Comparison between microwave and microwave plasma sintering of nickel powders

There is considerable interest in processing technologies which can lead to more energy efficient sintering of metal powders. The use of microwave sintering in particular leads to reduced energy usage during sintering as the volumetric heating process is considerably more efficient compared with resistance heating. In this study the use of a novel plasma microwave processing technology for the sintering of nickel powder discs is evaluated. The sintering study was carried out on 20 mm diameter by 2 mm thick pressed discs of nickel powder, with mean particle size of 1 μm. The discs were fired in a 5 cm diameter microwave (2.45 GHz) plasma ball under a hydrogen atmosphere at a pressure of 2 kPa. The same discs were also sintered using both non plasma microwave and tube furnace firing. The microwave plasma sintering is very rapid with full disc strength of approx. 1000 N based on 3--point bend tests being achieved within 10 minutes. In contrast the sintering time in the tube furnace involved treatments of up to 6 hours. The non plasma microwave system involved intermediate treatment periods of 1 to 2 hours. Another advantage of the microwave plasma treatment is that the degree of sintering between the individual nickel powder particles can be precisely controlled by the duration of the treatment time in the plasma. There was a broadly linear increase in fired pellet breaking strength with plasma treatment duration. In addition to breaking load, the mechanical properties of the sintered nickel discs were compared based on Rockwell hardness tests and density measurements. The morphology of the sintered discs was compared using microscopy and SEM. This study demonstrated that the plasma microwave sintered discs produced similar or superior performance (depending on processing conditions) to discs fired using the non-plasma microwave and furnace firing conditions. Accurate control of the sample conditions and structure can easily be controlled with the plasma system compared with the conventional systems. The apparent volumetric heating in the microwave systems give a more uniform heating at lower temperatures and allows for greater control and homogeneity.