The effect of fretting conditions on the lifetime of aero-engine dry film lubricants systems

Laolu-Balogun, Emmanuel (2023) The effect of fretting conditions on the lifetime of aero-engine dry film lubricants systems. PhD thesis, University of Nottingham.

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This study was initiated to address existing fretting failure modes of the aero engine fan blade root components which are protected through the application of a bonded MoS2-based dry film lubricant (DFL), with a focus upon understanding the wear of the DFL system as a route to its lifetime extension. Fretting wear tests have been conducted over a wide range of testing conditions on Ti-6Al-4V specimens which are coated with a DFL system to understand the driving wear and damage mechanisms and reveal pathways to which the extension of fretting lifetimes could be achieved.

Investigations of how humidity and oxygen influence the fretting wear behaviour and performance of the MoS2-based DFL coating demonstrated that, in low humidity conditions (0.5 – 3 % RH), sustained low friction and low wear capabilities were observed with or without the presence of oxygen in the environment which led to significant extension of fretting wear lifetimes. The presence of oxygen in the low-humidity environment is however shown to alter the wear interface with the formation of a cracked layer composed of molybdenum oxide. As humidity levels were increased, rapid wear of the MoS2 DFL occurred along with an increased coefficient of friction which shortened the lifetime of the system. A transition occurred from low friction adhesive wear in low humidity environments where limited damage on the MoS2 DFL coating was incurred to high friction abrasive wear after air with increased humidity was introduced to the system which resumed the progression of fretting wear damage. A graphite modified MoS2 DFL (where ~ 5 % graphite powder was added to the MoS2 paint matrix) was shown to provide an order of magnitude increase in the coating lifetime in atmospheric (25 – 40 % RH) and high humidity (60 – 75 % RH) conditions.

Examining the substrate surface conditions upon which the polymer-bonded MoS2 DFL is deposited on illuminated the effects of interfacial topography and substrate material type. The addition of a thermally sprayed Cu-Ni-In under-layer prior to the application of the DFL resulted in an extension of fretting lifetime and durability to the fretting system. Increased surface roughness provided a mechanical key for the retention of the DFL which prevents delamination and delivers a stable low friction interface through the generation of a metal-mixed layer for additional durability and increased lifetimes.

By incorporating additional fretting testing capabilities into the experiment program, an expansion of the testing conditions which included increased loading and alternate (flat-on-flat) contact configuration could be tested to gauge the performance limits of the both the MoS2 and graphite modified MoS2 DFL coatings. Two distinct fretting wear behaviours were identified; (i) a steady low friction wear which is characterised by maintenance of low friction for the testing duration and providing an extension of fretting lifetime; (ii) tests which are characterised as having an initial low friction plateau followed by a rapid increase of friction consequently followed by coating film failure.

A threshold limit was proposed using a PV concept to determine the fretting wear behaviours of the DFL coatings. Below the proposed PV limit, a steady state low friction is observed whilst high above the limit, high friction tests to early failure are observed. Under more aggressive loading conditions, the graphite modified MoS2 DFL was found to offer increased fretting lifetimes along with a rise of the threshold limit between these two regimes in both cylinder-on-flat and flat-on-flat tests but much more moderate enhancement in performance were observed when tested in flat-on-flat tests under equivalent conditions; the reasons for the difference in behaviour between the test types is not clear.

Alongside the laboratory testing, post-service fan blades were made available so that an investigation of the DFL degradation modes in service could be undertaken. As a result of this work, test programmes looking at coating durability where attritive wear is the dominant mechanism of failure (such as the cylinder-on-flat and flat-on-flat tests reported in this thesis) need to be reviewed, and more appropriate and representative tests designed and evaluated.

This work has provided unique insights into the damage mechanisms and fretting wear behaviours of bonded DFL coating systems by investigating the individual operational factors of environment conditions, substrate surface conditions and fretting loading conditions. Distinct fretting conditions that generate sustained low friction and low wear behaviours have been identified and the mechanisms that drive this behaviour dissected. This has provided a better understanding of how the extension of DFL fretting lifetimes could be potentially achieved and how the performance of the DFL system can be further increased with additives.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Voisey, Katy
Shipway, Philip
Keywords: Fretting corrosion; Solid lubricants; Titanium alloys
Subjects: T Technology > TJ Mechanical engineering and machinery
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 72204
Depositing User: Laolu-Balogun, Emmanuel
Date Deposited: 21 Jul 2023 04:40
Last Modified: 21 Jul 2023 04:40

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