Influence of Nitrogen Environment on the Surface of LM25 using GTA

Objective: The main aim of this project is to modify the surface of LM 25 ingot in order to have a refined microstructure, improvement in hardness and in the wear rate of the modified region. Methods: Surface modification of LM 25 is carried out using GTA method. Argon flow was minimised and simultaneously pure nitrogen (99.999%) was introduced into the environment. Microstructural observation was carried out using Zeiss Axiovert 25CA metallurgical microscope and the hardness was measured using Mitutoyo Vicker’s hardness testing machine. Wear test was carried out for the substrate and the modified region using pin-on-disc wear tester. EDAX analysis was carried out to find the presence of intermetalic compounds. Findings: From the microstructure, it was observed that there is grain refinement in the modified region. The hardness for the substrate was found to be 80.6 HV and 764.4 HV for the modified region. The wear rate in the substrate was 49.34x10^-4 mm³/m and 7.9x10^-4 mm³/m for the modified region. As the hardness increases, wear rate decreases. EDAX report confirms the presence of intermetalic compound in the form of silicon nitride (Si₃N₄). The presence of the silicon nitride is not reported previously using GTA as heat source on surface modified LM25. The increase in the hardness and decrease in the wear rate is attributed to the presence of silicon nitride in the surface. Application/Improvements: Due to the presence of silicon nitride it can be used as an insulator and chemical barrier in electrical circuits etc. It can also have wider automobile and aeronautics applications as its hardness has increased.