Development of a modelling and simulation framework to optimize layered bearing architectures

Abstract

In recent years, higher cyclic loading demands, linked for example to higher output and lower weight requirements for engines, must be considered in the design of new surface coatings. This requires improved fatigue performance as well as better mechanical performance at the higher loads now proposed for these surfaces. The use of multilayer coatings in a thin overlay coating is a potential approach to ensure this improved fatigue resistance requirement by further developing coating architectures. However, the design of these thin multilayer coating systems has to date been achieved empirically through trial and error due to the lack of mechanistic understanding of what controls the coating’s performance, especially the fatigue performance, and the corresponding numerical assessment method. A multilayer coating system containing two hard intermetallic (IML) layers within a soft metallic system (denoted as 2IML multilayer coatings) provides improved fatigue performance. Experimental investigations were carried out on this system to develop the mechanistic understanding of fatigue failure mechanisms in layered coating systems. Various coatings with different locations and thickness of the intermediate intermetallic (IML-Top) interlayer were manufactured. Their geometry characteristics, microstructure characteristics and mechanical properties were assessed experimentally. Their fatigue performances were tested and compared with each other, revealing the improvement of fatigue resistance by locating the IML-Top layer close to the top overlay coating surface or decreasing the IML-Top layer thickness. Post-test observations were conducted to understand the mechanisms providing these fatigue improvements. The mechanisms of improvement obtained by locating the IML-Top layer close to the surface were identified as (1) improved retardation of crack initiation and (2) improved crack growth resistance provided by the shielding effect of the IML-Top layer. The mechanisms of improvement through decreasing the thickness of IML-Top layer could be linked to: (1) the increased difficulty of surface crack initiation due to the limitation of the strain ranges experienced at the top surface and (2) the shorter crack propagation period in a thinner IML-Top layer. In addition, the surface initiation mechanism and a series of subsurface crack initiation mechanisms were also observed. These subsurface crack initiation mechanisms are classified as Ni-induced, Void-induced and Interface-induced subsurface initiation mechanisms respectively.
Complex 2IML multilayer coatings were then simplified into more easily analysed architectures of dissimilar materials consisting of only leaded bronze and steel layers. A fatigue crack growth analysis method in layered architectures (analysing the case of a surface-initiated crack) was ii established by modelling work conducted on this simple exemplar system. In this method, the shielded/anti-shielded crack tip fields arising due to the discontinuous material properties in layered architectures were considered. The crack path was determined by using the maximum tangential strain (MTSN) criterion step by step for cracks approaching a material interface. The crack behaviour at the material interface was determined by a two-step method (modifying the stress-and-energy-based cohesive zone method) to consider the change in direction of an interface penetrating crack. It should be noted that the crack path and the evolution of crack driving force can be predicted in this method. Combining them with the crack growth data for each material in layered architectures, a simulation tool to predict the crack growth life was established. A more detailed finite element (FE) model of the actual 2IML overlay architecture, together with this established fatigue crack growth analysis method and the simulation tool to predict crack growth life were applied in the 2IML multilayer coating system. The importance of the surface initiation mechanism in explaining the achieved fatigue performance improvement of 2IML multilayer coatings was further indicated by stress distribution analysis in various 2IML multilayer coatings without simulating cracking. Retardation of subsurface crack initiation mechanisms related to the IML-Top layer (Ni-induced subsurface initiation mechanism and Interface-induced subsurface initiation mechanism) that was achieved by moving the IML-Top layer location closer to the top surface was also indicated by the analysis of stress redistribution in the absence of cracking. The transition criteria between crack deflection and penetration at interfaces around the IML-Top layer and the overall crack path in multilayer coatings were simulated by this fatigue crack growth analysis method to better understand the effect of layer architectural parameters on the crack path. Based on this, the crack growth life was predicted by the developed simulation tool. In comparison with the experimental investigations carried out in this research, the predictions of the simulation framework successfully predict the ranking observed for the measured overall fatigue behaviour, indicating the validity of this simulation framework. Overall, a series of fatigue failure mechanisms in layered coating architectures has been revealed and a simulation framework was developed to assess the fatigue failure of layered architectures in this study. In optimising design of layered coating architectures studied in this research (2IML multilayer coating system), the findings are: (1) decrease the thickness of IML-Top layer; (2) locate the IML-Top layer closer to the top surface and (3) further decrease the occurrence of subsurface crack initiation via improvement of the manufacturing technique. The simulation framework enables the relative benefits of such changes to be assessed.

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

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