Plasma electrolytic oxidation (PEO) is a novel surface engineering technology, allowing relatively thick oxide coatings to be formed on metal parts. The process is superficially similar to the more familiar hard anodizing. The substrate is immersed in an aqueous electrolyte and a high potential, usually AC, is applied to it. The voltage between substrate and electrolyte rapidly rises as the native oxide thickens, and within a few minutes reaches several hundred volts. Sparks then start to appear on the surface, and this persists throughout processing. Recent work at Cambridge has confirmed that these are optically active plasmas, with durations ranging from tens to hundreds of microsecond, and peak temperatures up to ~10,000 K. These discharges allow oxide growth to proceed, so as to produce films with thicknesses of up to 100 microns or more. Moreover, the discharge events have a profound effect on coating microstructure, and hence on its physical and mechanical properties. Itâs thus possible to produce thick, strong coatings, extending the utility of such oxide films to encompass protection in tribologically and chemically aggressive environments, and also to offer significant thermal barrier function. The process is particularly effective on Al, on which it can generate thick, highly-adherent, hard and wear-resistant coatings.
Among the attractions of the process are that it involves very few health or safety hazards, with the electrolytes required containing neither the concentrated sulphuric acid nor the chromate ions necessary for hard anodizing. PEO coatings can be grown using solutions as simple and dilute as 0.02 M KOH, and are generally so mild that they do not require special chemical disposal, since they are less harmful than many household cleaning products. This is an advantage of increasing significance. Furthermore, coatings of uniform thickness can quickly and easily be produced on components with complex surface geometry, over a wide range of sizes, with no requirement for chambers or special environments. This cannot be said of most other coating techniques, such as thermal spraying, ion beam plating, sputtering etc.
In view of these advantages, PEO has recently attracted intense commercial and academic interest. Much of the research carried out hitherto has been aimed at characterisation of the coatings, and process optimisation through empirical observation. While several attempts have been made to explore the underlying coating formation mechanisms, many basic questions remain unanswered. Progress has been made in observation of discharge characteristics. Nevertheless, the development of an integrated and comprehensive model of the process remains to be achieved. In the project use of recently-developed plasma analysis techniques, focussing on single discharge events, will allow these characteristics to be monitored as a function of the processing conditions.
What can optical emission spectra tell us about PEO plasma in individual discharges? Optical emission spectra arise from the decay of excited electronic states of atoms and molecules in a hot gas or plasma. Provided that the plasma is optically thin, the strength of the emission lines is proportional to the product of the number of atoms or molecules in the excited state and the constant rate at which these decay to the lower state. If the decay rate constants are known, the measured strength of the lines can be used to estimate the relative abundance of the excited states. The relative strength of lines from different molecular species gives information about the composition of the plasma, while the relative strength of lines from the same species gives information about temperature.