The research work performed in this thesis has been carried out at the Production Tech-nology Centre where the Thermal Spray research group of University West has its work-shop and labs. This research work has been performed in collaboration with the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, India. First of all, I would like to express my sincere thanks and gratitude to my supervisors Dr. Nicolaie Markocsan and Dr. Nicholas Curry for their guidance, great support and valuable suggestions without which this work could not have been possible. I would also like to thanks Prof. Per Nylén for keeping faith in me and providing me an opportunity to work at PTC, which is a great place to perform research. It is my pleasure being their student and I wish I would keep learning from all of them, both on academic and personal grounds. I would also like to thank my colleagues at PTC Mr. Mohit Gupta and Mr. Stefan Björklund, for their help and support during this work. I would like to acknowledge the H.C. Starck Company for its financial support for the pro-ject; Dr. Filofteia-Laura TOMA at Fraunhofer IWS, Dresden to help us in spraying suspen-sion sprayed YSZ top coats, G Shivkumar from ARCI to help us in spraying solution pre-cursor sprayed top coats and Toni Bogdanoff, Jönköping University to help us in conduct-ing the LFA experiment

Dans le domaine des dépôts réalisés par projection thermique, les dernières recherches ont permis de développer des dépôts nanostructures par projection de suspensions (SPS) ou de solutions de précurseurs (SPPS). L’injection de liquide est une technique prometteuse qui a le potentiel pour devenir industriellement viable. Cependant, un meilleur contrôle des interactionsplasma/matériaux est nécessaire. En effet, les torches à courant continu mono-cathodes génèrent un plasma fortement fluctuant qui modifie les transferts thermiques et dynamiques vers les gouttelettes injectées, entraînant un traitement non-homogène de ces dernières. Cela influence directement les microstructures des dépôts obtenus et leurs propriétés. Des travaux antérieurs ont montrés les origines de ces instabilités. Une alternative à l’atténuation de ces instabilités est proposée : le renforcement et la modulation. L’ajustement des paramètres opératoires a permis d’obtenir un plasma pulsé laminaire avec modulation de ces propriétés. Une torche à courant continu modulé de faible puissance est utilisée avec de l'azote pur comme gaz plasmagène. Ce dispositif est synchronisé avec un système d'injection jet d’encre Drop-On-Demand pour reproduire les mêmes conditions d'interactions plasma / matériau pour chaque gouttelette injectée. Des solutions et des suspensions sont injectées pour réaliser des dépôts homogènes avec une microstructure contrôlée. Les objectifs de ces travaux sont d'abord de caractériser et de comprendre les transferts thermiques et dynamiques plasma / gouttelettes. Deuxièmement, de mettre en évidence l'influence de la synchronisation et des paramètres opératoires sur les dépôts obtenus. / In the field of thermal spray coating processes, research has led to the development of nanostructured coatings by suspension plasma spraying (SPS) and precursor solution plasma spraying (SPPS). Liquid injection are promising techniques with the potential to become industrially viable. However, a better control of plasma/material interactions is necessary. Mono-electrode DC torches indeed generate strongly fluctuating plasma that modifies the thermal and dynamic transfers to the injected droplet, resulting in an inhomogeneous treatment of the latter. This directly influences the texture and microstructure of deposits and subsequently their properties. Previous works have shown the origins of these instabilities. As an alternative to instabilities attenuations, a new approach is proposed: the reinforcement and modulation of the instabilities. The adjustment of process parameters has allowed obtaining a pulsed laminar plasma with a modulation of its properties. A low powered home-made modulated DC torch is used and operates with pure nitrogen as plasma forming gas. This device is synchronized with a Drop-On-Demand injection system to reproduce the same conditions of plasma/material interactions for each injected droplet. Solutions and suspensions are injected to make homogeneous coatings with controlled microstructure. The objectives of this work are firstly to characterize and understand plasma / droplet heat and dynamics transfers. Secondly, to highlight the influence of the synchronization and operating parameterson the coatings obtained.

Thermal barrier coating (TBC) systems are widely used on gas turbine components to provide thermal insulation and oxidation protection. TBCs, incombination with advanced cooling, can enable the gas turbine to operate at significantly higher temperatures even above the melting temperature of the metallic materials. There is a permanent need mainly of environmental reasons to increase the combustion turbine temperature, hence new TBC solutions are needed.By using a liquid feedstock in thermal spraying, new types of TBCs can be produced. Suspension plasma/flame or solution precursor plasma spraying are examples of techniques that can be utilized for liquid feedstock thermal spraying.This approach of using suspension and solution feedstock, which is an alternative to the conventional solid powder feed stock spraying, is gaining increasing research interest, since it has been shown to be capable of producing coatings with superior coating performance.The objective of this research work was to explore relationships between process parameters, coating microstructure, thermal diffusivity and thermal conductivity in liquid feedstock thermal sprayed TBCs. A further aim was to utilize this knowledge to produce a TBC with lower thermal diffusivity and lower thermal conductivity compared to state-of-the-art in industry today, i.e. solid feed stock plasma spraying. Different spraying techniques, suspension high velocity oxy fuel,solution precursor plasma and suspension plasma spraying (with axial and radialfeeding) were explored and compared with solid feedstock plasma spraying.A variety of microstructures, such as highly porous, vertically cracked and columnar, were obtained. It was shown that there are strong relationships between the microstructures and the thermal properties of the coatings.Specifically axial suspension plasma spraying was shown as a very promising technique to produce various microstructures as well as low thermal diffusivity and low thermal conductivity coatings.