Investigation of NBN coating and properties of steels by thermo-reactive diffusion technique

Surface engineering endeavors, undertaken with the objective of enhancing the longevity and performance of steel-based materials, have perpetually intrigued humanity. In the pursuit of this objective, a plethora of methods has been devised to augment the characteristics of steel materials. In specific cases, these methods may not induce any chemical composition changes on the surface of steel materials, yet in others, observable alterations in chemical composition manifest on these surfaces. In instances where studies induce chemical composition changes on the surfaces of steel-based materials, the result is the observation of the formation of a surface layer distinct from the base material. The ramifications of corrosion damages in steel materials can give rise to persistent and/or performance-reducing disadvantages. In our endeavor to mitigate these drawbacks, we have implemented a coating process on our steels. This application aims to eliminate the adverse effects associated with corrosion damages, contributing to the overall improvement of our steel materials. Within the framework of this study, experiments were conducted, NbN coating was applied to the surfaces of AISI 1010, AISI 4140, and AISI M2 steels using the thermo-reactive diffusion method. The initial objective was to obtain nitrogen-rich layers on steel surfaces to achieve NbN coatings. For this purpose, AISI 1010, AISI 4140, and AISI M2 steels were subjected to the nitriding process (at 530-530°C for 10 hour). Following the nitriding process, niobium was coated on the steel surfaces using the thermo-reactive diffusion method. The niobizing process took place in a solid environment at 1000°C for 2 hours, utilizing the box cementation method, which includes ferro-niobium, ammonium chloride, alumina, and naphthalene. After the niobizing process following the nitriding process, NbN coating layers were obtained on the surfaces of all three steels. The structural characterizations and phase analyses of the coating surfaces obtained using the thermo-reactive diffusion method were investigated. An array of analytical instruments, including an optical microscope and scanning electron microscope (SEM), SEM-EDS, MAP, line analysis, and X-ray patterns, were employed for this research. Within the scope of these experiments, data on the presence, thickness, homogenization, and diffusion depth of the coating layer were obtained. Steel samples subjected to both coating and nitriding processes were separately taken for each, followed by metallographic techniques. The steels were etched with 3% nital, and surface morphologies were revealed, examining the coatings under an optical microscope. Our examinations revealed a homogeneous and continuous layer on the surfaces of different steel types subjected to the coating process (AISI 4140, AISI 1010, AISI M2). The steel with the thickest coating layer and the deepest diffusion depth was identified as AISI M2 steel. The alloy elements within AISI M2 steel were effective in greater xxviii diffusion of the coating layer. In the mapping of nitrided and NbN-coated AISI 1010, AISI 4140, and AISI M2 steels, all steels exhibited homogeneous coating layers and surfaces. Nitrided steels had a high nitrogen content on the surface. Additionally, in the mapping of all niobium-coated steels, niobium was found in high amounts on the surface, and as one descended from the surface to the matrix, the nitride ratio increased. To observe the development of mechanical properties in the mechanical properties of the NbN coating layer, hardness tests were conducted. The hardness measurements of the coating layers, interface matrix, and main material matrix of NbN-coated AISI 1010, AISI M2, and AISI 4140 steels were measured under a 15 gr load using a Vickers hardness indenter. Among the NbN-coated steels, the steel with the hardest coating layer was determined to be AISI M2, followed by AISI 4140 and AISI 1010 steels. The main reason for the highest hardness value of AISI M2 steel is thought to be the alloy carbides (NbN, Nb2CN) formed on the steel surface. To observe the development of corrosion behavior caused by NbN coating layers on steel surfaces, corrosion tests were conducted separately for each of the three steel types: pure, nitrided, and niobium nitride-coated samples using a Gamry potentiostat-galvanostat device. These tests were performed using three-electrode technique with graphite as the auxiliary electrode and kalomel as the reference electrode. The area subjected to corrosion was determined as 0.2874 cm². The experiments were carried out at room temperature, and corrosive environments included 0.5 M NaCl, 1 M H3BO3, and 1 M H3PO4 solutions. As a result of these experiments, two different curves were obtained, Tafel extrapolation (TP) and electrochemical impedance spectroscopy (EIS). The corrosion current (Ikor), corrosion potential (Ekor), and corrosion rate values were determined from these curves. The graphs obtained from the tests were shown on the same graph for each environment and steel type, making it easier to observe the progression of corrosion. Using these parameters, the corrosion current (Ikor), corrosion potential (Ekor), and corrosion rate values were calculated using the Gamry Echem Analyst program. Corrosion tests were carried out using the EIS method, one of the alternative current methods, in the frequency range of 1 Hz to 300,000 Hz and 10 mV rms alternative voltage parameters. The Nyquist curves created between the real impedance and virtual impedance values obtained from the tests provide insight into the corrosion resistance of the NbN layers exposed to corrosion. The wide diameters of the curves indicate high corrosion resistance of the steels. Considering these data, it was observed that AISI 1010, AISI 4140, and AISI M2 steels coated with NbN have the highest corrosion resistance in all three environments (0.5 M NaCl, 1 M H3BO3, 1 M H3PO4). After the corrosion experiments, SEM and EDS analyses were performed using scanning electron microscopy to examine the microstructure of the elements formed in the corrosive region and the corroded region. While surface microstructures were examined with scanning electron microscopy (SEM), elemental analysis was performed to evaluate the presence of elements on the surface after corrosion. As a result of the evaluations, it was observed that untreated steel materials underwent more corrosion compared to nitrided and NbN-coated steels, and a high proportion of iron oxide was found on their surfaces. In contrast, on NbN-coated surfaces, corrosion progressed much more slowly. The study concluded that among the three steel types coated with NbN (AISI 1010, AISI 4140, AISI M2), xxix improvements were observed in corrosion resistance and mechanical properties. The developments observed on the coating surface of the steels have been examined and explained.