Electro-spark deposition in vacuum using graphite electrode at different electrode polarities: Peculiarities of microstructure, electrochemical and tribological properties

Abstract

Titanium and its alloys are widely used in offshore and costal infrastructure, but need protection against wear and corrosion in saline environments. Compared to other surface engineering methods, electro-spark deposition (ESD) does not have the drawbacks of small thickness and poor adhesion of protective coatings. However, ESD in air or inert atmosphere using a graphite electrode does not provide sufficient continuity of a protective layer due to a low efficiency of carbon transfer. To enhance carbon transfer, Ti was subjected to ESD in vacuum using a graphite electrode. The main difference between vacuum and air deposition is that, in addition to spark discharges, arc evaporation occurs. It was shown that the carbon content, microstructure, and phase composition of modified Ti layer can be well controlled by changing electrode polarity and pulse energy. At cathodic polarity, arc discharge occurs on graphite electrode, which leads to increased carbon evaporation and a higher carbon content in the coating. Resulting coatings protect Ti from wear and corrosion in 3.5 wt.% NaCl solution due to formation of TiC layer. Coatings with top graphite layer show the lowest values of friction coefficient and corrosion current density, as well as fast corrosion potential recovery during tribocorrosion tests.