Low-pressure plasma-induced physical vapor deposition of advanced thermal barrier coatings: Microstructures, modelling and mechanisms

Thermal barrier coatings used to protect the blades of aircraft and gas turbine engines are primarily ceramic coatings with unique microstructures result of the deposition method used., for example, plasma spraying. Over the last decade, low-pressure (50–200 Pa) plasma spray - physical vapor deposition (PS - PVD) has been recognized as a promising method for obtaining advanced high-temperature performance thermal barrier coatings on gas turbines or aircraft engines. However, challenges still exist in experimental measurements and numerical modeling inside very large supersonic fluid fields (at the meter scale) with multiple phases (at micron- or nano-scale) at temperatures over 12000 K and velocities of over 6000 m/s. The plasma spray flow and heat and mass transfer in a closed chamber are not fully understood owing to the complicated interaction of the supersonic flow, swirling flow, phase transition and shock waves. This paper reviews the current state of technology in thermal barrier ceramic coatings obtained using PS-PVD. The multiphase flow characteristics during low-pressure plasma spraying using ceramic nano-agglomerated powders at different chamber pressures are studied via computational fluid dynamics modelling and experiment. The self-shadowing effect of impinging particles and the intensification of heat and mass transfer from the low-pressure plasma plume to the substrate are demonstrated. The flash vaporization and atomization of the ceramic droplets induced by the plasma jet shock waves are clarified. The formation, effects and control of quasi-columnar ceramic coatings in the additive manufacturing process of low-pressure plasma spraying are studied. Finally, this paper concludes with the future outlook and outstanding problems in this topic.