| Authors: |
Jörg Oberste Berghaus* / Instituts des matériaux industriels | Industrial Materials Institute
Conseil national de recherche | National Research Council canada
Gouvernement du canada | Governement of Canada
, Canada
Basil Marple / Industrial Materials Institute,NRC, , ,, Québec, Canada
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| Abstract: |
Mullite (3Al2O3 ?2SiO2), an aluminosilicate with high thermal and chemical stability, low thermal expansion coefficient, low thermal conductivity and low oxygen permeability, has attracted considerable interest as a protective coating material in high temperature applications. It is used, for example, in heat exchangers, gas turbines and internal combustion engines i.e. diesel engines, often in combination with SiC or Si3N4 thermal barriers or alone. In the present study, mullite coatings were deposited by suspension thermal spraying of submicron feedstock powders, using a high-velocity-oxy-fuel gun (HVOF) operating on propylene fuel (DJ-2700). The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to coatings with low porosity (<1%), fine and homogeneous porosity distribution and smooth surface finish, making this process particularly suitable for creating thin layers with protective properties. Deposition efficiencies between 40 and 85% were attained. Conventional plasma sprayed mullite is generally fully amorphous and can show significant phase segregation. Recrystallisation during service at 700 to 1000°C has been reported to cause cracking and delamination from the substrate. In lowering the maximum particle temperature during spraying by using an HVOF system (flame temperature < 3000°C), as well as by dosing the evaporating liquid carrier, an attempt was made to delay the onset of overheating of the particles, reduce SiO2 evaporation and retain some crystalline phase in the coating. In-flight particle states were measured for a number of spray conditions of varying fuel to oxygen ratios and standoff distances and related to the resulting microstructure, stoichiometry, phase composition (EDS, SEM, XRD) and hardness (VHN 300g?f) of the coatings. Results indicate that this novel processing method can be tuned to create protective coatings with unique microstructure and phase composition to meet desired coating properties.
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