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.