Rapidly solidified alumina by thermal spray: Crystal structure and phase transformations.

摘要

Thermal spraying is a rapid solidification processing (RSP) technique which is known to produce metastable phases and unique microstructures. Thermal spraying of alumina is no exception; it produces a coating containing primarily the metastable phase called gamma. Two other phases are also present in the coatings, the stable alpha-phase (corundum), and there is a significant amount of amorphous content. Subtle changes to the phase content and crystal structure caused by changes in the processing conditions can be observed by the X-ray diffraction pattern. This could make it a useful material to develop detailed process maps of any thermal spray process.; Quantitative X-Ray Diffraction (QXRD) was used to determine the relative amounts of the phases present. The crystal structure, defect structure, preferred orientation and phase transformations of gamma-alumina is investigated using x-ray diffraction. A calibration curve is derived to determine the relative weight fraction of the a phase of any thermal sprayed coating.; Crystal structure of gamma-alumina is known to be a defect FCC spinel based structure; Fd3¯m. High quality intensity data, was gathered to investigate the occupancy of the aluminum sublattice. The vacancies were observed to be located in the octahedral sublattice. It has been found that there is considerable occupancy of the non-spinel sites, 16c and 48f Wyckoff positions. It has also been found that there is a significant amount of disorder in the aluminum sublattice especially in the tetrahedral positions.; Preferred orientation measurements are made using XRD rocking scans on several gamma-alumina and alpha-alumina peaks. The grains in gamma-alumina do not appear to have any preferred orientation, but the scans do provide insight into the formation of gamma-alumina from the melt. The alpha-alumina grains in the coating appear to be large and highly faceted and they do exhibit slight preferred orientation. High temperature x-ray diffraction was performed in order to track the phase transformations from the metastable-phase to the stable alpha-phase. Isochronal and isothermal scans were performed to track the gamma → delta transformation and delta → alpha transformation. It was found that the delta → theta transformation was suppressed in the case of thermal spray alumina. This also caused a decrease in transformation temperature of 50C compared with the boehmite derived gamma-alumina. Four different thermal spray systems were used in this research: EMC plasma (Electro-Magnetically Coalescence), RF-Induction plasma (Radio Frequency), DC plasma (Direct Current), and HVOF (High Velocity Oxy Fuel). Four different feedstock materials, fused and crushed alpha-powder, spray dried gamma-alumina powder, sol-gel precipitated boehmite powder, and liquid alumina sol are investigated.