Development of a Technique to Quantitatively Measure the Mechanical Properties of TiO2 Powder Particles and Coatings

Abstract

Indoor environment quality is starting to concern researchers who have found the concentration of indoor pollutants can be much higher than those outdoors. These volatile organic compounds (VOCs) can cause many health problems and recently photocatalytic oxidation has been seen an effective method that can be used for air purification. Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst, where light is absorbed by an adsorbed substrate. By using Titanium dioxide (TiO₂) as the photocatalyst, pollutants including organic and inorganic particulates can be broken down into less harmful compounds. In this study TiO₂ coatings were produced using two different methods, cold spraying and suspension plasma spraying to determine the parameters which would produce the most reliable coating for photocatalytic applications. Cold spraying of ceramics is a difficult procedure and to date only one powder form of TiO₂ has been successfully sprayed into a coating. It is thought that the two properties that determine the success of an applied coating are the mechanical properties of the powder itself and the substrate surface pre-treatment. A technique to quantitatively measure the mechanical properties of individual powder particles was successfully developed in this study using nanoindentation to measure hardness, elastic modulus and creep of commercial and lab-synthesised TiO₂ powders. Substrate surfaces were also measured before and after sand blasting to determine if this treatment alters the properties of the substrate at the coating interface. Results showed that the properties of the particles being sprayed are important for coating deposition, with increased ductility being the main property determining the success of a cold sprayed coating. Comparison of application techniques showed that suspension plasma spraying resulted in harder coatings than the cold sprayed technique. Further investigation revealed that this was partly due to the more dense structure of the suspension plasma sprayed coatings. Finally it was found that pre-surface treatment of the substrate through sandblasting not only increased the roughness of the surface, but also increased the hardness down to 150 μm of depth, most likely from work hardening.