluminum.The wear resistance, tensile strength and corrosive characteristics of the composites materials with different content and kinds of oxide/graphite particle were measured. The result showed that the tensile strength, the friction factor and the weight loss after wearing test descended along with the increase of graphite content. The anti-corrosion performance of CeO2 graphite and ZrO2 graphite reinforced composite materials was higher than that of alloy and SiO2 graphite reinforced composite materials. There showed a decreasing tendency of anti-corrosion performance with the increase of graphite content. The addition of graphite particles improve the possibility of pitting in composite materials.The results of wearing test showed that the oxide/graphite composite materials presented a higher self-lubricating performance comparing with Al alloy. The friction factor and the weight loss descended along with the increase of graphite content. But the friction factor increased when the content exceed 6%.Due to the chemical bond of CeO2/graphite/ZrO2 and reactive wettability with Al matrix, trace compounds were formed in the interface between Al-matrix and oxide. These compounds lead to an improvement of the mechanical and the anti-corrosion performance for the composite materials.
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