Droplet formation on steel substrates during cathodic steered arc metal ion etching

Abstract

Cathodic arc discharges are commonly used as metal ion sources to carry out, in vacuo, the precleaning procedure in cathodic arc evaporation and combined cathodic arc/unbalanced magnetron deposition processes. During this very efficient etching step, droplets are generated, which give rise to growth defects in the subsequently deposited hard coating. The number and size of droplets depend on the melting point of the metals used during ion etching. The present paper investigates the generation of droplets by target materials Al, Cu, TiAl, Ti, Zr, Cr, Nb and Mo with melting points in the range 660–2650 °C. Under the process parameters used, the TiAl alloy target showed the highest number of droplets generated (100 × 10 3mm −2) whereas the largest, up to 20 μm diameter and quite often ‘splash like’, formed during evaporation from Al targets. Both metals with melting points greater than 2000 °C show very similar droplet generation, with mean droplet deposition densities of 5−10 × 10 3mm −2 and diameters up to 5 μm. A comparison between the as-etched and as-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 case of Cr the number of defects on top of the coated surface were less. In general, following a non droplet producing unbalanced magnetron deposition process, defects were observed to extend from the substrate/coating interface to the coating surface.