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Title: Evaluating the effect of titanium-based PVD metallic thin films on nitrogen diffusion efficiency in duplex plasma diffusion/coating systems
Author: Yumusak, Gorkem
ISNI:       0000 0004 7655 4632
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Titanium is a very popular engineering metal due to its outstanding properties, such as low density and high specific strength. However, the wear resistance of titanium is very poor in many industrial environments. Wear-resistant hard coatings can be used to increase the service lifetime of manufactured products, but the effectiveness of these coatings on titanium is sometimes low due to the inadequate load-bearing capacity of the substrate. Therefore, titanium alloys benefit from a substrate strengthening thermochemical diffusion pre-treatment before the Physical Vapour Deposition (PVD) of hard ceramic coatings. In this work, triode plasma nitriding (TPN) has been applied to increase the load-bearing capacity of titanium alloys. It is known that the effective adhesion between PVD hard coatings and titanium alloy substrates can be improved significantly after substrate diffusion pretreatment. TPN treatments were used in this work because the diffusion of the nitrogen can be achieved more efficiently (than conventional nitriding techniques) at lower temperatures and shorter times, without the need for hydrogen in the gas mixture. Also, the (low) treatment pressure regime allows the coating and diffusion treatment stage to be combined and integrated into the same treatment equipment. Thus, the risk of hydrogen embrittlement is entirely avoided, and the treatment temperatures well below the alloy beta-transus minimise the grain growth (that can reduce the core strength and the fatigue life of the materials). The effectiveness of triode-plasma diffusion treatment can, however, be increased by applying a thin PVD metallic layer on titanium alloy substrates before the plasma nitriding stage. In this context, different compositions of β-titanium coating (stabilised by addition of Nb) were produced on a+β Ti-6Al-4V and β Ti-4Al-10V-22Mo substrate materials; the effect of formation of the β phase in Ti-Nb coatings before nitriding on diffusion treatment efficiency (and on nitride phase formation after TPN treatment at 700°C) was analysed. The hardening effect for Ti-Nb coated Ti-6Al-4V substrate was found higher than the other (uncoated and Ti coated) Ti-6Al-4V substrates after 4-hour triode plasma nitriding treatment. On the other hand, the hardening effect for Ti coated Ti-4Al-10V-22Mo substrate was found higher than other (uncoated and Ti-Nb coated) Ti-4Al-10V-22Mo substrates. Besides the hardness results, the wear coefficients of the Ti-Nb coated Ti-6Al-4V, and Ti coated Ti-4Al10V-22Mo substrates (after 4-hour triode plasma nitriding treatment at 700°C) were found approximately 16% and 31% lower (compared to their untreated counterparts) respectively. The enhanced performance of the duplex treated samples suggested that the coating deposition process (prior to diffusion treatments) needs to be optimised for each Ti alloy to attain adequate results.
Supervisor: Leyland, Adrian ; Matthews, Allan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available