Research On The Preparation And Performance Of Advanced Wear-Resistant Copper Matrix Composites

With the development of electronic, mechanical, aeronautic and aerospace industries, requires imperative demand of developing functional materials with not only high electrical conductivity, thermal conductivity, and excellent mechanical properties, wear-resistant, low coefficient of thermal expansion. Copper and copper alloys are the traditional high-conductivity, thermal conductivity materials, but because of lack of strength and heat-resistant Performance, its application is greatly restricted.In this thesis, to developing copper matrix composites with high electrical conductivity, thermal conductivity and good wear-resistant as the objective, the components of the composite systems and the preparation technique were designed. SiC particles reinforcement of Cu(-Cr)matrix composites were fabricated by mechanical alloying (MA)plus cold pressure forming and repressing and resintering process to find the best fabrication process and meet the high-strength, high-conductivity and excellent wear-resistant properties. The mechanical alloying process of powder and composites microstructure, mechanical, physics, wear and tribological properties were systematically observed and analyzed by means of SEM, XRD, TEM, and other test instruments.All of these works lay a solid foundation for development the utilization area of novel advanced wear-resistant copper matrix composites. The works include the following aspects:1. Cu (-Cr) supersaturated solid solutions were prepared by mechanical alloying, whereas such a binary system is known to be immiscible in solid and liquid state. The crystalline of Cu-Cr alloy powders were significantly refined and lattice strain were have serious lattice distortion during MA process. This suggests that the high density crystal defects, solute elements chemical potential decreasing and crystalline refinement due to MA may play a critical role on the supersaturated solid solution of Cr in Cu.2. Densification Process of Cu-Cr alloy powders was studies by means of cold pressing-sintering- repressing-resintering method. The influence of different pressure and temperature on the relative density was discussed systematically. The results showed that pressing and repressing can significantly improve the relative density, however, resintering did not evidently contribute to the relative density, and its main role reflected in the performance improvement and optimization.3. Through to test the different composition Cu-Cr alloys properties to analyze the influence of composition, temperature and repressing-resintering on the alloys mechanical and physical properties. Hardness and tensile strength of Cu-1.2wt%Cr alloy increased with the rising sintering temperature, and reached the peak value when 850℃and it decreased with further temperature rising due to Cr particles growth and coarsening, electrical conductivity and thermoconductivity slowly increase with the increase of sintering temperature. Over-saturated Cr atoms are the dominant factors accounting for strengthen by precipitation and grain refinement strengthening.4. Cu/SiC composites were fabricated by powder metallurgy. The effect of SiC volume fraction and sizes on the mechanical properties, physical, wear properties and microstructure of composites were investigated. On this base, the influence of various sizes reinforced particles on the properties of composites was investigated. The results indicated that under the same preparing parameters, the composites with SiC particle for 10μm possess the highest ultimate tensile strength, beyond 265.7MPa, and their breakage is due to tearing of matrix near Cu-SiC interface. Composites with large particle (≥21μm) possess finite strength owing to interfacial area limited and the space between SiC particles larged. Compared with tensile strength varying, the friction and wear properties of Cu/SiC composites obviously differ with the size of SiC changing. When the applied load was lower 120N, the bigger SiC are helpful to improve the wear resistance of Cu/SiC composites. However, composites reinforced with larger SiC particulates increase the counterpart steel wear rate, and wear mechanism is mainly abrasion. When the applied load increased, because larger size SiC particles mashed, the load carrying capacity decreased, which lead to delamination wear occurring. Because the SiC particle size reduced, Cu-SiC interfacial area increased and space between SiC particles increased, which impede the movement of electronic effects, leading to electrical conductivity and thermoconductivity decreased. The coefficient of thermal expansion composites increases with enhancing particle size. With the increasing in SiC particle size, the thermal stress trend to be improved and critical temperature with causing the coefficient of thermal expansion catararophe droped.5. SiC particles were coated Ni or Cu film by electroless deposition to modify Cu-SiC interfaces. The effects of modification different metals(Cu, Ni)on SiC particles surface on the interfacial bonding were investigated. The results indicate that the interface of composites is compact, improves interfacial bonding strength and loads can be transferred effectively between matrix and SiC by the surface modification with Cu and Ni. Therefore, the densification, bulk hardness and tensile properties of composites are improved. Matrix and coating Ni atoms can diffuse each other to form continual a film of solid solution, which is in favor of improving mechanical properties of composites. In the course of friction and wear, SiC particles effectively play the role of load carrying capacity and thereby improving the tribological properties.6. In order to combine particulate reinforcement with precipitation reinforcement, SiC particle were used to be fabricated Cu-Cr matrix composites. As a result, SiC particle can be more effective enhancing composites strength with the improvement of Cu-Cr matrix strength, and change the mechanism of the fracture, while softening temperature had large increased.7.The wear behavior of (Cu-Cr)/SiC composites under different loads,sliding velocities and distances was studied. The results showed that the wear resistance properties of composites increased with volume fraction of SiC particles, and delayed severe wear to hanppen. The wear mechanism of (Cu-Cr)/SiC composites changed with the extrinsic factors, such as load, sliding velocity and distance. The wear rate may increase abruptly at certain critical values. In the mild wear stage, the main wear mechanism was abrasion and oxidation wear. In the severe wear stage, mechanically mixed layer was detached and composites was softened by the frictional heat, and therefore massive materials transfer resulted in the abrupt increase of wear rate.8. The elevated-temperature wear behaviors of SiC particulate reinforced Cu-Cr matrix composites were studied. The results indicated that the critical temperature to occurring serious wear was increased effectively by the addition of SiC. Along with temperature elevation, the oxide film cracked and fell off from worn surface, leading to serious wear, and wear mechanism is mainly delamination and adhesive. The wear rate and friction coefficients of composites were reduced by further addition of Gr, improving the tribological characteristic in elevated-temperature environment.9. The nanometer SiC particles reinforced (Cu-Cr)/SiC composites were investigated. The results indicated that the hardness, tensile strength, electrical conductivity and friction and wear properties of composites were improved with the increase of nanometer SiC particles content when nanometer SiC particles was lower(0.5vol%-3vol%). When nanometer SiC particles content was 5vol%, the nano-particles were agglomerated, which would be harmful to the mechanical properties and lead to fracture mechanism of composites changing. In this thesis, Cu-Cr powders were fabricated by mechanical alloying, and developed forming techniques,obtained the best forming parameters.SiC reforcement Cu and Cu-Cr matrix composites, and the influence of different content, sizes of SiC particles and matrix strength on the properties of composites were investigated. The interface of Cu/SiC was optimized by electroless deposition, and the effects of interfacial modifying on the properties were stuied.The friction and wear behaviors of the composites under room-and elevated-temperature conditions were studied systematically, and micro-wear mechanism were analyzed. The reinforcement mechanism and strengthening effect of nano-SiC particles on Cu-Cr alloy was also studied elementarily. It is of great significances in theory and practice to develop the new type of wear-resistant copper matrix composites and to enrich the tribological research.