| Summary: | 碩士 === 明新科技大學 === 化學工程與材料科技系碩士班 === 102 === The main objective of this research is to agglomerate and spheroidize nano-scale core-shell WC-Co powder into powder size of 10~40 μm. Then, use high-velocity oxygen fuel (HVOF) technology to prepare the thermal spraying coating of the powder and to do characteristic analysis of the coating. The powder preparation process includes concentration adjustment of liquid recipe, powder sintering and degreasing after spray drying. The OM, SEM and XRD tests are performed to analyze the dimension and configuration, the powder structure and the phase composition of the spheroidized powder. The result will be adopted for the parameter adjustment in the powder process. Next, HVOF system is applying to prepare the thermal spray coating specimens using the spheroidized powder and using the multiple sets of commercial imported powders. Meanwhile, the OM, SEM and XRD are used to analyze the coated layers. The coating characteristic comparison is done in terms of packing efficiency, porosity, micro-hardness, WC and base Co basic phase composition, and their micro-structure. The experiment shows that the agglomerated WC-Co powder prepared by this self-made spray drying technique can be applied in the commercial thermal spray machine.
The experimental result of spray drying process indicate that during liquid adjusting for the ratios of solid content, polyvinyl alcohol(PVA) and dispersant(DP) in the slurry. It is found that in the weight ratio of H2O: WC-Co: PVA: DP=64.7: 31.3: 0.6: 0.6, the spheroidized agglomerated powder with average diameter of 16 μm can be prepared. Next, the spray-drying powder should go through sintering and degreasing thermal treatment processes. Using high-temperature atmosphere furnace with a controlled-atmosphere at 1.21 bar, firstly de-gluing for 1hr with temperature 350℃ then turn heat to 850 ℃ to sinter for another 1hr under a controlled-Ar atmosphere to effectively avoid WC oxidation problem.
The same thermal spraying parameters are controlled for the thermal spraying process in this experiment. The coating specimen of spray-drying powder is compared with commercial coating specimen. It is found that the XRD analysis presents a main phase of δ-WC, and without pure BCC W or pure HCP Co phase were found. The OM and SEM are used to observe film surface, cross section structure and crystal distribution status. The result shows that the hard coating prepared by spray-drying powder can meet the basic commercial requirements of film uniformity and hardness. This study shows that spray-drying method can be applied for localized production. It can also be promoted to commercial thermal spraying application for the purpose of surface coating film of high hardness and high wear resistance.
|
|---|
