Chemistry and physics of the ceramic core leaching process

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Adetunji, Daniel (2019). Chemistry and physics of the ceramic core leaching process. University of Birmingham. Eng.D.

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Abstract

The ceramic core leaching process is one of the final steps used in the investment casting process to manufacture air cooled turbine blades. The core is primarily composed of cristobalite and in a highly alkaline environment, depolymerisation of the tetrahedral SiO\(_2\) structure via siloxane cleavage is the chemical process via which leaching occurs. Industrially, this process is carried out in a low pressure autoclave using either NaOH or KOH solutions with a pressure swing cycle. The products from this reaction are either sodium or potassium silicates. The type of Group IA metal cation present had a significant influence on the dissolution process and the properties of the reaction products. Potassium silicates were found to be several orders of magnitude more viscous than sodium silicates at high concentrations. This led to a diffusion film controlled process in KOH and a chemical reaction controlled process in NaOH. A fluid film layer was formed on the cristobalite surface and consisted of potentially layered silicates which can absorb water, expand and cause cracking in the brittle ceramic core. The expansive nature of this gel has not been proven and requires some amount of calcium which exists only in trace amounts in the ceramic core. This is however still likely to be the primary physical mechanism responsible for core removal and is known as the Alkali Silica Reaction (ASR) cracking process.

Type of Work:

Thesis (Doctorates > Eng.D.)

Award Type:

Doctorates > Eng.D.

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