Technological developments around electric generation power plants aim to increase the thermal efficiency of conversion processes in steam turbines, developing materials able to resist ultra-supercritical (USC) conditions (600-620°C and 20-30 MPa). 9-12 %Cr ferritic-martensitic steels, commonly used for that application, tend to develop thick and non-protective iron-rich oxide scales. Therefore, partial or complete spallation of these oxide scales may cause serious consequences for the lifetime and safe operation of the components in contact with the steam and even the own plant. The solution to prolong their service life is to modify their surface, by means of protective coatings that retard interdiffusion mechanisms that take place at that temperature range.

Inconel 625 is used for its high strength, excellent fabricability (including joining), and oxidation resistance up to 982°C, so has been considered as one possible candidate for A-USC power plants (up to 700°C and 35 MPa).

In this research Inconel 625 coatings were deposited by High Velocity Oxy-fuel (HVOF) and laser cladding (LC) processes on a 12%Cr steel to improve its oxidation resistance. A high-temperature oxidation test under isothermal conditions up to 500 h was performed on uncoated and coated steel in a pure steam atmosphere at 600°C. The specimens were weighed monitoring at different times during the testsand characterized before and after oxidation tests by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). The results have shown that the application of Inconel 625 coatings reduces the weight gain of the steel by more than 50%. In addition, after 500 h of oxidation both coatings deposited by HVOF and LC exhibited an excellent oxidation resistance at 600°C in pure steam due to the formation of thin layers of protective Cr₂O₃ and NiCr₂O₄ oxides on the external surface of the coatings.

The reduction of carbon emissions as well as the improving of fuel efficiencies tied to modern aerospace and aviation have heavily invested in the development of lighter and more durable high temperatures materials. γ-TiAl bulk materials, with their low density and distinguished creep resistance, fulfill all these tasks till 780°C. Above 780°C, oxidation protection of γ-TiAl based alloys is a challenging task for the aero- and automotive industry. Here, especially thin films rise the possibility to protect these alloys while not affecting other material properties. Typically, ceramic like thin films are applied to defend the bulk materials against oxidation and corrosion attacks. Within this study, we applied a different approach utilizing metallic coating materials deposited by PVD.

Increasing temperature in light and heavy-duty internal combustion engines offers a straightforward solution for increasing engine efficiency and reducing CO₂ emission. Development of new Ni-based high temperature alloys is, however, burdensome due to the need for both high strength and high oxidation resistance. One solution is to apply corrosion-resistant coatings to high strength materials. A slurry aluminide coating was, therefore, deposited on commercial (alloy 31V) and ORNL-developed high strength gamma prime Ni-based superalloys. A significant improvement of the alloy cyclic oxidation resistance at 900°C and 950°C in air + 10%H₂O was observed for both alloys due to coating application. The coated ORNL sample exhibited more rapid oxidation rates than the 31V coated sample due to the higher Ti concentration in the former alloy leading to the formation of Ti-rich nonprotective oxides. High cycle fatigue testing was also conducted on bare and coated specimens to assess the coating impact on the alloy fatigue resistance at 800-900°C. Similar cycles to rupture were measured for the bare and coated samples, confirming that the coated ORNL alloy is a viable option for valve application up to ~900°C. Further optimization of the substrate/coating couple could allow for even higher valve operating temperature.

This research was sponsored by the U.S. Department of Energy, Energy Efficiency & Renewable Energy, Vehicle Technologies Office, Powertrain Materials Core Program.