Elsevier

Tribology International

Volume 44, Issue 11, October 2011, Pages 1271-1280
Tribology International

Tribological performance of thin overlays for journal bearings

https://doi.org/10.1016/j.triboint.2010.10.027Get rights and content

Abstract

This paper describes the analysis of overlay materials used in journal bearings. We conducted model tests using ring-on-disc test configuration and component tests on bearing test rigs. Appropriate test strategies were designed for investigating the running-in, stable operating, emergency running and break-down behaviour. We investigated three different types of overlays: PbSn18Cu2 galvanic, polymeric overlay (PAI matrix with MoS2-lamellae and graphite), PVD-coated AlSn20Cu-Sputter. Main results are: PbSn18Cu2 improves emergency running conditions. Polymeric overlay shows high need for running-in, where a particle structured surface is formed. AlSn20Cu-Sputter exhibits lowest wear and highest load capacity in component tests, but requires optimum running conditions.

Research Highlights

►We tested journal bearing overlays and employed damage analysis techniques. ►Designed methodology is able to resolve tribological processes in high resolution. ►Electroplated Pb-based overlay improves emergency running conditions. ►Polymeric overlay with MoS2-lamellae forms particle structured surface. ►PVD overlay exhibits lowest wear and highest load carrying capacity.

Introduction

Journal bearings in modern engines are subjected to increasing demands which are outcome of trends in engine design aimed at increased fuel economy together with significant reduction in engine emissions. In order to meet these requirements, engine designs incorporate among other things, higher firing pressures and usage of low viscosity lubricants, which both increase the mechanical and tribological loading on the journal bearings. Since materials with high strength do not possess good sliding properties, modern journal bearings are designed with a multi-layer structure, wherein each layer is tailored to meet a particular requirement [1], [2], [3], [4]. Schematic set-up of a journal bearing shell is depicted in Fig. 1. A journal bearing consists of one to four layers of sliding material plated onto a steel shell which carries out the supporting function. The lining material is Al or Cu based and has a thickness of up to 0.5 mm. For bearings which meet high tribological loadings, additional coatings known as overlays are applied. These overlays operate as either running-in or permanent layers. They improve the running-in behaviour, emergency running properties, load capacity and corrosion resistance. Existing overlays can be grouped mainly into three categories.

Electroplated overlays: Electroplated or galvanic coatings are widely used as overlays and consist of soft metals and since these overlays are rather thin, their shortcoming in terms of mechanical strength does not play a significant role. Lead based PbSn(8–18)Cu(0–8) system is a commonly used material [1], [5], [6]. The soft lead rich matrix is reinforced by Cu and Sn, which form harder particles and the latter also improves corrosion resistance. Additionally Indium might also be added to increase the fatigue and wear resistance. To avoid diffusion of Sn from the overlay into the copper substrate, an intermediate layer of Ni is applied. Although these lead based systems perform well, owing to EU legislation governing reduction or elimination of lead from components, Sn, Bi and Ag based systems are intended as potential substitute [1], [4], [7]. The wear resistance of these electroplated overlays can be improved by addition of hard particles [8]. An adaptive layer system which adapts its properties during operation is described in [4] and is composed of SnCu6 and SnNi. Compared with PVD-coatings, electroplated coatings are cost effective, exhibit good sliding properties, but possess inferior fatigue and temperature resistance.

PVD-coated: Physical vapour deposition (PVD) processes, commonly known as sputter technique, help produce material with fine grained structures. Atoms released from target material by ions, are deposited onto the substrate material [4], [9], [10], [11]. A typical material system used for this process is AlSn based alloy, of which the most widely used is the AlSn20 alloy. The formed layers are much harder compared with the target material itself. These sputter coatings possess high fatigue strength, high load carrying capacity and high wear resistance combined with moderate sliding properties. Hence, they find their use in highly loaded engine bearings, such as in diesel engines. One of the disadvantages of these coatings is their comparatively high costs.

Polymer based coating: Polymer based coatings with incorporated solid lubricants are predominately used as running-in layers, and in certain cases, as permanent layers as well. They are typically sprayed on the bearing surface and these layers improve the running-in behaviour as well as the geometrical adaptability. A widely used matrix material is polyamidimide (PAI) with MoS2 and graphite lamellae as filler material [1], [12], [13], [14]. As permanent layers, these composite materials reduce the friction to the rotating shaft when the lubricant film breaks down.

In the context of this study we investigated the tribological behaviour of one characteristic representative of each above mentioned types of overlays under boundary/mixed lubrication. These conditions of lubrication are characteristic for failure of journal bearings due to seizure and adhesive/abrasive wear. They can be hardly visualized with state-of-the-art component tests on bearing test rigs, but via a test methodology on model scale [15], [16]. However, we would like to emphasize that the testing of other properties like endurance and wear, does require component tests. Therefore, we also compared the results from model scale tests with those obtained from the component tests.

Section snippets

Tested overlay systems

The overlays tested are coated to a thickness of about 20 μm on a CuPb22Sn2 substrate, and are provided by Miba Bearing Group. The properties of the substrate material along with the overlays are presented in Table 1. In this study PbSn18Cu2 was considered as a reference material for the electroplated overlays and a microsection of the same is depicted in Fig. 2(a). Similarly, AlSn20Cu and polyamidimide (PAI) based material with MoS2-lamellae and graphite were chosen for PVD and polymer based

Employed test methodologies

This chapter presents the test methods employed for both model scale and component tests.

Results: tribometrics and damage analysis

Tribometric results from the overlay systems investigated are presented in here. These are compared with each other with the help of pv-diagrams and further on with the results obtained from endurance tests using bearing test rigs.

Discussion

The results indicate, that the designed test methodology on model scale elucidates the processes taking place in journal bearing overlays under boundary/mixed lubrication conditions with high resolution. We identified the following states: (1) running-in, (2) stable operation, (3) emergency running, (4) break-down.

Since results of model scale tribometry on journal bearing overlays are not commonly found in publications, a comparative summary based solely on our findings is given below. The

Conclusions

The findings in model scale experiments substantiate the goal of these experiments, which is to elucidate the tribological functionality of journal bearing materials under boundary/mixed lubrication. High quality of the ring-on-disc method concerning the visualization of the running-in and emergency running conditions has already been shown [16], [17]. The differences between the overlays investigated in this study are significant. The results form the basis for a profound optimization of the

Acknowledgements

The authors would like to thank the Christian Doppler Research Association for the financial support. The authors are also grateful to Miba Bearing Group for the declassification of the results.

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    The paper was originally presented at the World Tribology Congress 2009 (WTC IV) Kyoto, Japan, 6th–11th September 2009.

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