Characterisation of PI3 and RF plasma nitrided austenitic stainless steels using plan and cross-sectional TEM techniques

Type 304 and 316 austenitic stainless steels treated with plasma immersion ion implantation (PI3™) and RF plasma nitriding have been studied with transmission electron microscopy (TEM). A range of TEM specimen preparation techniques have been used to enable characterisation of the nature of the treated surface layers. Plan view TEM specimens, prepared by thinning from the back, highlighted the removal by sputtering of original surface features in steels during PI3 processing, while the less energetic RF plasma nitriding retained the original surface features. All treatments resulted in the formation of an austenite phase heavily expanded by the high levels of interstitial nitrogen. Decomposition of this expanded austenite into CrN and α Fe occurred at higher treatment temperatures, and was found to nucleate at the grain boundaries preferentially. Conventional cross-sectional TEM specimens also showed the importance of the surface as a nucleation site for this transformation. The transformation process and the products of expanded austenite decomposition were the same regardless of the treatment process used. The expanded austenite layer was aligned with the underlying steel, with no orientational difference across the interface observed. Focused ion beam milling has the ability to consistently section treatment depths of several micrometers, although further work is necessary to produce through-section specimens thin enough for EELS and EFTEM analysis.