Paper presented at Tribology 2006: Surface Engineering & Tribology for Future Engine and Drivelines, IMechE, London, 12-14 July 2006.
The use of high velocity thermal spraying as a method to form plain bearings for automotive engine applications was developed by a consortium of partners (that included the authors of this presentation) in a project funded by the DTI Surface Engineering LINK programme.  Further development has led to a widening in the range of bearing alloys deposited by this method, together with the launch of a commercial product by Glacier Vandervell. The presentation will give a brief summary of this approach to bearing production and highlight some of the more recent developments.
Plain journal bearings in automotive engine applications are required to offer a combination of properties, namely strength (for fatigue and wear resistance) and conformability, anti-seizure and embedibilty. As well as possessing good anti-friction characteristics, the bearings also need to be capable of withstanding cyclic loading, corrosion and temperatures in excess of 150°C for modern engines. As a consequence, alloys used for the bearing lining are highly engineered materials that must possess a complex balance of these properties. Modern bearing linings comprise of a composite microstructure with a moderate strength matrix containing soft phase regions. Traditionally there are two types of plain bearing, a trimetal and bimetal, both consist of a lining of about 0.25mm of bearing alloy on a steel half shell backing. The bimetal generally has a lining of aluminium-tin alloy to give an aluminium-based matrixwith a tin soft phase and is used principally for high volume gasoline passenger cars and some light truck applications. The trimetal has lead-bronze alloy as the lining, to give a copper-tin matrix with a soft lead phase, over which there is a thin, 0.015mm overlay of typically lead-based alloy to give improved resistance to seizure and is used for diesel passenger cars, heavy trucks and high speed gasoline engines where greater strength is need in the bearing alloy. For aluminium bimetals, the manufacturing route is casting of the lining alloy and roll bonding this to the steel substrate. Post-rolling annealing treatments are used to improve bonding and to recrystallise the bearing alloy for improved ductility. For trimetals, the bronze is cast directly onto a steel backing and following bearing manufacture, with the overlay deposited by electroplating or sputtering.
Deposition of bearing linings by HVOF spraying
Development activities at TWI Ltd, Glacier Vandervell and Nottingham University have now demonstrated high velocity oxyfuel (HVOF) spraying as an alternative process for bearing manufacture and as a route to produce novel bearing materials with microstructures that cannot be obtained using conventional casting and rolling. Using this approach it is possible to i) reduce bearing cost by simplifying the manufacturing process, ii) improve performance by refining the microstructure, iii) remove lead from bearings to help meet end of vehicle life legislation, iv) increase production flexibility and performance by removing the steel backing and depositing a lining directly on the journal surface.
In HVOF spraying, powder particles with a size range somewhere between 15-60mm, are injected into a high temperature and high speed gas jet within a specially designed spray gun. The jet is produced by the combustion of a fuel (gas or liquid) with oxygen at high pressures and flow rates within the gun. Powder particles typically attain velocities of 300-800 m×s-1 whilst reaching temperatures which allow them to be molten or semi-molten prior to impact. By scanning the spray jet across the substrate, a coating layer with low porosity is built up from the impact, bonding and solidification of successive particles, but with only minimal heating of the substrate. HVOF spraying is now well established and a number of systems are commercial available including the JP5000 from Praxair Surface Technologies, the Diamond Jet Hybrid from Sulzer Metco and the Met Jet II from Metallisation Ltd. The HVOF systems can differ significantly in terms of nozzle design, powder feed inlet position and type of fuel used in the high pressure combustion process. By using HVOF spraying with atomised metal alloy powders, it is possible to deposit bearing alloys as coatings on a steel backing and obtain ultrafine microstructures with a more homogeneous distribution of the soft phase than is possible by casting and roll bonding. Compared to other types of thermal spraying, HVOF is particularly suited for these lower melting point bearing alloys. This is because the very high spray particle velocity and good control of particle heating gives coatings with the required low levels of porosity and oxide, whilst maintaining the composition of the original atomised powder. It has been shown possible to deposit a wide range of alloy types for bearing applications including Al-Sn-Cu, Cu-Sn, Cu-Sn-Ni and Cu-Ni-In.
The development and optimisation of HVOF spraying for depositing Al-Sn based bearing alloys has been undertaken by the authors. [2,3] These alloys have been deposited using a number of HVOF systems of different design and atomised powders with a range of particle sizes. Coatings nominally 300mm thick were prepared and spraying parameters optimised within a 'design of experiment' approach. The optimised coatings are very dense with porosity measured well below 1vol% and oxide levels of about 1wt% or less. The deposited coatings were then heat-treated to develop suitable microstructures consisting of an aluminium alloy matrix with a fine dispersion of tin precipitates. A scanning electron microscopy (SEM) image of a cross section through an Al-12Sn-1Cu coating prepared using a liquid fuel HVOF system is shown in Fig.1 after heat treatment, with the lighter contrast phase being the tin precipitate.
HVOF Sprayed overlays for a trimetal bearing
Glacier Vandervell at their Rugby Technology Centre have developed and introduced a new trimetal engine bearing that uses HVOF spraying to deposit an Al-Sn-Cu overlay. This approach has also been part of a move away from lead-based linings and overlays without loss in bearing performance. A prototype spraying facility was set-up to develop and demonstrate spraying of overlays onto large batch production runs of half shell bearings. The spraying facility is shown in Fig.2, with views of the spray booth, HVOF spray gun and component-loading carousel. The Al-Sn-Cu overlay is initially applied as a 100mm coating over a bronze-based lining, then bored back to leave a thin layer of just a few microns, whilst also allowing the required dimensional tolerances to be achieved, Fig.3. The HVOF sprayed overlay has been successfully applied to both lead containing and lead-free bronze linings. Extensive laboratory testing has compared the wear, seizure and fatigue properties of bearings having HVOF sprayed overlays to those with a more conventional sputter overlay. The results obtained for a viper wear test and 'Sapphire' seizure test show that the HVOF overlay has a lower wear rate than the sputter overlay, while still matching its resistance to seizure. A lower wear rate is important to maintain clearances in the system and prolong overlay life to reduce the risk of seizure. Following these engine trials, a production facility has now been established in France.
Development of a sprayed bimetal bearing
Recent work at Nottingham University  has looked at improving the strength and performance of HVOF sprayed Al-Sn-Cu linings for bimetal bearings by using alloy additions. As an example, two alloys have been studied in detail, namely Al-20Sn-1Cu-2Ni andAl-20Sn-1Cu-7Si. The overall aim was to assess the coating microstructure as-deposited and following short annealing heat treatments, and then to investigate the microhardness and wear properties. Coatings were prepared using a Metallisation Met Jet II HVOF system and inert gas atomised powders with a nominal size range of 25 to 45µm. Coatings typically 200mm thick were deposited onto steel substrates, with the average substrate temperature maintained below 190°C. Spray parameters were optimised to minimise deposit porosity. Following spray deposition, the samples (coating plus substrate) were heat treated at 300°C for various times (1 to 5 hrs). This is above the melting point of pure tin to allow for controlled tin coarsening and precipitation of tin within the aluminium matrix. Wear testing of the heat-treated deposits was carried out at Glacier Vandervell, using the Viper test rig. Powder and coating microstructures and composition were determined using scanning electron microscope with energy dispersive X-ray analysis and by X-ray diffraction (XRD) analysis. It was found that the addition of 2wt.%Ni does not significantly affect the microstructure of the alloy, in both powder and coating form, whereas the addition of 7wt.%Si changes the microstructure in the powder, giving interdendritic Si and enlarged globular Sn. Following heat treatment, Sn coarsensin both alloys. In the Si containing alloy, the amount of Si phase slightly increases. In the Ni containing alloy small amounts of NiAl 3 form, as confirmed by the XRD spectra. These microstructural changes were found to have an influence on wear properties with improved results for the modified alloys, particularly the Si containing material. The scatter plot in Fig.4 is of the wear test results. This improvement was probably due to the presence of hard Si particles within the coating structure.