Real-time imaging of coating growth during plasma electrolytic oxidation of titanium

Abstract

The microdischarge characteristics during dc plasma electrolytic oxidation (PEO) of titanium at 20 mA cm −2 in orthophosphate electrolyte at 293 K have been investigated by real-time imaging. A relatively constant microdischarge rate of ∼100 cm −2 s −1 was revealed, with the sizes and lifetimes of microdischarges ranging between ∼70 and 380 μm and ∼35 and 800 ms, respectively. The average lifetime of the microdischarges increased with increased voltage, reaching ∼335 ms at 430 V in the main period of coating growth. The cumulative number of microdischarges during PEO was at least two orders of magnitude lower than the population density of pores on the surface of the final coating. Increased time of oxidation resulted in coarsening of the coating surface, due to the formation of relatively large pores. Microdischarges were observed with a bubble-free surrounding region indicative of generation of shock waves.