Erosion mechanisms of DLC coatings deposited on polyimide and silica substrates when exposed to a pulsed gas discharge

Abstract

The influence of spark discharges on the surface morphology and chemical composition of resistive diamond-like carbon (DLC) coatings designed to prevent breakdowns in gas electron multipliers (GEMs) has been studied. DLC coatings with thicknesses of approximately 200 nm and 66 nm were deposited in an inert argon atmosphere on polyimide substrates by high current pulsed magnetron sputtering and on silica (silicon oxide) substrates by vacuum arc method. Based on the analysis of images of optical and scanning electron microscopy, it was established that, the region of interaction between the gas discharge channel and the coating surface presents an accumulation of erosion craters with a characteristic linear size in the range of 20–40 μm. The value of current density passing in the region of interaction between the plasma in spark discharges and the sample is estimated to be of the order of 105 A/cm2, which is sufficient to ablate the coating and create thermal stresses of magnitude ~4 GPa at the interface between the coating and the substrate. Thus, pulsed surface erosion, insufficient adhesive strength of the coating to the substrate, and poor thermal conductivity should be considered as the key reasons for destruction of the DLC coating.