The effect of various silver tin oxide materials on contact performance under motor load (a.c.)
The new contact material AgSnO/sub 2/ which is replacing AgCdO in many applications, requires the addition of additives to improve electrical contact performance. These additives such as copper oxide (Cu/sub 2/O) and bismuth oxide (Bi/sub 2/O/sub 3/), can significantly affect contact performance. An investigation was undertaken to compare the changes in contact performance between silver cadmium oxide (internally oxidized) and three silver tin oxide based materials containing (1) no additive, (2) copper additive and (3) bismuth plus copper additives. Performance changes and physical characteristics of each material such as formability (tensile properties), contact resistance and contact material erosion (arc erosion) were evaluated. The silver tin oxide based materials containing copper additive exhibited significantly higher resistance to arc erosion than silver cadmium oxide based material and other silver tin oxide materials evaluated. The resistance to welding and changes in electrical resistance were dependent on the chemistry of each material. The results suggested that the chemistry of the material was the chief reason silver tin oxide based material with copper additive performed better than silver cadmium oxide based material under identical motor load.
Cohesion margin of copper
Abstract Measurements on the mechanical properties of copper-bismuth alloys show that the only effect of bismuth within the solid solution range is to reduce the true tensile strength of the alloys—there is no detectable effect on plastic behaviour or on the elastic limit. Intergranular fracture occurs by the formation of grain boundary crack nuclei, these being seen in alloys containing as little as 0·002 wt.-% Bi. In addition, by measuring the effect of bismuth on surface and grain boundary energies of copper, it is shown that bismuth is present at the grain boundaries at monolayer levels, as true Gibbs segregation. It appears that the brittleness is due simply to reduced grain boundary cohesion. The measured relative cohesion of the copper-bismuth alloy is approximately half that of pure copper: such a reduction in grain boundary cohesion therefore converts ductile copper into a brittle metal, and thus defines a margin of cohesion. Reduced grain boundary cohesion favours the nucleation of cracks where ...
Intergranular fracture in low carbon iron
The influence of grain size and temperature on twinning, yield and fracture has been examined in two low carbon irons. The evidence indicates that crack nucleation is the critical event in intergranular fracture and fracture can occur prior to general yielding or after subsequent work hardening. Twinning is not a prerequisite of intergranular failure, and the effect of impurity segregation at grain boundaries is to lower the cohesive energy of the boundaries rather than to increase the difficulty of slip propagation.