A COMPARATIVE-STUDY OF THE COATED FILLER METHOD AND THE ADMIXTURE METHOD OF POWDER-METALLURGY FOR MAKING METAL-MATRIX COMPOSITES

Copper-matrix composites were made by powder metallurgy (PM). The rein forcements were molybdenum particles, silicon carbide whiskers and tit anium diboride platelets. The coated filler method, which involves a r einforcement coated with the matrix metal, was used. In contrast, conv entional PM uses the admixture method, which involves a mixture of mat rix powder and reinforcement. For all the composite systems,the coated filler method was found to be superior to the admixture method in pro viding composites with lower porosity, greater hardness, higher compre ssive yield strength, lower coefficient of thermal expansion (CTE), hi gher thermal conductivity and lower electrical resistivity, though the degree of superiority was greater for high than low reinforcement con tents. In the coated filler method, the coating on the reinforcement s eparated reinforcement units from one another and provided a cleaner i nterface and stronger bond between reinforcement and matrix than the a dmixture method could provide. The highest reinforcement content attai ned in dense composites (<5% porosity) made by the coated filler metho d was 70 vol % Mo, 60 vol % TiB2 and 54 vol % SiC. The critical reinfo rcement volume fraction above which the porosity of composites made by the admixture method increases abruptly is 60% Mo, 42% TiB2 and 33% S iC. This fraction increases with decreasing aspect ratio of the reinfo rcement. Among Cu/Mo, Cu/TiB2 and Cu/SiC at the same reinforcement vol ume fraction (50%), Cu/Mo gave the lowest CTE, highest thermal conduct ivity and lowest electrical resistivity, while Cu/SiC gave the greates t hardness and Cu/TiB2 and Cu/SiC gave the highest compressive yield s trength. Com pared to Cu/SiC, Cu/TiB2 exhibited much higher thermal co nductivity and much lower electrical resistivity.