Influence of shaft coating thickness on dynamic behaviors of an automotive turbocharger

Turbocharger is a system that fitted to automotive engines for increasing performance and efficiency by taking air at atmospheric pressure, compressing to a higher pressure and passing the compressed air into the engine via the inlet valves. It consists of a turbine and a compressor impeller interconnected with a shaft supported in most cases by journal bearings. The shaft is one of most crucial component of turbocharger system and it operates at high speed. The shaft vibrations are inherent phenomenon and they are travelling to the surrounding structure effecting the stability and reliability of the turbocharger system. Selection of appropriate shaft material property is critical parameter amongst the parameters that affect vibration of the system. This study focuses on exploring whether it is possible to reduce the shaft vibration using different types of coating materials including Aluminum Chromium Nitride (AlCrN), Titanium Nitride (TiN) and Aluminum Titanium Nitride (AlTiN) each having thickness 2, 4, and 6 µm respectively for the shaft made of AISI 4140 alloy steel. The inherent natural vibration properties such as the resonant frequencies, damping, and mode shapes of the turbocharger shaft with and without coating were obtained by conducting the Experimental Modal Analysis through the measurements of Frequency Response Function which is about curve fitting the data using a predefined mathematical model of turbocharger shaft. The results were validated numerically by Finite Element Method. From overall results, it was observed that AlCrN, TiN, AlTiN coating thicknesses have a small effect on the resonant frequencies, but a good damping effect. The resonant response of the turbocharger shaft at resonant frequencies was suppressed remarkably by AlCrN and AlTiN coating especially having thickness of 6 and 4 µm respectively. In addition, 70 specimens of AISI 4140 shaft material were prepared and the mechanical and tribological properties of AlCrN, TiN and AlTiN coatings with thicknesses of 2, 4 and 6 µm were investigated. The coating surface, coating thickness from the specimen cross-section and worn surfaces were analyzed by scanning electron microscopy (SEM). The phase structures of PVD coatings were analyzed by X-ray diffraction and the crystal size and lattice distortion of the coatings were calculated. Microhardness tests were performed on the coatings. For tribological measurements, pi-on-disc tests were performed in oil and oil-free environments. In abrasion tests, the coatings were kept at 25±1 °C with 30%-40% relative humidity under a contact load of 5 N and the results were analyzed. According to the test results; it was observed that the coatings caused a decrease in wear rate and friction coefficient. TiN and AlCrN coatings with a thickness of 2 µm in lubricated and 4 µm in non- lubricated, respectively, had the lowest coefficient of friction. The 6 µm thick TiN coating with maximum hardness and reinforcement content showed the lowest wear track width.