| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 93 === Copper and copper alloys are well known for a combination of good corrosion resistance in a variety of environments, excellent workability, high thermal and electrical conductivities, and attractive mechanical properties at low, normal and moderately elevated temperatures. Copper and copper alloys thus are widely used in the electronic application and bearing materials. Metal matrix composites combine the advantages of the matrix and reinforcement to obtain the required properties, to increase strength, toughness and anti-wear. Among them, the composites incorporated with particles reinforcement, owing to better isotropic behaviors, were mostly used as bearing materials. However, the presence of foreign phase in composites often results in a worse corrosion behavior. Furthermore, in a wear and corrosion coexistence environment, the composites in general experience severe wear loss. This is due to the synergic effect of wear and corrosion which are more complicated than wear or corrosion alone.
The main object of this thesis is to adopt a hot pressing process, a powder metallurgy route, for manufacturing Cu metal matrix composites (Cu MMCs) with four additional particulates, i.e., SiCp, graphite, WCp and Cop. The experimental results verify the method can acquire high densification, sound composites.
The wear performance of Cu MMCs was studied using two different wear examinations associated with two different composites in this study. The Cu/SiCp/graphite composites uses a reciprocal (back and forth) wear machine while the Cu/WCp/Cop composites uses a block-on-ring wear tester. The sliding tests were performed under ambient conditions and without lubricant. Selection of foreign particles is based on the fact that SiC and WC phases can improve the bulk hardness of composites, graphite is commonly used in bearing Al matrix composites acting as a lubricant, and Co is helpful to increase shock resistant. Traditionally, harder composites exhibit better wear resistance due to their capability to endure contact load from being plastic deformation. By using different wear tests, the harder Cu/SiCp and Cu/WCp composites, indicate different results. The former get worse wear resistance than pure copper, while the later better do. Although graphite lowers hardness of composite, it increases wear resistance since graphite phase can lubricate a contact surface during friction. Besides, the addition of a Cop phase is also beneficial for improving wear resistance of Cu/WCp/Cop composites.
In this work, the corrosion test also executed to identify corrosion behavior after particles were incorporated into a copper matrix. The corrosion measurement in this study includes static weigh loss immersed in a corrosive solution and potentiodynamic polarization. The corrosive solution used in this examination was unified as 3.5 wt. % NaCl at pH 6.7 (simulative sea water). The electrochemical measurements were carried out using a three electrode system. Experimental results demonstrate that the static corrosive wear loss of Cu/SiCp/graphite increases, meanwhile, that of Cu/WCp/Cop composites decreases. Additionally, the mixing of particles into copper matrix depressed the anti corrosion behavior of Cu MMCs. The worse corrosion behavior of Cu MMCs can be explained mainly owing to the local corrosion (crevice and pitting) and the galvanic corrosion, while the influence of broken passive films can be referred to another factor.
The synergistic effect of wear and corrosion was also studied. The test was adopted by taking Cu/WCp/Cop composites for wearing in the same 3.5 wt. % NaCl solution at pH 6.7. The corrosion potential effect on the loss of composites can be concluded as follows: Cu/WCp/Cop composites exhibit a very small wear corrosion rate at a cathodic potential (relative to OCP) because corrosion seems negligible and mechanical wear dominates wear loss of the composites. However, at an anodic potential, the wear corrosion rate dramatically increases with an increase of applied potential owing to a synergistic effect of wear and corrosion.
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