Joining of advanced ceramics to metals

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Khene, K., 2003. Joining of advanced ceramics to metals. PhD, Nottingham Trent University.

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Abstract

A novel method for ceramic-to-metal joining, applicable for a wide range of emerging ceramic matrix composites (CMC) and cermets was developed. It combined singularity removal from the free surface and the introduction of a functional gradient zone, achieved by introducing a 'dome-flat' concept. This shaping was done by CNC electrical discharge machining (EDM) of the CMC by the preshaped metal part. These novel interfaces were modelled and the differential thermal expansion-induced stresses generated during brazing were predicted for 10mm, 20mm and 30mm diameter joints using finite element analysis (FEA). An experimental programme was conducted to validate the finite element analysis results as indicated by samples' residual strengths using tensile testing. It proved that removal of the excessive stress concentration associated with the peripheral region of simple flat interfaced joints, allowed useful joints to be manufactured with dome or dome-flat interface geometries in diameters up to 30mm in the Syalon 501/AISI 321 system despite its high coefficient of thermal expansion (CTE) ratio approaching 3.5:1. Thus, for lower CTE mismatch systems, diameters in excess of 30mm can be joined. For large joints, the strength limiting factor was shown to be more likely due to the decreasing strength attributable to increased ceramic cross section, than to the effects of induced differential thermal strains from joining. Active metal brazing was selected to manufacture the ceramic-to-metal joints and guidelines were presented for the production of strong joints. EDM machining proved invaluable for manufacturing the dome-flat geometry interfaces required with a good degree of accuracy and with near perfect interfacial conformity. For generating even more complex interfaces, non-contact EDM becomes the only viable machining method for generating ceramic surfaces of high complexity. Copper interlayers generally raised joints' strengths and resulted in more consistency in tensile tests. Techniques were developed for the accurate machining of complex interlayers and a tool design was proposed for producing precision cast or sintered metal interlayers on an industrial scale. Microhardness testing was used to monitor the differential thermal stress-induced strains at joints interfaces and, although braze- contamination hardening within the stainless steel and any copper interlayers accompanied the differential strain-induced hardening, the results provided evidence to support the predicted size effect on residual stress and strain in metal/ceramic joints. The strongest joints produced in this investigation had tensile strengths up to 52MPa and were found in 20mm diameter Syalon 501/AISI 321 stainless steel joints containing copper interlayers. The importance of balancing cooling rate across each interface of a joint was noted. For the soft copper interlayer used, the opportunity for recrystallisation to persist to as low a temperature as possible during cooling from brazing was an advantage. In addition it was beneficial to allow the full potential of the thermal stress-relaxing design features comprising the stainless steel featheredges and the copper interlayers to be realised to the limits of their strain absorbing capacity. Further work was proposed which would further increase the level of complexity of the interface by replacing the flat interface across the central region of the joint by a region of repeated interpenetration such as a 3-D sine wave, capitalizing on the ease of complex interface profile machining demonstrated through non-contact electrical discharge machining of ceramics.

Item Type:

Thesis

Creators:

Khene, K.

Date:

2003

ISBN:

9781369324914

Identifiers: