Cu-based and Cu-alloy-based diamond composites were made by high-pressure-high-temperature (HPHT) sintering with the aim of maximizing the thermal conductivity of the composites.Improvements in interfacial bonding strength and thermo-physical properties of the composites were achieved using an atomized copper alloy with minor additions of Co,Cr,B,and Ti.The thermal conductivity (TC) obtained exhibited as high as 688 W·m-1·K-1,but also as low as 325 W·m-1·K-1.A large variation in TC can be rationalized by the discrepancy of diamond-matrix interfacial bonding.It was found from fractography that preferential bonding between diamond and the Cu-alloy matrix occurred only on the diamond { 100 } faces.EDS analysis and Raman spectra suggested that selective interfacial bonding may be attributed to amorphous carbon increasing the wettability between diamond and the Cu-alloy matrix.Amorphous carbon was found to significantly affect the TC of the composite by interface modification.