Atom probe tomography characterization of engineering ceramics

• Main Author: Singh, Jaspreet
• Format: Dissertation
• Language: English
• Published: University of Oxford 2021
• Subjects: Atom-probe field ion microscopy; Ceramics; Materials science; Microscopy

This doctoral thesis aims to characterize the effects of carbon doping on alumina grain boundaries and the effects of flash sintering on YSZ and SiC microstructure using a variety of techniques with a particular emphasis on atom probe tomography (APT). Laser assisted APT, as applied to the analysis of ceramics, is a relatively nascent technique, so optimal operating conditions for the subject materials were rigorously investigated. A systematic and reproducible workflow with minimal subjective input for measuring the composition of features found in APT datasets was developed including a novel peak fitting ranging technique. Additionally, the effects of several APT reconstruction artefacts on compositional measurements were quantified, and the use of charge-state-ratios (CSR) was explored as a tool for minimizing these effects. Scanning electron microscopy (SEM) analysis of carbon-doped and undoped fracture surfaces demonstrated a quantitative increase in transgranular fracture with carbon doping. Grain boundary relative energy measurements made using atomic force microscopy (AFM) highlight the need for careful experimental parameter consideration alongside the limitation of the technique when applied to highly reconstructed surfaces. APT analysis demonstrates little evidence of carbon segregation to doped alumina grain boundaries contrary to previously published work, probably the result of an unconsidered peak overlap between carbon and magnesium [1]. APT analysis of conventionally sintered 3 mole percent yttria stabilized zirconia (3YSZ) alongside samples flash sintered in a variety of electrochemically reducing environments yields key microstructural insights and differences. Flash sintered samples demonstrate improved yttrium homogeneity which may indicate less susceptibility to often detrimental phase transformation. APT analysis of grain boundary chemistry demonstrates flash sintering in strong reducing environments reduces grain boundary segregation opening the door for grain boundary engineering. Bulk analysis demonstrates evidence of the first successful flash sintering of zirconium oxycarbide. Lastly, triple point analysis yields evidence of sub-structuring occurring along the length of the junctions, however electric field induced artefacts must be considered. The multi-scale analysis of conventionally and flash sintered SiC sintered with boron and carbon additives demonstrates fine-scale differences which may impact mechanical properties. Conventionally sintered samples subjected to long sintering times contain a bimodal grain structure with abnormal elongated grains, likely detrimental to mechanical properties. Electron backscatter diffraction (EBSD) analysis demonstrates no significant difference in polytype composition resulting from flash sintering, while energy dispersive x-ray spectroscopy (EDS) analysis shows microstructures with dispersed microscale boron/carbon rich inclusions. APT observation of nanoscale boron rich clusters prevalent in samples flash sintered using high power may help explain the anomalously high hardness values these samples demonstrate. Grain boundary analysis demonstrates the power used to flash sinter affects grain boundary segregation of boron and aluminum potentially due to differences in degree of sintering achieved and/or resulting grain size.