A. J. Sturgeon, C. Reignier
TWI Ltd, Cambridge, UK
I. Laing, C. Perrin
Dana - Glacier Vandervell, Rugby, Warwickshire, UK
Paper presented at International Thermal Spray Conference & Exposition, 5-8 May 2003, Orlando, Fl, USA
Aluminium-based plain bearings for gasoline internal combustion engines are traditionally manufactured by casting and rolling, followed by forming and boring. The application places severe demands on the bearing material and a combination of properties such as fatigue, seizure and wear resistance are required. These properties are achieved by using a multi-phase material comprising of a distribution of tin in an aluminium alloy matrix. HVOF has been investigated as an alternative process for bearing manufacture and as a route to producing novel bearing materials with microstructures that cannot be achieved using the conventional casting route.
The work reported describes the use of different HVOF spraying systems and powder types to develop aluminium-tin based coatings for advanced bearing applications. The coatings are described in terms of microstructure characteristics. The fatigue performance of the advanced sprayed bearings is compared with conventional cast bearings.
Aluminium-based plain bearings for gasoline internal combustion engines are traditionally manufactured by casting and rolling, followed by forming and boring. The selection of lining alloy is determined by the requirement for a combination and compromise of bearing properties, namely strength (for fatigue and wear resistance) and compatibility (for conformability, anti-seizure and embedibilty properties). Current bearing linings of aluminium alloys are cast, rolled and applied to their backing support by roll bonding, then formed into half bearings and machined to their final shape. These bearings consist of a multi-phase material comprising of a distribution of tin in an aluminium alloy matrix.
However, there is a limit to the load at which aluminium-based bearings made by this method may be used. Traditionally, higher loaded applications would require a more expensive 'tri-metal' bearing comprised of a steel backing, with a lead bronze lining and an electroplated layer of soft material at the running surface. There is a need to increase the load capacity of aluminium-based materials for use in a wider range of applications.
High velocity oxyfuel (HVOF) spraying has been investigated as an alternative process for bearing manufacture and as a route to producing novel bearing materials with microstructures that cannot be achieved using conventional casting and rolling.  In particular, to achieve a finer and more homogeneous distribution of tin phase than by casting and roll bonding. The HVOF process was selected over other thermal spraying processes because it offers the possibility to deposit coatings with the required low levels of porosity and oxide. The attainment of ultrafine microstructures in HVOF sprayed aluminium-tin alloys has recently been reported. [2,3] Furthermore, the HVOF process will allow for the application of the bearing lining directly onto a component of complex shape, such as the internal bore of a bearing shell, if required.
The work presented here was part of a larger activity that was undertaken with the objectives:
- To control powder manufacture and HVOF spray parameters, including fuel type, to achieve novel aluminium alloy bearings with desirable and predictable microstructures.
- To manufacture an aluminium-based lead-free bearing alloy with increased fatigue strength over existing materials.
Coating Preparation: Development trials were initially undertaken to optimise the deposition of aluminium-12% tin coatings using three commercial HVOF systems; the JP5000 and TopGun systems from Praxair Surface Technologies and the Diamond Jet Hybrid system from Sulzer Metco.
The three HVOF systems differ quite significantly in terms of nozzle design, powder feed inlet position and type of fuel gas used in the high pressure combustion process. Detailed descriptions of these differences are described elsewhere.  During the initial trials, Al-12Sn coatings, nominally 300µm thick, were sprayed onto carbon steel test pieces roughened by blasting with 60 mesh alumina grit. Coatings were prepared using a range of different powder types and spraying parameter settings developed within a 'design of experiment' approach (not reported here). The deposited coatings were heat treated to develop a suitable microstructure consisting of an aluminium alloy matrix with a fine dispersion of tin precipitates. Gas atomised powders were used with particle sizes within the range of 45 up to 125µm. The HVOF systems, fuel types and powder types used to prepare the coatings reported in this paper are given in Table 1.
Table 1: Selection of Al-12Sn coatings prepared by HVOF spraying
|Label||System||Fuel||Powder size, µm|
At a later stage, selected coatings were deposited directly onto a number of pre-formed steel half shells. These half shells were formed from steel strip of nominal dimensions 80mm long by 31mm wide and 1.65mm thick. Spraying of the coating onto the formed half shells was achieved using appropriate jigging and mounting of the HVOF spray gun on a robot arm. The coating was deposited to a thickness of approximately 500µm over the whole internal half shell surface. This coating was then bored back to a thickness of 250µm to achieve the dimensional tolerances required to test the bearing shells. A schematic of the HVOF lined bearing is shown in Fig.1.
Coating characterisation: Deposition efficiency was obtained by dividing the actual amount of material deposited on a test piece surface by that calculated to have impinged onto the surface over the same time period. Cross sections of the coatings after heat treatment were prepared and examined by optical and scanning electron microscopy. The level of porosity was measured from optical images of the cross-section using quantitative image analysis equipment. For each coating three measurements were made and a mean value calculated. The oxygen content was measured on samples of coating detached from the substrate and ground to a powder, as well as for the original powders. The level of oxygen was analysed by the inert gas fusion technique, using Leco TC 136 equipment. Estimates of the oxide level in the coatings were calculated on the assumption that the detected oxygen was present as Al2O3.
Fatigue properties: The fatigue strength of several bearing linings was measured using a 'Sapphire' test rig at Dana Glacier Vandervell, Fig.2. In this rig two test bearings are housed in a connecting rod big-end and run against an eccentric shaft. A load is applied to the test bearings for a period of 20 hours after which the bearings are examined. If there is no sign of fatigue damage in the aluminium lining, these are replaced in the machine, the load increased by 7 MPa, and run for another 20-hour period. This continues until the lining is seen to fail by fatigue.
Results and discussion
With correct selection of powder type and spraying parameter settings it was possible to deposit Al-12Sn coatings to a thickness at least 500µm using all three HVOF systems. This could be achieved without significant pick-up of the aluminium powder within the spray gun, allowing for consistent spraying times long enough to coat a batch of half shell components in a single spray run. Measured values for deposition efficiency, porosity level and oxide content are given in Table 2.
Table 2: Typical characteristics of Al-12Sn coatings prepared by HVOF spraying.
|Label||System||Deposition efficiency, %||Porosity, vol%||Oxygen level wt%||Calc. oxide wt%|
The measured deposition efficiencies show significant difference depending on the spray gun used. With the TopGun system deposition efficiency was over 60% compared to about 20% for the JP5000 and DJ2700 systems. This may be explained by the design of the TopGun spray gun giving a longer residence time in the gun, leading to greater heating of the powder particles compared to other HVOF systems. As a consequence, a larger particle size and narrower size distribution was required for the TopGun system to prevent excessive pick-up within the spray gun.
With all three HVOF systems the deposited coatings were very dense with porosity measured well below 1vol% and oxygen levels measured at about 0.5wt%, corresponding to calculated oxide levels of about 1wt%. For comparison, the oxygen level measured in all of the powders was below 0.1wt%. An optical image of a cross section through one of the Al-12Sn coatings prepared using the JP5000 HVOF system is shown in Fig.3
. After heat treatment, all coatings exhibited a fine dispersion of tin precipitates within the aluminium alloy matrix. Figure 4
shows an SEM image of the coating cross section in backscattered mode. The lighter contrast phase (higher atomic number) is the tin precipitate, which is typically fine and well dispersed throughout the coating.
The coating microstructures obtained on completion of these preliminary trials were considered suitable for the production of bearing linings and subsequent rig testing. Linings of Al-12Sn and also potentially higher strength aluminium-tin-silicon alloys were then prepared on steel half shells, pre-formed prior to coating deposition.
A coated half shell bearing in the as-sprayed condition and after boring is shown in Fig.5. The fatigue performance of a selection of bimetallic bearings lined by the HVOF spraying method are shown in Fig.6 along with a cast and roll bonded linings of similar Al-Sn and Al-Sn-Si alloys. The HVOF prepared Al-Sn linings can be seen to at least match the fatigue performance of a conventionally produced lining. Further modification to the alloy composition has led to the production of Al-Sn-Si HVOF sprayed linings with an approximately 20% increase in fatigue resistance compared to the conventional cast and rolled alloy.
Summary and conclusions
- HVOF spraying has been demonstrated as a viable process for the deposition of aluminium alloy bearing materials.
- Spray parameters necessary for the production of dense coatings with a good distribution of second phases have been identified.
- An improvement of approximately 20% in the fatigue strength of half shell bearings has been achieved by combining the HVOF spraying production method with the development of new aluminium bearing alloys.
- There may be potential for the development of stronger bearing materials manufactured by HVOF spraying through further modification of alloy chemistry and processing parameters.
The authors acknowledge the support of other partners in the DTI/EPSRC LINK Surface Engineering Programme entitled SURSOMSPRAY. These were University of Nottingham, Industrial Reclamation Services and Phoenix Scientific Industries.
- Harris, S. J., McCartney, D. G., Horlock, A. J, Perrin, C. and Sturgeon, A J. 'Forming A Plain Bearing Lining', US Patent 6,416,877, 2002.
- Harris, S. J., McCartney, D. G., Horlock, A. J. and Perrin, C. 'Production of ultrafine microstructures in Al-Sn, Al-Sn-Cu and Al-Sn-Cu-Si alloys for use in tribilogical applications', Materials Science Forum, vols. 331-337, 2000, 519-526.
- Kong, C.J., Brown P.D., Horlock, A.J., Harris, S.J. and McCartney, D.G., 'Microstructural characteristics of high velocity oxy-fuel thermally sprayed Al-12wt%Sn-1wt%Cu alloys' Inst Phys. Conf. Ser. No 168 Electron Microscopy and Analysis 2001 (ed. M. Aindow and C.J. Kiely), Institute of Physics, 2001, 227-230.
- Kreye, H., Gartner, A., Kirsten, and Schwetzke, R, 'High Velocity Oxy-Fuel Flame Spraying', Proceedings of the 5th HVOF Spraying Colloquium, Erding, Publ GTS, 2000, 19-28.