| AVS 63rd International Symposium & Exhibition | |
| Advanced Surface Engineering | Wednesday Sessions |
| Session SE+TR-WeM |
| Session: | Protective Coatings for Tribological Applications in Surface Engineering |
| Presenter: | Papken Hovsepian, Sheffield Hallam University, UK |
| Authors: | P.Eh. Hovsepian, Sheffield Hallam University, UK A.P. Ehiasarian, Sheffield Hallam University, UK Y. Purandare, Sheffield Hallam University, UK F.J. Perez, Universidad Complutense de Madrid, Spain M.I. Lasanta, Universidad Complutense de Madrid, Spain M.T. de Miguel, Universidad Complutense de Madrid, Spain A. Illana, Universidad Complutense de Madrid, Spain J. Juez-Lorenzo, Fraunhofer Instititute fur Chemische Technologie ICT, Germany A. Aguero, Instituto Nacional de Tecnica Aeroespecial (INTA), Spain |
| Correspondent: | Click to Email |
The demand for new materials to be used in supercritical steam power plants for efficient and clean coal utilization is ever growing. A significant reduction of CO2 emissions is expected by increasing the efficiencies of the steam turbines to ŋ > 50% which can be achieved by moving from subcritical low pressure/ low temperatures, (180 bar/540 oC) to high pressure/high temperature, (300 bar/600-620 oC) ultra-supercritical regime of operation. The main challenges faced by different steel components of the power plant with this approach however, consist of material failure due to high temperature oxidation, and phenomenon such as creep, erosion and descaling after a stipulated period of time. Over the years considerable research has been done in finding solution to the above problems in terms of protective surface layers with limited success.
In the current work, 4 µm thick CrN/NbN coating utilising nanoscale multilayer structure with bi-layer thickness of ∆ = 3.4 nm has been used to protect low Cr content P92 steel widely used in steam power plants. The uniquely layered coatings have a combination of nitrides of chromium and niobium which are not only resistant to aqueous corrosion, corrosion erosion and excellent tribological properties, but also have oxidation resistant in dry air up to a temperature of 850 °C. The novel High Power Impulse Magnetron Sputtering (HIPIMS) deposition technology has been used to deposit CrN/NbN with enhanced adhesion (critical scratch adhesion value of Lc= 80N) and very dense microstructure as demonstrated by XTEM imaging. These superior coating properties are achieved due to the unique high metal ion content (up to 90%) in the HIPIMS plasma, which allows particle acceleration and trajectory control by external electrical and magnetic fields thus delivering highly energetic material flux on the condensing surface.
P92 bare and coated samples were oxidised at 650°C in 100% steam atmosphere up to 2000 h, in order to simulate the future operation conditions of steam turbines employed in power plants. The oxidation kinetics was evaluated by mass gain measurements in a five decimal balance. In these conditions CrN/NbN provided a reliable protection of the P92 steel and outperformed other coatings such as ceramic Al2O3 and intermetallic Fe44Cr5Al used for the same application. The paper also discusses the effect of the growth defects and high temperature crack formation analysed by SEM and FIB- SEM techniques on the high temperature corrosion resistance in pure steam atmosphere thus revealing the coatings failure mechanisms.