Comparison of thermal and microwave-assisted plasma sintering of nickel-diamond composites

There is considerable interest in processing technologies which can lead to more
energy efficient sintering of metal powders. Microwave sintering has recently been
shown to reduce energy usage as the volumetric heating process is considerably more
efficient than resistance heating. RF plasma sintering meanwhile has been shown to
deliver heat via uniform excitation of the processing gas resulting in ion bombardment
of the workpiece. In this study the use of a rapid, novel microwave-assisted plasma
sintering (MaPS) technology for processing of nickel-diamond metal matrix
composites is evaluated. Nickel powder and polycrystalline diamond were mixed to
prepare 20 mm discs under uniaxial compaction pressures of 100, 200 and 300 MPa.
The discs were fired in a low pressure microwave plasma under a hydrogen
atmosphere. For comparison, discs were also sintered using conventional tube furnace
firing. The MaPS sintering is very rapid with full disc strength of >1000N, based on
3-point bend tests, being achieved within 10 minutes compared with 8 hours for
furnace treatment. This study demonstrates that the microwave-assisted plasma
sintered discs produced similar or superior performance to discs fired using furnace
firing conditions but with sintering cycle time reduced by up to 95%.