John W. Pugh

Image analysis for grain shape characterization in lamp filaments

Algorithms have been developed for appraising the quality of tungsten lamp filament wire microstructure with respect to its resistance to sag. They allow a numerical determination of grain aspect ratio, grain boundary contour, angle of boundary with wire diameter, and degree of grain boundary surface convolution. These values are combined algebraically to give a grain shape parameter (GSP) which reliably predicts how resistant the filament is to grain boundary sliding. Measurements are made of thermally etched grain boundaries on scanning electron microscopy (SEM) images of the surfaces on coiled filaments. Data are recorded and parameters computed by means of an image analyzer. The technique has been tested on samples made from the same wire modified in process to have varying strain after the last anneal. It has also been tested on samples purchased from various vendors. These applications indicate that the computed GSP is a sensitive predictor of filament creep resistance and reliably reflects variation in wire drawing strain after the last anneal.

Effects of vibration on the grain morphology

Pairs of halogen lamps were heated simultaneously at various rates of temperature increase through the recrystallization temperature and were held for five hours at 120 volts. One lamp in each of 20 pairs was vibrated during lightup. Examination of the grain boundaries of the filaments revealed that 63 pct of the boundaries in the vibrated lamps were “transverse.” In the lamps not vibrated, 52 pct of the boundaries were “transverse.” The greater number of “trans- verse” grain boundaries in the vibrated lamps is attributed to grain boundary sliding, which generates additional energy to allow the grain boundary to overcome the restraint to migration provided by the strings of potassium bubbles in the lamp wire.