Properties and failure modes of incandescent tungsten filaments

The behaviour of doped tungsten filaments is discussed with particular reference to the life-controlling processes in incandescent lamps. Since lamp failure is a complex phenomenon depending on many conditions, it appears that there is no single mechanism of universal validity by which the finite life of the tungsten filaments can be explained satisfactorily in all cases. It is suggested that, in addition to the nonuniform evaporation of the filament owing to local defects, other mechanisms, like migration and growth of the potassium-filled bubbles within the wire, and grain-boundary sliding promoted by unfavourable grain shape, may also contribute to the failure of incandescent lamps. Presumably, on the basis of a better understanding of the doping effect and failure mechanisms, further improvements can be achieved in lamp quality by optimisation of bubble dispersion, grain structure and other life-influencing parameters.