| High reinforcement volume fraction is important for discontinuously reinforced copper-matrix composites to obtain sufficient strengthening and low coefficient of thermal expansion (CTE), which are required for electronic packaging, electrical contacts, motor brushes and electrodes. The maximum reinforcement volume fraction depends on the composite fabrication method and the reinforcement morphology. In this dissertation, copper-matrix and brass-matrix composites were made by powder metallurgy (P/M). The reinforcements included molybdenum particles, silicon carbide whiskers and titanium diboride platelets. The method used was the coated filler method, in which a reinforcement coated with the matrix metal was used. In contrast, conventional P/M uses the admixture method, which involves a mixture of matrix powder and reinforcement. For all the composite systems, the coated filler method was found to be superior to the admixture method in providing composites with lower porosity, greater hardness, higher compressive yield strength, greater abrasion resistance, greater scratch resistance, lower CTE, higher thermal conductivity and lower electrical resistivity, though the degree of superiority was greater for solid-phase P/M (for copper) than transient liquid-phase P/M (for brass), and was greater for high than low reinforcement contents. In the coated filler method, the coating on the reinforcement separated reinforcement units from one another and provided a cleaner interface and stronger bond between reinforcement and matrix than what the admixture method could provide.; The highest reinforcement content attained in dense composites ({dollar}<{dollar}5% porosity) made by the coated filler method was 70 vol.% Mo, 54 vol.% SiC and 60 vol.% TiB{dollar}sb2{dollar}, compared to the values of 60 vol.% Mo, 33 vol.% SiC and 50 vol.% TiB{dollar}sb2{dollar} for composites made by the admixture method. In particular, the SiC whisker content of 54 vol.% is much greater than the previously attained maximum of 40 vol.%, thus resulting in exceptional hardness (260 Brinell).; Among Cu/Mo, Cu/TiB{dollar}sb2{dollar} and Cu/SiCw at the same reinforcement volume fraction (50%), Cu/Mo gave the lowest CTE, highest thermal conductivity and lowest electrical resistivity, while Cu/SiCw gave the greatest hardness and Cu/TiB{dollar}sb2{dollar} and Cu/SiCw gave the highest compressive yield strength. Compared to Cu/SiCw, Cu/TiB{dollar}sb2{dollar} exhibited much higher thermal conductivity and much lower electrical resistivity. |
