Figure 4.6 Steel bearing balls in a microwave experiment. (a) Experimental arrangement of bearing
balls on a ceramic substrate before microwave treatment; (b) rearrangement of bearing balls after
microwave treatment; (c) surface of bearing balls treated with microwaves; (d) high magnification
detail of contact area [16] (Figures 4.6(a), (c) and (d) reproduced by permission of European Powder
Metallurgy Association; Figure 4.6(b) courtesy of Dr Veltl)
particles can be sintered when in close proximity to each other, and physical contacts
between particles are not required [16]. The microwave field concentrates at cracks in the
materials, promoting material transport, and provides the possibility of reducing cracks
within the sintered materials with the use of microwave sintering [16]. A comparison of the
bending strength of precracked Distaloy AE samples sintered using a conventional furnace
and microwaves revealed an increase in bending strength from 242 N/mm2 in the case of
conventionally sintered samples to 270 N/mm2 for microwave sintered samples, demonstrating
the ‘crack-healing’ ability of microwaves.
Sintering of different metal powder compacts using single-mode microwave radiation and
conventional sintering was performed by Saitou to investigate the sintering behavior and
shrinkage parameter of the metals under different sintering conditions [17]. The activation
energy for the different metals sintered using a conventional electric furnace (EF) and
microwaves (MW) was evaluated based on Arrhenius plots for the shrinkage parameter. The
shrinkage parameter for microwave sintering is higher than conventional sintering at low
temperature and is sensitive to the particle size of the powder. Smaller particle sizes
displayed a higher shrinkage parameter than particles of a larger size. Also, during the
sintering of an iron powder compact, it was observed that the phase transformation
temperature for the transformation of BCC iron to FCC iron at 910 C (A3) under
conventional heating was reduced to 875 C under microwave heating, and the transformation
of FCC iron to BCC iron at 1390 C (A4) was also reduced by approximately 100 C by