ICMCTF2017 Session A1-3: Coatings to Resist High Temperature Oxidation, Corrosion and Fouling

MCrAlY type (M=Ni and/or Co) coatings are widely used for the protection of components in the hot sections of gas turbines at high service temperatures by forming a continuous α-alumina. A reliable criterion to estimate the capability to form α-alumina is of great importance to accurately evaluate coating lifetime. However, the traditional Al-concentration based criterion failed to properly predict the formation of a continuous α-alumina. Thus, a new life criterion, namely the critical Al-activity criterion, is proposed.

In this work, critical Al-activity to form a continuous α-alumina is calculated using Thermo-Calc software, based on literature survey of research results of critical Al-concentration to form α-alumina on binary Ni-Al and ternary Ni-Cr-Al system. Long-term oxidation test were performed to support the criterion: IN-792 superalloys coated with five different MCrAlY coatings were oxidized at 1000 ⁰C for various periods of time up to 10000 hours. The microstructural evolution of MCrAlY coatings were analyzed using Scanning Electron Microscope. The near-surface Al concentration and interdiffusion behavior between substrate and coating were measured using Energy Dispersive X-ray Spectroscopy. The new critical Al-activity criterion has been successfully adopted in α-alumina formation prediction, showing a good agreement with experiment results. Therefore, it can be concluded that the extrapolation of new criterion from binary and ternary systems to multi-alloyed MCrAlY system is reasonable. Furthermore, the partial pressure of oxygen (P_O2) in atmosphere has been taken into consideration by combination with Al-activity to calculate the Gibbs energy of formation of α-alumina. The potential applicability of the methodology to predict MCrAlY life is also discussed .

Ti₂AlN belongs to a family of ternary nano-laminate alloys known as the MAX phases, which exhibit a unique combination of metallic and ceramic properties. In the present work, the dense and high-stability Ti₂AlN coating has been successfully prepared on Ti6Al4V (TC4) substrates through combined cathodic arc/sputter deposition method, followed by heat post-treatment. The oxidation of Ti₂AlN coating and the TC4 substrates were investigated in air and in water vapor at 750 °C for 200h. The results indicated that the oxidation processes of both TC4 substrates and the coated samples were accelerated for the presence of steam, resulting in slightly higher mass gains. The oxidation behavior of the bare substrates under different atmosphere exhibited linear kinetics, which indicates a continuous oxidation during its exposure at high temperatures. In contrast, the mass gain was significantly reduced for the coated samples, suggesting that the Ti₂AlN coating can provide an effective protection for the substrates. Moreover, the Ti₂AlN phase can still be found after oxidation in air atmospheres for 200h and the oxide scale showed local Al₂O₃ and rutile TiO₂ growth, namely the oxide did not cover the entire surface of the coating. However, the Ti₂AlN phase disappeared after oxidation in steam condition and double layer scales formed in the water vapor atmospheres, consisting of an outer rich-Al₂O₃ layer and an inner rich-TiO₂ layer. The enhanced oxidation resistance achieved under different condition by the Ti₂AlN MAX phase coatings may satisfy the optimal requirements for many applications in the field of nuclear power plants and aerospace components.

Two coating architectures with distinct period number (PN) of Ti-N/Cr-N multilayer coatings were prepared by reactive magnetron sputtering process. Two coating architectures were designed as the multilayers TiN/CrN/…CrN/TiN/Ti/substrate (architecture T) and the CrN/TiN…TiN/CrN/Cr/substrate (architecture C). Four PNs with 1,5,10 and 15 were prepared in architecture T and C, respectively. This study investigates the effect of the coating architectures and PNs on the corrosion behaviors of the coatings. The corrosion rate were tested by immersion the coatings in the 3% HCl solution for 20 hours to measure the weight loss. The results show that the corrosion rate of the coatings were strongly related to the coating architecture and PN. In coating architecture T, the corrosion weight loss is decreased with PN increased. On the contrary, in architecture C, the corrosion weight loss is increased with increasing PN.