Abstract

A brief introduction will first be given to the plasma spray process, which is in extensive industrial use for the creation of (relatively thick) surface coatings. In order to illustrate some of the characteristics, a particular application will be covered – the production of (zirconia-based) thermal barrier coatings (TBCs) on gas turbine components. This technology is at the forefront of attempts to improve the efficiency of (aero-engine and power production) turbines by achieving further increases in the turbine entry temperature (TET). An overview will be given of how such coatings tend to degrade under service conditions, particularly in terms of sintering effects and consequent stiffening, which can promote spallation. This can be accelerated by the presence of impurities, which can promote grain boundary diffusion and may even lead to liquid phase sintering. In this context, there is a concern about the effect of ingested volcanic ash (VA) within gas turbine aero-engines. Some of the factors involved will be briefly described, including those affecting the likelihood of ingested particulate adhering to surfaces inside the turbine.

A brief overview will then be given of the plasma electrolytic oxidation (PEO) process, which evolved from anodization and can be used to create thick, adherent and durable coatings on certain metals, via repeated local dielectric breakdown (sparking) within an electrolytic bath. These convert surface layers of the substrate to oxide. While the process is very different from plasma spraying, the resultant coatings do have certain features in common. Although it is in quite extensive industrial use, discharge characteristics are affected by a number of processing variables and understanding of the process is incomplete. However, the inter-relationships between electrical conditions, electrolyte composition, coating microstructure and rates of growth, which are linked via the characteristics of the discharges, have become a little clearer over recent years and an outline will be provided of current scientific and technical developments.