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Title: Oxidation and wear of TiAlN/VN multilayer PVD hard coatings
Author: Zhou, Zhaoxia
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2010
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TiAlN/VN multilayers, with a layer periodicity of ~3nm, have exhibited superior sliding wear resistance (1.26xlO-17m3N-lm-l) and lower friction coefficient (~=0.4, pin-on-disc test, Ah03 ball counterpart) when compared to other commonly used wear protective coatings, e.g. TiN, TiAlN and TiAlN/CrN. They require excellent oxidation and wear resistance for dry high speed machining operations. This project investigates the microstructure of the as-deposited coatings, their oxidation degradation mechanism and their wear and friction properties at room and elevated temperatures. The microstructure of the starting films was studied in terms of intermixing between the TiAlN and VN individual layers caused by the absence of shutters during the industrial PVD deposition. A FEGTEM coupled with EELS revealed chemical distribution of individual layers at nanometre resolution. Cs corrected STEM allowed the composition of individual atomic columns to be imaged. It was also used to probe across the interface of TiAlN/VN with angstrom beam (1 A) using EELS which showed a 1±0.1 nm thick intermixing between TiAlN and VN. Film growth and elemental distributions were therefore theoretically predicted in association with substrate rotation. The experimental compositional profiles and the prediction showed good agreement. The coatings deposited with -75 V and -85 V substrate bias voltage were multilayer TiAlNNN, 37at%:::;V:::;55at%, 0.81:::;(Ti+Al)/V:::;1.73, which were used for subsequent oxidation and wear studies. The oxidation behaviour of these coatings in air was investigated using thermal gravimetric analysis up to 1000°C and compared to TiN and TiAlN. Static oxidation of TiAlN/VN films was studied in the range 550-700°C, and characterised by high temperature in-situ X-ray diffraction and STEM/EDXlEELS of selected surface cross-sections. The oxidation resistance of TiAlN/VN was found to be controlled by the VN layers and consequently oxidation was initiated at a lower temperature than TiN and TiAlN coatings. The onset for rapid oxidation of the TiAlN/VN coating was found to be 2550°C. At temperatures >600°C, a duplex oxide structure was formed; the inner layer comprised a porous region of Ti-rich and V-rich nanocrystallites, while several phases were observed in the outer region, including V20 5, Ti02 and AlV04. V20 5 was the dominant oxide at the outer layer at 2638°C. An Au marker study suggested roughly equal diffusivity of cations outward and oxygen inward diffusion occurred during oxidation. Further to the oxidation study, dry sliding ball-on-disc wear tests of TiAlN/VN (V 55.2at%, Ti 28.5at% and Al 16.3at%) coatings on flat stainless steel substrates were undertaken against Ah03 at 25°C, 300°C and 635°C in air to investigate the relation between the presence of V 205 and low friction. The friction coefficient was 0.53 at 25°C which increased to 1.03 at 300°C and decreased to 0.46 at 635°C. Detailed investigation of the worn surfaces was undertaken using site-specific TEM via FIB, along with FTIR and Raman spectroscopy. Microstructure and tribo-induced chemical reactions at these temperatures were correlated with the coating's wear and friction behaviour. The friction behaviour at room temperature is attributed to the presence of a thin hydrated tribofilm and the presence of V 205 at high temperature.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available