AbstractsChemistry

Highly effective surface engineering of structural alloys has been accomplished by infusing very high, non-equilibrium concentrations of nitrogen or carbon through the alloy surface. Earlier approaches used either plasma or gas phase methods. The current work explores a new technique called the encapsulation method. The workpiece is encapsulated together with a solid reagent. Upon heating, the solid reagent decomposes into molecular species that (1) activate the surface by removing the passivating oxide layer, (2) infuse interstitially dissolved nitrogen or carbon into the alloy.Amino-based compounds have been used previously for surface-nitrocarburization of austenitic stainless steel. When urea decomposes at the processing temperature, its products activate the steel surface and facilitate the infusion with nitrogen and carbon. Surface characterization and metallography of urea nitrocarburized samples demonstrated case formation, while surface hardness increased up to 4 times when compared to untreated austenitic stainless steel samples.In this project we discovered that guanidinium hydrochloride can nitrocarburize austenitic stainless steel samples. Using various C–N compounds, we showed that the nitrocarburization process of stainless steel highly depended on the chemical composition of the pyrolysis products and on their partial pressures in the reaction chamber. The variations in thickness and Vickers hardness of the nitrocarburized layer were analyzed in a set of experiments with urea and other C–N compounds. The correlation between nitrocarburizing conditions and case hardness was demonstrated in different types of steel, and optimal parameters for nitrocarburization were elucidated.Low-temperature nitrocarburized austenitic stainless steels possess remarkable mechanical and electrochemical properties. The nitrocarburization of austenitic stainless steels using urea and other C–N compounds, such as guanidine, can be applied to finished components by conformal, industrially relevant process. Advisors/Committee Members: Ernst, Frank (Advisor), Heuer, Arthur (Advisor).