Diagnostic tools for HVOF process optimization: Dissertation

In the thermal spray process the coating is built up from lamellas formed by rapid solidification of the melted or semi-melted droplets attached to the substrate. A typical structure for the coating is a pancake-like lamellar structure, where the flattening stage and adhesion between the lamellas, together with the coating material itself, define the main properties of the coating. Thermal spray coatings are often applied for better corrosion and wear resistance. Therefore, low porosity and good adhesion are desired properties for the coating. High velocity processes - especially HVOF (High velocity oxy-fuel) spraying - are the most potential methods for producing a good adherent coating with low porosity. From a scientific point of view, particle velocity and particle temperature, together with substrate temperature, are the main parameters affecting the deposit formation. They determine the deposit build-up process and deposit properties. Particle velocity and temperature affect the deposit efficiency as well as the microstructure. The aim of this work was to show the workability of diagnostic tools in the HVOF process. The focus was on first order process mapping, including on-line diagnostics and single splat studies. Nanocrystalline alumina composites and quasicrystals were selected, two materials that are complex to spray. With both materials the melting state of the particles must be well optimized in order to produce dense, well-adhered coating without unwanted changes in coating phase structure. The main focus was on the HVOF spraying of alumina. The target was to obtain a systematic understanding of the influence of the process conditions on the microstructure development in HVOF alumina coatings. Conventional limits of gas ratios and flows were exceeded to obtain a wide velocity-temperature range. The study aimed to produce information for a first order process map, and was carried out at a much deeper level than previously reported. Propylene and hydrogen as fuel gases were compared, and other variables, such as total gas flow rate, fuel gas/oxygen ratio, and standoff distance were also varied. The obtained data was applied for nanostructured alumina composite coatings, and the effect of the process conditions was compared on the obtained coating microstructure and properties. On-line diagnostic measurements, in which particle temperatures and velocities in the flame can be measured, were performed. The main work was carried out for alumina by using a DPV-2000 system. Two clear regions of different temperature and velocity arise from the use of different fuel gases. Single splat studies correlated well with the obtained coating properties, and a first order process map for alumina was created showing the window for the spray parameters producing best coating quality plotted against coating hardness and abrasive wear resistance. It was shown that diagnostic results can be correlated with the coating microstructure and coating properties in HVOF spraying. It was also demonstrated that the coating properties and coating quality can be improved by optimizing and carefully selecting the spray parameters.