Plasma sprayed thermal barrier coatings for industrial gas turbines : morphology, processing and properties

Abstract

Thermal barrier coatings out of fully or partially stabilized zirconia offer a unique chance in gas turbines to increase the gas inlet temperature significantly while keeping the temperature of the structural material of the component within conventional limits. The protection of combustor parts and transition pieces as well as of some stationary gas turbine parts however is state of the art. As a consequence of still insufficient reliability, the application for hot rotating parts is very limited. The introduction as a design element requires safe life within defined time intervals. These depend on the overhaul and repair intervals of the engines. For large land based industrial or utility gas turbines, for example, coating life between 25.000 and 30.000 hrs. is a minimum requirement. Premature failure of a coating by e.g. local spalling causes local overheating of the component with the consequence of its total destruction or even more expensive secondary damages. Life limiting is the corrosion rate at the ceramic-metal interface and the behavior of the coated system under transient operating conditions, where multiaxial strain and stress distributions are generated. Sufficient strain tolerance of the coating both under tensile as well as compressive conditions is required. The properties of thermal barrier coating systems depend strongly on the structure and phase composition of the coating layers and the morphology of and the adhesion at the ceramic-metal interface. They have to be controlled by the process itself, the process parameters and the characteristics of the applied materials (e.g. chemical composition, processing, morphology, particle size and size distribution). It will be reviewed, how properties and structures of coating systems correlate and how structures can be modified by careful control of the process parameters.

Microstructural characterization of yttria-stabilized zirconia plasma-sprayed deposits using multiple small-angle neutron scattering

“…However, TBCs have engineering reliability problems that may limit the trustworthiness and value of the systems into which they are incorporated [1,5]. An incomplete understanding of the complex processing-microstructure-property relationships in plasma-spraying, together with an insufficient microstructural characterization of the spray deposits themselves, lies at the heart of these reliability problems.…”

ZrO2 9.5Y2O3 5.6Yb2O3 5.2Gd2O3; a promising TBC material with high resistance to hot corrosion

“…Based on previous works, the splats possess intertwining columnar structures formed due to aligned freezing at elevated cooling rates [43]. The intra-splat cracks refer to the cracks in the splat structures.…”

Thermal Shock and Cycling Behavior of Thermal Barrier Coatings (TBCs) Used in Gas Turbines

“…As a result, properties such as; porosity, horizontal and vertical cracks and elastic modulus in TBC systems are key parameters that affect thermal cycling life. It is important to keep these parameters in optimum levels in service and to identify their relationship with each other carefully, for the coating system to resist thermal cycling [53][54][55].…”