Fatigue of Multilayer coatings for plain bearings

Abstract

The work described in this thesis considers the fundamental fatigue behaviour exhibited by multilayer coatings used in plain bearings. Modern plain bearing designs comprise a number of layers with very different mechanical properties in order to accommodate dissimilar requirements such as load capacity and embeddability. Due to the evolution of modern internal combustion engines, power output has dramatically increased, increasing the loads that plain bearings have to support. Therefore, bearings have also evolved becoming more sophisticated but also more complex, comprising micrometre multi-layered structures. The development of these systems has been carried out in a semi-empirical manner over the past years. In industry the bearings are tested in engine-like rigs, these are useful pass-fail tests but they cannot provide mechanistic insight. This thesis presents the results of a three year research program focusing on three bearing designs that comprise three different multilayer coatings electrodeposited on a leaded bronze interlayer bonded to a steel backing. Each multilayer coating consists of different combinations of layers made of Sn-Ni-Cu. A series of standard experimental characterization techniques, as well as more advanced methods such as nanoindentation, tomography and image analysis are applied to investigate the microstructure and mechanical properties of the material system under study. Likewise, the results of simplified fatigue tests, as well as newly developed methodologies adding strain gauges and infrared thermography techniques are presented, ranking the different coatings based on their fatigue performance and characterizing initiation and short fatigue crack growth propagation in these systems. A combination of characterization techniques and fatigue tests were used to study the interaction between microstructure and fatigue cracks in order to establish the key factors controlling initiation and early crack propagation, hence providing data to propose new routes for new coating designs and to optimize current designs. The first part of this research was dedicated to the study of the interlayer, where we found that fatigue cracks tend to initiate and propagate preferentially through lead (soft phase) and early crack propagation through bronze grain boundaries. Moreover, voids (defects) also affect crack propagation increasing crack tortuosity and crack deflection at the interlayer/backing interface due to shielding effect. During the second part of the thesis, we ranked the fatigue performance of three multilayer coatings, finding that the best performance was given by the structure with two hard layers, versus one and three hard layer structures. We also identified the factors affecting their fatigue performance (constraint, roughness, defects, creep and shielding), which were studied in the third part of this project, concluding that the best performance is shown by the coating with the best balance between crack initiation and propagation. This was then used to propose concepts to improve new coating designs.

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Identifiers

ORCID for Andreu Laborda Cabo: orcid.org/0000-0002-0696-1916

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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