SEM image analysis of droplet formation during metal ion etching by a steered arc discharge

Abstract

During cathodic arc metal ion etching a pronounced influence of the melting point of the target material on droplet formation was found. Low melting point materials showed quite an enhanced tendency to generate droplets with respect to number and size. Under the process parameters used the TiAl alloy target showed the highest number of droplets generated (100×10 3 mm −2). Even using target materials with high melting points such as Zr (MP: 1850 °C), Cr (MP: 1870 °C) and Nb (MP: 2450 °C) severe differences were observed. Cr played a special role as 90% of its droplets were smaller than 0.5 μm. For Zr with a melting point close to Cr the same percentage of droplets were less than 0.80 μm in diameter, clearly indicating a larger average droplet diameter. On the other hand, among the high melting materials the number of Cr droplets was 5 times higher than that of Nb and twice as high as that of Zr. Metals with melting points greater than 2000 °C showed very similar effects of droplet generation, with mean droplet densities of 3–5×10 3 mm −2 A comparison between the as-etched and UBM coated samples indicates that the number of droplets deposited during the etching phase with TiAl and Mo formed an identical number of growth defects in the coating. In the case of Cr the number of defects on top of the coated surface was less than the number of droplets deposited during the etching phase.