Turbofan jet engines are widely used as a means of propulsion in the aviation industry. The fan is a key part of the engine, providing a large proportion of the overall thrust, and is subjected to the harshest loading conditions in the engine. Fan blades are connected to the disc through a dovetail joint, where a low friction molybdenum disulphide surface coating is used. This coating enables the blade to move within the joint to achieve its least stressed location in response to service conditions. Should this coating fail, the fan blade is no longer able to move, and the blade root is subject to fatigue. One notable incident occurred in Australia in 2001, where the aforementioned fatigue of the fan blade led to a contained failure, highlighting the need to ensure this coating is well understood and works effectively. A literature review at the start of this work identified that there were gaps in knowledge surrounding the coating system used by Rolls Royce on fan blades, and that the exact mechanism and failure modes of the coating as it undergoes fretting are currently unknown. As such, the aim of this research project is to determine the functional tribological behaviour of molybdenum disulphide coatings when applied to titanium on titanium contacts, in the context of an aero-engine fan blade dovetail joint.
A failed blade root was tribologically assessed, along with biaxial samples coated with the molybdenum disulphide coating and tested on a Rolls-Royce developed representative test platform. From these components, the functional behaviour of the coating was assessed, and a failure analysis was performed. Following this, the response of the coating to a range of load and surface conditions was investigated, firstly through coated ball on flat samples on a tribometer, and secondly via a purpose built test platform, where contact conditions representative of the fan root slot were achieved. For each of the two stages of testing, in-situ force measurement allowed changes in the coefficient of friction throughout the test to be determined, with this dataset then coupled with full surface characterisation of the samples post-test. Finally, an ultrasonic based method was proposed to monitor wear in-situ on a fan blade root slot contact, and trialled on the aforementioned purpose built rig.
The wear mechanism of the coating was found to depend both on contact pressure as well as the surface topography. At moderate to high contact pressures, when wear debris was retained in the contact, a durable, low friction glaze layer was observed to form on the surface. This layer occurred as a consequence of compaction and sintering of wear debris, generated during the initial interaction between the two surfaces. With time, this layer was observed to lose ductility, and breakdown, forming a cracked, platelet structure, before ultimately being removed. Surface waviness was also found to play an important role in the formation of this glaze layer, with peaks in the surface profile concentrating contact stresses, improving compaction and sintering of wear debris. In cases where the contact pressure was lower, the coating was found to undergo wear, without the formation of the glaze layer. In such cases, the contact pressure was found to be too low to promote compaction and sintering of wear debris, and friction remained high. Finally, ultrasound was demonstrated to be a useful tool for monitoring wear of molybdenum disulphide coatings in-situ, however significant further work is required to develop the technique into a reliable condition monitoring tool.
