Investigation of tribocorrosion behaviour of nitride based ceramic thin film coatings in sea water

The simultaneous wear and corrosion (tribocorrosion) of the metals in the sea water environment is a major problem for the marine industry. In order to overcome this problem, stainless steels are preferred because of their superior mechanical and corrosion resistance despite their high prices. The aim of this study is to contribute the development of cost-effective and wear and corrosion resistance increased machinery and equipment for the marine industry. In this study, relatively inexpensive AISI 4140 carbon steel used for the construction of various machine parts in the marine industry with similar mechanical properties to stainless steels was employed. Nitride-based ceramic thin film coatings (CrN, TiN and AlTiN) were made by cathodic arc Physical Vapor Deposition (PVD) method to improve the surface properties of plasma nitrided and non-nitrided samples of AISI 4140 steel. Tribocorrosion behaviors of substrate material and the coatings in sliding contact against with the alumina (Al2O3) ball were investigated in a natural seawater environment with a ratio of 2,38% NaCl at room temperature. In order to be able to control the efficiency of the study, tribocorrosion tests were performed on AISI 316 stainless steel under identic conditions and comparative evaluations were made. The structural properties of the coatings were determined using scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-distributed spectroscopy (EDS) and optical microscope. Wear, corrosion and tribocorrosion behavior of AISI 316, AISI 4140 materials and CrN, TiN, AlTiN coatings of plasma nitrided/non-nitrided were investigated under 2,5 N and 5 N loads, -1 V, 0,3 V and Open Circuit Potentials (OCP) in natural seawater environment. Potentiodynamic polarization tests of all materials and coatings were carried out between 1 V and 2 V voltages and 1 mV/s scanning rate. The experiments were carried out with the reciprocating ball-on-disc type tribometer integrated with potentiostat. The elemental content of the electrolytes obtained from tribocorrosion experiments under open circuit potential (OCP) was analyzed by Inductive Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) method. Material losses from wear track were determined by optical microscope. As a result of the experiments and analyzes, it was determined that plasma nitriding before the coating and all coatings implimentation improved the wear, corrosion and tribocorrosion behaviors of AISI 4140 material. It was determined that the wear rate increased and corrosion potential decreased in all potentials with load increase. The formation of pitting corrosion under anodic potential (0,3 V) increased. In addition to mechanical damages such as fatigue-induced micro cracks and delamination fractures, corrosion damage caused by the leakage of Cl ion into micro cracks was also determined. It was determined that material losses were increased from cathodic potential to anodic potential. Plasma nitrided CrN coatings were observed to have the most successful results under all conditions when the material losses resulting from the tribocorrosion tests under different potentials were evaluated. The use of plasma nitrided TiN coatings in the seawater environment was found to be suitable in the second degree, while AlTiN coatings was not found to be suitable. As a result, it was determined that the use of AISI 4140 steel with plasma nitrided and coated with correct film instead of the stainless steels is technically and economically feasible in sea water environment.