Influence of gas type on the thermal efficiency of microwave plasmas for the sintering of metal powders

Microwave plasmas have enormous potential as a rapid and energy efficient sintering
technology. This paper evaluates the influence of both plasma atmosphere and metal
powder type on the sintering temperatures achieved and the properties of the sintered
powder metal compacts. The sintering is carried out using a 2.45 GHz microwave-plasma
process called rapid discharge sintering (RDS). The sintering of three types of metal
powder are evaluated in this study: nickel (Ni), copper (Cu) and 316L stainless steel (SS).
An in-depth study of the effects of the plasma processing parameters on the sintered
powder compacts are investigated. These parameters are correlated with the mechanical
performance of the sintered compacts to help understand the effect of the plasma heating
process. The substrate materials are sintered in four different gas discharges, namely
hydrogen, nitrogen oxygen and argon. Thermocouple, pyrometer and emission
spectroscopy measurements were taken to determine the substrate and the discharge
temperatures. The morphology and structure were examined using scanning electron
microscopy and x-ray diffraction. The density and hardness of the sintered compacts were
correlated with the plasma processing conditions. As expected higher densities were
obtained with powders with lower sintering temperatures i.e. nickel and copper when
compared with stainless steel. Under the power input and pressure conditions used the
highest substrate temperature attained was 1100
∘
C for Cu powder sintered in a nitrogen
atmosphere. In contrast under the same processing conditions but in an argon plasma, the
temperature achieved with SS was only 500
∘
C. The effect of the plasma gas type on the
sintered powder compact chemistry was also monitored, both hydrogen and nitrogen
yielded a reducing effect for the metal in contrast with the oxidising effect observed in an
oxygen plasma.