High temperature oxidation behaviour of nanostructured cermet coatings in a mixed CO₂ – O₂ environment

Nanostructured ceramic-metallic (cermet) coatings composed of nanosized ceramic particles (α-Al₂O3 and TiO₂) dispersed in a nickel matrix were co-electrodeposited and then oxidized at 500°C, 600°C and 700°C in a mixed gas using a Thermo-gravimetric Analysis (TGA) apparatus. The mixed gas was composed of 15% CO₂, 10% O₂ and 75% N₂. This research investigates the effects of CO₂ and O₂ partial pressures on time-depended oxidation rates for coatings and compared them to the results from atmospheric oxidation under similar temperatures. The increase in partial pressure of oxygen due to the presence of CO₂ at each tested temperature was calculated and correlated to the oxidation rate of the coatings. The results showed that the presence of CO₂ in the system increased the oxidation rate of cermet coatings when compared to atmospheric oxidation at the same temperature. It was also shown that the increase in the oxidation rate is not the result of CO2 acting as the primary oxidant but as a secondary oxidant which results in an increase of the total partial pressure of oxygen and consequently higher oxidation rates. The WDS and XRD analyses results showed that the presence of nanosized TiO₂ particles in a nickel matrix can improve oxidation behaviour of the coatings by formation of Ni-Ti compounds on oxidizing surface of the coating which was found beneficiary in reducing the oxidation rates for cermet coatings