Titanium Dioxide coatings obtained by Thermal Spray technlologies and their functional application

Abstract

The main subject of this thesis is the fabrication of functional titanium dioxide coatings by means of Atmospheric Plasma Spray (APS) and Cold Gas Spray (CGS). Functional role may be understood as the capacity of TiO2 surfaces to respond in a determined way under certain conditions.

Firstly, conventional coating processes, sensing mechanisms and overall efficiencies were deeply studied. As regards to experimental results, it was observed that H2 contained in the plasma mixture could reduce TiO2 towards non stoichiometric or stoichiometric compounds such as titanium sub-oxides (TiO2-x) or Magnéli Phases (TinO2n-1) respectively during the in-flight of the particles. Large accumulation of oxygen vacancies in the crystal lattice of rutile led to a donor level to the conduction band. Therefore, a corrosion-resistant ceramic material with a low electrical resistivity was obtained on ceramic tiles. This unexpected procedure led to deposit APS TiO2-x / TinO2n-1 coatings on stainless steel and apply them in electrochemical bi-polar batteries. Then, from the created feedback thin stainless steel and aluminium films, carbon-polymer composites or nickel foams as common standard electrode materials were selected and coated.

Produce the active layer of a metal oxide gas sensor using APS fed by TiO2 was still a target to be accomplished. With the aim of offering more innovation to conventional metal oxide sensors, it was determined to build-up the sensing layer on a thin polymeric flexible substrate. It was possible to reach certain spraying conditions that avoided thermal degradation of the polymer. Furthermore, heterogeneous disposition of the coating, where some areas were coated and certain spots uncoated provided electric contact between the electrodes and structure that eased elastic deformation of the film. Satisfactory performances were obtained testing the response of the device in front of a target gas and radiation.

Thenceforth, transition to thermally less-aggressive technologies was carried out. It was decided to focus the efforts on CGS, which does not require melting the material for being deposited. Subsequently, nanostructured anatase was used as feedstock in order to achieve photocatalytic layers with large specific surfaces for applying them in the degradation of different contaminants. It was used a powder able to create chemical bonds with the substrate and among the particles at the impact. Unfortunately, feeding system was repeatedly clogged because of the high agglomerating capacity of the powder. Blends were prepared with copper and microstructured TiO2 that flowed appropriately so as to avoid the obstruction of the pipelines. First, Cu/nano-TiO2 coatings were deposited using spraying conditions that favoured the deposition of nanostructured anatase at the top surface, which assured the development of the photocatalytic process. Samples successfully degraded toluene in gaseous phase. On the other hand, micro-TiO2/nano-TiO2 blend was not suitably deposited onto steel. Ceramic particles may not deform plastically. Thus, chemical bonds with the substrate and among particles had to be boosted for building-up the coatings. Substrate surface based on APS TiO2-x with controlled roughness provided composition, hardness and required geometry for adhering nano-TiO2 particles. In this way, CGS nano-TiO2 coatings were tested for degrading phenol and formic acid in liquid phase. The obtained results equalized or even improved the performance of sol-gel coatings.

Metallic Ti coatings were previously deposited onto the polymer by CGS for afterwards spraying nano-TiO2, following the know-how gained in CGS nano-TiO2 photocatalysts. Again, lower layer acted as a bond coat between the original substrate and nanostructured anatase. Osteblast cultures were tested on PEEK, CGS Ti on PEEK and CGS nano-TiO2 deposited on CGS Ti layer. Higher cell adhesion, proliferation and differentiation were obtained as long as CGS coatings were applied, which leads to an improved bioactivity of polymeric implants.

El principal objetivo de esta tesis es la fabricación de recubrimientos funcionales de óxido de titanio obtenidos por Atmospheric Plasma Spray (APS) y Cold Gas Spray (CGS). El rol funcional debe ser entendido cómo la capacidad de las superfícies de TiO2 de responder de una manera determinada ante ciertas condiciones.

El H2 contenido en el plasma podía reducir el TiO2 hacia compuestos no estequiométricos o estequiométricos como los sub-óxidos de titanio (TiO2-x) o las fases de Magnéli (TinO2n-1). Una gran acumulación de vacantes de oxígeno en la estructura cristalina del rutilo llevó a la formación de un nivel dador hacia la banda de conducción. Este inesperado procedimiento llevó a producir recubrimientos APS TiO2-x / TinO2n-1 sobre acero inoxidable y aplicarlos como electrodos en baterías bi-polares. Posteriormente, se decidió recubrir con este material electrodos típicamente utilizados como láminas finas de acero inoxidable y aluminio, compuestos de carbono-polímero y espumas de níquel.

Con la intención de ofrecer más innovación a los sensores convencionales de óxido metálico, se decidió fabricar la capa activa sobre un sustrato polimérico flexible. Fue posible alcanzar ciertas condiciones experimentales que evitaron la degradación térmica del polímero.

Se centraron esfuerzos en CGS, que no necesita fundir el material para producir el recubrimiento. De esta forma, anatasa nanoestructurada se utilizó como materia prima con el objetivo de lograr capas fotocatalíticas con gran superfície específica, capaces de degradar diferentes contaminantes. Se utilizó un polvo capaz de crear enlaces químicos con el sustrato. Se prepararon mezclas con otros polvos con el objetivo de mejorar su fluidez y evitar la obstrucción de las tuberías. Primero, recubrimientos Cu/nano-TiO2 fueron depositados utilizando condiciones que favorecieron el anclaje de las partículas de anatasa en la superfície del recubrimiento. Las muestras degradaron tolueno en fase gaseosa con éxito. Por otro lado, la mezcla micro-TiO2/nano-TiO2 no se depositó sobre acero. Se utilizó un sustrato préviamente recubierto con APS TiO2-x. Estos recubrimientos degradaron con éxito fenol y ácido fórmico en fase líquida.

Se decidió incrementar la bioactividad del PEEK (polyetheretherketone). Sin embargo, fue posible anclar partículas de TiO2 sobre el polímero previamente recubierto por Ti mediante CGS, obteniendo recubrimientos gruesos con una buena adherencia. Cultivos de osteoblastos fueron analizados sobre PEEK, Ti en PEEK y nano-TiO2 en PEEK. Se obtuvo una mayor adhesión, proliferación y diferenciación celular a medida que los recubrimientos CGS fueron aplicados.