A critical review of the current knowledge of PEO coating characteristics and properties is made, and several areas requiring more accurate or more detailed investigations are identified. A leading commercial product – the <i>Keronite<sup>TM</sup></i> coating for aluminium alloys – is the used as a basis for the investigation of the microstructure and properties of PEO coatings. Coating structure and morphology are studied quantitatively to investigate their growth mechanism. Composition is accurately determined for the first time, allowing predictions of physical, mechanical and thermal properties to be made. Particular attention is also paid to the presence of amorphous material and fine-scale porosity – properties which had previously been neglected. The latter is critical to the understanding of coating formation and the capacity for coating impregnation, and is measured and characterised using numerous porosimetry techniques. Mechanical properties of the coatings are characterised using indentation and macroscopic techniques such as beam bending. Correlations are established between the observed structure and measured physical properties such as hardness, local modulus and global stiffness. It is found that wear resistance can also be explained on the basis of the measured mechanical properties and structure. The discovery of low coating stiffness means that high-temperature applications, which had previously been dismissed on the basis of thermal expansion mismatch between the coating and substrate, may indeed be possible. The thermal stability of the coatings is therefore investigated and their stability up to 800°C is demonstrated. Residual stresses are measured and explained in terms of the postulated coating growth mechanism.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598226 |
| Date | January 2006 |
| Creators | Curran, J. A. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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