Residual stress in electrical discharge coatings

Abstract

Abstract Electrical discharge coatings (EDC) can be applied on complex metal components to repair or act as protective coatings. A variant of the EDM process, it can be used to make coatings up to several mm thickness on electrically conductive substrates using sacrificial electrodes. The technology can be classified under thin EDC which is a surface modification process or thick EDC, also known by the commercial name “MSCoating”, which is a surface coating process. Inclusion of a high current pulse in the discharge waveform results in high deposition efficiencies of between 40 and 200 μm/min, with a subsequent decrease in energy into the coating per volume of material resulting in reduced re-melting of deposited material; the thermal shrinkage phenomenon seen in standard EDM recast layers is not as significant here. This results in thick EDCs with consistent tensile residual stresses of between +70 and + 100 MPa. Exclusion of the high current pulse reduces the deposition efficiency to between 0.5 and 10 μm/min. The subsequent increase in energy into the coating increases the melting and quenching of material, resulting in a more nanostructured or amorphous layer. This results in thick EDCs with largely compressive residual stresses, with a range of between +6 and −140 MPa, explained by amorphisation induced volume expansion. In cases where the deposition efficiency is sufficiently low, below 0.1 μm/min, increased melting and quenching of coating material results in high tensile residual stresses due to the thermal expansion mismatch between the layer and substrate. This results in thin EDCs with average tensile residual stresses of between +440 and +490 MPa. It is proposed that the changeable and controllable directionality of the residual stress within these coatings is a positive attribute, ultimately allowing tailorable coatings with regions of favourable residual stress dependent on final application, e.g. for wear or fatigue.