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Title: The effects of Alumina purity, TICUSIL® braze preform thickness and post-grinding heat treatment on the microstructure, mechanical and nanomechanical properties of Alumina-to-Alumina brazed joints
Author: Kassam, Tahsin Ali
ISNI:       0000 0004 7658 0910
Awarding Body: Brunel University London
Current Institution: Brunel University
Date of Award: 2017
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Alumina-to-alumina brazed joints were formed using 96.0 and 99.7 wt.% Al2O3 ceramics in as-ground and in ground and heat treated conditions using TICUSIL® (68.8Ag-26.7Cu-4.7Ti wt.%) braze preforms of thicknesses ranging from 50 to 250 μm. Brazing was conducted in a vacuum of 1 x 10-5 mbar at 850 °C for 10 minutes. Joint strengths were evaluated using four-point bend testing and were compared to the flexural strengths of standard test bars according to ASTM C1161-13. Post-grinding heat treatment, performed at 1550 °C for 1 hour, did not affect the average surface roughness or grain size of either grade of alumina but affected their average flexural strengths, with a small increase for 96.0 wt.% Al2O3 and a small decrease for 99.7 wt.% Al2O3. Post-grinding heat treatment led to secondary phase migration, creating a fissured 96.0 wt.% Al2O3 surface. This affected the reliability of 96.0 wt.% Al2O3 brazed joints, in which braze infiltration was observed. As the TICUSIL® braze preform thickness was increased from 50 to 150 μm, the average strengths of both 96.0 and 99.7 wt.% Al2O3 brazed joints improved. This occurred due to a microstructural evolution, in both sets of joints, which was studied using SEM, TEM and nanoindentation techniques. An increase in the TICUSIL® braze preform thickness increased the amount of Ti which was available to diffuse to the joint interfaces. This led to increases in both, reaction layer and braze interlayer thicknesses. Excess Ti in joints that were made using TICUSIL® braze preforms thicker than 50 μm, led to relatively hard Cu-Ti phases in an Ag-Cu braze interlayer. Cu-Ti phase formation, which may have reinforced joint strength whilst also reducing CTE mismatch at the joint interface, also led to Ag-rich braze outflow at the joint edges. Brazed joints made using as-ground 96.0 wt.% Al2O3 consistently outperformed brazed joints made using as-ground 99.7 wt.% Al2O3, due to the formation of Ti5Si3 phases at locations where the Ti-rich reaction layer intersected with the triple pocket grain boundary regions of the as-ground 96.0 wt.% Al2O3 surface.
Supervisor: Ludford, N. ; Nadendla, H. ; Anguilano, L. Sponsor: Engineering and Physical Sciences Research Council (EPSRC) ; TWI Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Advanced ceramics ; Active metal brazing ; Silver-copper-titanium braze alloys ; Ceramic-to-metal joint performance ; Microstructural evolution in brazed joints