234 Prakt. Met. Sonderband 38 (2006) 
For obtaining desired combination of properties the microstructure of silver — tin oxide rev 
composite has to consist of high density of fine and uniformly distributed round oxide Sr 
particles (few ten nm in size). Namely, such morphology offers greater resistance to both ob 
arc-erosion and mechanical wear of contact material and assures stable switching ability the 
without causing welding. de 
de 
be 
ph 
sn 
bo 
lo 
200 um 200 um 
200 um 200 um _ | 
, u) 
Fig. 2: The microstructure of partially internally oxidized Ag-Sn (2 at.% Sn) at T=800°C 
and: a) t=5min; b) t=15 min; c) t=30min; d) t=45min 
In the internal oxidation process the size and the morphology of oxide particles are 
determined by: (i) Free energy of oxide formation; (ii) Concentration of the alloying element 
(iii) The rate of nucleation and the rate of growth; (iv) Internal oxidation temperature; and 
(v) The distance from the outer surface. So, by changing these parameters we are able to 
create the desired microstructure. For example, the microstructures in figure 4a and 4b F 
indicate the influence of distance. It can be seen that the particles are smaller near the free e 
surface and appear round. With increasing distance from the outer surface the particles S 
become coarser and have much more plate-like or acicular form. On the other side, the A 
influence of the internal oxidation temperature on the morphology of oxide particles is . 
evident form figures 4, 5, 6. The micrographs taken by the scanning electron microscopy : 
a) b) 
c) J